Downwind turn discussion

[Pilot DAR]

Elsewhere, thread drift has wandered to a discussion of the characteristics of a downwind turn at a slower airspeed. Passions smoulder on two sides:

One being that the wind has no affect on turn performance nor indicated airspeed, as the aircraft is moving in a parcel of air. The airplane does not know that the air is still or moving, just that it is flying with indicated airspeed, which is not affected by wind. What it was doing into the wind, it will be doing out of the wind once turned 180 degrees. Albeit with a slewed turn, and differing groundspeed.

The other side mentions characteristics of inertia of the aircraft, with the belief that it could be possible for the change from into the wind to out of the wind could occur faster than the inertia of the aircraft [in space, not the parcel of air] could be overcome by the turn acceleration. The possible result being a temporary reduction in indicated airspeed resulting from the turn out of wind, while the inertia of the aircraft is overcome and accelerated in space back to the original IAS, while groundspeed increases too.

Thoughts form the group? How is this concept taught and learned?

[Jan Olieslagers]

When turning my slow and light (and thus, very low on inertia) craft in pronounced steady wind, I certainly feel the effect of that wind, though I could not describe it.

But the wording of the question makes me doubt: "downwind" is one stage of the traffic circuit, are you discussing the effects of wind on how to fly the circuit? What exactly is meant by "a downwind turn", if not "a turn into the downwind section of the circuit"?

[Pilot DAR]

To clarify Jan, the term "downwind" is not relative to position in the circuit as much as having been flying directly into the prevailing wind, to now be flying away from it.

[Maoraigh1]

The difference is in the ground speed, and you must maintain air speed without being distracted by that.

[piperboy84]

A few years back I was in a ground school being taught by a freshly minted instructor. He was explaining that on downwind at an airspeed of 100kts if there was a tailwind of 20 kts the aircraft would have an airspeed of 120kts. During the break he joined me outside for a smoke, I took a cigarette paper and threw it into the wind and asked him if that wind was 10kts what is the papers airspeed? He said 10kts, I gave him a raised eyebrow and went back inside. After the lunch break (where he did not join myself and the other students) he came back into class looked over and just nodded with a sheepish look on his face. He proceeded to review the mornings session and correct any misapprehension any of us may have had.

[Chuck Glider]

I remember being confused by this when learning to fly.

I was taught your 'other side' which accounts for inertia, though it was not described in these terms, which might have helped my understanding of what I was being told. I couldn't see it, arguing the 'moving parcel of air' view and I recall my argument used the example of circling a hot air balloon.

When I eventually figured out by myself what the instructor was trying to convey I did at least acknowledge the possibility but maybe I've never made a sufficiently abrupt turn down wind but I cannot say I have ever noticed the effect of inertia in that situation.

[ShyTorque]

Wind shear effect? As you turn from into wind to downwind, the effect of sudden gusts changes. Flying into wind, airspeed is likely to increase with sudden gusts. Flying downwind, sudden gusts tend to decrease airspeed. The pilot of an aircraft flying close to the stall in a steep turn may well learn this quite suddenly.

[Jan Olieslagers]

Flying close to the (already much increased) stall speed in a (steep) turn is never a good idea, whatever the wind, whatever the gusts. Don't ask me how I know...

[Meikleour]

Pilot DAR (http://www.pprune.org/members/150561-pilot-dar): You might be interested in this observation. Many years ago I would routinely fly into Narita, Japan in B747s. In the winter it was common to have descent tailwinds of in excess of 100kts. The aircraft would usually be operated with the speed mode lockes onto 300kts. The first half of the arrival procedure, down to about 20,000ft. would have the full benefit of the entire tailwind. Next the arrival called for a 90 degree turn which resulted in the loss of the entire tailwind component. The aircraft would continue at 300kts. BUT the VSI rate of descent would increase hugely during, and for a time after the turn was complete, before returning to its usual value. Patently, inertia was at play here as the aircraft had to dive more steeply to recover/maintain the IAS. A rather unusual set of conditions no doubt but the "independent in its own block of air" view did not hold here.

[gondukin]

When I started gliding, it made sense to me that my airspeed is relative to the air, so surely by and large I would be carried along with any fluctuations. All this talk of wind gradients didn't make sense.

My first landing in a strong headwind dissuaded me of this notion. If any doubt remained, turning downwind off a practice cable break in a decent cross pushed the message home :)

[wiggy]

Jan

Flying close to the (already much increased) stall speed in a (steep) turn is never a good idea, whatever the wind, whatever the gusts.

:ok:

I know you'll all know this but just in case I'll offer up here what I offered in the other place (where TBH I think discusion on windshear and gusts and parcels of air may be confusing the matter):

Once upon a time UK CFS Bulldog students were demo'd the downwind turn "problem" when they started Low Level Nav flying/nav exes, 250 feet aal dual, perhaps, but not sure, 90 knots ish IAS). The training was nothing to do with coping with parcels of air or windshear; you can fly round at 45 AOB or more all day in your parcel of air at low level quite safely as long as the fuel allows and as long as you don't care where you end up, the point of the exercise was to make folks aware that it gets more complex when you need to turn to accurately to overfly a waypoint on the ground.

e.g. You are flying downwind, and it's a strong wind 30 knots plus, tailwind initially... you are going to turn so as to be on a new heading as you overfly a fixed point on the ground. You start off by using your "standard" 10 knot day angle of bank, standard IAS, standard 10 knot day amount of anticipation. :rolleyes:.... Of course the tailwind becomes an increasing beam wind and half way around the turn you see you are sliding sideways over the ground and are now struggling to get around to overfly the fix, something you really want to do that to get set up accurately for the next leg of the navex...so....the temptation is bound to be to tighten the turn ....."just a bit more bank"...."just a bit more pull / loading ( more g /AOA)"....."oh, darn, It's really windy and I'm not quite getting there even with 60 AOB, so perhaps I'll take just a bit more.......:eek: I suspect "wrapping" up a turn at low level has killed more pilots than parcels of air or windshear ( on "light" types)

Same problem/logic can apply to display flying ( without pointing fingers but e.g. trying to stay inside a fixed line such as a display boundary) and no doubt elsewhere.

[mikehallam]

FWIW.
Noticeable in a light a/c when turning say LH Base Leg (from Downwind) & the wind is regular & brisk on the runway heading, that that turn left actually exceeds 90 degrees in the air because the wind is pushing you to the right.

With reduced speed for the circuit, if careless, it could be as low as stall +30%. Yet there's a need to turn more steeply than in still air - just to maintain what is optically the familiar square pattern over the ground.

Steeper turn, at low speed is said to be a classic stall/spin scenario where one is too low to recover.

I always watch the ASI like a hawk in all stages of approach/landing & get down to safe flap speed before the turns to give extra margin over stall speed.

In stiff wind be aware of this optical illusion 'trap'.

mike hallam.

[Fitter2]

If you have a physicist friend, ask him about 'frames of reference'.

Depending on your latitude, you have a velocity (relative to the centre of the globe you are airborne above) of between 450 and zero metres/second, plus the vector sum of your groundspeed. Now, does turning downwind at the North Pole, and doing it at the Equator have a different effect? Discuss (as my lecturer used to say).

The bright ones will catch on, the rest go by whatever book they trust.

Wind gradient is very important, (as is the optical illusion of speed over the ground), but a totally different effect from 'inertia'.

[ShyTorque]

Wiggy, yes, been there, done that CFS "low level intro", at 250 feet in the old Bulldog. My staff instructor watched me fly my "give back" to him then quietly asked what height we ex-Heli pilots were used to flying at. I correctly answered along the lines of "down to 50 feet agl, in training areas, and no minimum height if carrying out a concealed approach or departure". He politely asked if I would mind climbing a bit then, as he wasn't used to being that low. Oops! I'd been doing my stuff much lower than he was used to. I'd just gone down to what I felt was a comfortable height.

[27/09]

The best way I can think of to illustrate this to a student is to place an object (pen) on a piece of paper on a flat surface (table).

The pen is the aircraft, the paper is the "parcel" of air the aircraft is flying through.

I move them both across the table with the pen move faster than the paper, thus representing what is happening when flying in a moving parcel of air.

I then stop the movement of both the pen a paper, letting go of the pen, I then tell the student to watch what happens if the speed of the "parcel" of air changes. I jerk the paper and the pen gets "left behind".

I then explain this is what happens to an aircraft when the parcel of air in which it's flying changes direction or speed. The result is manifested in a momentary increase or decrease in IAS while the aircraft catches up.

[megan]

What I posted in the thread Pilot DAR refers to.

Flying helicopters in the offshore world it was not uncommon to have 60 knots of wind when taking off from a platform. Climb speed in our particular aircraft was 75 knots and the turn to downwind while holding climb speed was visually spectacular if not seen previously. The point is, the aircraft doesn't care what the wind is, and if you are flying by reference to instruments you would have no idea what the strength of the wind is, or indeed, if there is any wind, save for the fact that you already have 60 knots airspeed prior to commencement of the take off.If you have a physicist friend, ask him about 'frames of reference'.One example we used to use by way of explanation was, imagine a Piper Cub flying a constant rate turn at 90 knots and such an altitude that its inboard wheel was resting on the bonnet (hood) of a convertible car, and the car drivers job was to maintain the position of the aircrafts wheel.

Now consider what the car driver experiences when no wind is blowing, and when say 60 knots of wind is blowing. Also what does the aircraft/pilot experience in both cases.

Aircraft inertia only comes into the discussion when talking with reference to gusts, which is not the subject under discussion.

[piperboy84]

Fascinating topic and posts. Just so I'm on the same page, the terms gusts and windsheer are interchangeable and mean the same thing?

And, 27/09's pen and paper demonstration method is limited to simulating horizontal sheer (gusts) and not vertical sheer?

And, at what point in the sheer does the inertia come into play ?

[9 lives]

I agree that the aircraft does not know what the air in which it is flying is doing, to a point. I have been taught that at the slow side of the aircraft speed scale, where wind speed is faster, inertia can begin to play a role in pilot perception, and then performance.

Flying helicopters in the offshore world it was not uncommon to have 60 knots of wind when taking off from a platform. Climb speed in our particular aircraft was 75 knots and the turn to downwind while holding climb speed was visually spectacular if not seen previously.

Though I have nowhere near Megan's experience flying helicopters, during my training, the balling out I received turning downwind after takeoff was memorable. In the SW300, I was climbing nicely after translation, and with lots of room under me (though not established in the normal [airplane] circuit yet), I gently turned crosswind then downwind. My instructor lit into me for that, explaining that the inertia of the helicopter might be overcome by the change in apparent wind direction relative to its mass, and would not accelerate fast enough with the wind to maintain the IAS and the rate of increase of groundspeed, and re-enter translation He went on to demonstrate the effect of allowing the helicopter to be caught by and apparent (to the helicopter) increase in tailwind, and begin to settle as more power had to be added to maintain the climb. It was a memorable demonstration. He is a 20,000 hour helicopter pilot, so I listened to what he said.

I accept that the effect is probably minimal to none with an airplane flying a normal circuit or at normal speeds, as it does simply move with the air. My experience with STOL equipped Cessna floatplanes has been that it is possible to get into an alarming situation turning downwind during a low speed climb, when trying to evade rising terrain. I agree that gusts, terrain effects, and increasing windspeed with altitude would be factors in the perception of decreasing performance. If the pilot could climb ahead safely, they probably would have. If they have chosen a low speed climbing turn, terrain is probably a factor in their decision. then they are flying with visual reference to that terrain, and may try to fly more slowly than they should in the climbing turn. Though a steady wind may not be affecting performance singularly, the combination of effects of wind and perception can get a pilot in trouble. I have experienced this in low powered skiplanes and floatplanes, where tight climbing turns in confined areas resulted in the sensation (or stall warning) of degraded climb performance at that speed. I've known two pilots who have reported to me after the accident that as they climbed in a turn, the wind caught them from behind, and they settled into the trees. In both situations, my senses allowed me to believe that they experienced a combination of terrain effect, and increasing windspeed as they climbed - but they still crashed in a downwind turn, where a climb ahead would have worked fine.

When dissected in physics, I'm sure there are purist answers which differ. From my training and experience, I fly and train others that this is a cautionary, or avoid if possible situation.

[RatherBeFlying]

I have been researching my own low level windshear incident with the assistance of GPS, Altitude, TAS and GS among other items at one second intervals from my recorder.

Much of the discussion of windshear is based on the blithe assumption that it's laminar - simplifies the equations;)

There's vortices lurking down there. When the air drops some 60 metres in 5 seconds, you're in the toilet bowl:uhoh:

[rotorfossil]

I used to have the student fly a low level downwind turn In two ways. First fly it looking out at the ground- result invariably a loss of airspeed. Second fly the same turn looking only at the instruments- result no loss of airspeed. The problem in this scenario is the visual effect of the apparent gain in speed as you turn downwind. HOWEVER, I have observed that if it is a climbing turn onto the downwind, that is climbing with an increasing tailwind component, maintaining airspeed does need a more nose down attitude to maintain airspeed.

[tmmorris]

Isn't the real effect of the downwind turn a mental illusion more than a physical phenomenon - at least in relatively light GA aircraft? Coupled with people flying circuits fixated on ground features rather than their relative position to the runway (leading to tightening turns instead of just accepting a wonky circuit shape). The latter can be due to poor instruction (I was certainly taught the circuit for a specific airfield first, rather than generic) or over-familiarity with the home airfield and concern for noise complaints.

[wiggy]

Just so I'm on the same page, the terms gusts and windsheer are interchangeable and mean the same thing?

Can't speak for what is taught in the light aircraft world but in the "heavy" world the answer is no.

A Gust is defined as a sudden change of windspeed at a fixed position, e.g. at the windsock/anenometer.

Windshear is a sudden change of windspeed/direction associated with a change in position - horizontal and/or vertical. (e.g. flying into the downdraft from a CB, or descending/climbing through a strong inversion).

[ShyTorque]

The shortfall of the "parcel of air" concept is that air movement is seldom constant. The aircraft will fly out of the first "parcel" and into another, with possibly very different movement. The problems of airspeed maintenance comes as the aircraft crosses the boundary.

[Fitter2]

ShyTorque

True, but irrelevant to the original question re 'inertia'

[Piltdown Man]

The question's terms of reference are vague. If we are talking about a homogeneous airmass then effectively we are talking about equivalent of a fly in a moving car or bus etc. It will have a 'strange' path across the ground but as far as it is concerned, it is flying from one side of the vehicle to another. But as soon as airmass changes speed or direction, or there is turbulence or shear layers then it becomes more interesting. Such as the detail about the 747 descending with strong tailwind, or low turns in a helicopter, or turns close to the ground in a glider following a cable break. I'm sure there is good maths to calculate what is safe and what is not. But there is also "pucker factor". Before I ever considered a low turn I always made sure I had plenty of airspeed. Anything above 0.1% PF had me worried.

PM

[Chuck Glider]

Keeping in mind that the OP was, I think, talking about a turn up in the air, ie. not connected with landing patterns or other ground features, what happens if you perform a 360° orbit on a day that has a wind blowing?
Is your airspeed fluctuating up and down as you head into and away from the wind? I'd say not.

[astir 8]

Mr Step Turn's description of flying into wind towards rising terrain and subsequent loss of height while turning away from the terrain sounds to me (as a ridge soaring glider pilot) a lot like sink being generated by the air mass flowing down the lee side of the mountain/ridge. And you can lose an awful lot of height turning in sink! Maybe not so much to do with the inertia issue being discussed

However returning to the inertia issue, if you're soaring close to a ridge in gusty conditions, for the sake of your health, you do it a lot faster than when it's calmer!

[9 lives]

An example of the reason which I don't accept the "parcel of air" concept to the exclusion of an inertial factor is as follows:

If, in my STOL modified C 150, with it's original 100 HP engine (so low power), I take off directly into a 25 MPH wind (north, let's say), I can be airborne, and initiating a turn at 50 MPH IAS. That will be 25 MPH groundspeed (GS). Yes, I know that the 'plane does not know its groundspeed, but the 'plane still is subject to the inertial forces of rigidity in space (or something like that, I'm not a physicist).

Now I'm turning away from north, with 50 MIAS, and 25 MGS. I will soon be flying west at 50 MIAS, and a changing GS. But, somewhere along that turn, before west, my GS along the north/south line will be zero. Then, moments later, I'll be flying south, 50 MIAS, and what will the GS be? The math says 75 MGS. For the purpose of this discussion, subject to expert physicist comment, GS is relative to rigidity in space for the 'plane, irrespective of the parcel of air. The 'plane still has inertia.

So, my question: Can we expect the lowly 100 HP C150, in a climbing turn at full power, to accelerate from zero groundspeed on the north/south line, to 75 MPH south, fast enough inertially, that no loss of airspeed is experienced with the perceived wind change? Yes the wind will contribute to the acceleration, south, but the 'plane does not have a sail on it either. Power is required to accelerate the 'plane from zero to 75 MGS against its inertia. That power is already being used for the turn and climb (and 150's are not known for excess power). On a runway, with zero wind, acceleration from zero to 75 MPH will use up at least 500 feet of ground run. A tailwind will help some, but will not negate the need for power to accelerate the 'plane, taking time and distance. During that time and distance, what had been the low speed climb/flight performance advantage of a headwind, is now the disadvantage of a tailwind, and must also be overcome.

This to me is an open issue. Irrespective of the theories of physics purity in the moving parcel of air, my experience (in the aforementioned C150) has been that if you're planning a low speed turn toward downwind, allow for a period of lesser performance as the 'plane accelerates inertially, to catch back up to the moving parcel of air. Right or wrong, this has been taught and demonstrated to me, and I teach it onward as I mentor. Purists will tell me I'm wrong - perhaps so, but I'm wrong so as to remind pilots to be conservative and safe about performance in a changing and demanding regime of flight.

[Heston]

There is an interesting conflict in standard ppl teaching. We bang on about the parcel of air and the aircraft not knowing where the ground is and so on - ie totally ignoring inertia.
Then we introduce the concept of wind gradient and what it does to airspeed on the approach - using the inertia of the aircraft to explain what is going on. So is the inertia of the aircraft important or not? No wonder folk get confused.

[Crash one]

The "pen and paper" analogy is an excellent description which would obviously work in the vertical as well, except it would not be possible to demonstrate.
As for the inertia part. The aircraft's mass/weight is defined/affected by gravity, not by the surrounding air parcel.
Therefore if the "pen" is replaced by half a matchstick, representing a very light aircraft rather than a heavy jet, when the "gust" is applied the very lightweight aircraft will react quicker (catch up quicker) it's airspeed will change relative to the air parcel, but by a smaller amount and for a shorter time than the heavy aircraft as the inertia, body at rest etc, takes over.
An excellent discussion and difficult to explain.
Perhaps if the parcel were to be a bubble of air suspended in a vacuum, then remove the bubble forwards and the aircraft has no air to keep it up and will fall through the vacuum until another bubble comes along in the right direction to replace the lift. Ground speed has nothing to do with it, until the bubble is replaced by a bubble of granite which produces very little lift!
Please excuse my gibbering.

[wiggy]

if the "pen" is replaced by half a matchstick, representing a very light aircraft rather than a heavy jet, when the "gust" is applied the very lightweight aircraft will react quicker (catch up quicker) it's airspeed will change relative to the air parcel, but by a smaller amount and for a shorter time than the heavy aircraft as the inertia, body at rest etc, takes over.

Indeed, the teaching/thought on windshear for years has been that airliners are much more prone to potentially fatal problems with windshear than light types, especially in the rapidly reducing headwind situation, due to the generally much higher "m" in the "mv" ....interesting to see some here talking about it having significant effect on light aircraft.

[Crash one]

Another thing I recall, ref inertia.
Take a small rowing boat in a heavy sea alongside another boat of the same size.
Both rise and fall together so jumping from one to the other is easy.
Then place a small boat against the ladder beside an aircraft carrier in the same heavy sea. The small boat is leaping up and down the ladder while the carrier stays relatively static, jump across at the wrong time and you will get very wet.
Don't ask how I know!!

[Heston]

So, my question: Can we expect the lowly 100 HP C150, in a climbing turn at full power, to accelerate from zero groundspeed on the north/south line, to 75 MPH south, fast enough inertially, that no loss of airspeed is experienced with the perceived wind change? Yes the wind will contribute to the acceleration, south, but the 'plane does not have a sail on it either. Power is required to accelerate the 'plane from zero to 75 MGS against its inertia. That power is already being used for the turn and climb (and 150's are not known for excess power). On a runway, with zero wind, acceleration from zero to 75 MPH will use up at least 500 feet of ground run. A tailwind will help some, but will not negate the need for power to accelerate the 'plane, taking time and distance. During that time and distance, what had been the low speed climb/flight performance advantage of a headwind, is now the disadvantage of a tailwind, and must also be overcome.

Step
The trouble with this argument is that it also applies to a turn in nil wind. In still air the aircraft and its engine have to accelerate the aircraft from say 75kias north to 75kias south. We dont have any problem thinking that the aircraft can do that without losing airspeed due to its inertia.
What we do is use the lift vector tilted by banking the aircraft into the turn to provide the required acceleration, and a bit of back pressure to increase the lift so that we maintain level flight. This increase in back presuure of course gives us a slightly greater AoA which may be noticed as a small reduction in airspeed (because drag also increases). But thats nothing to do with the wind because there isn't any.

To really convince yourself that for a turning aircraft the wind has no noticeable effect is to go for a flight in a glider (sailplane) on a good thermal soaring day with say a 15knot wind. Get well centred in the thermal with say a turn at say 45deg of bank. The glider will turn through 360deg in less than 30 seconds and the airspeed will be rock steady all the way round. You may hit areas of greater or less lift but these are seen and felt as changes to the climb rate - there is no change in airspeed so long as attitude and bank angle are maintained.

[9 lives]

Hmmm, I lack the experience flying gliders to know if I can equate my thoughts of a powered aircraft to an unpowered aircraft in this context. I do agree that a glider can perform a climbing turn without a loss of airspeed, but my very modest physics thinking about the inertia involved is incomplete when it comes to gliders.

I entirely agree that this discussion, in the context of gliders, is valid and interesting, though I'm not sure if it's the same discussion as that for powered 'planes.

[megan]

My instructor lit into me for that, explaining that the inertia of the helicopter might be overcome by the change in apparent wind direction relative to its mass, and would not accelerate fast enough with the wind to maintain the IAS and the rate of increase of groundspeed, and re-enter translation He went on to demonstrate the effect of allowing the helicopter to be caught by and apparent (to the helicopter) increase in tailwind, and begin to settle as more power had to be added to maintain the climb. It was a memorable demonstration. He is a 20,000 hour helicopter pilot, so I listened to what he said.If your flying at the nominated climb speed there is absolutely no way that a wind variation is going to make the aircraft enter the translation zone. Either the 20,000 hour instructor knew zip or had great trouble explaining what he was about. In my time of mountain flying in horrendous winds never ever came across a problem with maintaining a desired airspeed. Sure the airspeed bounced around a bit but nothing of much concern, turbulence in the vertical axis and yawing got your attention though.

[Flyingmac]

Step turn wrote.

This to me is an open issue. Irrespective of the theories of physics purity in the moving parcel of air, my experience (in the aforementioned C150) has been that if you're planning a low speed turn toward downwind, allow for a period of lesser performance as the 'plane accelerates inertially, to catch back up to the moving parcel of air. Right or wrong, this has been taught and demonstrated to me, and I teach it onward as I mentor. Purists will tell me I'm wrong - perhaps so, but I'm wrong so as to remind pilots to be conservative and safe about performance in a changing and demanding regime of flight. That about sums it up.:ok:

[oggers]

Step Turn, if you fly a coordinated turn it makes no difference to your airspeed whether you are turning into wind or downwind. This goes for rotary wing as it does for fixed wing. Please stop conflating this myth with the other hazards of low level maneuvering.

[Heston]

I'll try to expain my argument better:

Inertia is the property of bodies with mass wherein they maintain their momentum unless a force acts on them to change it (what we call acceleration - which can be in the direction of existing movement or not, depending on the direction that the force acts in). Newton's Laws and all that.

So imagine an aircraft flying north at 75kias in no wind. To turn it through 180deg and fly south at 75kias we have to provide an acceleration in a southerly direction capable of changing its velocity by 150knots.

Now imagine there is a 25knot tail wind. Ground speed is 100knots, with our same 75kias. To turn through 180deg and fly south, how much acceleration do we have to provide to maintain our airspeed of 75kias?

We know that after the turn our ground speed will be 50knots (75kias minus the wind speed of 25knots). So we have to accelerate the aircraft in a southerly direction by 100+50 knots = 150knots.

In both cases the change in velocity (ground speed) of the aircraft is the same. So we have accelerated it by the same amount in both cases. Which means that we have used the aerodynamics of the airplane in exactly the same way in both cases to provide the accelerating force to make the turn. So the airplane doesnt know it is flying in a wind.

If we are flying close to the ground we will see the turn as looking very different in the two cases of course..

[9 lives]

If your flying at the nominated climb speed there is absolutely no way that a wind variation is going to make the aircraft enter the translation zone.

I entirely agree. Both in the context of helicopter and airplane.

But, is that to extend the logic backward to an aircraft being [foolishly] flown at a much slower speed through the same maneuver? (Which I was doing in the SW300, at the time I was reprimanded).

The risk I see, and train against, is that of the pilot who has already made a poor choice about entering a turn which less than a full pocket of performance, and fails to make allowance for additional factors which may further degrade their aircraft's capabilities....

[Heston]

The risk I see, and train against, is that of the pilot who... ..fails to make allowance for additional factors which may further degrade their aircraft's capabilities....

Absolutely. In the circuit (ie close to the ground) turns have to be well handled and allowance has to be made for turbulence for example. But the "downwind turn" is dangerous because of the optical illusion that the speed has increased as the turn is completed and the pilot raises the nose to compensate, so reducing airspeed. Balanced turn, constant attitude and airspeed - thats what you teach, no?

[megan]

So imagine an aircraft flying north at 75kias in no wind. To turn it through 180deg and fly south at 75kias we have to provide an acceleration in a southerly direction capable of changing its velocity by 150knots.There is no acceleration in order to change the ground speed, because the aircraft is operating with respect to the airmass, not the earth below. A Boeing 777 cruising at Mach .84 at 38,000 has an IAS of 267, have a stall speed of 205 (cruise speed 1.3Vs) and may be flying in a jetstream of 200 knots. What do you think is going to happen if he turned from a headwind into a tailwind in those circumstances. Not that an airliner is going to fly in that sort of headwind.

A reading of how an INS works may give an understanding.

[Heston]

There is no acceleration in order to change the ground speed...

Oh dear I give up! I hope to God you dont actually fly a 777, megan. Please tell me what does change the groundspeed then?

[9 lives]

There is no acceleration in order to change the ground speed, because the aircraft is operating with respect to the airmass, not the earth below.:confused:

My car requires acceleration to change groundspeed, and it operates with respect to an airmass, (it's just not supported by it, or propelled within it - but it still sees it as drag or propulsion).

My airplane requires acceleration down the runway, and is propelled in the the airmass, but still must accelerate relative to the ground to increase both groundspeed and airspeed.

When I open the throttle, my 'plane will accelerate, but not instantly so, the propeller operates inefficiently while it overcomes the inertia of the aircraft to accelerate ithen things get better, as the propeller operates at greater efficiency. My car is a little better, and will press me into the seat, (no tire to ground inefficiency) but that's as it accelerates me inertially - I'm personally, not affected by the airmass.

I continue to cling to the belief that mass and inertia play a role in how the 'plane responds while moving in and with the airmass, at least at slower airspeeds.

[megan]

My airplane requires acceleration down the runwaySay your manual says rotate at 70 knots, say there is 70 knots of wind, what acceleration is there in order to rotate? Ridiculous example I know, but go back to my post re helo take off in 60 knots of wind. Turning downwind with 74 k climb speed meant our ground speed went from 14 to 134 knots. No acceleration applied to the airframe or pilot, as any accelerometer on the aircraft would tell you, for the simple reason an aircraft is operating, as I said, with respect to the airmass, not the earth.

Have a read on INS and in particular the frame of reference by which it works.

http://www.courses.netc.navy.mil/courses/14009A/14009A_ch7.pdf

[Heston]

Turning downwind with 74 k climb speed meant our ground speed went from 14 to 134 knots. No acceleration applied to the airframe or pilot, as any accelerometer on the aircraft would tell you

Total rubbish, as the document that you link to explains.

Since the INS works by measuring accelerations, in the example you quote, if there was no acceleration when you turn downwind then the INS would instantly be lost.

Im sorry, but this is basic Newtonian physics (again as the document you link to explains) which you learn at school when you are about 14 years old (in the UK anyway). But I'll let you off, because you aren't that old are you?

[Shaggy Sheep Driver]

I just don't believe this thread!

Experienced pilots thinking airspeed is in any way affected by turning up or downwind? Jeez! I've only ever before come across such gross misunderstanding among model flyers who remain earthbound and assign mass accelerations to their model rather than the air mass it is flying in!

Model flyers, leaning into a 20 knt wind, may be forgiven for not knowing that as far as an aeroplane is concerned there is no wind once it is off the ground and in the air. Aeroplane pilots should know better.

Good grief have you guys never flown a level constant bank angle 360 and noted no change in airspeed all the way round? Sure, you won't describe a circle over the ground unless you are in still air, but that because your groundspeed will change throughout due the wind.


This is REALLY basic stuff!

Two things you should be aware of:

1) Wind sure affects groundspeed, so watch your navigation, and of course it can make the transition between ground and air and back again a bit tricky.

2) All of the above applies to a steady wind only. Gusts, and wind gradients (or wind shear) have a very real change of airspeed effect which is dependent on the inertia of the aeroplane. But those are not steady-state wind conditions relative to the aeroplane.


Oh, and there's a third one, very relevant if you are flying low. Turning downwind in a strong wind gives the illusion of skidding during the turn as the groundspeed rapidly increases, and can temp a pilot to pull back to slow down as the ground, which was creeping past before, is now racing by. Look at the ASI.Your airspeed HAS NOT CHANGED, but your groundspeed sure has!

There are some posts on this thread that surprise me in that they have been posted by pilots I thought knew these basics, then some.

[maxred]

Well said SSD, but yes, pretty frightening that you actually had too. This thread I think shows quite clearly that basic groundschool during PPL theory training is a must. The lack of basic knowledge appears staggering. In fact, potentially fatal. We appear to have a number of pilots in the air, that do not understand the basics of what keeps them up there. Beyond belief.

[Heston]

I just don't believe this thread!

Experienced pilots thinking airspeed is in any way affected by turning up or downwind? Jeez! I've only ever before come across such gross misunderstanding among model flyers who remain earthbound and assign mass accelerations to their model rather than the air mass it is flying in!

Regrettably it doesn't surprise me at all. As well as model flyers its a common misunderstanding with hangglider flyers too. Sometimes, when teaching ex-hangglider flyers, its taken me several hours of demonstration to convince them, and then I'm sure they dont really believe the evidence of the instruments. As has been said, the trick to convincing people is go high, say 5000', so they cant visually reference ground features in the turn.

Another great misconception is what clouds do in the wind. You know those nice puffy summertime cumulus clouds? Did you know you can get out of the wind by flying upwind of them - they shelter you from it. Yes really!

[megan]

SSD and maxred :ok:

Regrettably it doesn't surprise me at all.Heston, you seem to have changed your tune with your last post. As SSD says,as far as an aeroplane is concerned there is no wind once it is off the ground and in the airwhich is what I've been saying all along, as in,Turning downwind with 74 k climb speed meant our ground speed went from 14 to 134 knots. No acceleration applied to the airframe or pilot, as any accelerometer on the aircraft would tell you, for the simple reason an aircraft is operating, as I said, with respect to the airmass, not the earth.PS: You'll need to study the principle of INS a little closer.

A little bedtime reading from a highly trained individual.

http://www.australianflying.com.au/news/dragons-of-the-downwind-turn

[Heston]

Right that's it. I'm out.

[ShyTorque]

ShyTorque

True, but irrelevant to the original question re 'inertia'
No, it's a problem because of inertia!

If there's no inertia, there's no problem. The larger the mass of the aircraft, the more of a problem there is. Tiny objects, such as small insects, don't suffer a problem.

[mikehallam]

Extrapolating from Shy T.

Inertia ?

A flying machine is propelling itself due North into a 100 mph airstream at 100 mph airspeed. [GS = 0].

It flies into a space of 'magic' vacuum & falls vertically straight down under 'G'.

Same machine going due South [GS = 200 mph], ditto 'vacuum, but this time continues forwards at 200 mph in a descending arc.

IMHO a high (mass X speed) 'plane has sufficient inertia not to change its ground speed as quickly as a change of wind.

mike hallam (whose light a/c is contrary & simply does what the wind does - mostly!)

[Fly-by-Wife]

PS: You'll need to study the principle of INS a little closer.
Actually Megan, it's you that needs to do the study. You seem to be confusing your frames of reference.

An INS has a unique point of reference, that being a known, fixed starting point - effectively the point on the ground at which the system was initialised. Everything that subsequently happens to the aircraft, as far as the INS is concerned, is measured as an acceleration / deceleration in one of the 3 axes. Whether that's taxi, take-off, climb, cruise, turning or whatever.

Put simplistically, it's the aggregation of the effect of all the accelerations over time relative to the starting point on the ground that provides the position information.

As an example, you fly at a constant airspeed on a constant heading in a zero wind condition so your airspeed and groundspeed happen to be the same. At this point the INS detects no acceleration or deceleration while in this zero wind situation, so a constant velocity relative to the reference point on the ground is recorded.

A headwind develops, so your groundspeed decreases, but your airspeed remains the same. The accelerometer in the INS does measure a deceleration, relative to the reference point on the ground, however, and can therefore compute that the groundspeed has changed. The aircraft's velocity WRT the air is unchanged, but the aircraft's velocity WRT to the ground has indeed changed.

So an INS knows nothing about what the air is doing, other than accelerating or decelerating the aircraft in space, relative to the reference point on the ground.

FBW

[piperboy84]

Sound like an amazing bit of kit, is it subject to errors and if so how do you recalibrate it enroute?

[megan]

The accelerometer in the INS does measure a deceleration, relative to the reference point on the groundAs has been tirelessly said an aircraft turning from downwind to intowind, or the reverse, does not experience any acceleration in the fore/aft/longitudinal axis. An aircraft can fly around the world once in the cruise and theoretically not experience any acceleration in the fore/aft/longitudinal axis. That's why the system includes a gyro to measure angular rate. The output from the INS is combination of information from acceleration, as measured by the aircraft, and the angular rate of transport.

The start point is not a frame of reference, INS works by using a global or body frame of reference. What I think you're trying to say is the INS tracks the position and orientation of the aircraft relative to a known starting point, orientation and velocity. The known starting point may not be the point of departure, but an update from a positive fix during flight to remove accumulated errors.

[cats_five]

Extrapolating from Shy T.

Inertia ?

A flying machine is propelling itself due North into a 100 mph airstream at 100 mph airspeed. [GS = 0].

It flies into a space of 'magic' vacuum & falls vertically straight down under 'G'.

Same machine going due South [GS = 200 mph], ditto 'vacuum, but this time continues forwards at 200 mph in a descending arc.

IMHO a high (mass X speed) 'plane has sufficient inertia not to change its ground speed as quickly as a change of wind.

mike hallam (whose light a/c is contrary & simply does what the wind does - mostly!)

You are using the ground as your frame of reference, you should be using the air.

[wiggy]

Warning: thread drift to answer piperboy84 question.

Sound like an amazing bit of kit, is it subject to errors and if so how do you recalibrate it enroute?

TBH there's are a few shortcomings in descriptions of how they work in some posts I've seen here ...:hmm: however FWIW:

INS/INAS 101: Yes, it is an amazing bit of kit, FWIW they've been around since the 60's, if not earlier ... they do "drift" away from an accurate position/velocity but these days you generally don't "recalibrate" the INS itself in flight ( though some systems do or did allow it....oh, that was fun......) but they're normally only one of several data sources into the nav systems on modern airliners, along with GPS and ground based radio aids so they're not usually the only source of info/data on position etc.

....Next week we will discuss Schuler tuning.....:8

and back to the thread: I think SSD had it in one, back permalink #47.

[Meikleour]

Leaving aside all the theoretical arguements about this subject - everyone seems to agree that rapidly changing wind/shear does affect IAS. This, of course, is because the rate of change of the wind/gust exceeds the rate at which the inherent momentum of the aircraft ie. it's inertia can change. So,now, why is it that the "one parcel of air" concept can totally ignore inertia? Is it because the rate of change is usually so minor at usual speeds?
Perhaps we are straying into the old Bernoulli versus Newtonian argument where neither theory is totally and or wholely at work?
I would also like someone to explain to me the cause of the large change in VSI indications at CONSTANT IAS when turning into or out of a very strong wind with respect to the TAS that I cited earlier. The only cause I can come up with is the large inertial change in speed of the aircraft wrt to it's own TAS.

[PaulisHome]

I'd just got through tearing my hair out about the lack of physics and flying knowledge shown here when I came across SSD's post (#47).

I can glue it all back on now. He has nailed it.

All those who aren't physicists - just read that and try to internalise it.

Paul

[Mixed Up]

I haven't had time to read all the above posts, but people are obviously getting rather worked up by the debate.

This debate has been going on for decades:

I remember an AAIB report touched on this matter. If I recall correctly, c. 1990 a C150 flown by a father with his son as a passenger, crashed when turning 180 degrees from a headwind to a tailwind. The flight was part of a low-level navigation competition in Hampshire and the aircraft was flying slowly so as be precisely on time. Tragically both were killed in the crash. The AAIB report, if I recall correctly, gave credence to the theory of the proposing side of this argument. Perhaps somebody will find the AAIB report? It generated a huge debate.

Whatever, as a practical (as opposed to a theoretical) pilot, when I do a similar turn, my eye is always steadfast on the ASI!

[oggers]

The last word on downwind turns (http://www.flyingmag.com/pilots-places/pilots-adventures-more/last-word-downwind-turns-really)

[Crash one]

I'm going to mention stall and speed in the same sentence to SSD and really wind him up!

[PaulisHome]

Leaving aside all the theoretical arguements about this subject - everyone seems to agree that rapidly changing wind/shear does affect IAS. This, of course, is because the rate of change of the wind/gust exceeds the rate at which the inherent momentum of the aircraft ie. it's inertia can change.

Basic laws of physics. Known since Newton. A moving body will keep moving in a straight line unless a force acts on it. (Newton's first law) If there is a force it will accelerate at rate proportional to the force, and inversely proportional to its mass (F=ma - Newton's second law).

So if an aircraft moves (say whilst in a descent) instantaneously from a head wind of say 30 knots to one of 20 kts, the airspeed will decrease by 10 kts. If the pilot is trying to fly at a given speed he will now need to accelerate by 10 kts. That takes time - and that's what we mean by inertia.

So,now, why is it that the "one parcel of air" concept can totally ignore inertia? Is it because the rate of change is usually so minor at usual speeds?
Perhaps we are straying into the old Bernoulli versus Newtonian argument where neither theory is totally and or wholely at work?

No it really isn't either of those. It's because if you're in a moving layer of air, it really doesn't matter how fast it's moving. There aren't two sides to this - there's a correct view and an incorrect view. And the idea that somehow turning downwind makes a difference due to inertia doesn't come into it.

I would also like someone to explain to me the cause of the large change in VSI indications at CONSTANT IAS when turning into or out of a very strong wind with respect to the TAS that I cited earlier. The only cause I can come up with is the large inertial change in speed of the aircraft wrt to it's own TAS.

Because generally when you do this you're in turbulent air. The high wind creates updrafts and downdrafts particularly in the boundary layer. Talk to any glider pilot. Fly in a high wind at a higher altitude and you won't see this.

I remember an AAIB report touched on this matter. If I recall correctly, c. 1990 a C150 flown by a father with his son as a passenger, crashed when turning 180 degrees from a headwind to a tailwind. The flight was part of a low-level navigation competition in Hampshire and the aircraft was flying slowly so as be precisely on time. Tragically both were killed in the crash. The AAIB report, if I recall correctly, gave credence to the theory of the proposing side of this argument. Perhaps somebody will find the AAIB report? It generated a huge debate.

Slow turns at low level are not a good idea, and lots of people have killed themselves doing them (see also spinning in from the final turn) but the reason for that is nothing to do with 'inertia'. See SSD's post #47 for the main reasons, but I'll add one. One effect of doing a low level turn is that your lower wing can be in air moving at a different speed to the air in which your upper wing is moving - due to wind shear. That can cause one wing to stall, followed by a spin and an impact with the ground.

Paul

[maxred]

One effect of doing a low level turn is that your lower wing can be in air moving at a different speed to the air in which your upper wing is moving - due to wind shear. That can cause one wing to stall, followed by a spin and an impact with the ground.

I think I may now have seen it all........

[worrab]

There seems to be some confusion between speed and velocity - the latter being speed in a particular direction. To change velocity requires a force. If you apply a force to a mass it accelerates.

Thus, a turn changes the velocity (ie changes the direction) even at a constant speed. Where does the force come from to change the direction of an aeroplane? It comes from the lift of the wing. We bank the aircraft so that a part of the wing's lift acts to change our direction.

It's impossible to tell whether uniform air is moving or not without reference to something. When airliners travel in the jetstream, their groundspeed can be pretty high, but whatever measuring device you attached to the airframe it would still record the speed of the aircraft through the air. So in uniform air, if you put up the screens and focus on the instruments you will be unable to tell whether the air mass around your aeroplane is moving relative to something else or not. You'll simply execute the turn and it will be completely undramatic. Try it some time (we all have!) at altitude - it's a total non-event.

So what happens closer to the ground? Well, there are a couple of factors.

- Close to the ground the wind tends not to be uniform. Friction slows down air nearer the surface and turbulence arises from obstacles. Each of these in their own way mean that the aircraft is no longer in a uniform block of air. We typically compensate for these (and gusting wind) by slightly increasing our airspeed so that our angle of attack at any point across the wing remains lower than the stall angle.

- At low levels we often have (as GA pilots) constraints of circuit shape, ground features to avoid, runway to aim at etc etc. Our flying is consequently done with reference to the earth and not to the air mass (ie we take our eye off the ASI). The change in reference is dangerous and I suspect it's that which ultimately leads to stall/spin.

Keep safe.

[PaulisHome]

Quote:
One effect of doing a low level turn is that your lower wing can be in air moving at a different speed to the air in which your upper wing is moving - due to wind shear. That can cause one wing to stall, followed by a spin and an impact with the ground.
I think I may now have seen it all........

It's real, though, doing the sums, probably not very big. There's a bigger effect due to the wings doing different speeds due to the geometry of the turn.

Take an aircraft with 20m wingspan doing 25 m/s (50kts) in a 45 degree turn.

Radius of turn = 63.7m (v^2/1.g = 25^2/9.81)

Difference in height between wingtips, and also difference in turn radius between wingtips = 20/1.4 = 14.3m

If we have a wind at height of 40 kts, and a 30% reduction across 300 ft due to the wind shear (4kts/100 ft or a little over 1 kt between the wing tips for our aircraft above). So not huge. But that 30% is representative of relatively flat smooth country. In rougher areas (mountains for example), it would be more.

However, the difference in turn radius between the upper wing and the lower wing is responsible for about 11 kts difference between the two tips (the top wing is doing a turn of radius 63.7+14.3/2 m in the same time as the lower one is doing a turn of radius 63.7-14.3/2 m).

So adding the two together if the aircraft is doing 50 kts, one tip can be doing 56 kts and the other 43 kts. That really is enough to make a difference.

Then into that, we can add gusts from various sources.

All of which says speed is your friend close to the ground (but it's still nothing to do with inertia).

Paul

[dsc810]

..and here is the effect "demo'd" so to speak on a long wingspan glider close to the ground.
Yes it does spin and hence the video is age restricted and you have to log in to you tube to view it.
https://www.youtube.com/watch?v=_xCct8cDtyk

[Geriaviator]

Turning downwind in a strong wind gives the illusion of skidding during the turn as the groundspeed rapidly increases, and can tempt a pilot to pull back to slow down as the ground, which was creeping past before, is now racing by.I'm with SSD on this. I have forgotten the tech stuff so ably explained on this thread, but not the lessons from my patient and brilliant CFI on the queen of training aircraft, the Tiger Moth. Before going solo he taught me to spin and recover from every attitude until I could recognise the brief slackening of controls in the instant before the wing dropped. He considered it was even more important to recognise and correct the incipient spin than the spin itself.

One breezy day he showed me low-level tight circuits and even though he had warned me about misleading visual cues I still skidded downwind with the slip needle off the scale as he let me fall into the trap, taking control only at the last moment. It was a lesson that stood me in good stead over many happy flying years and one which I remember to this day, 50+ years later. Is it true that spinning is no longer on the PPL syllabus?

[Meikleour]

PaulisHome (http://www.pprune.org/members/345527-paulishome)

Paul, your second paragraph agrees with the argument that inertia plays it's part with windshear!
With regard to my Bernoulli/Newton reference I was only referring to similar long standing arguments/discussions - not what was being discussed here. Sorry if that was not obvious.
You may not have read my earlier post but the problem with my observation was that it usually happened around 20,000ft with smooth conditions and therefore had absolutely nothing to do with the conditions you quote.
So, anyone out there with a relevant explanation for me?

[cats_five]

Slow turns at low level are not a good idea, and lots of people have killed themselves doing them (see also spinning in from the final turn) but the reason for that is nothing to do with 'inertia'. See SSD's post #47 for the main reasons, but I'll add one. One effect of doing a low level turn is that your lower wing can be in air moving at a different speed to the air in which your upper wing is moving - due to wind shear. That can cause one wing to stall, followed by a spin and an impact with the ground.

Just how low are you thinking of? I fly gliders at a site prone in a typical wind direction to turbulence & wind shear, I like to turn onto finals at 500' or more. It's easy to burn the height off if necessary.

Another optical illusion that can cause problems is a rising horizon, especially in the turn.

[bookworm]

One effect of doing a low level turn is that your lower wing can be in air moving at a different speed to the air in which your upper wing is moving - due to wind shear. That can cause one wing to stall, followed by a spin and an impact with the ground.

The first sentence is correct, as Paul demonstrates. But airspeed doesn't cause a wing to stall. The AoA is the same for both wings, unless you're in a climbing or descending turn, in which case there might be a tiny second order effect.

[oggers]

The AoA is the same for both wings,

You are correct that the low airspeed does not cause the wing to stall. But it does cause it to drop thereby increasing its angle of attack.

[piperboy84]

Oh bugger all this calculations, theory and inertia stuff, If you’re trying to land a spam can in gusty or wind shear conditions just do what I do.

Keep the throttle firewalled and fly her balls to the wall all the way round a fattened out level circuit pattern so there’s no slow steep turns, then once lined up on short final pull the power, drop the flaps and elevator her down hoping for the best while praying that nobody’s watching.

[bookworm]

But it does cause it to drop thereby increasing its angle of attack.

As Paul's sums show, there's already an 11 knot difference between the wingtips without any windshear. That requires some out-of-turn aileron to maintain angle of bank. An extra knot of difference simply increases the out-of-turn aileron required by a small amount, which is probably imperceptible as the extra knot of difference will kick in over the course of 180 degrees of turn. Compared to maintaining control in a moderately gusty wind, the vertical gradient of wind speed is hardly challenging.

The significant issue with low-level downwind turns is in the visual perception of the pilot, as SSD said a couple of pages ago.

[9 lives]

I had not realized the passions on this topic, or the genuine concerns about the "terrifying lack of safety knowledge" (I'm not sure which side of that line I'm on), I'm advocating safe, conservative maneuvering in slow speed turns, rather than the assumption that there will be no surprise effects resulting from the relative change in the wind, as the airframe and flight controls might see it.

The only time I have damaged an aircraft in motion was during a turn toward downwind, while doing a step turn in my flying boat, on a lake. I do concede that I was in contact with the water at the time, but, at 45MIAS, there is still some flying happening too. I entered the turn with adequate rudder effectiveness to maintain control in the turn, and all aspects of the turn remained constant all the way around (until I lost control), so the only variable was the effect of the wind. Once I came out of wind, I did not have enough rudder effect to control the turn, and the result was a waterloop. I have the wrinkled wingtip float to prove it. Had it been a floatplane, I would have flipped and wrecked it in that event.

From my first post on this topic, I have advocated caution, and consideration of the possible effect of a downwind turn. My knowledge of physics is even less than my knowledge of flying, so I can still learn a few things. Though I don't feel comfortable learning to disregard the possible effects of a downwind turn. All things working out well, the aircraft safely moves with the parcel of air, and there is no IAS change, nor effect on the handling. I do believe that can, and usually does happen. Or, you get a little cocky, and wrinkle a bit of the 'plane, because you just got too close to the edge of effective control...

Whatever the right answer is, 2230 some people read this topic, without commenting. Perhaps they still feel that they do not have a certain answer, but they've renewed their thinking about it, and that's a good thing...

[PaulisHome]

Meikleour

Paul, your second paragraph agrees with the argument that inertia plays it's part with windshear!

Yes. If the aircraft is moving from an airmass moving at one speed, to an airmass moving at another, then inertia is an issue in the sense that it will take time for the aircraft to accelerate to recover its desired airspeed. That's what happens as we go through wind shear on approach.

Where that doesn't happen is if the aircraft stays in one airmass, whatever speed it is moving at relative to the ground. It's fairly easy to do the sums to demonstrate this, and it doesn't matter which frame of reference you use, the result is the same, though the algebra is a little trickier in one case than the other.

The Bernoulli/Newton reference (for lift from a wing) is not really a good analogy. They are both ways of explaining lift that pilots use, but neither of which actually explain what's going on. (Listen to John Finnemore's Cabin Pressure for a very funny sketch on this). Newton's laws of motion are hugely accurate, at least until you get to relativistic speeds, and I don't believe we're built an aircraft like that yet.

I went back and read your comment. Don't know is the answer, although if this was happening during the descent that would be entirely reasonable since you can get different winds at different heights (and then see my first paragraph above).


The AoA is the same for both wings,

You are correct that the low airspeed does not cause the wing to stall. But it does cause it to drop thereby increasing its angle of attack.

Good points both - I stand corrected. Thinking more about it, I suspect that gusts are quite a big issue, particularly when air may not be moving horizontally, thus seriously modifying the AoA. And it's gustier closer to the ground. But you can see this effect in when thermaling a glider at a high angle of bank close to stalling speed - it's not uncommon to find the glider starting to auto-rotate as one wing hits an adverse gust. Easily fixed with some forward stick and opposite rudder, but you really need to be able to do it from feel.

cats_five
Just how low are you thinking of? I fly gliders at a site prone in a typical wind direction to turbulence & wind shear, I like to turn onto finals at 500' or more. It's easy to burn the height off if necessary.

Sounds sensible. For those of us that do mountain flying it's entirely possible to be turning within a low few hundred feet of the ground in rather gusty conditions. But I think the key is if you'r going to put yourself in a position where a spin is possible (ie slow, gusty, turning), then being high enough to recover might be smart. So we do the low turns quickly.

[Hotel-Mama]

This thread takes me back! I recall a very similar discussion in pages of the British Hang Gliding Association magazine in the 1980s. Being pre-internet, it raged on for months in the correspondence pages, and someone who ought to have known better even wrote a stroppy article rather embarrassingly advertising his ignorance. It is sad to hear that decades later:it is still a common misunderstanding with hangglider flyers Of course, hang gliding is particularly prone to speed perception errors when face down and turning low over a hill in a strong wind, which has unfortunately led to more than a few accidents, the cause of which wrongly attributed by those a with a dodgy grasp of basic physics.

[oggers]

Bookworm

As Paul's sums show, there's already an 11 knot difference between the wingtips without any windshear. That requires some out-of-turn aileron to maintain angle of bank. An extra knot of difference simply increases the out-of-turn aileron required by a small amount, which is probably imperceptible as the extra knot of difference will kick in over the course of 180 degrees of turn. Compared to maintaining control in a moderately gusty wind, the vertical gradient of wind speed is hardly challenging.

The significant issue with low-level downwind turns is in the visual perception of the pilot, as SSD said a couple of pages ago.

I didn't mention vertical gradient. I simply pointed out that if you get that 11 knots assymetry across the wings you are likely to get a wing drop in which case the AoA will no longer be the same for both wings. Yes, of course you can hold the wing up with out of turn aileron but that in itself will increase the AoA at the tip and increase the drag induced yawing moment. These are the classic ingredients of autorotation and an incipient spin. It is not as simple as "the angle of attack is the same for both wings".

[n5296s]

Whatever the right answer is, 2230 some people read this topic, without commenting
Yep, been keeping out of this one, but it has been a fun read.

It did make me think of something from one of my favourite books, The Concorde Stick and Rudder Book. Apparently landing Concorde in a strong headwind could lead to a very nasty surprise because of the huge relative height difference between the back of the wing and the rest of it. So the trailing edge is much more in ground effect than the rest. As it gets very close to the ground, the headwind reduces due to ground friction. At some point the part of the wing that is doing the most work drops out of - well, not the sky, but where it is.

I don't pretend to follow the detailed math/aerodynamics, but the net effect is a "did we land or were we shot down" landing.

[Right Hand Thread]

..and here is the effect "demo'd" so to speak on a long wingspan glider close to the ground.
Yes it does spin and hence the video is age restricted and you have to log in to you tube to view it.
https://www.youtube.com/watch?v=_xCct8cDtyk



What is clear from that video is the effect of the onboard air-conditioning fans being switched off before landing resulting in a loss of thrust.



<cough!> Windsock.

[terry holloway]

The tendancy to sink on final approach can be overcome by having the cabin full of birds which are flapping their wings. I note from today's paper that a Saudi airline is doing just that by carrying a large number of falcons, so it must be correct!

[cats_five]

Meikleour



Yes. If the aircraft is moving from an airmass moving at one speed, to an airmass moving at another, then inertia is an issue in the sense that it will take time for the aircraft to accelerate to recover its desired airspeed. That's what happens as we go through wind shear on approach.

Where that doesn't happen is if the aircraft stays in one airmass, whatever speed it is moving at relative to the ground. It's fairly easy to do the sums to demonstrate this, and it doesn't matter which frame of reference you use, the result is the same, though the algebra is a little trickier in one case than the other.

The Bernoulli/Newton reference (for lift from a wing) is not really a good analogy. They are both ways of explaining lift that pilots use, but neither of which actually explain what's going on. (Listen to John Finnemore's Cabin Pressure for a very funny sketch on this). Newton's laws of motion are hugely accurate, at least until you get to relativistic speeds, and I don't believe we're built an aircraft like that yet.

I went back and read your comment. Don't know is the answer, although if this was happening during the descent that would be entirely reasonable since you can get different winds at different heights (and then see my first paragraph above).



Good points both - I stand corrected. Thinking more about it, I suspect that gusts are quite a big issue, particularly when air may not be moving horizontally, thus seriously modifying the AoA. And it's gustier closer to the ground. But you can see this effect in when thermaling a glider at a high angle of bank close to stalling speed - it's not uncommon to find the glider starting to auto-rotate as one wing hits an adverse gust. Easily fixed with some forward stick and opposite rudder, but you really need to be able to do it from feel.

cats_five


Sounds sensible. For those of us that do mountain flying it's entirely possible to be turning within a low few hundred feet of the ground in rather gusty conditions. But I think the key is if you'r going to put yourself in a position where a spin is possible (ie slow, gusty, turning), then being high enough to recover might be smart. So we do the low turns quickly.

You will need to turn finals a lot higher than 500' to be able to recover a spin.

[n5296s]

You will need to turn finals a lot higher than 500' to be able to recover a spin.
Hmmm... I'm certainly not advocating low level spins, but pretty much everything I've spun recovers from a one-turn spin in about 500'. And that's one full turn. If you do a regular recovery as soon as you feel the stall/spin breaking away, 500' is plenty. Not, again, that I'm recommending going out and trying it at that altitude.

The problem isn't that it's impossible to recover an incipient spin in less than 1000', it's that most people have never experienced one and so aren't spring-loaded to start the recovery, and probably go through a significant panic time before they recall some vague, distant memory of what they're supposed to do. And that WILL kill you.

Which is why imo getting some spin training (and other unusual attitude stuff) is a REALLY good idea, once you have a couple of hundred hours under your belt. (It is dangerous though - it led me to a seriously expensive aerobatic habit).

[Crash one]

Anyway, it is not wind shear it is wind gradient, wind shear is when the wind is traveling in a different direction at a lower level, not slightly slower due to surface friction.
Landing towards high trees or a hill, you have a headwind on final and a sudden rotor generated tailwind just before you land. That's the shear.

[megan]

Next week we will discuss Schuler tuningAfter you've explained the relativistic effects of Eötvös and its impact on aircraft performance wiggy. ;)but the net effect is a "did we land or were we shot down" landingThe usual explanation was "the copilot landed it" (not Concorde specific). :{

[BEagle]

The usual explanation was "the copilot landed it" (not Concorde specific). :{

Not always. The VC10 could be a bit of a handful in a cross-wind. One day we were doing a Brize/Kinloss/Aldergrove/Kinloss/Brize trip. The weather at Aldergrove was pretty demanding, but the landing was impeccable.

"That was a very nice landing, Captain" said the ATC chap in the tower.
"Not me, mate", I replied, "That was the co-pilot's landing - and yes, very good it was too!"

[PaulisHome]

You will need to turn finals a lot higher than 500' to be able to recover a spin.

I agree entirely Cats 5, which is why we don't fly slow in a finals turn.

But in principle I agree with n5296s too. It should be entirely possible to recover from an incipient spin in much less height than that (and is). It's just that a small but non-zero stream of pilots (with spin training) get themselves into a situation where they don't.

Paul

[PaulisHome]

And at the risk of confusing people, I thought I'd share the following.

Some sea birds have a rather neat trick called dynamic soaring. It uses wind gradient to stay airborne without flapping their wings. Very neat trick if you can do it.

Close to the sea the wind is going slower than the wind a bit higher up. By climbing through the wind gradient, airspeed increases and they can extract energy. Then turning downwind, they descend through the gradient, increasing airspeed in the dive. Repeat doing linked semicircles and they stay airborne and move along without flapping.

It has been done in a glider, but it's a bit tricky.

Paul

[Shaggy Sheep Driver]

n5296s, re Concorde. What you describe is a wind gradient effect. As I said in my earlier post, these changes in windspeed with height near the ground (unlike a steady-state wind relative to the aircraft which has no effect) do indeed have very real effects on aeroplanes.

[BossEyed]

<cough!> Windsock.

Yes, that is a little tiny bit important to the discussion.

[mikehallam]

But Concorde or not, 'Ground Effect' is not wind speed change it's simply ground effect !

mike hallam

[Shaggy Sheep Driver]

mikehallam - very true, and particularly effective on Concorde where there is lot more wing area, and a lot closer to the ground on landing, at the back of the aeroplane rather than the front.

A Concorde captain described to me landing the beautiful white bird as easy if you maintain the attitude. But as the aeroplane flies into ground effect this acts much more at the back of the aeroplane because of the above, and tends to force the nose down just before touchdown. This must be countered by the pilot easing back just enough to prevent that nose drop and maintain a constant attitude, whereupon the main wheels, he said, will touch down in elegant style.

Thus one does ease back on landing as in a conventional aeroplane, but it's not a flare, it's just to counter that ground effect... effect!

[Mike Flynn]

I was taught in my PPL training back in 1981 that low and slow was an area to avoid.

Height and speed is my motto and I am still here.

In my early helicopter training I was stressed to focus on the avoid curve.
Helicopter Aviation (http://www.copters.com/pilot/hvcurve.html)

[cats_five]

I agree entirely Cats 5, which is why we don't fly slow in a finals turn.

But in principle I agree with n5296s too. It should be entirely possible to recover from an incipient spin in much less height than that (and is). It's just that a small but non-zero stream of pilots (with spin training) get themselves into a situation where they don't.

It should be, indeed it is at altitude when there isn't the pressure of being in the circuit and planning a landing. As you so rightly say, if one doesn't fly too slowly one isn't going to spin and the problem with all the near the ground stuff, and with the rising horizon stuff, is it can sucker the unwary into slowing down.

[maxred]

If you do a regular recovery as soon as you feel the stall/spin breaking away, 500' is plenty. Not, again, that I'm recommending going out and trying it at that altitude.

Not wishing in any way to dampen a thoroughly entertaining read, but has anyone, witnessed, completed, heard of, a spin recovery, in the circuit, from 500':rolleyes:

I have read hundreds of reports where, let's say, all,perished from a loss of control/spin in, within the circuit pattern.

[Crash one]

Not wishing in any way to dampen a thoroughly entertaining read, but has anyone, witnessed, completed, heard of, a spin recovery, in the circuit, from 500':rolleyes:

I was once on final in a glider with an instructor, at 400 ft he said "I have!" And spun it losing 200 ft. "You have" he said as two Tornadoes roared overhead at 4/500 ft. I recovered it and landed, along with "what the f**k". So it is possible.

[maxred]

I am sure it is, but with respect a glider may handle differently to a powered aeroplane. I have only had one flight in a powered motor glider, so would not be qualified to confirm the above. Sounds like an interesting experience you had......

[terry holloway]

Not wishing in any way to dampen a thoroughly entertaining read, but has anyone, witnessed, completed, heard of, a spin recovery, in the circuit, from 500':rolleyes:

I have read hundreds of reports where, let's say, all,perished from a loss of control/spin in, within the circuit pattern.
I can claim to recovering from a spin at that height over a school playing field next to an airfield on final approach. If the football net had been up I would have been in it! I frightened my passenger and me! I also wrote it up for "I learned about flying from that" and it was published in "Flyer". It happened 25 years ago and my experience was such that it shouldn't have happened.......
.....it happened because I pulled a lot of G in a hard turn.

[n5296s]

I have read hundreds of reports where, let's say, all,perished from a loss of control/spin in, within the circuit pattern.

I've told you a million times not to exaggerate. Seriously? Hundreds? i've probably seen half a dozen. I don't claim to have read every accident report ever written, far from it, but hundreds sounds like a lot.

If you read what I wrote, it wasn't, "it's easily survivable, anyone can do it." It was, "someone who is familiar with spin recovery should be able - in any reasonable type - to detect and recover form an incipient spin in less than 500'". I then went on to say that most low time pilots, who aren't familiar with spins, will probably panic for long enough to make the fatal difference.

I experienced one incipient spin while I was training, and solo (practising power on stall recoveries). It took me a second or so to realise what was going on, and recover. At 3000' it made no difference. At pattern altitude maybe it would have.

[maxred]

The myth.......

spin recovery in fixed wing aircraft (http://www.pilotfriend.com/training/flight_training/fxd_wing/spin_recov.htm)

It's as big a myth as the EFATO turn back

[Maoraigh1]

Having done spinning in a Chipmunk, I was at first startled at the height the Instructor deemed safe for spinning a Tiger Moth. There are big differences between aircraft in the height needed for safe spin recovery.
And C of G will affect it.

[Thud105]

"I was once on final in a glider with an instructor, at 400 ft he said "I have!" And spun it losing 200 ft. "You have" he said as two Tornadoes roared overhead at 4/500 ft. I recovered it and landed, along with "what the f**k". So it is possible."

If true, I sincerely hope this instructor NEVER flew again, let alone instructed.

[9 lives]

Not wishing in any way to dampen a thoroughly entertaining read, but has anyone, witnessed, completed, heard of, a spin recovery, in the circuit, from 500':rolleyes:

I have not. I did witness an airshow pilot attempt to demonstrate a low altitude spin. His attempt was fatal. :

https://aviation-safety.net/wikibase/wiki.php?id=39240

Which left a lasting impression that such maneuvers are foolish

[Thud105]

Exactly Step. Deliberately spinning at low-level is like practicing bleeding.
Its breathtakingly stupid.
In the glider example cited I don't understand the relevance of the two Tornadoes either. If the instructor's intention was to lose height quickly, a spin is not the correct manuver.
I call bulldust. I was tempted to ask "what sort of glider" but then realized its a moot point, bulldust is bulldust.

[Crash one]

"I was once on final in a glider with an instructor, at 400 ft he said "I have!" And spun it losing 200 ft. "You have" he said as two Tornadoes roared overhead at 4/500 ft. I recovered it and landed, along with "what the f**k". So it is possible."

If true, I sincerely hope this instructor NEVER flew again, let alone instructed.

I'm afraid I had the greatest respect for that particular instructor.
So did any other pilot in the club.
He was no cowboy, and taught me and many others very well.
He spun the thing as the quickest way to lose height without over speeding, go around not an option. The Tornadoes were on a collision course and hadn't seen us.
Please don't jump to conclusions. It certainly was not done for fun!!
The actual manoeuvre amounted to an upside down nose down rapid loss of 200ft followed by recovery. As we got to about 45deg he handed it back.
It was a K13 glider.
Whether you wish to call it a true spin I really couldn't say, it happened too quickly.
It certainly rotated around its long axis with the nose down. Call it what you like, but bull dust it certainly wasn't.

[maxred]

bulldust.

I really like that term. Think I will use it in future when I come across any.......unfortunately, that is pretty frequent.

[9 lives]

He spun the thing as the quickest way to lose height without over speeding

I'm not a glider pilot, I admit that, 'only flown them a few times (and really enjoyed it!). But I am rather experienced at intentional spin entries. For my experience, a spin can be entered from slow deceleration, which will take planning, patience and time, or, from an abrupt maneuver, which will generally see the required decay in speed as a gain in altitude first. Therefore, as a rapid evasive maneuver to lose altitude, a spin entry would not come to mind. Nose down and a sideslip would....

[Thud105]

Sorry, I'm not buying it. So, you're on the approach at say 400ft and 55kt in a -13. Want to descend quickly? Full brakes and lower the nose - a 13's brakes are speed limiting. Or, as Step suggests - a very steep side-slip with full airbrake.
The whole thing sounds incredibly fraught - if not dare I say unbelievable. Upside down at 300ft, and then he handed control back........

[Crash one]

Sorry, I'm not buying it. So, you're on the approach at say 400ft and 55kt in a -13. Want to descend quickly? Full brakes and lower the nose - a 13's brakes are speed limiting.
The whole thing sounds incredibly fraught - upside down at 300ft, and then he handed control back........

It wasn't upside down when he handed it back, it was at about 45 deg right side up, still decending and under control.
After the event the instructor called Leuchars military and asked, pretty much, what the hell they were playing at. They denied all knowledge.
At the weekend the two Tornado crews arrived at the gliding club, apologised profusely for an error of "cross a big lake, between two hills, down the valley and turn left". And spent the evening buying the beer.
They picked the wrong two hills and flew over our airfield by mistake.
I remember the incident very well even though it was '85.

[Thud105]

I believe the event happened. I don't believe you spun at 400ft, or were inverted at 400ft. Either event in a K-13 would, IMHO only end with a crash. Very steep side-slip with full airbrake is the most plausible scenario - IMHO of course.

[Crash one]

I believe the event happened. I don't believe you spun at 400ft, or were inverted at 400ft. Either event in a K-13 would, IMHO only end with a crash. Very steep side-slip with full airbrake is the most plausible scenario - IMHO of course.

Believe what you wish, I was there you were not, I recall the rotation all the way round rather than a reversal. I do, and did, know the difference between inverted and slipping.

[Pilot DAR]

So we're finished with the discussion about Tornado avoidance maneuvering in the K13, one way or the other?

[dsc810]

Well from my memories of spinning K13 - from annual check flights, were that to get it to spin you needed - for a spin to the right:
Approach the stall speed from straight and level.
Before you reach stall pull the elevator full back (thus gaining altitude) and at the same time apply full left aileron to get the right wing to stall.
While doing this apply full right rudder.
So you entered the manoever holding the stick hard back and to the left with your right leg fully extended to hold the right rudder in.
This would create a sort of flick roll entry which is most cases would then become a spin - though on some occasions if the timing of the control inputs was a little out it would instead enter a spiral dive.

Recovery was the usual opposite rudder while moving the stick forward with ailerons neutral and recover from the ensuing dive.
Notice I said 'while' in the preceding sentence not the usual power aircraft default of opposite rudder wait and then stick forward.
If you held the stick hard back (at my CofG loading anyway) and just applied opposite rudder (left in this case) then the direction of the spin immediately reversed and you would need to apply in this example right rudder again plus stick forward to get it to come out.
I hope this is of some vague interest - if anyone is remotely bothered.........

[terry holloway]

Well, in that case you weren't at 400ft. And I don't need to have been there to know that either spinning or being inverted at that height in a K-13 could only end in a crash, whereas you claim he then "gave it back to you." Note also that - as Step points out - to spin abruptly would require a gain in altitude.
I spent a lot of time in the air as a gliding instructor in the 80s and 90s, which included a fair amount of time in K13s. The K13 has a fairly benign spin which is easy to recover from and it does not lose a lot of height. Certainly one wouldn't deliberately enter a spin to avoid an approaching jet! I believe the instructor in this instance "turned away from the threat" and either pulled too hard and unintentionally entered a spin(unlikely), or conducted a fairly vigorous manoeuvre which became a spiral dive, and that might have felt like a spin. Undoubtedly a very unpleasant experience and there are still too many instance if powered aircraft (civil and military) infringing gliding sites. Whilst they are a hazard to gliders, the 2,000 feet of cable in the air offers a significant hazard to powered aircraft!

[cats_five]

I remember the incident very well even though it was '85.

I am struggling to find any record of Tornado squadrons at Leuchars as early as 1985.

[Crash one]

I have always been quite good at recalling details, I was trained to do that in the military. I am also not guilty of elaborating on facts as I remember them.
This manoeuvre did not involve waiting for the stall, followed by what may be considered normal intentional spin entry procedures.
This was a rapid application of left aileron until beyond 90deg followed by a hard pull as the rotation continued, at the point of about 45deg before returning to level and with the nose starting to rise, but still well below horizonal, control was given back to me.
I continued the rest of the roll and nose levelling and then landed.
Perhaps it was not a true spin in the normal sense of climbing stall, rudder, aileron or whatever, followed by opposite rudder, forward stick recovery. But, the aircraft rotated through 360deg around the long axis, lost about 200ft in the process, enough to avoid the threat. I do recall seeing the condensation around the Tornadoes dead ahead.
We certainly lost the height far quicker than any normal upright manoeuvre of dive, side slip, air brakes or stalling would have produced.
Please don't insult my intelligence by telling me I was not inverted but in a side slip instead. And at half a field short of the touchdown point I was not far away from 400ft. Though I am not in the habit of staring at the instruments at that point in the landing process.
Edit: the Tornadoes were crewed by Americans from somewhere else.

[BossEyed]

You must be misremembering; the US has never operated Tornado.

[9 lives]

This manoeuvre did not involve waiting for the stall, followed by what may be considered normal intentional spin entry procedures.
This was a rapid application of left aileron until beyond 90deg followed by a hard pull as the rotation continued,

If the maneuver did not involve waiting for a stall, it was not a spin entry (to be proud of, anyway). If it involved a steep roll and a pull it has the signs of a spiral dive, or some kind of wingover.

It is very important for pilots to recognize and correctly react to one vs the other, in recovery. The recovery for one, applied to the other situation, could be messy. I accept that the other pilot flying the glider flew a purposeful evasive maneuver (which worked), but as I understand the description, if that pilot was asked if they had deliberately spun the glider, I doubt that answer would have been yes.

[jgs43]

Crash one.

Leuchars operated Lightnings until around 1969.

F4 Phantoms followed for the next several decades with Tornados only becoming operational from that Airfield in April 2001.

Since you mention the lake (loch) you must have been flying from the Scottish Gliding Union at Portmoak - I am left wondering who the instructor was as I must admit to never having heard of this incident. Perhaps you could PM me with the name as I do have an interest as a past CFI and safety officer at Portmoak.

It is possible that the aircraft were indeed Phantoms as the Americans also operated this type within the UK and were, quite often, involved in the low flying and other military operations with the Scottish region.

The usual route to/from Leuchars was to the North of Bishop Hill between that and the Ochils. Americans, unfamiliar with the area, might well have aimed for the gap between Bishop and Benarty Hills although that track would also have taken them low over Glenrothes airfield to the East of Portmoak.

Leuchars were well aware of Portmoak and there was a letter of agreement between them and the Scottish Gliding Union re their circuit requirements in Easterly winds as they came close to our own circuit in those conditions.

A somewhat out of context post but I am intrigued by your experience.

As an aside low slow or steep turns in gliders close to the ground, especially long span ones, were never a good idea. This was not so much because of the danger of the lower wing stalling from being in the lower slow moving mass of air on turning into wind but more from the differential lift between the tips at differing airspeeds. The lower wing in slower wind speed due to wind gradient effect plus moving at lower speed than the outer wing equals a considerable amount of extra lift on the upper wing. The early long span gliders were a bit lacking in aileron effectiveness and thus might not have had powerful enough ailerons to actually level the wings for landing.

[Fly-by-Wife]

My last dual flight before solo in a K13, Lasham, 1990 - instructor pulled off the winch launch early, 900 feet, but told me to keep the nose up into a stall, then bank to the left, which precipitated a spin - normal spin recovery, lost 400 feet with just over 1 revolution, so about 600 feet when recovered, just right for a normal circuit. He cleared me to solo after that.

FBW

[Big Pistons Forever]

My last dual flight before solo in a K13, Lasham, 1990 - instructor pulled off the winch launch early, 900 feet, but told me to keep the nose up into a stall, then bank to the left, which precipitated a spin - normal spin recovery, lost 400 feet with just over 1 revolution, so about 600 feet when recovered, just right for a normal circuit. He cleared me to solo after that.

FBW

Personally I think this borders on criminal stupidity. Good instructors can effectively demonstrate all of the required skills while maintaining a good reserve of safety margins to allow for the unexpected.

BPF
Canadian glider License with Instructor and Aerobatic Instructor ratings.

Sorry for the thread drift but I had to comment.

Back to the thread topic......my 2 cents

I found the thread rather entertaining and as usually happens it forced me to evaluate and consolidate what I knew.

However flying is an inherently practical exercise in hands and feet an coordination. for light aircraft I say again for light aircraft I firmly believe that there inherent low inertia means that all of the physical effects discussed at length in this thread exists to such a small extent that they are effectively theoretical not actual.

The one effect that is very real and regularly kills is the visual illusion.

SSD summarized it nicely on page 3

Turning downwind in a strong wind gives the illusion of skidding during the turn as the groundspeed rapidly increases, and can temp a pilot to pull back to slow down as the ground, which was creeping past before, is now racing by. Look at the ASI.Your airspeed HAS NOT CHANGED, but your groundspeed sure has!

[megan]

You just can't convince some people BPF. Earlier I made a post re a 777 operating in a 200 knot jetstream and statedThere is no acceleration in order to change the ground speed...to which Heston repliedOh dear I give up! I hope to God you dont actually fly a 777, megan. Please tell me what does change the groundspeed then?Your quoting of SSD has it absolutely correct Look at the ASI.Your airspeed HAS NOT CHANGED, but your groundspeed sure has!YourThe one effect that is very real and regularly kills is the visual illusion.is the killer.

For Heston, I never flew a jet, a humble ex helicopter pilot me, and the downwind turn is a killer in helicopters as well, though for different reasons to a fixed wing. As I related earlier we took off at times in 60 knot winds with a climb speed of 75, meaning a GS of 15 into wind and 135 downwind. Most of our copilots only had a bare CPL on joining, and the demonstration of a take off in those conditions, with a bank angle of 60° applied at climb speed to downwind gave an extremely impressive demonstration of the need to fly instruments and ignore the visual. The ASI remained nailed on climb speed, and the ball remained centred throughout.

It matters not if you are in a 777 in the jetstream, or a bug smasher at sea level. The physics are exactly the same.

[PaulisHome]

You just can't convince some people BPF. Earlier I made a post re a 777 operating in a 200 knot jetstream and stated

Quote:
There is no acceleration in order to change the ground speed...
to which Heston replied

Quote:
Oh dear I give up! I hope to God you dont actually fly a 777, megan. Please tell me what does change the groundspeed then?

Sorry Megan, Heston is right.

If you change the ground speed, you have accelerated. You can accelerate without changing your airspeed. You do it every time you make a turn - the aircraft is accelerating towards the centre of the turn. Acceleration is rate of change of velocity - and velocity (unlike speed) is a vector - that is it has a direction as well as a size. So if we change direction, we accelerate.

That's Physics for you.

But you're quite right about the rest - make your turn whilst ignoring the visual illusions and so on, and the ASI will stay nailed.



Paul

[Crash one]

Isn't acceleration an increase in speed relative to the medium that the body is suspended by? Rather than an acceleration relative to something else?
You are accelerating towards the centre of the turn but the centre of the turn is moving, so what ever else you use as a reference is irellevent.
You cannot use two points of reference at the same time.

[cats_five]

You cannot use two points of reference at the same time.

And using the wrong one is the fatal bit... Use the ASI, not the ground.

[maxred]

Acceleration = the rate of change of velocity per unit of time

That's Physics for you.

Sorry, thats bollox for you.

If you change the ground speed, you have accelerated. You can accelerate without changing your airspeed.

The problem with that is that you have NOT changed your airspeed. The airspeed is the medium you require to pay attention to. That is the medium that you are flying in. The physics you quote, is only applicable on the ground, i.e. in the same medium.

Take a car. If the car turns, there is an acceleration toward the new direction, we can call the forward acceleration of the car a "linear acceleration", which passengers in the car might experience as a force pushing them back into their seats. When changing direction, we might call this "non-linear acceleration", which passengers might experience as a sideways force. It is FORCE, not acceleration.

The fact that your aeroplane may increase groundspeed, is, IMO, and that of Newton I think, not acceleration.

Thats physics for you...

[Hadley Rille]

My instructor teaches speed by attitude with ASI as a delayed and sometimes erratic backup. The ground doesn't even come into it and I don't notice it in the circuit. Horizon and aiming point are what matters.

[PaulisHome]

The problem with that is that you have NOT changed your airspeed. The airspeed is the medium you require to pay attention to. That is the medium that you are flying in. The physics you quote, is only applicable on the ground, i.e. in the same medium.

Take a car. If the car turns, there is an acceleration toward the new direction, we can call the forward acceleration of the car a "linear acceleration", which passengers in the car might experience as a force pushing them back into their seats. When changing direction, we might call this "non-linear acceleration", which passengers might experience as a sideways force. It is FORCE, not acceleration.

The fact that your aeroplane may increase groundspeed, is, IMO, and that of Newton I think, not acceleration.

Might I hazard a guess, Maxred, that you're not a physicist?

That force that car passengers experience? Caused by acceleration - whether it's along the line of travel or sideways. (Newton's second law - F=ma). That 2g you feel in a 60 degree banked turn - it's the aeroplane pushing your backside so you accelerate along with it.

I didn't claim for a moment that your airspeed would change. Other things being equal, in a turn, it won't.

But if your groundspeed is changing, you will be accelerating. Even if the airspeed stays the same. And no matter what speed the wind is.

And the idea that the physics is only applicable on the ground will, I suspect, be a surprise to most physicists! They do rather cling to the idea of the physics holding true no matter what frame of reference you use.

Paul

[mm_flynn]

Acceleration = the rate of change of velocity per unit of time

That's Physics for you.
Sorry, thats bollox for you.acceleration is by definition the rate of change of velocity, as measured in an inertial frame of reference (ie a non-accelerating frame). The issue people seem to be having is what is 'velocity', which I think is the root of the differences of opinion.

The conventional frame for measuring Velocity is the ground (which is very close to inertial after adjusting for a 1 g offset from gravity). So, if an aircraft has changed its ground velocity, it has by definition accelerated.

If the air mass is moving at a constant velocity (with respect to the ground), measuring velocity relative to the air mass will give exactly the same acceleration as measuring vs the ground.

If you do the maths for two cases of a level constant rate turn
1 - flying North at 60 knots and turning to South at 60 knots
2 - flying North at 0 knots (into a 60 knot headwind) and turning to South at 180 knots

You will find the answers are all identical, except the track over the ground is displaced by a constant distance of 60 nautical miles every hour in the second case.

In both cases the air SPEED (not air VELOCITY) will remain constant. The Relative Air Velocity (measured relative to the wing, which for normal rates of turn is effectively an inertial frame) will remain constant. It is this relative Velocity that defines the performance of an aircraft wing.

If the air mass is not moving at a constant speed, the aircraft will require accelerations to maintain the relative air velocity, hence the importance of the aircraft's inertia when exposed to gusts and shears.

[maxred]

Thanks MM. There you go Paul.........apologies, I am not a physicist. Got a well earned B in my A level though.....I am teasing a bit, obviously, and Paul, I admire your argument based on pure physics. I am sure everyone is now totally bored and baffled with the physics, but the scenario is a serious one. If anything this thread could achieve, is that it gets folks thinking about it......

[piperboy84]

The ground doesn't even come into it and I don't notice it in the circuit.

You may want to have a butchers every now and then.

[eckhard]

You wake up, not sure where you are. You see that you are in a small, windowless room. Apart from your chair, the room is empty.

On the floor, you find a small model aeroplane with a battery-powered motor driving counter-rotating propellers. A simple instruction sheet tells you that when the motor is switched on and the 'plane is hand-launched, the controls are fixed so that it will fly perfect circles until a timer turns the motor off after 30 seconds.

You launch the model and watch, pleased and impressed, as it flies circles around you.

After the flight has ended, you notice that there is actually a window blind in the wall behind you. You open the blind and are amazed to find that you are in fact inside a carriage being pulled by a train along a perfectly straight, smooth track at 60mph.

You consider the fact that the model aircraft was flying in a parcel of air which was moving across the ground at 60mph. To an observer not on the train, it was, in effect, flying in a 60mph wind. This is a speed which is about five times greater than its own flight speed, yet the model did not seem to be affected at all! It did not, for example, exhibit any signs of 'stalling' or losing height when it was turning 'downwind'.

You come to the conclusion that the model was simply unaware of its location and speed relative to the tracks, as were you until you looked out of the window.

[maxred]

And the idea that the physics is only applicable on the ground will, I suspect, be a surprise to most physicists! They do rather cling to the idea of the physics holding true no matter what frame of reference you use.


Just to keep the record straight. I did not actually state that.........

[Geriaviator]

The ground doesn't even come into it and I don't notice it in the circuit. That's fine, but I suggest this may not be the best policy if the weather closes and you have to make a low-level circuit on a windy day :ooh:

[PaulisHome]

Well, if you really want your head to hurt.....

As I'm sitting typing this, I am in fact accelerating at 1g upwards, pushed by the force from my chair. Relative to me, the inertial frame of reference is falling towards the centre of the earth at 1g, as I would be (following a straight line in space time), were it not for the earth getting in the way.

But then we'd be having a physics conversation, not a flying one. And we'd be using Einstein's physics, rather than Newton's. The latter works quite well for our purposes, and the former might make the PPL exams a bit tricky.

Paul

[9 lives]

The thinking power being brought to bear on the acceleration topic brings to mind a question I've had in my mind for years. It's really an astronaut question, and I came close to being able to ask Chris Hadfield at his presentation, but he sure is popular with the crowd! Anyway:

The space shuttle, which can maneuver in orbit, is orbiting the earth, and the astronauts inside are floating around, also orbiting the earth (weightless, but held in orbit by G? - no that's not the question). If the space shuttle pilot maneuvers the space shuttle in its orbit (I think they roll it in and out of the sun for heating/cooling?), would he have to tell all the astronauts to hang on? Then they would experience acceleration as they change direction with the shuttle? Otherwise, it would maneuver around them - and maybe hit them, as they remained in the original orbit...

When my daughter was quite young, she could be entertained until I felt airsick, with my placing Mr. Bear on the glareshield, and then bunting over in near zero G so he seemed to jump into her lap all on his own. Was I placing Mr. Bear in a very crude orbit for a few seconds, surrounded by a C 150?

[worrab]

The thinking power being brought to bear on the acceleration topic brings to mind a question I've had in my mind for years. It's really an astronaut question, and I came close to being able to ask Chris Hadfield at his presentation, but he sure is popular with the crowd! Anyway:

The space shuttle, which can maneuver in orbit, is orbiting the earth, and the astronauts inside are floating around, also orbiting the earth (weightless, but held in orbit by G? - no that's not the question). If the space shuttle pilot maneuvers the space shuttle in its orbit (I think they roll it in and out of the sun for heating/cooling?), would he have to tell all the astronauts to hang on? Then they would experience acceleration as they change direction with the shuttle? Otherwise, it would maneuver around them - and maybe hit them, as they remained in the original orbit...

When my daughter was quite young, she could be entertained until I felt airsick, with my placing Mr. Bear on the glareshield, and then bunting over in near zero G so he seemed to jump into her lap all on his own. Was I placing Mr. Bear in a very crude orbit for a few seconds, surrounded by a C 150?
Yes, and yes.

The shuttle direction change is an acceleration which requires force. The astronauts must have some force applied for them to stay with the shuttle's new direction.

Zero G occurs when you accelerate downwards at 9.8ms^2. Google the vomit comet for another example.

[maxred]

Think sideways force, and constant acceleration.......

[Hadley Rille]

That's fine, but I suggest this may not be the best policy if the weather closes and you have to make a low-level circuit on a windy day :ooh:
Maybe I should have spelled it out that I was referring to turns in the circuit in VMC and maintaining healthy airspeed.

[megan]

PaulisHome, hate to tell you but Heston, as are you with

But if your groundspeed is changing, you will be accelerating. Even if the airspeed stays the same. And no matter what speed the wind is.
are incorrect. You are not accelerating. There are two frames of reference, in both of which Newtonian physics apply. One frame is the earth, the other is the airmass, and for an aircraft in flight the relevant frame is the airmass. The only acceleration an aircraft experiences when making a turn, while maintaining a constant airspeed, is that of "g" in its vertical axis, being 2 "g" for a balanced 60° banked turn. Have a read up on Galilean invariance. eckhard in his post re the model in a train exemplifies exactly the Galilean invariance.

[fujii]

People here seem to be confusing speed and velocity.

Acceleration is the name we give to any process where the velocity changes. Since velocity is a speed and a direction, there are only two ways for you to accelerate: change your speed or change your direction—or change both.

An aircraft in a turn is accelerating.

[PaulisHome]

Hi Megan

You seem confused. On the one hand you say "You are not accelerating". On the other, you say "The only acceleration an aircraft experiences ... is that of "g" in its vertical axis, being 2 "g" for a balanced 60° banked turn" Which is it? No acceleration or 2g?

Let me help. It's the second. That 2g isn't pointed upwards relative to the earth (or we'd be accelerating in a loop), it's pointed at 60 deg from the vertical. That resolves into 1g vertical to the earth which counteracts gravity, and about 0.87g pointing towards the centre of the turn. It's the second one which accelerates us around the turn. (and just FYI, the radius, velocity and acceleration are described by the formula a=v^2/r).

And Fujii is entirely correct in #142 (http://www.pprune.org/private-flying/590360-downwind-turn-discussion-8.html#post9667400)

I don't think you've interpreted Gaililean equivalence properly. What it means is that the laws of motion apply no matter what frame of reference you use. There is no correct frame of reference. Using the airmass frame of reference makes the sums easier, sure, but it's quite possible to use the ground, or any other, and it won't change what actually happens. mm_flynn demonstrated that in #130 (http://www.pprune.org/private-flying/590360-downwind-turn-discussion-7.html#post9666879)

Paul

[megan]

Paul, I thought the discussion was in reference to velocity ie in the fore/aft axis. So the question is, what reading would an accelerometer placed in the fore/aft axis read while maintaining a constant airspeed and turning from into wind to downwind? + something, - something, zero? Forget the vertical and lateral.

[Geriaviator]

Maybe I should have spelled it out that I was referring to turns in the circuit in VMC and maintaining healthy airspeed. And I should have noticed that you are under instruction, HR, I'm sorry if my comment seemed flippant. I'm enjoying this learned discussion but I still think myself very fortunate to have had a brilliant ex-RAF instructor who demonstrated the misleading visual cues which have entrapped so many pilots when turning downwind at low level, especially with a misty horizon or none at all. It also helped that the nearest controlled airspace was 30 miles to the west, and Blue Two five miles above. Happy days ... I wish you just as many in your flying career although I understand ATC tends to frown on low-level instructional circuits over the parked Airbuses, in strong winds or otherwise.

[PaulisHome]

Paul, I thought the discussion was in reference to velocity ie in the fore/aft axis. So the question is, what reading would an accelerometer placed in the fore/aft axis read while maintaining a constant airspeed and turning from into wind to downwind? + something, - something, zero? Forget the vertical and lateral.

It would read zero. All the acceleration is normal to the aircraft as you imply. Airspeed would stay the same, groundspeed would vary (assuming there was a wind).

Still true to say that if the ground speed varies, the aircraft is accelerating. That isn't the same as saying that its airspeed is changing. It isn't though necessarily true to say that if the groundspeed stays the same, the aircraft isn't accelerating.

(I can think of only one, rather extreme, example where the first of those isn't quite true, and that's if we head north/south, in which case the speed of the ground moving under us due to the rotation of the earth changes).

But we're having a physics discussion at this point, not a flying one, and we need to use our terms carefully.

;)

Paul

[Pontius]

That isn't the same as saying that its airspeed is changing

And THAT is the only point that needs making in this absolutely ridiculous string of threads born from the Perth Mallard crash. The stuff that is passing over the wing and keeping you up in the air is NOT changing and all the rest of the chatter that surrounds this one fact is just chaff. The downwind turn is a myth generally spouted by inexperienced instructors, believed as true by their even more inexperienced students and so continues to evolve as 'truth'. It's all complete and utter bollox.

Keep the speed right (IAS), keep the ball in the middle and don't pull further than the light buffet.

[PaulisHome]

Still true to say that if the ground speed varies, the aircraft is accelerating
Force = mass X acceleration or acceleration = force/mass

From whence does the force commeth?

From the wings. I'm using the proper definition of acceleration here (not just confined to the fore/aft axis). So if the ground speed is varying then either the airspeed is varying, or the direction of the aircraft is varying and there's some wind. In either case there is acceleration.

P

[gondukin]

The discussion so far has mostly focussed on still air, or a parcel of air moving at a consistent speed. The advice that in these conditions turning up or downwind makes no difference would tie up with my limited experience - I haven't found the need to "dolphin" around a thermal!

However, what about in gusty conditions? If I turn into wind, as I fly through a gust head-on I would expect the ASI to flicker up (although by the time it registers I'm probably out the other side!). However, if I turn downwind, would I not "overtake" a gust, which should register in a brief drop in airspeed? I would also have thought that it would take longer to pass through the gust, as it's travelling in the same direction. So in these conditions, "average" airspeed would be lower turning downwind that up?

[fujii]

PaulisHome. Re post #146, don't accelerometers read one when at rest or is it different for non aviation accelerometers?

[megan]

don't accelerometers read one when at restYes, if you are referring to measuring the "g" in the vertical axis, but when placing an accelerometer in the lateral or longitudinal axis they would read zero when the aircraft is at rest. For example, in the longitudinal axis it would record a value as the aircraft accelerates for take off, or de-accelerates upon landing, and any other the time the aircraft increases/decreases speed. One placed in the lateral axis would record a value when making a turn during taxi, or if slipping or skidding in a turn during flight ie the turn was not balanced. Police measure a "g" following accidents to record the available braking performance of the road surface ie the de-acceleration available.

[India Four Two]

Yes, if you are referring to measuring the "g" in the vertical axis, but when placing an accelerometer in the lateral or longitudinal axis they would read zero when the aircraft is at rest.Here's a real example that you can try at home. The free 'geemeter' iPhone app displays the readings from one of the three accelerometers in the iPhone.

The three pictures below show the display with the iPhone in three different orientations:

1. Held vertically in front of you +1.0 G
2. Held upside down -1.0 G
3. Flat on a table 0.0 G

An alternative is to use the free iSeismometer app and look at the output of each accelerometer as you rotate the iPhone about all three axes.

[Flyingmac]

To quote Elmer Shakespeare. "Methinks y'all doth think too much". Nerd Forum (http://nerdforum.org/)


PS. What's de-acceleration ?

[PaulisHome]

However, what about in gusty conditions? If I turn into wind, as I fly through a gust head-on I would expect the ASI to flicker up (although by the time it registers I'm probably out the other side!). However, if I turn downwind, would I not "overtake" a gust, which should register in a brief drop in airspeed? I would also have thought that it would take longer to pass through the gust, as it's travelling in the same direction. So in these conditions, "average" airspeed would be lower turning downwind that up?

Good question. I think it depends on how you model the gusts. If we model the gust as instantaneous say +/- 5 kts in the horizontal layer, say for 100m (don't ask how for a moment), then despite the airspeed changing the aircraft is still moving at the same speed as the overall airmass frame of reference (that inertia thing). So it would take the same time to move through the 100m no matter which way the gust was going.

If you think about a thermal - the model for that is a vortex smoke ring - so depending on where you hit it you can find air going up, down, in or out. But the thermal as a whole will be moving at the same speed as the airmass, so if you go through it symmetrically and relatively quickly, you'll spend the same time with air gusting towards the nose as away from the nose.

P

[Hadley Rille]

No problem Geri. I should also point out that my instructor is reluctant to teach when the horizon isn't clear until the student has the 'feel' for the right level, climbing and descending attitudes. I feel happy learning with the ASI as backup only.
Maybe this is why I don't understand the influence of the ground on the perception of airspeed.

[megan]

What's de-acceleration ?deacceleration
English
Etymology

de- +‎ acceleration
Noun

deacceleration (plural deaccelerations)

The act of deaccelerating; retardation

[fujii]

decelerate. Look it up.

[Flyingmac]

I'm familiar with Deceleration. Just hadn't encountered Megan's term before. De-acceleration.
Thought it might mean something different. Like returning to a constant speed?
That's what happens when you try to apply logic to English grammar.:)

[Crash one]

As this has turned into a physics discussion of two frames of reference, I've been having a think, dangerous thing to do!
A ship sailing across an aqueduct displaces its 100tons weight in water, no additional weight added to the aqueduct. The ship sinks 2inches in the middle of the aqueduct and is resting on the bottom, does the aqueduct collapse under the extra 100tons weight? Or does it still weigh the same? Or does it weigh more by the difference in displaced water?

[Fitter2]

It's no longer a distributed load (applied by depth of water x density x g) but a point one, so a different loading case.

Now, what about banging on the side of a 15cwt truck to transport a ton of budgies?

[eckhard]

Where does the displaced water go? If 100tons of water is sloshed over the sides, no change of total weight. If 100tons of water is displaced by raising the level of the aqueduct but all the water is still in situ; net gain of 100tons I would have thought.

If 100tons of water is displaced longitudinally (along the length of the aqueduct) then maybe there would be no weight gain in the section under discussion. Is the length infinite? Does it have to be? My brain hurts.

[Tolka]

Is it not a volume of water that is displaced rather than a weight of water equal to the weight of the ship.

[LOMCEVAK]

Tolka,

True, but the weight of water that is displaced has a volume so it has to go somewhere and eckhard is correct in what he says. I believe that you need to consider the water in the aqueduct to be part of a sealed system with a constant volume of water (i.e. including the 'ocean' at either end). The weight of water displaced by the ship will have a volume that is re-distributed throughout the sealed system resulting in an overall surface level rise, including across the aqueduct. Therefore, there will be a theoretical increase in the total load on the aqueduct - but it will be veeeeery small!

So, the budgies in the truck. Once airborne, is their weight transferred back to the truck's floor by the air downflow caused by their beating wings (assuming a sealed truck)?

[worrab]

Crikey! Isn't Physics wonderful that its laws apply to boats and aircraft.

The boat displaces its own weight in water (the Eureka! thing). An aqueduct typically isn't an enclosed space so if you lowered the boat from a crane, water would move out at each end and the overall level of the entire canal would rise just a smidge. If the boat grounds on the aqueduct then the aqueduct is subject to an increased load that corresponds to the weight of the boat less the weight of water that it is displacing even in its sunken state.

The most marvelous incarnation of this is the Falkirk wheel which looks great from the air. (It's a good few years since I circled it, but IIRC it's under the Edinburgh CTA). It has effectively two bathtubs, one on each end of a long bar. The tubs fill with water and the bar balances. A boat sails in and displaces its weight in water. Consequently, the bar is still balanced. I don't know how much power is needed to move a boat from one level to the other but I imagine it's tiny. A true marvel of engineering.

[terry holloway]

If 2,000 dead chickens each weighing 5lbs are loaded in an aeroplane it has a payload of 10,000lbs. If the same chickens are alive and flap their wings vigorously does the payload decrease and what is the affect on aircraft performance?

[megan]

decelerate. Look it upSynonymous with de-accelerate, as Flyingmac said,That's what happens when you try to apply logic to English grammarThe use of the de prefix creates a compound word eg decapitate. In forming compounds the de is normally joined without a hyphen or space. If the second element begins with the letter e, or a capital letter, a hyphen is used. It is also preferable to use a hyphen if the compound brings together three or more vowels, as in de-accelerate, though scientific papers, among others often use deaccelerate, and ignore the preferable rule regarding the hyphen usage. Even my spell check allows de-accelerate, but flags deaccelerate.

Do a search and you'll find the word used in many places, scientific papers etc.

[worrab]

If the same chickens are alive and flap their wings vigorously does the payload decrease and what is the affect on aircraft performance?

I suspect that if the aircraft had a mesh fuselage it would actually decrease the load on the aeroplane and as long as the birds remained out of contact with the airframe it would perform as if empty. Whether they'd keep up with the accelerations of flying is anyone's guess. On the other hand it rather defeats the purpose!

Are we getting more than a little abstract here? If we carry on like this someone will be asking whether a cat left in the hold is alive or dead (that one for Cabin Pressure afficionados).

[TheOddOne]

If we carry on like this someone will be asking whether a cat left in the hold is alive or dead

According to Erwin Schrödinger, both.

Now we're talking PROPER physics!

TOO

[TheOddOne]

Back to the OP's discussion...

There were letters in AOPA's magazine a while ago propounding the 'inertia' theory from someone styling themselves 'Bunbury'. I took it to be a hoax using the pseudonym employed in Oscar Wilde's 'The Importance of Being Ernest'.

Next..

'Flat Plate' theory of lift.

For those of you who aren't familiar, this is where all those fancy aerofoil sections are regarded as so much bunk and all that you need is a flat plate to generate lift.

If we keep this thread running long enough, it'll soon be 1st April, which is where all this stuff belongs.

TOO

[eckhard]

all that you need is a flat plate to generate lift.

Well, that is true. After all, a paper dart flies quite well, as does a kite.

The fancy aerofoils result in greater aerodynamic efficiency and also help the structures and design people to have room for spars, tanks, wheels, etc.

[Mixed Up]

The tailplanes on many aircraft are simply flat plates.

[terry holloway]

The tailplanes on many aircraft are simply flat plates.
But not if it's at the front as a canard!

[SATCOS WHIPPING BOY]

That said Terry, the canards on Gripens and Typhoons are practically planar and rely on flat plate lift.

[terry holloway]

That said Terry, the canards on Gripens and Typhoons are practically planar and rely on flat plate lift.
True! Actually I was thinking of smaller and slower things like longeze and e-Go. The tiny canard on e-Go produces a huge amount of the total lift at cruise speed.

[keith williams]

Getting back to the original subject of this thread:

The following post which was provided by a contributor using the name “theheadmaster” in the thread concerning the Mallard crash in the Pacific Forum covers the matter very well.

Momentum is a vector value. The formula is p=mv, p being inertia, m being mass, v being velocity. In our example mass is constant.

Nil wind, aircraft 100 knots, turning through 180 degrees, the velocity change is 100 knots forward to 100 knots in the reverse direction (both airspeed and ground speed), so a 200 knot vector change.

If you are going from a 50 knot headwind to a 50 knot tailwind, the airspeed change is still 100 knots ahead to 100 knots in the opposite direction. So, 200 knots velocity change. If you want to work in groundspeed, it will be 50 knots (100 kts airspeed less 50 kts headwind) to 150 knot in the opposite direction (100 kts plus tailwind of 50 kts). So, the velocity change is still 200 kts, the exact same value as nil wind. As mass is constant and velocity change is the same, the change in inertia is identical in both cases.


For completeness the tailwind scenario is as follows:

If you are going from a 50 knot tailwind to a 50 knot headwind, the airspeed change is still 100 knots ahead to 100 knots in the opposite direction. So, 200 knots velocity change. If you want to work in groundspeed, it will be 150 knots (100 kts airspeed plus 50 kts tailwind) to 50 knot in the opposite direction (100 kts minus headwind of 50 kts). So, the velocity change is still 200 kts, the exact same value as nil wind. As mass is constant and velocity change is the same, the change in inertia is identical in all three cases.

The use of the word MOMENTUM rather than the word INERTIA in the above statements would have been more accurate, but the meaning is quite clear.

So regardless of whether we use the ground or the air mass as our reference frame, and whether we use still air, headwind or tailwind, the aircraft will experience the same acceleration.

Everything which has mass has inertia, so whenever an aircraft is manoeuvring its inertia will affect its performance. But this effect will be the same regardless of whether the air is still, a headwind or a tailwind. So it is not true to say that the inertia of an aircraft does not affect its performance in a downwind turn. It is simply that the effect is the same as that in any other manoeuvre.

The danger when turning from downwind to upwind, is that half way through the turn the aircraft will be flying across the wind. This will cause it to drift downwind. If the pilot interprets this as having insufficient bank angle, he/she will be tempted to bank further into the turn. At low speeds this risks entering a stall/spin.


Megan you have argued that the only reference frame which is relevant is the air mass, because this is the only one which affects the performance of the aircraft. It is certainly true that the interactions between the aircraft and the air mass are the only ones which influence lift and drag. But the purpose of flying is usually to get from one place to another. For this purpose the most relevant reference frame is the ground. You have also stated that you are only talking about accelerations in the fore and aft direction. The fore and aft direction is a valid coordinate if your reference frame is the aircraft, but it is not valid if you are using the air mass reference frame. Whatever reference frame we choose to use, the coordinate system must be fixed relative to that frame. The fore and aft axis of an aircraft is not fixed relative the air mass in which it is flying.

Brian you asked:

Force = mass X acceleration or acceleration = force/mass

From whence does the force commeth?

It comes from the wings. In a 60 degree banked turn the aircraft experiences a 2g acceleration. The vertical component of lift is equal to the weight of the aircraft and the horizontal component is approximately 1.7321 times the weight of the aircraft. (in the triangle of forces we have 1 squared + 1.7321 squared = 2 squared). That horizontal force of 1.7321 times the weight, is accelerating the aircraft towards the centre of the turn. One factor which is often forgotten is that the wings generate much greater forces than the engine and propeller. Any fixed wing aircraft can be supported by wing lift, but how many can hang vertically on the propeller thrust?

[eckhard]

One factor which is often forgotten is that the wings generate much greater forces than the engine and propeller. Any fixed wing aircraft can be supported by wing lift, but how many can hang vertically on the propeller thrust?

Quite!

I blame this on the way that the 'four forces' are displayed in most diagrams.

The four arrows are shown as roughly equal in length, whereas for a typical small propeller aircraft, the vertical forces should be about ten times bigger than the horizontal forces.

The problem is fitting the forces in to a conveniently-proportioned diagram on the text-book page and displaying them to scale.

[cats_five]

The danger when turning from downwind to upwind, is that half way through the turn the aircraft will be flying across the wind. This will cause it to drift downwind. If the pilot interprets this as having insufficient bank angle, he/she will be tempted to bank further into the turn. At low speeds this risks entering a stall/spin.

Surely the danger is that the pilot uses the ground as their frame of reference when they should be using the air and their ASI? Well-banked turns are not dangerous unless airspeed is inadequate - or is that simply glider pilot speak? (we regularly fly 45 degrees of bank)

[gondukin]

If the pilot interprets this as having insufficient bank angle, he/she will be tempted to bank further into the turn. At low speeds this risks entering a stall/spin.


In gliding though we're taught that a well banked turn increases the stall speed but decreases the spin risk. It reduces the speed differential between the wing tips, and the pilot is less able to over-rudder. In a slow, shallow turn, particularly if being blown downwind or running low on height, the pilot may be tempted to try and use rudder to increase the rate of turn.

I was doing a standard BGA exercise in a Puchacz (which has a reputation for spinning) a couple of weeks ago. At 20, 40 and 60 degrees of bank the glider was stalled and the airspeed noted. Stall speed increased significantly at 60 degrees, but it showed no inclination to spin. A slow shallow turn with a little extra rudder and it won't hesitate.

[piperboy84]

A slow shallow turn with a little extra rudder and it won't hesitate

Yeap, skidding turns low and slow can be bad for your health.

[India Four Two]

I blame this on the way that the 'four forces' are displayed in most diagrams.
eckhard,
I agree. In the glider case, specifically my club's DG-1000 two-seater, the maximum AUW is 760 Kg and the best L/D is 46.5 to 1. So the total drag in 1G flight is 16Kg! I've read that opening the cockpit window and putting your hand out, can double the drag.

[megan]

G'day Kieth, you're correct, but extending the argument beyond that to which I was referring. That was, changes taking place while turning from into wind to downwind while maintaining a constant ASI.you have argued that the only reference frame which is relevant is the air massI was speaking strictly re the case of turning from upwind to downwind. Nothing else or more.the purpose of flying is usually to get from one place to anotherAs you say usually, but not always. The time when ground reference comes into the picture is answering what's my drift, GS and ETA. The fore and aft direction is a valid coordinate if your reference frame is the aircraft, but it is not valid if you are using the air mass reference frameI know what you are saying, if an aircraft accelerates there will be an incremental difference seen between the body frame and air mass frame due the change in AoA. The point I obviously did not make well enough is that, there will be no acceleration sensed in the longitudinal axis, with respect to either the body or air mass plane in an aircraft making a turn from upwind to downwind while maintaining a constant ASI.

An example of taking something out of context.how many can hang vertically on the propeller thrust?Helicopters do it every day on every flight, and not only hang, but climb. :p

pro·pel·ler (prə-pĕl′ər)
A device consisting of a series of twisted blades mounted around a shaft and spun to force air or water in a specific direction and thereby move an aircraft or boat.

[9 lives]

how many can hang vertically on the propeller thrust?

Helicopters do it every day on every flight, and not only hang, but climb. http://cdn.pprune.org/images/smilies/tongue.gif

But Helicopters are also known as rotorwing aircaft. I can think of just a couple of rotorwing aircraft in history which were also equipped with propellers. Otherwise, they are propelled by a lift vector?

[pilotmike]

if an aircraft accelerates there will be an incremental difference seen between the body frame and air mass frame due the change in AoA. The point I obviously did not make well enough is that, there will be no acceleration sensed in the longitudinal axis, with respect to either the body or air mass plane in an aircraft making a turn from upwind to downwind while maintaining a constant ASI
WFT??!!!????:ugh:

What's an "air mass plane"? Is it an aeroplane made out of thin air? Maybe made from a mass of air? Can you make it plain for the masses? We're not on the same plane, that's plain! It's Sunday, I'm off to mass. Err!

It beautifully sums up the total confusion some seem to have about a simple concept when they try to over-think it and end up over complicating it when trying to explain their confusion to others!

[cats_five]

It beautifully sums up the total confusion some seem to have about a simple concept when they try to over-think it and end up over complicating it when trying to explain their confusion to others!

If we all stick to using the ASI to tell us how fast we are flying instead of looking at the ground all will be well. I do look at the ground to judge height - a necessity for a field landing in a glider, and indeed for the variation in the circuit between windy and still days.

[keith williams]

If we all stick to using the ASI to tell us how fast we are flying instead of looking at the ground all will be well.

That is quite true. Looking at the ground will not tell you your airspeed.

But:

I do look at the ground to judge height - a necessity for a field landing in a glider, and indeed for the variation in the circuit between windy and still days.

And therein lies the risk. It is quite natural for a pilot to want to monitor his/her height, position and trajectory over the ground. But unless he/she is aware of the wind conditions and fully understands the effects that this will have on his/her ground speed and direction, he/she may misinterpret the signs. If this causes him/her to and bank too far into the turn or use too much rudder, the consequences may be fatal.

Megan
I really do think that you need to go back and read your posts throughout this thread and see what impact they have had on the readers. The exasperation registered by some contributors should make it obvious to you that many of your comments have added nothing to the general understanding of this subject.

[cats_five]

And therein lies the risk. It is quite natural for a pilot to want to monitor his/her height, position and trajectory over the ground. But unless he/she is aware of the wind conditions and fully understands the effects that this will have on his/her ground speed and direction, he/she may misinterpret the signs. If this causes him/her to and bank too far into the turn or use too much rudder, the consequences may be fatal.

Maybe glider pilots are more aware of the wind? After all the effects are very obvious in a thermal or when soaring a ridge.

However why is a well-banked turn dangerous in a Cessna (for example) and not in a glider? Simple answers only please!

In gliding though we're taught that a well banked turn increases the stall speed but decreases the spin risk. It reduces the speed differential between the wing tips, and the pilot is less able to over-rudder. In a slow, shallow turn, particularly if being blown downwind or running low on height, the pilot may be tempted to try and use rudder to increase the rate of turn.

I was doing a standard BGA exercise in a Puchacz (which has a reputation for spinning) a couple of weeks ago. At 20, 40 and 60 degrees of bank the glider was stalled and the airspeed noted. Stall speed increased significantly at 60 degrees, but it showed no inclination to spin. A slow shallow turn with a little extra rudder and it won't hesitate.

[Heston]

A well banked turn is no more dangerous in a Cessna than a glider, provided the ball is kept in the middle. But glider pilots are more familiar with turning at high bank angles and what to do with the rudder.

[cats_five]

A well banked turn is no more dangerous in a Cessna than a glider, provided the ball is kept in the middle. But glider pilots are more familiar with turning at high bank angles and what to do with the rudder.

Cessna pilots could learn?

[Heston]

Cessna pilots could learn?
Could and should 😀

[Crash one]

Could and should 😀

A bit presumptuous I think.

[Tankengine]

A bit presumptuous I think.

Why?
Icreasing skill is usually a good thing! ;)

[Crash one]

Why?
Icreasing skill is usually a good thing! ;)

Presuming the Cessna pilot needs his skill improved more than the pilot of any other aircraft.

[Heston]

Presuming the Cessna pilot needs his skill improved more than the pilot of any other aircraft.
It wasn't me that first invited the comparison between Cessna pilots and glider pilots. I'm assuming we meant Cessna to be shorthand for 'the average ga pilot flying for leisure on a ppl'.
And I can tell you that more often than not, the average ga pilot, when invited to demonstrate a steeply banked turn (at altitude) during a biennial flight manages about 30degrees of bank maximum. Glider pilots have to be comfortable at much higher angles for continuous turning in thermals.

[cotterpot]

Another sweeping generalisation. I fly a C172.

I was taught 60 deg steep turns and still do them - now it seems 45 deg is regarded as a steep turn.

[cats_five]

It wasn't me that first invited the comparison between Cessna pilots and glider pilots. I'm assuming we meant Cessna to be shorthand for 'the average ga pilot flying for leisure on a ppl'.

That was exactly my meaning, and mentally I was including 3-axis microlights as well.

[Crash one]

I just don't agree with these generalisations. There is never any accuracy in them.

[pilotmike]

Brian Abraham, as you appear to be wading in to defend some of the mixed up nonsense that megan comes out with, and you boldly state that this mysterious "air mass plane" that megan refers to is:Plane of reference I take it to be. Don't know how anyone could assume anything else from my reading
Please explain EXACTLY what - in your own words - an "air mass plane" is, and what its relevance - if any - is to this discussion about aircraft turning in 3D space.

I'm sure the Wiki page you referenced will help to remind you that a plane in this context is a 2 dimensional flat surface. How does any such 2D plane become an "air mass plane", and what EXACTLY is the significance of the "air mass" label given to any such 2D plane? What relevance is any "AIR MASS" to any of the infinite number of 2D planes in space? What is this "Plane of reference" you take his "air mass plane" to be?

Probably, both megan and you are confusing the notion of a 2D PLANE with an inertial FRAME of reference, which could be a valid starting point for a sensible discussion to start from. Other than both words happening to rhyme, there is no connection between any such inertial FRAME of reference and any PLANE that you and megan seen keen to claim has any relevance.

A few contributors to this thread make accurate, valid scientific statements and give equally valid answers and facts; Keith W and PaulisHome to name just two. They are outnumbered by others who like to mix up jumbles of half-truths and myths using occasional precise scientific terms in inappropriate, inaccurate and confusing ways, which unfortunately spreads their own confusion to others.

Unless you can justify your bold claim that you cannot understand how the term "air mass plane" could be confused by anybody, then you are placing yourself squarely in the latter category, along with megan.

Keith W and PaulisHome are correct in what they say. In contrast, the repeated offerings from megan contain little of substance, and much of it is simply wrong. How odd that you should come out in support of some of megan's muddled nonsense, whilst apparently ignoring Keith W's very valid reprimand to megan that their contribution adds little, if anything of value to the discussion.

Edited - it appears that Brian Abraham has chosen to delete his post in support of megan's nonsense rather than try to explain his theories about a "Plane of reference" being an "air mass plane". Pity, I was looking forward to the explanation.

[scifi]

Has anyone mentioned 'Momentum' yet..?


Consider two cases... Airplane flying North at ASI of 100 kts with 99 kt tailwind, and second Airplane flying North at 100 kts into a 99kt headwind.
This makes the ground speeds 199 kt and 1 kt, respectively.


The first has considerable Momentum (Mass x Velocity) the second has almost none.
So if they are both disturbed by a similar force (a gust of wind, or a control movement.) The first aircraft will be deflected from its course by one or two degrees, whereas the second will change its direction vector by a very large amount.


.

[Crash one]

No they won't. Because they only have the momentum of their individual 100knots in their individual air mass.
You will see things differently because you are still on the ground.
I'm goin up the pub!!!!

[BossEyed]

scifi, if that were correct then aircraft would be more agile when flying downwind than upwind.

And they aren't.

[scifi]

Hi Boss Eyed... No I think you have got it completely the wrong way round...


If you are going North downwind at GS = 199 kts a small control input could change your course by one degree... (your still effectively going North.)
But going into wind at Ground Speed =1 kt, the same control input could make your course change by 90 degrees (i.e. you fly west.)


In both cases your heading remains about the same, but the Course works out very differently.


For those not familiar with the Jargon, 'Course' is the track over the ground.
'Heading' is the way the front of the aircraft is pointing.


.

[Crash one]

In answer to Pilot Dar's original question starting this thread.
It obviously isn't.
I've read some rubbish over time, but some of this really takes the biscuit.
All this krap about Mass X V = the square root of an orange to the 10th power of a pile of horse ****e.
There is nothing to it.
An aeroplane flies round in circles in the air.....full stop. FFS.

Edit: Is there a misconception here about the term "downwind"?
As far as the wind or air mass is concerned , there is no "downwind".
Into wind or downwind are referenced from the ground.

[Heston]

Hi Boss Eyed... No I think you have got it completely the wrong way round...


If you are going North downwind at GS = 199 kts a small control input could change your course by one degree... (your still effectively going North.)
But going into wind at Ground Speed =1 kt, the same control input could make your course change by 90 degrees (i.e. you fly west.)


In both cases your heading remains about the same, but the Course works out very differently.


For those not familiar with the Jargon, 'Course' is the track over the ground.
'Heading' is the way the front of the aircraft is pointing.


.


If there was a way to do it I think the mods ought to link the thread at this point back to the beginning, so creating an endless loop of horsesh1t, that all posters are forever doomed to read without ever breaking out.
Groundhog Day and Sisyphus spring to mind.
So while I still can, I'm going to duck out for a second time.

[worrab]

... though if you touch down at 45 knots into a 45 knot headwind you will have zero kinetic energy :) .

Kinetic energy is of course proportional to Velocity squared so even a 10kt headwind will reduce your landing energy by around a half for an airspeed of 45kt since your groundspeed will be around 35kt.

The point is that the aircraft doesn't have any knowledge of what's going on around it. As an observer you can choose your frame of reference. You can make the maths slightly more complex by using an external reference and including the rotation of the earth (in the UK we have a linear speed of around 700mph because of that) or the speed of the earth round the sun (approx 70,000mph). You could decide that the whole of space was moving relative to the aircraft and that pulling back on the stick forces the earth down.

The key lesson from this long, tortuous path is that close to the ground, airspeed can be a friend. A quick glance at the ASI and timely correcting action with the stick and/or throttle can save your life.

[Crash one]

The key lesson from this long, tortuous path is that close to the ground, airspeed can be a friend. A quick glance at the ASI and timely correcting action with the stick and/or throttle can save your life.

I'm sure that procedure used to be called, landing the aeroplane.
I stand to be corrected of course.

[PaulisHome]

For those wanting to talk about flying, rather than physics, look away now.

Has anyone mentioned 'Momentum' yet..?

Consider two cases... Airplane flying North at ASI of 100 kts with 99 kt tailwind, and second Airplane flying North at 100 kts into a 99kt headwind.
This makes the ground speeds 199 kt and 1 kt, respectively.

The first has considerable Momentum (Mass x Velocity) the second has almost none.

So if they are both disturbed by a similar force (a gust of wind, or a control movement.) The first aircraft will be deflected from its course by one or two degrees, whereas the second will change its direction vector by a very large amount.

scifi, if that were correct then aircraft would be more agile when flying downwind than upwind.

And they aren't.

Hi Boss Eyed... No I think you have got it completely the wrong way round...

If you are going North downwind at GS = 199 kts a small control input could change your course by one degree... (your still effectively going North.)
But going into wind at Ground Speed =1 kt, the same control input could make your course change by 90 degrees (i.e. you fly west.)

In both cases your heading remains about the same, but the Course works out very differently.

For those not familiar with the Jargon, 'Course' is the track over the ground.
'Heading' is the way the front of the aircraft is pointing.

Scifi, Most of what you say is true, but your conclusions are wrong - at least if you're trying to imply that the two examples' agility are different.

In particular, the statement "The first aircraft will be deflected from its course by one or two degrees, whereas the second will change its direction vector by a very large amount." is wrong.

You've used the term "direction vector". Difficult to know what you mean by that - but if it's velocity (which is a vector), then the change in it is the same for both aircraft. Similarly the change in momentum (which is just the velocity times the mass, and is also a vector) is the same for both aircraft. The course change (or track as I think we'd conventionally call it), changes dramatically, but that's just a direction and not a vector quantity, nor does any law of conservation apply to it.

That the momentum of your two examples is different is just a consequence of the frame of reference you are using - in that case that of the ground. In the air mass frame of reference, the momentums are the same.

And the fact that you've chosen a different frame of reference doesn't change the agility of the two aircraft. They are both equally agile - it's just that the ground is moving in respect of the airmass frame at 99 kts.

Paul

[piperboy84]

All this theory and scientific stuff is way over my wee heed, it reminds me of a driving trip to Vegas where I pulled over at a truck stop for lunch. Bellying up to the breakfast counter I was within earshot of 3 gentlemen who were obviously truckers, the sporting of clothing with head to toe advertising logos of Kenworth and Mac was a dead giveaway. While eagerly awaiting my very first try of the days special Chicken fried steak with lashings of biscuits and gravy I picked up the topic of their conversation being their respective educational backgrounds and the reasons for each getting into the trucking business. One guy in fantastic southern drawl stated adamantly that the reason he left high school early was he realized he was wasting his time attending a maths class as he observed

"How the hell are you supposed to add A's, B's and C's and get something? "

I thought to myself well done that man, there's no point trying to get your head around a subject you know fine well is way over it, which is pretty much how I feel about this thread.

[cats_five]

I thought to myself well done that man, there's no point trying to get your head around a subject you know fine well is way over it, which is pretty much how I feel about this thread.

So true. Almost all that is needed is to watch the ASI (and act on what it's telling you!) and keep the ball in the middle. Of course planning needs to take cross-winds into account, just as sailing needs to take the tide or current into account, and landing (or rounding bouys) is where you have the complication that you are moving in one frame of reference but going to a target in another. Still air / no tide or current = not that hard. Blustery conditions / strong tide = another kettle of fish.

[scifi]

Hi again, its me.... Unfortunately you are overlooking the fact that the ASI only measures forward velocity (pitot tubes only face forwards.)
So the statement by Worrab... ... though if you touch down at 45 knots into a 45 knot headwind you will have zero kinetic energy http://cdn.pprune.org/images/smilies/smile.gif Is not absolutely true...
To land, you also need a downward velocity, so many ft/sec, so you can still have considerable downward Kinetic energy, which can compress your suspension and bounce you back into the air... (been there, done that..)
Also the pitot tube will not pick up any sideways velocity, which you will have when you bank in a very strong headwind.


A Velocity Vector is what is displayed on an ATC Radar screen, and shows the direction and speed that the aircraft is moving... It gives them some indication of the position you will be in in the next few minutes.
.

[BossEyed]

As well as regularly amazing me with what I am reading, this thread reminds me that I miss the definition of "sciolist" that used to be at the foot of each PPRuNe page.

[Crash one]

Scifi.
This is not the ladies cross stitch forum.
Some of us actually already knew that pitot tubes point forward.
Your conclusions are all wrong.
An aircraft landing, progressively runs out of kinetic energy in all directions in the air as its airspeed slows down. It has no kinetic energy in the ground reference until it makes contact with the ground.
It can have vast amounts of speed in one frame of reference and nothing in another.
It is the transition from one frame of reference to another that causes all the problems.
How simple can it get?

[mikehallam]

C1,

But if you keep power on as required to hold 40 kt into 40 kt breeze ready to land but say 25 ft up & still too high, you hover, (after a fashion).

When you decide to fly nearer the deck it will land and whilst shutting down, remaining facing into wind as you, brakes on, wait for a wing walking team.

Deck being the operative word as it's how high performance naval a/c manage short runway landings - with carrier 'steaming' into wind plus arrestor hooks. u.s.w.

Come to think of it a suitable bit of elastic stretched across any runway could be fun & solve many landing possibilities.

mike hallam.

[worrab]

Still on my to-do list is to fly backwards. Looking out might be a bit tricky.
There are rumours of someone having done a take-off into a strong wind, reduced throttle, flew backwards and then landed on the runway they took off from. I guess in something like a Chevron this is a very real possibility?

[Crash one]

C1,

But if you keep power on as required to hold 40 kt into 40 kt breeze ready to land but say 25 ft up & still too high, you hover, (after a fashion).

When you decide to fly nearer the deck it will land and whilst shutting down, remaining facing into wind as you, brakes on, wait for a wing walking team.

Deck being the operative word as it's how high performance naval a/c manage short runway landings - with carrier 'steaming' into wind plus arrestor hooks. u.s.w.

Come to think of it a suitable bit of elastic stretched across any runway could be fun & solve many landing possibilities.

mike hallam.

I remember the Ark Royal having to do 5knots astern in a howler in Biscay Bay trying to recover two Fouga jets that we're having difficulties catching up with us.
They had hooks down but I don't think they used them!

[Fitter2]

I thought it might help to point out to the confused that it's actually change in velocity that should be considered (i.e delta v in any frame of reference, rather than v in an arbitrary frame). But on second thoughts, they are clearly sufficiently confused already.

[cats_five]

Hi again, its me.... Unfortunately you are overlooking the fact that the ASI only measures forward velocity (pitot tubes only face forwards.)
So the statement by Worrab... ... though if you touch down at 45 knots into a 45 knot headwind you will have zero kinetic energy http://cdn.pprune.org/images/smilies/smile.gif Is not absolutely true...
To land, you also need a downward velocity, so many ft/sec, so you can still have considerable downward Kinetic energy, which can compress your suspension and bounce you back into the air... (been there, done that..)
Also the pitot tube will not pick up any sideways velocity, which you will have when you bank in a very strong headwind.


A Velocity Vector is what is displayed on an ATC Radar screen, and shows the direction and speed that the aircraft is moving... It gives them some indication of the position you will be in in the next few minutes.
.

So no varios in light aircraft?