Downwind turn discussion
 

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?

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"?

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.

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

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.

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.

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.

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...

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.

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 :)

Jan

: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.

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.

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'.

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.

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.

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.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.

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 ?

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.

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.

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:

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.

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.

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).

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.

ShyTorque

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

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

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.

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!

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.

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.

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.

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.

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!!

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.

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.

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.

Step turn wrote.

That about sums it up.:ok:

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.

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..

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....

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?