Downwind turn discussion

[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.

[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?