Downwind turn discussion

[oggers]

The last word on downwind turns

[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

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.