The Downwind turn
 

This is more an aerodynamic question that a flight testing one, but maybe somebody in the fraternity could help me. Among light aircraft pilot's there is a belief that a downwind turn from a strong headwing causes the aeroplane to lose airspeed and stall. Many accidents have occured like this. Yet aerodynamically this should not be so, or should it? Is it just a preception or is there another explanation.

Have checked this one for a while now, waiting for someone better qualified to reply. None forthcoming so here goes. This has been around a LONG time and in the early sixties there was a fairly long-winded series of letters to Auntie Mary's Flying magazine "Air Clues". There were more lift arrows, drag arrows, thrust arrows and general aero-dynamic theories than one could shake the proverbial stick at. Finally it was nailed by - I believe - someone from Boscombe Down who added the magic ingredient INERTIA. He postulated that this was related to the earth's mass and not to the block of air in which the aircraft was operating. The downwind turn therefore required a nose-down pitch input to compensate for the relative speed loss. The normal visual cues (rather than instrument reference) which most light pilots used would not provide the necessary input and the resultant speed decay could, and did, occasionally, harvest its reward !
That's all from memory and it may not be terribly accurate. The originals may still be available but I doubt it :)

It's a fact, though it goes aginst most basic flying training guidance.

Essentially, as has been pointed out, the a/c momentum keeps it at the same relative groundspeed, so that a turn downwind results in a loss of airspeed. Conversely, if you stonk along with a strong tailwind and turn into wind, the airspeed will increase. Try it next time you have a chance, it is very clearly seen in any more than about 25 kts of wind with a light a/c (or any helicopter).

Interesting subject bandied around for years. The turning downwind accidents at low level still happen. To prove something to myself, I flew a Cessna 152 into a steady 40 knot headwind. IAS 80 knots. 2000ft. Then ripped into a 180 degree limit turn downwind at full throttle (to minimize speed bleed off) Took about 5 seconds. The IAS remained unchanged at 80 knots.

I suspect the point is that close to the ground one tends to use ground cues to a greater extend, rather than pure attitude cues (which are less reliable because of terrain changes) and instrument changes (because lets face it, we should be flying with our heads :mad: out :mad: in the circuit).

I found myself getting mildly thrown a little while ago in a slow (45kn) aircraft as I turned downwind (wind was about 15kn down the strip). Once I disciplined myself to maintain attitude, and not do what "seemed" to look right from the ground cues I found my airspeed remained fixed, just like Hudson. I could see how easily I could have stalled, but it was me, not the aeroplane.

It would be an interesting trial to do (with a safety pilot with their head inside, plus an aircraft known to lose very little height at the stall), ask various pilots to turn downwind at around 600 - 800 ft with a reasonable wind down a friendly runway following ground cues alone and see what happens.

G

[This message has been edited by Genghis the Engineer (edited 03 February 2000).]

G2,

Happy new year.

Any aerodyne will lose speed in a turn because, ceteris parabus, the supporting vector shifts from the straight and level position to produce a centripetal component. The frame of reference is the air mass within which it is flying.

Wind is only an earth-referenced concept.

Humans regularly injure themselves (ither physically or mentally) when they attempt dynamic activities in the incorrect frame of reference. I used to demonstrate this problem at reasonable speeds in the earth's boundary layer by getting the student to observe the IAS and attitude in turns flown on the clocks and then in turns flown by reference to the ground. They generally had no problem thereafter recognising that they had to actively prevent the visually-inspired desire to maintain a constant ground speed from interfering with the need to maintain air speed.

All and any reference to the issue should be to "the downwind turn illusion" so that the urban myths scattered throughout this forum and the minds of the ill-informed are contained rather than perpetuated.


------------------
Stay Alive,

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A downwind turn in moderate wind with a friendly strip, at 600-800ft, in an aircraft known not to lose too much height in the stall, and all by ground reference only.....?

Anyone been gliding recently? (...ever?)

I would agree that this is more to do with the visual experience, but the fact that all our gliding sites are not littered with broken aircraft and pilots (well not many anyhow) would suggest that there are better trees to bark up in the interests of saving life.

One more ingedient for the pot: let's remember to think about AoA in this maneouvre, not just IAS. Give this one some thought.

I think I said it would be interesting, not groundbreaking.

I also think you miss the point; most GA pilots haven't flown gliders (or microlights which would give the same benefit) and thus do not have the sort of pure-handling benefit that flying those sort of very simple aircraft at low speeds gives.

Wow Ghengis, that sounds like a compliment! Let's hope none of us experience 'groundbreaking' aviation.

Youn obviously haven't seen me fly (I must say that the Boeing doesn't respond quite as rapidly as the Discus)!!!!

Those who have taken part in this discussion might like to read the similar thread in the flight instructors' forum.

[This message has been edited by Fokjok (edited 09 February 2000).]

Stalling in downwind turns

Sorry I am so late coming to this party

I think this is a very simple issue that has got a bit out of hand over the years (and more recently in this thread)

So let’s use the general knowledge and common sense that all pilots have!

If a turn is maintained at a constant angle of bank and constant height and constant power in air that is not moving over the ground (an academic rarity if ever there was one!) the aircraft is going round and round in a state of constant energy. That energy being made up from the sum of its kinetic energy (due to speed over the ground) and potential energy (due to height above the ground).

NB I have deliberately ignored the effect of the loss of weight due to burning fuel in the turn because it is so small - unless you are grunting round turn after turn in full reheat at the local airshow, which is not what we are talking about here.

On the other hand, if the air is not still, but moving at a steady speed over the ground, the aircraft will suffer a fluctuating ground speed after the “steady ” turn is established at “constant” airspeed. This ground speed (and its related kinetic energy) will be a minimum when flying into wind and a maximum when downwind.

This energy change and the problems it produces (thanks to inertia) is what is at the heart of the so-called downwind turn stall issue. I say so-called because this effect (while always having some transient effect on airspeed, related to the transient change in ground speed) will only cause a STALL in quite EXTREME CIRCUMSTANCES. Normally there will be no more than a minor reduction of height or a slight loss of airspeed (both usually so small that I recommend you don’t even think of wasting your time airborne by trying to measure them in real air, which is anything but steady)

OK so what are these extreme circumstances?

An example was what applied to the Bristol Fighter at the Farnborough airshow in 1962 and it will do it every time! Perhaps some of you even saw it hit the ground then?

Imagine you are standing on the tower at Farnborough. It is a lovely day, the wind is nicely down 25 and you are watching the vintage Bristol F2B (D8096) doing its thing. Every day the same, it flies majestically downwind along runway 07 about 50 ft up, then just before the control tower it pulls up into a stall turn yawing round at say 350 ft. Nicely done it goes vertically up (how else) before the rudder goes in, round she goes, check going vertically back down, then pull out of the dive. Fly back down 25 and into the next pass. (Ah Hendon! Those were the days sort of thing).

Let’s think about that. Any stall turn takes the aircraft from its entry heading to its exit heading – a 180 degree change – in the time it takes the nose to yaw from straight up to straight down. Not long. (2-4 secs say) If the aircraft is like the Bristol Fighter above and going downwind when it pulls up it will necessarily be recovering from its dive facing into wind. Thus the airspeed that builds up in the dive, before pull out, gets a nice little addition courtesy of the surface wind when you fairly abruptly level out into that wind.

So now you can guess what happened! On the last day at Farnborough 1962 the wind turned and blew down 07. The Bristol Fighter however was flown to exactly the same visual (ground) cues and so when it tried to pull out from the nice vertical dive it not only didn’t have the benefit of a headwind, quite the reverse, it had some 12 kts up its backside. Fortunately the height margin the pilot had in hand was nearly enough to compensate and so it just clobbered the ground hard at the bottom, modified the undercarriage a fair bit, then bounced back into the air for a wobbly airborne rethink of the eventual landing strategy.

So, is there anything behind this (sudden) downwind turn and stall thing? - yes. Does it normally affect today’s aviator? - NO.

I agree absolutely with your analysis John - it's about inappropriate reliance upon ground cues.

Does it affect the modern aviator, well it's happened to me once for exactly that reason, the wind shifted and strengthened when I was practicing circuits and flew the original ground cues whilst practicing circuits and not watching my AoA / IAS enough until I got some buffet and told myself not to be so daft.

That said, I usually fly very slow aircraft, often in winds up to half my cruising speed - so perhaps I'm more likely to see it than somebody doing 85 knots around the circuit in a Tomahawk.

G

Thank you J F - that should nail it nicely. Your erstwhile secretary (at EGTD) sends her best wishes.

Stalling in downwind turns

Sorry I am so late coming to this party

I think this is a very simple issue that has got a bit out of hand over the years (and more recently in this thread)

So how about this:

If a turn is maintained at a constant angle of bank and constant height and constant power in air that is not moving over the ground (an academic rarity if ever there was one!) the aircraft is going round and round in a state of constant energy. That energy being made up from the sum of its kinetic energy (due to speed over the ground) and potential energy (due to height above the ground).

NB I have deliberately ignored the effect of the loss of weight due to burning fuel in the turn because it is so small - unless you are grunting round turn after turn in full reheat at the local airshow, which is not what we are talking about here.

On the other hand, if the air is not still, but moving at a steady speed over the ground, the aircraft will suffer a fluctuating ground speed after the “steady ” turn is established at “constant” airspeed. This ground speed (and its related kinetic energy) will be a minimum when flying into wind and a maximum when downwind.

This energy change and the problems it produces (thanks to inertia) is what is at the heart of the so-called downwind turn stall issue. I say so-called because this effect (while always having some transient effect on airspeed, related to the transient change in ground speed) will only cause a STALL in quite EXTREME CIRCUMSTANCES. Normally there will be no more than a minor reduction of height or a slight loss of airspeed (both usually so small that I recommend you don’t even think of wasting your time airborne by trying to measure them in real air, which is anything but steady)

OK so what are these extreme circumstances?

An example was what applied to the Bristol Fighter at the Farnborough airshow in 1962 and it will do it every time! Perhaps some of you even saw it hit the ground then?

Imagine you are standing on the tower at Farnborough. It is a lovely day, the wind is nicely down 25 and you are watching the vintage Bristol F2B (D8096) doing its thing. Every day the same, it flies majestically downwind along runway 07 about 50 ft up, then just before the control tower it pulls up into a stall turn yawing round at say 350 ft. Nicely done it goes vertically up (how else) before the rudder goes in, round she goes, check going vertically back down, then pull out of the dive. Fly back down 25 and into the next pass. (Ah Hendon! Those were the days sort of thing).

Let’s think about that. Any stall turn takes the aircraft from its entry heading to its exit heading – a 180 degree change – in the time it takes the nose to yaw from straight up to straight down. Not long. (2-4 secs say) If the aircraft is like the Bristol Fighter above and going downwind when it pulls up it will necessarily be recovering from its dive facing into wind. Thus the airspeed that builds up in the dive, before pull out, gets a nice little addition courtesy of the surface wind when you fairly abruptly level out into that wind.

So now you can guess what happened! On the last day at Farnborough 1962 the wind turned and blew down 07. The Bristol Fighter however was flown to exactly the same visual (ground) cues and so when it tried to pull out from the nice vertical dive it not only didn’t have the benefit of a headwind, quite the reverse, it had some 12 kts up its backside. Fortunately the height margin the pilot had in hand was nearly enough to compensate and so it just clobbered the ground hard at the bottom, modified the undercarriage a fair bit, then bounced back into the air for a wobbly airborne rethink of the eventual landing strategy.

So, is there anything behind this (sudden) downwind turn and stall thing? - yes. Does it normally affect today’s aviator? - NO.

Thanks genghis and Cornish, and mine to hers

Sorry about the finger trouble. That is why I have quit aviating...

Working on the autobiography now John?

Nicely put, John, and an interesting 'take' on the issue.

Nice to hear someone respected confirming the energy theory. Those that say wind is earth referenced are quite correct, however the a/c total energy is an absolute, and not relative, concept which will be affected by wind. Why does airspeed fluctuate with turbulence? a/c momentum.

I particularly like the experiment where the pilot uses full throttle to prevent speed loss during the turn, but then finds that on rolling out his speed is unchanged. That's a bit like demonstrating a spin but avoiding rudder and elevator inputs to avoid entering a spin.

PS. Why do people assume that if someone is pushing a theory that they do not agree with (understand?), that person must be ignorant or uneducated (read the same threads in "flying instructors")? It's jolly rude.

If one of my students flies a perfect downwind turn I suspect he/she is instrument flying! If they are looking out then I expect a small loss of IAS. It's all in the visual cues.

If the conservation of groundspeed theory is right then take the situation of flying at TAS 90kt into a 30 kt headwind now if we do a (theoretical) instantaneous turn the TAS needed to maintain the G/S is 30kt ie a loss of airspeed of twice the wind speed!! This is certainly mathematically correct - is it correct in practice?

Well Martair, it's all been answered above but maybe another analogy would help:

Let's take an aircraft ideally trimmed to a known IAS, with the thrust set to the correct setting for that speed. Now let's carry that aircraft up to say 10,000ft under a balloon and drop it. Due to its stability, after falling a while the aircraft will stabilise and fly level at the planned IAS.

The same aircraft, subject to a loss of airspeed ( which is what happens in your inertial mathematical model ) provided the thrust is not touched, will fall as before and regain its stable speed - provided there is enough altitude. The effect of the wind change is transient and is compensated for by a change of altitude.

Now let's put in a pilot. He can do one of three things:

He can do nothing and let the aircraft fall;

He can maintain altitude but let the IAS drop and trim it to the lower IAS (stall IAS in an extreme case) or;

He can maintain altitude and as a part of his scan maintain IAS (and incidentally trim) by adjusting the thrust appropriately.

The third case is of course the ideal one and yet an extreme wind change, especially if unexpected may lead to the pilot adjusting the thrust too little and too late.

Modern aircraft are designed to fly at a margin above stall speed but older pioneering types such as the some of the Shuttleworth ancients were hard pushed to fly at all and were very close to the stall.
On these types the problem was more likely to arise.

Having said that, wind changes will always affect aircraft to some extent as we see every time we fly an approach in gusty weather - IAS and/or glidepath are affected.