Myth busting! I MIGHT STALL IF I TURN DOWNWIND!

[Guest davidh10]

That's a very good and informative article which I found helpfull, Except for me I found the myth of the "downwind turn temporarily reduces your airspeed" and the Real issue of wind shear a bit of a contradiction.I think they are both real but in the downwind turn is the loss of airspeed due to inertia isn't realised because the turn isn't rapid enough so the turn on it's own doesn't pose any problems. (not including the other influences like apparent increase of airspeed while viewing the ground etc).

The two situations described are different, which is where confusion can arise.

 

• The fact that wind does not affect a turn relates to a steady state condition where the wind is constant, with the absence of gusts or turbulence.
   
   
  
• Gusts and turbulence affect the lift generated by changing the airflow speed over the wing and / or the angle of attack (due to change in the angle of the airflow relative to the aircraft. These effects are usually momentary and thus an aircraft with greater inertia will take longer to adjust to the new airflow direction and speed. The effects can be of longer duration in situations like a thermal or patch of falling air or more extreme in a microburst (stay away from microbursts).
   
   
  
• When close to the ground (several hundred feet), such as in the take-off and landing phases, the drag of the Earth on the wind causes wind shear* and elevated objects (terrain, trees, buildings) generate turbulence.
   
   
  

 

 

When you turn an aircraft, lift is tilted and the vertical component decreases, which is usually compensated for by increasing angle of attack and taking you closer to a stall. A climbing turn will unequally increase the angle of attack taking the high wing closer to a stall than the lower wing. In these situations, encountering gusts or turbulence is more likely to generate an aerodynamic stall and if you are close to the ground with insufficient altitude to recover, its all over red rover.

 

Also refer to JB's tutorial section " Don't stall and spin in from a turn", for a more detailed explanation of AOA in turns.

 

*Wind usually travelling faster at altitude than at ground level. Thus as you climb or descend the inertia of the aircraft impairs the ability of the aircraft to adjust to the changing relative airspeed. I have seen situations where wind was stronger at circuit height than at higher altitudes, but you will still get a drag effect of the Earth within that air layer. In such a case there will be wind shear near the ground and also at the boundary between the higher altitude slower air and higher speed air at circuit height. I have personally experienced up to three distinct layers travelling in different directions at different speeds while descending to land.

 

Hope that helps.

 

 

[REastwood]

Actually you will not observe the ball to be travelling faster than light because as an object approaches the speed of light with respect to an observer, time slows down for that object as seen by the observer.This effect* has to be compensated for by the GPS satellite clocks which as observed by us on Earth, run a few microseconds slow each day. If not corrected, it would cause the computed position on the Earth's surface to be in error by an amount that would grow by about 10km/day.

 

*Actually there are two effects that have to be compensated; time contraction due to the velocity of the satellites and warping of space-time by the gravitational field of the Earth, which is greater at the surface than at orbital altitude.

There is another, more controversial*, theory developed from data returned from the Cassini and Voyager probes as well as the GPS network that indicates the speed of light in a vacuum is variable and that time is constant. The difference between clocks on Earth and those in orbit are due to the effect gravity exerts on time measuring devices (atomic clocks) as apposed to gravity slowing down time itself. The variability of light would dispense with the need for dark matter & dark energy (which no one can find) thus stopping the runaway complexity of current theories trying to explain observed discrepancies.

 

* Controversial because it means Einstein was wrong - Google "Einstein was wrong" and in between a lot of rubbish there is some respected scientific study indicating that the invariance of c is false.

 

 

[Guernsey]

Hey guys this thread has now proved to be extremely dangerous. I went flying this morning in perfect weather conditions with nobody else in the circuit. I took off into a 25knot wind...was very relaxed but as I started to turn downwind (I was doing circuits) I started to think of everything that has been said on this thread.My head started spinning then without realizing it I started to pull back on the controls to offset my increased ground speed the right wing stalled violently and I spun vertically into the ground. The impact was unbelievable and then I felt a reassuring hand on my shoulder and heard my wife saying "wake up dear are you alright"

 

Alan Marriette.

 

 

[poteroo]

Hey guys this thread has now proved to be extremely dangerous. I went flying this morning in perfect weather conditions with nobody else in the circuit. I took off into a 25knot wind...was very relaxed but as I started to turn downwind (I was doing circuits) I started to think of everything that has been said on this thread.My head started spinning then without realizing it I started to pull back on the controls to offset my increased ground speed the right wing stalled violently and I spun vertically into the ground. The impact was unbelievable and then I felt a reassuring hand on my shoulder and heard my wife saying "wake up dear are you alright" Alan Marriette.

Actually Guernsey, there's some truth in your statement re the d/w turn.

 

And, the explanations re wind shear and higher/lower wing are all plausible too.

 

A stall situation is more likely to happen if you make the d/w turn at lower level, and more so if you are level. The reason is in the wind illusion - which is telling you that the aircraft is slipping into the turn. Your response might well be - I'll apply some more rudder to prevent this. But, the increase in rudder increases the angle of bank - causing your nose to fall into the turn even further, and decreasing your total lift too. So, you apply some back pressure on the elevators to raise the nose - but that just lowers your airspeed. And, you already have an increased stall speed because of the a-o-b. Solution is to not look down into the turn - especially to the left - and to fly in balance via your pair of posterior balance sensors attached to the seat.

 

Remember that it's also a question of how much energy your aircraft has in reserve. If you are fast - then there's margin to steepen the turn and still not load up your wing and reach a resultant high stall speed. But, if you start with low energy, as in a climb after a strip inspection, or you are flying slower in marginal visibility - then you need to increase power into the turn. When you increase power - you'll be rolled further into a left turn than anticipated, which accentuates the slip illusion due to the d/w turn.

 

In just about every case - you need to reduce the a-o-b - because you either don't have the energy or the power to make it around an ever increasing a-o-b turn. And this needs to be done in balance - or you are again reducing lift while still in the turn.

 

Low level, downwind turns are not the manoeuvre you attempt when you are still not managing your balance in turns.

 

happy days,

 

 

[Guernsey]

Thanks for the input Poteroo I am aware of all that you have stated having flown many types over a 50 year period...I was just trying to add some humor to the debate however I am sure the info will be useful to others on this forum even those of us who have logged many hours but don't fully understand the mechanics involved.

 

All the best....Alan.:thumb_up:

 

 

[APEppink]

"The aircraft is borne by the wind, and reference to GROUND SPEED has no impact at all on aerodynamics."

 

Impossible. By F = m*a, an applied force is required to effect a linear accelleration. HOW does the airmass impart this force? SPECIFICALLY?

 

 

[kasper]

"The aircraft is borne by the wind, and reference to GROUND SPEED has no impact at all on aerodynamics."Impossible. By F = m*a, an applied force is required to effect a linear accelleration. HOW does the airmass impart this force? SPECIFICALLY?

But unfortuantely it shte NUT behind the stick that is the problem ... the mk1 human eye when attached to the mk 1 human brain is not a logical combo in this situation ... the lower you are the more the perception of speed hits the visual brain and IF you are flying by visual clues as to speed by reference to bits of static landscape whizzing past your window you may pull back and then it can get messy.

If you are flying by visual reference to the horizon out the front then the whizzing is not an issue as the aispeed/attitude/poweer linkups that are actually keeping the gods of gravity at bay are all in balance ;-)

 

 

[SDQDI]

But unfortuantely it shte NUT behind the stick that is the problem ... the mk1 human eye when attached to the mk 1 human brain is not a logical combo in this situation ... the lower you are the more the perception of speed hits the visual brain and IF you are flying by visual clues as to speed by reference to bits of static landscape whizzing past your window you may pull back and then it can get messy.If you are flying by visual reference to the horizon out the front then the whizzing is not an issue as the aispeed/attitude/poweer linkups that are actually keeping the gods of gravity at bay are all in balance ;-)

I've mentioned quite a few times on here just how important I think it is that we all put in some time in doing LL training. For sure 99% of the time most of us might not need it BUT the one time we get caught with a need for a 300ft circuit for whatever reason it could make a huge difference.

I was always a well balanced pilot (no matter what others say!) at altitude and was always relatively capable in the takeoff and landing phases but with just a mild 10-15 knot wind my first ll turns were amazingly pathetic. We can't help the way we are wired but IMO a little ll training can make a huge difference in our safety in out of the ordinary emergency low level manoeuvres and also helps with the safety of our landings and takeoffs.

 

 

[OzBirdy]

Amazing.

 

Scarey to think, there are pilots, with licenses, who fly machines capable of carrying more than just the pilot, who dont get it.

 

How do you get a licence to opperate a passenger carrying aircraft, when you dont understand the simplest of phisics?

 

 

[APEppink]

E

 

Hi Mike thanks for your response. There is one thing though that has not been taken into account in this explanation. The inertia of the aircraft. how about we talk about turning from downwind into a headwind. . If the aircraft was to suddenly turn into the?u wind wouldnt it pick up some speed for a moment because it would take time for the inertia of the aircraft to slow down. Or we can think of it in another way. Lets first say there is a 50 knot westerly. Now lets imagine an aircraft is climbing verticly up in the air until it almost stalls at this point the pilot kicks in full rudder and the nose comes around pointing straight down at the ground. When the pilot has some speed he pulls sharply out fo the dive into the tailwind. now with that sudden change in direction wouldnt the aircraft loose some speed until it ha time to catch back up with the wind. Or what about if he pulled out of the dive into the wind. wouldnt he suddenly have a 50 knots of extra airspeed that may last for 3 seconds or so until the inertial of the plane has been slowed back to the oringinal airspeed.

Everybody's ignoring F = m*a. An applied force is necessary to effect a linear accelleration. The force has to come from the air mass in the form of an initially reduced IAS. How else can it be?

 

 

[APEppink]

More

 

yep so wouldnt it be correct to say the aircraft has more inertia when landing downwind? isnt that why it takes longer to stop

Momentum.

 

 

[M61A1]

E

 

Everybody's ignoring F = m*a. An applied force is necessary to effect a linear accelleration. The force has to come from the air mass in the form of an initially reduced IAS. How else can it be?

Maybe you could explain a little more clearly how you think F=m*a is at work here.

 

 

[APEppink]

"I think someone stated that an aircraft cruising at 45 knots into a 45 knot headwind would have zero kinetic energy, that is not correct. Kinetic energy relates to mass multiplied by speed squared and the speed for an aircraft in normal flight is the true airspeed, so any airspeed above Vs1 can still be swapped for gravitational potential energy even if the aircraft was moving backwards relative to the ground."

 

No. It is correct. Even tho Kinetic Energy is a scalar (V^2 = V*V is a dot product). Kinetic energy exists only where there's a relative speed between the objects in question. Thus the guy sailing at 45kts IAS into a 45 kt headwind has no kinetic energy relative to the earth.

 

 

[M61A1]

Right....it's all about "relative"

 

 

[APEppink]

Maybe you could explain a little more clearly how you think F=m*a is at work here.

The plane accellerates relative to the ground, thus Newton's Law obviously comes into play.

 

 

[APEppink]

Ideas c variance are tenuous but some people think it does and are working on it.

 

 

[M61A1]

The aircraft flies relative to the air, not the ground. Problems occur when you try to fly relative to the ground without concern for your relation with the air.

 

 

[APEppink]

True, but how's that relevant?

 

 

[M61A1]

True, but how's that relevant?

It's what the whole thread is about......

 

 

[turboplanner]

I've mentioned quite a few times on here just how important I think it is that we all put in some time in doing LL training. For sure 99% of the time most of us might not need it BUT the one time we get caught with a need for a 300ft circuit for whatever reason it could make a huge difference.I was always a well balanced pilot (no matter what others say!) at altitude and was always relatively capable in the takeoff and landing phases but with just a mild 10-15 knot wind my first ll turns were amazingly pathetic. We can't help the way we are wired but IMO a little ll training can make a huge difference in our safety in out of the ordinary emergency low level manoeuvres and also helps with the safety of our landings and takeoffs.

Great post to separate reality (even if it's perceived by the pilot eyeball reality) with theory. If you follow the rules and stay above 500 feet, the most likely time to get caught out is the first time you are forced down by weather into doing a genuine precautionary landing, where you're flying low circuits to inspect the landing area you've chosen.

 

 

[facthunter]

Your speed has to be relative to something. To keep your plane happy it has to be relative to the airmass you are in, which may be moving relative to the earth. and takes you with it..

 

If you are inside a railway carriage, which is doing 100 mph and you walk in a small circle, or spin on a merry go round, relative to the ground you are moving faster and then slower describing quite weird shaped path but as far as you are concerned you may as well not be moving except relative to the inside of the carriage. Nev

 

 

[M61A1]

Great post to separate reality (even if it's perceived by the pilot eyeball reality) with theory. If you follow the rules and stay above 500 feet, the most likely time to get caught out is the first time you are forced down by weather into doing a genuine precautionary landing, where you're flying low circuits to inspect the landing area you've chosen.

Or on your first engine failure....not a good time to have to be learning what you should have been taught early on.(and encouraged to practice)

 

 

[dutchroll]

Reading through this thread both made my head spin, and led me to the same topic on the "Straight Dope" message boards, which made it spin even faster to the point it was lucky not to fall clean off and whiz across the room. One problem is using poor analogies, and another is getting excessively bogged down in physics. (I'm truly surprised no one has invoked Einsteins Theory of General Relativity.....oh wait, I think they did!)

 

Simply:

 

I can't imagine a normal, correctly executed turn onto downwind, would lead to a stall in constant wind conditions. Unless you're flying out at perilously close to the power-on stall speed - and that's not the wind's fault.

 

So what therefore, do we think of windshear? Is this not a sudden change of headwind into tailwind or vice-versa which can cause tragedy? - I see the heads nodding furiously.

 

Well, yes it certainly is. But equating the way the aircraft responds to a wind change during windshear (especially a severe one) to the wind change on a downwind turn is a case of "apples versus oranges".

 

A windshear event where headwind very quickly turns into a tailwind causes a transient loss of lift. This is because the inertia of the aircraft prevents it adjusting to that wind change instantaneously. Normally your power and attitude would give you a set performance (and thus IAS). This is true in whatever static headwind/tailwind conditions you like. But a rapidly changing wind results in the aircraft briefly needing to play "catchup" in the absence of power and/or attitude changes to force it. Mild windshears are not too much trouble. Severe ones are different, and need accompanying severe action.

 

A constant wind velocity during a normal downwind turn simply doesn't result in an adverse lift change that quickly, at least not one which can't easily be controlled by a modest adjustment in attitude. Some people on message boards have said "yeah well what if you did a 180 degree turn instantaneously in a 60 knot headwind?" WTF? Ok yeah sure, you're probably "simulating" the equivalent of a severe windshear then, and yeah theoretically you could be in trouble if you want to maintain level flight (if you haven't already g-stalled it). However this discussion doesn't involve advanced aerobatics. We're talking "normal" flight.

 

It's all about how a wind change affects your lift and how you respond to that. What could cause you to stall turning onto downwind is poor flying technique (eg, inattention etc), much like stalling on base, and that's another discussion, and also not the wind's fault.

 

 

[facthunter]

Since I am the only one who made analogies perhaps you will elaborate as to why they are not relevent or are POOR. You concentrated your argument on windshear which sidesteps the issue of turning downwind. Windshear is a problem mainly with changing level usually quite close to the ground. Nev

 

 

[dutchroll]

Actually I was specifically referring to some really poor analogies made on the Straight Dope message boards on this topic. For a message board which reckons it gets straight to the truth, they just got more and more confusing as they went on. Even delving into formulas for momentum, etc which really didn't help anyone understand the reality of what was happening when the relative wind changes. Don't be so paranoid Nev!

 

I talked about windshear versus a steady state wind because a number of people online tried to argue that wind changing from headwind to tailwind makes no difference to aircraft performance. True or untrue? Well that depends.

 

• If you took a plane with a set power and attitude, and 100 knots headwind, its IAS would be exactly the same if it had 100 knots tailwind.
   
   
  
• If you took a plane with a set power and attitude, and 100 knots headwind, then very rapidly changed that to 100 knots tailwind, its IAS would drop sharply and it would be in trouble.
   
   
  
• Turning downwind in a steady state wind isn't really enough to do that (for the purposes of our discussion and in the planes we fly).
   
   
  

 

 

They had the correct conclusion, but the wrong way of reaching it. A wind change from headwind to tailwind can have dramatic consequences, but again it's that "apple versus oranges" thing.

 

Just as a minor point of curiosity, most people think the big wind changes only happen close to the ground. Not true. It happens more often at altitude during climb or descent. It has the most serious consequences close to the ground. The most dramatic windshear I've ever encountered was in a B767 at 20,000 ft during climbout from Sydney. It caused us to overspeed the aircraft (exceeded VNE of 360 knots IAS) in about 15 seconds, with klaxons and warnings going off everywhere, full speed brake out, still climbing. It's quite a difficult situation because you generate a massive rate of climb to control the speed, but at some point you have to undo that when the wind change stops.

 

On both the B767 and the Airbus A330, you need to continually watch for wind changes on descent from cruise altitude or again you can overspeed the aircraft if you're distracted when it happens. Descending into a reducing tailwind could do it, whereas descending into a reducing headwind would cause the IAS to decrease which is easily fixed with a pitch down and increased rate of descent.