Angle of attack while banked

[Gingerbread Man]

I apologise if this is not suitable for this forum, but as I have posted before, I know you chaps know about this sort of thing. I have been told that in a level banked turn, the angle of attack is not the same for both wings. Also, whilst flying last week, my instructor asked me which wing had a greater AoA in the turn we were doing. I used my head to try and think about it and provided the wrong answer. I don't doubt the theory, but I have difficulty working out why there is a difference in AoA by just going through it in my head. Can anyone explain it?

Many thanks

GBM

[John Farley]

Good grief! What some people come up with to berate/confuse students….

I can only think of one way this topic should be discussed (for the benefit of all that is rather than to ‘prove’ some point) So here goes

In a turn the outer wing is going a bit faster than the inner one.

The only two terms in the lift equation that can be different between the two wings are lift coefficient (ie AOA) and speed. We have agreed the speed of the outer one is up so it must have less AOA.

In point of fact the only thing that can vary the AOA between the two wings (when the bank is not changing) is the ailerons.

So yes the tip of the outer wing will have its aileron up a bit more than its opposite number.

Unless he was joking feel free to give him a kick from me. A proper kick that is.

[B Fraser]

Thanks Gingerbread for asking the question and thanks John for answering it. That question has been puzzling me too.

I guess then that the AOA must vary along the length of each wing too.

Thanks also John for your articles in a certain monthly mag, fascinating stuff.

[John Farley]

I guess then that the AOA must vary along the length of each wing too.

No. Just that part of the wing in front of the aileron

When they go down ailerons act to increase the curvature of their bit of the wing so give more local AOA. Likewise they reduce curvature and AOA when they go up.

Re articles thank you. BTW Don't hesitate to suggest topics for same.

[Gingerbread Man]

Thanks for the replies, as usual the mathematics explains all! The question arose because we were practising the stall on the turn from base to final. After saying the tendancy of most people was to roll wings level before recovering, he was trying to explain which wing would drop as you rolled out at the point of stalling. I don't know if that makes it sound any fairer or not! I quite like the idea giving him a kick and saying "That's from John Farley", just to see the bewildered expression that followed .

Thanks again.

GBM

[saudipc-9]

No. Just that part of the wing in front of the aileron

John are you sure about that? The angle of incidence difference from the wing root and wing tip would add up to a difference in AOA too.

Here's another skill testing question- The rounded tips on the P-3 Prop vs the squared tips for a C-130. Why the difference? I know for the P-3 it is for a better top end speed and for the C-130 better takeoff ability but how does the prop tips affect this difference??

[kebab kid]

i think john was making it clear that aileron defection doesn’t affect the aoa elsewhere on the wing, only the bit in front of the moving surface, increasing or decreasing its aoa and lift coefficient. of course, during the turn the aoa varies from wing root to tip, but this is due to the difference in turning radius (and therefore airspeed).

re: the wing incidence angle changing from root to tip thereby changing the aoa - not quite sure what you mean by that, but if you're referring to the washout built into the wing structure, that is a constant, for all intents and purposes. as aoa is relative to the aircraft's reference line (which applies to all wing stations - irrespective of washout) this effect is bit of a red herring here. thats if i correctly understood what you were getting at - the term 'incidence angle' sometimes means different things to different people.

kk

[John Farley]

John are you sure about that?

Yep

[john_tullamarine]

"After saying the tendancy of most people was to roll wings level before recovering, he was trying to explain which wing would drop as you rolled out at the point of stalling"

There are so many things affecting local spanwise stall (and consequent rolling moments) that to pontificate on this one or that is to set one up for a surprise. Aileron deflection is but one of the players ...

No doubt JF will elaborate but I should think that stall recovery is a far easier (and more predictable) exercise right way up for those who don't spend most of their flight time wrong way up ...

[John Farley]

Sorry chaps…and thank you John T... we seem to have finished up talking about more than one thing here. Which may be my fault.

Time for a recap.

At the top Gingerbread Man asked a question without any explanation of the circumstances in which it was asked. I saw it as a clever dick issue and responded accordingly.

Then I was asked if I was sure that only the wing in front of the aileron had its AOA changed by use of the aileron. So I replied Yep. That still stands.

Sadly when I did that Yep I had already missed Gingerbread's background to his instructor’s original query - which puts a totally different slant on the whole thing. In that context his instructor was doing an excellent job and trying to advance safety in low speed flight - which some will know is a good cause I am always banging on about.

In my view the small AoA difference between the inside and outside wings in a turn due to the lower speed of the inner wing (meaning that the outside one has to have a tad less AoA than the outer one if the bank angle is constant) is a total red herring here as the difference is so small. In fact looking at the way wings are made I would think there was a better chance of asymmetry between the two wings stalling characteristics due to manufacturing tolerances than such a theoretical point.

When it comes to stalling in a finals turn trying to decide which wing will drop first is far from easy. Just look at some of the issues (variables if you will) that could be involved.

1 First and foremost is the ball in the middle?

2 What are the type dependant slipstream effects round the wing (slipstream swirls and affects the two wings differently depending on power and airspeed)

3 What about flap effects which could add or subtract to the point in 2.

4 What about pilot technique? (Is the pilot using large coarse aileron inputs both ways to fly the turn or smooth low gain low amplitude inputs)

5 If there is turbulence what about an up-gust that could hit one wing more than another.

So the only way to treat all this is to teach good low speed flying technique. This should be aimed at making the pilot’s instinctive reaction to an unexpected wing drop one of checking the yoke forward (to kill the stall dead) while using rudder just enough to stop any yaw (not to pick the wing up) simultaneously slamming full power and then rolling the wings level. Then and only then try to recover the flight path. If close to the ground use all the height available to fly out of the dive.

Now, if your instructor suggested that one had to be prepared for the inside wing to drop and if that happened then most people’s instinctive reaction would be a handful of aileron before anything else (very bad) then full marks to him.

If he suggested that the inside wing is most likely to drop JUST because of the original thing about AoA in a turn then I would suggest that was an oversimplification. But not one that deserves a kick!

JF

[shortandsmelly]

What happens if GBM's already kicked him, then?

[LOMCEVAK]

JF,

Overall, I agree totally with what you have said, but with just one small caveat. "...slamming full power.." may not be a good idea as some types have a strong pitch up with power (which could make recovery more difficult) and some produce so much yaw (due to propwash) and torque that you can have insufficient aileron and rudder authority for control. Perhaps "...select full power as quickly as practicable..." would be more appropriate.

Rgds

L

[BOAC]

Typical Harrier pilot...

[7FF]

Excellent discussion.
From my Jet Provost instructor days I can remember preaching that it could be a dangerous practice to increase the AOB when turning finals to regain the extended centre line, after flying through it for some reason, when the airspeed is low.
Because of the dihedral on the JP, the down going wing would have the AOA increased and despite the negating aileron effect at reducing the AOA on that portion of the wing in front of the aileron and the washout, the rest of the low wing could stall.
Full power, unload and when out of the buffet roll the wings level and start the climb as gently as the distance from you and the ground will allow.
Even in a steady rate turn I think the lower wing have the AOA increased due to the sideslip and hence having a greater reason to stall first? Which is possibly the worst case scenario.
Demonstrating this effect at height often lead into an incipient spin and great fun.......and a salutary lesson applicable to every type of aircraft.

[John Farley]

L

You are quite right of course. I am afraid I was thinking more about low powered club aircraft.

Loved your recent back for tea and medals pic with IH

Regards

J