Stalls in turbulence

[dr.aero]

White_knuckle...

Can you fill me in on what the real reason is ? I'm also curious as to why my instructor, and the examiner after him, were content to let me perform the ol' pitch, rudder, power trio. My instructor was no spring chicken, was an AME as well and seemed to hae a pretty good handle on all facets of flying. Is it just easier to throw out the 1,2,3 procedure since it works in a 172 99.2% of time ?

The real reason why you don't want to use aileron in stall recovery, until the airflow is reattached, is simple - in a straight wing (without washout) there is a good chance you'll stall the outboard portion of the wing whose aileron is deflected downwards. This is because it increases the airflow separation at that point.

A stall is simply significant airflow separation that results in a significant decrease in lift and significant increase in drag. Let's say the wing is at an AoA of 14 degrees and the critical AoA is 15 degrees. All I'm trying to illustrate is that in this example the airplane is very close to stalling. If the pilot were to snap aileron input in, it would most likely produce enough airflow separation for the wing whose aileron was deflected down to stall. If the airplane is already stalled, doing this would take one wing further into the stall, resulting in a similar wing drop.

Many high performance airplanes aren't designed to be as stable and resistant to stalls as a Cessna is. Lancair and Cirrus all make aircraft that are easier to stall than Cessnas. Their wings are made for high speed cruise - washout increases drag at high speed, reducing the top speed of the airplane. Manufacturers like Cirrus don't want to do that. They build in some protection but nothing like what Cessna has. If you get used to how you can man-handle a Cessna, you'll have a pretty high chance of killing yourself in a high performance airplane like a Cirrus or Lancair when operating in the same flight regime.

[Rich_Pa]

During flare, after using the crab method to compensate for a crosswind, when you de-crab you are cross-controlling close to stall speed, why there's no wing drop or even an incipient spin behaviour? I figure out it may be a slip here, but you are wings levek, not banked as in a slip and being so close to stall, why opposite aileron doesn't increase the angle of attack of the opposite wing?

[Steve Pomroy]

White_knuckle...

Instructors (myself included) will tell you to not use aileron in a stall recovery and I'll make sure you understand the real reason why you shouldn't do that. I found it pretty ineffective when my PPL instructor tried to show me that it was dangerous to use aileron in the stall recovery and the airplane happily rolled in the direction the instructor put the ailerons - not what he was trying to illustrate!

Can you fill me in on what the real reason is ? I'm also curious as to why my instructor, and the examiner after him, were content to let me perform the ol' pitch, rudder, power trio. Is it just easier to throw out the 1,2,3 procedure since it works in a 172 99.2% of time?

You can get a pretty detailed answer here: http://www.flightwriter.com/2011/05/fly ... -wing.html. Here's a snippet from the article:

So we have three possible aileron behaviors in a stall: they can reverse, they can be ineffective, or they can work properly. Which of these behaviors shows up will depend on aircraft design features (planform shape being a major one) and the depth of the stall. This stall-depth dependency creates some uncertainty regarding aileron response in many aircraft types, and is ultimately the reason for the "neutralize ailerons" doctrine of stall recoveries.

If we knew, with certainty, that the ailerons would work properly, we could just use them as usual. If we knew, with certainty, that the ailerons would be neutralized, we could just ignore them and not spend so much time learning not to use them. If we knew, with certainty, that the ailerons would reverse, we could just use them backward.

For most aircraft types, the aileron response will depend heavily on how deep the stall is. As a result, the ailerons can be considered unreliable—we don't know how they'll respond until we try. Since inadvertent stalls occur at low altitude, and minimum-altitude recoveries are paramount, spending time experimenting is ill-advised. So neutralizing the ailerons for stall recoveries is the best course of action.

[white_knuckle_flyer]

During flare, after using the crab method to compensate for a crosswind, when you de-crab you are cross-controlling close to stall speed, why there's no wing drop or even an incipient spin behaviour? I figure out it may be a slip here, but you are wings levek, not banked as in a slip and being so close to stall, why opposite aileron doesn't increase the angle of attack of the opposite wing?

Would you not be banked if you were cross-controlled ? Perhaps not noticeably in a light crosswind. But if performed correctly, isn't the crab kicked out at the last possible second anyway, so there wouldn't be any altitude for a spin to develop. Plus I would think that ground effect would also be helping to keep you above the stall. I also believe, and I hope someone corrects me if I am wrong, but as long as you are tracking straight, there is no lift differential between the wings since each wing is travelling the same horizontal distance per foot of descent.

[Rich_Pa]

Would you not be banked if you were cross-controlled ?

No, if you don't use crab to sideslip transition, the tehcique is just to kick the grab out, so you use opposite aileron to prevent opposite bank due to rudder application to kick the crab out. So you are not banked, you are wings level.

[dr.aero]

why opposite aileron doesn't increase the angle of attack of the opposite wing?

That's really not clear. Too many opposites!

I think you're asking why the aileron deflected down doesn't increase the angle of attack on that wing... is that correct?

Technically, you're not changing angle of attack when you deflect aileron down or put flap down. You're changing the coefficient of lift curve. I'd recommend you think about it as deflecting aileron down increases lift and brings the wing closer to a stall (airflow separation).

So, ANYTIME you deflect an aileron down you are increasing the lift and bringing the wing closer to a stall (airflow separation). Just because it doesn't drop (stall) doesn't mean that it didn't get closer to the stall.

[Rich_Pa]

I'd recommend you think about it as deflecting aileron down increases lift and brings the wing closer to a stall (airflow separation).

I can't figure on my own why is so. Why increased lift brings the wing closer to a stall? Due to increased drag also?

[dr.aero]

I'd recommend you think about it as deflecting aileron down increases lift and brings the wing closer to a stall (airflow separation).

I can't figure on my own why is so. Why increased lift brings the wing closer to a stall? Due to increased drag also?

That's not what I meant.

Deflecting an aileron will - 1) increase lift, and 2) bring the wing closer to stall.

Not... Deflecting an aileron will increase lift, which will bring the wing closer to stall.

Do you understand the difference between those two sentences?

[Rich_Pa]

Ok, so why does it bring the wing closer to stall?

[white_knuckle_flyer]

Ok, so why does it bring the wing closer to stall?

Is it because the aileron changes the "camber" of the wing which, although it doesn't change the AOA, it does lower the critical AOA of the wing and therefore brings the wing closer to a stall ?

[dr.aero]

Because it increases airflow separation. I'm going to probably get hung out to dry here by the resident PPL-instructor know-it-alls if I don't mention that for a PPL level, all that's required to be known is that aileron deflection down will bring the wing closer to stall or further stall it if it's already stalled. Obviously you want more details though.

It's complicated to explain. I'm not sure what your background is so I'm not sure exactly what to say.

Due to the air having viscosity, lift (the 'turning' of air) is produced. If air had no viscosity, the wing would have nothing to 'grab onto' when it encountered the air and would therefore not be able to 'turn' the air - meaning zero lift.

Look at this image: http://cache.io9.com/assets/images/8/20 ... -layer.png

The arrows show the relative velocity of air as you move away from the surface of the wing. The image is blown up and in reality the height of the green section is about 5 mm to a few cm above the wing as you move towards the turbulent section. As you get closer to the wing, the arrows get smaller indicating the slower velocity of the air at that point. The air is slowed down because of the friction with wing-air and air-air. This slowing down of the air will bend the air that's going faster, towards the slower air. In this case that's a downward deflection. This explains a bit about why the air wants to 'stick' to the wing.

Then there is the effect of a relative low pressure on top of the wing that causes an acceleration of the air over top of the wing.

When you deflect an aileron down, due to the properties of air (a gas), it wants to fill in the void created by the aileron deflection. Static pressure forces, combined with the viscosity of the air (friction) will ensure that the air tries to fill in that void and follow the wing. Since the air is 'turning' the air downward more, more lift is created. Newton's laws can help explain that. Eventually, the air can't be turned more and it separates in a turbulent manner with vortices and eddies being produced. These vortices and eddies substantially increase drag and the reduction in the downwash angle, due to the vortices and eddies, substantially reduces lift.

It's the same concept as a wing being brought to a higher AoA. You could think of aileron down deflection as that if it makes it easier but I will say that if you think like that with regard to flaps, you'll get very confused when you look at Cl vs AoA graphs depicting changes with flap deflection!

This video shows a tiny bit of what is happening right near the wing during a stall. It doesn't give you a 3D view of it but it might be helpful: http://www.youtube.com/watch?v=WFcW5-1NP60

There are other videos on YouTube with airfoils in windtunnels that might help you visualize it better.

[dr.aero]

Ok, so why does it bring the wing closer to stall?

Is it because the aileron changes the "camber" of the wing which, although it doesn't change the AOA, it does lower the critical AOA of the wing and therefore brings the wing closer to a stall ?

Pretty much!

Rich...

Look at Figure 2 in this PDF: http://www.boeing.com/commercial/aeroma ... 12/aoa.pdf

It shows you what I was talking about with regard to flaps. Ailerons and flaps, in this regard, are exactly the same. Deflecting a flap down vs deflecting an aileron down really has no difference with regard to changes in lift.

You'll notice that the critical AoA with flaps down is less. As I was mentioning in the previous post, the AoA doesn't change when you deflect aileron down (or flaps down). AoA is the angle between the chord line and the relative airflow - the chord line is always for the clean wing and so doesn't change when you deflect aileron or put flap down.

The graph also shows that if you maintain the same AoA and put flap down, the lift is increased. This explains why your attitude with flaps down at the same speed is lower! If you fly a flaps up approach at the same speed as flaps down, you'll notice the nose is way high.

[Rich_Pa]

Ok, I think you lost me in theory, my fault, I'm don't have such deep knowledge on the issue. However, coming back to basics, whn you deflect aileron, the wing with the aileron coming down it is more prone to stall at tip if the airplane is not designed to stall first at wing root and still having roll control at the point of stall, right?

[dr.aero]

Ok, I think you lost me in theory, my fault, I'm don't have such deep knowledge on the issue. However, coming back to basics, whn you deflect aileron, the wing with the aileron coming down it is more prone to stall at tip if the airplane is not designed to stall first at wing root and still having roll control at the point of stall, right?

In any case where the aileron is deflected down, you're closer to the stall on that wing.

With an airplane which has washout, the wings are twisted so that the outer portion is at a lower angle of attack than the inner portion. A Cessna is a good example. When you stall these airplanes, most of the time you're just stalling the inside portion of the wing and the nose drops and you recover before the outside wing portion stalls. The reason is washout. That's why you have aileron control (most of the time) while in the stall in a Cessna. If the airplane had more elevator authority (a bigger elevator would do that) at the point of stall, it might be possible to bring the Cessna into a deeper stall (by pitching higher) which would at that point have the entire portion of the wing stalled - if you used left aileron in this case, the airplane would roll to the right.

[Rich_Pa]

Ok, now it's clear, so the conclusion on the initial issue?

During flare, after using the crab method to compensate for a crosswind, when you de-crab you are cross-controlling close to stall speed, why there's no wing drop or even an incipient spin behaviour? I figure out it may be a slip here, but you are wings levek, not banked as in a slip and being so close to stall, why opposite aileron doesn't increase the angle of attack of the opposite wing?

[dr.aero]

Ok, now it's clear, so the conclusion on the initial issue?

During flare, after using the crab method to compensate for a crosswind, when you de-crab you are cross-controlling close to stall speed, why there's no wing drop or even an incipient spin behaviour? I figure out it may be a slip here, but you are wings levek, not banked as in a slip and being so close to stall, why opposite aileron doesn't increase the angle of attack of the opposite wing?

So you're flying an approach coordinated (crabbing) and then you put the airplane into a sideslip to align the airplane's longitudinal axis with the runway to touchdown?

The main reason you lose lots of altitude nicely while in a slip is because of the extra drag you're producing by having the airplane's longitudinal axis angled to the relative wind. You also lose a bit of lift because the airflow is not going straight back from leading to trailing edge. But just because you have less lift, doesn't mean the wing is closer to stall! Having spanwise flow on a wing doesn't change the AoA of the wing - it just reduces lift.

[Rich_Pa]

Having spanwise flow on a wing doesn't change the AoA of the wing - it just reduces lift.

Ok, but what about the aileron application we just debated and we concluded that the aileron is deflected down, you're closer to the stall on that wing?

And I'm confused even about the fact that rudder application in a sideslip doesn't change the angle of attack of one wing. In a skid, is the same thing? What differs is just the lift between wings and not the angle of attack?

[dr.aero]

Having spanwise flow on a wing doesn't change the AoA of the wing - it just reduces lift.

Ok, but what about the aileron application we just debated and we concluded that the aileron is deflected down, you're closer to the stall on that wing?

And I'm confused even about the fact that rudder application in a sideslip doesn't change the angle of attack of one wing. In a skid, is the same thing? What differs is just the lift between wings and not the angle of attack?

Yes! Aileron deflection will put you closer to the stall.

If AoA is changed on one wing, the other will also change.

There is a lot to talk about with regard to dynamics and how a slip is a lot safer than a skid at slow speed. The slip is essentially a stable flight mode where as the skid is an unstable flight mode. If you want to understand why then it'd involve discussing some complicated aerodynamics which you wouldn't understand.

It's great that you're trying to understand this stuff but I'd suggest just taking away what you have so far and come back to it at a later date. You'll eventually get a better understanding of it.

[Rich_Pa]

It's great that you're trying to understand this stuff but I'd suggest just taking away what you have so far and come back to it at a later date. You'll eventually get a better understanding of it.

Thank you very much for explanations, I really appreciate. I agree with you and I usually don't want to bother with so complex things hard to understand, however, I was interested because I think it's crucial to understand some things like those which might put me at risk in some flight attitudes. For example, assuming what if someday I use more aileron than usual at decrabbing in flare and that leads to a hard wingdrop? The outcome will be really bad. It was just an assumption as an example, but generally speaking, that's why I want to figure out those things which seem to be tricky in some circumstances.