Aerodynamics Question

[Sparks]

If an aircraft is stalled in a turn, why does the outside wing stall first?

Ex: Stalled in a left turn; plane drops to the right.

Can anyone explain this to me? This always puzzled me through training.

 

[Please Hire Me]

Which wing stalls first depends on the coordination of the aircraft. If the outside wing stalls first, it is because the airplane was in a slip (yawed to the outisde). The outside wing will stall first because the relative wind is creating a higher angle of attack for that particular wing. I hope I make sense.

 

[Please Hire Me]

OK, I think I figured out where you're coming from (yes, I am kinda slow).

The airplane doesn't always stall to the outside of a turn, so forget that.

First of all, during a turn the outside wing is creating more lift than the inside wing. A by-product of lift is drag, so without correcting for it, the aircraft will yaw to the outside of the turn. (called adverse yaw). So this will "slow down" the outside wing and bringing it closer to a stall.

Maybe someone can explain this better?

 

[Sparks]

That's an interesting idea to consider. I have more questions with it though:

You say that it is not always the outside wing that stalls first, but I have seen first hand the consistancy of the manuever. I've never seen the inside wing stall first, or even both wings stall at the same time. There is always a significantly higher angle of attack on the outside wing, significant enough that the roll to the outside is often rather violent once the wing stalls.

Regarding your explanation, I can't see how the outer wing would be developing more lift once the turn was established and the ailerons were nuetral. I keep running through possibilities, but nothing seems to add up. There must be a factor I'm not considering.

I don't mean to dog your response. It's well thought-provoking. I just don't see how it all ties into the equation.

 

[Illini Pilot]

That's an interesting idea to consider. I have more questions with it though:

You say that it is not always the outside wing that stalls first, but I have seen first hand the consistancy of the manuever. I've never seen the inside wing stall first, or even both wings stall at the same time. There is always a significantly higher angle of attack on the outside wing, significant enough that the roll to the outside is often rather violent once the wing stalls.

Regarding your explanation, I can't see how the outer wing would be developing more lift once the turn was established and the ailerons were nuetral. I keep running through possibilities, but nothing seems to add up. There must be a factor I'm not considering.

I don't mean to dog your response. It's well thought-provoking. I just don't see how it all ties into the equation.

you need to consider the fact that that outer wing is actually traveling faster through the air than the inside wing. think about the radii that the inside and outside wingtips travel. Since the outer wingtip is traveling faster, it does have a higher airspeed than the inner.

 

[Sparks]

That makes sense. Good point. Thanks.

 

[172driver]

If the outside wing wasn't developing more lift, you wouldn't be turning...it wouldn't be up there.

More airflow over the outside wing would make it stall later rather than sooner, so that theory goes out the window. The reason it stalls first is because it is at a higher angle of attack.

Also, relative wind is coming from the inside of the turn...more directly hitting the inside wing and enabling it to keep lift as the outside wing stalls.

The outside wing will NOT always stall first. Try applying a bunch of inside rudder as the stall approaches...and hang on.

 

[PTWOB]

Remember that a wing always stalls at the same angle of attack, regardless of aircraft (wing) speed.

 

[enigma]

I'm probably just going to show how much I have forgotten, but. What makes you the original questioner, think that the outside wing is stalling first? I will surmise that since the stall breaks to the outside of the turn, you are guessing that the outside wing stalled first. You would be wrong if you were to come to that conclusion. I assume that you are speaking of a stall from out of normal upright flight, something like the stalls the one practices for his private license. In that condition, gravity is pulling the aircraft toward its bottom (as opposed to acting towards the top if the stall occured when inverted), and centrifugal force is pushing the aircraft to the outside of the turning arc. Once the wing loses lift, the forces opposing gravity and centrifugal force are no longer present. Now, gravity acting on the CG of the aircraft, and the remaining centrifugal force combine to pull the aircraft outward and down. When this happens, it seems as if the outside wing stalled first because the nose pitches towards the outside wing. If really didn't, and when you learn spins, you will find that the outside wing may well come up and over as the nose drops, leaving you with a short upside down sensation.


hope I made sense.
regards,
8N

 

[enigma]

If the outside wing wasn't developing more lift, you wouldn't be turning...it wouldn't be up there.

More airflow over the outside wing would make it stall later rather than sooner, so that theory goes out the window. The reason it stalls first is because it is at a higher angle of attack.

172, your profile says CFI. Please tell me that this post is a joke.

Airplanes do not turn because of unbalanced lift between the two wings.

Airplanes turn because of the horizontal component of lift pulls them around.

When the bank is initiated, the outside wing will momentarily develop more lift, but as soon as the bank is established the ailerons are neutralized and the imbalance goes away. IF the aircraft stalls from an uncoordinated condition, it could be possible to see differences in angle of attack, but for a normal coordinated stall the wings are both flying/stalling equally.

regards,
8N

 

[profile]

Well, nice tries, but I didn't see one that hit the mark.

The outside wing WILL stall first, all else equal. This is because the aircraft is not only turning about the vertical axis but also actually rolling. In a left turn with the nose up, the aircraft is actually rolling to the right, the higher the nose, the more the roll effect. Vice versa if the nose is down with a negative AoA.

 

[avbug]

An aircraft with constant bank is not rolling, period. This is true regardless of weather neutral ailerons are required in the turn, or aileron is required to into or out of the turn.

Which way the airplane will "break" once stalled has nothing to do with the bank. Airplanes may break "over the top" or tuck under by rolling to the inside, depending on several factors.

One of the most decisive factors is rudder useage. Some feel that the rudder deflection is what what breaks the airplane one way or the other, but that's not it.

When yaw is applied outside of a balanced condition in flight (inclinometer ball in the center), a yawing action is introduced. This may occur due to rudder input, or lack of rudder input. This may be due to adverse yaw owing to aileron deflection. Many possibilities exist.

The inclinometer tells you if you're slipping or skidding. In either condition, you will experience not only chordwise airflow over your wing, but also spanwise. The airflow patterns over each wing are altered, and are not symmetrical one wing to the other. Reduced aileron effectiveness will exist on one side of the airplane, and reduced airfoil effectiveness on the other, blanked slightly by the fuselage, with additional interference drag from the fuselage.

Put in left rudder in a left turn, and skid the airplane. You have options. You can apply right aileron, lowering the left aileron, increasing adverse yaw to the left and induced drag on the left...you'll get a left break in most cases. Additionally, the left wing will be faced with increased drag, and reduced airflow over the inner wing and more spanwise flow over the outer wing, increasing the effect of the aileron. The right wing, already experiencing reduced drag from the aileron reflexed upward (to a point) and increased spanwise airflow, will be operating at a reduced aerodynamic angle of attack.

It's important to realize that AoA represents more than the simple angle between the chordline of the wing and the relative wing. Local AoA varies from that of the free airstream, and configuration changes alter the aerodynamic chordline of the wing, alterning the critical angle of attack, or the point at which the stall will occur.

It's also important to note that the wing does not stall evenly, nor does it experience a uniform change in angle of attack. Local AoA is affected by the wing shape, thickness, etc. It is affected by the wing structure, and proximity to other parts of the aircraft. The wing structure adjacent to a nacelle, tank, pod, etc, will experience different airflow than the wing two feet from that item.

Wing planform has a lot to do with the stall behavior to be expected. Large rectangular wings will exhibit different general stall behavior than eliptical wings, tapered wings, or modified taper. The addition of stall modification devices such as stall strips, boundary layer energizers, vortex generators, slots, slats, etc, will all make a difference. The assymetrical effects of changes in these devices makes a great deal of difference.

Finally, no airplane is perfectly symmetrical. Slight variances in structure, past repairs and damage, or the simple aging of the airplane leads to subtle differences one side to the other. Some airplanes, all else being equal, will tend to break one way or the other, where an identical make and model will not.

When I was eighteen, I began flying ag, crop dusting in Cessna AgTrucks, Pawnees, etc. My second operator had three trucks, each of which were identical except for color. We operated these airplanes heavy, meaning close to gross, with minimal performance. We made low level steep turns in them, such that feeling the stall buffet in each turn wasn't uncommon. Because of the proximity of the ground (75') in the turn, a strong association was able to be made between what the airplane was doing, and the slipping and skidding action. The perephrial image, as well as the buffet and the other things associated with the turn, lead to a very sharp intuitive sense of what the airplane was doing, and needed to make it do something. In short, seat of the pants type flying.

I found out very early on that each airplane had a personality. They did not fly the same. For someone flying them around the pattern or taking them on a cross country, the difference might not have been noticable. However, we used the full operational envelope for the airplane on every flight, often every 30-60 seconds during the course of a flight, and the differences were painfully obvious. I've noticed this time and time again when flying for companies or operators who have a fleet of similiar of seemingly identical airplanes. Each one has distinct personality.

Simply because an airplane appears to break left or right, does not mean that a left turn causes an airplane to break over the top. It's not true. Most likely I think the origional poster needs to look at aircraft control to determine the cause. I believe that upon inspection, he or she will find that it's operator error causing the break. Secondly, he or she needs to compare this behavior to other similiar airplanes. If no difference is found, it's still not evidence that an airplane breaks over the top in a left turn...because that's not true. It's then evidence of operator error...the real reason that the majority of airplanes that do break one way or the other, do so. Have a ball.

 

[profile]

Avbug-

You are flat out incorrect. Read Sammy Mason's "Stalls, Spins and Safety" if you need to.

Easier than that, take an airplane model and put it in a nose up attitude. Bank it and rotate it about an axis that is perpendicular to the ground (note that this is NOT the vertical or z axis, as that would be running at an angle with the pitch up). Due to the pitch up attitude, the outside wing is actually descending and the inside wing ascending. The more extreme the pitch up the more pronounced this is, until you have an aircraft pointing near vertical so it is virtually all roll.

 

[avbug]

Well, I'm not exactly sure how to post a response to that, but since you won't be able to hear me laughing my ass off, how's this:

Yeah, right.

I haven't read Mr. Mason's fine treatise, which must be right after Aerodynamics for Naval Aviators and the Illustrated Guide to Aerodynamics for masterful industry standards on the topic. However, as soon as I run across a copy, I'll be sure to give it a thorough perusal.

In a banked turn, the outside (upper, or higher) wing is descending?

In a constant bank turn, implying that no movement exists about the longitudinal axis, the airplane is rolling?

An airplane pointed staight up isn't turning. It may be turned about the vertical axis, but this is a roll. An aircraft in a banked turn is not rolling, nor effecting movement about the longitudinal axis...which is, after all, what defines a roll.

You may want to go back and read that fine publication again. Either the author is very confused, or you forgot what you read.

A spin is a constant roll coupled with yaw. A snap roll is the same thing. A constant banked turn is not a roll, nor does it involve action about the roll axis, or the longitudinal axis.

Z axis? Is that french for "the axis"? Or something only an aerodynamicist or mathematician would understand? I guess I need to read that treatise after all, or go back for that high school education that's thus far eluded me (been too busy flying).

Now where was I? Oh yes. Laughing my ass off. Excuse me...

 

[profile]

I'm sure others can grasp the concept. How can you not know what the Z axis is, or is aero not your forte?

I stand by my post, as will every test pilot. Are you the one that claimed to use rudders at altitude to counter dutch roll in Learjets sometime back?

Best-

 

[avbug]

Are you a civillian test pilot, then?

Yes, I have used rudder to counter dutch roll, along with aileron in the Learjet. Would you prefer that one simply lean in one direction, instead?

Dutch roll in the Learjet can occur with, or without the yaw damper engaged. Especially in the older 20 series. It is made worse by pilots trying to correct for it. Usually a series of worsening oscillations, and with only a few repitions I have seen pilots roll the airplane right over. I take it you haven't.

Rather than fight it, small inputs on the rudder can stop it, as can slightly crossing the controls, or in some cases, freezing them and letting the osciallations stop. Sometimes, it's easy to stop it simpy with a little aileron input.

Forget the Z axis. Concentrate on the longitudinal axis of the airplane. During a constant bank turn, where does the rolling action come in again? I missed that. Even though you're a test pilot with a z axis kind of understanding and an aero forte, explain to me once more, in kiddie terms, how you get a roll out of a constant banked turn...or any motion whatsoever about the longitudinal axis of the airplane.

 

[172driver]

Now, gravity acting on the CG of the aircraft, and the remaining centrifugal force combine to pull the aircraft outward and down

I will buy this explanation provided we call the force inertia rather than centrifugal force. There is, after all, no such thing as CF.

 

[avbug]

That's centripital, actually. But that's purely semantics; the result is the same. Centrifugal is a common-use explaination of a force vector, where the more correct centripital is the vector in opposition, and in truth exists only in theory.

 

[skiddriver]

Some interesting comments here, not all concerning aerodynamics....

A few notes from an out of practice mil test pilot-

Depending on aircraft characteristics, it may be necessary to hold aileron into the turn or away from the turn to maintain a constant bank angle. This is the product of the individual airplane's spiral mode, and whether the pilot has established a balanced flight condition using rudders. Aircraft designers/manufacturers consider neutral to positive spiral mode desireable, so the aircraft won't keep trying to increase bank angle in a turn. To be a complete geek, the spiral mode is governed by dihedral effect, yaw rate damping, directional stability, and roll due to yaw rate - expressed in coupled pairs. If the product of the first two is greater than the product of the second, spiral mode is stable.

Some factors that effect which wing drops first in an accelerated (steady-state turning) stall are - in no particular order - spiral mode, prop slipstream (for prop airplanes - duh!), lateral CG postioning and CG shift in a turn (interconnected wing fuel tanks), pitch rate due to post-stall pitching characteristics and associated roll and yaw coupling with pitch rate (including gyroscopic coupling), sideslip angle (caused by the turn and rudder positioning), and aircraft rig (is it bent?).

Remember that aerodynamically, stall in level flight is just a special case of stall in a turn (bank angle = 0). As soon as you start the stall phenomenon, a lot of the same things are going on as far as dynamic coupling. Ever have a wing drop during a straight ahead stall? Was it caused by angle of bank, or had you inadvertantly generated a sideslip through rudder miscontrol?

Sparks, there really isn't a rule about which wing will drop in a turning stall. I personally believe that wherever possible, the manufacturer is going to strive to develop flying characteristics that encourage return to controlled flight following a stall. Nose down pitch following stall and wing leveling in a turning stall are two that come to mind immediately, and it may be that the aircraft you're flying have the desireable flight characteristic of dropping the outside wing by design. Or it could be the way that you personally fly the maneuver. Either way, remember that at this point in the aircraft's design life (well past test flight and into customer use) the point of the maneuver is to show you that the aircraft will stall at a higher IAS and with less warning. The post-stall gyrations are just an added value item to reinforce in your mind the undesireability of stalling the airplane in the first place.

 

[profile]

Not logged on for a while, so catching up. Certainly all good stuff in that last post, but I do stand by what I said wrt the differential AoA due to the effective roll rate in a nose-up turn.

As for the Lear cross control thing, I had the opportunity to speak with Pete Reynolds (VP of Flight Test, Bombardier) on this topic last spring when we were both panelists at a NASA Handling Qualities workshop. I asked him about this and he stated that it was certainly not a procedure the manufacturer would endorse and he went on to tell me that there are a number of similar urban myths out there on Learjets.

Don't believe me? Call him and ask. I'll not post the phone number, but it's easy enough to obtain if you are interested in facts.