No adverse yaw?

[MtrHedAP]

If you do an aileron roll while the airplane is unloaded is there adverse yaw? I went up for an intro to aerobatics and that is what he told and showed me. I am trying to wrap my head around this. Since the lowered aileron creates more lift and drag, and causes the airplane to roll, i dont see why it matters if the aircraft is unloaded or not there would still be adverse yaw.

Anybody able to clarify?

 

[Boch]

adverse yaw is still there but is less visible because the when the airplane is "unloaded", ZeroG, the angle of attack is as close to 0degs as its ever going to be. so with a 0 deg angle of attack (or 0 Alpha for the brits) both ailerons are equaly effected.

the above is only 100% true if the wing is completly semetrical and the ailerons move up as much as they do down. but even with an unsemetrical wing and ailerons that move up say 12 degs and down 9 degs the principal is the same.

Clear as mud?

 

[MtrHedAP]

Ok thanks. That clarifies some things, it does make sense. By the way it was in a Pitts and he took me for a hell of a ride. I had a real good time and definately started to grey out at times.

 

[Headwind]

Another way to look at this: In order to have adverse yaw the wing must be creating lift. If you unload the wing to zero lift........ it want be there.
On a vertical line (up or down) you have no wing lift and no adverse yaw. If you do get adverse yaw when vertical, then you not at zero lift or perfice vertical.
If the wing is not creating lift theres no HP air on the bottom side....so the down going aileron has the same amount of drag as the up going aileron.
The wing must be creating lift to get adverse yaw.

 

[d.fitz]

Another way to look at this: In order to have adverse yaw the wing must be creating lift. If you unload the wing to zero lift........ it want be there.
On a vertical line (up or down) you have no wing lift and no adverse yaw. If you do get adverse yaw when vertical, then you not at zero lift or perfice vertical.
If the wing is not creating lift theres no HP air on the bottom side....so the down going aileron has the same amount of drag as the up going aileron.
The wing must be creating lift to get adverse yaw.

Anytime an aileron is deflected you are creating lift, that is how an aileron works (local changes in angle of attack). The ailerons on a Pitts deflect more in the up direction than in the down direction (at least visually, I have never measured it). Therefore even at zero lift, you will still have some adverse yaw. It may not require much attention, but I'd bet if you are doing vertical rolls, you are good enough to counteract it instinctively.

My question is: Why was your instructor talking about this on an intro flight? If you end the slow roll off heading, chances are you need (more) opposite rudder through the first three quarters of the roll. The roll should be uncoordinated. Its all about how it looks to the judges...

 

[Boch]

I think he was talking about an aileron roll, or balistic roll. your correct about the local change in angle of attack but un like in level flight the 0 alpha means no high press air under the wing. so when both ailerons are deployed neither gets any high press air and neither gets any low press air. the lack of the difference at 0 alpha = no adverse yaw. the only way you can get adverse yaw at 0 alpha is when the airplane is set up so the ailerons move in one direction more than the other. say the left aileron deflects upward 10 degs and the right deflects down only 7 degs. the only "adverse yaw" would be a function of the extra drag produced by those 3 degs of difference. this is not really adverse yaw in the true sence though, more just differences in wing drag.

this kinda talk on an intro flight is OVERKILL. just go fast, pull up, unload, stick left, give a yah hoo, and recover.

 

[d.fitz]

your correct about the local change in angle of attack but un like in level flight the 0 alpha means no high press air under the wing. so when both ailerons are deployed neither gets any high press air and neither gets any low press air.

We are definitely splitting hairs, but the ailerons change the camber of the wing, which creates lift. If you are exactly at zero lift, one aileron will be creating lift in one direction, and the other aileron will create lift in the other direction. A change in lift equals a change in drag - specifically induced drag. Differential drag from one wing to the other is the definition of adverse yaw. Being at zero lift doesn't stop adverse yaw.

My brain hurts...

 

[Headwind]

Most aerobatic instructors, that I know, do teach this on the first ride.
They will express that to do a slow roll, that scores well, you must use both rudders and elevator.
For a first roll it's a balistic roll because it's simple. The aircraft does not yaw - it just finishes more nose down than it started.

 

[MtrHedAP]

Interesting thoughts from everybody. I did not think it was too much to learn on an "intro" flight. I am not really interested in learning aerobatics at this time I just wanted to go on a good ride. Afterwards it got me thinking as I am a instructor and I always teach adverse yaw anytime ailerons are deflected, so I guess I was wrong in this case. So is it just action - reaction in this case for the ailerons?

 

[Boch]

I we need an aerodynamic physicist stat. d.fitz you are a formitable opponent, but the battle has already started.

...the ailerons change the camber of the wing, which creates lift. If you are exactly at zero lift, one aileron will be creating lift in one direction, and the other aileron will create lift in the other direction...

lets think about this. d.fitz is very correct that the change in camber creates lift and drag. duh. but the change in camber and there for lift and drag on one wing is going to be equal to the change in camber, lift and drag on the other wing. lets put it in to numbers (that may be very far) off to show my point. At 0G (and 0 alpha) a crazy half drunk "stunt" pilot punches the stick left. this changes the relaive alpha of the left wing to -2degs and changes to relitive alpha of the right wing to +2 degs. the net of the alpha change is 0 degs( -2 + +2=0 ) back to where we started. because there is no difference in alpha between the two wings there is no more lift or drag on one side of the airplane. and dare I say it... no adverse yaw. Gulp.
ps my iespell is not working and my computer is doing the thing where it deletes letters when you try to add some to the text. so deal with it.

 

[Boch]

, but I'd bet if you are doing vertical rolls, you are good enough to counteract it instinctively...

lets be clear, a vertical roll is a 1G move, but it is also 0 Lift angle manuver.

I think it may be the only way in an airplane to have 0 lift angle while still having 1G.

 

[Headwind]

Bosh that makes sense.

 

[jafo20]

edit

The amount of yaw you feel (rudder required) depends on a lot of things. The downard deflected aileron will always* generate more induced drag (adverse yaw), but this can be offset by the proverse yaw on the descending wing.

*You can actually stall your deflected aileron under certain, very specific conditions. If that happens you may feel a lot more or a lot less adverse yaw based on how it stalls. You may also feel lightening or hard-over on the stick. It's pretty hard to do this.

So, loaded or unloaded, the downardly deflected aileron will generate some adverse yaw. The opposite, downwardly moving wingtip will also generate a little more lift and little more yaw. So what you sense varies on a number of factors.

Some of these variables are average angle of attack on entry, types ailerons, how far outboard the airlerons are, maximum roll rate, wingspan, etc.

Your original understanding of adverse yaw is correct.

Your aerobatic instructor was correct for describing little to no sensed adverse yaw while the plane was unloaded.

All the technical stuff falls away as you simply do what you need to to do make the airplane do what you want.

It's actually kinda cool to see how different airplanes perform and feel through the same maneuver.

Tangent:
Did you have fun?

 

[d.fitz]

lets be clear, a vertical roll is a 1G move

??? How's that? The typical "1-G" description is 1-G measured parallel to the vertical axis. Like a 1-G roll while holding a glass of water....

and you are correct that you are on the zero lift line when vertical (if done perfectly), but you are not generating zero lift when you deflect the ailerons...

 

[Boch]

what is the "vertical axis" realitive to exactly?

1g is 1 x the earths gravity. go up, down, sideways what ever just as long as you are not changing your vector you will have 1g. velocty + change in vector = G
you generate lift when you deflect the ailerons this is always true, but as I said before the lift you do get is negated by the equal* and opposite lift you get from the other wing.

as for you jafo, you sir speek the truth. I was trying to keep it comprenendible for a mear mortal man. you must be some kind of aerobatic highlander. I commend you, just dont try to chop my head off.

 

[VxClimb]

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[VNugget]

what is the "vertical axis" realitive to exactly?

The airplane.

1g is 1 x the earths gravity. go up, down, sideways what ever just as long as you are not changing your vector you will have 1g. velocty + change in vector = G

What you're talking about is the total acceleration summed up on all axes on the plane, which is best represented by a coordinate system fixed to the horizon.

But that's not really useful, as most of the time people talk about G-force they're just talking about the force along the vertical axis of the airplane, which depends on the lift produced by the wings, and affects the structural stress (and ultimately if you're gonna black out or pop yer eyeballs).

Upright straight & level = 1 G along vertical axis
Inverted straight & level = -1 G along vertical axis
Vertical upline = 0 G along vertical axis, but 1 G longitudinally
Knifeedge flight = 0 G along vertical axis, but 1 sideways (lateral axis)

 

[belchfire]

We are definitely splitting hairs, but the ailerons change the camber of the wing, which creates lift. If you are exactly at zero lift, one aileron will be creating lift in one direction, and the other aileron will create lift in the other direction. A change in lift equals a change in drag - specifically induced drag. Differential drag from one wing to the other is the definition of adverse yaw. Being at zero lift doesn't stop adverse yaw.

My brain hurts...

Aileron travel difference would be the only cause for different change in camber from wing to wing and it is typically a fairly small difference-maybe 14 degrees down and 17-20 up-and possibly differential-ie the difference increasing towards maximum deflection but not normally more than about 30% at max deflection.

Remember that differential aileron travel is specifically intended to counteract adverse yaw. If you have asymmetric aileron travel on an aerobatic airplane that has normal category lineage (say a 150 aerobat uke: for instance) that travel would be intended for normal flight regimes-max 30 degrees of bank or so-maybe 1.3G. If the aircraft were unloaded-0G you are sticking more control surface into a relative wind of equal pressure you could come to the conclusion that the adverse yaw would be in opposite direction of normal!

It's my opinion that it's an effect that would vary greatly between aerobatic types given the wide difference in control configuration and wing shape and probably isn't a hard and fast rule...

And you thought your brain hurt!

 

[Boch]

What you're talking about is the total acceleration summed up on all axes on the plane, which is best represented by a coordinate system fixed to the horizon.

But that's not really useful, as most of the time people talk about G-force they're just talking about the force along the vertical axis of the airplane, which depends on the lift produced by the wings, and affects the structural stress (and ultimately if you're gonna black out or pop yer eyeballs).

Upright straight & level = 1 G along vertical axis
Inverted straight & level = -1 G along vertical axis
Vertical upline = 0 G along vertical axis, but 1 G longitudinally
Knifeedge flight = 0 G along vertical axis, but 1 sideways (lateral axis)
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The airplane could care less what the Gmeter in the dash has to say. Untill the limits are exceded.

Inverted straight & level = -1 G along vertical axis

G is a multiplier of gravity how are you multiplying gravity by flying straight and level while upsidedown?

 

[VNugget]

The airplane could care less what the Gmeter in the dash has to say. Untill the limits are exceded.

Uhhh, what the airplane cares about is exactly what the G meter says. That's why there's a G meter in the airplane. The more G's you pull, the more stress you put on the structure. Exceed the G limit, too much stress. What are you getting at?

G is a multiplier of gravity how are you multiplying gravity by flying straight and level while upsidedown?

You are multiplying it by -1 since it acts in the opposite direction it normally does. While upside down, gravity is acting up relative to the airplane, the G-meter reads -1, and all the junk hits the ceiling; same as if you turn your house upside down.