Question about adverse yaw

[Crashnburn]

I know adverse yaw is the nose moving in the opposite direction of the turn when you use ailerons to bank into a turn..

My first instructor said it was because the outside turn aileron moves down into denser air so creates more drag than the inside aileron moving up into less dense air (Bernoulli).

This FBO says the increased drag comes from the outside aileron moving down and creating more lift, which causes induced drag. That was my answer yesterday on my Ground part of the check ride.

I didn't think to ask, but what about what happens with the inside turn aileron moving up?
Does it kill lift by channeling the air up, so induced drag decreases, if not disappears?

 

[dmspilot]

The density of the air around an airplane doesn't change until you approach the speed of sound. First instructor wasn't too bright.

Yes, one aileron creates more lift and more drag, the other creates less lift and less drag.

 

[Dan Thomas]

The aileron is part of the entire wing airfoil. It's not just an "air deflector." It changes the camber of the wing, and the wing's angle of incidence, in the span covered by the aileron. Deflecting the aileron down accelerates the air over the top of the whole wing in that area, increasing the pressure differential between the top and bottom of the wing, thereby creating more lift, and deflecting the aileron down slows the airflow over the top, reducing the pressure differential.

The angle of incidence is that angle between the airplane's waterline (longitudinal axis) and the wing's chordline. That chordline runs between the leading and trailing edges. Dropping an aileron moves the aft end of that line down, changing the angle of incidence. In effect, the angle of attack of the wing changes there.

 

[MRC01]

I know adverse yaw is the nose moving in the opposite direction of the turn when you use ailerons to bank into a turn..

My first instructor said it was because the outside turn aileron moves down into denser air so creates more drag than the inside aileron moving up into less dense air (Bernoulli).

This FBO says the increased drag comes from the outside aileron moving down and creating more lift, which causes induced drag. That was my answer yesterday on my Ground part of the check ride.

I didn't think to ask, but what about what happens with the inside turn aileron moving up?
Does it kill lift by channeling the air up, so induced drag decreases, if not disappears?

#1 is wrong, #2 is closer to right. Ailerons work by changing the AoA of the wings, increasing one side and reducing the other. All else equal, more AoA means more lift and more induced drag, and vice versa. So you get slightly more drag on the wing moving up and slightly less drag on the one moving down.

Adverse yaw - Wikipedia

en.wikipedia.org

 

[Capt. Geoffrey Thorpe]

The fundamental law of the universe: Ain't no such thing as a free lunch.

To pick up a wing, you have to increase lift - that comes at a cost - and the cost is drag. On the other wing, you are trowing away some lift which tends to shed drag (OK. There are ways to reduce lift and increase drag at the same time, but this is not one of them.)

Whatever your instructor told you about Bernoulli is wrong. Bernoulli's equation is simply the conservation of energy along a streamline in an ideal fluid. Unless the explanation started with "Circulation" (which it didn't) then anything that followed about lift and Bernoulli is just one of those fairy tails for pilots.

I would direct you to a Youtube playlist, but it is mostly my videos so clicking on this would be a bad thing.

 

[GaryM]

My first instructor said it was because the outside turn aileron moves down into denser air so creates more drag than the inside aileron moving up into less dense air (Bernoulli).

Ask your first instructor to calculate the difference in air density in a gentle turn where there is a 10 foot elevation difference between the high and low wing.

 

[Crashnburn]

The aileron is part of the entire wing airfoil. It's not just an "air deflector." It changes the camber of the wing, and the wing's angle of incidence, in the span covered by the aileron. Deflecting the aileron down accelerates the air over the top of the whole wing in that area, increasing the pressure differential between the top and bottom of the wing, thereby creating more lift, and deflecting the aileron down slows the airflow over the top, reducing the pressure differential.

The angle of incidence is that angle between the airplane's waterline (longitudinal axis) and the wing's chordline. That chordline runs between the leading and trailing edges. Dropping an aileron moves the aft end of that line down, changing the angle of incidence. In effect, the angle of attack of the wing changes there.

Thank you for the excellent explanation.

 

[BillTIZ]

Fly a long winged glider to truly experience adverse yaw. The statements of the down vs. up ailerons is correct. The upward moving wing, right wing in a left turn, has a downward deflecting aileron that "creates more lift" and "increases drag". The upward moving aileron does decrease lift and drag. Air density does not change from tip to tip.

To extend the argument, why in a stall recovery is the "stick forward and centered, rudder to pick up a low wing"? If left wing is low on the stall and recovery, use the right rudder to pick up the low left wing. Rudder increases the tip speed or airflow over the opposite wing, increasing lift. Right rudder moves the left wing tip forward to increase the lift at the left tip than the left experienced at the right tip.

If you were to use aileron to pick up the low wing, that would put that aileron down, increasing drag, creating adverse yaw that would further stall the wing and could over power the rudder until airspeed is substantially increased making the rudder more effective to overcome the adverse yaw.

 

[brcase]

The aileron is part of the entire wing airfoil. It's not just an "air deflector." It changes the camber of the wing, and the wing's angle of incidence, in the span covered by the aileron. Deflecting the aileron down accelerates the air over the top of the whole wing in that area, increasing the pressure differential between the top and bottom of the wing, thereby creating more lift, and deflecting the aileron down slows the airflow over the top, reducing the pressure differential.

The angle of incidence is that angle between the airplane's waterline (longitudinal axis) and the wing's chordline. That chordline runs between the leading and trailing edges. Dropping an aileron moves the aft end of that line down, changing the angle of incidence. In effect, the angle of attack of the wing changes there.

Correct, think about how the Wright brothers did it, they actually twisted the wing to increase or decrease the angle of attack. As I recall they had a lot of Adverse Yaw to deal with.
The Aileron is just a simpler way to accomplish the same thing.

Brian
CFIIG/ASEL

 

[Ed Haywood]

Down aileron increases the AoA of the wing, which increases lift and drag. Up aileron does the opposite.

Keep it simple.

 

[MRC01]

... To extend the argument, why in a stall recovery is the "stick forward and centered, rudder to pick up a low wing"? If left wing is low on the stall and recovery, use the right rudder to pick up the low left wing. Rudder increases the tip speed or airflow over the opposite wing, increasing lift. Right rudder moves the left wing tip forward to increase the lift at the left tip than the left experienced at the right tip. ...

Simply put, "Step on the high wing".

 

[Crashnburn]

Fly a long winged glider to truly experience adverse yaw. The statements of the down vs. up ailerons is correct. The upward moving wing, right wing in a left turn, has a downward deflecting aileron that "creates more lift" and "increases drag". The upward moving aileron does decrease lift and drag. Air density does not change from tip to tip.

To extend the argument, why in a stall recovery is the "stick forward and centered, rudder to pick up a low wing"? If left wing is low on the stall and recovery, use the right rudder to pick up the low left wing. Rudder increases the tip speed or airflow over the opposite wing, increasing lift. Right rudder moves the left wing tip forward to increase the lift at the left tip than the left experienced at the right tip.

If you were to use aileron to pick up the low wing, that would put that aileron down, increasing drag, creating adverse yaw that would further stall the wing and could over power the rudder until airspeed is substantially increased making the rudder more effective to overcome the adverse yaw.

Simply put, "Step on the high wing"

Fly a long winged glider to truly experience adverse yaw. The statements of the down vs. up ailerons is correct. The upward moving wing, right wing in a left turn, has a downward deflecting aileron that "creates more lift" and "increases drag". The upward moving aileron does decrease lift and drag. Air density does not change from tip to tip.

To extend the argument, why in a stall recovery is the "stick forward and centered, rudder to pick up a low wing"? If left wing is low on the stall and recovery, use the right rudder to pick up the low left wing. Rudder increases the tip speed or airflow over the opposite wing, increasing lift. Right rudder moves the left wing tip forward to increase the lift at the left tip than the left experienced at the right tip.

If you were to use aileron to pick up the low wing, that would put that aileron down, increasing drag, creating adverse yaw that would further stall the wing and could over power the rudder until airspeed is substantially increased making the rudder more effective to overcome the adverse yaw.

Thanks for expanding on why to step on the high wing. I always heard "step on the ball" but they are functionally the same, and you don't need to have your head in the cockpit.

 

[infrequentFlyer]

I've got another question about adverse yaw:

Does it only occur during the "roll-in" phase?

Once established in a turn the wings remain at the same relative height, so I would conclude that there is no longer a difference in lift (or else your bank would continue to steepen). No more difference in lift means no more difference in induced drag, therefore no more adverse yaw?

Unless... just thinking out loud here... I think planes generally have a tendency to upright themselves. So perhaps there is still a difference in lift just to fight this rolling-back-to-wings-level tendency.

 

[RyanShort1]

I've got another question about adverse yaw:

Does it only occur during the "roll-in" phase?

Once established in a turn the wings remain at the same relative height, so I would conclude that there is no longer a difference in lift (or else your bank would continue to steepen). No more difference in lift means no more difference in induced drag, therefore no more adverse yaw?

Unless... just thinking out loud here... I think planes generally have a tendency to upright themselves. So perhaps there is still a difference in lift just to fight this rolling-back-to-wings-level tendency.

It is directly related to the use of the ailerons, so it is happening anytime you are moving the ailerons...

 

[MRC01]

Simply put, "Step on the high wing".

@Crashnburn said: "Thanks for expanding on why to step on the high wing. I always heard "step on the ball" but they are functionally the same, and you don't need to have your head in the cockpit."

Besides keeping your eyes out the window, another benefit to "step on the high wing" over "step on the ball" is the ball has a response lag, so you are always behind the airplane. The butt-o-meter also has instantaneous no-lag response, so pilots should learn to use that "instrument" too.

 

[Dan Thomas]

I've got another question about adverse yaw:

Does it only occur during the "roll-in" phase?

Once established in a turn the wings remain at the same relative height, so I would conclude that there is no longer a difference in lift (or else your bank would continue to steepen). No more difference in lift means no more difference in induced drag, therefore no more adverse yaw?

Unless... just thinking out loud here... I think planes generally have a tendency to upright themselves. So perhaps there is still a difference in lift just to fight this rolling-back-to-wings-level tendency.

Never heard of, or experienced, the overbanking tendency?

The outside wing is moving faster than the inside wing in a banked turn. Since lift is governed by both angle of attack and airspeed, the slightly faster wing will generate a little more lift, and the bank will want to increase. Most of us are using a tiny bit of opposite aileron to counter that, or we're being sloppy and letting the airplane slip a little, which also counters it.

Go do a 40-degree banked turn while keeping the ball centered and see what happens with the ailerons held neutral.

https://www.aopa.org/news-and-media/all-news/2008/march/flight-training-magazine/steepen-that-turn

 

[infrequentFlyer]

Yes - I thought about overbanking tendency while writing my comment but considered it not to be relevant. I was asking about an already established and fixed bank angle turn.

In other words, once you have established your turn and taken whatever corrective action to maintain a fixed bank angle (including aileron correction to overcome overbanking tendency) your wings should each have equal lift. If there was not equal lift, and raised wing continued to have more lift, it would keep lifting.

I'm not saying there is no overbanking tendency at this point, but that you have already applied aileron input to counter it.

At this point when you have done whatever is required to keep the wings at a constant bank angle, I am saying there should be no more adverse yaw. (I mean, I was asking - I don't claim to be correct. Maybe I'm missing the connection between adverse yaw and overbanking tendency).

 

[BillTIZ]

It is directly related to the use of the ailerons, so it is happening anytime you are moving the ailerons...

Exactly... fly a Grob 103, that 17.2meter wing span is about 1/3rd aileron. You can always tell new pilots to the Grob 103 by the "yaw dance" that gets set up with the nose hunting back and forth with every minor wiggle of the aileron.

I demonstrate adverse yaw in most gliders it works the same. In the Grob 103 the experience is amplified.
Feet on the floor, quickly move the stick left or right to roll into a turn and watch the nose swing opposite the desired direction before it finally comes around.
Then get on the rudders and establish a 45degree bank coordinated turn. Take your feet off the rudders and try to roll wings level. Again the nose goes the wrong way, it moves more into the turning direction, when you want it to stop moving and get wings level. Again, put your feet to the task to make it happen.

 

[MRC01]

Yes - I thought about overbanking tendency while writing my comment but considered it not to be relevant. I was asking about an already established and fixed bank angle turn.

In other words, once you have established your turn and taken whatever corrective action to maintain a fixed bank angle (including aileron correction to overcome overbanking tendency) your wings should each have equal lift. If there was not equal lift, and raised wing continued to have more lift, it would keep lifting.

If we define lift as normal to the Earth, in a turn, the wings don't have equal lift because due to dihedral, the inside wing is more horizontal than the outside wing, thus more of its lifting force is normal to the Earth. You can think of this as the inside wing holds you up and the outside wing turns you (of course that's an exaggeration). However, if lift is defined as normal to the wing then your description is correct - they must be equal else the airplane's bank angle would be changing.

Note: most but not all airplanes have dihedral. For example the Cessna 170A has no dihedral, in which case the above distinction doesn't apply - in a turn, both wings have the same angle to the Earth. I have a few hours flying the 170A and it does feel different in the turns.

At this point when you have done whatever is required to keep the wings at a constant bank angle, I am saying there should be no more adverse yaw. (I mean, I was asking - I don't claim to be correct. Maybe I'm missing the connection between adverse yaw and overbanking tendency).

To summarize the article quoted by @Dan Thomas:
In gentle turns the natural tendency is underbanking.
In steep turns the tendency is overbanking.
In between these is a range where the airplane can be trimmed for a hands-off constant rate turn.
The bank angles where the plane transitions between these tendencies varies, depending on the design of the airplane.

 

[infrequentFlyer]

it moves more into the turning direction, when you want it to stop moving and get wings level

I think I understand this. When you are rolling out naturally the lowered/inside wing needs more lift to come back to level - hence more drag.

Still mentally processing your responce MRC - I didn't consider dihedral before.

 

[MRC01]

I think I understand this. When you are rolling out naturally the lowered/inside wing needs more lift to come back to level - hence more drag.

Yep, it's all relative. For example to return to straight & level flight from a constant rate coordinated turn to the L, you will apply R aileron and R rudder. Relatively speaking, it's similar to entering a R turn from level flight.

Still mentally processing your responce MRC - I didn't consider dihedral before.

Dihedral is part of the airplane's inherent stability: it helps create a tendency to return to level flight from normal (not too steep) turns.

 

[Ed Haywood]

Again, no need to think on it too hard. When you deflect the ailerons, you need rudder in the same direction to stay coordinated. Amount of rudder is proportional to amount of aileron. So you need a lot of rudder when rolling into and out of a bank, and a small amount of rudder to hold a bank angle.

Best way to learn to apply is Dutch Rolls. Pick a point on the horizon, and begin rolling your aircraft from 45 to 45 bank with full aileron deflection, while keeping your aircraft pointed at that point on the horizon. Don't pause in a bank; as soon as you reach 45 bank angle, roll the other way. I put a dot of colored tape on the windshield as a "gun sight" to help me be more precise.

Try it without rudder first and you will slew abruptly from side to side. Your ball will go crazy too. Then do it with rudder, and you will learn quickly how much rudder you need and when. Feeding in rudder at the same rate as ailerons, and taking it out at the same rate, will keep you aligned.

I start every aerobatic practice session in my Decathlon with Dutch Rolls. It is a fundamental stick and rudder skill. The effect is more noticable with tandem seating because you are sighting down the center line of the aircraft.

 

[Dana]

Note: most but not all airplanes have dihedral. For example the Cessna 170A has no dihedral, in which case the above distinction doesn't apply - in a turn, both wings have the same angle to the Earth. I have a few hours flying the 170A and it does feel different in the turns.

The aerodynamic interaction between wing and fuselage on a high wing airplane gives some dihedral effect even with a flat wing. OTOH, the fuselage above the wing is destabilizing in roll on a low wing aircraft, one reason most low wings have more dihedral than most high wings.

My Kolb Ultrastar had zero dihedral and no fuselage, either, just a tailboom... it had zero roll stability and had to be flown all the time.

One other thing dihedral does is give yaw-roll coupling. Kick the rudder on a plane with dihedral, and it will roll in the direction of the yaw due to the relative wind hitting the underside of the outer (now slightly forward) wing from an angle. Some ultralights (and many R/C models) have no ailerons at all, just rudder... and lots of dihedral. My Kolb with its flat wing wouldn't roll at all from rudder input. OTOH the Quicksilver has ailerons (the early ones didn't) and lots of dihedral. If you use the rudder in one direction and the ailerons in the other, the rudder wins and you roll in the direction of the applied rudder.

 

[Silvaire]

...The butt-o-meter also has instantaneous no-lag response, so pilots should learn to use that "instrument" too.

The term "flying by the seat of your pants" comes from the old biplanes when pilots normally sat behind the wing. In a typical modern GA aircraft you're going to be sitting pretty much right at the center of lift so you're not going to get much of it. Your passengers in the back seat, on the other hand, will feel it.

 

[MRC01]

The term "flying by the seat of your pants" comes from the old biplanes when pilots normally sat behind the wing. In a typical modern GA aircraft you're going to be sitting pretty much right at the center of lift so you're not going to get much of it. Your passengers in the back seat, on the other hand, will feel it.

It may be true that old vintage biplanes had an enhanced effect, I don't know since I haven't flown one.

But in "modern" aircraft (I use that term loosely, since many common trainers are 40+ years old) you can most definitely feel whether the airplane is coordinated. If many pilots don't notice, it's not because it can't be felt, but because they never learned to pay attention to it. If you turn without enough rudder (lazy feet), you'll feel your butt pushing to the inside of the turn. If you use too much rudder you'll feel your butt pushing to the outside of the turn. Apply just enough rudder so your butt isn't being pushed L or R, but the force is straight back into your seat.

A few minutes in the practice area turning with rudder only and with ailerons only will make this clear. Then it's just a matter of paying attention to what the butt-o-meter is telling you.

 

[Pinecone]

A friend was partner in a Cherokee 180D. One day were going somewhere and I was flying from the right seat. I hear him muttering. Final I figure out he is saying S**T over and over. I asked him what was up. He said, "D**m you, you fly my airplane 4 MPH faster at the same power setting.

Difference was I was using the rudder to keep the nose straight in some light chop.

 

[Maxnr]

None of the responses mentioned differential ailerons. They have been around for awhile. Noticeable if you mostly fly the oldies. You know, the ones with all that adverse yaw. When you finally fly one with differential ailerons, its a revelation.
I was building a Thorp T-18 way back when and John Thorp designed it with differentials. They can be easily designed into most airplanes. In the case of the Thorp, it was all in the 4 arm aileron Bel cranks. One arm was (carefully) designed longer than its opposite. Also the angle between the arms was carefully engineered.
I was not able to complete mine. I was a Nat Guard pilot and was called up in the fall of 68. My project didn't survive a flooded cellar while I was otherwise occupied. It had to be scrapped. However, another member of my EAA chapter built one and flying it was a delight. Not at all like all the Tripacers, T-crafts and Champs.

 

[Dan Thomas]

Not at all like all the Tripacers, T-crafts and Champs.

All 7-series Champs and Citabrias have differential ailerons.

https://drs.faa.gov/browse/excelExternalWindow/2B3FA1C355E95F9A862578380053D423.0001

 

[Maxnr]

All 7-series Champs and Citabrias have differential ailerons.

https://drs.faa.gov/browse/excelExternalWindow/2B3FA1C355E95F9A862578380053D423.0001

My 1946 7AC had adverse yaw.

 

[Dan Thomas]

My 1946 7AC had adverse yaw.

So did my uncle's 7AC, and so did the 7EC, 7ECA and 7GCBC I instructed in.

Imagine the adverse yaw without the differential...

The 7 series typically have around 28 degrees up and 18 down. A Cessna 150 or 172 has 20 degrees up and 15 down. Less down travel means less adverse yaw. The washout of the Cessna wing also figures into it. Look at the twist in the aileron sometime.