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Yaw question

Old 23rd Dec 2015, 04:53
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Yaw question

All the text books I read tell me that the predominant secondary effect of roll is yaw IN THE SAME DIRECTION OF BANK. They mention adverse yaw as an initial secondary effect, but emphasise that the adverse yaw is nowhere near as strong as the tendency to slip then yaw towards the lower wing.
My question is this: In every aircraft I have flown, the need to hold same side rudder to combat adverse yaw is obvious and very noticeable, yet I have never needed OPPOSITE rudder whilst turning. If the slip then yaw issue was so prevalent, wouldn't we all need to be turning with opposite rudder held on all the time?
Thanks in advance to any aerodynamics afficionados..
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Old 23rd Dec 2015, 10:55
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I'll give it a crack, but if history proves correct I've got a >90% chance of being incorrect.

Adverse aileron yaw would only be present during the entry and exit from the turn while the ailerons are displaced, but either way this probably isn't a huge factor in most of the aeroplanes you fly thanks to having either differential or frise ailerons, and due to yawing tendancy created by the aircraft slipping. Once established in the turn, the ailerons are neutral or possibly against the turn to overcome the overbanking tendency; now the horizontal component of lift provides the centripetal (turning) force, and now that the heading of the aircraft is constantly changing, the rudder is required to keep the aircrafts longitudinal axis aligned with the direction of flight.

Confused? I know I am, but I'll blame it on the wine...
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Old 23rd Dec 2015, 13:15
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the forces you are talking about are mere percentages of the major forces.
dont obsess about them, just know that you need to fly the aircraft and use control inputs appropriate to the desired flight path and occasionally these may be different. you can't use a recipe. left turn so push rudder to here each time.
you have to fly the aircraft.

in a climbing turn to the left I need to apply right rudder.
it is a characteristic of the aerodynamics of the fuselage shape and probably peculiar to the W8 Tailwind.

the other aircraft I believe has non standard FAR23 type handling is the Transavia Airtruck. the flying fuselage on that crop duster is designed so that a 60 degree banked turn needs no rudder input. again it is an effect caused by the fuselage aerodynamics.

don't let CAsA spook you. it's good fun.
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Old 23rd Dec 2015, 21:10
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Plow King is in the right direction ... if the texts call it an effect of roll, they're wrong - adverse yaw is an effect of aileron - and during most of a turn, ailerons are only slightly deflected, so only slight effect.

Plenty of aircraft display adverse yaw - especially older or special purpose aircraft. Pop down to your nearest gliding club and arrange a bit of a joy flight - then see adverse yaw. The (few) gliders that I've flown have displayed quite spectacular adverse yaw - you must lead a turn with rudder, or you get all sideslip and no turn!

I count gliders as special purpose - your average GA aircraft is designed to be easy to fly, and a bit of extra drag from Frize ailerons is acceptable. A glider is designed to fly well, no extra drag thanks, so you just have to learn to fly it well.
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Old 24th Dec 2015, 00:09
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in a climbing turn to the left I need to apply right rudder.
it is a characteristic of the aerodynamics of the fuselage shape and probably peculiar to the W8 Tailwind.
So do I but I think it is due to torque effect, P factor and slipstream effect rather than anything unique to the aircraft type. Also, you need less right rudder in a left turn compared with climbing straight ahead so you actually reduce right rudder pressure as you are entering the turn.

Last edited by uncle8; 24th Dec 2015 at 00:20.
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Old 24th Dec 2015, 02:37
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Thanks all
I guess I'm curious as to why the textbooks talk about the need to counteract yaw from slip (ie opposite rudder required) when it doesn't seem to be remotely evident, whereas adverse yaw is quite evident in aircraft even with differential ailerons. I have seen recommended briefing notes for instructors, they all mention slip then yaw as secondary effect of aileron, some barely mention adverse yaw...
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Old 24th Dec 2015, 06:48
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I think that you are reading it incorrectly. There is no opposite rudder required. In normal level flight, if you bank the aircraft without touching the rudder it will bank, start turning and slip. To overcome the slip (and any adverse yaw) you apply rudder into the turn so that the ball is centred.
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Old 24th Dec 2015, 07:04
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Try banking while using enough opposite rudder (yes opposite) to stop the turn (aka the yaw!) completely, then you will see what effect the bank and induced sideslip has on yaw in the same direction.

Adverse yaw is a secondary aerodynamic effect of the ailerons not roll or bank. Don't confuse them.
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Old 24th Dec 2015, 07:15
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To expand on the above, I think the textbooks just talk about what the aircraft is likely to do with a continuously held aileron deflection as opposed to what any sensible pilot would do.

Initially you'd expect adverse yaw as the roll commences, then as sideslip develops into the turn the weathercocking effect of the fin creates a pro-turn yaw, leading to the good old spiral dive.

No need to over-think this one.
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Old 24th Dec 2015, 07:46
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Originally Posted by jgray View Post
All the text books I read tell me that the predominant secondary effect of roll is yaw IN THE SAME DIRECTION OF BANK. They mention adverse yaw as an initial secondary effect, but emphasise that the adverse yaw is nowhere near as strong as the tendency to slip then yaw towards the lower wing.
My question is this: In every aircraft I have flown, the need to hold same side rudder to combat adverse yaw is obvious and very noticeable, yet I have never needed OPPOSITE rudder whilst turning. If the slip then yaw issue was so prevalent, wouldn't we all need to be turning with opposite rudder held on all the time?
Thanks in advance to any aerodynamics afficionados..
I believe that the books are referring to sort of a "bigger picture" than just the adverse yaw.

Step back a couple of steps and look at what an airplane does in a turn.


It yaws. A lot.


Take a plane on a heading of north. Roll right, and hold the turn till your heading is 090, During the course of that turn, your plane yawed (rotated around it's vertical axis) 90 degrees.

That 90 degrees of yaw is waaaay more significant than the +/- 5 degrees of adverse yaw that you may or may not get, depending one the aircraft model.

Even if you had 5 degrees of uncorrected adverse yaw, your airplane would still have yawed 85 degrees.

I believe that is what the text books are referring to as the predominant secondary effect.
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Old 24th Dec 2015, 08:10
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Last edited by Radix; 18th Mar 2016 at 03:04.
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Old 24th Dec 2015, 09:31
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What Radix said.

Gidday jgray, and welcome to the forums.

My question is this: In every aircraft I have flown, the need to hold same side rudder to combat adverse yaw is obvious and very noticeable, yet I have never needed OPPOSITE rudder whilst turning.
There is no roll occurring during a level turn. The roll is at the initiation of the turn, but then ceases. For the remainder of the turn, the aircraft is in continuous yaw and continuous pitch.

The pitch is up, and the yaw is towards the inside of the turn.

drpixie said it first: adverse yaw is a function of aileron application, and nothing to do with roll.
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Old 24th Dec 2015, 10:50
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I have seen recommended briefing notes for instructors, they all mention slip then yaw as secondary effect of aileron, some barely mention adverse yaw...
Notes are crap.

It sounds like you have seen EXACTLY what happens and you are all over it.

FGD

There is no roll occurring during a level turn. The roll is at the initiation of the turn, but then ceases.
Correct!

For the remainder of the turn, the aircraft is in continuous yaw and continuous pitch.
NOT CORRECT!

Yaw is rotation around the normal axis. In a constant bank, the normal axis is tilted at a constant angle. The aircraft turns, yes, but if the aircraft was in fact in a yaw during a turn its nose would be increasingly below the horizon, as if the pilot had used (and held) too much rudder in the direction of turn.

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Old 24th Dec 2015, 12:08
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Originally Posted by FGD135 View Post
For the remainder of the turn, the aircraft is in continuous yaw and continuous pitch.
Originally Posted by Horatio Leafblower View Post
NOT CORRECT!

Yaw is rotation around the normal axis. In a constant bank, the normal axis is tilted at a constant angle. The aircraft turns, yes, but if the aircraft was in fact in a yaw during a turn its nose would be increasingly below the horizon, as if the pilot had used (and held) too much rudder in the direction of turn.
Ummm, yes, he has it *exactly* correct. The airplane is both yawing and pitching. in a banked attitude, the vertical component of yaw which would tend to cause the nose to drop below the horizon is counteracted by the vertical portion of pitch.

It's just silly to say that an airplane doesn't yaw during a turn. How else can the nose point in a different direction without yaw??? ( Aside from a 90 degree banked turn, in which the heading change would be all pitch)
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Old 24th Dec 2015, 13:24
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Originally Posted by uncle8 View Post
I think that you are reading it incorrectly. There is no opposite rudder required. In normal level flight, if you bank the aircraft without touching the rudder it will bank, start turning and slip. To overcome the slip (and any adverse yaw) you apply rudder into the turn so that the ball is centred.
If it was slipping in the turn, the ball would be displaced away from the centre of the turn, not towards the centre of the turn surely - hence requiring rudder away from the turn, not into it...
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Old 24th Dec 2015, 13:44
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Ok, it's all making sense now I think. Adverse yaw on application of aileron, requiring rudder towards roll direction (if we wish to stay 'balanced'). Followed by yaw towards lower wing during roll, which would require opposite rudder if we wanted to stay 'balanced' - this is due to a side slip developing.
However in a constant turn with no aileron deflection, neither of these effects are evident as they relate to aileron application, not bank.
Am I there yet?
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Old 25th Dec 2015, 03:04
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Originally Posted by jgray View Post
If it was slipping in the turn, the ball would be displaced away from the centre of the turn, not towards the centre of the turn surely
No, that's skidding, not slipping.

A slipping turn will have the ball out of center toward the center of the turn.
A skidding turn will have the ball displaced to the outside of the turn.


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Old 25th Dec 2015, 03:38
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Originally Posted by jgray View Post
Ok, it's all making sense now I think. Adverse yaw on application of aileron, requiring rudder towards roll direction (if we wish to stay 'balanced'). Followed by yaw towards lower wing during roll, which would require opposite rudder if we wanted to stay 'balanced' - this is due to a side slip developing.


Am I there yet?
Not quite. you're thinking of yaw as only relative to aircraft coordination, ie: whether or not the ball is centered. Yaw is more than that that's the "big picture" I referred to earlier. Even in perfectly coordinated flight, a turning airplane is yawing. 90 degrees of heading change is 90 degrees of yaw. Doesn't matter whether you're holding rudder into the turn or opposite the turn, if you're turning, you're yawing.



As far as the sequence of the events as the turn is initiated, you seem to be a little off on your understanding. You are describing it as an aileron input produces adverse yaw requiring rudder into the turn *then* a sideslip develops, requiring opposite rudder. That is not quite correct. Yes, it is true that on most planes, an aileron input creates adverse yaw, but the adverse yaw *is* a sideslip. If you turn using ailerons only, the ball will be displaced to the center (or centre, if you prefer) of the turn. That's a slip. The correct response, as you have said, is to apply rudder into the turn. But where you err is in saying that then a slip develops and you need opposite rudder. That's not true, the slip developed immediately with the adverse yaw, and you have already corrected it with inside rudder.


Consider this. You can turn the airplane with only ailerons. It's not good practice, and it will cause your instructor to whack you about the head with a rolled up chart, but it will work. You can just roll the ailerons in the desired direction and leave the rudder pedals alone. Initially the airplane will yaw in the opposite direction, creating a sideslip. As you increase the bank, the sideslip will create forces which will overcome the adverse yaw, and the airplane will start yawing in the direction of the bank. Te airplne will be turning, so it will be yawing. It won't be a pretty turn, but it will be a turn.

Of course, what you're aiming for is to not have the adverse yaw, so you use your rudder. As you begin to roll into a bank, you also apply rudder into the bank, ant juuuust the right amount to counteract adverse yaw. That rudder application causes the airplane to yaw. not a lot. not so that the plane is uncoordinated and the ball out of center, but just enough yaw to match the angle of bank.

Bottom line is that if you bank the wings, the airplane will eventually yaw in that direction. You can do nothing and let side-slip produce the yaw (not recommended) or you can proactively use rudder into the bank to cause that yaw yourself (preferred) but either way, the airplane *will* yaw.


Does any of that help?

Last edited by A Squared; 25th Dec 2015 at 05:09.
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Old 25th Dec 2015, 04:27
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Bottom line is that if you bank the wings, the airplane will eventually yaw in that direction. You can do nothing and let side-slip produce the yaw (not recommended) ...
And that yaw is the "secondary effect of roll" that you were first asking about. Good work, A Squared.

Ok, so there is no roll during a level turn. During climbing and descending turns, however, there has to be, and there is, ... roll! So for these manoeuvers, the aircraft is yawing, pitching and rolling.

On the climbing turn, there is roll "away" from the inside of the turn. Turning left, for example, the aircraft is rolling "right".

On the descending turn, the roll is "towards" the inside of the turn. Turning left, for example, the roll is to the "left".
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Old 25th Dec 2015, 06:36
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Last edited by Radix; 18th Mar 2016 at 03:03.
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