Aileron Drag

[AIRWAY]

Hello all,

Could someone please explain to me the above effect ( Aileron Drag ) im a bit lost regarding this effec on aircraft...

Many Thanks for your help

[df1]

Hi AIRWAY,

Regarding aileron drag:

This is a product of the movement of ailerons as in a turn, for example. If the pilot wishes to effect a left turn the ailerons move such that the right aileron will move down and the left will move up. The affect of the right aileron moving down will increase the lift over that wing, whilst the left aileron in moving up will have a net affect of reducing lift on that wing. This then, is essentially going to start the roll (to the left in this case).

There are some secondary affects of this. The right wing, in generating more lift will also generate more drag! The opposite will happen with the left wing. So, we now have a situation where there is a yawing tendancy as the right wing generates more drag. So although we are effecting a left bank we have a right yaw - which would need to be countered by applying left rudder. This situation can be dangerous at or near the stalling angle!

To overcome this situation aircraft can be designed with a number of features which include:

Frise ailerons: A patented device that when an aileron is deflected up (to reduce lift) there is a lip protrusion into the airflow. This increases drag on that wing. In the case of a down-going aileron the design is such that it is smooth and flush so as to reduce any drag.

Differential Ailerons: These are designed so that the down-going aileron moves through less of an angle that the up-going aileron.

Spoilers: These panels which can be raised on the upper wing surface. They can be designed to operate on the down-going wing in conjunction with a bank - again increasing drag on that wing to counter the "adverse yaw".

There are others.

Regards,
df1

[AIRWAY]

Thanks df1 for your time and explanation :ok:

[bookworm]

...The affect of the right aileron moving down will increase the lift over that wing, whilst the left aileron in moving up will have a net affect of reducing lift on that wing. This then, is essentially going to start the roll (to the left in this case).

There are some secondary affects of this. The right wing, in generating more lift will also generate more drag! The opposite will happen with the left wing. So, we now have a situation where there is a yawing tendancy as the right wing generates more drag.

OK, here's the bit I don't get. The wings are actually producing the same lift. If they weren't, the aircraft would be accelerating about the roll axis, wouldn't it? Perhaps that happens for a tiny fraction of a second while the rate of roll is established, but after that the rolling moment is zero.

In a constant rate roll, the couple from the ailerons is balanced by the higher AOA of the downgoing wing causing that to have greater lift. Thus overall, the lift on each wing is the same. In a slip, the couple from the ailerons is balanced by, for example, the greater lift on the into-slip wing due to dihedral. So again, overall, the lift on each wing is the same.

But don't we feel aileron drag for much longer periods than that, in fact, whenever the ailerons are deflected? So can we really blame the induced drag that comes with differential lift for aileron drag? :)

[Keith.Williams.]

As with most aspects of aerodynamics, adverse yaw can be explained in many ways. Different ways appeal to different listeners, but all of the useful explanations (at least appear) to match the observed effects.

Some people explain adverse yaw in terms of the different pressures above and below the wing. Pressure is higher below than above. So the down-going aileron is acted upon by comparatively high pressure, while the up- going aileron is acted upon by a lower pressure. This causes more drag on the down-going aileron, so the aircraft yaws away from the roll. The problem with this explanation is that it appears to ignore Bernouli's argument that total pressure is constant.

More significantly perhaps, dynamic pressure, which only acts downstream is greater in the high speed flow above the wing. So the up-going aileron should produce more drag...but it doesn't???

Another explanation is based on changes in the relative airflow and the effects of these changes on the direction of the total aerodynamic reaction generated by each wing.

Total reaction does not act at right angle to the relative airflow, nor at right angles to the chord line. But its angle is dependent upon both of these. When an aircraft rolls, one wing goes up and the other goes down. Let's assume (as a starting condition) that the relative airflow is horizontal and the total reaction is at an angle of 110 degrees above the relative airflow. This means that the total reaction is angled 20 degrees rearwards. And for the sake of simplicity let's assume that the angle between relative airflow and total reaction remains constant (yes I know it won't really do that but let's try to keep it simple).

The up-going wing (with its down-going aileron) experiences a downward airflow. This tends to tilt its relative airflow downwards. If the relative airflow tilts 10 degrees downwards, then the total reaction will now be at a rearward angle of 30 degrees. This reaward tilting of the total reaction will increase the drag. The down-going wing (with its up-going) aileron experiences an upward flow. This tilts the total reaction in a less rearward direction, thereby reducing its drag. So the aircraft yaws away from the roll.

Readers may of course pick holes in this theory, and could (and probably will) come up with other explanations.

Personally I prefer the argument that the Pink Pixy likes to keep us all guessing.

[ShyTorque]

What has been explained just about covers it. I can only offer a practical example in an attempt to reinforce the principle.

Bear in mind that an aircraft in a balanced turn is yawing and rolling in the same direction (and also pitching a little too, but we can ignore pitch for our purposes at smaller angles of bank; it does become more significant in a steep or max-rate turn but that is not a relevant factor in this discussion).

I have an old radio controlled model slope soarer glider, a semi-scale model of a Breguet Fauvette. The model has a V tail with elevators only, working as a pair (only up or down in unison). It has NO steering rudder surfaces as such, simply because it was designed as a 2 channel model.

The full size aircraft had a control mixer giving the surfaces the ability to offer rudder as well as elevator - "ruddervator".

The aircraft model has long wings and full wing-length ailerons. Because of this arrangement, to make it turn properly, it really does need some "in-turn rudder" to give the necessary yaw into the turn.

By using conventional aileron travel (one up / one down), the RISING wing suffered more drag than the falling wing so the nose yawed OUT of the turn, such that the aircraft flew cross-controlled as it was rolling into the turn.

Adverse yaw is very noticeable on a long winged aircraft because drag changes on the outboard end of the wing give a large moment about the yaw axis.

This adverse yaw reduces the initial rate of turn, something not good in a fast slope soarer. Once the turn was established, (aircraft no longer rolling by application of ailerons), the adverse yaw stopped and the weather-cock effect took over, allowing the aircraft to maintain the turn reasonably well.

I had to adjust the ailerons linkages to become more and more differential until the aircraft is now controlled in roll ONLY by the aileron on the inboard side of the turn i.e. by using UP aileron only. The rising elevator does two things. It reduces the lift on the "inside" wing and it also causes DRAG on the "inside" wing, giving roll AND yaw in the correct direction for the turn. It flies really well now.

Do bear in mind that adverse yaw is a TRANSIENT effect only, when the aircraft is rolling due to the application of aileron.

[CBLong]

Bookworm,

I'll take the bait... :)

The clue to the answer to your conundrum is the fact that the downgoing wing is operating at a higher AoA than the upgoing wing. With the ailerons deflected, we effectively have a different aerofoil section on each side, and while the roll rate remains constant, we also know that each wing is producing the same amount of lift. However, there's no reason why two different aerofoils operating at two different angles of attack should also produce the same amount of drag, just because they happen to be producing the same amount of lift - the amounts of drag from each wing are very likely to be different.

Pursuing this line of thought... the downgoing wing is a lower-camber aerofoil operating at a higher AoA, while the upgoing wing is a higher-camber aerofoil operating at a lower AoA... for the same value of L, would one expect the high-camber/low-AoA combination to always have a worse L/D ratio...?

I'm getting out of my depth, help!!

:)

cbl

[ShyTorque]

Chaps,

It is mainly a TRANSIENT phenomena. If it wasn't, the aircraft would continue to yaw during a continuous aileron roll until it was flying sideways!

To accelerate the aircraft in roll does require a difference in the lift that is produced by each wing and hence the difference in drag experienced by the wings, causing the adverse yaw to begin.

However, the aircraft also has stability by virtue of it's wing dihedral. To oppose this and hold the aircraft into the turn, some aileron will need to be held, prolonging the phenomena.

Any small increase in lift causes a large increase in drag (increase in drag = increase in lift squared).

[bookworm]

...we also know that each wing is producing the same amount of lift. However, there's no reason why two different aerofoils operating at two different angles of attack should also produce the same amount of drag,...

Yes, I agree with your line of thought. The point, I think, is that it's a second order thing.

Looking along its span, the aileron-down/upgoing side produces mostly a little less lift and then, close to its tip, much more lift. The aileron-up/downgoing side produces mostly a little more lift and then, close to its tip, much less lift. Because of the shape of the drag/lift curve with aileron deflection as a parameter, the latter is more efficient (less draggy) than the former.

It is mainly a TRANSIENT phenomena. If it wasn't, the aircraft would continue to yaw during a continuous aileron roll until it was flying sideways!

To accelerate the aircraft in roll does require a difference in the lift that is produced by each wing and hence the difference in drag experienced by the wings, causing the adverse yaw to begin.

I'm not convinced that it's a transient thing. Roll damping has a typical characteristic time of the order of 0.1 sec. That means that if you apply aileron very abruptly and start accelerating it in roll, you reach steady state roll rate in about 0.2 sec. I think that aileron drag persists longer than that, and is pretty much dependent on aileron deflection.

[Tinstaafl]

ShyTorque, I disagree that aileron adverse yaw is a transient thing. You argue that the a/c would continue to yaw until it was flying sideways.

The a/c's stability in yaw is still present & opposing the adverse yaw. The scale of this restoring force is proportional to the amount of yaw. Adverse yaw seems t be proportional to aileron deflection. At some point the two opposing forces will reach some degree of equilibrium (I'm ignoring any other forces or second/third/whatever order effects) and the yawing motion will cease. The a/c would still have some amount of slip present.

[kabz]

Would it be too simplistic to say that moving the ailerons for the purpose of banking the aircraft simply lessens the lift/induced drag on the downgoing wing, and increases the lift/induced drag on the upgoing wing ?

The mismatch in drag causes yaw and needs to be countered by appropriate use of the rudder.

[ShyTorque]

Kabz,

I think you are probably correct, the "KIS,S" principle is best for we pilots.

However, some like to complicate things further....

I think we are beginning to become involved in the cross-coupling effects of other aircraft movements. I perhaps need to re-state my case.

By deflecting the ailerons, we increase both profile and induced drag elements. Induced drag, as you will all know, is the drag caused by inducing lift, profile drag, in simple terms, is caused by something being stuck out in an airflow. The induced flow on the upgoing wing increases while extra lift is being produced, so it occurs while the wing accelerates in roll or has to provide more lift than the other for another reason. The opposite is true for the downgoing wing. Profile drag can be varied by a number of means, already mentioned. These methods (for example differential deflection) are employed by the aircraft designer to prevent or at least to minimise adverse yaw, as Tinstaafl has just intimated.

Adverse aileron yaw is the secondary effect of aileron. Once the adverse yaw has occurred, in obtaining the primary effect of roll to the required bank angle, it gets quite complicated because other effects come into play.

Sideslip occurs next, but the fixed fin surface is there to prevent it. Without it, there would be no way the aircraft could be turned in the conventional sense and the aircraft would continue to adverse yaw much further (if it were continued to be accelerated in roll) to the extent that control would be lost.

Most aircraft have good stability in roll. This must be opposed by some extra lift on the upper wing, produced by in-turn aileron being held by the pilot. This results in extra induced drag still being produced so some adverse yaw effect would be apparent without the pilot's input of in-turn rudder. That is what I meant by stating that adverse yaw is mainly a transient effect. I did use the term "MAINLY".

From a pilot's point of view, this requires a relatively large rudder movement while the aircraft is being rolled into a balanced turn, followed by a lesser amount of rudder being held all the time the aircraft is turning, we do this every time we turn the aircraft.

If you wish to obtain the max rate of roll into the max rate of turn such as in an emergency break for collision avoidance, you may need to input almost full rudder initially to keep the aircraft anywhere near in balance during the turn entry.

Airway,

Hope you aren't now more confused than before you asked this question.

:uhoh:

[AIRWAY]

Heheeh

Well, i´m still on the lower level in terms of knowledge, but i will get there and posts like these just helps me to learn things at a quicker rate and also with heaps more of information than i bargained for, so yes it´s a bit confusing in the bigining but it makes sense in the end. :ok: i guess i just have to keep burning more brain cells :E Anyway i need to go and sleep i have an 9 am flying lesson :yuk:

Thanks all

[John Farley]

Perhaps that happens for a tiny fraction of a second while the rate of roll is established, but after that the rolling moment is zero

Not quite, the rolling moment due to the ailerons remains (so long as they are deflected) and is eventually opposed by the roll damping term such that roll accel stops and a steady roll rate remains.

[bookworm]

Would it have helped if I'd written "but after that the net rolling moment is zero" -- in other words the rolling moment from the ailerons is equal and opposite to the rolling moment from the roll damping. Looking at what that means for the wings, it implies that they are producing the same lift.