dredhea

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I think I'm in the right place for this... My other choice was the beginners forum. I'm not a beginner, but I have what might be considered to be a beginner question. Why does differential in the ailerons help correct adverse yaw? I've never bothered to find the answer to this as I just coordinate rudder into my turns, but since I'm going to be doing some "depot level" maintenance on my 1/4 scale Cub, I'm considering adding differential. Also, if I do, how much should I add? Or is that dependant on the airframe? For an idea of the amount of yaw I deal with, at half throttle, banked about 60 degrees with no rudder input, the plane will nearly stall before it will turn. Since I always get quality answers here, I'll thank you all in advance.

gboulton

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mboland

The down going aileron always produces more drag than the upward going aileron, thus dragging the nose to that side.

dredhea

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Thanks, Guys! The answer was a lot simpler that I thought it was going to be. I feel like beginner asking something so simple, but if ya don't know, ya don't know and usually the fastest way to earn is to ask. So now the only question is "How much?", but it sounds like that's more of a trial and error type of adjustment. I'll take care of that when I get it to the field.

gboulton

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For what it's worth, I've found that reducing the travel of the "down going aileron" is more effective (that is to say, produces results with less adjustment).  Which, given the forces at work, makes sense.  *heh*

otrcman

A safe place to start with aileron differential would be 2:1 (upgoing aileron travels twice as far as downgoing). That is often a minimum, and you will probably need more than that. As an extreme, the full scale Tiger Moth has zero downgoing aileron when the upgoing is at full travel.

The amount of differential required is a function of a number of things. If your airplane is high wing (you did say, "Cub"), then it will need plenty of differential. If you have extra wing dihedral, you will need even more differential.

Have you thought about mixing in a bit of rudder? Aileron/rudder mix accomplishes pretty much the same thing as differential. If you have a computer transmitter, the mixing function is readily adjustable and you can tune the airplane to be just right to your tastes.

Dick

dredhea

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Thanks for the starting point. yeah, it is my Cub, so it'll probably end up being a bit more than 2:1. I'll play with it when I get flying it. Weather forecast is for continued building. I just need to give the plane some TLC before she goes up this year and thought that I would address the yawing while I had her skin off. That was before I found out that it'll be a trial and error procedure.
I correct the yaw with a rudder mix now. It's carefully adjusted during every turn with my left thumb

BMatthews

It's actually not as simple as you think. The amount of adverse yaw you encounter will depend on the flying speed. The worst condition for adverse yaw is near the stall point where the down traveling aileron moves that wing to a high lift coefficient and a high induced drag as a result. But if you're ripping along at a high cruise speed where the wing is operating at a very low lift coefficient then you will see extremely little or no adverse yawing. In fact at high speeds with a strongly cambered airfoil such as found on most Cubs it's quite possible that the up traveling aileron may force that side to such a low lift coefficient or even to a negative Cl that it produces a PROverse yaw. See? It's not quite as clear as you may think.

Anyhow since it's a Cub I'm assuming you spend more time putting around closer to the stall than you do trying to emulate a pylon racer. As such a 2:1 up to down would not be a bad thing at all. But even with that if you're trying to do a low and slow turn out at the end of the runway it's a wise choice to use only small control inputs. After all, even with some differential in the system if you're near the stall the half travel on the down side could be the straw that breaks the camel's back and you end up with a tip stall. When flying slow it's a wise pilot that moderates the aileron use and decides to use just a little and be more patient with the model's roll rate.

dredhea

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My line about carefully adjusting the rudder mix with my left thumb was an attempt at humor, meaning that I currently just coordinate rudder into my turns. The casual observer would not understand what I meant by adverse yaw as they would never see it (hopefully). I know what you mean, though, as my rudder input varies depending on airspeed. The yaw problem is more pronounced at lower airspeeds, where I usually fly. Your post makes me think that perhaps I'll just leave well enough alone and continue to fly with both hands.

HarryC

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Adverse yaw is a fact of life caused by rolling. It is not caused by the aileron drag. If it were then it would be very easy to avoid on all full size planes by using differential and saving the pilot having to operate the rudder. But it isn't designed that way full size, because differential can't work! It has been tried though a long time ago when people didn't know much. Someone gave the example of the Tiger Moth which has a clever bellcrank so that the downgoing aileron starts going down then comes back up again. Yet as Pilot magazine remarked in an article about flying the Tiger Moth about 2 years ago, the phrase "adverse yaw could have been invented for this plane". But it has enormous differential so how can it have such appalling adverse yaw?

Adverse yaw comes from the rolling motion. Lift force acts at right angles to the direction of travel of the wing. When the plane is rolling the upgoing wing is travelling along an up slope so its lift force is angled backwards by the same amount and pulls it backwards. The downgoing wing's path is angled downwards so its lift force is angled forwards, pulling it forward. That is adverse yaw. It is not caused by ailerons in the air stream and aileron differential can't cure it. If you fly slowly but try to roll at the same rate the path of the wings is steepened so the change in angle of the lift force is also changed that much more. Long spans mean that for the same roll rate, the tips travel a steeper slope than for a short span and once again the change in angle of the lift force is greater. That is why long spans and slow speeds cause greatest adverse yaw.

The cure is to use rudder to counter the advesre yaw. Use your tx to mix in enough to do the job. Some full-size planes even jet fighters have aileron to rudder mix and if its good enough for hot-shot F-4 Phantom fighter pilots it's good enough for us!

Many model fliers confuse an offset rolling axis with adverse yaw. They see the roll looks odd, they have heard the phrase "adverse yaw" so they put the 2 together quite wrongly. Differential can cure an offset rolling axis, so these modellers put in differential, the offset axis is cured and they proclaim that differential cured their adverse yaw!

What about Frise ailerons that have downgoing bit of the upgoing aileron to counter adverse yaw? Firstly, that is not aileron differential. Secondly, the downgoing snub is a pure drag creator, like a mini-airbrake. It has been shown that the increase in drag from using the rudder is less than the increase in drag from Frise ailerons which is why they are mostly not used, as overall aircraft/glider performance is therefore slightly better when rudder is used.

dredhea

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Welcome to the discussion HarryC!. That's quite a technical answer. It'll take awhile (and several readings) for me to digest all of that! I'm not sure I know what you mean by "offset rolling axis". Could you please expand on that?

HarryC

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Here's a drawing that shows the inclination of the lift angles in opposite directions as the wing rolls http://images-mediawiki-sites.theful...2414313322.png

An offset roll axis is an axis that is not along the middle of the fuselage for example it is slightly out to one side often due to an unequal change in the lift of up and downgoing ailerons. So the aircraft does not roll along the centre of its fuz, it rolls around an axis that is offset and possibly even outside the fuz. This gives it a slightly barrelly appearance to the roll and many model fliers wrongly call this adverse yaw. Making the aileron travels unequal can shift the axis back towards the centre and make the roll more axial and less barrelly. Aileron differential has corrected it, but what it has corrected is not adverse yaw!

kerwin50

I'd go 30 percent or more.

pimmnz

Actually, the only way to correct 'adverse yaw' is to use rudder. All the other fixes mentioned will reduce the effect, but at the expense of some reduction of control authority. Mixing a bit of rudder/aileron in your computer tranny then playing with the amount until you have a nice balanced turn will be all that is required.
Evan, WB #12.

tomfiorentino

I understand that both are correct; ailerons cause yaw and so does the tilting lift components due to roll.

The down aileron increases camber, increases lift and increases drag. The opposite is true; up aileron decreases camber, decreases lift and decreases drag. The delta in the drag causesthe yawing moment.

I don't know which one accounts for a greater percentage of yaw...ailerons or lift tilt...but I imagine there are other combinations and dependent variables.

Tom

HarryC

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Not with a symmetrical section, aileron in any direction increases camber for both wings, yet symmetrical wings still get adverse yaw. Forget aileron differential, it's a model fliers myth, that's why fullsize planes designed by very clever people who actually know what they are doing, don't bother with it.

tomfiorentino

I agree with the increased camber in both directions on a symetrical section. But as you know, in order for a symetrical section to create lift in the first place, in needs AOA. And in that instance, aileron deflection still contributes toadverse yaw.

As I am sure you know, at the end of the day, ailerons bank an airplane by increasing lift onone half of the wing relative to the other half. I don't know how lift can increase without also increasing drag. More lift on one sided equals more drag on same side equals a contribution toward adverse yaw. Percentages I don't know.

It's not new that this isone of those aerodynamic items in constant change as AOA, air velocity and relative wind are in a constant state of flux. As usual, it's never ONE thing...always several...and I'm sure the percentage allocation betweendrag vs lift tiltcontributing for the total yaw will change accordingly on the same airplane.

Tom

rmh

Gee whiz-
It looks to me that -it's all about the differential in work being done.
If one considers a craft accelerating vertically -then rolling- the work is equal
-so - no adverse yaw in the roll. Just as lift is nothing more than pressure differences at work-
adverse yaw is is simply a result in differences involved.
There -that wasn't so tough was it?

HarryC

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Do the calculations and you find it is simply two ways of describing the same thing - change of induced drag (drag due to lift) is the same as the change in drag due to rearward and forward inclined lift vectors.

What is is not is drag due to ailerons being deflected into the airflow which is why aileron differential doesn't work, and it is not drag due to change in camber per se otherwise you can't explain why symmetrical sections get adverse yaw.

One myth that keeps popping its ugly head up, and thankfully (though suprisingly) has not yet been put forward in this thread, is that the downgoing aileron is going into higher pressure air and the upgoing into lower pressure air so the form drag from the ailerons is different. It's remarkable how many model fliers cling to that explanation but for good reason since it is often repeated by model magazines etc. Why is it false? The equation for drag is 0.5*air density*V squared*coefficient drag*area. Note that pressure is not in the equation. You can change the pressure up and down and all over the place but every time you run the equation the answer is the same because pressure isn't in it!

rmh

actually - doing the aileron differential does work-
the inboard panel increases in drag-the outboard with NO deflection does not change much in drag - In that it is travelling further - it does do more work but all in all a good dose of differential does work-
better yet -you can proove it in actual application -

tomfiorentino

I went back and re-read the section on adverse yaw in my reference book. It very clearly describes adverse yaw as being caused by both items and I am not sure what to do from here. I mean, the science is the science no?

Adverse yaw is caused by:

a) aileron / camber / drag etc.
b) aft lift tilt etc.
c) Both a and b
d) None of the above

To my way of thinking this can't be an unresolved, unproven, unstudied etc. aeronautical enigma.Take the two brightest of bright on the subject and ask them the question. You mean to tell me this is a toss-up? The jury is still out on this one?

BMatthews

I've actually found that most science is whatever the particular book's authour felt like saying. Usually they won't say anything wrong but it's not at all unusual for some authours to present one part more fuly while downplaying or managing to avoid some other part either from a biased sense of belief or in some attempt to water down a subject to make it more understandable to the masses. For example someone taking pilot's training doesn't need to know all the ins and outs of Prandtl's lifting line theory to be able to fly. So the books often give the generally watered down Bernoulli explanation or perhaps mention a little about Newtonian downwash. It depends on which book you're reading.

From actually flying I can say that the amount of adverse yaw is related to the aspect ratio. Longer gives the AY effects more leverage to work with. It is also related to the flying speed. Or more accurately how far up the lift vs drag coefficient curve the wing is operating at. At the higher lift coefficients adding camber and angle of attack with a down aileron movement will add a lot of drag for a small angle change at this point in the operating range. Meanwhile if you look at the typical chart shapes reducing camber and angle of attack on the up aileron side doesn't make as big a change in the drag. So the response at slower speeds is to generate a lot more AY than at higher cruise speeds where an angular and camber change makes less of a difference in drag.

I had a buddy locally that wanted to fly an electric scale SPAD XIII at "scale" (in other words super slow) speeds for his model. In his efforts to avoid AY he used a control linkage that generated only up travel in the ailerons. Yet he still had AY and required some rudder to get a smooth turn entry. I finally convinced him to just try flying a little faster. He found that the AY went away with about a 5 mph increase in flying speed. The only effect was to bring the lift coefficent down to where the wing with no aileron travel and the one with the up traveling aileron were both generating about the same drag by being down more into the vertical region of the lift drag curve.

rmh

Yes basic facts - if load/work is the same - no net difference
change load/work- and you get a difference- adverse yaw OR a decreasing radius turn.
No math needed

tomfiorentino

Thank you for that post. You are probably right about the bias in textbooks which is unfortunate, but I can see that being the case.
I understand your comment on aspect ratio too, and it speaks to the prior point that it is a combination of a lot of things.

So let me link two of your references above...Bernoulli vs Newton and Adverse Yaw with the SPAD. I'm not trying to be smart and I know this kind of links two threads together.

It's interesting that the effort to avoid AY was to generate only UPtravel in the ailerons; and in doing so I'm sure the airplane still rolled.

But if only down aileron was used, would the airplane roll and if so who was the culprit....Newton or Bernoulli?

rmh

Neither of those guys had the chance to fly the SPAD
So there is no blame to be assessed to either .
The interest in assignment of flight cause/effect to one or the other- is a mystery to me- Those old cats were simply the first to record the obvious. In fact - I would venture that the Egyptians knew the basics of some simple laws discovered by Newton- (action-reaction) and the Chinese likely noted speed/pressure relationships -
Let's face it -many discoveries are simply rediscoveries and some recent ones ( airplane/telephone /light bulb etc., were predictable discoveries and occurred in various places at practically the same time.
Just something to ponder -