Hello,
I have a hopefully simple question for you: I am thinking (and searching on the Net) about differential ailerons, and I can't figure out the REAL reason for they to exist in a pattern plane. All people say that they act in order to contrast inverse yaw. But, what I can't understand is this: if we have a symmetrical airfoil (in a pattern aircraft) and we are doing a vertical roll, what is the wing that is producing morre lift (and hence more drag)? Morover, let's suppose we hare differentiating ailerons in order to contrast inverse yaw. Well, when we are in inverted flight and we do a roll, they act in the opposite direction! The aileron goes down much more, obviosuly.

These argumentations suggest to me that inverse yaw is not the real motivation, at least referring to pattern aircraft, where aileron differentials are still necessary!

I hope you will have the time to help me with this little mistery (for me)!!!

Thank you in advice and good flying!

It's no mystery to me. I will use it (Sometimes) on a trainer plane (Where vertical and inverted rolls are few), but that's about it.

It's like so many things in this hobby (and others) where some people feel the need to over-engineer something, and then others flock to it like sheep when it's really not necessary.

I don't use it on pattern planes -- or any other aircraft with symetrical airfoils -- I use the rudder to counter adverse yaw (if any). You are correct that it negatively impacts inverted flight.

It is definately usefull in some applications, such as the case of a beginner trying to learn to fly with a Cub, or similar aircraft.

I do use differential ailerons in ALL my designs-----symmetrical airfoils all!

I've done so since 1961 when I started Pattern. IMO you ALWAYS need more up than down aileron. My starting point is 10 to 12% then dial in from there----either more or less until the A/C flys as you like. The only designs with zero dialed in are with top hinged ailerons which means that it is 'in' mechanically. If you can find the old Ed Kazmirski instructions for the Orion and/or Taurus you will have as adequate a description of 'why' as there is. Yes, I know that the Taurus used a semi-symetrical NACA 2419 mod.

To further confound the 'purists' I also use differential on elevator------season to taste.

I use this on the 'in line' designs' (I prefer low wing) and on low wing designs. I just flat do not like the way the in lines fly------simply personal preference.

With the 'tricks' allowed with computer radios it is a simple and rewarding procedure to set three flight modes and then go fly. Frankly, the hell with the formulae we are interested in results and we want the A/C to fly to fit our 'style'. Do it, get the results you want----then having them you or anyone else can hypothesize as to WHY. This may make for some interesting discussions but in the meantime your A/C is flying as you wish.

I'm with Dick Hanson on aerodynamics------the full scale stuff isn't the same (and yes I do understand Reynlods numbers). First use whatever works-----THEN philosophize as to why.

Differntial when upright becomes anti-differential when inverted.

It's out of doubt that differential ailerons are still needed on pattern airplanes: for example, Sebastiano's Silvestri suggest about 20% differential on his Funtana 40. Christophe Paysant Le Roux used them also on great Top Line. So, the question is not IF we have to use them on pattern, but WHY.

Since for symmetrical airfoil the inverse yaw is of little entity and when flying inverted it is actually a bad thing I think that there must be ANOTHER reason for they to exist in pattern airplanes. I agree with you that in a general purpose trainer they can be used to contrast inverse yaw, but in patterns...
someone has an explaination?

I think for planes where the wing is not mounted on thrust line, or the stab isn't - like on the Funtana, differential may just be needed to counteract the little bit of coupling that comes from this, especially on a vertical roll. Go do your vertical rolls, and change the differential, and see which setting makes it the most axial..

I agree with ONEWASP......it has alot to do with what an individuals flying style is all about..........I, for the most part, fly heavy and high wing -loaded warbirds.....and differential ailerons just seem to fly much better.....just "Feels" crisper......I also have experimented with drooping the ailerons just slightly......about 1/16th of an inch and that seems to also help response as well......trying different ideas is what this hobby is all about...........and what works well for one pilot might not fit with another..................Bill......

I think John is right: once I heard a great pattern pilot telling something about the difference between thrust line and wing placement. In fact it seems that differentials are more needed, in order to do axial rolls, on low wing airplanes. This is not in contraddiction with the inverted flight, whereas inverse yaw is.

So, assuming that this thrust-wing line difference is the cause that makes the need for differentials, I put another question: WHY?

I am not an aerodynamic expert, so I ask this to you.
To me it seems that it must be the opposite, but I think my reasoning is not correct: if the upward moving aileron "sees" a greater air flow (than the downward one) due to the propeller, shouldn't it generate more drag? So, if this is correct, the downward moving one should have more travel (instead with differential it's travel is reduced)

Where am I wrong?

Thank you for your answers...and sorry for my bad english

Part of the reason is, the upper aileron is operating in a low pressure area, so needs more throw, opposite with the lower aileron.

Ouch, my brain hurts[sm=bananahead.gif]

Sorry, not trying to be mean, but there's no truth to that.

What makes planes fly then ?

Confusing, isn't it. If a pattern plane is so symmetrical, then why does it need aileron and elevator differential. Maybe we should examine the validity of the basic premise, that pattern planes are symmetrical.

In fact, they are not, and they are not even close. Certainly the rigging angles (incidences!!) are mostly zero, and most things are nicely in line and the airfoil has no camber (symmetric), but practical considerations conspire to prevent it being perfect.

The fin sticks up more than down. The wing is on the bottom, below the motor and tailplane. If these were not the case you could not fly it - you wouldn't know which way up it was in the air (see early 80's design called Balus - the flying banana). Landing would be a problem too (see Balus).

If that's not enough, a wing with zero camber requires a positive angle of attack to generate lift, so for level flight a pattern plane needs to be trimmed to overcome the pitching moment generated by the positive AA. This of course is not right for inverted flight, so the pilot must hold down elevator inverted. Furthermore, since the wings are at a +ve AA it cannot be expected that the aileron response will be symmetric, and this is exactly what practical experience proves. Even vertical, at zero AA, aileron response is not symmetric because the aircraft's rigging is tuned for upright flight at +ve AA.

To further complicate matters, GRAVITY conspires to force us to tune the aircraft so that it is not symmetric.

So what IS the true motivation for differential ailerons. On a pattern plane it's simple - they fly better. Why - who cares?, but most likely due to the asymmetric response resulting from the aircraft being tuned for upright level flight.

jfitter, you're right when you say that pattern airplanes are non symmetrical, obviously. The good designer tries to make decisions in order to make the airplane more neutral in every flight configuration. I think that these decisions make an airplane a good pattern one or not...

What I really wanted is to KNOW THE REASON why differentials are needed...

Thank you all for your replies!

Very straightforward.

Let's start with a wing having no camber, flying along minding it's own business at 2deg angle of attack, flying level and supporting the weight of the airplane.

The pilot decides to do a roll to the right. The right hand side aileron deflects up, changing the camber of the right side wing and effectively reducing its angle of attack to (say) zero. The left hand side aileron deflects down and the effective AA of the left side wing increases to (say) 4deg.

Induced drag is nature's "price to pay" for generating lift. There are no "free lunches" in nature, and the result of generating more lift is more induced drag. The right hand wing, which has its AA reduced, generates less lift than before and so its induced drag is reduced. The left hand wing, which has its AA increased, generates more induced drag.

We see from this that even if there was no nett change in total drag on the wing, there is a redistribution of drag towards the rising wing (left hand side). This will cause a yaw of the aircraft in the direction opposite to the intended turn.

This simple explanation ignores the effects of spanwise flows, vortices, fuselage interactions, prop wash, etc. etc. Purists stay away!!

How to fix it (in plain english)?

We see that the problem is the up-going aileron which reduced the lift on that half wing and consequently the drag, causing a drag asymmetry. If we made this aileron go up by a large deflection then two things would happen. Firstly, because the aileron is an inefficient camber changing device, it will add extra drag beyond that of the other aileron. Second, if the deflection is large enough, we may be able to get the wing to develop substantial negative lift (ie. push down) and consequently generate lots of drag. Both of these effects will tend to neutralize the drag asymmetry.

So, the method is to arrange for the up-going aileron to go up further than the down-going aileron goes down (jeeeez, there must have been a better way to say that!!).

There is a problem. It doesn't work when the airplane is inverted. Well it does work, but it works against us. See, the problem exists because of gravity. In level flight the wing starts at some finite positive AA, because it needs to support the weight of the airplane. If it were not for this then there would be no asymmetry and differential would not be required.

The best solution is to eliminate gravity. I'll let you know when I succeed!!

Oh!! Almost forgot.

Instead of differential, use coordinated rudder when starting to roll. That works anyway up - just don't push the rudder the wrong way.

jfitter, thanks for the explaination! But this is IMHO an explaination of the inverse yaw. As you say, in inverted flight differentials act against a good roll (if I understood correctly your post...did I?

But the contraddiction is that in pattern airplanes we need differentials and they help ALSO in inverted flight rolls.
I'm pretty sure this is true: in my pattern plane, if I remove differentials rolling capabilities are bad even for rolls starting from inverted flight!

jfitter appears to have the most accurate and informative post. Gravity is always toward the earth and the wing must lift opposite to gravity. When the plane is upright lift is generated on the 'upper surface' of the wing and aileron differential is normally set for this situation. If the plane is inverted lift must be generated on the 'lower surface' of the wing and differential should be readjusted for this condition but rarely is.

I would assume that the benefit of aileron differential would be most notable when flying 'slower' because the wing must generate lift at a higher angle of attack, maximizing the influence of aileron differential throw. Conversely, flying at higher speeds would probaby minimize the influence. But in both conditions, properly set aileron differential is required to some degree by the purist.

I don't get it, the whole thread [X(]...

Why in the trim charts used to trim IMAC planes and Pattern planes they specify to test for aileron differential while doing rolls on a VERTICAL line. If the roll is not axial while going vertically, defferential must be added or removed.

The part I don't get is the use of trim charts is to make the aircraft fly BETTER, not worse [:-].

I think that gravity has nothing to do with aileron differential.

Differential on many pattern planes is required or your model will not roll axially.....

I have never noticed a negative effect on inverted flight, or vertical rolling manuvers...in fact if you dont use it on some models the heading deviation during 45 degree/vertical climbing rolls is very significant.

No theoretical reason as to why, just 1000's of flights of experience doing it.....

3D joy, I definitely AGREE with you!

This gives more credit to the possible explaination given above, which I think it is correct (but don't understand why), i.e. the difference between thrust line - wing line.

Hmmmmm --- maybe I'll put a bit of differential into my models.

Usually only takes 1/2-1 degree difference in throw to get a solid improvement.

I put 5-10% in my IMAC planes usually. Don't know the amount in degrees though...

Jfitter's explanation of the reason for aileron differential is correct for almost all situations. It's used to compensate for adverse yaw when ailerons are applied.

The reason you start adding aileron differential to different aerobatic aircraft is not necessarily due to some adverse yaw, but to compensate for a bunch of other adverse affects. Aircraft, because they must fly in air and support their weight against gravity have a large variety of asymmetries. You can't make a model act symmetrically about all axes at all times. Thus, the various ways to compensate for them.

Sometimes, you can use aileron differntial, sometimes you have to trim both ailerons up...or down...from the regular in-trail position, you have to add mixing to fix knife-edge behavior, alter thrust lines, move the canopy, and on and on and on. Remember how many flights it took to get your Pattern model dialed-in to just the way you needed it so that it would handle and present properly? That's working out all of the little asymmetries you wound up building into it in spite of the high level of care and craftsmanship you put into it.

All of this is why aircraft geared to a certain event tend to start looking alike. The top fliers start finding out which aircraft configuration best performs the given tasks...the designs start trending towards a certain optimum. When Pattern changed from the high-speed, one-maneuver-per-pass style to the early turnaround style, it didn't take very long before the models changed dramatically. Designed for one kind of flight, the old models just didn't do the new sequence particularly well.

It comes down to you do what you have to do to get the model to fly the way you want....just build it absolutely straight first...that eliminates a host of problems!

bax

(edited for typos)