Hi pattern guys!

I can't get a straight answer to this question, perhaps you could help me!

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 more 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. Needless to say that differential ailerons do help when doing rolls, and not the opposite!

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!

Aileron differential is deflects the downward moving aileron less to help alleviate the adverse yaw caused by the increased lift of that wing. It will only work either right side up OR upside down due to the aileron orientation in the given flight attitude. If you dial in aileron differential for flying straight and level right side up as soon as you roll inverted the down aileron just became the up aileron and you have increased the yaw.

I hope that helps.

BTW - I personally don't use differential on any of my planes.

The difference with a symetrical airfoil is angle of attack....and angle of incidence if it isn't installed at 0 degrees. Some planes call for some amount of positive incidence.

Another reason is that the trust line of the engine is normally not at the center of the wing. As is the center of gravity, this is also normally not at the center of the wing. The plane however will roll around a line combined by the the CG and the trust line. The further the wings is away from this line, the more differential is needed to roll axial.

Compare a real low-wing plane (CAP) or a high wing (Decathlon) plane with a symatrical wing. The need a reverse differntial to roll axial. The low wing plane needs more down then up aileron, the highwing needs more up then down.

At least, that is my understanding.

I always thought it had to do with the fact that lift coefficients were changed when the aileron deflected, meaning the downward moving aileron INCREASED lift, and the upward moving increased DRAG, thus causing the yaw. ALthough most pattern airplanes these day have the wing location on the fuse correct to offset this, years ago, it was a real problem (lower wing placement on fuse increases need for differential)

BUT I could be wrong, here....

Part of the problem is the design of the airplane. If everything was set at zero that won't reduce the need for differential. The shape of the wings in relation to the ailerons and the shape of the ailerons are the main factors, but also the tail moments and fuse design.

Thank you all for the replies!

I think wvr is right about thrust line - wing line difference: once I heard a top pattern pilot giving this explaination.

But I can't figure out WHY such a difference would need differentials. Also, my pattern plane has a quite low wing placement and still need "positive" differentials, not "negative" (I mean that the upward movement must be greater than the downward, in order to do axial rolls)

So it seems the right explaination is missing...

For those of you interested in the topic, I posted my question to the Aerodynamic forum too.

I was said that it is used because of different angles of attack on the different wings. Since the airplane flies always at some positive AOA or else it would descend (with symmetrical airfoil), therefor the rising wing would have a positive number plus something and the descending wing something minus the same amount (which is less then the rising wing) and the result- the rising wing produces more lift and drag.
What I don't understand is why Petec says it would be different in inverted flight, since I at least don't change the airplane's trim in inverted flight.

Yes you do, you must put some down elevator to fly inverted...

Maybe.
I've ran C.G. far enough rearward on a 3d plane to negate down elevator on inverted. Of coarse, then I needed LOTS of down elevator on landing. And, every upright stall became a nasty snap.

I know one guy who did this on a pattern plane! Said he didn't like it because if he hit some turbulence then plane would just continue on its new path as there was very little natural stability.

Is it not simply because the lifting wing (down aileron) is producing more drag therefore the up going aileron has to move more to balance the drag. However, on my Synergy I need a load of differential to get it to roll axially. It rolls great from inverted and that is with the now down going aileron moving significantly more than the now up going aileron. Weird eh. If someone knows why this is then I’d like to know.

Angus

Angus, thanks for your post! Now we have a confirmation that even on modern pattern differrentials are still needed to reach good roll capabilities.

Just for curiosity, how much do you use on your Sinergy?

These day I'm thinking again and again to the problem, and I have noticed an intersting thing that someone in the thread has mis-understood. That is: both on airplanes with top wing and on airplanes with low wing aileron differentials are needed in the same direction (I mean, less downward than upward). Interesting!

The only case where differentials are needed in opposite direction (more down than up) seems to be the top-hinge aileron case...

To me this thought seems to go against the thesis of wing-thrust line difference: in fact, in top-wing planes the wing is in the top of propeller wash, in low-wing planes the wash goes on the top of the wing... so it seems that this explaination is also wrong???

Thank you all for your replies... but it seems we need an aero expert here! Several days ago I posted the question to George Hicks, on his forum... but it seems George is away from a computer!!! I will post a message in case of a reply...

I use +22% on my JR 10X. In terms of movement thats (rates off) roughly 45mm up and 37mm down.

My Alliance is only +7%

Angus

Guys,
this problem is making me crazy ...

Perhaps I have done a little step forward toward the solution, or at least we can say that thrust line is not a player in the problem.

Infact, some pilots suggest to check differentials on a vertical dive WITHOUT throttle. I tried with my plane, and of course I found that even these diving rolls were good as a climbing one with full throttle...

So it seems that thrust is not an issue.

Angus, I was thinking about your differentials difference in your Synergy vs your Alliance.

Looking at the some pictures of these planes, it seems that there is not a great difference in terms of thrust-wing line. Can you confirm this?

If so, this again proves that thrust line is not the explaination

PS: two pictures of these planes can be found for example here: http://www.singahobby.com/mkp02.html

Guys,
perhaps I got the solution. I think wing incidence is the motivation. Let me explain why.

Let's imagine a wing with a great AoA needed to fly straight level (great so that we can visualize it into our mind). This could be the case of a nose heavy airplane. Well, in such a configuration, suppose we apply full ailerons in one direction. Now, the aileron that moves downward "sees" a laminar air flow, whereas the aileron that moves upward "sees" a little turbolent air flow (the more incidence, the more turbolent as air flow will leave wing surface at increasing AoA).
So, if I am correct up to now, the solution is simple: the downward generate more lift and hence more drag, so its throw must be reduced. So far, so good.

Now let's imagine an inverted flight condition. The theory remains the same: even in this situation, the upward aileron (which previously was moving downward) sees a laminar flow so its throw must be reduced.

If these theory is correct, we can say that greater wing incidence will make the need for greater differetials.

A little check can be done now: Angus, you told us that in your Synergy you have 22% diff. and only 7% in your Alliance. What can you say about wing incidence of your planes? Is perhaps greater the one of Synergy (cosidering of course also elevator trim)?

Also, I think that wing airfoil must be considered too. Perhaps an airfoil produces more turbolence at low AoA (tipically required to do a level flight, i.e. 0.5-1 degree) than other, so the comparison between two different planes is not significant...

What do you think?

Wing Incidence: My Alliance is as per the fuz moulding. The Synergy has been packed down by 3mm at the leading edge.

The Synergy has got very thin tips on mine though (just under 8%) so I don't know if it's that thats causing it to need more differential. Who knows.

Angus

Even if we think about symmetrical airfoils, this is very relative, the curvature of an airfoil is a physical measure, you can say it is identical on both sides, but in order to generate lift, an airfoil needs to have more curvature on the top of the wing(I mean surface looking up, so it doesn't matter if it flies inverted), because this is a fixed value then AOA(angle of attack) will increase the curvature, so even if your wing and stab are at 0deg on a symmetrical airfoil, in flight it will be positive AOA.

So inverted or upright will need the same differential, but during verticals, the prop wash or P factor also affects the differential, also the engine torque might affect this, there are airplanes that depending on the design, it will not react as desired, so, some times after differential has been adjusted to one side you need to adjust to the opposite direction with ATV.

When adjusting differential be careful not to use too much that it will actually change the heading.

In a few words, don't stress, it is not magical or perfect, just adjust until you get the best roll reaction possible without changing heading(you can see this during verticals) and to the usual upright rolls.

I hope this helps.