Differential throw on ailerons--how important?

[Lon Enloe]

I'm building my first aileron trainer, and I've read in several venues that differential throw on the ailerons is desirable. How much of a benefit is it? Critical? Important? Nice to have, but not a show-stopper?

[vintage1]

It's not at much of an issue on a typical trainer.

On some biplanes its 'as much as you can get'

The less inherent stabilty the model has the less important it is to have the differential.

[Andy W]

It depends on the model, there's no one value that's correct for all models, even models of a similar style..
..a

[shaneyee]

maybe he should have asked "how will i know that my plane needs differential?"

[vintage1]

When aplication of aileron results in opposite yaw and zero or negative rolling moment?

[Lon Enloe]

I went ahead and built the wing without differential throw, finished it this afternoon, got in a couple of flights before sundown. Ailerons worked fine--no wierd cross-coupling that I could detect with my admittedly limited skills. The second flight was a real adventure (see my "crazy but true story" in the Beginner Training Area forum, if you're amused) but that wasn't the ailerons' fault--they probably saved the day.

[adam_one]

Flat bottom wings are more likely to get adverse yaw due to aileron drag than symmetrical ones.

[PGR]

Every 5-channel (flaperons) plane I own benefits from aileron differential. The most noticeable benefit is axial aileron rolls but the differential basically cleaned up the entire flight envelope with little or no sacrifice of aileron authority or roll rate. I've also messed with mechanical differential on some of my planes with a single aileron servo but it's hard to beat the ability to fine tune that dual servos and a computer radio offer.

Pete

[Lon Enloe]

Pete,

Are you using differential throw on symmetrical as well as flat-bottomed airfoils?

[Andy W]

It's still not that simple.

Sailplanes (full-scale) have very efficient wings, yet still require significant rudder input to initiate turns.

Don't look for a single answer. Fly your model, and see if it needs it.
..a

[PGR]

Are you using differential throw on symmetrical as well as flat-bottomed airfoils?Flat-bottomed airfoils? Whazzat?

;)

Pete

[Lon Enloe]

I agree with Pete, nothing beats trying it out--that's the reason I'm building my own designs, so I can learn by doing--so I put in an 80%/20% differential throw in my linkage, gave it a try today, and it felt a bit cleaner in the turns. It was probably more wind than I should have been flying in, but it still felt comfortable. Rolls seemed a bit better, too, although I can't really say that they're pretty. At least I can get the ship out of a roll generally pointed in the same direction it was going when I started, rather than 90 degrees away! :rolleyes:

I think I understand the effect of a downward-pointing aileron's leading to adverse yaw--basically, the drag on that side is trying to turn the plane in the wrong direction from where you're trying to make it go. This makes sense for a plane that spends most of its time right-side-up.

What about a symmetrical-airfoil, no-dihedral ship--don't you want equal throws to let it axial roll without pulling one way or the other? And, if you're inverted, doesn't the argument of "more up than down" on the differential throw go right out the window?

[adam_one]

What about a symmetrical-airfoil, no-dihedral ship--don't you want equal throws to let it axial roll without pulling one way or the other? Symmetrical airfoils seldom use differential ailerons.
Flat bottom wings are more likely to get adverse yaw because the aileron that moves upward has less effectiveness because it's acting in the disturbed flow from the wings' upper side, whereas the one that moves downward increases the drag.
This is more likely to happen during take-off and/or landing approach when the airspeed and AOA may be near stall.

And, if you're inverted, doesn't the argument of "more up than down" on the differential throw go right out the window?Yes that's true, but you are not supposed to land or take-off inverted… :)

[Mike Taylor]

A more reasonable way to describe it is that the downward-moving aileron creates more lift and therefore more drag. The upward-moving aileron is reducing lift and drag. The draggiest side is on the outside.

Adding more up deflection than down induces more drag by hanging the inside surface out father into the airflow... I believe that un-symetrical lift & drag work only when right side up however.

[adam_one]

A more reasonable way to describe it is that the downward-moving aileron creates more lift and therefore more drag. The upward-moving aileron is reducing lift and drag. The draggiest side is on the outside.That's true in case of a flat bottom wing.
But with a symmetrical airfoil the aileron that moves upward causes the same drag as the aileron that moves downward (in level flight).

[Mike Taylor]

Not neccessarily so, Adam. Depending on the AOA, when you deflect ailerons, one side will cause more lift and more drag, the other will produce less lift and less drag. Creating lift creates drag - its not just sticking the surface out that matters...

[PGR]

Symmetrical airfoils seldom use differential ailerons.I'm using computer-mix aileron differential on my BiG Bubble, my Dystraction, my Little Banchee E-3D, and my Mamba. I'm using mechanical aileron differential on my Formosa and my Straight Edge. The aileron differential has improved the flight characteristics of each and every one despite the fact that they all have symmetrical airfoils.

Pete

[adam_one]

Not neccessarily so, Adam. Depending on the AOA, when you deflect ailerons, one side will cause more lift and more drag, the other will produce less lift and less drag. Yes, that's why I wrote "in level flight".

I'm using computer-mix aileron differential on my BiG Bubble, my Dystraction, my Little Banchee E-3D, and my Mamba. I'm using mechanical aileron differential on my Formosa and my Straight Edge. The aileron differential has improved the flight characteristics of each and every one despite the fact that they all have symmetrical airfoils.
Note that I wrote "seldom used" because fully symmetrical airfoils are often used in aerobatics where the pilot wants the plane to behave the same way both upright and inverted.
When you fly inverted the differential gives the opposite effect.

[electroboy]

Yes, that's why I wrote "in level flight".



But initiating roll is not "in level flight". Level flight is disturbed.

^camber (down aileron)= ^ lift.
^lift=^drag.
differential drag on a long moment arm = adverse yaw.

At least thats how my FAA written expressed it.

I hafta agree with Mike. ;)

[adam_one]

Well, how much the level flight is disturbed depends on how your plane rolls.
With a near “perfect” axial roll and a low aspect ratio the drag difference may be insignificant.
That’s why symmetrical airfoils used for aerobatics seldom have differential ailerons.

The benefit of differential ailerons depends much on which airplane type you have.
For instance, a glider with high aspect ratio is more likely to get adverse yaw than a plane with a low aspect ratio. ;)