Folks:
Is there consensus on aileron differential for aerobatic aircraft? I set up my trainer (HS 60) with a bit of mechanical differential, and Sig 4*60 with a bit of computer differential, due to the dihedral in wings of both aircraft, and the fact that they are both generally upright!. I'm now building an Extra 300s, which by nature I may fly inverted more, which would make differential dialed in for upright flight counterproductive for inverted flight.

If no differential, do most of you use aileron->rudder mix, or is adverse yaw in a capable aircraft not so much of an issue?

Thanks in advance

Bill

Consensus? Probably not. Theory of the value of it's application.... yeah.........

It's really not necessary for most aerobatic models. They're usually designed to be as neutral as possible. Not to have an UP or DOWN preference. They've got symmetrical wings for example, and aileron differential is mostly used to help dihedral winged airplanes with cambered airfoils. As you move from those types to the neutral ones, the need for differential lessens to zero.

All the Caps, Extras, etc I've had benefitted from a bit of mixing to tone down the rudder couples, but I've never had one need aileron differential. They all had symmetrical profiles however.

Personally, I've never put a mix into the radio for adverse yaw from the above reasons, but will trim with the radio in a heartbeat to tone down rudder couple. And that's probably because of the types of models and how I use them. It just seems appropriate to do it that way.

I have seen an Ultimate Biplane fly with aileron differential, and it did not make the rolls very pretty. They weren't very axial. I can only assume that because one aileron (or sets of ailerons in this case) deflects more, it pushes that wing with greater force. Thus moving the center of rotation. The differential was removed, and the rolls tightened up and looked much better. Also I have never used any aileron to rudder mixing in any of my aerobatic planes, but I don't fly any competitions either. I think they fly fine without it. I have used it in some non aerobatic models though, so I know the advantage of using it.

Competition pilot here. We don't fly Airl to Rudder mix (Airl is the master, rudder is the slave). That mix might be for warbirds or scale personal aircraft like what you'd see at an FBO. It's used mostly for procedure turns. However, that mix will affect so many other manuevers that your Extra 300 is able to do. As an example, most new pilots want to learn a slow roll. With that mix, your plane will have the rudder deflected and cause your tail to wobble in roll. Your roll will look like a snap roll. Or, on an up line if you roll your plane your heading will change because your rudder deflected during the roll. None of the top guys are running that mix.

Airleron differential..... Let me say that the Extra 300 was born to fly aerobatics, an I'm assuming that you are building it to do some kind of freestyle type flying with rolls, loops, spins, 3D, snaps, 4-point rolls, precision flying, etc. After all, if you wanted to just fly around and land, you'd stick with the 4star-60. So off to "aerobatics 101" we go. Differential on this type of plane is used to keep the plane from falling off heading during a full roll, not just a 45 degree bank turn. Here's how to trim out the plane for the proper differential, and so it will help you fly better on all lines (up lines, down lines, straight, and 45 degrees up). Start out with NO differential at all (airleron goes up the same amount as down, measure and get it exact). Get the plane up to speed and pull a radius to a vertical up line, as in going into a hammer head. Be sure the plane is straight up. Do one full roll right , the plane will need a rudder correction to get the nose heading back straight up. Go ahead and give that rudder command and then land. After landing turn the plane on and deflect the airlerons to the right. Take the right airleron and add throw at max deflection , take the left and subtract throw. Add/ Subtract about 5% to start and then retest in the air. Repeat until the plane will roll on a vertical upline without needing any rudder correction to stay on heading. The differential will take away the need to give a rudder correction after rolling. In aerobatics you are looking for the plane to roll, AND ONLY ROLL when the airlerons are put in. Differential allows for roll only.

May I suggest hooking up with a pattern or IMAC style pilot at your local field for other trimming ideas. The above is just one trimming suggestion. There's a lot more to get your new Extra completely "in trim".

Hope this helps, Danny.

Danny
Thank you - very helpful post

The only planes I have that I need aileron differential on are the Piper Cub Look Alikes

If you use the procedure describe above by Danny, you will see that virtuallyall models need at least a bit of differential, and some need a lot in order to roll axially.

Keep in mind that when ailerons are deflected they initiate not only a roll but also a bit of yaw opposite the direction of the roll. Aileron differential compensates for this yaw by creating a little - or a lot - more drag on the "up" aileron, pulling the nose of the model back to the intended heading.

I have a Seagull Super Decathlon (some similarities with Cub) which needs 50% aileron differential, and even then aileron rolls are not quite axial. This is a straight wing (no dihedral) with semi-symmetrical airfoil. There are three reasons I can think of for this flight characteristic: 1) It is a high-wing design, so the point of rotation is well below the wing; 2) the fuselage has a huge amount of side area, which exacerbates the yaw component when it is presented; and 3) the model has a very short tail moment which reduces its tracking ability.

I'm sure there are other (perhaps many?) such as the wing's AOA, nose moment, CG, etc. etc., but it is easy to see that even the best design probably needs at least a teensy bit of differential.

If you use the procedure describe above by Danny, you will see that virtuallyall models need at least a bit of differential, and some need a lot in order to roll axially.

Keep in mind that when ailerons are deflected they initiate not only a roll but also a bit of yaw opposite the direction of the roll. Aileron differential compensates for this yaw by creating a little - or a lot - more drag on the "up" aileron, pulling the nose of the model back to the intended heading.

I have a Seagull Super Decathlon (some similarities with Cub) which needs 50% aileron differential, and even then aileron rolls are not quite axial. This is a straight wing (no dihedral) with semi-symmetrical airfoil. There are three reasons I can think of for this flight characteristic: 1) It is a high-wing design, so the point of rotation is well below the wing; 2) the fuselage has a huge amount of side area, which exacerbates the yaw component when it is presented; and 3) the model has a very short tail moment which reduces its tracking ability.

I'm sure there are other (perhaps many?) such as the wing's AOA, nose moment, CG, etc. etc., but it is easy to see that even the best design probably needs at least a teensy bit of differential.

Interesting opinions
 a really lightly loaded model  needs none and for really complex aerobatics - differential can get in your way

  It is easy for  one's own flying techniques  and basic cg setups to  mask roll errors 
 for example :
some guys hold in a smidgen of  up or down elevator  all the  time  in upright flight
 seen it over  -n over  etc.. when doing a roll  - the  elevator trim causes the roll to look goofy  . 
they fiddle with differential trying to  get  the  wallow out of the  roll.
  The  ATV setup on some radios will  affect the  throw (up vs down )and cause  aileron response  which again appears to need  differential.
 The reason for  differential in the  first place - was  to compensate for  the  yaw -which occurred on just about any  full size sport plane  and  the  rudder  /aileron blending was used to  fix this  skidding
 Now here is  a neat trick if you have not tried it
 On a design with as much fuselage side area  (effective side area) as  wing area - you don't use  ailerons to bank n turn - only to roll . so  NO differential wanted or needed
 to turn these  you just  add rudder  - that's it 
 the  fuselage  provides  the  lift normally lost in a turn
 I know this will cause  some  to  faint dead away  in disbelief but  -it does work.

I used to keep a little down trim in the elevator to keep a little pressure on the stick in level flight, which allegedly kept the model flat when exiting from loops, and to reduce the amount of push when inverted.

But now I use neutral elevator. I figured out that if the plane was ballooning it was because either the plane wasn't set up properly or I wasn't relaxing the stick at the proper time. And I adjust CG so that inverted push is minimal, and put the fuel tank on CG to reduce elevatortrim changes.

I use (to a certain extent) the technique you mention to turn the Decathlon. I initiate a bank with the ailerons, but use only rudder and perhaps a bit of opposite aileron to keep the wings at the desired bank angle. With the amount of differential I have dialed in, the plane needs no elevator to maintain altitude.

Because of its large amount of fuselage side area, the Decathlon requires only a breath of rudder for knife edge, but a ton of opposite aileron.

It worked wonders to clean up the rolls with a high wing, semi-symetric airfoiled Midwest Aerosport model. The aileron pushrods were positioned at 10:00 & 2:00. Best advice I ever got at the old club field.

Anyone else notice a trend here?  All the models that benifited from some differential have a few common charactaristics.  A flat bottom or otherwise cambered airfoil and often are high wing designs.  The 4-40 is an exception on the high wing issue but it has the cambered "lifting" airfoil".  It also had a longer aspect ratio wings than most aerobatic models.  It's also a model that tends to be flown a little slower on most occasions.  All those issues add up to contribute to a more noticable adverse yaw issue that is helped by adding some differential.

Another little factor.  Adverse yaw shows up more when the model is flown at higher lift coefficients.  Sort of like high wing trainers by students that are learning.  And it'll also show up when you ask the down aileron wing to suddenly do a lot of work.  Like with extra throw and full travel commands to perform a quick roll.

But the Laser and other such designs use a symetrical airfoil  So when both ailerons alter their angle plus and minus the airfoil is still symetrical from side to side and they both just altered their angles of attack by almost the same amount other than the angle of attack for the model to stay in the air.  So they pretty much generate the same drag for the same travel be it positive or negative.

Now near to the stall adverse yaw will still be an issue and some rudder added in will help.  But it's not a linear problem.  At lower lift coefficients, like when flying fast, you won't need any mixed in rudder because the adverse yaw will be so minimal as to not be noticable.  So the effect will be to hurt the maneuver at that time.