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Old Dec 22, 2014, 01:50 PM
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United States, AL, Huntsville
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Adverse Yaw - Aileron Differential or Rudder?

I'm just about to build a sailplane and it's my first full house build. I've been reading up on the setup and I'm interested to find that most people add aileron differential to combat adverse yaw. When flying full scale (I have a PPL) you always use rudder to do that and I'm wondering which way is better? Or does it even matter? Either is straightforward on my Tx (Futaba 14SG) so I'm looking for pros and cons of each approach. Thanks.
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Old Dec 22, 2014, 02:20 PM
R2R
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On my full house planes, in cruise mode, I use both. In thermal mode, I increase the rudder coupling and decrease differential.
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Old Dec 22, 2014, 03:06 PM
Kurt Zimmerman ≡LSF 4461≡
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Montrose, NY
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Although I have great latitude with my radio I still like to fly 2 hands, one on airerons and the other on the rudder. I do have them mixed for my launching/landing modes and have the ability to disable it during the thermal mode.

Kurt
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Old Dec 22, 2014, 03:36 PM
Scott J.
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Rudder or Differential?

If you search RCG you can find a dialogue with Dr. Drela on this topic. Dr. Drela will mention coupled rudder or diffrential, not both. He goes further to say that rudder is prefered to differential since it causes lift while differential causes drag. I would add when you start flying a new plane, begin with no mix and then systematically try various settings /mixes. Hope that is helpful.

S
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Old Dec 22, 2014, 06:28 PM
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SJ, first, they can both help you but it can be dependent on design and airfoil. Drela's airfoils use much less differential than lets say Epplers or Selig. When I set up a full house ship, I try to get the diff. so that I can give a uncoupled aileron input and get a fairly decent axial roll, and that is with a a bit of speed. Now, as you get slower, coordinated rudder becomes more important and that is not using a mix but learning to fly with your left hand. Yes, I will have some rudder mixed in cruise and thermal mode, but a minor amount as a starting point, and then fly coordinated with rudder input. As you know, you know that flying full scale, that once you start a bank and have established a turn, you will be holding opposite aileron and direct rudder, same thing here. In very lite conditions, I will fly rudder only at times even to allow the wing to stay in the profile, cause every input to ailerons when it is lite makes you loose time.

Marc
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Old Dec 23, 2014, 11:10 AM
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That's really helpful, guys - thanks. I must admit that I'm in two minds as to whether to mix in any "aids" rather than just flying with my left hand. Sometimes I think that all the cool mixes you can introduce with a computerized Tx actually detract from the simple essence of flying the ship!

I'll have a look for Drela's discussion, but my full scale flying has rudder somewhat ingrained so I can't help thinking of aileron differential as a "cheat" that you could implement mechanically before computerized Tx's allowed you to do it "properly." I'm thinking now that I might put a rudder mix on a switch so that I can flip it in if I just want to cruise with my brain on other things.
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Old Dec 23, 2014, 11:31 AM
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Do not think of diff. As cheating. I am guessing that full size sailplanes have it and so do most GA aircraft.

Marc
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Old Dec 23, 2014, 11:46 AM
Peter, irl
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It was flying a Cub (RC, not full scale) that helped me get my head around adverse yaw and turn coordination. As do all my fixed-wing RCs (except the flying wings) it has differential ailerons. In my opinion this is not "cheating" or artificial in any way. Some wings just don't like much down-aileron. Besides, differential aileron could theoretically be achieved with a non-computerized radio and a single servo (with a little geometric wizardry).

Mixes like flap-->elv or throt-->elv are sometimes required to get a model flying well, but ail-->rud or rud-->ail are a bit cheat-ey (again, just my opinion here). If you turn with rudder, you probably need to maintain rudder deflection longer than you'd need to give aileron inputs to keep your desired bank. If you were running rud-->ail, your roll control would be going to keeping your desired bank (as it should) *and* fighting against the rud-->ail mix (detracting from the flight experience).

So even on one of the worst planes for adverse yaw (the Cub), I prefer (even enjoy) flying without any ail/rud mixing. It helped me develop the skills to execute nice, coordinated thermal turns on my 4-ch DLGs, and to recognize and correct little bits of slip and skid on any rudder-equipped plane. Nothing more fun than carving it up in a bubble of lift

If you want to fly on one stick, there are lots of choices in 2-channel (Radian-like or some RES without the 'S') or even bank-n-yank (elevator and aileron). But if you're going to have all three primary flight controls (as you would in a full-house), might as well fly them, no?
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Old Dec 23, 2014, 11:48 AM
Peter, irl
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Is aileron diff cheating? No! Sailplane airfoils are not symmetrical, so why should your aileron deflection be?
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Old Dec 23, 2014, 12:39 PM
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Ya, I use to use a lot of diff n coupling. After reading the articles zephirus mentioned, I reduced to minimum as used my left thumb more. More work yet not only fun but much more efficient especially in light air.

Jared
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Old Dec 23, 2014, 01:26 PM
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Quote:
Originally Posted by Itchy Thumbs View Post
But if you're going to have all three primary flight controls (as you would in a full-house), might as well fly them, no?
Yes, that's my thinking. It' not like I don't already know how to fly that way, and to me it's more fun to face the challenge of coordinating the controls than relying on mixes.
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Old Dec 23, 2014, 01:40 PM
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Quote:
Originally Posted by Itchy Thumbs View Post
Is aileron diff cheating? No! Sailplane airfoils are not symmetrical, so why should your aileron deflection be?
Agreed, but that's not quite where I'm coming from. When you input an aileron command you certainly want the wings to produce equal and opposite forces so that the net effect is in roll only. If that requires differential aileron input then that's absolutely needed. But that's about wing design and balancing moments.

Adverse yaw in a turn is a bit different though - it's not about wing asymmetries so much as flow asymmetries in the turn and that needs to be corrected for. In my full-scale experience that is done using rudder - you step on the ball to stay coordinated. So my original question was, isn't that the right way to do it for RC too?

Or am I wrong about the source of the issue?
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Old Dec 24, 2014, 09:40 AM
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There is no substitute for the rudder stick ... ask a heli guy
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Old Dec 24, 2014, 07:52 PM
volare est vivere
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hi there from Toledo

Yes, SJBrit, you are correct. The subtle differences involved can turn into longer flights and higher duration scores. It is that last 2% of performance where winners and losers are defined during a contest. But still it requires practice, lots of practice, until it becomes second nature.

ciao -rjf
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Old Mar 23, 2015, 10:09 AM
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Just to follow up on this thread after finishing this build and getting some hours of flying in (after a major rebuild - doh!): I am very happy flying without an AIL-->RUD mix. It feels very natural to me and as Marc said further up this thread, I'm using a lot of opposite aileron to maintain the turn radius - having the rudder automatically coordinate with opposite aileron just seems like a very bad idea.
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