Originally Posted by vespa

Yeah the pitch influence is another factor. Aileron differential is essentially the addition of reflex across the entire wing and as such it has a pitch effect.

Target, your reduction of rudder mix with speed makes sense but I can't think of any advantage in increasing differential. What is the behavior that inspires that?

My thinking and experience from the slope makes me feel that when flying slower, the overall lift needed from the wing is greater. As you have written above in the first passage, more differential kills more lift from the wing through this pitch effect.
So, when flying slower and asking for more lift, less diff, but more needed rudder to keep things straight...
When flying faster, the wing is already at a lower angle of attack, and doesn't need to make as much lift, so a little extra diff seems like less of a penalty, and means less rudder mix necessary.

I cannot say if this works or not, especially for the reasons I state; its just a feeling I have, and that is the way my flight modes are generally setup.

If I can prove or be proven to that it should be the other way, I will for certainly consider changing my setup.

R,
Target

Yes, you're right about not wanting to reflex the wing (by way of differential) when flying slower (at high CL) but you also don't want to deflect an aileron down too far either because it may cause the wing to stall. Hence my suggestion of just enough diff to avoid the stall, but no more than that.

"extra diff seems like less of a penalty, and means less rudder mix necessary."

This seems to indicate that you view rudder mix as an inferior means of yaw control and try to minimize it in favor of differential when in fact, the opposite is true. Rudder and differential accomplish exactly the same task, upright or inverted, but the rudder does so far more efficiently. As usual, don't take my word for it, take the good doctor's. As mentioned with tip stalls, reflex, nonlinearities, etc. there may be a noticeable handling difference between a diff-dominant setup and a rudder-dominant setup so it could be a pleasing, jarring, or disturbing change and a good balance must be found thru iteration since there are few cases where some combination of both is not preferred.

Darwin: When an aileron rises slightly the drag of that wing is reduced (reflex) and the result is more adverse yaw. Thus you can see how aileron differential actually aggravates this problem *causing* adverse yaw for small deflections since it's not until the aileron has gone up far enough to become a drag brake that a proverse yaw force is finally produced. Thus your solution of reverse expo on the rudder mix agrees perfectly with theory.

No, I don't think that aileron diff is a better means of yaw control, but I do think that when you are talking about V-tails (which I am, when speaking about slope planes), using a lot of rudder mix is a bad thing when you can use some diff instead.

Also, when you cut a sentence in half, the full meaning as intended is NOT there...

What I said (and meant).
"When flying faster, the wing is already at a lower angle of attack, and doesn't need to make as much lift, so a little extra diff seems like less of a penalty,(less penalty flying faster than it is when flying slower and trying to fly closer to max lift) and (as an added benefit) means less rudder mix necessary".
That is not the same as just the second half of that sentence......


BTW, an aileron rising doesn't cause less drag. It makes that wing come forward because the lift vector of that wing is forward of vertical, and the lift vector of the wing with the down-going aileron is rotated aft. That is what makes the adverse yaw, according to Drela.

R,
Target

Differential should never be used as the primary method of reducing adverse yaw. Rudder mix first, then diff. if necessary. V tails, T tails and crossies.

Mike

Target, I agree that when flying faster (low CL) less "stuff" is needed and understood that part of your statement. What I highlighted was your mention of "extra diff" and "less rudder", which seemed to indicate a preference toward differential even when outside of the stall regime. I didn't intend to take your statement out of context but even with your clarification I still see this interpretation. Again, the partial sentence in reference to V-tails seems to convey the same point: "...rudder mix is a bad thing when you can use some diff instead." To which I'll note that while V-tails are certainly less efficient at producing yaw forces than cross tails they are still more efficient than aileron differential which acts as both a drag brake and a spoiler. And yes, an ail-rud mix with a V-tail produces a roll force in the opposite direction, but still, it's more efficient than scrambling the wing's lift distribution and deploying speed brakes.

A rising aileron most certainly reduces drag and you know this -- that's why I reminded you with the "(reflex)" reference. Drela points out that the major cause of adverse yaw is usually lift vector rotation, much to our surprise, but this doesn't mean that aileron effects are insignificant, they may be nearly so with a good F3J pilot, but in other cases (F3F for example) aileron input is frequent and so the induced and profile drag effects of the aileron can be very significant.

Originally Posted by FLY F3B

Differential should never be used as the primary method of reducing adverse yaw. Rudder mix first, then diff. if necessary. V tails, T tails and crossies.

Mike

Thank you! :-)

Originally Posted by vespa

For high CL flight such as pylon turns or thermal circling you want a lot more differential to avoid tip stalls.

In thermal CL (.6-.7) unless the bottom deflecting ail is already maxed out (perhaps > 25 degrees deflection, won't happened unless running reverse diff), it is hard to tip stall any of the current sailplanes unless incorrect wing design or not mixing in correclt flaps to ail. We should be nowhere near max ail deflection on the donwgoing ail.

Also, there's a higher penalty for deflecting the ail up while in any high CL flight. If we reduced the down going ail, to get the same control or roll rate, we must deflect the up going ail higher = higher penalty. That's why we add camber to thermal modes.
Most people wan't realize it, but adding camber is the same as reducing diff in thermal mode.
And yes, I still fly with little/no differential in speed and very slight amount in thermal. This is due to me setting the max ail throws at +-15 degrees from cruise speed mode. That marks the envelope of throw for all other modes.
And finally seeing others doing the same thing makes me feel like I kinda know what I am talking about .
But please, don't let this fool you. I still barely know what I am doing.

Didn't this same conversation happen about 4 months ago?

Joe

Originally Posted by target

I must be doing it wrong, I usually increase diff with speed, and decrease rudder mix with increased speed.
This has been (seemingly) working for me so far, but I haven't tried the opposite yet.

R,
Target

Target,
That may work for you in F3F where you guys tend to pull up for a turn (and during roll the pos. Diff makes the fuselage point downward into the turn).

In F3B this is not that good: I try in a speed and distance run never to pull up, the plane is always monotonously descending. The nose is always pointed downwards, at every moment in the run at an almost fixed rate. Therefore I want an axial roll, would it be slightly pointing down, it would loose too much altitude in each turn (pulling up would destroy speed). This sounds anal, but if you fly 30+ laps and loose a feet every turn, then that costs too much altitude. The same in anticipation for a speed turn, I don't want to loose too much altitude.

Further you can keep the plane for a moment in knife edge to anticipate a turn, even in medium speed distance, when the roll is axial and has no up or down pitch. If the differential would be slightly pitching, it is much harder to keep the glider in knife edge, even for a short moment.

This is how I leaned it from Quabeck's book and the Boehlen Bros, and it works for me well.

Best,
Reto

Originally Posted by J. Wydronek

Didn't this same conversation happen about 4 months ago?

Joe

Joe, this convo happens every 4-6 months.

Alrighty then...
Here's the next question:

Why use any aileron differential at all?
Don't tell me its for fine tuning.
Tell me what it IS for, and why we shouldn't just fly with only down aileron travel, or at the least no diff in any flight mode, and just rudder mix to cancel any unwanted yaw with any aileron input.

Thanks!
PS. Reto's explanation of why it COULD work for me (other than me smoking crack) is the best one so far....

Hey Target, from the practical side of flying our RC sailplanes, I can and have answered that question many times. If you are really willing to listen and learn my approach I'd be happy to share it with you.

Is it needed? Yes.

All the time? NO.

Does it help make it easier to fly the plane in a more coordinated manner with less pilot work load? Yes.

More detail, PM me.

Mike

Thanks Mike.
Maybe I will talk in person to you about it Sunday, if I don't get time to PM.

R,
Target

Happy to talk to you about this stuff Targ....

but I may not be out there on Sunday. If I am I may have to arrive late, so I have to see if the CD will allow it.

Mike

Aileron/Rudder mixing on the right stick. Entry into a turn activates both surfaces into the turn. What happens if, to maintain the bank angle, you need to apply a bit of opposite aileron throughout the turn, aren't we also apply opposite rudder when it isn't needed? I've always wondered if it's something I needed to compensate for with the left thumb, or, maybe so little deflection is needed that it doesn't affect the coordination of the turn? I suppose more practice with the left thumb is useful. I try to keep the fuse as parallel to the horizon as possible with the rudder. Make sense?
Thanks,
Jim