Greetings,
After trying knife edge with my new World Models Ultimate 120 a couple of times, I've got a question about coupling between control surfaces. When I crank the rudder in, this model needs aileron and elevator correction to maintain heading. I'm assuming this is normal (I've never had a biplane before.....or a model with this much rudder authority), but what I'm wondering is how the rest of you handle it. Do you exploit the mixing capabilities of your computer trannies (which I'm about to play around with), or do you rely on the mixing capabilities of your brain? If you use the tx mixers, do you use a switch to turn the mix off until you need it (to avoid unwanted aileron or elevator input while on takeoff etc)?

I have a cap with a lot of coupling...both roll and pitch!! I have a lot of rudder throw also !! I mixed them both out with the computer radio ! I first put them on a permanent mix...but then changed it to a switch. I changed to a switch because I didn't like it on while landing.

The biggest problems with trying to mix it all out perfectly.....is

the amount of coupling is different depending on.
1.....the speed of the plane
2......the amount of rudder deflection

for example....if you mix the coupling out for a fast knife-edge pass using little rudder.... then you try a very slow pass with lots of rudder deflection it wont be perfect anymore!! If you set it up for a slow knife-edge pass then try a fast pass, it will be off again!!

I think what I am trying to say is it is not a linear function and has the variable of the speed of the plane also thrown in to make it more complicated

It is however a lot better than doing nothing at all, which is what you are faced with now

I've tried the mixing for knife-edge. It did make it a bit easier. However, I do think that switching the mix is necessary, since rudder corrections for wind or whatever in vertical manoeuvres (stall turns, square loops, etc.) will be affected by unwanted elevator or aileron inputs.

I tend to forget where all the switches are set, so I just fly the plane. It just seems more natural than having to change setup for each manoeuvre. I don't fly in competitions yet, and can barely get through the basic sequences so you may want to hear from more experienced flyers.

I just fly the plane. You can mix it all out but it does have some trade offs. You sure as heck won't 3D it with the mix on.

Bean

MrBonk, as you can see, there are differing opinions. Personally, I used the trans to mix coupling out. Outsider is right, mixing can be dependent on surface deflection, depending on the plane. My Extra has small pitch coupling with rudder, and the same setting works regardless of rudder deflection. My H9 Cap has some pitch and roll coupling, but the percentage of correction needed for low rate rudder deflection is a lot less than that needed for correcting high rate coupling. Fortunately, my 8AUS can do separate mixes for low and high rate, so it's not a problem. My Cermark Pitts biplane has the strongest rudder authority of any plane I've flown, but also has the strongest roll and pitch coupling.

Now........for me, mixes are similar to using a calculator. I CAN count, but using a calculator allows me to do it better, faster, and with fewer errors. What I'm trying to do is have more pure reactions where yaw is YAW, rather than being yaw+pitch, or yaw+pitch+roll. My mixes aren't switched, rather are on all the time, because I ALWAYS want pure reactions. If I give a rudder correction for a crosswind landing approach, I don't want a simultaneous pitch to the belly, or a roll. If I'm vertical in a hammerhead, humpty-bump etc. and use the rudder to wind correct or correct for torque, I don't want a simultaneous pitch or roll, all I want is YAW. And, remembering outsider's good point that correction needed might be dependent on surface deflection, if you have a mix set that corrects at one point, then the amount of error you get when you give higher rudder throw is STILL less than the error you'd have if you had ZERO mix. Again, some radios (my 8AUS for example) allow for different mix settings with you dual rate settings.

For me, I've found that setting up your plane to give pure reactions allow me to fly the MANUEVER, rather than flying the PLANE! Once you get both knife edge flights correct, then you're now ready for those super slow (15+ second) county to county slow rolls. Now you don't have to hurry through 4 point rolls, rather you've got a tool that behaves through 8 pointers, where each point is held for 2 seconds, or 16 point rolls at one second per point! You can now move from elevator-only rolling circles (fast roll) to graceful 2 turn rolling circles and if your got enough rudder authority, even one turn rolling circles.

Here's the effect that NON-CORRECTED roll and pitch coupling have on a slow roll: You feed a bit of right aileron to establish your roll and slowly feed a bit of top rudder (left) and pull elevator to keep the nose centered on the "string". Past 45 degrees pull elevator is eased out and more top (left) rudder is eased in approaching the knife edge at 90 degrees of rotation. The more rudder you give, though, the more the plane pitches to the belly, requiring more pull elevator. Additionally, the more rudder you give, the slower the roll rate becomes, due to the roll coupling, so you have to compensate with more aileron to maintain a steady slow roll rate. Your past knife edge at 45 degrees and easing off the rudder and also need a bit of push elevator to maintain altitude and heading. You've got a bit of a breather when you get inverted at 180 degrees, but as the roll continues, you feed a bit of top rudder (now right) with a bit of push elevator to maintain heading. At 225 degrees, (like at 45 degrees) the more rudder you add to maintain altitude and heading, the more the plane wants to pitch to the belly, requiring some pull elevator, and also, the roll rate again changes, this time INCREASING causing you to decrease aileron input to maintain the consistent roll rate. You get past the knife edge at 270 degrees, and gradually decrease rudder while adding some pull elevator to keep the nose "on the string" and you finish. In this case, you not only had to fly the manuever, but also you had to make corrections for things that the PLANE was doing. Making the reactions pure takes the plane out of the equation. Even if your skill level isn't there now to do some of this stuff, at least the plane won't fight you when you ARE ready. After all, it's easier to learn to drive a car if the wheels are PROPERLY aligned than one that's pulling to the left.

If you think about it, the BEST flying model planes out there are the PATTERN PLANES, because they've been designed to FLY well, and having PURE REACTIONS is a part of that design! Our semi-scale planes are designed to LOOK like real airplanes, at the sacrifice of some flying characteristics. Mixing makes up some of this difference.

Full scale aerobats suffer the same adverse coupling issues, but since the pilot is in the plane, he has a LOT more sensory inputs and information about what the plane is doing, and has the BEST computer (brain) to make corrections. We're flying our little planes from afar, and I don't know about you, but I need ALL the help I can get! (yeah, call me a wimp!)

Try the mixing, you may find that on a biplane you like it on all the time. I have a Byrons Pitts that has quite a bit of roll and pitch coupling that I mixed out. What I found is that all maneuvers became easier, including landing and take off. When I give it a little rudder on landing or take off the coupling is gone making it easier. Most computer transmitters today have multi point mixers making it less likely that it will do something you don't want at different rudder deflections. As 3drc has stated mixing may make some 3D a little harder, if not impossible. Today most people do not use a biplane to 3D anyway because of the coupling issues.

I eventually took the mixing off the switch after discovering that a couple of times my sloppy flying (ok...more than usual) was caused from the switch being off.