Hi all:
I have a 60 size Super Skybolt powered with an OS 1.08.
Flies great, but when I attempt to fly knife edge it wants to roll out, requiring lots of aileron to correct. Is this 'normal' for this plane?

Tom

Yes and no. Most planes will tuck to the canopy or belly more than it will roll. Since you didn't mention this, I will assume that it is a combination effect you are seeing. Try to mix out the pitch coupling to see just how much roll coupling you really have.

If the plane doesn't curve toward the belly or the canopy, and just rolls, your dihedral is wrong. If it rolls over to inverted you need more dihedral - if your plane levels out again, so that it attains normal flight position, you need less dihedral.

i dont think dihedral has anything to do with it

Dihedral very much has something to do with it. If you apply rudder in level flight to a plane with ailerons, it banks. Ie, you get a roll coupling with rudder. If you are in a 90degree bank, you still get the same roll coupling with rudder.

It sounds like dihedral is the problem. You can also get a roll to inverted from the fact that the majority of rudder lift is above the roll axis. In other words, the rudder is acting like an elevator with an assymetric lift factor which results in a roll when in a knife edge.

BTW, one of the most pronounced causes in pitch coupling (not roll coupling) in a knife edge is prop "P" factor. In larger models, the "P" factor is so strong that the plane will pitch toward the canopy in a left wing low knife edge and pitch toward the belly in a right wing low knife wedge. This can cause a lot of confusion if your not aware of the phenomenon.

Dihedral, sweep back and a high wing location cause roll in the direction of the rudder.

Anhedral, sweep forward and a low wing location will cause roll opposite to the rudder.

Your Skybolt has sweep back and 1 wing high so you are probably going to have to mix out the roll. That's about your only options and keep it scale.

40-60 size Stick, straight wing type planes need in the neighborhood of 3 inches total anhedral to compensate for the high wing location and get good knife edge.

I have moved a lot of rudder area around trying to keep from cutting a wing in two and fix the lack of dihedral in a low wing plane. The area is located so close to the centerline, making the moment arm so short, that it only makes minor differences. If you have a very slight roll, this might work, but something like taking the knife edge roll out of a Big Stick, for example, you'll need anhedral.

yeah, i assumed the skybolt had zero dihedral being a aerobatic model.
if it has no dihedral then i dont think its an issue, i would guess that the rudder area above and below the centre line is incorrect.
what im trying to say is what pmw has summarised in the above post!

Coupling is do to control surface loads, wings, tail surfaces, fuselage, and many other means of aerodynamically dragful components being unbalanced in relationship to the aircraft's datum line. I've designed several aerobatic planes with zero roll and pitch coupling using aerodynamic load calculations and then simply balanced the forces. Everything needs to be considered aerodynamically to make this work. people here are trying to make it like it's one or tow things like dihedral or not, that just part of the equation. The hard part about designing coupling out of a plane is that everything effects ever thing in one way or more, usually more:-)
iflircaircraft, It's normal with most bipes to have roll coupling. The reason for it is that the full scale plane has to incorporate a human in the seat. That means the designer has to think about blind spots, visibility, and functionality. Some times the airplane design has to suffer a aerodynamic flaw or imbalance; if you will, in order to made the plane more functional an user friendly.
Coupling use makes you a better pilot. I'd like to see someone go to a national IMAC competition with a scale model Stearman or Waco and do the advanced patterns. It would take a truly awesome pilot to pull off a first or second place finish due to the battle of controlling a plane through complex sequences with a plane that's fighting you the hole way. Some people mix out the coupling with computer radios and theres nothing wrong with it, But it's much more rewarding to me when I counter it your self.

I have found that when I don't mix out couplings and I fly the plane to make it fly right, I learn bad habits and it makes it bad for me when I fly a different plane. I would like to mix out the bad habits so that the plane flies the way it is suppose to.

I have a computer radio, but don't use it to it's full capability.
I prefer to use my thumb, although pre-historic.
I must say the Skybolt flies very good and I'm learning more about counter control more than ever.
I feel it's a good skill.
The bottom wing, however has approx 3 degrees di-hedral in each wing panel.
The top wing 0 degrees.
I also enlarged the rudder somewhat...being larger than the scale version.
I really don't have a problem with it wanting to roll out of knife edge. I just wanted to know why!
If it's inherent of bipes than I can live with it, if it's something I'm doing, I cannot.
Thanks for all your input.

Tom