What are the aerodynamics, and design qualities of an airplane with really nice axial rolls? They say a plank wing generally won't roll axially... why?

Thanks,
av8tor

Basically keep the wings, tail and power on or near the center line. Keep the fin/rudder short and wide. Use a symetrical airfoil and balance the model far enough back that it's pitch neutral. No dihedral and even side area above and below the center line of the fuselage......

Sounds sort of like a pattern model doesn't it?

What do you mean by "plank wing"? If you mean a constant chord wing then there's nothing that says it won't do a decent axial roll.

Thanks for the answer. Yes, constant chord was what I meant by "plank wing". A swept wing definitely seems to roll better. I'm wondering why. You don't want dihedral in a wing for good rolling characteristics, but sweep adds dihedral effect. I'm missing something on the wing sweep concept. That's what I'd like to learn.

Thanks again,
av8tor

The often quoted "dihedral effect" of sweepback is very different from real dihedral in one important aspect. It has the same effect whether the plane is upright or inverted. This makes a huge difference to the roll ability.

Steve

One of the best indications of an airplane with either physically or aerodynamically drooping airlerons is it's inability to do clean axial rolls. When aileron neutral position is set properly, even a simi-symetrical shoulder winged airplane should do very clean axial rolls.

Differential ailerons can be tailored to do axial rolls very well on semi symetrical, flat bottomed and undercambered wings.

Mostly likely you're getting the nose pitching down in your rolls and end up needing lots of down elevator coming thru inverted to keep them straight. It's a Cg issue most likely. Had the same problem on my TopCat jet until I took some ballast out of the nose. The the rolls cleaned up nicely. Seen it on other airplanes too. Get your airplane to where you only need slight forward stick when inverted, then see how it rolls. Then go for the other suggestions.

No, I'm not really having any problems with my planes. A while back I went through and really checked out my Big Low Stick custom design. It wasn't rolling as axial as I thought it could. Come to find out one side was a little heavy, but more importantly it had a little downthrust in it that it didn't need. Took that out, retrimmed, and now it rolls better.

Actually, it was more of a question in general about axial rolls. I'm going to design something in the 72 to 80 inch range for use with small gas engines, and was debating in my head whether to bother with sweep. That got me to wondering about the actual aerodynamics of the question.

Thanks for the help and info though.

AV8TOR