Left Rudder -> Right roll  
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Left Rudder -> Right roll - 5/16/2008 7:25:50 PM   
subtleguru


 

Posts: 16
Joined: 2/8/2008
From: Reigate, UNITED KINGDOM
Status: offline 		I have a small CAP 21 (53" wing) that I built from plans that has a tendency to roll sharply to to right with a yaw left. Rolls by themselves show no yaw coupling.

It is my first scratch build so there is lots that could be improved on yet the wing looks straight and untwisted.

I assume right yaw will result in left roll but I haven't actually seen this. I do notice that I have some slight up trim and some slight aileron trim for level straight flight. The wing has some slight diheral and is semi-symetrical.

The drop of the wing tip is fairly sharp but correctable with aileron if I am expecting it.

Before I mix in some reverse aileron with rudder I want to understand what could be causing this.
       Post #: 1

RE: Left Rudder -> Right roll - 5/16/2008 7:48:26 PM   
Red B.


 

Posts: 935
Joined: 8/29/2002
From: Jonkoping, SWEDEN
Status: offline 		Adverse roll coupling like you describe is often caused by insufficient dihedral. Try increasing it.

_____________________________

/Red B.

(in reply to subtleguru)
       Post #: 2

RE: Left Rudder -> Right roll - 5/16/2008 11:32:42 PM   
Tall Paul



Posts: 4606
Joined: 6/23/2002
From: Palmdale, CA, USA
Status: offline 		I have a scale Stearmann bipe that does that.
Bad close to the ground.
It has to do with the location of the force on the vertical relative to the longitudinal axis of the forces on the plane.
When the vertical force is some distance above that axis, the deflected rudder acts like an aileron.
Right rudder forces the left wing down, etc.

_____________________________

Sparky Paul
http://www.angelfire.com/indie/aerostuff

(in reply to Red B.)
       Post #: 3

RE: Left Rudder -> Right roll - 5/20/2008 2:23:05 AM   
buzzard bait


 

Posts: 982
Joined: 1/1/2003
From: Ithaca, NY, USA
Status: offline 		Either one of the explanations above could be correct, but in this case I think insufficient dihedral is the reason. It is a low wing plane, and when yawed, pressure will increase at the juncture of the fuse and the outboard wing, which will roll it in the opposite direction of the yaw. The CAP has little or no dihedral.

The CAP has most of its rudder area low down, which should reduce the rudder-as-aileron effect.

The Great Planes CAP actually has more dihedral than the scale amount in order to correct this problem. They tell modelers NOT to take out the added dihedral.

Jim

(in reply to Tall Paul)
       Post #: 4

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