Jim Cubbage wrote:
>I am curious, most of the methods mentioned seem to me, and I am really no expert,
>to create a left momentum. What I mean by that is that it seems to me that
>the tendency for the planes will be to roll and continue to roll to the left,
>probably until the plane is in a knife edge position. That means, you will
>have to have some means counter the left tendency with a right tendency,
>otherwise your plane would continue to fall to the left.
The tendency for a plane to steepen its bank simply
means it is spirally unstable. All aileron gliders
I'm aware of are spirally unstable to some degree,
and hence must be "flown" constantly to keep them
in a circle.
The most common way to eliminate spiral instability is
to use sufficient dihedral -- at least 8-10 degrees or so
is usually needed (the exact threshold number depends
on the tail length, rudder area, and a few other things).
Many aileron gliders have some dihedral to make them LESS
spirally unstable. Their bank will still steepen, but not
as fast. All free-flight models and most properly set up
RC poly ships are spirally stable. They will hold their
airspeed and turn radius without pilot input.
One way to eliminate spiral instability _without_ dihedral
is to drive the ailerons though a rate gyro which senses
yaw and roll rate. Blaine Rawdon and Bill Watson did
some nice work on this:
http://www.rc-soar.com/tech/spiral.htm
Blaine's Plane Geometry spreadsheet and document also
delves into spiral stability and dihedral a lot more.
- Mark Drela
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