Wing Placement and CG Location

[WrongBrother]

Hi All,
Well after carefully selecting an airfoil, and sizing all surfaces and so on for a small UAV I've come to a little roadblock. Placement of the wing on the fuselage. I have gone through Perkins and Hage and determined the stick fixed neutral point and know that the CG has to be in front of this point for a statically stable configuration (which I desire). I also know that the CG needs to be in front of the aerodynamic center - and I think this makes it dynamically stable, although the texts have been pretty elusive on this point - maybe it's me. I have also seen that a static margin of about 5% is recommended. So how far ahead of the aerodynamic center should the CG be placed? As I recall, static margin is the lift curve slope divided by cmalpha (the negative of that I think).
Any help would be appreciated.

 

[davidjh]

Wrongbrother;

Experience says sooner or later in the vehicle's operational life there will be changes/additions that will shift the CG. So plan on a "CG range" within which you can meet basic stability criteria.

 

[aviat]

With the CG ahead of AC it forms a couple which tends to rotate nose down, which is what you want when engine quits. This couple is offset by the tail.

Bascially, if the CG is fwd it will give better stability, but there is a limit where the elevators will not be able to lift nose for landing etc. With CG too far aft you will problems with spin recovery.

The folowing site might provide you with some help.

http://www.zenithair.com/kit-data/ht.html