Model Forum / Radio Controlled / Air Models / July 2008

Suggestion:  Build a couple of examples of your tube-wing joint, and
test them to destruction.  You want your wing falling apart at about the
same time the tube is bending -- sooner and you're not getting maximum
use out of the tube, later and you're too heavy.

And if it's light as a feather and still breaking the tube -- either
congratulate yourself, or get a lighter tube!

The actual center of pressure on ordinary aircraft is ... uh ... hold
on a second while I google ...

Uh, I think it is behind the CG in intrinsically stable designs:

"The center of gravity must lie ahead of the neutral point for positive stability
(negative static margin). If the center of gravity is behind the neutral point,
the aircraft is longitudinally unstable (the static margin is positive), and
active inputs to the control surfaces are required to maintain stable flight.
Ultimately, the position of the center of gravity relative to the neutral point
determines the stability, control forces, and controllability of the vehicle."

http://en.wikipedia.org/wiki/Longitudinal_static_stability

When a plane is set up this way, it will tend to fall nose-first after
a straight-ahead stall, which helps with stall recovery.  

So the "ordinary" spar location (thickest part of the airfoil, normally
around 25% to 33% of the MAC) is not necessarily where the strongest
forces are applied.  It is where you can get the best tradeoff that
Tim is talking about: strong enough to hold everything together;
light enough to let you get off the ground and have fun flying.

Somewhere in my files, I've salted away a very beautiful advertisement
for a built-up jet wing.  It shows how the designer created a spar-and-rib
design for a two-piece swept wing.  It's a work of art and a thing of
beauty.  AH!  Here it is:

http://www.bvmjets.com/pages/kits/balsabandit.htm

What a great design!

                    Marty