LAST EDITED ON May-08-06 AT 10:17 AM (CST) Seems like there have been a lot of assumptions based on scant information.
Most of the comments about the CG and the leadout position are likely valid but until we know where the CG really is "NOW" everything is strictly speculation.
I'd like Bill to check that out for us. He likely already is familiar with the method but just in case...
Bill, pick the ship up with your index fingers, one at each tip in the same location back from the leading edge. Move them back and forth until the ship balances and doesn't want to flip forward or backward. Have a friend make a small pencil mark where your finger is on the left wing and measure the distance aft of the leading edge.
Next, measure the distance between the leadouts at the wing tip and make another pencil mark halfway between them. Measure how far aft of the leading edge the center point of the leadouts are.
Finally, measure the distance from the leading edge to the trailing edge (the "chord" of the wing).
Tell us what the measurements are and we can make a better informed response.
Theoretically, the CG should be forward of the midpoint of the leadouts an inch or so and the CG should be around 10% of the chord aft of the leading edge.
Now tell us how much it weighs, what length lines you are flying it on and what the lap times are (how fast it is going when you experience the problem).
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A quick comment about the farther forward CG than you normally hear discussed. You usually hear us smart aleck designers talk about CGs around 15 to 25% of the average chord (more correctly Mean Aerodynamic Chord or MAC. In this case of course it is the same thing since the wing isn't tapered)
A flying wing is a bit different animal from a conventional mainplane/tailplane design. In a conventional layout the pitch control comes from lift variations in the tailplane causing the wing to pitch in the desired direction.
Also, in a conventional airplane the location of the CG can be further aft because the neutral point or aerodynamic center of the entire ship is further aft. The bigger the tail the further aft the neutral point and the further aft the CG can be and remain stable and, therefore, flyable. The distance between the CG and the neutral point is called the static margin and is a measure of the stability of the airplane ... its willingness to track straight ahead primarily, in our cases, in the pitch axis.
These luxuries aren't available in a flying wing. First of all the only thing that controls the pitch of the wing is the flap on the trailing edge. Note I didn't call it an elevator ... for a very good reason.
A flying wing is in actuality a very poor scheme for ambitious pitch control; the primary reason that flying wings enjoyed a very limited period of popularity in combat before being replaced by the now ubiquitoes wing with a flying tail.
The manner in which a flying wing "turns" in pitch is the result of deflecting the flap which cambers the airfoil in a positive or negative direction. A "cambered" airfoil has a characteristic known as "pitching moment". The aerodynamic effect of this moment is to pitch the wing in the direction of the camber; nose up when the flap is deflected up and nose down when down.
As a result the ship will appear to be flying like a conventional layout but in so doing it is using the wing area in a very inefficient manner. It is like doing outside loops with a Piper Cub with a flat bottomed Clark Y sort of airfoil. I.e., given sufficient tail authority, horsepower and altitude in could be done but I'd not volunteer to be the test pilot.
This is why combat flyers quickly gravitated toward the modern flying tail concept so that the area of the wing can do its thing as efficiently as possible. Much tighter corners and much less drag result.
Back to the CG. Stability requires that the static margin be "X" distance between the CG and the N.P. (neutral point). With a flying wing the neutral point is very far forward compared to a conventional layout because of the lack of a tail or aft fuse. It is nothing but a wing and the neutral point will be located based on only the wing.
To be stable the CG must be forward of that point the same distance it would need to be forward if the N.P. was well aft due to a long, large conventional tail.
A CG near the aerodynamic center of the wing (roughly 25% of the chord on a symetrical wing) will be way to close to the N.P. and the flying wing will be very touchy and potentially unflyable. To make it work at all (and still inefficiently) the CG must be well forward and the 10% location is probably a safe place to start and anything much aft of that should be approached in small steps.
Ted
Important ps for regular stunt planes.
The phenomenom we talked about which causes the flying wing to "turn" is exactly the same phenomenom we talk about when we speak of the negative pitching moment the tail must overcome when flying a flapped ship. It's worth noting the pitching moment is enough to allow a flying wing to fly reasonably respectable maneuvers.
This is the reason we (me in particular) spend so much time talking about getting the CG back to around the aerodynamic center of the wing so as to minimize the demands on the tail to overcome the pitching moment. Once again, this is particularly important when flying in high winds and the G forces build up in maneuvers.