CG and High-Speed Snap
 

I have a .90 sized Edge that mildly snaps during loops with full low-rate elevator. My major concern was eliminating knife-edge pitch coupling. I have moved my CG a bit rearward to eliminate the pull to the canopy successfully. The snapping tendancy did not change much with the CG change, but it got me wondering about the relationship between the CG and high speed snaps.

It would appear to me that with a rearward CG, the wing Angle of Attack would be lower, giving more time before critical wing angle of attack is exceeded during loops etc...

However, common concensus says that a forward CG will help reduce high speed snap tendancies.

Can someone set me straight?

RE: CG and High-Speed Snap
 

Hello,
these are my thoughts.

A more forward CG will yield a more STABLE plane. This means that it is harder, for the control surface, to give a boost to force a snap-roll, in which a wing has to reach its critical AoA.

Conversely, a more reaward CG will yield a less pitch stable plane, so the control surface has more power to force a snap-roll condition.

Changing CG we have the power of changing airplane stability.

Among the pattern guys it is also known that a more forward CG will help snap & spin performance, since the stall is more defined and the plane tracks better after the manouver.

Regarding your plane, if the snap is obtained even with low throws, it could be due to the fact that the wing constantly flies "near" the critical AoA - i.e., the plane could be HEAVY.

RE: CG and High-Speed Snap
 

I think you'll find that moving the CG aft will accentuate any bad habits the model has since the tailfeathers now have more authority. I believe what you are experiencing is what the model guys call a "tip stall".

It is in fact a stall of one wing. Some airframes are more prone to this, but usually with a model it has to do with setting it up properly with the incidence's set, control deflections matched, and no warping of any of the flight surfaces to cut down on the tendency for one wing to quit flying before the other one does.

Yes a forward CG will improve stability in "normal" flight modes including landings and hurt 3D stuff, but I suspect that as stek79 has mentioned, you're heavy... :eek:

IF you've got a high wing loading and you try to pull too tight a maneuver, it's going to quit flying no matter what you do with regard to trimming.