I have recently put together a TuffFlight Panther flying wing with elevons, set up for SSC. I am noticing a significant "tuck under" when I am making a left hand turn. I have not noticed this happening in a right hand turn, only left. When I roll left and apply up elevator, the nose will pitch down hard leading to a snap or a roll to the inverted. This tuck under tendency seems is sudden not predictable, and unrecoverable. I crashed out in three times in three rounds today with the Panther doing this. I must give credit to the TuffFlight boys.... the Panther came out unscaved and the homeplate motor mount design did it's job every time. Any insight on what the problem might be would be most helpful. I wanted to email TuffFlight, but something is wrong with their website.

Regards,
GPB [&o]

The TufFlight web site is too popular right now. There is a lot of attention for the 4D videos lately.

You most likely have a geometry problem on the elevons. Check to make sure your servo arms are 90 degrees to the push rods when the plane is trimmed to fly neutral (not just the way you programmed them). You should always re-adjust the push rods for good mechanical symmetrical geometry after you get the plane trimmed. Otherwise, you'll notice problems that sound just like what you're describing.

Uneven roll trim often mascarades as a pitch dive in one direction and not the other.

Look on p 46 of the instruction manual under this:

"plane dives or climbs momentarily when initiating a roll"

Joe

Hi Joe,
Thanks a million. I have been going over the instructions and saw this. Off to square the control arms!
Regards,
GPB

Another possibilty is a thrust line or vertical stab out of alignment. Many of my planes will "tuck" in a tight turn, rolling one way in to an inverted situation like you describe, when something is out of alignment. The problem often doesn't show up at normal speeds.

If you turn hard to the right, does the plane kind of spiral up and try to start rolling out of the turn if you hold it long enough? It's usually not nearly as violent as the "downwards" tuck, but if you get that same effect, it's worth looking at.

Do check the pushrods and such as Joe said though, that's the first place to look.

Hi Kirk,
We met at Andersonville this past summer. You are correct regarding right hand turns.... they did tend to climb. The vertical stab is pretty floppy side to side. I am going to stiffen the stab as well. Even at SSC speeds, the forces on the stab are bound to make it move side to side, affecting stability.
Regards,
Mike

Yeah, though I'm not so sure it's the side-to-side forces that cause that so much as the effectiveness of various surfaces changing at different rates as AOA increases. So at high speed, a little thrustline or rudder trim is not an issue, or cancelled out by the ailerons, but at high AOA, the rudder seems to matter more. I'll also say that my scale planes, with the smaller vertical stab area seem to be more prone to this than the open ships with lots of vertical fin. On some planes, I've gone so far as to stick some music wire in the fin horzontally to allow me to bend in some rudder trim and have it stay there. A little right trim seems to really help.

If you can increase your vertical area, or add some sub-fin, that might also help.

You really know you have the problem if your plane likes to slew to the left on launch. That plus the tuck in the turn is my signal to apply some right rudder trim.

Mike,
My first Falcon exhibited the same problems. The solution was in the elevon set up. I slaved the rudder channel to the aileron and did not set equal amounts of movement in them (by accident). Consequently the left aileron (rudder) was moving a lot more than the right. This was also true when the elevons were acting as elevators. You might want to check the ATV on your rudder, and see if it matches your ailerons. Also you might need to check expo for both channels.

Good luck, Lou.

Well, good news, fellows. Your suggestions regarding servo arm alignment plus a careful engine alignment check has got the Panther flying quite nicely. I almost know what I am doing with these wings, now! Thanks for your suggestions!
Regards,
GPB