I have a modified a Gremlin by adding a coroplast tail assembly. I have a Magnum .28 on it and it flies great. I flew it a few times until I broke in the engine and I then added a mousse can muffler and then went and flew. First of all, man,... that lil engine will really haul. But as I was doing a High Powered flyby. I heard the bbbrrrrrrrrrrpppppp............. sound of flutter. My question is how can I tell if the flutter is comming from the ailerons or elevator. The plane didnt bump up or down while fluttering so that would make me think that it could be the ailerons but that coroplast does flex. I will include a pic of the plane. Now I have trimmed the stab quite a bit with a good swept so that the only thing different from the pic. I was thinking of Trimming the outboard aileron tips at an angle appx 2 1/2 - 3inches inward to reduce any possible aileron flutter. My other option would be to build the tail assembly out of balsq but it would add unwanted weight. I just really wanted to stay with this design cause its alot more stable than a standard Gremlin and is more manuverable, in my opinion.

Thanks,
FUZZ

From the pic, it looks like your ailerons are not tapered at their tips. Try trimming the outer 7"-8" down to a point. That should eliminate your flutter.

HTH,
D

Fuzz, I just built my first gremlin with a tail and kinda modeled it after your first post a week or so ago. The coroplast tail is really stiff, I doubt that's where the flutter is.

Btw, how big is the feather on the elevator and how much movement does it have?

I can't tell from your picture, but do you have anything in the stab flutes to stiffen the coro? You might try a bamboo skewer or two if you suspect the elevator.

If your vertical is also un-stiffened, it's possible that it's doing the fluttering as well, though not likely.

How was the roll trim? When I had aileron flutter problems, it was like flying in gusty wind, the wings would very slightly rock, and sometimes the trim would drift.

But, it is possible to flutter any control surface and not have control problems.

If you are feeling brave (reckless?) then you can try to do a low hot pass close in and see if you can actually see the fluttering surface. I did that once, though it wasn't planned, it just happened that I was doing a highspeed pass when I saw the ailerons moving.

Also, double check your linkages and make sure there is no slop or movement anywhere.

Well,..... I went ahead and trimmed the aileron wing tips and the outside edges of the elevator. Im not sure of exactly where the flutter came from but my guess would be the ailerons. All the linkages are slop free and I think I may also try stiffining up the stab with the bamboo as suggested. Ive added a pic of the new modifications. Hopefully,... I'll be able to see if it helped this weekend.

Thanks,
FUZZ

I've had the rudder flutter when made from coroplast. I drive a pointed fiberglass rod across the flutes at an angle. That really stiffens them up. I usually have to pound it in with a hammer for the last bit though!
The diving low flyby should show it up, it's a good idea to have someone else watch for the flutter so you can react quickly if something odd happens. Chop throttle and pull up!