I was thinking about putting this in aerodynamics but that forum is a little too hardcore for me. Plus I'm not sure it's an aerodynamics issue. Plane is a Tower Fun-51 with a OS LA 46, Futaba with standard servos. Great plane overall but odd thing I don't get. Full power, full elevator loops from level flight. Pretty tight but what happens is it pitches about 45 deg and then stops pitching for a fraction of a second and then starts pitching again. It does this all the way around so it kinda has the same feel as a 8-point roll but in a loop. There's still full aileron control and no snapping in the loop and the it's still a pretty tight loop considdering the plane and speed. I've mixed in a little opposite flaps (flapperon) to the elevator channel and it doesn't make a difference. Totally not a big deal but does anybody have an idea why this is happening?

thanks,
Mike

I have NO idea why it's happening, but I'd suggest that you just let everyone think it's intentional. (Then challenge the other pilots to try and duplicate the manuever. )
Dennis-

It sounds a little like you are 'stalling' the control surfaces.. Such as in fwd flight and you decrease power and increase elevator until the nose drops..

You are entering a fast fwd loop with all the elevator.. that creats a lot of drag and turbulance..

Try the same thing with just 3/4-back elevator... I'd bet the loop smooths out...

After that, if you want a faster loop, some people set up a '3D' plane to have flap input with the elevator.. You could mix down flap with up elevator, this will help push the plane through a tight loop...

You can also set up dual rate for fast fwd flight and use something like 70% total control throw.. Just remember to switch back for slow-speed flight

Let us know how additional tests work out..

TEBerg has it right. The wing is stalling then unstalling etc. You are lucky you have a wing that won't tipstall as thats the usual outcome of turning to tight. Flaperons will certainly help, also to do nice square corners. I'll wager it will take a lot to tipstall it on landing.

I think it could be one of a couple of things.

It could be a mild stall:
Basically, you increase the AOA of the wing when you pull the elevator.
This causes an increase of lift as you'd expect.
But you also get a BIG increase in drag, causing airspeed to fall off.
This loss in airspeed leads to a mild stall, causing a small loss of lift which causes the nose to fall and the loop to open up.
But when the loop opens up, the drag goes down, the engine pulls the plane to a higher airspeed, so the lift goes back up, and the loop tightens.

It could also be the elevator getting in to the wake of the wing. Basically, as the nose goes up, the tail drops. There is a downwash of turbulant air coming off of the wing. If the elevator gets in to this air, it becomes less effective. So, the elevator looses effectiveness, the loop opens, the elevator regains it's effectiveness just long enough to get back in to the wings wake, and the cycle repeats. (I actually think this is less likely than the first though)

For a solution to either one, try a lower pitch prop. This will allow the engine to pull the plane at a more consistant airspeed though the loop rather than slowing quite as much as you pull on the elevator.

You can also try moving the CG back a little. This will make sure that if you ARE stalling, it will be a lot more obvious . Actually, what it will do is make the elevator have to work less to pull the nose up, making the effects less obvious.

Also, try easeing in the elevator. I don't know, but I'm going to guess that you start your loop pretty suddenly, going from level flight to full deflection in one movement. Also, as the plane gets vertical, ease off the elevator, and let gravity help you over the top of the loop. A nice round loop often requires adjusting the amount of elevator you hold though the loop.

Thanks for the help. That makes sense. I do have some flapperon mixed in but it's only on 50 (whatever that number means) and results in only about 1/4 to 1/2 inch movement at the TE. I'll crank it up to and see what happens.

I am sure that the guys have it, but also check your elevator pushrod and make sure it isn't flexing. I am sure it isn't that because if it was, it would just be a larger loop, but you never know.

Sounds more like stalling the tail.

What prop are you using?

APC 11x6. I tried an APC 12x5 but wasn't too thilled with it so went back to the 11x6.