Was given an older trainer. Believe it was built in the late 70's. Still in great shape and hardly used. Having a problem flying inverted. Need a ton of down elevator, when inverted, to barely keep the nose up. Elevator throws are about 1.2" in each direction. Am running an OS 40 LA engine and the plane has way enough power. The wingspan is about 48 inches. Normal, level flight needs about 1/4", up trim, delection to keep the model flying upright, straight and level. The CG is balanced on the main wing spar. Would it help to bring the CG aft or is the plane so inherently stable, with its design, that inverted flight in not likely? Maybe a different elevator design? the pic is provided below. Any input will be appreciated.
Thanks.........


[8D]

Glowfuel,

With out going into a ton of aerodynamics.....I think it is the flat bottomed wing, not really a good design for inverted flight. It should loop and roll just fine, but not really very good for sustained inverted. The airfoil design makes it want to lift more to the top of the wing....hence the more up elevator when inverted.

I'm sure the experts will follow with the aerodynamics of a symetrical airfoil......[X(]

Hope this helped........and it is a nice looking airplane.....!

EDIT: Spelling...should have spent that singing lesson money on a dictionary......

I have to agree with Heli 001. In the pic I can see that the wing is flat bottomed and in the inverted flight, what you have now is about all you are going to get. Your next plane needs a full air foil and inverted will be much easier. Good Luck, Dave

Heli is correct about the flat bottom airfoil not wanting to fly inverted very well. But I think if you adjust the CG enough to get the elevator trim neutral at level flight you will have much less down elevator pressure when inverted. If you lighten the nose a bit the tail will be more effective throughout the flight envelope. Don't go to far back, make adjustment small and test fly until it is better, then quit. The old saying goes: A nose heavy plane will fly poorly, a tail heavy plane will fly ONCE.

The flat bottom wing will not provide much lift in an upward direction in inverted flight. It will fly inverted for a while, but it want to keep slowing down as you feed in down elevator to hold it up, with the eventual stall from loss of airspeed.

That's the nature of the old Clark "Y" airfoil. They are great for trainers and thermal gliders, but not too good for aerobatics.

This talk about flat bottomed wings and inverted flying reminded me of the time RCM had a construction article on the RCM Funster. For those of you who don't know, the Funster was a 70 or 72" low winger, a big slow flying sport plane, but it had flaps, and the article said "the Funster can be flow inverted all day long with those flaps down, or up if its inverted." I thought they were just having fun with us, but a years later I ordered a Funster kit from Hobby Lobby and when I unrolled the plans there they were, flaps! I built the wing as designed, flaps and all, and guess what, the first time I pushed out of a loop half way up the danged thing flew out of it inverted just as easy as if it had a semi-symetrical or full semi airfoil! I didn't have to struggle to maintain level flight with it either. They were right, I could fly inverted all day if I wanted too. Matter of fact I'm still flying the wing, although on another plane, a sort of semi-scale Crop-Duster. The Funster really lived up to it's name alright. I still have the plans and may someday build another one. "Famous often heard words!"

Glowfuel....

Quote: "Normal, level flight needs about 1/4", up trim, delection to keep
the model flying upright, straight and level."

Look at the horiz. stab. It is mounted incorrectly...being that it is glued
onto the top of the fuse, at the rear....with a wild up angle. [X(]
Normally it would be mounted at the same angle or incidence as the
wing....however, in this case....being a flat bottom trainer type plane,
the wing shoulg have about 2 degrees positive incidence at the front.
(about 3/16" will do)

That's why you have to give up-trim for level flight....and that trim is
making the inverted flight even worse.

Dave.

See how....on this typical airplane....the horizontal stab is in
alighnment with the wing ? In this case, it is fastened straight
onto the bottom of the fuse....but in most cases....it would be
set into the fuse on top, but would be alighned with the bottom
of the fuse and the wing.

Dave.

Dave,
Could it be that the model is so nose heavy that the trim is needed to maintain level flight, I have seen it. It is hard to say the incidence is off by looking at a picture, for me at least. That's not to say it isn't off, just that it is hard to tell from looking. As far as CG goes it is stated to be balanced on the main spar which for that plane is a safe starting point, very safe. So in my opinion it is stll just a CG problem if you want to call it that. It is however where I would make the first adjustment before getting the incidence meter out.
Not trying to step on any toes here, just looking for the easiest fix first.
Tim

The CG is probably your problem but also check your motor position. If it is set positive by several degrees this could cause the problem. Right or left thrust will make no difference.

The tail feathers should be mounted more like this. If the plane is
balanced on the spar....and if the plane has positive incidence in the
wing.....which some flat bottom planes have....

....you will need "down trim" in the elevator, because flat bottom
winged planes climb like mad at speed....not "up" trim.

More power, more elevator surface deflection (differential to augment "push" authority) and cg further back will help but will destroy the trainer characteristics the further you take these steps. Anyway no matter what you do, unless you change wing airfoil, will be less than desirable for most.