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, deflection 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.........

Your description sounds like it's simply nose heavy. If you intend to fly it like a trainer, a smaller engine is probably the solution.

The aircraft was designed to be inherently stable in both roll and pitch. There's probably positive incidence in the wing (most likely a flat-bottom or semi-sym airfoil) and down thrust in the engine that make it pitch stable. All that works against you when you want to fly inverted. There's not much you can really do to improve this without making some fairly drastic trim changes that will likely make it somewhat squirrely.

Sounds like you're ready to move up to an intermediate or better plane capable of more aerobatics that your trainer.

The plane is just nose heavy, that is why the up elevator to fly flat and level. Move the CG back 1/4" at a time and try until you need almost no elevator to stay inverted. That will put CG where you want to be. Watch out for overcontrol. As CG gets nearer where it should be, the plane will become much more responsive, and your elevator throws may be much too much.

Les

LesUyeda;
I agree with you on moving the CG back, but IMO you want to move it too far back. Making a flat-bottom trainer fly nearly-hands-off inverted would probably call for a lot of down elevator when flying straight and level upright. I would move the CG back a bit, but not too much.


One of the problems you meet when trying to make this type of plane fly inverted, is that it (because of the wing profile) will need a huge angle of attack to fly level (inverted). It's a good thing you have plenty of power, because this AOA will create lots of drag. Most likely the airplane will be easier to fly inverted at high speed, and terrible at low speed. My $.02. Good luck.

The airfraft has a flat bottom,Clark Y, airfoil .

It's because of the flat bottom wing. Just crank in a lot more down elevator throw and it will be fine. With enough down throw you should be able to do outside loops.

While I agree with the others that it sounds like it IS nose heavy you will also be fighting the flat bottom airfoil.

To TRY to compensate you can set up the controls so there is more down elevator travel than up as well as moving the balance back a bit at a time. Don't be surprised when you find you need to add down trim to compensate for the rearward CG shifting. Just keep adding down until the model is trimmed closer to neutral.

Do a search in the Sailplanes forum for "dive test". That will describe a method for sneaking up on the neutral handling CG location.

But in the end it IS still a flat bottomed trainer that you are trying to make into a stunter. I WILL NOT be happy about it and it'll constantly remind you that you're expecting too much from a model that was designed for a different mission.

Hi,

Does the model climb quickly at full throttle? If so, you can adjust the elevator linkage and/or trim to give enough down elevator at neutral stick position so that the model climbs only slowly at full throttle. Then when inverted, not so much forward stick will be required to maintain level flight. You may not have enough movement available on the elevator to allow all the down trim which will be required.

Another approach, if the wing attachment system allows, shim the TRAILING EDGE of the wing UP about 1/16 to 1/8 inch at a time until the model will only slowly climb when at full throttle with neutral elevator trim. It may require quite a thick shim. With the correct shim, even a flat bottom or Clark Y wing can be made to do outside loops, given enough down elevator control.

Either way should give the desired result. Moving the CG back too far can cause lots of excitement. A SLIGHTLY tail heavy model is VERY pitch sensitive; just a little too tail heavy and it will take off very quickly and crash almost as quickly. I would suggest not more than about 1/3 back from the leading edge to begin with.

Any thing you do will be a compromise; sounds like you need a semi-symmetrical or fully symmetrical wing.

Have fun.

Look at the size of the horizontal tail... it is much bigger in relation to the size of the wing then most airplanes... the CG position depends on the size and moment of the horizontal tail, it's not always at the main spar or 25% chord.... I may open up a bucket of worms here, but with that big of a tail I bet the CG should be about 30-35% wing chord and the tail will be lifting up in level flight, rather then down.

Ty

glowfuel....look in here, you might find the answer....

http://www.rcuniverse.com/forum/%2AW...1398041/tm.htm

Please acknowledge when you find an answer to your satisfaction.
We need to remove the duplicate thread from the Clubhouse....

Dave.