RC Plane Trim Problem

[Jon H]

Hello all. I wasn't quite sure where to post this but here seems to fit the bill best.

I have a question about my radio controlled plane. It is an electric powered high-winger of about 0.9m span. I think it would be classed as sport/aerobatic. It has 4 channels with a brushless motor and LiPo batteries. I admit it may be a little over powered.

The trouble is that at full power and neutral trim (or anywhere near it) the plane pitches up dramatically and climbs almost vertically. The trim is not enough to hold the nose down. At low throttle settings (and all the down trim) it will drop it's nose and go into a shalow-ish dive.

I was wondering if my problem lies with the centre of gravity or with the thrust line of the motor. Or is it something completely diffenent?

I would appreciate anybody's views before I start modifications. There must be some RC-ers around here somwthere...

[DaveSims]

Hmm, you've already figured out the two most likely culprits, the center of gravity and thrust vector.  I would start with the center of gravity.  Did you build the model or was it ready to fly?  Hopefully in the book there is something to show you about where the center of gravity is supposed to be.   Based on what you've told us, I would wager the center of gravity to be too far aft, which reduces the effectiveness of the elevator.  That is an easy fix, just by shifting or adding weight.

[Splinter562]

Aside from checking the C.G. (which should usually be located near the 1/4 chord of the wing). As you said, you can check the incidence of the thrust line, but also the incidence of the tail. If the prop is tilted up a bit, or the tail tilted down a bit, it can cause that sort of issue.

[Jon H]

[quote]Hmm, you've already figured out the two most likely culprits, the center of gravity and thrust vector.

[Jon H]

[quote]Hmm, you've already figured out the two most likely culprits, the center of gravity and thrust vector.

[DaveSims]

You may be right, I have trouble keeping it straight.

[Splinter562]

Actually my understanding is that the further back the centre of gravity, the MORE effective the elevators become, and the model becomes unstable in the pitch axis. Is this not so?

That is correct.

[Jon H]

That is correct.

Ah thank you...

Well I've Increased the amount of down thrust on the motor by about a degree and I'm just waiting for some decent weather to try it out again. Wish me luck.

[Boss_BlueAngels]

Any luck?  I usually just adjust my control linkages (adjusting pushrod length, clevis holes) before mucking around with any complicated CG/engine mount problems.  Unless it's really significant anyway.

[JLangholzJ]

[quote]Any luck?