My scratch build doesn't fly very well !!!

On 29 Apr 2007 04:07:43 -0700, snipped-for-privacy@hotmail.com wrote in :

Blunt, rounded leading edges and fat airfoils are better for avoiding snaps than sharp leading edges on thin airfoils.

Check the incidence of the wing against the incidence of the horizontal stab. Your need for up-elevator may come from having them working against each other.

Go ahead and try some nose weight. If it helps stabilize the plane, that's good. Planes that are too nose heavy can often be landed OK; planes that are tail heavy often can't.

You might want to try putting a little washout in the wing tips or using stall strips on the inner 1/3rd of the wing. This sometimes makes tip stall less severe because different parts of the wing stall at different angles of attack.

Don't slow down too much on landing until you figure out the problem. A touch plane like yours is a prime candidate to stall, snap, and spin while attempting to land.

Marty

Like he said: Blunt leading edge and washout are the more forgiving.

Sounds to me as if you have too much elevator throw.

Can you tell us the incidence angles of the engine, wings and tail? Also, what is the wing loading? If you don't know that, how much does it weigh and what is the wing area?

One thought: how much verticle stab. do you have? If you tried to make it scale like it may be too little for a model. mk

Those sound like classic symptoms of a too-rearward GC.

Have you done the dive test? Assuming that you can get the thing high enough in a steady wind, get it trimmed for a slow glide. Then nose down into a fast glide and let go of the sticks. If it tends to zoom up when you let go then the CG is forward. If it tends to tuck under into a steeper dive then the CG is definitely too far back. If it stays the same then the CG is right on the knife-edge stability point. For sport flying, you probably want to go about 1/4 inch ahead of the neutral CG point; if you're a hot rodder then leave it right there and learn to be gentle with the elevator (and enjoy the flick rolls).

If you don't like the dive test, then check to see how it flies inverted compared to upright. If you can use the same elevator trim in canopy-down flight as canopy-up then you're definitely at the neutral CG point. If you have to stick close to the slope this may be an easier test to do.

The other suggestions that you've gotten will all help to tame the plane down when you get a sudden excursion in pitch, except for reducing the elevator throw which will compensate for the CG position somewhat. Don't discount them, but I'd get the CG right.

Thanks for the info. Will push CG forward and check relative angles of incidence on tail and wing. Point about scale vertical stab being too small is very interesting. If I re-build I will look at bigger vertical stab and enlarge the wing tip slightly over scale.

Thanks Mike

You can test the vertical stab notion by building a slip-on extension. If it makes a noticeable improvement then you can think about increasing its area (or seeing if there's a later version of the original that used a dorsal fin).