I have built two "Transatlantic" slope soarers from 1990 RCM plans. One for myself and another for a friend. They both seem unstable and prone to stalling and poor penetration. I have had no luck finding anyone else with hands-on experience with these planes. They have full floating stabilators. My friend has used the top of the fuse as his zero reference and decided the wings had nearly 6 degrees of positive incidence and adjusted it down to about 1/2 degree positive. He is more pleased with the way it handles now but I am not convinced.

My questions are: What is the best reference to measure incidence on a floating stabilator equipped plane? If the top of the fuse does make the best reference, why would a designer want that much positive incidence?

Thanks for looking, Cliff

The fuselage is just there to hold the tail and wings. Incidence measurements relative to it are meaningless unless it easier that way and subtract the two. You measure the full flying tail relative to the wing. Set it at a couple of degrees negative initially and fly it. Adjust as required because with a full flying tail the incidence is defined only at the time you have your hand off the stick.

Stability and penetration are more a issue of CG location and weight. Move the CG forward to get stability, add some weight for penetration. There is more to it that that of course but it is a start.

A full flying tail with several degrees of negative incidence and a forward CG will have a strong angle of attack stability tendency. Move the CG aft and the incidence will go toward 0. Get one of the lazer incidence measurng devices.

The fuselage is just there to hold the tail and wings. Incidence measurements relative to it are meaningless unless it easier that way. The aero of the airplane cares about tail relative to the wing.

I appreciate your input. I forgot the instructions mentioned to add weight to increase penetration. I reviewed them after posting. I don't like the idea of adding dead weight. I also calculate a wing loading higher than that of my Gentle Lady with this plane. While I'm sure the Gentle Lady is not the ultimate base for comparison, I do have hundreds of flights on it and can tell that it penetrates better with a lower wing loading. I imagine there are factors at work that I do not understand as to the differences between the two planes...airfoil etc.

Maybe I am over complicating the problem. The more I think about it the more I realize they do fly as if they are a little tail heavy. In the end for the best efficiency would you expect the plane to fly with the wing and the stab. at the exact same angle once the C.G. is at it's optimum location?
Thanks for the help, Cliff

"They both seem unstable and prone to stalling and poor penetration. "
Pitch instability is the main indicator of the cg being too far aft. Add enough nose weight to get the plane pitch stable and then see how you like it. The correct cg is the position where the plane is pitch stable enough for the way you like to fly. I have not run across a plane that could not be made pitch stable by moving the cg fwd.
Did you try a dive test?

I must admit that I don't know exactly what a dive test is. I have searched this forum to save asking but I only find discussion of the results not the actual method of performing the test. Can you give me a procedure and some hints as to what to look for?

Thanks again, Cliff