I had a Sparman P1 (since deceased), a swedish trainer scratch built which was almost impossible to fly even by excellent pilots. It balanced well, had great moments for a model. I have since read that the full sized had "cross control" problems. Can anyone enlghten me on what this may mean? Would it apply to a model?
PS I have scratch built lots of scale models which all fly well.

I think I have your answer.

The Sparman had only one wing, not two.

It had an inline engine, not a round engine.

It did not have two wings, nor a round engine, therefore it was not a real airplane.

Next problem...............

Looking at a small image, the plane apparently had no dihedral.
For a low wing plane which was apparently intended to be a trainer, this is .... unusual.
The plane would certainly fly strangely.
The rudder would be very powerful, and possibly generate enough side force when turning to require the ailerons held in the opposite direction to the desired turn.

I did a model like that once - and that was enough
the full scale had an inverted inline engine and a flat wing on bottom of fuselage it looked like a badly done Ryan.
I don't know what the designer had in mind unless it was to make sure the survivors could fly any POS design---

Thanks for the answers. I'll stop wondering what I did wrong in my engineering. Da... I do build some unreal planes; this winter a Van's RV3.

You are building a model of it I assume....or are you building an actual full scale RV3?

Model