Dick the airplane is a set of inertias, a cg location all driven by a set of aerodynamic parameters. For a roll to look good to us you want it to roll with the fuselage not varying in pitch as it rolls. You can do a lot of things during that time with aero inputs to keep the fuselage straight.

Or you can design an airplane that doesn't need the inputs. Take the modern FAI pattern airplane design. They have everything inline - thrust line, wing, horizontal tail, No dihedral, etc. it makes things simply easier.

The Clipped Wing Monocoupe won't do a roll in a straight line because the aero forces are undergoing a large change as it rolls the full 360 degrees. For example that configuration will have a different roll stability upright compared to inverted. For a constant aileron deflection that means the roll rate will vary during the maneuver. The roll coupling due to inertia will cause a change in sideslip angle which also feeds into the roll rate and pitch angles. If you are going to try to slow the roll then through the inverted part you need some inverted lift "up". To get that requires a large angle change because of the flat bottomed airfoil. That makes another input into the roll asymmetry. And so on.

Our eyes aren't a good instrument to measure things like this. Also if you tell me it's going to be an axial roll, I like everyone else is probably going to believe you and the halo factor will come into play.

On the F-15 project we tested the machine pretty throughly. It can look like it was doing a perfect roll but when you looked at the instrumentation it really didn't. We weren't surprised since we had wind tested and computer simulated the machine beforehand. It lines up things pretty nicely but there is enough asymmetry in the airplane as in the Monocoupe that it doesn't roll straight.

But the thing is that the physics of the maneuver means that some airplanes will roll better than others. Again I point to the FAI designs. They do roll on a line.

Why don't large scale models use the setup? Simply, there are no full scale models that look that way so if you want to fly in any large scale contest you have to fly the other machines. If there were they would be flown in the XFC.

Back to the CAP and downwash angles. Right, it will tumble and I have seen one knife edge right off the grass forever it seemed, but for a given elevator deflection - up or down - the resulting diameter of the loop will be different due to the difference in elevator effectiveness due to the change in downwash angle at the tail through the loop.

On the F-15 project we tested the machine pretty throughly. It could look like it was doing a perfect roll but when you looked at the instrumentation it really didn't. We weren't surprised since we had wind tested and computer simulated the machine beforehand. It lines up things pretty nicely but there is enough asymmetry in the airplane as in the Monocoupe that it doesn't roll straight.

Ben