Reply to Stek79:

If you intend to minimize the effect of spiral slipstream, and if you desire to present a steady velocity flight, then it follows that you need to use a prop pitch that will minimally pull your plane at that velocity during the most power-intensive part of your routine while providing the capability for extra margin needed for control and adjustments during the flight.

Prop drag, like wing airfoils, is composed of parasitic and induced drag. Parasitic drag increases approximately with he square of prop velocity and induced drag increases primarily with pitch relative to the airstream the airfoil sees. In regular flight, the comparison is when you increase pitch angle and get "begind the power curve". The more total drag you generate, the more spiral effect and torque you will feel and the more power you will need from your engine. Virtually all airfoils have a most efficient angle of attack, at which they produce the highest ratio of lift (thrust for Props) to drag. If you desire to minimize "wasted power" going to produce torque and spiral slipstream that is not generating lift(thrust), you need to select a prop pitch that is optimized for the airspeeds you intend to use. In most cases relating to aerobatic aircraft, that will be a pitch that provides the greatest static thrust near the engine's peak torque rpm.
If the peak thrust prop does not provide enough forward speed for certain portions of the intended flight, you will need to increase pitch as needed, while maintaining a "flying airfoil" at the minimum flight speed you intend to use.

In most cases, you will be balancing lots of factors. Longer props mean more tip velocity, more noise, but they generally make more lift from available horsepower because accelerating a large volume of air by a smaller change in velocity is more efficient than accelerating a smaller volume by a larger change in velocity. We previously discussed that all things being equal, a smaller prop might generate a smaller amount of spiral slipstream, but that might have to be traded to get the other factors you need.

Reply to Dick Hanson: BTW, the glue that holds the chaw in that tight spiral is good ol' surface tension. Let's hear it for the fearless leader just going out and slaying those dragons. You can hear the dying gasps as we speak. "Zero, Zero, Zero ain't all bad". You'd think someone plans to fly with other than right side up orientation. That's just terrible, why how can you suggest - -- --- --? You have an aerobatic design, symetrical as you can make it, zero wing incidence, zero stab incidence, engine crank on the trust/drag lines, shoulder wing at center line, airfoil stabs at center line, big wing tapers with a deep cord at the fuse, straight leading edge, constant cord ailerons so the ratio of aileron to wing cord varies as we move outward from the center of rotation, neutral stability about the cg in all axis of rotation, no aerodynamic stability, stays where it's put, goes where it's told, lightest possible prop. Sounds a lot like an Edge 540! Flies the same upside down or upright, Right KE or left. Same magnitude corrections, adjusted for attitude. Gotta love it!

Reply to Multiflyer: Is this a great hobby or what? We can choose how we want to tumble at the top of those maneuvers, whether by aerodynamic inputs to the surfaces in the propblast, or by building and using a 2 lb prop so the gyroscopic precession does it for us!

All the best in '06