Brenner

Hey Chuck,

You are right about about the airflow over the rear prop being faster than the airflow into the front prop. I was considering this when I originally designed the Contra Drive, and I calculated that in a steady state vertical upline the flow over the back prop would be about 10% faster than the flow over the front prop.

When the plane is held statically there is a much larger difference because the flow essentially accelerates from zero to the blast of air that we all experience when we try to hold a plane at full throttle. During a steady state high speed pass the change in flow velocity is much smaller, probably less than 1%.

My original thinking was to try and optimize the front and rear props so that the pitches are matched during a steady state vertical upline, because this is the flight condition where we spend most of our time at full throttle, and under these conditions the best combination should be the 20X18 on the front, and a 20X20 on the rear like you said.

However at the time all of this was just theory, and we had genuine concern about low speed performance with high pitch props that was harder to predict because the props were going to be partially stalled until the plane got up to speed, so we made the decision to stick with a single pitch and use it as a starting point. Of course in hindsight, now that I'm flying the Contra, all my concern about low speed performance has turned out to be a bit foolish, but regardless, using a 22X18 on both the front and rear has turned out to be an excellent starting point, and I am extremely pleased with the performance of these two props so far.