RCU Forums - View Single Post - Contra Rotating Propeller Drive for f3a 2m Pattern Planes
Old 07-23-2010, 09:41 PM
  #52  
Brenner
 
Join Date: Jul 2007
Location: Bridgman, MI
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Default RE: Contra Rotating Propeller Drive for f3a 2m Pattern Planes

Attached is a table that shows relative performance of various Contra prop setups. I'm showing it because there was a general concensus amongst pilots at the Nats that I needed a setup that had more speed. The way that Dave Snow explained it to me was that just because you are capable of flying fast, doesn't mean that you have to fly fast, but it helps to have the extra speed in reserve for those days when the wind is strong, and it's a ninety degree crosswind. It was actually like that on the Sunday before the start of Nats competition, so I'm fully aligned with what he's telling me.

Anyhow, I thought that this table would be interesting to show, because it shows some of the benefits of going to a Contra setup from a single prop setup, and it shows the various trade-offs for different Prop diameters and pitches.

First off, the general conclusion seems to be that bigger is better as far as diameter is concerned. This is because it looks like efficiency is increased without any real trade-off with respect to other performances. Secondly, there seems to be a direct trade-off between prop stall speed, and maximum level flight speed. Also, it seems that there are diminishing returns with regard to steady state vertical rate of climb, with the maximum being reached with a prop pitch somewhere between 21" and 22".

All of this suggests that the best combination might be either a 22X20, or a 22X21 pair of props, or somewhere in the vicinity. However, all of this is a very quick first pass look at the trade-offs. I plan to do quite a bit more work before I make a final choice for our second design iteration.
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