RE: Contra Rotating Propeller Drive for f3a 2m Pattern Planes
Yes, I think that making the front and rear props different sizes would help reduce the synchopating rhythm noise. However, making the front prop smaller would also increase its rpm, because the planetary gearbox automatically shares the power going into each prop so the rpm of the front prop would increase in order to compensate for the reduced diameter, which could possibly have a negative effect on noise.
Also, one of the primary benefits from using a contra rotating prop system would be to have each prop cancel out the gyroscopic effects of the other, and this effect might be reduced if the two props were different diameters. I'll have to look at the equations for gyroscopic moment and power to see if this true, or if the increased rpm of the first prop equalizes the gyroscopic moment regardless of differencs in prop diameter. (within reason of course ..)
My current thinking is that noise can be affected in the following ways:
1/.. Mismatched prop diameters. (see above..)
2/.. Reduce rpm, and increase stalling of blades when plane is held statically by increasing prop diameter, pitch, and gearbox ratios. (reduces ground clearance and reduces performance at slow speeds)
3/.. Reduce rpm, and increase stalling of blades when plane is held statically by increasing pitch, and gearbox ratios. (reduces performance at slow speeds)
4/.. Increase separation distance between front and rear props. (reduces counter whirl effect, and distance is limited by need to keep the length of planes under 2m)
5/.. Mismatch prop pitches. (This would mean that each prop has a different operating point, and would spin at different rpms, both of which might reduce efficiency)
Right now I'm thinking that the thing I can most easily trade off is low speed performance, because with the power available I currently have no performance issues at low speed, so this leads me to think that increasing prop pitch and the gearbox gear ratio is the best place to start. I figure I can drop about 200 rpm by going to a pair of 20X20 props and still maintain prop efficiency and flight performance in the air.