ORIGINAL: Jim Thomerson
I think F1C is down to either 4 or 5 seconds engine run these days. What kind of HP, more or less, does it take to drive that size prop at 8,000 RPM? There was a letter to the editor in Aeromodeller which showed that the downhill changes in engine run in F1C have fairly closely tracked increases in engine power.
Prop dia 12.8
Prop geometric pitch 11.8
Prop RPM 8,000
Prop Efficiency 80%
Gearing 4:1
Engine RPM 32,000
Reduction efficiency 92%
Engine output 1.15 BHP
Power absorbed by prop 1.06 hp
Estimated thrust, sea level 7.60 lbs.
Thrust velocity 90 mph
Now assume it was direct drive, and your engine was still producing 1.15 BHP and you needed 90 mph thrust velocity
Prop dia 6.5
Prop geometric pitch 2.95
Prop RPM 32,000
Prop Efficiency 67%
Gearing 1:1
Engine RPM 32,000
Reduction efficiency 100%
Engine output 1.15 BHP
Power absorbed by prop 1.15 hp
Estimated thrust, sea level 4.74 lbs.
Thrust velocity 90 mph
See the difference in thrust
Now when I see that, I assume the engine is being run rather conservatively, and perhaps would have an active life of more than a few flights - very interesting -- anyone has pictures.