Posted Monday, March 26, 2007 @ 04:54 AM
Hi Folks,
A good way to reduce propeller noise if you have an engine capacity more than the original 1200cc is to go to a coarser pitched propeller, thus getting lower rpm.
I now have 1400cc pistons and cylinders on my Rectimo. When I first flew it with the original Hoffman HO-11 133s propeller, the prop tips were very noisy.
I now use a Hoffmann HO 11-145 B 80 L propeller (originally supplied for the early RF5 with the 1700cc Limbach motor). It is a little longer than the original, so there is slightly less ground clarance. I helped this by pumping up the main wheel tyre to a higher pressure, and fitting a smaller tailwheel.
To my amazement, not only was this propeller quieter, and greatly reduced the engine rpm, but it improved the climb rate and ALSO increased the top speed. Take-off roll is a little longer though (but still shorter than with the original 1200cc engine). Data below:
9th March 2006, Temp 30°C, QNH 1010 HPa, 1400cc engine. Standard Hoffmann 133/70 propeller, no spinner, 17 litres fuel (aerobatic weight)
Take-off rpm 3,200. Climb 500-1,500 feet 1:47 @ 60 knots and 3,200 rpm = 560 fpm. Max IAS 100 kts @ 4,000 rpm
Same conditions, new Hoffmann 145/B/80L propeller, no spinner, 16 litres fuel (aerobatic weight)
Take-off rpm 2,800. Climb 500-1,500 feet 1:34 @ 60 knots and 2,850 rpm = 638 fpm
9th March 2006, Temp 28°C, QNH 1010 HPa, 1400cc engine, Hoffmann 145/B/80 propeller, no spinner, 36 litres fuel (maximum weight)
Take-off rpm 2,800, Climb 500-1,500 feet 1:40 @ 60 knots and 2,850 rpm = 600 fpm, Max IAS 110 kts @ 3,600 rpm
Just for interest.
Yours, Bob
