Light Plane Philosopher
- Dec 16, 2007
- Port Townsend WA
I think the C-185 seaplane is around 1100 fps at takeoff. Not much choice with direct drive.Metal may be able to handle 1,000 fps, but the propeller may be extremely noisy; my experience in the propeller industry (I did prop aero at a little company that made propellers in Windsor Locks, Connecticut) is that most commercial operators (ATR-42, ATR-72, etc) keep tip speeds down to about 700 fps, with a couple of outliers to about 800 fps. We didn't do propellers small enough to be swung by an auto-derived V-8 when I was there, so maybe small props have different limits.
Advance ratio is simply airspeed (in ft/sec) divided by the product of prop rotation rate (in rev/sec) times diameter (J = v/(nD)); it's one of the four important non-dimensional prop parameters (the others are thrust coefficient, power coefficient, and activity factor)
But with a gear drive the designer has the option to choose lower tip speed.
Did the aircraft or prop designer recommend an optimal prop rpm so the builder could then choose a proper gear ratio?
I think the gear ratio should be calculated after optimal prop rpm is determined.