Erik Hofman writes: > You know how fast the aircraft goes at a certain propeller RPM. > Now you want to know the propeller RPM at a certain speed.
It's not quite so simple. A fixed-pitch propeller (or a constant-speed propeller at any given pitch) has a measurement of how far the propeller will move through the air in a single rotation without creating drag or thrust (i.e. as if it were a screw in something more solid). My propeller has a 60-inch pitch, which is typical, so it will advance 60 in (5 ft) for each rotation. That makes it easy to draw up a table -- just divide the fps by 12 (60/5) to get the calibrated airspeed in fps, and divide again by 1.69 to get knots: 1500 rpm = 125 fps = 74 kcas 2000 rpm = 167 fps = 99 kcas 2500 rpm = 208 fps = 123 kcas That's easy enough. The problem with windmilling is that the propeller does not spin all the way up to its neutral speed, but drags somewhere behind; for example, idling at 74 kcas, you're more likely to see around 1100 or 1200 rpm (I've never shut down the engine in flight, but I imagine it would be a couple of hundred rpm lower in that case). We need to figure out the balance between engine friction and compression (slowing the prop down) and the oncoming airstream (speeding the prop up). All the best, David -- David Megginson, [EMAIL PROTECTED], http://www.megginson.com/ _______________________________________________ Flightgear-devel mailing list [EMAIL PROTECTED] http://mail.flightgear.org/mailman/listinfo/flightgear-devel
