Calculating Propeller Efficiency
Going a little farther, we can actually set up a speed trial to determine
how fast an aircraft is going and then determine propeller efficiency using
those numbers (time over distance).
So let's say you time your aircraft on a 100 yard (300 feet) course (upwind and
downwind to make it even). The average time is 2.7 seconds.
Convert the distance covered to miles by dividing distance covered in feet by
number of feet in a mile. There are 5,280 feet in a mile.
300 ÷ 5280 = .0568 miles
Convert elapsed time to hours by dividing time in seconds by seconds in an
hour. There are 3600 seconds in an hour.
2.7 ÷ 3600 = .00075 hours
Find Miles Per Hour:
.0568 ÷ .00075 = 75.7 mph
If our timer was accurate and the distance is accurate then that speed will be
accurate. An easier way is to use a radar gun, but then you don't get to
do all this fun math.
Going back to the previous example, let's determine the overall
loss of efficiency and then, for convenience, blame it all on the propeller.
Divide actual speed by the theoretical speed using a 100% efficient propeller
and an aircraft having zero drag:
75.7 ÷ 99.4 = 76.16% efficiency
Unless we have an onboard tachometer, we do not really know what the RPM of the
engine is. Also, the lack of efficiency could very easily be attributed to
design - not necessarily the fault of the propeller. Still, it is something to play
around with if you are so inclined.