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HOW TO CHECK FOR THE CORRECT PROPFrom CP55, Page 10 (April, 1988)
With any of the RAF
designs, matching a perfect prop to your new plane is not real easy but it is not all that
difficult, either. The main problem is that the stall speed and the maximum cruise speed
in level flight are quite far apart, and getting a fixed pitch prop to cover the whole
range perfectly, is not possible. You have to accept a compromise.
With the engine thoroughly warmed up, park on a clean piece of
hard surface in the runup area, lock the brakes, and smoothly go to full throttle. If you
are at a high density airport, you will have to lean slightly to get maximum static RPM.
If you do not see at least 2350 RPM, your prop is already suspect! 2400 RPM is better and
2500 RPM is not unacceptable. Of course, all of this assumes you have an accurate, easily
read tach without which this test cannot be conducted. Now, taxi out and make a normal
full throttle take-off. Observe the RPM during this take-off roll (make sure you watch
where you are going, use only an occasional glance at the tach). The RPM during the roll
and early part of the climb should be the same, or actually increase slightly. Maybe 100
or so RPM above static.
Use a normal climb speed, best rate or even a little higher for
good engine cooling and better visibility. The RPM should hold at your static or a little
better all the way to 8500 feet MSL. As a rule of thumb, your normally aspirated aircraft
engine will develop approximately 75% power at 8500 feet at full throttle, mixture leaned
to peak RPM.
At full throttle, leaned to best power, concentrate on
maintaining exactly level flight at 8500 feet (altimeter set at 29.92) fly for several
minutes in this condition to allow the airplane to accelerate to its maximum speed. When
you are certain it won't go any faster in level fight, read the indicated airspeed and OAT
(just for your own reference, since this is your maximum 75% cruise speed and you can
figure your true airspeed if you know the airspeed calibration error and instrument
error). The RPM at this point should be 50 to 100 RPM over the factory recommended maximum
RPM. If it is not, you will probably never realize the full take-off and climb potential
of your airplane. Now, obviously, most people would not want to fly at over the factory
red line, and that is good, you don't have to. This is just a test to see if you have as
close to an optimum prop as possible. If your prop meets the above RPM limits, you have
the best possible prop for all around performance, good take-off and climb and good high
You must now decide if you want to sacrifice maximum speed and
shortest take-off and climb performance for something more of a cruise prop, say 2300 RPM
static and 2700 RPM at full power at 8500 feet. This will let you cruise at reasonable
speed, good economy and a fairly quiet cockpit but you will give up take-off and top speed
performance. On the other hand, a 2500 RPM static and 2900 or even 3000 RPM flat-out at
8500 feet will give you excellent take-off and climb and a very high top speed - it all
depends on what you want! You can not have everything with a fixed pitch wood prop but at
least you do get a reliable, safe, economical , easy to maintain prop.
You must do this test at 8500 feet because 75% power is the
reference point for maximum cruise speed on all light planes and because this is the
easiest way for you to know you are putting out 75% power. Doing all of this at 3000 feet
or 5000 feet really does not tell you anything at all unless that is where you always
intend to fly.
Keep in mind that this is a quick and easy rule of thumb type
method that will work well for most homebuilders. It is not the absolute epitome in
accurate testing methods but it does give surprisingly close results.
One other comment: We have seen some props that only turn up 2100
rpm static and 3000 or 3100 rpm at Vh (maximum speed in level flight). The performance
results with these props, in our opinion, are not acceptable and the designers of these
props need to go back to basics and learn how to really design a good wood prop, such as
the three prop manufacturers we have listed in this CP, produce.