diceco

In order to accurately compare prop RPM data at different atmospheric temperatures and pressures, normalize all the data to "standard atmospheric conditions". There are many sets of these depending on what organization is defining them but I use these: 59 deg F ambient temperature, 14.7 psi (29.9 inches mercury (Hg)) ambient pressure, and zero humidity.

To normalize RPM, a correction for temperature is all that is needed.

This correction is:

RPM measured divided by the square root of theta (the Greek letter. Sorry, there may be a way to type a greek letter on this keyboard but I haven't learned it yet!).

theta = (Tamb deg F + 460) / 519 or (Tamb deg C + 273) / 288

So to correct to standard conditions -> corrected Np (the letter "N" is often used to denote speed) = measured prop RPM / sq rt [(tamb+460)/519]

So, you can see that when Tamb is 59 deg F (standard temp) there is no correction. When Tamb is colder than standard temperature the corrected prop speed (Np) will be greater than measured prop RPM. Conversely when the ambient temperature is greater than standard, Np will be less than measured RPM.

For example:

Prop RPM measured = 6600 rpm
Ambient temperature measured at the time of the test = 40 deg F

theta = (40+460) / 519 = .9634
Np = 6600 / sq rt. 9634 = 6600/.9815 = 6724rpm

If you record the air temperature with your RPM data for all the runs you make, then you can normalize the data and more accurately compare it to different runs. This should give more meaningful results when you compare engine configuration changes. If you are able to make your runs all at the same temperature then you will get meaningful comparative results for those runs of course. But it's a great advantage to be able to compare runs on different days for it's often not possible to make all your performance measurements on days that have exactly the same ambient temperatures.

BTW the correction for thrust measurements is Fn = Fg x (sq rt theta) / delta
where:
Fn - Net thrust
Fg - Gross (measured) thrust
delta - (the Greek letter) = Pamb/Pstandard = barometric pressure, inches of Hg / 29.91

I usually just log onto a local weather Internet site and get the current barometric pressure. It's not the most accurate way to do it but most folks don't have an accurate barometer at hand and the local weather data is pretty close.

Humidity can be corrected for as well but it's effect is pretty small and can be ignored for these sorts of tests.

diceco