The size designation of all propellers is comprised of two basic measurements: Diameter and Pitch, both measurements.
APC & EMP Propellers
On APC and EMP propellers, those numbers appear a Diameter X Pitch (e.g., APC 8 x 6: the diameter of the propeller is 8 inches, the pitch is 6 inches).
On GWS propellers (as well as some Emax propellers) the numbers appear as a four-digit cluster (e.g., GWS 8060: this propeller has a diameter of 8 inches and a pitch of 6 inches).
On GWS 3-bladed propellers, that four-digit cluster is appended with “x3” to indicate the number of blades (e.g., GWS 8040×3 = 8 inch diameter, 4 inch pitch, 3 blades). Sometimes the diameter and/or the pitch of a propeller is measured in both whole numbers and fractions.
APC & EMP
In the case of APC and EMP propellers, this measurement appears in whole numbers and decimals (e.g., APC 9 x 3.8: this propeller has a diameter of 9 inches and a pitch of 3.8 inches; APC 5.5 x 4.5: the propeller diameter is 5.5 inches, and the pitch is 4.5″).
But what does “pitch” actually mean and how does it pertain to the performance of the propeller and the aircraft on which it is mounted?
The measurement for pitch does not pertain to a physical measurement of the propeller itself, but rather it indicates the distance in inches of forward travel per revolution of the propeller. For example, a propeller with a 6″ pitch will travel 6 inches forward for every revolution of the propeller. From that information, one can determine the airspeed of the plane upon which the propeller is mounted — provided that one also knows revolutions per minute (RPM) of the motor spinning the propeller.
Determine the RPM
The best way to determine the RPM of a motor/propeller is to use a tachometer while running up the motor on the ground or on a workbench.
Once you have the RPM, you can calculate speed using the following formula:
- Multiply the RPM by the propeller pitch (e.g., RPM 5699 x 6 = 34194)
- Divide the sum of the above calculation by 1056* (e.g., 34194 / 1056 = 32.380)
- The dividend of 32.38 is the speed in miles per hour.
*1056 = 12 (inches per foot) x 5280 (feet per mile)
If you do not have a tachometer, you can use the known thrust of a motor/propeller combination to get a fairly decent estimate of the RPM. Most of the motor listings in our web store include prop data tables that indicate the amount of static thrust produced by various propellers in combination with the motor.
If you know the thrust power (ounces) produced by a specific propeller on your motor you can use the calculator at the following link to determine the RPM:
(you will need to select the propeller brand, size and pitch and enter the thrust ounces; leave the altitude at the default of 800)
Once you have the RPM, refer to the above formula to determine the propeller pitch speed.
Please Note: The calculation of pitch speed does not account for real-world environment variables such as aircraft weight and aerodynamics, wind or air density nor prop slip. Pitch speed is therefore only an estimate and actual flight results may vary. Real-time flight-speed is best calculated either by radar on the ground or a telemetry system onboard the aircraft.