last updated 

Radio Control Club of Rochester
AMA charter 465       since 1957
DEDICATED TO RESPONSIBLE R/C FUN
SAFETY BY CHOICE   NOT BY CHANCE

Propeller Efficiency Calculator P. Ponk Aviation CLICK HERE
Dave Patrick, in his book Aerobatics For Everyone,
says prop tip speed of 560 feet per second (.515 MACH or 382 mph) is very quiet,
and 650 feet per second (.598 MACH or 443 mph) is where it starts to get noisy.

To determine propeller tip speed:
Prop Diameter X Pi / 12 X RPM X 60 / 5280 = Tip Speed in MPH
Example: 86" prop turning at 2800 RPM
86 X 3.1416 / 12 X 2800 X 60 / 5280 = 716.4 MPH

To determine the speed of sound:
Square Root (absolute temp + ambient temp) X 33.4 = Speed of Sound
Example: 59 degree F. day
Square root (460+59) X 33.4 = 760.9 MPH (speed of sound)

To determine propeller tip mach speed:
Tip Speed / Speed of Sound = Tip Mach Speed
716.4 / 760.9 = .942 MACH (too fast)

Airspeed Calculation:
This is an approximation based on the propeller pitch and engine rpm. It assumes that the rpm of the engine increases enough in flight to make up for the fact that thrust goes to zero at the calculated airspeed (based on static rpm measurement). It also assumes that you have done a reasonable job of matching your engine and propeller to the plane.

Velocity in miles per hour = 9.47E-4 x RPM x Pitch

RPM = propeller speed in revolutions per minute.
Pitch = propeller advance rate in inches/revolution.

Notes from the SIG Factory Fliers on ..... SIG PROP CHARTS (from the SIG catalog)

Both the "PROP CHART FOR 2-STROKE ENGINES" and the "PROP CHART FOR 4-STROKE ENGINES" are intended to provide an R/C sport flier with a safe, dependable starting propeller to use on a typical sport/trainer type model airplane. While the "STARTING PROPELLER" listed may not deliver optimum performance in every single case, it should get the model off the ground and flying nicely with the engine operating in a safe RPM range. This will provide a starting point from which other size props, either from the "ALTERNATE PROPELLERS" list or from the engine manufacturer's instructions, can be tried and compared. The model's size, weight, drag, wing loading; the type of engine being used (sport, pattern, racing, etc.) and its actual power curve; the type of fuel being used; and even the altitude at which you are flying, are all factors in finally determining the optimum propeller for each different airplane. The optimum propeller can be determined only by flying with different props and noticing any differences in the model's speed and climb.

In general terms, a higher pitch prop will pull the airplane faster in level flight. A lower pitch prop will cause the airplane to take off quicker and climb faster. Some full-scale airplanes have adjustable pitch props so they can use the most efficient pitch in each situation. The pilot will select low pitch for the takeoff and climb to altitude, and then switch to a higher pitch for better level flight speed and fuel economy. It's exactly like switching gears in a car! Low gear provides quick acceleration from a stop, while high gear is used for better fuel economy after the car is up to cruising speed. Even owners of full-scale airplanes with fixed pitch props, like a J-3 cub, can choose between at least two different FAA approved propellers - one called a "climb prop" (lower pitch) and another called "cruise prop" (higher pitch).

On a model airplane, you should not only try different pitch props, but different diameters as well. For example, let's say you are running a .60 2-stroke engine and start out with an 11-7 prop. The model will very likely fly real nice.

Next, put on a 12-6 prop, readjust the needle valve and fly again. Watch carefully! This time the model should be able to takeoff in a little shorter distance and you will be able to pull the nose up a little steeper on the climbout without stalling. However, the level flight speed will probably be slightly less than with the 11-7. So while the engine turns both propellers at approximately the same RPM, the flight characteristics of the airplane are slightly different with each prop. Don't be afraid to try another different size propeller and note any further changes. Usually the changes will be very small, and there will always be a tradeoff of some kind - what you gain in one aspect of performance, you may lose in another. By trial and error you will eventually determine which size prop best suits your particular model and how you want it to perform. For safety, balance all propellers before use. Discard propellers with nicks, cracks, or visible defects of any kind.
 

To get a really neat converter program
download it from Josh Madison
1 cu.in = 16.38706 cc
.06102375cu.in. = 1 cc