This is from the Knife Edge forum website:
Quote from "dhk79":
"Selecting the right prop for a plane is probably the biggest factor in improving its performance. On the other hand, the wrong prop is best way to ensure you have a dog in the air.
All prop manufactures have a chart to select the correct sized props to match engines. Below is a copy of Top Flite's. Notice anything that is omitted from the chart? If you said the type of plane, you're right on the money because the chart only addresses the engines.
As an example, say we have two scale planes both with .90 engines. One plane is a scale bi-plane with full rigging and guy wires and the other plane is a low drag pattern aircraft with retracts. Even if by some wild stretch of luck these two planes weighed the same, you would not expect them to fly anywhere near alike. So why would you expect the same propeller to work for both airplanes? The answer is, of course, it won’t.
The way out of this dilemma is to use a ratio of diameter to pitch to help rate propellers to determine the type of aircraft it will be best suited on. For best aircraft performance use a prop that is in the power range for your engine, but match the prop ratio to the type of aircraft that the engine & prop is going on.
Draggy/3D - 3:1
Trainer - 2:1
Sport - 1.5:1
Racing - 1:1
Using the above ratios, let's put the same .40 engine on several different planes. The engine manufacture says you can use a 10x4 to a 11x6. Well a biplane would do well with something in the draggy range, so it would take a 10x4 (ratio of 2.5:1). Now a hot pattern plane needs a prop in the racing range, so it would get a 10x8 (1.25:1). Sitting in the middle, a trainer could use a 11x6 (1.8:1)."
Image upon request...
