A good place to start is to calculate your pitch speed. Many pieces of software, such as ThrustHP can do that. The pitch speed is just the speed that the propeller throws the air back at, basically the prop''s pitch x the rpm x a conversion factor for mph or kph.

That gives you a good number to start with. My AX46 powered mustang has an 11x6 prop. It turns it at 13,000 rpm, give or take a couple hundred for the weather. That returns a pitch speed of 74mph. That''s at static ground RPM.

A plane in flight is gonna achieve a different speed than this for a few reasons. First, in the air, the engine "unloads" and achieves better RPM''s. So the pitch speed goes up. We think the ''stang''s engine is turning about 15K in the air, and that would up the pitch speed to 85mph. However, the plane has drag, so it won''t likely achieve quite that. The draggier the plane, the less it''ll achieve. You know you''ve got a draggy airframe if you put it in a full power dive and the engine barely speeds up, it still has to work to pull the plane down. Slick airframes will wind their engine right out, quite an impressive sound. Some are slick enough to over-rev the engine and can occasionally blow it up.

When someone asks me how fast my planes are, I give them the static ground pitch speed as an idea of things.

I have that hotwheels radar gun to try this summer, though. We''ll see how close my guess was (''course, it''s a toy, with poor range and accuracy, but trials in teh car have been promising.... If it can see the plane, it should be within about 5%)

J