(RPM) X (PROP PITCH) X .0007 = AIRSPEED
Got this off another forum, don't no if it is true or not.
Ken

There is such things as slippage and drag that makes the plane slower

i plugged this formula in and figured without factoring slippage and whatnot a 31cc ryobi on GGL spinning roughly 8200 rpm spinning a 16x8 prop it would only go about 50 mph!? this formula must be a bit out of whack b/c i know a guy that has a big 'ol cub with a similar setting as this, and his plane is alot faster than that! no radar to prove it, but that plane would fly circles around my trainer which is a lt 25 with a webra 40 on it.

Just manually calculating it I multiplied the pitch in inches times the rpm, divided by 12 inches in a foot and then multiply by 60 for one hour. To then divide through by 5280 ft in one mile, I get 62 mph

But this does not allow for inefficiencies, drag, and slippage.

then maybe i'd better get a bigger prop for my engine when i get a plane built for it

Here's a copy and past of another post I did, in answer to a guy with a small gas conversion engine on a Mustang. He had very good static thrust, but poor flight performance. I used the Thrust HP program to get the speed numbers, but they can be calculated mathematically as well. It is not an exact science, both because of slippage factors varying greatly depending on airframes and props, and due to the fact that it is not usually known exactly how much the engine unloads and speeds up in flight in any given case. You can get close enough for good performance though, by matching your pitch speed to your airplane, and fine tuning by experimenting with props in flight.

Here's the other post:

Something for those new to gasoline engines to think about; Pitch Speed . A normal sport glow engine likes to turn in the 10 to 12000 rpm bracket, and unless you are into 3D, will run a prop with around a 6 to 8 pitch . The gasoline engines have a lot of torque, but usually don't turn such high rpms, with 6 to 8500 rpms being more normal. So here's an example of pitch speed .

.60 size glow engine, 11x8 prop at 11,000 rpms = approx. 83 mph in the air.

20cc gas conversion engine, 16 x 6 prop at 7000 rpms = approx 39 mph in the air.

Quite a difference as you can see. 39 mph would be not all that much above the stall speed for a heavily wing loaded Mustang, especially in a .60 size plane, and very doggy in the air even at full throttle. Power absorption is a function of diameter and pitch . Once you have a target rpm, you need to select pitch to match your airplane, and then select a diameter that will let the engine turn that rpm. I don't know what prop you are using torkwrenchmon, but to begin experiments with the combo you had, I would be looking at a 13 x 10 or 14 x 8 prop to start, and hoping for around 7500 to 8000 rpms.

Your comment that "it felt like it was going to jump out of my hand" but then didn't fly well, suggests to me you had a lot of static thrust but not enough pitch speed which would indicate a prop that was too large in diameter and had too little pitch .

Good luck,
AV8TOR