grnd0

My Feedback: (4)

I have a 18-6 Zinger now, which I like, but would like to
know if the Pro would perform better. Thanks

Larry

h82crash

You'll get higher rpm but that's because it is grabbing less air. It will have less thrust at the same size/pitch but it may have more speed.

Geistware

My Feedback: (16)

I have always understood that the Pro will give you more thrust because you will get a higher RPM but because the volume of air is less, you will get less speed compared with the same pitch propeller.

h82crash

Thrust comes from the volume of air the prop can pull on, speed comes from the way the prop screws itself through the air. Higher pitch=faster at a given rpm. If you reduce the pitch the load will be lighter and rpm will increase but probably not to the point of keeping up the same speed. The prop will be moving more air however and have more force. If the same pitch is turning faster because the load is lighter it will cut through the air faster and go faster. If your theory were true then a 15x6 would have more thrust because the same engine could turn it at 12000 rpm's. But no, it will take a lighter plane a whole lot faster.

Geistware

My Feedback: (16)

You can adjust the diameter and pitch to get the same loading, RPM, and volume of air with different propellers but the propeller with the larger diameter will have more thrust and the propeller with the higher pitch will give the plane more speed.

As an example:
mathmatically at 20x9.75 propeller and a 21x8 propeller will have the same static loading and hence the same speed on a given engine (fixed RPM). The higher pitch propeller will allow more speed. The longer diameter will give more thrust.

Here are the formulas from AMA mag Oct 86, play with the numbers:

Load = Prop Diameter^4 * Pitch

Speed = Pitch * rpm * 0.000947

Horse Power = Load * rpm^3 / 1.4 * 10^17

Static Thrust = 0.00000000000283 * rpm^2 * Prop Diameter^4 * Air Density/29.92 * CF value

h82crash

Yeah! What I said.