Pusher prop, why not just turn the propeller around?

On Fri, 30 Jul 2010 02:42:17 +0000, John Doe scribbled:

you can do that if you can reverse the rotation of the motor. With an electric, that is easy. With a gas/nitro motor, not as easy.

What if you want the airflow to go away from the motor? Isn't that what a pusher propeller does, pushes air away from the motor instead of over the motor? Can you produce the same effect by just reversing an ordinary propeller? Is the only difference the way the propeller is drilled, but the center is still the center no matter which way you mount it? The propeller fits the mounting hardware better when it is mounted the right way?

I tried to restate the same question, hopefully to be clear.

Thanks.

On Fri, 30 Jul 2010 03:38:08 +0000, John Doe scribbled:

You will still need to reverse the direction of rotation of the motor to get the air to go the other way. Otherwise, it is going to flow the normal direction with high in-efficiency.

On Fri, 30 Jul 2010 03:38:08 +0000, John Doe scribbled:

A pusher propeller has the blades pitched the opposite of a normal propeller. With a normal propeller, it has to rotate counter-clockwise for air to flow though it properly. To reverse the airflow, you need to rotate the propeller clockwise. To make it efficient rotating clockwise, you need to turn the propeller around.

It would seem that way. However, no matter which way you install the propeller on the engine, if the engine shaft rotates counter clockwise the prop will blow the air over the engine. Take a model propeller, look at it edge on, then rotate it 180 degrees and you will see the same angle of attack between the airstream and the propeller blade. Only when it is reversed, the airfoil shape is very inefficient.

Ron Kelley

Got it, thanks.

If you simply flip the propeller, it _will_ _not_ reverse the direction of the airflow. It will continue to "propel" the air in the same direction as before the flip, i.e. over the motor (Just try it, if it is not obvious to you.) The only thing that will change is that the intended leading edges of the blades will become trailing edges (and vice versa), which will essentially destroy the efficiency of the prop.

Without a special pusher prop, the only way to turn a puller setup into a pusher setup is to reverse the direction of the motor. This will also turn the leading edges of the prop into trailing edges etc., which is why you also have to flip the prop to get back to the normal efficiency.