Someone on ezone proposes reversing his brushless motor in flight, to reverse prop rotation sense, for purposes of slowing down his airplane. I'm not familiar with cam folders, so I'm not sure how the cam works. Does it extend the prop blade regardless of the sense of rotation of its driving shaft?

I'm thinking that reversing a folder will simply cause it to fold even more quickly than the airstream would (as it's designed to do) in the case of a stopped driving shaft.

From : Don Stackhouse

I'd advise against this. You can get plenty of drag from a prop just by applying a brake to it while allowing it to windmill, without actually reversing its direction.

Trying to run the prop in reverse will potentially put huge bending loads on the blade roots and hub during the reversals of rotation, possibly damaging them and setting the prop up for a blade failure, probably just after advancing the throttle for takeoff. The airplane will start shedding high-energy bits while the remaining bits will start flopping around and try to eat everything within reach, including the rest of the plane, and possibly even the launch person's arm. In addition, if it does succeed in reversing the direction of rotation, the prop will almost certainly fold.

In the case of these Graupner props, "CAM" stands for "Computer Aided Manufacturing", and has nothing to do with the unfolding sequence. The blades are simply hinged at their roots and open due to centrifugal force and thrust loads when the prop starts to rotate. They can stay open while windmilling if the opening effect of the centrifugal force is greater than the closing effects of the windmilling drag (which is why you usually need an ESC equipped with a brake to get them to close in flight. However, if the prop was actually powered in reverse, I doubt that you could get it to stay open, the centrifugal forces would no longer be strong enough. The blades would close as the RPM decayed through zero, and then they would not want to reopen as the prop started rotating backwards.

Don Stackhouse
DJ Aerotech