Username Protected wrote:
Seems like the weight issue is easily solved, but it must have a lot of asymmetrical loading on the crankshaft since the thrust developed is nowhere near the center of rotation.
The centripetal force dominates the forces on the prop.
For example, a 15 pound prop (7.5 lbs per blade), 15" center to CG of blade, going 2,500 RPM is 22,000 lbs centripetal force. The thrust force on a Cub is about 400 pounds.
You can build a slight coning angle to the prop, basically tilt the blade forward ever so slightly, such that the centripetal force cancels the thrust force. Given the ratios of forces above, that angle for this example is 1 degree. If you do that, there is no difference in the force this one blade prop generates versus a two blade prop on the crankshaft.
You can see coning angles in helicopter rotors when under lift. If the rotor is not moving, and you pick up the helicopter by lifting on the center of the rotor blades, you will break the rotor blades off. They depend on the centripetal force to stabilize the rotor disc.
The bending strength of the rotor blade is often limited by the off case, it can't break sitting on the ramp. Some helicopters, particular big ones, have droopy rotors when not in motion, the rotor can barely support its own weight, but in motion, it can lift the whole helicopter, due to centripetal force.
Attachment:
mi-26-droopy.png
Picture of rotor head. Note the swivels to remove any bending moment on the main rotor shaft.
Mike C.
