Take a look at this figure of two different rotor blades:

The top one is just spinning around the upright rotor mast. The bottom one has flapped up, due to dissymmetry of lift.

If we assume a fairly regular rotor blade, the center of mass of the rotor blade is halfway out. The green line represents the center of mass of the unflapped blade. The blue line represents the center of mass of the flapped blade. Notice that the center of mass has moved in when the blade has flapped.

Conservation of angular momentum demands that as the radius of the center of mass decreases, the rotation rate must increase. Thus the flapped blade must be spinning faster than the unflapped blade. A similar effect is seen when a spinning ice skater pulls in her arms; the rotation rate of her body increases dramatically.

Return to Aerodynamics page