There could be two possible explanations for this:
1. (Most likely) Rotor blades act like wings, and when spinning, they start to produce aerodynamic lift which will support their own weight and stop them from drooping.
2. (Not particularly significant to this example) As a body is spun, the centripetal force acts in a plane to which the axis of rotation is perpendicular. This pulls the blades outwards, and to a certain extent, if they are drooping, this could appear to 'lift' them, even though the downward component of their weight remains the same.
As you should be able to see from this, simply spinning a heavy object around an axis does not result in any net transational force. If the object is out of balance, a net force will be produced, but the fact that the force is changing direction constantly means there is still no net acceleration.
The only known way to produce a force on an object is to push it. This invariably means it either interacts with an external body or medium (e.g. air) or simply ejects matter to produce a momentum change.
Pie
[This message has been edited by Pielander (edited 30 June 2001).]