excessive flap in slip flight
 

I remember seeing the old army video about mast bump that said out of trim flight could cause excessive flap and lead to MB. Why does the rotor flap more in out of trim flight? And how would this lead to mast bump? Thanks in advance.

I don't think actual quantity of flap would be increased, just relative to aircraft. Slipping causes fuselage to roll relative to disk...

remember that the helicopter just hangs under the blades. so when the aircraft is turned sideways the aerodynamics is gone and the drag on the air frame holds it from moving through the air as fast as it needs, tiping the heli sidewards,lowering the disc in direction of movement, increasing sidward air speed, increasing drag on the frame, to stop the disc nosing down(in the direction of travel)too much oposite cyclic will be necisarie. and if maximum cyclic(sidewards)is achieved, mast bumping would be geting close. <img src="confused.gif" border="0">

vortecy is on to the reason, I think.

The helicopter has a balance of pitch axis forces, with the fuselage, the horizontal tail and the rotor (through longitudinal cyclic) all part of the sums. In a sideslip, the tail and fuselage balance is upset, and the horizontal tail can be reduced in effectiveness due to the angle of sideslip and also if any fuselage blanking takes place. Blanking means that the tail can be placed in the wake of the fuselage, which reduces ite effectiveness.

This can strongly effect the need for cyclic pitch to rebalance the aircraft, so much more flapping can be needed.