For example in forward flight what could cause the blade to flap lower than parallel to the CF at the front of the aircraft ?
The first thing that comes to mind does not occur in forward flight. That would be a hard landing. Depending on the rotor design and the amount of articulation and the landing gear type, a hard landing can result in some serious problems, notably, Ground Resonance.
In-flight, it may be possible to achieve with a rapid unloading of the rotor (Zero - G) plus cyclic input. One adverse condition which causes negative flapping is Mast Bumping. Usually, if not always fatal. Mast bumping takes a specific set of conditions and control inputs, to take place. It is typically occurs with 2 bladed teetering rotor systems at zero G.
First, why does the blade need to flap "negatively"?
- In forward flight the rotor disk must be tilted forward to produce the thrust required to overcome drag.
- In order to tilt the rotor disk forward the rotor blade must be at the lowest point in its range of flapping when it is over the nose (12 o'clock), and at the highest point in its range of flapping when it is over the tail (6 o'clock).
- For the above to occur the blade must also be at the neutral point (exactly halfway between the highest and lowest points) at the 3 and 9 o'clock positions.
- Therefore the blade must be flapping "negatively" when it is between 3 o'clock and 9 o'clock.
The flapping of the rotor blades in forward flight is primarily controlled by cyclic feathering --- in other words, the pilot must hold forward cyclic to maintain forward airspeed.
I don't believe that mast bumping is a concern with fully-articulated rotors.