Originally Posted by
Uplinker
As a fixed-wing pilot who is interested in all things technical, please could someone briefly explain what helicopter mast bumping is, and why teeter head unloading is dangerous on the R22/44.
A teetering head is a Rotor Head, which has a central undamped teeter hinge which freely connects the Rotor mith the mast and subesquently the cabin. This means the cabin is just suspended freely underneath the disc and it can act as a pendulum. In contrast to that rigid rotor systems or fully hinged rotor systems do transfer moments between disc and mast/cabin (due to excentricity of the hinge points). When the rotor disc is tilted (due to cyclic input or external force) it will apply a moment on the mast/cabin forcing it to tilt into the same direction. A teetering rotor does not. It lets the disc freely tilt while the cabin stays as it is. To limit this tilting on the ground and while starting up there are mechanical stops on the teetering head which contact the mast when a certain tilt angle is achieved. In flight the cabin is kept vertical simply by gravity (or centrifugal force when in a turn). If a low G situation occurs, this centralising effect of the weight of the cabin 'hanging' underneath the teeter hinge will be reduced/gone and the rotor being unloaded it will produce less lift, i.e. also produce less horizontal force opposing any sidewards force from the tailrotor. Now disturbances (like the tail rotor pushing sideways on an axis higher than the cg) are able to push the cabin out of the vertical, If now the pilot wants to counteract this tilting of the cabin by opposite cyclic input the disc can and will unopposed and easily tilt against the direction of the cabin without applying any force to take the cabin with it. That is up to the tilt angle where the teeter stop hits the mast. So there is a sudden change from no moment to 'rigid' force transfer. This in many cases happens so violently that the thin rotor mast will receive a massive dent and in many cases finally sheer off. That said the angle at which this happens is so big that combined with the coning hinge travel plus elasticity of the blades the excursion will be sufficent for the blades to chop through the cabin and/or the tail boom.