To: av8rbpm
There is another thread on rotorheads: "Ways to put it across".
This thread deals with the best ways to get a point across to students about the various aspects of flying helicopters. The poster suggests analogies and minimalist training aids. I will do the same. All it takes is imagery and visualization on your part.
Take a plywood sheet 4 X 4. Now take a metal plate 4 X 4 and lay the plywood sheet over it. Connect the respective corners of the plywood sheet and the metal plate with springs. Install a padeye at the exact center of the plywood sheet and connect a line to the padeye. Using some means of lifting device connect the line to the lifting device. Lift the plywood sheet and the springs will extend to the point that they will collectively lift the metal plate. The plywood and the metal plate will remain parallel to each other (within limits). Lets assume that the springs stretched three inches.
Now install a mechanical linkage between the plywood sheet and the metal plate. This linkage is 2 long and installed left of the center and slightly forward of the centers of the plywood sheet and the metal plate. Again lift the plywood sheet. The mechanical linkage will immediately cause the metal plate to rise and in the process the plywood sheet will pivot about the connection point of the mechanical link so that they are no longer parallel to each other. As the plate and plywood sheet continue to rise the springs will extend in order to keep the metal plate and plywood sheet parallel to each other.
Now apply the same situation to the helicopter. The springs are the elastomeric supports. They can be compressed by the lifting force but to a limit. So, they allow limited movements of the transmission. However the actual mechanical lifting force is provided by the lift link which is not on the actual center of lift (Main rotor shaft). Any movement of the transmission due to control inputs will result in the transmission pivoting about the lift link connection. This applies loads greater than the actual lifting of the helicopter on the elastomeric supports causing them to wear and with the end result of pylon rock. With new elastomeric mounts the pylon rock is still there but it is subdued.
