To: helmet fire
Response to (B)
Safety Notice SN-11 states in part, If the pilot attempts to stop the right roll (experienced during zero G) by applying left cyclic before gaining main rotor thrust, the rotor can exceed its flapping limits and cause structural failure of the rotor shaft due to mast bumping or allow a blade to contact the airframe.
The key word is/are “flapping limits”. As I had indicated in my last post helicopters that have blade flapping capability employ some type of stop that can be unlocked as the rotor comes up to speed which allows the blade to flap above and below the static non rotating position. Therefore, the movement of the blade is virtually un- impeded that is, until the blade comes up on its’ flapping limit stop. If the blade hits this stop during maneuvering the pilot will experience a very heavy beat and he will back off on his control input. In order to hit this stop the pilot would have input excess cyclic. Hitting the stop is not a bad condition as long as the pilot responds in a timely manner. The Robinson on the other hand has the flapping limits set by the static (droop) stop built into the rotorhead. Any coning or flapping will be above this stop. If the flapping level is very high as in an incorrect application of left cyclic in recovering from zero G the blade will flap down until it hits the stop and at that time there is a mechanical linkage between the blade and head and it in a sense is like one half of a Bell blade. If both blades hit their respective stops during rotation the lock up can apply severe bending loads to the blades and the lock up can force the rotor to teeter to the point that it contacts the mast or a blade strikes the fuselage.
Response to (D)
What makes the Robinson head different from a Bell head is that the Robinson head employs coning capability separate from the teetering capability, which is common to both types of rotorheads. The cone hinges were incorporated on the Robinson head to minimize the bending loads on the blades during application of collective or during maneuvering. With the incorporation of the cone hinges the Robinson head further deviated from the design of the Bell rotorhead in that the pitch horns had a 72-degree lead as opposed to the 90-degree lead of the Bell pitch horn.
When the Bell is rigged for fore and aft cyclic the rotor blades are disposed over the lateral axis of the helicopter. Rigging for left and right lateral the blades are placed over the longitudinal axis of the helicopter. On the Robinson when rigging for fore and aft the blades are placed 18-degrees ahead of the right lateral axis of the helicopter with the retreating blade being 18-degrees behind the left lateral axis of the helicopter. This places the pitch horn on the longitudinal axis. Rigging for lateral control the blades are placed in a similar manner in respect to the longitudinal axis. Control input relative to swashplate movement is the same for both the Bell and the Robinson.
Both the Bell and Robinson behave the same relative to zero G but it is the Robinson design which permits flapping that allows the exceedance (SP) of flapping limits as opposed to the Bell which the blades can flap in unison with one going up and the other going down but not at the frequency and range of movement that allows contact with the stops resulting in mast bumping. The Bell system must flap a great deal by comparison before the Bell head hits the mast.
In summary, it is the fact that the blades can flap and that the limiting stops enter into the equation when left cyclic is applied during zero G recovery. Also the same applies to sideslip and out of trim flight which can cause excessive flapping to the point of contact with the stops.
Response to (E)
I believe it is easier to get into zero G difficulties in a Robinson than in a Bell although both are susceptible to mast bumping. This can be due to the weight difference in the respective airframes and the fact that the Robinson blades can flap and hit the stops.
Response to (G)
As previously stated if in fact the 18-degree offset comes into play then this is what happens. Because of the offset the pilot would have to apply right cyclic to compensate for the offset which means that if under this condition the pilot encountered zero G and he effected the recovery by moving the cyclic straight back without inputting any lateral control per the POH instructions he, would in fact, already have a right cyclic bias (for compensation for the offset) and this right cyclic bias would add to the right roll component generated by the tail rotor. This takes a rapid roll and upgrades it to a violent roll.
Response to (H)
First of all I do not know if some of what I propose for the test is possible. With that said, here is what I propose.
All movement in the first two elements of the test are made relative to the rigged neutral position of the cyclic stick with this position noted in the maintenance manual regarding cyclic rigging.
With the helicopter on the ground and the rotor turning up to speed move the cyclic gently forward and on a line with the rigged neutral position and observe the movement of the disc. Does it tip down over the nose or, does it tip down and to the left?
Lift the helicopter up several feet and trying not to compensate for cg shift or propeller effect move the cyclic forward on a line with the rigged neutral cyclic position and hover taxi at a slow speed. Does the helicopter fly forward or to the left? Propeller effect may cause left movement so note the disc position (see above) to see if it is tipped down over the nose or, down and to the left.
Noting the rigged neutral position of the cyclic bring the helicopter up to speed where you have passed through inflow roll and you have compensated for blow back. Note the position of the cyclic stick relative to the rigged neutral position. Is it on a line with the rigged neutral lateral position or, is it to the right of that position.