To: helmet fire
Let me give you a bit of history and then I will respond to your questions. Back around 1994 the FAA and NTSB became alarmed because of the high number of crashes of R22s and R44s resulting from main rotor separation. The FAA commissioned the University of Georgia Aeronautics department to evaluate the crash data and to develop a computer model that would point out exactly what was causing the problem. Although the report was never fully completed there was enough data to show that under certain maneuvering conditions the rotor system would develop high levels of flapping to the point that the rotor head would contact the mast or a blade would contact the fuselage. As a result of the report the FAA issued a priority letter AD which was issued on 13 January 1995. The FAA and NTSB negotiated with Robinson on what should be done to limit the possibility of flapping excursions. Robinson did two things, they initiated the safety course and an SFAR was issued outlining the safety requirements. They also modified the POH by including the FAA AD however they never put a page number and as such the full impact of the AD was never fully enforced as it took on the guise of only being a suggestion and not a hard requirement. In some cases in the UK the page was not included in the POH. From what I was told by the CAA they were going to rewrite it and make it a safety sensitive requirement.
I don’t know how this requirement is addressed in the safety course nor do I know if it is explained why the changes were made and why the Robinson helicopters are the only helicopters with the stated restrictions.
Response to your post:
A) Does not require a response
B) The POH does not state only during zero G. This is what it says in Safety Notice SN-11: If the pilot attempts to stop the right roll by applying full left cyclic before regaining main rotor thrust, the rotor can exceed its’ flapping limits and cause structural failure of the rotor shaft due to mast bumping or allow blade contact with the airframe. It goes on to say that in recovering from a zero G incident the pilot should gently bring the cyclic aft to regain main rotor thrust before applying lateral cyclic. I would consider the POH to be flawed only if the pilot has to correct for an 18-degree offset in the controls. If this were the case there would be a right bias in the controls so that when the pilot brought the cyclic back he could add to the right roll of the tail rotor. If it is eventually proven that there is no offset then there is no problem in what the POH directs the pilot to do.
C) The reason for the high level of flapping is covered in the history paragraph. I personally believe the high flapping loads are generated due to the design of the main rotor and that mast bumping is caused by the design of the main rotor. On other helicopters that incorporate flapping hinges the blades are allowed to flap above and below the static position of the blade at rest. This is allowed by the use of unlocking stops that are centrifugally operated. On the Robinson the blade is restrained from dropping lower than its’ at rest static position by a stop. I believe that when there are large flapping excursions the blade can contact its’ static stop coupling with the main rotor and driving a blade into the fuselage or causing mast bumping.
D) There are no basic differences in cyclic movement between the Bell and the Robinson unless it can be proven that the pilot of the Robinson must compensate for the 18-degree offset. If there is no requirement to compensate then the problem is mooted. The flapping hinges are not the problem (assuming 18-degree offset problems) it is the fact that the flapping hinges and not the flapping action that causes the 18-degree offset. The Bell has a phase angle of 90-degrees and the Robinson has according to Frank Robinson and Nick Lappos a 72-degree phase angle. I believe that the Robinson like other helicopters has a 90-degree phase angle and because of this, the pilot must input right cyclic to fly forward. Nick tried very hard to convince me but I respond to the physical and not the theoretical and as such I hope to perform a test in the next few weeks. The results of that test will be published on these threads no matter which way it goes. If it proves Nick right I will acknowledge that too and will in the process have learned something.
E) It is not the 18-degree offset on the Robinson that causes the problem it is the entry into zero G and improper technique in countering the problem. It is the same for the Bell.
F) No comment
G) As previously stated if there is an 18-degree offset then in the recovery from zero G in accordance with the POH the pilot will add to the right roll thrust caused by the tail rotor. If there is an 18-degree offset in the control input then if the helicopter is flying straightforward and the pilot brings the helicopter to a hover by moving the cyclic straight back with no lateral input then he should move to the right. This will be proven or disproved in the test.