Dear WhoNeedsRunways.
Here is the question you mentioned in your posting.
Robinson / FAA Why are two bladed rotor systems susceptible to "Zero G" maneuvers and three or more rotor blade systems are not? Why was a two bladed system selected over a multi-blade system for use on a helicopter that would be operated by pilots who have minimal "stick" time or students learning to fly helicopters?
The first part of the question sets up the second part of the question. Anyone that has learned helicopter theory of flight can answer the first part of the question. My intent was to get them to answer the first part and then explain why they designed a rotor system that that would place pilots in jeapordy.
Regarding your thoughts on the safety of the Robinson design, there have been 32 loss of rotor or fuselage incursion accidents on R22 and R44 helicopter. The most recent was about three weeks ago in California. The Instructor pilot had many hours in the Robinson as well as thousands of hours in other helicopters and fixed wing aircraft. He was also a member of an acrobatic team the flew 22s and a 44. He not only lost control, he lost his main rotor. If we assume that both helicopters have collectively flown 1.000.000 hours we can divide that figure by 32 we get a mean time between rotor loss or fuselage incursions of 31,250 hours. The FARs require that single point failures be designed out of the system and if they can't be, it must be statistically demonstrated that the occurance of an accident involving loss of life should occur no more frequently than 1 10-9 or, 1.000.000.000 hours. In this case, it is the design of the rotor head that sets the up the accident. Regarding low blade inertia, you forgot one other thing and that is, excessive pitch, which can induce rotor stall,not retreating blade stall as that is an entirely different subject that we can get into another time.
The very nature of the rigging procedure and the design of the rotor head can cause the mechanic to rig in too much pitch.
If you wish to demonstrate the design defect of the Robinson rotor head do the following.
Place the blades over the longitudinal axis and move the cyclic from its' rigged neutral position forward and aft. The blades will move. Now, rotate the blades so that the pitch link is directly in line with the same axis. Move the stick in the same manner. The blade won't move.
Now, do the same thing with the blades over the lateral axis and move the cyclic from its' rigged neutral position both left and right. The blades will move. Now, place the pitch link directly over the lateral axis and move the stick left and right. The blades will not move. Now, do the same thing with a Bell. With the blades in the positions described above and the cyclic moved as described above the blades will not move. If they do, it will be very slight due to possible pitch coupling in the rotor system.
Regarding the fact that the rotor system has a design defect you might wonder why the FAA approved the design. It might be that Frank Robinson was the FAA Designated Engineering Reoresentative or, DER. FAA regs state that the DER cannot have a vested interest in the design or a financial interest in the company. This was brought to the attention of the FAA by the NTSB but neither agency took any action to correct the infraction of the regs. If the helicopters had been subject to the regs relative to certification you wouldn't be flying in an R22 or R44. Another point is that the R22 and R44 are the only aircraft that are restricted from flying out of trim or being sideslipped. Yet, the certification requirements state that sideslips of 90 degrees should be demonstrated as well as flying out of trim +/- 10 degrees. If the two helicopters were put up for certification with those restrictions which were mandated by an FAA AD the two helicopters could not be certified.
Low rotor inertia and mast bumping were the reasons , according to Frank Robinson, why the helicopters were lost. That way, they could blame it on the pilot and not on the real culprits, Rotor head design and the rigging procedure.
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The Cat
[This message has been edited by Lu Zuckerman (edited 25 October 2000).]
[This message has been edited by Lu Zuckerman (edited 27 October 2000).]