To: Arm out the window
You said,
“Hmmm.
I see where you're coming from, Lou, but I'm also skeptical that an aircraft can get through the certification process without being subjected to amounts of out-of-balance flying and sideslip which would have to have shown up any untoward characteristics long before the machine was allowed to be sold commercially.
Surely in sideward flight the sideways airflow component must have been at least 20 kts or so (I don't know what the flight manual limit is, having not flown the R22, but I'm guessing it must be in that order of magnitude?).
If that's the case, the amount and direction of flapping associated with that must brought the aircraft into the 'danger zone', for want of better words”.
That is exactly my point. In order to certificate, the helicopter had to comply with the requirements of FAA Advisory Circular 27-1 which state that the helicopter had to demonstrate a sideslip of 90-degrees left and right at .6VNE and 10-degrees left and right out of trim at the same speed. Assuming these tests were demonstrated they should have realized that the flapping loads were excessive and made note of it in the test reports handed over to the FAA. You must also understand that the FAA does not oversee all of these tests. The tests are conducted and documented by what is known as a DER (Designated Engineering Representative). These individuals are employees of Robinson Helicopters and in some cases are officers of the company who have a vested interest in getting the helicopter certified. Frank Robinson was a DER for some time on the R22 and R44 certification programs. How is that for a conflict of interest?
During the timeframe of certification to the issuance of the FAA restrictions there were 31 rotor incursion / mast separation accidents. After the issuance of the restrictions and the implementation of the higher training standards there were no rotor incursion/mast separation accidents until last year where there were four and this year possibly one.
The Georgia Tech investigation pointed out that sideslip and out of trim flight could induce these high flapping excursions and result in mast bumping or rotor incursions. This was mainly a computer analysis and it pointed out the problem. Why did they discover this fact and the actual certification tests did not?
I pointed out the following on another post. The design of the Robinson head incorporates an internal droop stop, which is called a tusk. The tusk contacts a bolt in the rotorhead, which prevents the blade from drooping when at rest. Upon spin-up and the introduction of collective the blades cone up and the tusk moves away from the limit stop (bolt) thus allowing the blade to flap. During normal maneuvering the blade will always be in the flapping zone. However when the rotor head is placed in conditions of violent maneuvering or the flapping excursion exceeds the normal range the tusk will contact the stop bolt. When this happens the flapping energy of the blade is mechanically coupled to the rotorhead and it causes an instantaneous teeter taking the rotor system out of its’ rotational plane. If the kinetic energy of the opposite blade is strong enough then the blade will bend and most likely hit the tail boom or the fuselage in the cabin area. This can’t happen on a bell blade because it can’t flap.
Another point to consider is that when the blades flap they want to lead and lag. This tendency to lead and lag is partially absorbed in the inplane bending of the blades but the primary lead lag loads are reacted by the cone hinges and in part by the teeter hinge. With the introduction of the high flapping excursions, the energy of the lead lag increases plus and minus (inplane) and this may cause the blades to twist and aerodynamically fly out of plane. Also, this can exert higher than normal twisting loads on the rotor mast, all of which can cause mast bumping, mast separation or rotor incursion.
Again I say this can’t happen on a Bell system because the blades don’t flap.
The requirements of FAA AC 27-1 state that when a new and unusual design is proposed for certification that design must undergo a high degree of scrutiny and it must be proven to the FAA that this new design is both safe and reliable. The Robinson rotorhead meets these criteria but the question begs asking did they comply with this requirement of 27-1?
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The Cat