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Old 6th Dec 2001, 08:04
  #29 (permalink)  
Lu Zuckerman

Iconoclast
 
Join Date: Sep 2000
Location: The home of Dudley Dooright-Where the lead dog is the only one that gets a change of scenery.
Posts: 2,132
Question

To: helmet fire

“To Lu: your sweeping statements are why I love this forum so much. Lets look at your call that >>something bad will happen if the pilot violates the cautionary statements in the POH<< “.

This is probably the tenth time I have said this. The cautionary notes in the Robinson POH were incorporated as a result of an engineering study made by the Georgia Tech Aero department and the FAA commissioned the report. The FAA wanted to know the causes of over 22 mast separations and rotor incursions on Robinson helicopters since they entered into service. The report was made just prior to 1995. Although the engineering study was never completed there was sufficient evidence that prior to the mast separations or the rotor incursions the helicopter experienced violent flapping excursions. (I will explain the design problem later in this text). It was determined that these flapping excursions could be initiated in three different ways.

1) By flying out of trim

2) By sideslipping the aircraft

3) By the application of left cyclic while recovering from a zero G incident.

In 1995 Robinson incorporated an unnumbered page in section 4 of the POH outlining restrictions from performing the above (1-2-3) and instead of making the information mandatory they made it in the form of a suggestion. The CAA indicated to me that they would make it mandatory in all POHs for G registered Robinson helicopters. That was about 8 months ago and nothing has been done. During the past year or so there have been several mast separations in GB.

This is what I meant by the pilot being protected by words and not design.

Regarding the design problem on the Robinson head it involves the “droop stop” which is referred to as a tusk. The tusk establishes the low position of the rotor blade while static. Once the rotor is up to speed and collective applied the tusk moves downward from its' limiting stop. Under normal flight conditions the tusk will never contact the stop. However, when you experience extreme flapping excursions and the tusk hits the stop then it turns the rotor system into a first class lever with the fulcrum at the teeter hinge. If the kinetic energy is sufficiently strong the force exerted by the blade will cause rotorhead contact with the mast.

Regarding your comment about sweeping statements this is what I get paid for as a RMS Engineer and that is to identify and eliminate if possible design problems that will impact Reliability, Safety and even Maintainability.

No helicopter should be designed and certified if there is a possibility of experiencing “jack stall” or to have a mechanical part of the rotorhead that under certain circumstances can cause the loss of the helicopter.

Now, here is something to think about. If the Robinson was restricted from certain flight conditions in 1995 because these conditions could result in the loss of the helicopter, how could the helicopter pass certification if these same situations had to be demonstrated (sideslip and out of trim).
Lu Zuckerman is offline