How rapid will rotor RPM decay?

[Dr. Michael K. Hynes]

When helicopter certification regulations were first written back in the 1950s, all of the helicopters had high inertia rotor blades. Engine failure autorotation entry tests with these helicopters had some builtin safety because of the high inertia rotor systems. Later on, helicopter rotor designs with much lower rotor inertia were certified. In some cases, factory test pilots are now admitting that the helicopters were not able to meet the certification requirements re. rotor RPM decay after engine failure. Some US FAA test pilots are also confirming this. If the helicopter manufacturers stated in their flight manuals what the RPM decay was...XX% per second...and also stated what the minimum RPM before blade stall was, would that help as a wake up call to pilots and be useful information? Do you know what these conditions are for the helicopter you fly?

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Dr. Michael K. Hynes

[MaxNr]

Hi Doc....Call me a "bluff old cove" but all helo` pilots should know at what main rotor speed the old girl stops flying. The FRC`s and flight manual will have a section of limitations showing both power-on and power-off Max and Min Nr, and demonstrations of Nr droop/decay will "hopefully" have been given during training. As for Nr/Sec lost, again, I`m sure the Test Pilots could,if asked, produce a comprehensive table and DA/Airspeed graphs up the ying yang, and once a year I`d demonstrate my ability to read them. But if the engines go BANG, that levers going down, and just hear that Nr wind-up to MaxPwr-Off, plus a bit for the wife and kids in the flare.

[This message has been edited by MaxNr (edited 23 August 1999).]

[This message has been edited by MaxNr (edited 23 August 1999).]

[HeliAv8tor]

Dear Doc, I have to agree with MaxNr. All the info you need about max and min rotor rpm (Nr) should be in the aircrafts Aircrew Manual. This apart the instrument panel will have an Nr gauge clearly marked with the normal safe power-off range. Should the Nr droop below the minimum rpm indicated, all is not lost. It is possible with the collective fully down and by a hard flair of the aircraft to increase Nr back into the safe range from an Nr figure as low as 80%.
Wishing you all a happy and safe PFL when you need it. HeliAv8tor.

[This message has been edited by HeliAv8tor (edited 26 August 1999).]