helicopter power off critical main rotor rpm(lower)

[jagannath]

sir
i want to know the factors/empirical formula to arrive at power off critical rotor rpm (lower). we know that in a single main rotor/tail rotor configuration especially for a hingeless rotor, at the instant power to the rotor shut off then if pilot doesn't react (like putting down collective) the rotor rpm will decay. How far can pilot can tolerate the lower rotor rpm below which it is impossible to recover.

thanks

Jagannath

[Miklavz]

Hello!


Your question is a little bit confusing or hard to understand; however, for my type, B412 there are stated lowest RPMs in AR; bellow this RPMs rotor is unable to recover without engine power.
These RPMs are stated in RFM - keep rotor RPM in AR between these and these RPM. During training for OCF/FCF instructor told me different number - lower, of course. But this number was empirical and not stated in RFM.
Is this the Q and A you are expecting? As the number differs from type to type.
RGDS, Mike

[jagannath]

sir
What am looking is that "how designers arrive at lowest critical rotor rpm" below which as you said engine power is required to recover. Is there any formula to arrive at that.

regards
jagannath

[GipsyMagpie]

There is a formula for that in Alastair Cooke and Eric Fitzpatrick's book on helicopter flight test. Para 2.12.1

I will to put up an extract if you need it

[Camp Freddie]

75% more or less :)

[AnFI]

CF I think you may be overlooking their 'error' in that this 'critical RRPM' may well not be recoverable even with the engine. Since the angles of attack increase rapidly with the increased descent rate. (weight dependant it might be very much lower in some aircraft but who wants to try it?)

Or are we saying that there is an auto RRPM that will not self-sustain (ie will continue to reduce) that can be recovered with engine? (straight question)

[Camp Freddie]

Mr Anfi,

I did say "more or less" to give an average of the different scenario's

What do you think the relevant situations / figures would be?

[AnFI]

Sure the "more or less" covers it well.

The straight question really is a straight question? Your thoughts?

Personally I doubt if there is any (in any reasonable sense of the word) overlap or buffer between unrecoverable RRPM without engine and Unrecoverable RRPM with engine.

I don't think it helps the cause to discuss the really extreme end of the envelope 'in public' and it is a good thing people are 'over cautious' in general. Even then it's not always enough.

There are all sorts of extreme things possible but of no practical benefit, and a parachute is required for extensive examination of these numbers. (In short your answer is fine but was alarmed at the apparent belief (by the others) that there is a (significant) window where the engine will help you after you have found unrecoverable auto RRPM)