Rotor RPM decay

[Dr. Michael K. Hynes]

With sudden engine failure, how quick does the main rotor decay? Helicopter certification tests may not reflect real life events. If actual pilot reaction time is longer than what certification testing assumes, and main rotor RPM decay rapid, the potential for a successful autorotation is nil. Should each manufacture provide a table that shows rotor RPM rates and blade stall RPM? Any comments?

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Dr. Michael K. Hynes

[Schooner69]

Well, Doc: if the engine quits and the rotor starts to decay, all the tables in the world are going to be of little use to you unless you get the collective down, even if you have dutifully commited said tables to memory. Consider our fixed wing counterparts: what if a pilot, upon experiencing engine failure in a twin engine aircraft, takes longer than the test pilot to react to the emergency? Should we have some tables to show what happens if you're five seconds slow in feathering the engine?

I don't personally see a need for more tables which I won't have time to read when the engine quits anyway. It's a little late to be worrying about decay rates/stall rpm at that point.

What has prompted your question?

Sorry I don't have time to re-read this and tidy it up: She-Who-Must-Be-Obeyed wants to go out for breakfast NOW!

John

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FixedWing,FlingWing,GoldWing

[HeliAv8tor]

Doc, rotor rpm decay depends on various factors. As pointed out previously, No1 is how quick you get the collective lever to min pitch, coupled to this is the inertia in the head and blades i.e. a Hughy's head and blades hold a lot of inertia whereas an R22's does not, the density altitude at which the engine fails and the rotor rpm (Rrpm)starts to decay and whether the pilot carries out the correct drills. To be honest the average pilots reactions are about 2 seconds (this statement will get a bite!) by which time the Rrpm will have decayed to approximately 90% - 95%, lower on some helis. Rrpm can be brought back up by flairing or turning the aircraft. As to tables, no thanks all thats needed is plenty of pilot training and practice so that in the event an engine failure the drills are carried out quickly, smoothly and in the correct sequence. Flare, level, check and kiss it on straight and smooth, just an advanced co-ordination manoeuvre.

[This message has been edited by HeliAv8tor (edited 21 August 1999).]

[4dogs]

Doc,

Q1: Each manufacturer will have data on Nr decay rates. So will NASA and the US Armed Forces.

S1: In almost all cases, they do not reflect real life. Certification is about establishing benchmarks that vaguely relate to but do not replicate real life.

S2: If there is no pilot intervention and Nr decay is such that Nr or aircraft control is lost, then your statement is correct. In real life, I am much slower to react than a certification TP because I don't know that it is coming. I fly helicopters with Nr decay rates best described as poisonous. However, whether it all happens in the worst place or the best, at the worst speed or the best or indeed at the worst attitude or best is in the roll of the dice. Given that you are a Doctor of something, I presume that your conditional probability analysis emphasises the distinction between "necessary" and "sufficient"...

Q2: Provide the table to who - those who already know that the lever should have already been on the floor or the litigants afterwards? The data is probably already there to be "discovered". I would be loathe to publish it because there will always be some dunce who will try to finesse some self-invented demonstration or manoeuvre and kill someone.

Q3: Eeffa! (BTW, that is a contraction of "E" for "All of the Above"


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Stay Alive,

[email protected]

[sloppylink]

I believe the TPs allow something like 2 seconds before they react to a situation like engine(s) failure (either the only one or both). This is to try and simulate what the reaction time will be for a typical pilot.

Going back to my time in the mob - all EOls were initiated by a verbal command and then when certain of making the EOL area, the throttle was retarded to ground idle. A demonstration of Nr decay was given to students early on in their flying training, by initiating the EOL by retarding the throttle without any warning, however it was pre-briefed and set up so the student knew what was happening. Nonetheless, I do recall that most beefers used to pull the throttle and then push the lever down.

In my company we practice true autorotation by retarding both engines to ground idle and I am slightly surprised by the number of very experienced pilots who need reminding of the basics of Nr control whilst in autorotation. Some forget to use the flare and others seem to freeze on the collective whilst watching the Nr decay!!

All this suggests that the recent enquiry into reaction times which I read about recently, suggests that the 2 seconds may be realistic for your average pilot to react to a situation, but he will probably be about 1 second behind the drag curve.

[ENG OUT]

I've had a turbine wheel go bang on a b206 .The bang and shudder gave the game away that it was n't going to stay up any longer. Reaction time to lowering the lever felt like less than one second and rpm decay was minimal,but imagine hot and high at max gross weight on departure near the end of a long day and it decides to let go then, that would make life interesting,trying to keep it sunny side up .