Minimum autorotation rpm

September 28, 2013 by Tim McAdams

When a helicopter is in autorotation (that is, gliding without the benefit of engine power) rotor rpm must be maintained. This is done when entering the autorotation by lowering the collective control. If the rotor rpm approaches an upper limit, the collective is raised to add pitch. This increases drag and slows the rotor rpm. A low rotor rpm situation is just the opposite, lower the collective pitch to reduce the drag and allow the rotor rpm to speed up.

Rotor rpm in autorotation is a function of several factors like density altitude, gross weight and airspeed. An over speeding rotor is easy to manage since the collective control was lowered on entry there is plenty of movement upward to add drag. However, if a pilot enters autorotation and lowers the collective control all the way down and the rotor rpm is still too low this could be a problem. Typically, in this case, the main rotor pitch is set incorrectly. The helicopter’s maintenance manual has a procedure to adjust this by either lowering the collective control’s down stop or adjusting the main rotor blades’ pitch links. To accomplish this, a mechanic will note the helicopter’s weight and the density altitude and then reference a chart to get the correct rotor rpm. A flight test will then be performed at those conditions and the actual rotor rpm will be noted with the collective all the way down. If it is not at the correct rotor rpm stated in the chart, the mechanic will make an adjustment.

This is done to insure that in the worst case scenario (light helicopter, high density altitude) the pilot will be able to lower the collective control far enough to guarantee an acceptable rotor rpm in autorotation.

9 Responses to “Minimum autorotation rpm”

  1. David from Alabama Says:

    If you can’t get acceptable rotor rpm due to light helicopter and high DA, could you bank steeply to bring up rotor rpm?

  2. Alan Barnes Says:

    Yep. Turning and forward/aft cyclic are also used to control rotor rpm.

    Some helicopters have a minimum pilot weight to fly solo. One of the reasons for this is that you might not be able to maintain a high enough rpm in an auto due to the gross weight being too low. I think this is mainly an issue in smaller helicopters though.

  3. Joe Connell Says:

    Years ago (1961) I received Air Force helicopter training at Stead AFB near Reno, NV. One of the helicopters was the twin-rotor H-21. Using steep banks was a very effective way to boost rotor RPM prior to round out and touch down…

  4. Stephen Dines Says:

    The thinner the air (higher DA), the higher the RRPM. So, worst case for low RRPM is light weight with low DA.

    On initially lowering collective in forward flight, the secondary effects can reduce RRPM, sometimes dangerously. A flare will pick up the temporarily lowered RRPM allowing the autorotative descent to be stabilized safely.

  5. Stephen Dines Says:

    A long time ago, on the first flight of the day, I checked auto RPM as was my habit. The needle headed down with no sign of stopping, so I recovered with power and took the machine back to the mechanic. He didn’t believe me because I was “only a pilot” but I refused to resume ops, so he agreed to come up “for a look”. He almost had a heart attack as the needle headed down through the lower red.

    The pitch link rod ends had been changed overnight; the responsible mechanic said he had counted the threads to ensure that the pitch links went back in the same as they had come out, but the overall length of the links had changed! Red faces all round the workshop, but I was pleased that my habit had paid off.

  6. Alan D. Resnicke Says:

    I’ll be pleased to recount the entire tale of a broken main driveshaft and simultaneous tail rotor driveshaft failure on a USAF UH-1N at night over the Mojave Desert unaided (no NVGs) if anyone’s interested. But from personal experience I’ll tell you that a Huey’s rotor decays at about 10% per second of delayed collective response (depending upon how much collective is in use at time of engine or drive shaft failure). When my brain finally registered what was happening and I reacted to bottom the collective, our Nr was at about 80% and slowing. The resulting retreating blade stall was pretty darn impressive. The quick, sound thinking of my fellow pilot, Capt. John (Augie) Augustine, recalling that right turns in a Huey during an autorotation built rotor rpm, was all that kept us from falling like a homesick brick. We regained Nr well before we entered the flare.

  7. Jean-Gabriel LARROCHE (France) Says:

    What so crucial this subject you wrote about Tim !

    I’m flying in France and except my first CFI who was “obsessed” by the rotor RPM management (we were flying first R22s without RPM governors (beginning of the 90′s)), my other instructors (most of them high timers (15000+ hours) and mostly turbine experience from the french army) from the private through commercial than CFI trainings never emphasized the simple but vital aspect of rotor rpm management, particularly during an engine failure. We used to fly governor assisted helicopters and we practiced as hundreds of students pilots the standard way of entering the autorotation : down, right, and…….maybe a bit of aft cyclic to prevent the nose to dip too much….but the first and most important thing you had to do was to reach the target airspeed for a “safe” autorotation. Then, years after, I had the great opportunity to fly with Pete Gillies of Western Helicopters in Rialto (CA) then, I realized the simple fact that, had I not succeeded in catching up my falling rotor RPM in the case of a real, without any clues before, engine failure, the “game was over”, I was dead. Then I really understood the tremendous importance of the cyclic action (backwards for instance) even with a small airspeed in order to not losing my “wings” and so being able to deal with the NEXT other aspects of the maneuver (find a spot to land then have enough speed just before the flare to have something to trade for the next rapid rotor RPM decay).
    It may be incredible for most of my fellow helicopter pilots, but that’s the entire truth. Many high time CFI don’t emphasize this fact.
    As a parallel topics, after the Air France Rio-Paris Airbus crash where the aircraft was lead to stall due too numerous factors, what was a surprise for many of my fixed wings CFI colleagues (in every flying school from PPL to ATPL level training) to receive a report from the Airbus chief test pilot himself explaining that the first reaction to have in order to counteract the stall was to “reduce the angle of attack of the wing” !!!!!. The article was titled “back to basics”.
    So that’s why I’m very pleased to see people reacting and dealing with the rotor RPM management, especially with the cyclic, in case of a power failure and maybe going “back to basics” as far as autorotation entries are taught in many, many schools.
    So please find this links to better learn on that subject :

    http://ainonline.com/aviation-news/aviation-international-news/2013-05-01/astar-accident-shines-light-autorotation-training

    http://www.morningtonsanfordaviation.com/articles/Enough-skill-to-kill-yourself.html

  8. Chris Says:

    Pilots who fly helicopters with externally mounted equipment (i.e. cargo baskets, fixed fire fighting tanks, spray rigs) should always try an autorotation with the equipment installed as these will almost always cause a change in autorotational RPM and/or glide. In most cases the external equipment will have a negative effect on the autorotation from what is considered the norm.

  9. Chris St.Germain Says:

    David asked if banking would raise the RRPM. Of course, in a bank, the load factor increases, making the aircraft effectively heavier. This will raise the RPM, while the turn is in progress. When we roll out level, the load factor decreases, and the RPM will, too, all else being equal. Banking and pitching up will raise the RPM, but these are transient states.

    I really like Stephen’s idea to go into auto daily to check for proper RRPM. Better to find out that way, as he did, then in a real auto where the power can’t be brought back in to correct the below-limits RPM.

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