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.

8 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. 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.

  8. 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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