Ask Ray Prouty

By Staff Writer | December 1, 2008
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Sikorsky’s Black Hawk In Autorotation

Mr. Prouty’s writings provide great insights into all of the forces acting upon a helicopter. I have two questions regarding ("Gross Weight’s Effect on Autorotation" October 2007, page 44). While autorotation is a complex exercise in energy management, his detailed analysis and explanation (in an easy-to-read format) helped to separate fact from fiction (and intuition).

My questions relate to the specifics of the matter, from an operational perspective:

  1. Flight test data proves the theory that a heavier aircraft descends in autorotation at a slower rate. By how much? For example, for a constant external configuration, what would be the resulting steady-state rate of descent for an 18,000 lb Black Hawk versus a 22,000 lb Black Hawk?

  2. Regardless of what happens at altitude, the flare, recovery and landing portion of the autorotation will determine ultimate success and survival. Since the avoid region of H-V diagrams gets bigger with increasing gross weight, is there any practical benefit to having a lower descent rate during the descent portion of the autorotation? If time permits, my preference is to get rid of any excess weight prior to the flare (especially if it’s ordnance or fuel).

With thanks and respect,

Sid Hatcher

Patuxent River, Md.

It can be seen that the calculated rate of descent decreases between 12,000 lb and 18,000 lb as predicted by simple theory, but that at the two highest weights the trend reverses. The calculations were done for the Black Hawk with its original blades that were designed to give good performance at a mission gross weight of about 16,500 lb. For this study, at the two high weights, the chord is too small to prevent some of the blade elements encountering stall, thus requiring a higher rate of descent to maintain autorotation.

Figure 1. I have used my autorotation computer program to answer Mr. Hatcher’s first question. The results are:
Gross weight lb Min rate of descent, ft/min Speed/kt
12,000 2,039 60
14,000 1,927 65
16,000 1,877 70
18,000 1,853 75
20,000 1,859 80
22,000 1,882 85

Later Black Hawk versions such as the UH-60L and UH-60M have wider chord blades that make flying at 22,000 lb more feasible.

Mr. Hatcher’s second question is somewhat confusing. The successful landing from a two-engine-out autorotation, has nothing to do with the H-V diagram, which primarily has to do with getting into autorotation in the first place. In my opinion, the advantage of having a lower rate of descent due to a high gross weight is slight. One might argue that it gives the pilot a slightly better chance to judge at what altitude he should start the cyclic flare, but few Black Hawk pilots have done this enough times to know what this altitude is.

Pilots of single-engine helicopters must be prepared for a total power loss. They learn when to start the cyclic flare and then the collective flare under the tutelage of experienced instructors. For twin-engine helicopters, it is generally assumed that simultaneous engine failures are so rare that training for them would break up more helicopters than would be saved. During the development of the Apache, a full power-off landing was made only once just to show that it could be done. Are their any instructor pilots who are proficient with this maneuver on Black Hawks or Apaches?

Two factors that make the simultaneous failure of both engines unlikely is that each engine has its own fuel tank so simultaneously running out of fuel is unlikely and designers of the Black Hawk and the Apache were required to assure that no single 12.7mm (50 cal) round could knock out both engines.

I will leave the discussion of Mr. Hatcher’s suggestion of jettisoning ordnance or fuel to those who have opinions on this.

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