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If a hovering helicopter's engine fails, can the helicopter autorotate as it could if it were moving?

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  • $\begingroup$ Do you have a general altitude in mind as to when the engine fails during the hover? $\endgroup$
    – user22445
    Commented May 12, 2023 at 2:39
  • $\begingroup$ @757toga no, I hadn't thought of that. I'm interested in theoretical possibility more than practicality, so assume there is enough altitude for any maneuvers to be made. $\endgroup$
    – Someone
    Commented May 12, 2023 at 2:57
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    $\begingroup$ Vertical autorotation is not a good idea $\endgroup$
    – Koyovis
    Commented May 13, 2023 at 6:43

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Look up the 'Dead Man's Curve' for a helicopter. It gives the zones for safe flying (in event of engine failure) as a function of speed and altitude. Hover is the left-side of the curve (where V=0).

Very low, a helicopter can safely hover and have an engine failure -- from low altitude, it won't pick up enough speed to have terrible consequences when it reaches the ground.

Up high, a helicopter has enough altitude to use that potential energy to safely autorotate.

There is a region of altitudes in-between where the helicopter can not safely hover in the event of an engine failure.

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  • $\begingroup$ Wow, I've never seen this diagram, that's very insteresting. Is that the reason why helicopter don't take off and land vertically (except for the last bit) but basically follow some kind of "glide slope" (in lack of a better term)? $\endgroup$
    – kruemi
    Commented Nov 19 at 8:03
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    $\begingroup$ @kruemi Yes it is. It is also the reason you usually don't see helicopters hovering at altitude (also lower fuel consumption) -- or example a Police or News helicopter will usually orbit the point of interest instead of hover at a single vantage point. $\endgroup$
    – Rob McDonald
    Commented Nov 19 at 16:34
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The helicopter will be able to maintain rotor speed using the same "windmill" effect. However, unless the autorotation is initiated just slightly above the ground, the descent rate will become too large to be survivable (and control may be lost due to vortex ring state).

If the helicopter is moving too slow (KTAS < 50), but is above the H-V curve in altitude, the pilot would accelerate to a survivable autorotation speed where (1) the rate of descent will be smaller, (2) range will hopefully allow the pilot to find a suitable landing spot, and (3) forward speed will be used to boost lift and rotor speed in the flare, just before landing.

A pilot should be aware of many airspeeds related to this: (1) the minimum airspeed to use in autorotation, (2) the speed that will provide the minimum descent rate in autorotation, (3) the speed that will provide max range in autorotation and (4) the max speed in autorotation.

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Yes it could, because the process of auto-rotation does not rely on the forward motion. Think of auto-rotation as gliding, but in a helical path.

A glider has to constantly descend¹ to maintain airspeed - when descending, a component of its weight counters the drag. This allows the plane to stay in motion, thereby maintaining the airflow needed by the wing to produce lift.

In a helicopter, this wing rotates while descending, and is called a 'rotor'. Other than this difference, the mechanics are identical to that in gliding. Since the blades follow a helical path, a component of helicopter's weight opposes the "drag" (or torque) of the rotor.

This allows the rotor to maintain its rotation speed, providing the blades the airflow they need to produce lift. Since blades don't rely on forward motion of the helicopter to produce this lift, auto-rotation is perfectly possible without forward motion.

¹The glider must constantly descend relative to the ambient air, not necessarily relative to the ground.

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    $\begingroup$ I think this answer misses the point. Forward speed helps immensely during the landing flare. As forward speed bleeds off, it adds energy to the rotor (just like an autogyro) and that energy can be used to arrest the vertical descent. While you can autorotate without forward speed, you will impact the ground without much flare. $\endgroup$
    – Rainer P.
    Commented May 12, 2023 at 9:19
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    $\begingroup$ @RainerP. You are right, but OP just asked if it was theoretically possible to have auto-rotation without forward speed; he didn't mention anything about landing. $\endgroup$
    – Aditya Sharma
    Commented May 12, 2023 at 10:11

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