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If a helicopter is flying relatively low, say for sightseeing, can it still land using autorotation if required?

What about if there was an issue during a routine landing? Is there any cutoff point where autorotation is no longer feasible?

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What you're looking for is the height/velocity curve, some times called the 'dead man's curve'. This shows the altitude and speed requirements for a safe autorotation, plus the height and altitude conditions under which an autorotation is probably not possible.

A lot of variables go into this, and the h/v curve is specific to the particular helicopter, and in particular, whether the helicopter has a high inertia rotor system or low inertia rotor system.

For example, the Robinson R22 has a low inertia rotor system, meaning that the rotor blades are fairly light, and thus don't have a lot of inertia to remain spinning in the case of engine failure. A similar size helicopter, the Enstrom F28, has a high inertia rotor system, so the h/v curve for the Enstrom looks quite different from the Robinson, but is somewhat similar to another popular high inertia helicopter: the Bell 206 JetRanger.

As the article notes, Robinson doesn't recommend a climb out under 45 knots speed, while Enstrom allows it at 17 knots. This limitation is governed by the h/v curve, not performance. While the Robinson could climb out at lower airspeeds, it wouldn't be safe in the event of engine failure.

Disadvantages of a high inertia system are that it uses more fuel, and is slower to accelerate under power in low rotor speed conditions (which are also potentially deadly).

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There are different techniques for low altitude autorotations and high altitude autorotations. If your altitude is low, you most probably have to land directly ahead and you will only have time to lower the pitch, flare, then use the last rotation on pitch to have power, then land the helicopter. If you have altitude, you can direct the helicopter to where you have decided to land, you can use the airspeed and rpm of the rotor system to reach to your landing area with a safe airspeed for a flare and landing. As far as I know there is no unfeasible point for a autorotative landing. A safe autorotative landing is completely about the airspeed and altitude which you had the engine failure and your landing area. High altitude autorotative landings may change your flare height.

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