The foregoing paragraphs have discussed the factors affecting the rate of rotor speed decay following an engine failure and noted how timely action by the pilot can establish the helicopter in a stable autorotation. The flare manoeuvre has been described and using a relatively simplistic analysis of the trajectory of the helicopter it has been possible to determine the optimum height/speed combination at which to execute this manoeuvre. It is now necessary to consider the effect of airspeed on the risk associated with operating a single-engined helicopter close to the ground.

Given an appropriate combination of airspeed and height (AGL) the pilot will be able to manage the balance of kinetic energy (stored in both the rotor and the fuselage) and the potential energy to arrive at a gate condition from which a safe EOL is assured. However at low speed the pilot may have insufficient height, or potential energy, available to accelerate the aircraft to the gate speed. Alternatively when operating at low speed very close to the ground, say in a high hover, there may be insufficient kinetic energy available in the rotor to reduce the rate of descent to a survivable value. Equally a transit at high speed and low height may not give the pilot sufficient time to react to the engine failure cues and initiate a zoom climb to the EOL gate condition.

Consequently for most helicopters there exists a set of height-airspeed combinations which should be ‘avoided’ to prevent hazarding the aircraft in the event of total power loss. These critical parameters are typically presented graphically in an avoid curve or height-velocity diagram. The similar shape of most avoid curves makes it possible to produce a generalized non-dimensional curve, see Fig. 2.37, that can then be applied to a variety of rotorcraft and also scaled to take account of density altitude and gross weight [2.28].

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