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RETREATING BLADE STALL
The limit to a helicopter’s forward speed is reached when the retreating blade’s outer parts reach their stalling angle.
15.4 AUTOROTATION
If a helicopter suffers engine failure it may be trimmed for autorotation, which is equivalent to gliding with a fixed wing aircraft. In this condition, instead of air being
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Fig. 15.12 Reversed flow on retreating blade
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drawn down through the rotor disc the rotor as a whole is inclined at a slight positive angle to the airflow, which streams upwards through the disc. The conditions on an individual blade are such that they continue to turn rapidly enough to provide support. As shown in Figure 15.13, the forces on an autorotating blade are identical with those on the whole wing of a glider, a component of the lift being set off against the drag to keep the aerofoil moving through the air. The helicopter as a whole then may glide some distance before being compelled to touch down ami it may be steered with the cyclic pitch control.
