Autorotation & the Blade Regions

In powered flight the engine turns the rotor and air is drawn downwards through the disc. When the drive is lost, the freewheel unit disconnects the stopped engine so it cannot brake the rotor, and as the helicopter descends the airflow reverses and passes upwards through the disc. That reversed flow is what keeps the rotor turning.

The blade divides into three spanwise regions. The stall region, roughly the inner quarter, operates beyond the stalling angle and produces drag. The driving (autorotative) region in the middle is where the upward flow tilts the total aerodynamic force forward of the axis of rotation, giving a component in the direction of travel that accelerates the blade. The outer driven (propeller) region has the force tilted rearward and decelerates the blade. Rotor RPM is steady when the accelerating and decelerating torques are in balance, and the pilot controls that balance with the collective.

This is why the first action on losing drive is to lower the collective immediately. A high blade angle increases drag on the blades and decays the rotor RPM quickly; once RPM is lost it cannot be recovered, and the stored rotational energy is the only thing available for the flare and touchdown. Near the ground the pilot flares to convert forward speed and rotational energy into lift, then uses the remaining inertia to cushion the landing.

• Try this: Press Engine failure with the collective still high. Watch the rotor RPM decay, then lower the collective. How quickly must you act, and why can lost RPM not be recovered?
• Try this: In autorotation, vary the collective slowly. Which way do the driving and driven regions move along the blade, and what happens to the RPM?
• Try this: Establish a stable autorotation, then press Flare. What happens to rotor RPM and rate of descent, and where does that energy come from?