Question

I have read that if helicopter's engine fails, you set the pitch to minimum but not reverse pitch. The rotor would have to be tilted back to provide an angle of attack on the blades so it glides like a fixed wing aircraft but with the rotor spinning to keep the blades from buckling under the weight of the craft. I think it would have to go into a slight reverse pitch to spin the same way. That would also provide more angle of attack on the receding blades where it is needed because of the lower air speed over those blades. How does it keep rotating in the same direction with the air going up through it if it can't go into a slight reverse pitch?

Answer 1

In an autorotation during helicopter engine failure, pilots manage the pitch, but not to the extent of reverse pitch, to allow the blades to rotate with upward airflow, converting gravitational potential energy into rotational kinetic energy. A tail rotor or counter-rotating blades provide stability and prevent torque-induced rotation of the helicopter body.

Step-by-step explanation:

When a helicopter engine fails, the rotor blades must still rotate to allow for a controlled descent. The process used here is called an autorotation. In an autorotation, the helicopter's rotational kinetic energy and gravitational potential energy are critical factors for a safe landing. The blades do not go into reverse pitch; instead, the pitch is adjusted to allow air to flow upwards through the rotor system, causing the blades to spin due to aerodynamic forces.

During normal flight, a helicopter stores large amounts of rotational kinetic energy in its blades, critical for sustaining lift. The engines are designed to maintain this rotation level, not to bring the blades from a slow state back to operating speed quickly. Hence, pilots avoid letting the blades slow too much. In the event of an engine failure, pilots use the helicopter's altitude to convert gravitational potential energy into kinetic energy, by aligning the blades at an angle that allows the air to push them around as the helicopter descends. This is unlike fixed-wing aircraft that glide forward; helicopters descend vertically when in autorotation.

As for helicopter stability, a small propeller, known as the tail rotor, is used to stabilize the helicopter against the torque reaction created by the rotation of the main blades. For helicopters with two sets of lifting blades, these blade sets rotate in opposite directions to cancel out the torque that would otherwise require a tail rotor.

Understanding the principles behind helicopter autorotation and blade rotation involves concepts from Newton's third law and the conservation of angular momentum. Combined, these principles facilitate controlled flight, even in the face of engine failure.