A helicopter can land safely without engine power by using a technique called autorotation.
Understanding Autorotation
Autorotation is a clever way that allows a helicopter to descend and land safely even if its engine fails. Here's how it works:
- Normal Flight: In normal powered flight, the engine spins the rotor blades, creating lift.
- Engine Failure: When the engine stops, the rotor blades no longer have a power source driving them.
- Autorotation Begins: The pilot takes specific actions to initiate autorotation. The downward movement of air through the rotor blades is what provides the spin of the rotors. This is opposite to normal powered flight, where the engine spins the rotor, creating thrust.
- Rotor Thrust: As the helicopter descends, the upward flow of air spins the rotor blades, meaning they are not being spun by the engine. According to the provided reference, "[a] vector of the rotor thrust in a helicopter is used to give forward thrust in powered flight; thus, where there is no other source of thrust in a helicopter, it must descend when in autorotation."
- Controlled Descent: By carefully managing the rotor speed and the helicopter's descent, the pilot can control the rate of descent and bring the helicopter in for a smooth landing.
- Final Flare: Just before touchdown, the pilot uses the stored energy in the rotor blades to "flare" or pitch the helicopter upwards. This reduces the rate of descent just prior to landing, which is critical to a successful landing.
Autorotation in Detail
| Aspect | Powered Flight | Autorotation |
|---|---|---|
| Rotor Power Source | Engine spins the rotor blades. | Upward airflow spins the rotor blades. |
| Rotor Thrust | Thrust provided by powered rotation. | Thrust maintained by forward motion and rotor speed. |
| Descent | Usually controlled by rotor thrust. | Controlled by managing rotor speed and descent rate. |
| Pilot Input | Throttle and collective control primarily. | Requires immediate and precise control management. |
| Typical Scenario | Normal operation. | Emergency procedure after engine failure. |
Key Points
- Autorotation is not just a free fall; it's a carefully controlled descent using aerodynamic principles.
- It allows for a safe landing, despite complete engine failure.
- Pilots undergo extensive training to master autorotation techniques.
In summary, the downward movement of air through the rotor system is used to create lift in autorotation. Because there is no engine providing thrust, the helicopter will descend, and the rate of descent can be controlled by the pilot.
