How they Fly?

What happens when the helicopter engine fails ?

Wrote for the Helicopter History Site by Mick Spiers


  • autorotation

Well, in a multi-engine helicopter, the remaining engine will still be able to power the rotors and therefore normal flight will be able to be maintained. However this document will answer the question you have probably have by now already asked yourself;
"What happens if I lose my only engine or god-forbid, I lose all my engines?"

The simple answer to this question is that the helicopter will enter autorotation.

What is autorotation?


Autorotation is a condition where the main rotor is allowed to spin faster than the engine driving it. How is that achieved? It is actually quite simple.
All helicopters are fitted with a free wheeling unit between the engine and the main rotor, usually in the transmission. This free wheeling unit can come in different forms but one of the most popular is the sprag clutch. The free wheeling unit will allow the engine to drive the rotors but not allow the rotors to turn the engine. When the engine/s fail the main rotor will still have a considerable amount of inertia and will still want to turn under its own force and through the aerodynamic force of the air through which it is flying. The free wheeling unit is designed in such a way to allow the main rotor to now rotate of its own free will regardless of engine speed. This principle is the same reason that if you are in your car and you push your clutch in, or put it into neutral while the car is still moving, the car will coast along under it's own force. This occurs regardless of what you do to the accelerator pedal.

Controlled Descent ? The next question you are probably asking yourself is: "Does the pilot retain control of the helicopter?" The answer is yes. The pilot will still have complete control of his descent and his flight controls. The majority of helicopters are designed with a hydraulic pump mounted on the main transmission. As the rotor will still be turning the transmission, the pilot will still have hydraulically assisted flight controls. The pilot will be able to control his descent speed and main rotor RPM with his collective control stick. He will be able to control his main rotor RPM by increasing the collective pitch, which will increase drag on the rotor blades and thereby slow the main rotor. If he needs to increase his rotor RPM, he can decrease his collective pitch therefore decreasing drag.
The pilot will usually be able to find a suitable area for a safe landing by normal manipulation of his cyclic control stick and his directional, or tail rotor pedals.
Larger helicopters will usually have a generator mounted on the transmission that will still provide electrical power for flight and communication systems.

What happens to Torque Effect ? Torque effect is the aircraft's tendency to rotate in the opposite direction to the main rotor due to Newton's third law "Every action has an equal and opposite reaction". This is the reason why we need a tail rotor or some other form of anti-torque control. The question at hand is what happens to torque effect during autorotation? Well torque effect is directly proportional to the amount of force driving the main rotor, so when when the engine fails the amount of force driving the main rotor instantaneously decreases and therefore the torque effect decreases. This being the case the fuselage of the helicopter will tend to rotate due to the sudden lack of torque effect. The pilot will therefore have to immediately manipulate his directional pedals to overcome this problem and retain control of his aircraft.

Conclusion So in conclusion if your helicopter's engine/s should fail it is not just possible, but quite easy for the pilot to retain control and land safely and gently. This is the reason I believe that helicopters are far safer and more fun to fly in than fixed wing aircraft. A fixed wing aircraft will always need forward speed to safely land, with or without an engine operating. A helicopter can be made to land with zero forward speed whether the engine is operating or not.




Autorrotacion en Español (In Spanish)

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