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Helicopter auto rotation

Updated on December 4, 2015
Helicopter hovering near a giant explosion
Helicopter hovering near a giant explosion

What happens if you lose your only engine during flight of a helicopter? When helicopters were something of a novelty, many people assumed that if the engine should stop, the helicopter would crash. Fixed wing aircraft in the event of engine failure may continue to glide and make a fair approach for landing as wing produces certain amount of lift during it's glide. Helicopter has rotating wing not a fixed wing, hence rotor need to be rotated to generate lift and the source of power to rotate the rotor is lost when the engine is failed. It is important to remember that having an engine failure in helicopter is extremely wade an operation. It is a part of the basic training for all helicopter pilots to perform "engine off landing" (auto rotation) very safe and well controlled.

Main rotor makes the flight possible

Before we go in to the details of what to do if engine fails, let us brush up the function of main rotors, the life line of a helicopter. Main rotor is power driven by a single engine or by a twin pac power plant. Helicopter will take off when lift produced by main rotor is greater than the weight of helicopter. Helicopter will remain stand still flying (hovering) when lift equals weight and it can further climb only by increasing lift and that is done by pulling collective control lever further up.

Helicopter flies forward when forward thrust generated by main rotor is greater than the drag induced by helicopter itself. When rotor is tilted forward (see the picture on the right) it acts like a propeller more or less. Large amount of air is accelerated rearward by the rotor, as a result an opposite force is generated forward. That is the forward thrust developed by the main rotor and Newton's third law of motion explains.

What is auto rotation?

When rotor is rotated solely by the action of air on to the blades, the rotor is said to be windmilling. Helicopter auto rotation is however a condition where the main rotor is allowed to rotate faster than the engine driving it. Rotor produces lift with this freely rotating airfoils by means of aerodynamic forces resulting from an upward flow of air. In a helicopter the rotor is in auto rotation only when the aircraft is descending with the blades at a low pitch. Auto rotation is demonstrated in this video, watch it carefully.

What happens when engine fails?

At the instant of engine failure when the helicopter is in flight, the main rotor blades are producing lift and thrust from their angle of attack and speed. The pilot immediately lowers the collective control lever, an immediate action must be taken in case of engine failure. This is to reduce the angle of attack on the blade so as to reduce the drag caused by them. When drag on the blade is decreased blade will want to run at an increased speed.

Helicopter crashed during auto rotation
Helicopter crashed during auto rotation

All helicopters are fitted with a free wheeling clutch mechanism coupled on to it's drive train between engine and main rotor. Free wheeling unit will make the engine to drive the main rotor but does not allow rotor to drive the engine. Both ends on the clutch unit will rotate at same speed during the normal operation. But when the engine fails, the driving end of free wheeling unit from engine side slows down while the driven end of the rotor maintaining an increased speed. This dissimilarity in engine and rotor speed at the free wheeling unit will centrifugally cut of engine free from main rotor and allows the main rotor to continue on its rotation due to inertia. We do not want to lose this free wheeling rotor speed but want to take advantage of it. The helicopter will descend drastically as no lift is produced from the rotors because pilot has lowered the angle of attack on the blades. This situation causes an upward flow of air through the rotor system (See the picture above). The upward flow provides sufficient thrust to maintain rotor to windmill through out the descend. Pilot will be able to control his descend speed and maintain the rotor speed by using his flight controls.

Pulling collective lever up will enable him to decrease the rotor speed by putting some angle of attack (pitch) to the blades and lowering the collective will enable him to maintain the rotor speed by reducing the drag on to the blade. Thus the pilot is in complete control of the descend of the helicopter even though engine is not rotating the rotor.

During descend by using the cyclic pitch control, pilot brings the aircraft to a comfortable place for landing. Prior to touch down pilot pulls the collective to add some pitch on to the rotors in order to take the benefit of ground cushion for gaining a gentle touch down.

Conditions for employing auto rotation

The video indicates a helicopter making regular approach and practicing Auto rotation. Almost at 0.16 th second the pilot closes the engine throttle to simulate engine failure and pushes the collective control lever fully down to zero pitch. Watch the helicopter falling down under pilot's control with rotor windmilling in full speed in flat pitch. The sound you hear is the sound of engine running in idle speed and the sound of the rotor rotating.

Auto rotation will be employed in the helicopter in one of the following circumstances.

(1) When pilot wishes to descend at a faster rate than he could do with power applied to the rotor.

(2) When pilot is practicing auto rotation descend.

(3) When the engine has failed and the pilot is using auto rotation in order to make a safe landing.

Conclusion

So in conclusion if the helicopter engine fails the pilot can make a safe and gentle touch down by auto rotation. Helicopters are far safer and interesting, unlike a fixed wing aircraft will always need forward speed to land safely with or with out engine operating. You have seen a helicopter can make a free fall by it self and land with out any forward speed whether the engine is operating or not operating.

Picture indicates

1. Engine failed.

2. Helicopter descends drastically.

3. Pilot manipulates cyclic control to reach to the right spot.

4. Pulls collective little up so as to create ground cushion from rotor down wash for a gentle touch down.

5. Makes a successful touch down with out engine

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