What is autorotation?

I was asked to define the term autorotation today. It is the procedure that allows a helicopter to land safely when it has lost its engine power. This is how it works.

First, what allows an aircraft to fly? The rotor blades act as the wings on an airplane, ie, air passing over the blades or wings provides lift allowing the aircraft to fly. Imagine sitting in a car with your arm extended out the window, palm down. Sitting still, nothing happens. Raise the front edge of your hand so it tilts, front edge higher than the back edge. Still nothing happens. Now add power to the car so it moves forward. (The car speed over the ground is groundspeed. The rate the air moves over and under your arm is airspeed. When flying they are usually different but not always.)

As the air moves faster and faster past your arm you feel the pressure on the front edge of your hand but still, nothing happens. Now raise the front edge of your hand slightly and your hand will suddenly fly upward. You just created lift using airspeed and angle of attack (called pitch or pitch angle)with your hand which represents the wing or rotor blade. The top of an air foil (wing or rotor blade) is flat while the bottom (palm of your hand in the example) is curved. Moving forward, the air moving under the wing has farther to go than that passing on top of the wing in order to get to the rear of the wing. Therefore the air under the wing must move faster than the air on top of the wing. That greater speed creates lift and allows the aircraft to overcome gravity and fly.

So a helicopter can sit on the ground with blades moving at full speed and nothing happens as long as there is no pitch (angle of attack) in the blades. Change that pitch angle and she begins to fly because there is sufficient airspeed over the rotors and the rotors are angled so that they “bite” into the air. Bear in mind the pull of gravity is always there but is being overcome by lift created by airspeed and pitch angle of the blades. Decrease the pitch angle and you descend. Increase it and you will climb. Cut the power source driving the blades and gravity takes over instantaneously.

Autorotation is this: The engine quits. Gravity wins and down you go. Take the pitch out of the blades immediately. With no pitch angle the air rushing upward past the blades as you fall will actually drive the rotor blades causing them to spin. You can control the speed of the blades by raising or lowering the nose of the aircraft. If you do not get the pitch out of the blades the weight of gravity pulling against the pitch angle in the blades will cause the blades to cone upward and tear off, leaving you and your aircraft a falling rock.

That said, with the pitch out of the blades (blade angle flattened) you can fly the aircraft. You have no engine power to drive the blades but you do have airspeed driving them and providing lift and allowing you to fly under control. You will go down but you can fly the craft down and land safely—once—if you do everything correctly.

You have one landing stored in the speed of those blades and one only. Remember that adding pitch to the blades will cause you to fly but with no power source you can add enough pitch to fly only once. So the idea is to find a nice flat spot where you can land into the wind, fly to that spot and land. All aircraft are different to some degree, but in the Huey, which I flew, the autorotation went like this:

Flying along straight and level, fat, dumb, and happy. Engine quits. Sometimes a loud bang, sometimes not. Sudden silence. Bottom falls out and down you go really fast. Take the pitch out of the blades (uh, you might want to do this in a nano-second). Make sure your airspeed is such that the blades are spinning fast enough but not too fast. Find a place to land within your estimated range of flight before you get to the ground. Maneuver yourself into the wind if possible. Fly to that spot. At 75 feet from the ground, pull the nose up high (this slows the aircraft). At 15 feet off the ground, abruptly pop in a bit of pitch angle. This will stop all forward movement. Push the nose down to level the aircraft. Pull in all the pitch you have causing the aircraft to fly and land softly as it contacts the ground. You will have landed nicely (hopefully) and at the same time killed off all rotor speed. The blades will now barely be turning and will stop completely in two or three rotations. Shut the battery switch off and get out. Go have dinner.

Ray Kenneth Clark

There are other things to do when you have no place to land except in trees or water but basically this is how it works. Let me know if you have further questions. And do stay in touch.

Ray

April 22 · No Comments
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