So, I was learning about helicopter and how they auto rotate and I learned that when the engine fails the helicopter starts to fall and the air is thrown up at the blades which turns them like a windmill. I though about this for a little and found a issue
This image shows that the blade is being pushed back …….
In the wrong direction
The blade would have to be moving the other way the create lift. wouldn't it?
I also found another issue. If this force was applied to every blade wouldn't the forces equal out and the blades stay at the same speed.
The video I was watching about this said that the blades and span like a windmill is span in the wind, but the way a windmill spins would push the blades the wrong way.
I tried to google windmills to see if another voo doo magic happened in them but, most results explained how a wind turbine makes electricity which was not very helpful.
I am sorry about the quality of the picture I drew
Best Answer
You are missing one essential detail: The incidence of helicopter rotor blades can be changed. This is called collective pitch.
When power fails, the pilot changes the blade angle such that the spinning direction is maintained but now the rotor extracts energy from the upwards airflow instead of pushing air down. Note that the blades will still create lift, but because of the fact that the flow into them has an upward component, the lift force on the blades is tilted forward such that the horizontal component keeps the blades spinning. Now the helicopter flies much like a glider, continuously trading altitude for forward and rotor speed. This is the same for all blades, and each blade will pull the rotor forward.
Close to the ground the pilot will now pull on the collective pitch lever and increase lift on the blades momentarily. This is a dynamic maneuver and needs to be well coordinated. Now the lift force increases, but the lift vector on each blade will point slightly backwards, so the rotor spins down. Now the pilot is trading the stored rotational energy of the rotor for increased lift in order to reduce sink speed and come to a soft landing. This requires a minimum amount of rotor inertia to have enough energy available for deceleration.