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Why should a helicopter necessarily have two propellers?

Why should a helicopter necessarily have two propellers?

Grade:12

3 Answers

SAI SARDAR
1700 Points
7 years ago
Dear kalyan,
If the helicopter had only one propeller, then due to conservation of angular momentum, the helicopter itself would turn in the opposite direction.
Naga Sandesh
19 Points
6 years ago
Actually when helicopter had only top propeller, the body of helicpoter would rotate in opposite direction(to conserve Angular Momentum) to propeller’s spin when we start the engine.
But when you have the other propeller also, the body doesn’t rotate. This is not because it conserves angular mometum, because the other propeller doesn’t contribute any component of its angular momentum. As the two propellers are exactly perpendicular to each other.
Here the side propellercreates pressure difference across it. Since every fan creates pressure difference due to differnce in speeds of air across it. Therefore there is an external force directing from high pressure to low pressure region. This force obviously creates torque. Helicopters are designed in such a way that the direction of external torque is vertically opposite to direction of angular momentum of top propeller. Since external torque is acting, Angular Momentum is not conserved...
Upendra Yadav
33 Points
6 years ago
If the helicopter had only one propeller, then due to conservation of angular momentum, the helicopter itself would turn in the opposite direction.

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