Forces acting on a helicopter

Thursday, January 27, 2011

 The three main forces acting on a rotor blade are the lift force, the drag force, and the centrifugal force. Lift is the upward force caused by the interaction between the air flow and the airfoil. Drag is the force of the air resisting the movement of the airfoil. The centrifugal force represents the tendency of the rotor blade to fly away from the center. Because of the circular motion, the air velocity is a lot higher at the tip of the blade than at the base. Because of the quadratic relation between lift and speed, and drag and speed, the lift/drag increases quadratically with respect to the distance to the center.

 Because of the forces on the rotor blades, the blades have a tendency to cone. This means they tend to tilt upward during flight. This is caused by the combination of lift and the centrifugal force. The centrifugal force tries to make the blade as horizontal as possible, while the lift tries to move the blade up. The combination of these forces mean that the helicopter blade is rotated slightly upwards. When the pitch of the blade is changed, the lift generated by that blade changes too, this means that there's a relation between the angle of attack and the coning angle. A tilted swashplate thus results in an asymmetric cone.

The motor of the helicopter makes the blades spin relative to the helicopter fuselage. However, because of action and reaction, this means that the fuselage has a tendency to spin in the opposite direction: this is called anti-torque. To alleviate this, another force needs to be generated outside of the central axis of the helicopter. In most helicopters, this force is the thrust from the tail rotor.

While in forward flight, the thrust generated by the blades depends on their position. Some areas will generate more lift than others, which means that the net result isn't a vertical force anymore. This net result can be split up in lift and forward thrust. The sum of the lift and the weight determine if the helicopter goes up, down, or hovers. The sum of the forward thrust and the total drag determine at what speed the helicopter moves forward.


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