A helicopter is moving to the right at a constant horizontal velocity. It experiences three forces →Fgravitational,→Fdrag and force on it caused by rotor i.e. →Frotor. Which of the following diagrams can be correct free body diagram representing forces on the helicopter?
Fgravitational acts downwards.
Fdrag acts opposite to the direction of motion.
Now let us see how Frotor acts.
Wings(rotor blades) of helicopter will push the air perpendicular to their plane of rotation as shown in figure (a). From third law, air will apply force Frotor on the wings as shown in figure (b).
There is no force ma. Basically ma is the resultant of all the forces acting on a body. Moreover, here a is zero because velocity of helicopter is constant.
The right answer has to have Gravitational force downwards, drag force opposite to direction of motion and rotor force as the resultant of these two in the opposite direction to balance it out.