Answer:
If the helicopter is not moving, then it is not being accelerated.
By second Newton's law we have that:
F = m*a
Force equals mass times acceleration.
Then if we do not have acceleration, we have:
F = m*a = m*0 = 0.
Then the total force must be zero.
So if the gravitational force (pulling down) is 115N, then the force of the propeller must be such that:
Total force = F - 115N = 0
F = 115N
The force of the propeller must be also of 115N, but in the opposite direction.
