I was wondering about the force that a helicopter produces to descend to the ground being always upwards. How does that happen? I understand that it must be upwards but can't imagine it, how do the engines do it?
The engines power the rotor, which in turn produces lift.
Lift is produced due to the fact that the rotor blades have an aerofoil profile, similar to the profile of an aircraft wing.
In order to descend, the lift force of the rotor is simply set to be smaller than the weight pulling the helicopter down.
The lift in a helicopter is caused due to the air pressure difference above and below the blades. This pressure difference is the consequence of relative motion between the air and blades of the helicopter. The magnitude of lift force increase and decrease with increase and decrease in relative velocity between blades and air. So to lift a helicopter upwards, the speed of blades is set in such a way that the lift would be grater than the weight. And while landing it, the speed is set such that the lift is less than the weight.