Generally engines turn the chemical energy in fuel to kinetic energy, in a helicopter that kinetic is turned into gravitational potential energy when it goes up. Now let's imagine a helicopter hovering above the ground at a fixed height. To stay where it is the engine has to keep supplying a force to counter gravity so there is kinetic energy being supplied to the system of helicopter and earth but the gravitational potential energy is not changed. Energy is conserved so what is that kinetic energy being turned into?
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The kinetic energy is being turned into the kinetic energy of the air, and also energy dissipated as heat and sound by frictional forces internal to the helicopter. By constantly using your rotors to push air to provide lift, you need to constantly give the air around you kinetic energy. Imagine you have a helicopter in vacuum and it has no frictional forces. There would be no energy (no fuel) needed to maintain the rotor's motion - once you got it in motion it would just keep spinning at a constant rate. Now imagine you're in the air and your helicopter has internal friction. Now you need fuel and energy to maintain your rotor's motion because otherwise your rotors would stop spinning eventually.
enumaris
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Actually, the kinetic energy does not go anywhere, since the propeller could maintain the same RPM and the same kinetic energy, but it does take energy to keep the propeller spinning.
This energy is required to overcome the resistance of the air (ignoring internal friction) and is passed to the air, which accelerates toward the ground, its reaction force keeping the helicopter from falling down.
Eventually though, all the energy passed to the air will turn into heat.
V.F.
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Good from the answers we agree that the propellers using newton's second law provide lift, the energy turns into heat and motion in air, forget about the helicopter. Say a rocket in space away from gravitational pull all the energy supplied by the engines becomes kinetic energy, heat and expanding gas now if we bering that rocked on earth so the that it hovers above the ground the same amount of energy that was turning into heat or light in space is still turning into heat and on earth but there is no gain of gravitational potential energy or kinetic energy. The engine output is the same, the amount of energy that was going in the expanding gas and heat is the same but here is the problem In space the energy was being to tunred to heat and motion and the sum of those energies was equal to the energy output of the engine while on earth even thou the engine output did not change nor did the amount of gas or heat produced change but that gain in kinetic enegry for the rocket is not here and the total output of the engine is greater than the heat and sound and gad expantion on earth what happens to the extra energy?