Moving a vehicle, may it be on the ground, on the water, or in the air, requires a force to overcome the friction and inertia forces and to lift the vehicle to a different elevation. A force can be created from any kind of energy, like the energy contained in liquid fuel for an internal combustion engine, the electric energy stored in a battery or like solar energy being transformed by solar cells into electric power. To actually move an object, the force must be must be translated by some sort of engine into power, pushing the vehicle forward. Burning fuel in an open pan does not create a force and having a bottle of compressed air lying around creates a force on the bottles walls, but does not create any power output.
Power equals force times distance per time .
Process of converting stored energy into power which can be used to move a craft. The lack of powerful, lightweight engines was one of the reasons for the failures of early attempts to fly. Using natural sources of energy like thermals requires quite sophisticated aircraft, which were of course not available in the beginning. When steam engines became available, they proved to be too heavy for flight, compressed air engines or twisted rubberbands could be used for smaller demonstrations, but were not practical or to heavy for man carrying airplanes. Finally the piston engine, internally burning fuel, was to become the first successful powerplant for aviation.