All aircraft require one or more engines that are responsible for generating propulsion and lift, allowing the vehicles to overcome their weight for takeoff. Some use jet engines, while others use reciprocating engines, both of which consume energy to create propulsion. To remain steady during flight, the thrust provided must be equal to and in the opposite direction of aircraft drag. The majority of heat engines convert heat energy into mechanical energy, as a fluid mass like air flows through the engine. Once the working pressure is higher than the atmospheric pressure, heat energy is released.