Chapter 1 Aircraft Engines General Requirements Aircraft require thrust to produce enough speed for the wings  to provide lift or enough thrust to overcome the weight of  the aircraft for vertical takeoff. For an aircraft to remain in  level flight, thrust must be provided that is equal to and in  the opposite direction of the aircraft drag. This thrust, or  propulsive force, is provided by a suitable type of aircraft  heat engine. All heat engines have in common the ability to  convert heat energy into mechanical energy by the flow of  some fluid mass (generally air) through the engine. In all  cases, the heat energy is released at a point in the cycle where  the working pressure is high relative to atmospheric pressure. 