Aircraft flight mechanics

Aircraft flight mechanics are relevant to fixed wing (gliders, aeroplanes) and rotary wing (helicopters) aircraft. An aeroplane (airplane in US usage), is defined in ICAO Document 9110 as, "a power-driven heavier than air aircraft, deriving its lift chiefly from aerodynamic reactions on surface which remain fixed under given conditions of flight". Note that this definition excludes both dirigibles (because they derive lift from buoyancy rather than from airflow over surfaces), and ballistic rockets (because their lifting force is typically derived directly and entirely from near-vertical thrust). Technically, both of these could be said to experience "flight mechanics" in the more general sense of physical forces acting on a body moving through air; but they operate very differently, and are normally outside the scope of this term. A heavier-than-air craft (aircraft) can only fly if a series of aerodynamic forces come to bear. In regard to fixed wing aircraft, the fuselage of the craft holds up the wings before takeoff. At the instant of takeoff, the reverse happens and the wings support the plane in flight. In flight a powered aircraft can be considered as being acted on by four forces: lift, weight, thrust, and drag. Thrust is the force generated by the engine (whether that engine be a jet engine, a propeller, or -- in exotic cases such as the X-15 -- a rocket) and acts in a forward direction for the purpose of overcoming drag. Lift acts perpendicular to the vector representing the aircraft's velocity relative to the atmosphere. Drag acts parallel to the aircraft's velocity vector, but in the opposite direction because drag resists motion through the air. Weight acts through the aircraft's centre of gravity, towards the centre of the Earth. In straight and level flight, lift is approximately equal to the weight, and acts in the opposite direction. In addition, if the aircraft is not accelerating, thrust is equal and opposite to drag. In straight climbing flight, lift is less than weight.