in straight climbing flight, lift is less than weight. at first, this seems incorrect because if an aircraft is climbing it seems lift must exceed weight. when an aircraft is climbing at constant speed it is its thrust that enables it to climb and gain extra potential energy. lift acts perpendicular to the vector representing the velocity of the aircraft relative to the atmosphere, so lift is unable to alter the aircraft's potential energy or kinetic energy. this can be seen by considering an aerobatic aircraft in straight vertical flight (one that is climbing straight upwards or descending straight downwards). vertical flight requires no lift. when flying straight upwards the aircraft can reach zero airspeed before falling earthwards; the wing is generating no lift and so does not stall. in straight, climbing flight at constant airspeed, thrust exceeds drag.