Aircraft can be categorized in three ways: by basic design as either lighter-than-air or heavier-than-air, by propulsion as either power driven or non-power driven, and by wing design as either aeroplanes with fixed wings or rotorcraft with rotary wings. Lighter-than-air aircraft rely on gases for lift while heavier-than-air aircraft use air flowing over wings. Power driven aircraft use engines for propulsion and non-power driven use gravity and aerodynamic forces. Aeroplanes have stationary wings and rotorcraft have rotating wings.