Tailplane - Equilibrium

Equilibrium

An aeroplane must be in balance longitudinally in order to fly. This means that the net effect of all the forces acting on the aeroplane produces no overall pitching moment about the center of gravity; i.e. the overall center of lift of the aircraft must be close to it. In a conventional aircraft, the center of gravity is ahead of the center of lift of the main wing, which would cause the aircraft to pitch forward without the downward force of the tailplane to balance this.

Not all aircraft have tailplanes. On many flying wings such as the Fauvel AV.36 and tailless deltas such as the Dassault Mirage III, the downward force is instead provided by the trailing edge of the wing. On some swept wing flying wings it is provided by the wingtips. In these cases, because the force being applied is acting on a short moment arm, the forces must be larger, which incurs a larger induced drag penalty that may outweigh the reduction in weight the lack of a fuselage brings, particularly for larger aircraft where weight is less critical than drag. Some canard types use their foreplane as a stabilizer, in which case it exerts an upward force.

On some airplanes built before World War I, such as the Bleriot XI, the center of gravity is between the center of lift from the wings and the tailplane, which instead of providing a downward force, provided an upward one. However there are severe handling issues with this arrangement that were beyond the capabilities of designers at the time to fix, and the approach was eventually abandoned. Examples of aircraft that had this setup include Charles Lindbergh's Spirit of St. Louis, the Sopwith Camel and the Gee Bee Model R Racer - all aircraft with a reputation for being difficult to fly. With computer controls this is no longer a problem and aircraft as different as the Airbus and the F-16 are flown in this condition. The advantage to this is a significant reduction in induced drag caused by the tailplane, and in the case of the F-16, improved maneuverability.