a fully movable tail, the entire assembly acts as a control surface. Wing-stabilizer interaction The upwash and downwash associated with the generation of lift is the source of aerodynamic interaction between the wing and stabilizer, which translates into a change in the effective angle of attack for each surface. The influence of the wing on a tail is much more significant than the opposite effect and can be modeled using the Prandtl lifting-line theory; however, an accurate estimation of the interaction between multiple surfaces requires computer simulations or wind tunnel tests. Horizontal stabilizer configurations Conventional tailplane In the conventional configuration the horizontal stabilizer is a small horizontal tail or tailplane located to the rear of the aircraft. This is the most common configuration. On many aircraft, the tailplane assembly consists of a fixed surface fitted with a hinged aft elevator surface. Trim tabs may be used to relieve pilot input forces. Alternatively, some light aircraft such as the Piper PA-24 Comanche and the Piper PA-28 Cherokee have an all-moving stabilizer known as a stabilator, with no separate elevator. Stabilators are also found in many supersonic aircraft, where a separate elevator control would cause unacceptable drag. Most airliners and transport aircraft feature a large, slow-moving trimmable tail plane which is combined with independently-moving elevators. The elevators are controlled by the pilot or autopilot and primarily serve to change the aircraft's attitude, while the whole assembly is used to trim (maintaining horizontal static equilibrium) and stabilize the aircraft in the pitch axis. In the Boeing 737, the adjustable stabilizer trim system is powered by an electrically operated jackscrew. Variants on the conventional configuration include the T-tail, Cruciform tail, Twin tail and Twin-boom mounted tail. Three-surface aircraft Three-surface aircraft such as the Piaggio P.180 Avanti or the Scaled Composites Triumph and Catbird, the tailplane is a stabilizer as in conventional aircraft; the frontplane, called foreplane or canard, provides lift and serves as a balancing surface. Some earlier three-surface aircraft, such as the Curtiss AEA June Bug or the Voisin 1907 biplane, were of conventional layout with an additional front pitch control surface which was called "elevator" or sometimes "stabilisateur". Lacking elevators, the tailplanes of these aircraft were not what is now called conventional stabilizers. For example, the Voisin was a tandem-lifting layout (main wing and rear wing) with a foreplane that was neither stabilizing nor mainly lifting; it was called an "équilibreur" ("balancer"),