Definition: Airplanes possessing only one primary lifting surface.
Significance: Monoplanes are less expensive to build, more efficient in flight, and are capable of higher speeds than two-winged biplanes. After their early structural problems were solved, monoplanes quickly became the favored configuration for transports (in the 1920′s) and for light airplanes and fighter aircraft (in the 1930′s).
The earliest practical airplanes were biplanes, with low-powered engines and very large wing areas. The braced-wing biplane design was considered the lightest and strongest aircraft configuration, especially for the thin wing sections then thought to be necessary.
The first airplane to cross the English Channel, however, was Louis Bleriot’s wire-braced monoplane, the Bleriot XI, on July 25, 1909, six years after the Wright brothers flew their first biplane at Kitty Hawk, North Carolina. In 1912, the streamlined Deperdussin monoplane established a world speed record of more than 100 miles per hour.
Biplanes continued to predominate during World War I, because their rapid climb to a fighting altitude and their maneuverability for fighting were favored over high speeds. However, a few monoplanes, such as the Fokker Eindecker, the first to have a machine gun synchronized to fire between propeller blades, were used. Early monoplane designers did not appreciate the twisting to which a wing is subjected in flight, and there were a number of structural failures due to the elimination of external bracing to reduce drag.
In 1927, the greater efficiency of the monoplane was decisively demonstrated by Charles A. Lindbergh’s New York-to-Paris flight in his Ryan monoplane, the Spirit of St. Louis. In the 1930′s, the development of the Douglas DC-1 and DC-2 models, using the modern configuration of a single low wing, a retractable landing gear, streamlined engine cowlings, and flaps for good low-speed performance, instantly made all biplane transports obsolete. ADC-2 carrying passengers nearly won the 1934 London-to-Australia race against specialized racing machines. The military, requiring extra strength and maneuverability from its aircraft, took longer to be convinced of the monoplane’s advantages and still maintained a few biplanes at the beginning of World War II in 1939.
The 1937 Piper J-3 Cub, a strut-braced, high-wing monoplane with an inexpensive four-cylinder opposed engine, was far less expensive to produce and fly than were biplanes with much more powerful radial engines. This configuration has dominated the lower end of light-plane flight ever since.
In the traditional monoplane design, a single large wing is followed by a much smaller horizontal surface containing both a stabilizing surface and a pitch-control surface, or elevator. For pitch stability with this configuration, the airplane’s center of gravity must be sufficiently forward that the horizontal tail generates a downward force, or negative lift. Pitch stability means that the aircraft will tend to maintain a constant nose attitude relative to the horizon, even when disturbed by atmospheric turbulence. However, a monoplane’s wing can be made into a very efficient lifting surface, because it does not compete with another nearby lifting surface, as does a biplane’s wing.
Different monoplane designs differ in their relative vertical locations of the wings on the fuselage. If the wing is mounted on top of the fuselage, as in high-wing aircraft, the critical upper surface of the wing is minimally disturbed by airflow around the fuselage. The placement of the primary lifting surface above the center of gravity also enhances lateral stability because of the pendulum effect, in which the airplane tends to return to wings-level flight if it is banked. High-wing aircraft give pilots and passengers a particularly good view of the ground but the upward and sideways views are typically restricted when the aircraft is banked.
The most efficient location for a monoplane’s wing, from an aerodynamic standpoint, is considered to be in the middle of the fuselage. In this mid-wing configuration, the interference drag between the wing and the fuselage can be minimized. Therefore, many racing airplanes use this efficient design. The pilot typically must sit close to the center of gravity, about one-quarter or one-third of the way back from the leading edge of the wing to the trailing edge. The pilot’s field of view is thus severely restricted, and the pilot’s location obstructs any carry-through structure for the wing spar.
The low-wing airplane is favored for most high-speed airplanes, because the wing provides a good place to house retracted gear. To minimize interference drag, both the leading and trailing edges of the wing normally require rather elaborate fairings. To obtain lateral stability, the wing of a low-wing airplane must incorporate more of a dihedral angle, the upward tilt of the wingtips, than that of a high-wing airplane.
In the 1980′s, a number of canard-type airplanes, efficient aircraft with a horizontal lifting surface in front of the main wing, were designed for both low- and high-speed flight. The low-speed canard-type aircraft are mainly those linked to Burt Rutan’s very successful VariEze and later designs. Canard aircraft with two nearly equal wings, the dragonfly configuration, may be thought of as either two-surface monoplanes or biplanes with a great deal of stagger. High-speed military aircraft often use a canard for extra pitch control at high angles of attack. Propeller-powered canard aircraft normally use pusher propellers, which tend to be less efficient because they operate in the wake of the wing. A few three-surface aircraft, with both a canard surface and a conventional tail surface, have also been designed and flown; they have the advantage of placing the pilot and passengers ahead of the wing.