Definition: An airplane with two levels of wings.
Significance: Most early aircraft utilized the biplane configuration, and biplanes remain popular for sport flying and aerobatics.
Reasons for the Biplane Configuration
From the early, pioneering flights of Orville and Wilbur Wright in 1903 through the 1920′s and 1930′s, biplanes represented the most practical aircraft configuration for both structural and maneuverability reasons. By the 1940′s, they remained a practical choice only for training aircraft. Since then, biplanes have retained a certain popularity as sport and aerobatic and air show aircraft.
Until sufficiently light and powerful aircraft engines were developed, a large wing area was required to keep an aircraft aloft, and the biplane structure provided the most strength with the least weight. It was initially thought that thin wing sections were necessary for efficient generation of lift. In a biplane configuration, interplane struts and wire bracing provide a bridge-like strength and rigidity to the wing. Biplanes can use lesser wingspans, and both wings can use ailerons, resulting in the added advantage of maneuverability. Thus, for the first few decades of flight, biplanes were the configuration of choice for training aircraft, sport aircraft, military fighters, military bomber aircraft, and transport aircraft.
Famous Biplanes
America’s best-known aircraft during World War I was the Curtiss JN “Jenny” trainer. It used a four-bay wing with eight interplane struts and many bracing wires, but it could fly two people with only a 90-horsepower OX-5 engine. After the war, Jennys were surplused and became the barnstormer’s choice of airplane. The most famous World War I fighters were biplanes. In England, the De Havilland Tiger Moth was the trainer of choice between the world wars. In the 1930′s, the Curtiss P-6E Hawk fighter biplane delighted the eye. In World War II, the best-known U.S. trainers were the Piper Cub monoplane and the Boeing-built Stearman PT-17 biplane. The Stearman had a reputation for indestructibility in the air and remains a popular sport biplane. When the Stearman was declared to be surplus after the war, it became the favorite of crop dusters, who took advantage of its great strength and load-carrying ability. It also survives as a popular sporting aircraft.
The biplane flowered in the interwar period. Travelair, which began producing biplanes in 1925, bettered the Jenny in control, comfort, speed, and safety. The Travelair D-4D is arguably the best-looking open-cockpit biplane ever built. During the 1920′s and 1930′s, the Waco Aircraft Company of Troy, Ohio, was by far the largest airplane manufacturer in the United States, building thousands of open-cockpit and cabin biplanes. The first Waco biplanes were built in 1922, but the Waco 9 appeared at the same time as the first Travelair and was highly regarded. The Waco Taperwing, using a tapered wing planform on both wings, remains popular.
Disadvantages of the Biplane Configuration
One disadvantage of the biplane is related to the extra drag of its wires and supporting struts and the interference drag between its two wings, which result in reduced cruising and top speeds for a given engine power. Another disadvantage is a poor lift-to-drag ratio that results in poor glide angles. By the 1920′s, the most efficient aircraft were monoplane designs, such as Charles A. Lindbergh’s Spirit of St. Louis. A monoplane is more simple and less costly to build. When aircraft designers learned how to make strong, internally braced aircraft entirely of aluminum,and when powerful and relatively light engines became available, the monoplane replaced the biplane as the configuration of choice for all high-speed aircraft.
The primary lifting surface of a wing is its upper surface, so the lower wing suffers the most from this; the gap between the wings is therefore usually made at least as large as the wing chord. If the wings are set at different angles (decalage), the relative loading of the wings and stall characteristics can be adjusted. Often the upper wing is mounted ahead of the lower wing, an arrangement known as positive stagger. This is particularly true for open-cock-pit biplanes in which the front cockpit is under the wing and the rear cockpit, for stability reasons, is not placed too far back on the fuselage. However, the famous Beechcraft Stagger wing has a closed cabin and uses negative stagger. A biplane that has a smaller lower wing is known as a sesquiplane.
Most biplanes use the lighter tail wheel configuration for their landing gear, but the higher center of gravity and the poor view for the pilot upon landing mean that the directional instability of the tail wheel configuration requires significantly more pilot alertness and skill. Usually, the lower wing has a dihedral angle to provide lateral stability and to keep the tips farther from the ground, whereas the upper wing is straight, to simplify its construction.
Sport, Aerobatic, and Air-Show Biplanes
The biplane configuration has long been preferred for aerobatics because of its inherently good roll rate and because the extra drag of brace wires and struts prevents a rapid buildup of speed in the diving aspect of maneuvers. The 1920′s and 1930′s Great Lakes Trainer biplane was considered the best aerobatic aircraft of all U.S.-manufactured aircraft until the arrival of the Pitts Special. When the Great Lakes Trainer was first flown, it was found that its center of gravity was too far aft. This problem was corrected most simply by giving the upper wing rearward sweep. This correction had the side benefit of making the airplane a better snap-roll performer.
Biplanes remain favored aircraft for many air-show pilots, because of their extra visibility to spectators and because of the additional possibilities for wing-walkers. Only in the last decade of the twentieth century did monoplanes begin to dominate aerobatic competition at the highest levels. The appeal of the open-cockpit biplane, a sort of motorcycle of the air, will live on indefinitely, as pilots feel the sheer joy of flying between two wings in warm summer air.
Biplanes are so called because they have two parallel levels of wings; these offered early aviators the largest wing area and strength for the least weight.
