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A monoplane has inherently the lowest drag of any wing configuration and is the simplest to build. However these advantages may be offset by its inherent weight and need for higher power.
Monoplane vs. biplane
The wing of a monoplane must nominally have twice the area of the equivalent biplane wing, because the biplane has two of them. This makes the monoplane larger and less manoeuvrable. However, in practice the biplane's wings interfere with each other and the handicap to the monoplane is reduced.
A biplane wing is usually braced to stiffen the structure and allow it to be much lighter and to fly slower. However even a braced monoplane will still be more efficient and human-powered aircraft, which are among the slowest and lightest of flying machines, are monoplanes with very large wings for their weight.
Support and weight
The inherent efficiency of the monoplane can be realised in the unbraced cantilever wing which carries all structural forces internally. By contrast the braced wing gains additional drag from the exposed bracing struts and/or wires. On the other hand, the braced wing has greater structural efficiency and can be made much lighter. This in turn means that for a wing of a given size, bracing allows it to fly slower with a lower-powered engine, while a heavy cantilever wing needs a more powerful engine and can fly faster.
A cantilever wing can be made lighter by making it thicker. This increases internal storage for fuel, retractable undercarriage, armaments and in some cases even passengers and crew. A thick wing can also be given greater curvature of its upper surface and so create more lift than a thin wing, and some American bombers of WWII had unusually thick wings.
But thickness, like bracing, also increases drag, especially above the speed of sound. Supersonic aircraft have thin wings which are much heavier, and the wheels and fuel must find storage elsewhere.
Besides the general variations in wing configuration such a tail position and use of bracing, the main distinction between types of monoplane is how high up the wings are mounted in relation to the fuselage.
A low wing is one which is close to the bottom of the fuselage.
Placing the wing low down frees up the central fuselage from the wing spar carry-through structure and also allows good visibility upwards. By reducing pendulum stability it makes the aircraft more manoeuvrable. It also allows a lighter structure because the fuselage sides carry no additional loads and the main undercarriage legs can be made shorter. It is common on jet airliners.
A mid wing is mounted mid-way up the fuselage. It is aerodynamically the cleanest and most balanced, but the carry-through spar structure can reduce the useful fuselage volume near its centre of gravity, where space is often in most demand. It is common on high-performance types such as sailplanes.
Placing the wing above the midpoint increases the useful volume beneath and also allows good visibility downwards. It provides pendulum stability by placing the centre of gravity below the centre of pressure. It also gives increased ground clearance for the propellers of multi-engined aircraft, for underwing stores and for ground handling. Many light transport aircraft have shoulder or high wings.
A high wing has its upper surface close to or above the top of the fuselage. It has all the advantages and disadvantages of the shoulder wing, and even more so.
On light aircraft the wing is sometimes located on top of the pilot's cabin.
A parasol wing is a high wing which forms a separate structure above the fuselage and is not directly attached to it, with structural support being provided by either several cabane struts or a single wider pylon. Additional bracing has to be provided by underwing struts extending either side of the fuselage.
The supports and bracing create extra drag so the parasol wing is not often used. Compared to a biplane it has less bracing and lower drag, and the parasol wing was popular only during the inter-war transition years between biplanes and monoplanes.
The first attempts at heavier-than-air flying machines were monoplanes.
Although the first successful aircraft were biplanes, many pioneering aircraft were monoplanes, for instance the Blériot XI that flew across the English Channel in 1909. Throughout 1909-1910 Hubert Latham set multiple altitude records in his Antoinette IV monoplane, eventually reaching 1,384 m (4,541 ft).
The equivalent German language term is Eindecker, as in the mid-wing Fokker Eindecker fighter of 1915 which for a time dominated the skies in what became known as the "Fokker scourge". The German military Idflieg aircraft designation system prior to 1918 prefixed monoplane type designations with an E, until the approval of the Fokker D.VIII fighter from its former "E.V" designation. However the success of the Fokker was short-lived and World War One was dominated by biplanes. Towards the end of the war the parasol monoplane became popular and successful designs were produced into the 1920s.
Nonetheless, relatively few monoplane types were built between 1914 and the late 1920s, compared with the number of biplanes. The reasons for this were primarily practical. With the low engine powers and air speeds available, the wings of a monoplane needed to be large in order to create enough lift while a biplane could have two smaller wings and so be made smaller and lighter.
Towards the end of the first world war, the inherent high drag of the biplane was beginning to restrict performance. Engines were not yet powerful enough to make the heavy cantilever-wing monoplane viable and the braced parasol wing became popular on fighter aircraft, although few arrived in time to see combat. It remained popular throughout the 1920s.
On flying boats with a shallow hull, a parasol wing allows the engines to be mounted above the spray from the water when taking of and landing. It was popular on flying boats during the 1930s; a typical example being the Consolidated Catalina. It died out when taller hulls became the norm during WWII, allowing the wing to be attached directly to the hull.
As ever-increasing engine powers made the weight of all-metal construction and the cantilever wing more practical, they became common during the post-WWI period, the day of the braced wing passed, and by the 1930s the cantilever monoplane was fast becoming the standard configuration for a fixed-wing aircraft. Advanced monoplane fighter-aircraft designs were mass-produced for military services around the world in both the Soviet Union and the United States in the early-mid 1930s, with the Polikarpov I-16 and the Boeing P-26 Peashooter respectively.
Jet and rocket engines have even more power and all modern high-speed aircraft, especially supersonic types, have been monoplanes.
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- Crane, Dale: Dictionary of Aeronautical Terms, third edition, page 379. Aviation Supplies & Academics, 1997. ISBN 1-56027-287-2
- King, Windkiller, p. 227.