Asymmetrical aircraft have left- and right-hand sides which are not exact mirror images of each other. Although most aircraft are symmetrical, there is no fundamental reason why they must be, and design goals can sometimes be best achieved with an asymmetrical aircraft.
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Invisibly asymmetric aircraft
Rotating propellers and engines are asymmetric and so such aircraft may have asymmetric dynamic behaviour, even though their airframe is outwardly symmetric.
Single-engined propeller aircraft are inherently asymmetric, as the propeller rotation direction and the resultant propwash gives a strongly asymmetric effect over the tail control surfaces, especially at low speeds during takeoff. As engines became more powerful, around the end of World War II, the last single piston-engined fighters used contra-rotating propellers both to handle this great power, and also to reduce their asymmetry.
Rotary engines, used in World War I, had a pronounced asymmetric effect from the rotational inertia of their heavy rotating crankcase and cylinders. Some aircraft, particularly the Sopwith Camel and its relatively heavy Clerget 9B engine were noted for having a faster turn to one side than the other, which influenced combat tactics both with it and against it. In contrast, the corresponding German engine, the contra-rotary Siemens-Halske engines were more balanced.
Twin-engined aircraft with their propellers rotating in the same direction are also asymmetric. Counter-rotating propellers avoid this, either by building pairs of engines to rotate their crankshafts in opposite directions, or by using a reversing gear in one of the propeller reduction gearboxes. Handed engines have rarely been used, owing to cost, but were sometimes used for naval aircraft such as the Sea Hornet, to simplify their handling across a narrow carrier deck.
Pioneer years
The first aircraft to fly, the Wright Flyer had an asymmetrical arrangement of pilot and engine. Both needed to be close to the centre of gravity above the front of the wing, so each was moved to one side to make room for the other. The propellers were symmetrically placed, so one engine drive chain was longer than the other. The longer, port drive belt was also twisted across itself so that the propellers rotated in opposite directions.
World War I
During World War I, Swiss-born Hans Burkhard designed several asymmetrical aircraft. Burkhard obtained German Patent number 300 676 for his design on 22 September 1915. The Gotha G.VI first flew in 1918, but did not reach production before the war concluded.
World War II
During World War II, German designer Richard Vogt experimented with several asymmetrical aircraft, including:
The BV 141 was heralded by the Germans as the first asymmetric aircraft, an evaluation batch was built, but it was never ordered into full-scale production.
Modern aircraft
The NASA AD-1 includes oblique wings, and flew 1979-1982.
Rutan Boomerang is an FAA-approved concept aircraft of the 2000s, which was restored to flying condition in 2011.
Engine torque
A problem with powerful propeller-driven aircraft is that the engine torque driving the propeller round one way also tends to push the aeroplane round the other way.
On some types such as the Sopwith Camel this was used to advantage to allow very fast turning in one direction, although it was at the expense of sluggish turns the opposite way.
But in general it causes more problems than it solves.
On some single-engined types such as the Italian Ansaldo SVA it was counteracted by lengthening one wing to provide a counter-torque.
On some pioneer types driving twin propellers from a single engine shaft, one of the drive belts was crossed over to cancel out the two propeller torques. Examples include the Wright Flyer described above and the early designs by J.W. Dunne.
On a twin-engined type the asymmetrical torque is most easily counteracted by having left-and right-handed engines, which rotate in opposite directions on each wing, making the craft fully symmetrical.
Variable sweep
The mechanism for varying the angle of a swept wing is complicated, heavy and expensive. The two halves must be synchronised and each supported at one end. A single oblique wing may be supported at its centre and needs no linking gear. The idea has been tried successfully on the NASA AD-1.