Asymmetric aircraft

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.

Symmetry is never truly complete. Though aerodynamics might require balance, all aircraft have some level of asymmetry. Rotating propellers and engines are asymmetric and so such aircraft may have asymmetric dynamic behavior, even though the airframe is outwardly symmetric.

Single-engine 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.

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.

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Wikipedia