Chine (aeronautics)

In aircraft design, a chine may be one of several features:

A chine shaped fuselage may be described as a fuselage featuring sideways extensions, or long extensions of the wing roots along the fuselage. Chines first appeared on the Lockheed A-12 precursor of the SR-71 Blackbird, where they formed forward extensions of the wing roots along the fuselage sides into which they blended.

The Lockheed Martin F-22 Raptor has chines that lead to the leading-edge extensions that are blended into the engine air intakes.

A small horizontal surface forming a fillet between the main wing root and the fuselage, more usually called a Leading Edge Root eXtension (LERX) or Leading Edge eXtension (LEX), is also sometimes called a chine.

Large sideways extensions of a chined forebody can have a significant effect on aircraft lift, drag, longitudinal balance and directional stability.

The chine shaped fuselage of the Lockheed Blackbird series extends about 40% of the aircraft's length and contributes useful additional lift at supersonic speeds. The chines may be understood as enhancing the lift generated by the forebody, acting as a low aspect ratio canard surface. In order to further increase this lift contribution, the forward fuselage is set with a positive incidence relative to the wing.

The chine lift increases with the square of the Mach number, helping counterbalance the rearward shift in the lift of the main wing in supersonic conditions. If a tailless (Delta) wing is trimmed for safe subsonic flight, at high speeds it gains excess trim drag in pitch and becomes excessively stable resulting in poor manoeuvrability. The destabilising effect of a forward surface is provided by the chines where it is needed most, at high Mach numbers.

Forward chines also act as Leading edge root extensions (LERX) at low speeds and high angles of attack, generating a downflow over the inboard wing to reduce its angle of attack and delay the stall, and also generating vortices which provide additional lift.

The chines also increase directional stability, by reducing the adverse effects of crosswinds or yaw on the forward fuselage. Unlike a conventional fuselage, the chines allow the crossflow to travel smoothly over their profile and beyond, avoiding the side forces due to flow separation and stagnation. Again the effect is stronger at higher speeds, and reduces the size of the vertical stabilisers (tail fins). The YF-12A lacked the foremost section of the chines seen on the SR-71 and consequently needed extra vertical tail surfaces.

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