Chine (aeronautics)

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Chined fuselage of a SR-71 Blackbird
The front view of the A-12 showing forebody shaped into chines

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

  • On a fuselage of any aircraft, a chine is the longitudinal line of sharp change in the cross-section profile
  • In seaplane design, a hard chine [note 1] is the longitudinal line of sharp change in hull cross-section, meeting of bottom plane with lateral panel, as in the hull of a powered planing boat.
  • The longitudinal sideways extensions of a chined-shaped fuselage are usually called chines ; this kind of fuselage is also called "chined fuselage", "chined forebody", "chine shaped forebody".[note 2] The usual appellation is "chine" or sometimes strake.[1] This article discusses this type of chine.

Configuration[edit]

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.[1]

The Lockheed Martin F-22 Raptor has chines that lead to the leading-edge extensions that are blended into the engine air intakes.[2]

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.[1]

Forward chines effects[edit]

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

Effect on lift, drag and longitudinal balance[edit]

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,[3] 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.

Directional effect[edit]

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.

Lateral effect[edit]

The improved crossflow behaviour also benefits lateral characteristics by reducing yaw-induced roll, especially during landing and takeoff of delta-winged aircraft. This in turn helps reduce roll-yaw coupling and any tendency to Dutch roll. However chines have also been found to reduce lateral stability in some configurations, due to abrupt asymmetric vortex breakdown effect.[4]

Stealth effect[edit]

Blending the chines into both the fuselage and the main wing avoids presenting corner reflectors or vertical sides to radars.[5] This has led fifth-generation jet fighter designs to replace low-stealth canard surfaces with chines, when helping to generate vortex lift over the main wings.[citation needed] (An exception is the Chengdu J-20, whose canards are mounted inline with its chines.)

See also[edit]

Notes[edit]

  1. ^ Angled chine, different from soft (rounded) chine
  2. ^ NASA-aiaa-98-2725 Impact of fuselage cross-section on the stability of a generic fighter uses "chined-shaped fuselage cross section, chined forebody, fuselage with ... included chine angle" expressions. NASA CR 189641 and AIAA 2008-6228 use "Chine forebody and Chine fuselage"

References[edit]

  1. ^ a b c David Godfrey (30 August 1973), "Blackbirds from the skunk works", Flight International: 383 
  2. ^ F-22 Raptor Manufacturing, globalsecurity.org, 7 August 2011, ...the F-22's chine, a fuselage edge that provides smooth aerodynamic blending into the intakes and wings. 
  3. ^ Forebody lift is about 17 to 20 % of total lift. AIAA 2008-6228 report, p.15 fig. 8
  4. ^ Chambers, Joseph R. "Modeling Flight." NASA, 2009, p.143.
  5. ^ Rebecca Grant (political expert) "The Radar Game" Mitchell Institute, 2010