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In aeronautics, downwash is the change in direction of air deflected by the aerodynamic action of an airfoil, wing, or helicopter rotor blade in motion, as part of the process of producing lift. [1] In helicopter aerodynamics discussions, it may be referred to as induced flow .
Lifting line theory supposes wings that are long and thin with negligible fuselage, akin to a thin bar (the eponymous "lifting line") of span 2s driven through the fluid. . From the Kutta–Joukowski theorem, the lift L(y) on a 2-dimensional segment of the wing at distance y from the fuselage is proportional to the circulation Γ(y) about the bar a
For complete aircraft configurations, downwash data is also included. When compared with modern methods of computational fluid dynamics, Digital DATCOM may seem antiquated. However, in its day, the program was an advanced estimation tool, and certainly much faster than plowing through pages and pages of engineering texts.
Since the tail is operating in the flowfield of the wing, changes in the wing incidence cause changes in the downwash, but there is a delay for the change in wing flowfield to affect the tail lift, this is represented as a moment proportional to the rate of change of incidence: ˙
where is the tail area, is the tail force coefficient, is the elevator deflection, and is the downwash angle. A canard aircraft may have its foreplane rigged at a high angle of incidence, which can be seen in a canard catapult glider from a toy store; the design puts the c.g. well forward, requiring nose-up lift.
The Kutta–Joukowski theorem is a fundamental theorem in aerodynamics used for the calculation of lift of an airfoil (and any two-dimensional body including circular cylinders) translating in a uniform fluid at a constant speed so large that the flow seen in the body-fixed frame is steady and unseparated.
The downwash is associated with induced drag and is a component of the system of trailing vortices. [ 4 ] The horseshoe vortex model is unrealistic in that it implies uniform circulation (and hence, according to the Kutta–Joukowski theorem , uniform lift) at all sections on the wingspan .
Washout is commonly achieved by designing the wing with a slight twist, reducing the angle of incidence from root to tip, and therefore causing a lower angle of attack at the tips than at the roots.