Downwash

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The effect of downwash (non-engineer's meaning) from a hovering helicopter is clearly visible on the surface of water below.

The term downwash has two meanings within the field of aerodynamics.

1. One meaning, used most often by non-engineers (in particular pilots), refers to the forcing of air downward during the creation of lift. This usage is most common with regard to helicopters where the effect is most dramatic.
2. The other meaning, used most often by engineers, refers to the flow of air behind a wing. The downwash immediately behind a wing is a consequence of the wing vortex system (also see horseshoe vortex). This downwash is experienced by a horizontal tail placed in the flow behind a wing. The primary influence of downwash on the horizontal tail is an increase in angle of attack on the tail. (The horizontal tail generates lift in the downward direction, unlike the wing. A disturbance that causes an increase in angle of attack on the wing will cause a decrease in angle of attack on the horizontal tail, and vice versa.)

These meanings are closely related. They differ primarily in terms of scale. The "pilot's meaning" is a much larger-scale effect than the "engineer's meaning".

Downwash is also related to the difference in pressure between the top and bottom of a wing. At the tip of a wing, this difference in pressure will "leak" around the tips from the bottom to the top, resulting in wingtip vortices.

Downwash is a component of the vortex system generated by a wing (see horseshoe vortex). The energy stored in trailing vortices is the product of induced drag multiplied by true airspeed.

[edit] Upwash

If an airfoil's vortex produces downwash, somewhere else the airframe must also produce a similar amount of upwash. In "stable flight" this occurs approximately within the airframe surface envelope; it can also occur outside of the airframe envelope under special circumstances. The term upwash is used to refer to regions around an aircraft or airfoil where the air is moving in the opposite direction to downwash.

This image of the vortex trailing from the right wingtip of an agricultural aeroplane clearly shows the region of downwash to the left of the vortex core, and the region of upwash to the right of the core. (It also shows air above the core moving left, and below the core, moving right.)

The wingtip vortices induce an upwash outside the wingspan of an aircraft or airfoil balancing downwash produced by upper wing surfaces. Migratory birds make use of this upwash when they fly in a V formation.

Upwash is particularly important in the case of a canard aeroplane because the upwash from the canard increases the angle of attack on the outboard ends of the wing, promoting wing tip stall. This must be avoided by suitable precautions in the design of the wing to ensure separation of the flow over the wing commences close to the wing root, and the ailerons remain functional.

As the air approaches the wing it is given an upward component of velocity and this is described as upwash, in contrast to downwash as the air leaves the wing. This upwash and downwash can be considered a consequence of the bound vortex in the wing, causing lift.