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 behind a wing is a
consequence of the wing trailing vortex system. This downwash is
experienced by a horizontal tail placed in the flow behind a wing.
The primary influence of downwash of on the horizontal edge is the
reduction of tail angle of attack, and consequently the reduction
of the tail lift. This reduction depends on the longitudinal and
vertical distance between wing and horizontal tail. A horizontal
tail that is close to the wing is more affected by the wing
downwash than one that is far from the wing. In aircraft design,
this influence must be avoided by locating the horizontal tail far
and high enough compared with the location of the wing. In front of
the wing the term upwash is used instead of downwash. Upwash is
particularly important in the case of a canard airplane
design.
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, which is required for lift. 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.
Trailing vortices and downwash are caused by the same phenomenon.
The energy used to create trailing vortices is a component of
induced drag.
