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National Advisory Committee for Aeronautics, Report - Investigation of Downwash, Sidewash, and Mach Number Distribution Behind a Rectangular Wing at a Mach Number of 2.41

In order to assess the stability characteristics of an aircraft
or missile configuration, it is necessary that the nature of the
flow field ezn'sting in the region of the tail plane be known.
As far as the subsonic flow is concerned, much attention has
been given in the past to this problem (e.g., ref. 1) and, for
the conventional subsonic aircraft configuration, downwash
effects can be predicted with sufficient accuracy to serve the
practical needs of designers. For configurations in which the
ratio of wing span to tail span is small, as is the usual case
for supersonic-missile configurations, the downwash problem
is not so fully developed. Only fairly recently have research
investigations in this important field been undertaken with
any degree of thoroughness.

The application of the linearized theory to the problem of
predicting the downwash field existing behind wings of vari-
ous shapes has been studied by Lagerstrom, Graham, and
Grosslight (refs. 2 and 3) using the method of superposition,
by Mirels and Haefeli (ref. 4) using a surface of potential dis-
continuity (represented by a. distribution of vortices over the
vortex sheet), and by Lomax and Sluder (ref. 5) using the
discontinuity as represented by a distribution of doublets.

In addition, several experimental investigations of the
downwash field behind lifting surfaces in a supersonic flow
have been made within the past year. In reference 6 a some-
what indirect approach is made to the problem, the average
downwash over a tail surface being measured by force tests
of a. variable configuration of a body, wing, and tail. In
references 7 and 8, direct measurements at a Mach number of
1.53 of the downwash angle of the flow have been made at a
limited number of locations behind a rectangular wing and
behind a triangular wing. In addition a survey of the pitot
pressure across the wake of the two wings was made. A
similar investigation of the flow in the region of the tip of a.
trapezoidal wing at a Mach number 1.91 was made in
reference 9.

Except for these few studies, experimental data pertinent
to the problem are very limited. The primary purpose in
undertaking the present investigation is to supplement the
available experimental data. For this investigation a number
of measurements of both the downwash and sidewash compo-
nents of the flow angle have been made in the Langley 9-inch
supersonic tunnel for locations in the plane of the wing and
in planes above and below the plane of the wing. These
measurements were obtained by using a. floating vane that
would weathercock in the stream in the direction of the local
flow angle.