TUNNEL-WALL EFFECTS ASSOCIATED WITH VTOL-STOL MODEL TESTING
ADVISORY GROUP FOR AERONAUTICAL RESEARCH AND DEVELOPMENT PARIS (FRANCE) PARIS France
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Wind-tunnel investigations of VTOL and STOL airplane models involve configurations in which a large amount of power is being used to generate part of the lift through the medium of propeller slipstreams or jet exhausts directed downward at large angles to the free-stream direction. For many configurations the propellers or jet exhausts are arranged, for example, as in the jet flap, to cover the entire span of the wing and thus to assist the wind in its natural process of producing so-called circulation lift. This arrangement results in the streamlines in the vicinity of the wing also being turned through large angles to the free-stream direction of flow. The presence of the tunnel walls, however, imposes the conditions that the streamlines at the tunnel walls must be parallel to the free stream. Thus, the problem of tunnel-wall effects in VTOL-STOL model testing is similar to that associated with conventional model testing but differs greatly in degree. Experience has shown that, in addition to these usual tunnel-wall effects, flow separation on the model can also be induced by the tunnel walls. The experiences of the Langley Research Center of N.A.S.A. related to these problems in closed-throat wind tunnels are reviewed.