THE EFFECT OF WIND-TUNNEL WALL INTERFERENCE ON THE PERFORMANCE OF A FAN-IN-WING VTOL MODEL

摘要

A fan-in-wing model with a 1.07-m (42-in.) span was tested in seven different test sec¬tions with cross-sectional areas ranging from 2.2 m2 to 265 m2 (24 ft2 to 2857 ft2). The data from the different test sections are compared both with and without correction for wall inter¬ference. The results demonstrate that extreme care must be used in interpreting uncorrected VTOL data since the wall interference may be so large as to invalidate even trends in the data. The wall interference is particularly large at the tail, a result which is in agreement with recently published comparisons of flight and large-scale wind-tunnel data (NASA CR-2135) for a propeller-driven deflected-slipstream configuration. The data of the present investigation verify the wall-interference theory of NASA TR R-124 even under conditions of extreme inter¬ference. A method given by Tyler and Williamson in AGARD CP-91-71 yields reasonable esti¬mates for the onset of Rae's minimum-speed limit.nThe present investigation shows that the rules for choosing model sizes to produce negli¬gible wall effects, as given by Cook and Hickey in NASA SP-116, are considerably in error and permit the use of excessively large models. Even simple momentum theory appears to yield more nearly correct performance estimates in transition flight than uncorrected wind-tunnel data when the model span approaches one-half of the tunnel width. The "fan-induced" lift indi¬cated by a number of previous studies in which the model was of similar relative span appears to be largely the result of wall interference that was not accounted for in reducing the data.