A METHOD TO FIND SURFACE DEFECTS WHICH MAY DISRUPT LAMINAR FLOW

Abstract

The elegant wings of sailplanes in flight deflect noticeably and twist slightly. Accordingly, there must be a degree of distortion of the airfoil contour - which may have been close to perfect when measured while the aircraft was not subjected to flight loads. This paper describes a technique for finding such localized distortions of the wing under simulated flight conditions, seeking those that can cause the airfoil boundary layer to transition prematurely from laminar to turbulent flow with consequent increase in drag. With the use of the relatively easy to make, yet adequate instruments here described, it is possible to find discrepancies of the contour of templates,plugs, female molds and the surfaces of the wings made from them. It should take fewer hours to define those errors of surface curvature which could cause transition to turbulent flow than any other method known to the author. The critical measurements should be made with the wings deflected, ballasted and not ballasted, approximating flight loads. These devices do not measure the coordinates of the airfoil, instead, they do locate faults and define them so they may be eradicated by repair or redesign. It would be improper to examine the top side of wings that are not deflected to simulate at least the "one g" flight condition. Until the "inverted and deflected condition" is found to be impractical or unnecessary, the bottom-side-up wing with weighted tips ideally should be set up to the same "45" bank/twisted deflection." The exploration of a deflected ("45" bank" right-side-up), twisted and ballasted wing will probably differ significantly from an examination of that wing's plugs, molds and non-flight deflected (e.g., drooping wings). With the method and tools described in this paper and the improvements that might be developed with practice in the years to come, it should be possible to create wing surfaces to further extend laminar flow, to learn more about what causes it to break down and, by this survey practice, possibly, to enhance performance by establishing the skin stiffness requirements to avoid surface buckling due to at least 45 degrees of bank.