Lift Measurements on Small-Scale Flat Planing Surfaces

Abstract

Because of the need for design information for seaplanes and planing boats, the hydrodynamics of planing has been the object of much experimental and some theoretical study in the last twenty-five years. More recently the subject has become of interest in the field of hydroballistics. For practical purposes the chief source of information has been the extensive tests made with small models in towing tanks, both on specific hulls and on simple geometric forms. These latter tests are useful for under· standing the basic phenomena of planing, and experiments on flat plates have been particularly helpful in this respect. An immediate question for the experimenter is the nature and magnitude of the so-called scale effect, if any, which may appreciably influence the test results. The term "scale effect" as used in planing work includes both viscous and surface tension effects, but not gravity effects, which are modeled by maintaining the proper Froude number. The problem was first considered by Sottorf, who carried out a systematic series of tests on flat plates of different scale at a constant Froude number. Sottorf concluded that the hydrodynamic pressure distribution on the plate was not noticeably affected by scale, while the shear forces, on the other hand, were very sensitive to the size of model, both because of the usual viscous effects and also because of the alteration of the flow near the edges by surface tension. Thus, although the force parallel to the plate, which largely determines the drag at small attack angles, cannot be modeled properly, the force normal to the plate {or for small angles, the lift force) is unaffected by scale so long as the proper Froude number is maintained. In spite of Sottorf's conclusions on this point, however, there have recently been expressions of doubt as to the validity of lift force measurements made with small models. For example, some tests made by Falkemo and Adlercreutz have been reported3 in which the measurements on a model 5 inches wide were of the order of 20% higher than that value predicted from an empirical formula based on Sottorf's data. The discrepancy was ascribed to scale effect though Sottorf had successfully used a model of 3-in. width for a similar measurement. In view of the importance of small models in planing work, a program of systematic tests on scale effect has been initiated in the Free Surface Water Tunnel of the Hydrodynamics Laboratory. The results of tests on flat plates to date are reported herein.