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Although mini-tufts are considered generally to show the surface flow direction on windswept surfaces, there is some uncertainty about their correct interpretation, particularly as three-dimensional flows approach separation. This note provides some guidance on this controversial question, based on experience with mini-tufts on wings, fins and canards in the DRA 13 ft x 9 ft low speed wind-tunnel at Bedford.An illustration of the value of the technique is given. Mini-tuft photographs are used to identify the trailing edge condition which relates to the classification of the steady and fluctuation pressure distributions observed a swept wing with a conical, separated flow.

SummaryThis report describes a series of steady and time dependent pressure measurements on a half-model of a rectangular wing of aspect ratio 4. The wing was mounted on a halfbody attached to a sidewall of the RAE 8 ft x 8 ft windtunnel and the tests were made at Mach numbers of 0·60, 0·70 and 0·80 with fixed transition and a Reynolds number of about 2·4 x 106 at the highest Mach number. The wing was oscillated about its mid-chord axis at four frequencies giving frequency parameters up to 0·27 at M = 0·80. The static angle of incidence was varied from -10° to + 10°; here interest is centred on angles of incidence for which the flow on the upper surface of the wing is separated.Selected mean and oscillatory measurements are offered as a challenge to computational fluid dynamicists for this simple threedimensional configuration. The measurements are of particular interest because the transonic flows include both incipient and well developed separations.

The new requirement to test wind tunnel models of combat aircraft at high angles of incidence and high kinetic pressures has led to a review of the factors controlling the model stability. The review suggested that dangerous motions might occur (possibly without prior warning) on models at high angles of incidence unless special preventive measures were taken. An internal tuned damper and balance bump stops were recommended to limit the responses. The bump stops would also prevent the moment limits of the strain gauge balance from being exceeded. The effectiveness of both devices was confirmed by tests on a swept wing model which experienced dangerous bending oscillations in a vertical plane at a Mach number of 0.50 in the incidence range from about 27–29° together with dangerous yawing oscillations in a horizontal plane above an incidence of about 35°. Further research is recommended to ensure the safety of other models. For sting supported models in a conventional wind tunnel, it is shown by analysis that the structural damping in the sting bending mode needs to be about 4 to 6% critical damping. In a cryogenic wind tunnel corresponding levels would need to be 7 to 10% critical damping because of the possibility of increased negative aerodynamic damping relative to ambient conditions.

The use of sweep-frequency excitation for rapid measurement of time-dependent pressures on wind-tunnel models is examined. Results obtained from two different wind-tunnels covering the Mach number range from 0.2 to 0.85, and a wide range of flow conditions, are compared with measurements made using the slower, traditional method of discrete-frequency excitation. It is concluded that the sweep-frequency excitation method can reduce testing time in certain flow conditions with no significant loss in accuracy.

SummaryThis paper describes a series of steady and time-dependent pressure measurements on a half-model of a rectangular wing of aspect ratio 4. The wing was mounted on a half-body attached to a sidewall of the RAE 8 x 8 ft wind tunnel and the tests were made at Mach numbers of 0·20, 0·42, 0·70, 0·80 and 0·85 with fixed transition and a Reynolds number of about 2·5 x 106 at the highest Mach number. The wing was oscillated about its mid-chord axis at four frequencies giving frequency parameters up to 1·15 at M = 0·20 and 0·25 at M = 0·85. The static angle of incidence was varied from –10° to + 10°.Selected mean and oscillatory measurements are offered as a challenge to computational fluid dynamicists for this simple three-dimensional configuration. Comparisons are made with calculations by means of subsonic linearised theory and recent transonic small perturbation methods. The measurements are of particular interest at transonic speeds, where both attached and incipient separated flows are considered.

Semiconductor strain gauge pressure transducers, operating on the Wheatstone bridge principle, are widely used in wind/tunnel tests. However, a major disadvantage is their inherent sensitivity to variations of the model temperature. This is particularly serious when accurate steady measurements are required. A method is described by which the thermal stability of the transducer may be improved, using integrated circuit operational amplifiers.