Autorotating wings: an experimental investigation

Abstract

The autorotation of a flat plate about its spanwise axis was experimentally studied. Most of the work was done with a wing mounted in the University of Michigan 5 × 7 ft low-speed wind tunnel. The measurements consisted of the unsteady lift, drag, angular acceleration and the wing rotation rate. The flow pattern was studied by means of smoke, tufts and a small model in a water tunnel.The flow pattern was very different from that over a static wing. The wing did not stall until it was nearly perpendicular to the free stream and the flow did not reattach to the lower surface until the wing had rotated well past zero angle of attack.The maximum and average lift, drag and angular acceleration were measured for Reynolds numbers from 25 000 to 250 000. At Re = 240 000 the maximum lift coefficient was 4·50 with an average value of 2·20, while the maximum drag coefficient was 4·30 with an average value of 1·60. The angular acceleration was small; the wing rotated at an almost constant angular velocity. The non-dimensional wing rotation rate was measured for Reynolds numbers from 1300 to 280 000 and approached an asymptotic limit of 0·35 for sufficiently high Reynolds numbers.The effect of applying driving and retarding torques to the wing was studied. As the rotation rate was increased above the free autorotation rate, the lift and drag increased. When the rotation rate was reduced by a retarding torque they both decreased. The power developed by the rotating wing was considerably less than for a windmill of the same frontal area.A variety of wing configurations were tested, including different airfoils and aspect ratios and spoilers mounted at various locations on the wing. However, except for spoilers that were so large that they prevented autorotation, none of these changes had a major effect on the gross properties of the autorotation phenomenon.Freely falling wings were also studied. For Reynolds numbers above 4000 the average lift and drag coefficients were comparable to those observed in the fixed axis tests and it appeared that the flow pattern was similar.