Loads attenuation using compliant surfaces

The efficacy of compliant surfaces for load alleviation during aircraft manoeuvres or in turbulent gusts was investigated. A rectangular wing with a NACA0012 profile and a semi aspect-ratio of sAR = 5 was tested in a wind tunnel at a chord Reynolds number of 300,000 by means of lift, drag, wing root bending moment, flow fields, and compliant surface deformation measurements. The compliant surfaces with both ends fixed on the suction surface near the leading edge formed a hump which acted like rigid spoilers for separation control and led to substantial lift reduction. Similar lift reduction performance was also observed with leading-edge inverted compliant surfaces that exhibit steady large deflection to induce massive flow separations, thus led to a maximum lift reduction of 44% at the angle of attack of α = 10°. The trailing-edge inverted compliant surfaces with both steady deflections and limit-cycle flapping motions demonstrated a nearly constant lift reduction of up to Δ C L ≈ -0.28 by deflecting its wake flows upwards and reducing the circulation of the wing. When α increased beyond a critical value, both self-activated compliant spoilers and rigid spoilers with compliant hinges flipped over towards the upper surface, created massive flow separations over the wing and led to a maximum lift reduction of 32% at α = 10°. However, hysteresis loops in lift and drag appear when decreasing the angle of attack as both compliant and rigid spoilers remained on the upper surface of the wing and this requires future investigations. Nevertheless, to the best of our knowledge, this was the first time that compliant surfaces were employed for loads alleviation, and it provided a very cost-effective mechanism that can potentially eliminate the need for heavy mechanical surfaces such as trailing edge flaps or fluidic actuators.