Segmented-Freewing Concept for Gust Alleviation

Abstract

Design ForumDESIGN FORUM papers range from design case studies to presentations of new methodologies to speculations about emerging design trends. They vary from 2500 to 12,000 words (where a figure or table counts as 200 words). Following informal review by the Editors, they may be published within a few months of the date of receipt. Style requirements are the same as for regular contributions (see inside back cover).A promising technology for gust alleviation is the free wing. A free-wing design allows the wing to adjust itself in pitch about a spanwise axis in response to aerodynamic loads rather than being rigidly attached to the aircraft fuselage. The extension of the concept proposed here is called a segmented free wing and differs from a conventional free wing by sectioning the wing into multiple independent segments. This allows the wing to adjust to not only timevarying gust fields but also to gust fields that vary across the span. Initial wind-tunnel results showed an almost 40% reduction in rolling moment for a segmented free wing when subjected to an asymmetric velocity field, when compared with a traditional free-wing design. A 13.3-ft-span model was constructed, and experimental tests of this model showed a divergent oscillatory roll mode that appears with increasing velocity. An analytical model of the experimental test was developed that successfully predicts the instability. Comparison of the analytical model with the experimental results shows an overprediction of the stability of the system by the analytical model, and causes for the overestimation were investigated. Using the analytical model and experimental model, a successful solution to the roll instability was devised and tested. Nomenclature a = hinge location as fraction of half-chord b = half-chord C = material damping C ij = influence matrix elements C L = coefficient of lift C L = lift-curve slope C M = moment coefficient G = shear modulus of elasticity h = aerodynamic plunging I xx = rolling mass moment of inertia I yy = pitching mass moment of inertia J = polar area moment of inertia K = spring force L = lift L B = beam length L 0 = lift per unit span M = moment M aero = aerodynamic pitching moment M x = rolling moment M y = pitching moment M 0 = moment per unit span m = mass of wing segment m cw = mass of counterweight s = distance traveled in half-chords, Ut=b t = time U= velocity X ac = aerodynamic center in percent chord X 1 = unsteady aerodynamic term X 2 = unsteady aerodynamic term x = segment width y = distance from segment center to origin = angle of attack = circulation x = center-of-gravity offset distance = pitch angle = air density seg = material density of wing segment = roll angle