Measurement of transonic dips in the flutter boundaries of a supercritical wing in a wind tunnel

Abstract

Flutter experiments were performed on a supercritical transport type wing to investigate the transonic dip in the flutter boundaries and to obtain data for verification of a calculation method for unsteady aerodynamic loads Complete transonic dips were measured at three wing angles of attack At the highest wing angle of at tack, a second dip was found Simultaneously, the mean pressure distribution was measured in one chordwise section which could explain the flutter characteristics. Using a flutter damper device, flutter onset could be measured accurately and rapidly This paper deals with the experimental test setup, describes the on line data reduction, and presents the results. Nomenclature c/c r -damping ratio (percent of critical damping) Cp -mean pressure coefficient C. PO 95 = mean pressure coefficient at 95 % of chord / = model frequency MH M 22 = modal mass of modes 1 and 2, respectively MO, = Mach number P 0 = stagnation pressure x/c = dimensionless chord a 0 = adjusted wing angle of attack a w = measured wing angle of attack I Introduction D URING the last decade a large pait of the aeroelasticinvestigations at the National Aerospace Laboratory (NLR) focused on the determination of unsteady airloads on oscillating supercritical wings in transonic flow * These investigations aimed to contribute to the development of an advanced short to-medium haul transport aircraft to be built by the Netherlands Aircraft Factories Fokker B V One of the topics was the development of an engineering type method at NLR for three dimensional unsteady transonic airloads applicable to large aspect ratio wings to be used in the prediction of flutter boundaries in transonic flow In sum mary, this method combines calculations of aerodynamic loads on oscillating two dimensional (2 D) wings in transonic flow with three dimensional (3 D) corrections due to wing span and taper calculated with 1 D and 3 D doublet-Lattice methods An outline of this so called quasi three dimensional (Q3D) method is given in Ref 2An experimental validation of the Q3D method was considered necessary The most direct way would have been the performance of pressure measurements on an oscillating model, but a flutter model test was preferred because tran sonic flutter characteristics of the wing could be studied, especially the presence of transonic dips in the flutter boundaries For that reason it was decided to perform a flutter test on a supercritical wing model for which many data of steady pressure model tests were available already The flutter model was designed as a study model and did not represent the aeroelastic behavior of any full-scale designThe first flutter test was carried out in 1979 in the high speed wind tunnel (HST) of NLR with partial success In particular, complete transonic dips could not be measured because of unexpected instabilities at low Reynolds numbers 2 After modification of the model a second flutter test was carried out in the same wind tunnel in April 1982 An im proved data acquisition and reduction system also was used This...

Flutter Computations for a Generic Reference Aircraft Adopting CFD and Reduced Order Methods

“…Various flutter computation assessments for transonic transport configurations have been reported in the literature, most of them adopting just a small number of test cases, see table 2. Unfortunately the most interesting ones are based on proprietary models, for example the Fokker model, 23 YXX, 24 Mavric 25 and AMP. 26 Their restricted availability reflects that windtunnel tests were very expensive, and despite their age these data are still of high strategical use.…”

“…If for increasing Mach number the flutter mechanism switches from one mode to another, a second dip-like minimum of the flutter boundary has been observed. 23 The typical behavior of the transonic flutter boundary we are looking for is sketched in figure 3. For validation our configuration should also exhibit the phenomena of Limit Cycle Flutter and Buffet, but the main issue is the transonic dip and its variation with different flow conditions.…”