Tiltrotor Research Papers - Academia.edu (original) (raw)

Three generations of small, semispan, subscale tiltrotor wind tunnel models were designed, fabricated, and wind tunnel tested. The goal of this project was to develop a series of flutter tests at small scale and low cost that would experimentally validate a series of analytical models developed in-house; this required a wind tunnel model that had as low a flutter speed as was feasible, in order to permit it to be tested to the point of instability within the operating limits of the current facility. The first-generation model consisted of a hollow plastic wing rapid-prototyped from ABS plastic, with a three-bladed rotor consisting of constant-chord wooden blades. Five different configurations of this first-generation model were tested, but only one configuration exhibited whirl flutter within the test facility, at a speed of 115 ft/s; however, this configuration was only able to exhibit whirl flutter through the use of a one-pound steel mass mounted aft of the wing trailing edge. For the unstable configuration, the center of gravity (c.g.) of the aft-mass was located 5.5 in. aft of the wing elastic axis. The second-generation model used the same wing, but featured composite rotor blades; this second-generation model exhibited whirl flutter at a tunnel speed of 113 ft/s, but was also incapable of experiencing an instability without the use of the aft-mass. The third-generation model consisted of a composite wing and composite rotor blades, with an integrated wing spar that acted as a flexure; two configurations of this third-generation model exhibited whirl flutter within the test facility, at tunnel speeds of 95 and 105 ft/s, and showed excellent correlation with the analytical model. NOTATION a = airfoil section lift-curve slope, /rad c = blade chord, ft EI beam = beamwise stiffness, lb f − f t 2 EI chord = chordwise stiffness, lb f − f t 2 GJ = torsional stiffness, lb f − f t 2 I b = single-blade bending inertia, slug − f t 2 [M] = wing/rotor system mass matrix [C] = wing/rotor system damping matrix [K] = wing/rotor stiffness system R = rotor radius, ft [X] = wing/rotor degrees of freedom vector V ∞ = freestream velocity, ft/s β = flapwise degree of freedom γ = Lock number, dimensionless δ 3 = kinematic pitch-flap coupling angle, deg. λ = eigenvalue µ = advance ratio, V ∞ /ΩR, dimensionless ν β = rotor flapwise frequency ρ = freestream density, slug/ f t 3 Ω = rotor shaft speed, rad/s ω = circular natural frequency, rad/s ζ = lagwise degree of freedom; damping ratio