Design, fabrication, test, and evaluation of small-scale tiltrotor whirl flutter wind tunnel models

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 (e.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.