Design, Fabrication, Test, and Evaluation of Small-scale Tiltrotor Whirl Flutter Wind Tunnel Models

Open Access
Author:
Costa, Guillermo Jorge
Graduate Program:
Aerospace Engineering
Degree:
Master of Science
Document Type:
Master Thesis
Date of Defense:
May 07, 2015
Committee Members:
  • Edward C Smith, Thesis Advisor
Keywords:
  • tiltrotor
  • rotorcraft
  • aeroelasticity
  • wind tunnel testing
  • whirl flutter
Abstract:
The modern tiltrotor is an aircraft capable of efficient hover as well as high-speed forward flight. The large, flexible rotors needed for good hover performance are susceptible to an aeroelastic phenomenon known as whirl flutter in high-speed flight: high rotor inflow and the flexible nature of the rotors will result in negative aerodynamic damping when the tiltrotor is in airplane mode, causing the coupled wing/rotor/pylon system to become unstable. Traditionally, the tiltrotor whirl flutter margin has been increased through the use of thick wings of low aspect ratio, which maximize the stiffness of the wings at the expense of increased structural weight and reduced aerodynamic efficiency. To increase the top speed of tiltrotors, new methods of analyzing and mitigating the whirl flutter phenomenon must be developed. This study focuses on the design and development of a semispan wind tunnel model that permits the testing of whirl flutter stability in a controlled, low-risk environment. The wind tunnel model was dynamically tested in the Penn State Hammond Low-Speed Wind Tunnel. Five configurations were tested for modal damping variations with respect to tunnel speed. Two of these configurations ("Gen-2") were modified versions of the first-generation ("Gen-1") wind tunnel model designed and tested by S.C. Johnson in the 2012-2013 timeframe. Three configurations of an all-new model ("Gen-3") were developed to increase the realism of the test specimen, and included features such as tuned wing modal frequencies, higher-Lock number rotor blades, and blade twist. Most importantly, the Gen-3 models were designed to flutter without the use of an added mass or unconventional center of gravity placement. Gen-3 was developed through the use of an in-house computer code, which generated damping predictions for the wing modes at various tunnel speeds. Model components and support equipment were fabricated in-house by the author, with some components (e.g. those requiring computer-controlled machining) outsourced to local manufacturers. Wind tunnel tests were performed in the Hammond tunnel from October 2014 through January 2015; these tests showed that the experimentally-measured flutter speeds of the Gen-3 models were within 5-7% of the predicted values, with the Gen-3a (untwisted composite blades) fluttering at approximately 101 ft./sec., and the Gen-3b model (twisted composite blades) fluttering at approximately 97 ft./sec. The present work has further validated the feasibility of whirl flutter testing at a small scale. The Gen-3 model was able to exhibit whirl flutter without the use of an additional mass, and will permit the testing of devices designed to enhance the whirl flutter margin within the current facility. The techniques used to fabricate the Gen-3 model show potential for introducing features such as blade flexibility and an modular wing root flexure for a wider test regime.