Flutter Testing. Wind Tunnel Testing (excerpts from Reference 1)

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1 Flutter Testing In the early years of aviation, no formal flutter testing of aircraft was performed. Flutter was usually discovered by accident during flight of the aircraft. The pilot flew the aircraft to its maximum speed to demonstrate the integrity of the design. Hopefully, flutter did not occur and cause the airplane to crash! Today, engineers and technicians test aircraft design with scaled models in a wind tunnel before flight-testing. Wind Tunnel Testing (excerpts from Reference 1) The desire to achieve faster flight forced vehicles in the direction of ever-lighter structures and thinner, more flexible lifting surfaces. This trend continued to make aeroelasticity an important technical field for flight. As vehicles approached and exceeded transonic speeds, the need for experimental assessment of aeroelastic behavior grew substantially because of the pronounced effect of transonic aerodynamics on phenomena like wing flutter. [Click here to learn more.] At the time that the transonic flight regime was being conquered, the ability to theoretically determine unsteady aerodynamics for use in the prediction of flutter did not exist. This inability to handle transonic aeroelastic effects was one of the major considerations that led to the idea of the NASA Langley Transonic Dynamics Tunnel. NACA (the predecessor to NASA) converted an existing wind tunnel to the Transonic Dynamics Tunnel (TDT) in Langley Virginia (now Hampton, VA) in the late 1950s to meet the needs of engineers. Right from the beginning, the TDT was to play a critical role in solving a severe aeroelastic problem. In late 1959 and early 1960 the Lockheed Electra aircraft experienced two catastrophic crashes. Testing verified the cause was propeller-whirl flutter. [Click here to view movie.] The TDT has served as a workhorse for experimental aeroelastic research and flutter clearance testing ever since. Testing has included such varied aeroelasticity concerns as buffet, divergence, gust loads, flutter, limit cycle oscillations, and other types of dynamic response. In addition to testing for these phenomena, many passive and active control studies have been carried out in the TDT to demonstrate methods of overcoming aeroelastic obstacles to flight. Most military fighters and commercial transports built in the United States have been tested in the TDT at some time in their development history. Today, the TDT is still a very unique facility dedicated to aeroelastic testing. Below are some photos of models in the test section of the TDT.

2 Figure 1. Lockheed C-141 Model Mounted in the TDT ( Figure 2 F/A-18 Model Mounted in the TDT

3 Figure 3 Wind Tunnel Test Model in the NASA Langley Transonic Dynamics Tunnel ( You can also watch some videos of wind tunnel tests: 1. A-6 Wing Flutter Failure 2. Anti-Symmetric Flutter on a Boeing Empennage flutter on a Lockheed C-5 Flight Testing (Extracts from Reference 2) Even though aircraft have been analyzed and tested for flutter, flight testing is still hazardous for several reasons. First, you must fly close to the flutter speed before imminent instabilities can be detected. Second, sub-critical damping trends cannot be accurately extrapolated to predict stability at higher airspeeds. Third, the aeroelastic stability may change abruptly from a stable condition to one that is unstable with a very small change in airspeed. Three components of flight flutter testing are discussed below: structural excitation, response measurement, and data analysis for stability.

4 Excitation of the structure can be accomplished by several methods. The pilot can pulse the controls, causing a sudden movement of the control surface. Two benefits of this type of excitation are that no special excitation equipment is required and that the transient response signature of the structure is easy to analyze for stability. However, the pulse may not be a large enough disturbance to excite critical flutter modes. Manual control surface pulses are used today on most small aircraft and sailplanes because this is usually the only affordable type of excitation for these aircraft. However most modern fly-by-wire flight control systems, used in high-performance aircraft and commercial airliners, do not allow the control surfaces to respond to this high-frequency input. So other methods must be employed. Some of these are commanded oscillations (computergenerated commands input into the control system), small solid propellant thrusters, inertial exciters (rotating eccentric weight and oscillating weight), aerodynamic vanes, and flying through atmospheric turbulence. Each of these has advantages and disadvantages that must be carefully weighed before choosing the most appropriate approach. Click here to watch flutter in flight. The instrumentation used to measure the structural responses of an airplane to excitation is another critical component of flight flutter testing. The response data must be measured at enough locations and be of high quality. The most commonly used transducers to measure the response are accelerometers and strain-gages. The data are sent to a ground receiving station for processing and analysis. Engineers on the ground use computers to manipulate and display the flight data. There may also be an on-board system for use by the crew. The response signal can consist of random responses, transients (short duration events), or steady-state responses. Different algorithms are employed to filter-out signal noise and to obtain frequency response functions. Engineers use these frequency-domain results to estimate damping or the system. Figure 4 shows a typical data-analysis flow. The typical approach to flight flutter testing is to fly the aircraft at several stabilized test points arranged in increasing order of dynamic pressure and Mach number. The data obtained are used to establish a damping trend as a function of airspeed. Information is then extrapolated to predict the stability of the next test point. However, flutter phenomena can be non-linear and therefore attempts to predict behavior may not be accurate. Engineers are researching new methods to permit a reliable determination of flutter speed at a speed that is well below the actual one. Improvements in analysis accuracy and signal processing speed will help to make flight flutter testing safer.

5 Figure 4 Typical Modern Flight Flutter Test Process (Reference 2) Sources: 1. Stanley R. Cole, Thomas E. Noll, and Boyd Perry III, Transonic Dynamics Tunnel Aeroelastic Testing in Support of Aircraft Development, Journal of Aircraft, Vol. 40, No. 5, September-October NASA Technical Memorandum 4720, A Historical Overview of Flight Flutter Testing, October 1995.