Vibration damping doesn’t have the gee-whiz, sci-fi appeal of some aerospace engineering research pursuits, but it’s critical. Unwanted structural vibration can cause serious trouble, including overstressed components, accumulated fatigue and damage, pointing errors, and noise. Interacting with airflow or active control systems, structural vibration may also result in potentially disastrous instability.
A familiar example of airflow-driven vibration is a metal traffic sign fluttering in a strong wind. Eventually, repeated bending can lead to fatigue and failure. Likewise, high flight speeds can cause the wings of an airplane to flutter, and wind gusts can cause vibration. Together, flutter and vibration may contribute to the wing’s eventual failure. The F/A-18 Hornet, a Navy multi-role fighter jet, provides an example. When the jet is flying at high angle of attack, its shed vortices buffet the twin vertical tails downstream, resulting in greatly reduced fatigue life. In the jet’s early days, the fatigue life of the twin tail at high angle of attack was only about 200 flight hours. Eventually the problem was solved using a combination of aerodynamic and structural damping modifications.
George Lesieutre, a member of the Penn State aerospace engineering faculty for 20 years and now head of the department, has spent a lot of time considering the best ways to effect passive damping of vibration, meaning damping vibration with the use of materials, design or both. Currently, he is looking at methods of using passive damping to increase the durability of integrally-bladed rotors (IBRs or blisks), a major component of turbine engines.
Most commercial and military aircraft are powered by turbine engines, which are designed to extract energy from a fluid flow. In a typical engine, there are multiple stages of fixed nozzles, each directing air at a bladed disk, causing it to spin. In the last 20 years, some manufacturers have replaced the traditional multi-component bladed disk with the blisk – a one-piece bladed rotor disk machined from a single piece of material.
A blisk’s simplicity makes it more efficient than the traditional bladed disk. When it comes to vibration, though, the blisk has a disadvantage in that the complexities that degrade efficiency – mechanical attachments, for example -- also damp vibrations. When the blade passage frequency associated with a nozzle stage matches a blade resonance frequency, the pulsing airflow can induce high blade vibration levels. With inadequate damping, the result is high cycle fatigue and eventual failure.
One way to damp vibrations might be to introduce piezoelectric materials, which can change shape in response to an electrical charge, or generate a charge in response to a change of shape. A piezoelectric element bonded to or embedded in a blade would damp vibration by converting some of the energy associated with vibration into electrical energy. That energy could then be dissipated in an electrical impedance. Manufacturing such a blade, however, would pose several challenges. With piezoelectric elements integrated at key locations, the blade would still have to maintain high aerodynamic efficiency and endure the force of thousands of g’s of acceleration. Blades constructed of composite materials could potentially incorporate piezos while meeting these requirements.
The lead-lag vibration of helicopter rotor blades is another subject of ongoing study for Lesieutre. Whether a helicopter is on the ground or in the air, the interaction of the blades’ motion with the pitch and roll of the fuselage can be destabilizing. In fact, as Lesieutre puts it, “Things can go wrong quickly.” Damping the blade’s lead-lag motion tends to stabilize the interaction and thus the helicopter. One way to accomplish this might be interposing a block of lossy elastomeric material between the blade and the rotor hub.
George Lesieutre first became interested in structural vibrations when he encountered problems with spacecraft structures while working at Rockwell Satellite Systems in the 1980s. Later, as a graduate student at the University of California at Los Angeles, he studied structural vibration and damping, eventually writing his dissertation on finite element modeling of viscoelastic structures. Since then, he and his colleagues have seen some of their research evaluated and used by industry for helicopters and spacecraft.
Growing up, Lesieutre was more interested in playing baseball and reading mysteries than he was in airplanes, but he does remember building model rockets, visiting Nike missile sites as a cub scout, watching the moon landing in 1969, and following the Apollo and space shuttle programs. After graduating from high school in Elgin, Ill., he spent eight months canoeing 3,000 miles from Montreal to the Gulf of Mexico as part of a reenactment of the French explorer Robert de La Salle’s 17th century expedition.
Members of the La Salle crew not only canoed, they told stories and performed songs before community and school groups. As an undergraduate at the Massachusetts Institute of Technology, Lesieutre continued to pursue his new musical interest, taking up guitar and joining the a cappella group, The Logarhythms. He tried rowing crew as well, but didn’t like it because he couldn’t see where he was going. So he took up lacrosse and rugby.
In 1981, Lesieutre took advantage of another post-graduation lull to bicycle from Boston to Minneapolis, then he married his high school sweetheart, Annie, moved to Southern California to work for Rockwell International, and in 1982 enrolled at the University of California. He joined SPARTA, Inc., in 1983, as dynamic systems scientist, and earned his doctorate in aerospace engineering from UCLA in 1989. Today he commutes on foot to his Penn State University Park campus office, accompanied by Annie and their Norwegian elkhound, Pippin.
Besides serving as head of the department of aerospace engineering, Lesieutre is associate director of the university’s Center for Acoustics and Vibration. His administrative, research, and teaching responsibilities keep him busy enough that he frequently answers e-mails after midnight. But he hasn’t entirely given up his athletic and musical pursuits either. As a member of a fundraising team for the Centre Volunteers in Medicine, Lesieutre has trained for and run three Boston Marathons. He also plays rhythm guitar for a rock band that has included other faculty members and friends.
In 2009, Lesieutre ran his fastest marathon ever, 3:12:24, completed a 50-mile ultra-marathon, and was named a fellow of the American Institute of Aeronautics and Astronautics.
2010 Director At-Large, AIAA Board of Directors
2009 Fellow, American Institute of Aeronautics and Astronautics (AIAA)
2009 UCEA Mid-Atlantic Program & Activities Award (Wind Energy)
2008 ASME Best Paper (Adaptive Structures)
2008 AIAA Sustained Service Award
2007 Hirschorn IAC Best Paper Prize (Inst. of Noise Control Eng’g.)
2006 CIC (Big 10) Department Executive Officers Program Fellow
2001 AIAA Zarem Educator Award
2000 ASME Best Paper (AIAA Structures, Structural Dynamics and Materials Conference)
2000 Penn State Engineering Society (PSES) Outstanding Research Award