Aeroelasticity and Structural Dynamics
August 1, 2003
Aircraft do not behave as rigid bodies in flight. Structural deformation, particularly of wings, causes the quasi-static airloads to differ substantially from what they would be were the aircraft rigid. Also, because of wing and control surface flexibility, there are a variety of unstable behaviors, both quasi-static and dynamic, which lead to degradation of structural integrity over time; these can shorten the life of the aircraft, or even catastrophically destroy it.
Aircraft are also subject to dynamic structural deformation (vibration) from gusts and turbulence. The interdisciplinary field of aeroelasticity, which deals with interactions among aerodynamics, structural mechanics, and dynamics, addresses such phenomena. The closely related field of structural dynamics examines interactions between structural mechanics and dynamics. Serious study of aeroelasticity began only a half century ago; but in the last 20 years the integration of controls and optimization has been an important development.
At Georgia Tech, research in aeroelasticity and structural dynamics (ASD) spans the areas of fixed- and rotary-winged aircraft as well as spacecraft. Sponsors for this work include NASA, the U.S. Air Force, the U.S. Army, and industry. Georgia Tech researchers are developing computational methods for dynamics of systems of flexible structures that are interconnected. These are called multi-flexible-body dynamics. This important area of computational mechanics enables engineers to accurately and reliably predict the dynamics of complex flexible multi-body systems such as rotorcaft or spacecraft undergoing large changes of orientation. Other work relating to rotary-winged aircraft aeroelasticity concerns vibration reduction, improved methods for calculation of stability, methods for modeling composite rotor blades, and investigation of the effects of elastic coupling on blade performance and stability. The facilities and specific activities related to this important topic are discussed under the Center of Excellence for Rotorcraft Technology (CERT).
Fixed-wing area projects focus on computational methods, analytical and experimental studies of buffeting, and aeroservoelasticity of composite-winged aircraft with wings of high-aspect ratio. Computational aeroelasticity may require the utilization of high-order aerodynamics methods to capture the correct flow physics. This requires the marriage of computational fluid dynamics (CFD) and structural mechanics (CSM). Georgia Tech researchers are developing computational methodologies which incorporate these aspects, and are utilizing this technology to explore the vitally important effects of structural flexibility and dynamics. Using ENS3DAE, a transonic tightly-coupled aeroelastic methodology developed as part of a consortium (Georgia Tech and Lockheed-Martin) through Wright Laboratory, researchers are exploring the mechanisms driving aeroelastic phenomena which lead to Limit Cycle Oscillations (LCO), flutter and fatigue problems. Georgia Tech researchers have also developed a methodology, under Wright Laboratory funding, to couple existing rigid CFD codes and CSM codes to examine static aeroelastic problems. This methodology, Fluid and Structure Interface Toolkit (FASIT), can also be used to extend the usefulness of existing CFD codes, and as an interface mechanism between disciplines. Finally, flexible multi-body dynamics codes are coupled with OVERFLOW, a CFD code developed at NASA Ames Research center, to study rotorcraft aeroelasticity problems.
Aeroelasticity and structural dynamics issues for aerospace systems have traditionally been addressed late in the design process. Unfortunately, this can lead to late-program “surprises” (such as flutter or vibration problems that show up on the prototype system) which delay production and add to the cost of new systems. At Georgia Tech's Aerospace Systems Design Laboratory (ASDL), research is underway to incorporate these disciplines earlier in the design process using state-of-the-art computational techniques.