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| Aerospace Ground Test |
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Ground Vibration Testing >>
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Introduction
Ground testing of a new aircraft design or an aircraft that has undergone significant structural modification, is a prerequisite before any flight-test program can begin. In turn, ground testing follows and complements an extensive structural modeling program, which encompasses flight loads, material fatigue, structural dynamics, airborne and structure borne acoustics and more.
A design life or “life expectancy” goal, in flight cycles (takeoffs and landings) or flight hours, is established early in the development of a new aircraft. Due to their extreme operating environments, military fighter aircraft may have design life expectancies only in the high thousands of flight hours. For civilian transport aircraft, the design life goal is typically in the high tens of thousands of flight cycles. Before first flight, a significant number of these cycles are accumulated during ground testing performed on a full-scale aircraft structure. Knowledge of the anticipated flight load spectrum enables pressure cycling of the fuselage, as well as hydraulic loading of the wings, empennage and other principal structures. Large data acquisition systems enable monitoring of the applied pressures and loads and resultant structural deflections and strains. Periodic inspections with nondestructive testing equipment often accompany this process to monitor for any resultant crack growth.
As part of the ground test program, ground vibration testing (GVT) is also performed. The purpose of GVT is to obtain experimental vibration data for the entire aircraft structure for validating and improving its structural dynamics model. Assessment of the aircraft structure’s linear or nonlinear behavior is also performed. During GVT testing, the aircraft must be in a configuration as close as possible to flight test. Successful correlation of structural frequencies and mode shapes between GVT and structural modeling enables assignment of accelerometer mounting locations to support the subsequent flight test program. In addition, GVT results provide inputs to predict flutter behavior to ensure the safety of these flight tests.
New engine development to support aircraft design requires its own test program beginning in test cells and culminating with “on-wing” testing in a test bed aircraft. Once the qualified engine(s) become integrated as part of the new aircraft, their noise influence on their surrounding environment and the passenger cabin must be assessed. Therefore, airborne and structure-borne acoustic transmission paths are also characterized as part of the aircraft ground test program.
The preceding has focused on aircraft. Ground testing of rotary wing aircraft and space launch vehicles follows the same pattern. Testing and structural modeling must correlate on the ground to assure structural integrity and safety in subsequent flight tests..
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