The ground vibration tests are used to find the mode shapes of the aircraft. Typically it excites the vibrational modes by using electrodynamic shakers. It ensure that the aircraft will avoid flutter. GVT helps to validate the understanding of an aircraft and validate it’s computer mode. The benefit of fast GVT testing is that it minimizes the time that the aircraft needs before it can fly. If the aircraft is in a lifted state for the GVT, then it must be isolated with a bungee system, or airbags to prevent the harness fixture from creating artificial stiffness that does not exist on the aircraft. The first few modes with the lowest frequencies are some of the more important modes to test. These may be further characterized as fuselage bending, wing bending, wing, torsion, control surface rotation, or any other type of description. These tests can use single or multiple exciters to assess the nonlinear behavior of the structure. GVTs identify eigenfrequencies, eigenmodes, damping ratios, masses and transfer functions. GVTs are usually performed under time pressure due to limited availability of the aircraft. After a model analysis the frequency responses are transferred into a SQL database. then the databases for the individual tests must be correlated. GVT is a modal test of the entire airframe, this determines the resonant frequencies, mode shapes, and damping.[^4] This is then compared with a finite element model for flutter predictions.[^4] Pre-test planning is important to ensure testing efficiency.[^4] Frequency response signals can be measured from impulses, random noise, sine sweeps, and constant amplitude oscillations.[^4] GVT’s allow you to extract the mode shape.[^4] The test model modes should represent a structure with proportional damping throughout.[^4] FE models can only estimate the damping.[^4]
[[Electrodynamic Shakers]]
[[Vibrational Modes]]
[[Rotorcraft GVT]]
[[GVT Pre Test]]
Effective Independence Method – find locations for sensors
[[Amplitude Dependency]] – amplitude changes with varying force
[[Frequency Dependency]] – frequency changes with varying force
[[Affixing Accelerometers for a GVT]]
[[HL-10 Vibration Test]]
[[AH-56 Cheyenne]] – First Vehicle off the assembly line was used for GVT
[[F-18 Lateral Auto Flap Up]] – structural notch filters are based on modes identified during ground vibration tests
[[BP7.2]] – models should be supported by GVTs
[[H145 GVT MAC Model]]
[[X-56 Ground Vibration Tests]]
[[Flexible Aircraft Equations of Motion]] – determined using GVT
[[B-2 Ground Vibration Test]]
[[Stepped Sine]]
[[Lissajous Display]]
Modal Assurance Criterion
Sources
- X-56 Full-Body Ground Vibration Tests, (Nov. 21, 2016). Accessed: Apr. 07, 2023. [Online Video]. Available: https://www.youtube.com/watch?v=qwylNMeJHp4
- B. Supersonic, “Boom – FlyBy – What is Ground Vibration Testing?,” Boom. Accessed: Jul. 17, 2023. [Online]. Available: https://boomsupersonic.com/flyby/what-is-ground-vibration-testing
- C. Ciavarella, M. Priems, Y. Govers, and M. Böswald, “AN EXTENSIVE HELICOPTER GROUND VIBRATION TEST: FROM PRETEST ANALYSIS TO THE STUDY OF NON-LINEARITIES,” 2018.
Backlinks
[[Finite Element Method]]
[[Flutter]]
[[Frequency Response]]
[[Vibration]]
[[X-56 Ground Vibration Tests]]