For the first test setup using the soft support system, also called the free-free setup, the team installed accelerometers at 127 different locations, which allowed them to measure 318 degrees of freedom (DOF). They distributed the accelerometers along both sides of the airplane fuselage, wing spars, control surfaces, motors and soft-support frames (located at the main landing gear and the fuselage nose) using the global coordinate system of the airplane. For the on-tires setup, they removed the accelerometers from the 14 locations on the soft-support system, leaving them with a total of 113 accelerometer locations, measuring 276 degrees of freedom.
For both setups, they used both uniaxial and triaxial accelerometers, but the majority of the accelerometer locations were set up to measure two degrees of freedom. At these locations, they installed two uniaxial accelerometers or a triaxial accelerometer. The accelerometers around the motors were very difficult to install because of the limited access and real estate around the electric motors. In addition to the GVT external accelerometers, they acquired data from the flight accelerometers using the airplane’s instrumentation system and compared this to data gathered from the nearest GVT accelerometers.
To acquire data from these accelerometers, the NASA team used a 300+-channel LAN-XI data acquisition system from HBK. The LAN-XI is a modular data acquisition system and analyzer platform capable of acquiring sound and vibration data from more than 1,000 channels, with a wide dynamic input range, used in many demanding measurement applications. The NASA AFRC LAN-XI system included four mainframes, two LAN-XI 11-slot mainframes near the nose of the airplane, where the main GVT control station was located, and two LAN-XI 5-slot mainframes at the rear of the airplane. The mainframes were daisy-chained together via a network switch.
Key components of the GVT data acquisition system included:
- Type 3660 Mainframes. This test system used both five-slot and eleven-slot mainframes. The 3660 mainframes provide both power and connectivity, as well as protection, for LAN-XI modules
- Type 3053 12-Channel Input Module. This module was chosen because it provided a compact and cost-efficient solution for this application
- Type 3160 Generator Module. This module was used because this application required system excitation. It is available with either two inputs and two outputs or four inputs and two outputs. All input and output channels have a frequency range of DC to 51.2 kHz