Aerospace & Defense Technology - August 2022 - 23

Test and Simulation
mass simulators were used instead of
energized batteries.
One challenge for the test team was to
design and build simulators that have
the same size, mass, and center of gravity
as the actual batteries. They also had to
be mounted to the fuselage in the same
way as the actual batteries. Another challenge
was that two battery control modules
(BCMs) were not ready in time for
the GVT. To get around this problem,
NASA engineers designed and built two
BCM mockup units. The BCM mockup
units were lighter than the actual units,
so the engineers added shot bags to bring
the weight up to the expected flight
weight. Weight is a crucial factor for
modal characteristics, so the test team
had to track missing weight and ballast
areas when needed.
The airplane configuration for the GVT
contained power cables in the wing in
addition to other flight instrumentation
cables and sensors (flight accelerometers
and strain gauges). All the components
were installed before the airplane arrived
at NASA Armstrong. The airplane weight
(including all of the GVT locking devices,
hardware, and ballast) was measured at
2,782 pounds prior to installing the GVT
external accelerometers.
Two Test Setups
Because the objective of this GVT was to
test two main boundary conditions, there
were two different test setups. The first
setup, called the " soft support " system,
was designed to simulate a free-flight environment.
It used bungee cords to suspend
the airplane at underside airplane hard
points. This was a real challenge because
manufacturers do not normally measure
the characteristics of bungee cords for use
in these kinds of tests. Prior to the GVT,
NASA first performed numerous bungee
characterization tests to ensure that the
bungee cords selected could both handle
the load and have a low enough frequency
response to ensure separation from the
first airplane modal frequency.
The second GVT setup was called the
" on-tires setup. " For this test routine, the
airplane had its landing wheels installed,
and during the test, the aircraft rested on
the ground. The tires were inflated to
normal service pressures, and the airplane
crew performed a shakedown of
Aerospace & Defense Technology, August 2022
Overview of GVT setup on the X-57 Maxwell Mod II Configuration. (Image: Courtesy of NASA)
the airplane nose, tail, and wingtips to
allow the struts to settle at equilibrium
prior to testing.
Gearing Up
For the first test setup using the soft
support system, also called the freefree
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 locaLAN-XI
Mainframe #3 & #4 (two 5-slot mainframes).
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Aerospace & Defense Technology - August 2022

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