Chin, Alex

Spivey, Natalie D.

Truong, Samson

English

NASA Technical Reports Server

Advanced Air Vehicles Program
Advanced Air Transport Technology Project
X-56A Structural Dynamics Ground 
Testing Overview and Lessons Learned
Alex Chin, Samson Truong, & Natalie Spivey
alexander.w.chin@nasa.gov
AIAA SciTech 2020
Orlando, FL
January 6th-10th 2020
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https://ntrs.nasa.gov/search.jsp?R=20200000421 2020-03-11T13:37:07+00:00Z
Advanced Air Vehicles Program
Advanced Air Transport Technology Project
Outline
• Background and Motivation
• Ground Vibration Testing
• Troubleshooting and Resolution
• Moment of Inertia Testing
• Lessons Learned
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Advanced Air Vehicles Program
Advanced Air Transport Technology Project
Background and Motivation
• Next generation aircraft will incorporate 
cutting-edge technologies that enable 
higher performance, while increasing 
structural efficiency through weight 
reduction.  
• However, reducing weight often means 
reduced stiffness in the structure. Increased 
flexibility can make aircraft more vulnerable 
to various aeroelastic phenomena, such as 
flutter, buffet, buzz, divergence, and adverse 
gust response. 
• The X-56 research vehicle is designed as a 
high risk aeroelastic aircraft to demonstrate 
active flutter suppression and gust load 
alleviation.
• Accurate structural modeling is critical for 
successful control of a highly flexible 
aircraft. 
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Advanced Air Vehicles Program
Advanced Air Transport Technology Project
X-56 Research Vehicle
• Funded by Air Force Research 
Laboratory
• Designed by Lockheed Martin 
Skunkworks 
• Delivered to NASA for continued 
research efforts
• Complete Research System
• 2 Center Bodies (Fido and 
Buckeye)
• 1 Stiff Wing Set
• 3 Flexible Wing Sets
• 1 Ground Control Station 
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Advanced Air Vehicles Program
Advanced Air Transport Technology Project
Structural Dynamics Ground Testing
• Lockheed Martin performed initial series of ground tests on Fido 
centerbody and wing sets
• NASA performed additional series of structural tests on Buckeye 
centerbody and wing set to address potential fabrication differences 
and configuration changes
• Buckeye fuselage had increased mass, change in mass distribution, 
shift in CG 
• Wings were made from different batches of composite materials. 
Needed to verify consistency of fabrication.
• Required high confidence in finite element model (FEM) due to its 
integral role for developing models directly used in controller 
development
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Advanced Air Vehicles Program
Advanced Air Transport Technology Project
Wing-only Strongback GVT
• Compare and validate frequency 
response of both left and right 
wings.
• Determine any manufacturing 
differences.
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Advanced Air Vehicles Program
Advanced Air Transport Technology Project
Wing-only GVT results
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Conclusion: The left and right wings had similar structural dynamic properties
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Advanced Air Vehicles Program
Advanced Air Transport Technology Project
Full Vehicle Ground Vibration Test
• Conducted multiple GVTs in 
different aircraft, test, and fuel 
configurations 
• Acquired damping, frequency, and 
mode shape for each GVT test 
configuration
• Data used for FEM model update 
and tuning in order to reduce 
model uncertainties between 
numerical and experimental modal 
data Free – Free Full Fuel Configuration: One Bungee Suspension Assembly
Advanced Air Vehicles Program
Advanced Air Transport Technology Project
X-56A Aircraft GVT Data Collection 
• GVT accelerometers
 119 accelerometer locations
 227 aircraft channels
 32 soft support system channels
• FOSS
• High-speed photogrammetry
 Only for the left wing
 250 frames/sec
• Aircraft flight accelerometers
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Advanced Air Vehicles Program
Advanced Air Transport Technology Project
Progression of Soft Support Set-up
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Original Three Bungee Suspension 
System with Spreader Bar
Modified Three Bungee Suspension 
System without Spreader Bar
Single Bungee 
Suspension System
Challenges in the soft support boundary condition:
 
Advanced Air Vehicles Program
Advanced Air Transport Technology Project
Primary Mode Shapes 
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Mode 
number GVT mode shape
Difference 
between FEM and 
test: Empty fuel, 
percent
Difference between 
FEM and test: Full 
fuel, percent
1 Symmetric Wing 1
st
Bending (SW1B) -6.09 2.01
2 Antisymmetric Wing 1st Bending (AW1B) -6.39 -3.31
3 Symmetric Wing 1
st
Torsion (SW1T) -0.62 0.36
4 Symmetric Fore-Aft (SFA) -0.04 -1.25
5 Antisymmetric Wing 1st Torsion (AW1T) 1.63 1.27
6 Symmetric Wing 2
nd
Bending (SW2B) -1.31 -0.21
Advanced Air Vehicles Program
Advanced Air Transport Technology Project
Mode # Mode
GVT 
(normalized 
freq)         
FEM 
(normaliz
ed freq) 
7 SW1B 0.997 0.977
8 AW1B 1.659 1.714
9 SW1T 3.539 3.526
10 SFA 3.901 3.950
11 AW1T 4.166 4.113
Mode # Mode
GVT 
(normalized 
freq)         
FEM 
(normalized 
freq) 
7 SW1B 1.000 1.061
8 AW1B 1.622 1.726
9 SW1T 3.561 3.539
10 SFA 4.001 4.000
11 AW1T 4.190 4.122
GVT Data Troubleshooting
• Added fuel mass should decrease the first bending frequency due to 
increased inertia 
• Frequency shift from fuel weight was observed in the FEM and during 
subsequent flights 
• The fuselage contributes significantly to the SW1B mode; therefore any 
external factors that can affect the fuselage dynamics can affect SW1B
Notes:
* Free-Free GVT results calculated from single bungee configuration.
* Baseline FEM only models vehicle, no GVT lifting hardware is included
* All frequencies are normalized with respect to measured SW1B empty fuel
Empty Fuel Full Fuel
% Frequency Shift due 
to Fuel Load
GVT FEM
-0.33% -7.93%
2.25% -0.71%
-0.63% -0.37%
-2.50% -1.25%
-0.58% -0.21%
Primary Mode
Secondary Mode
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Advanced Air Vehicles Program
Advanced Air Transport Technology Project
Bungee Set-up Scrutinized
• Vehicle was still free to pivot about sling-
bungee connection (offset vehicle Y-axis)
• XCG (fore-aft) of the vehicle shifts along the 
direction of the vehicle pitching motion 
between the empty and full fuel condition 
• Metal wire slings will readjust attitude of 
vehicle to ensure vehicle CG is directly below 
bungee
• A sensitivity analysis on the bungee X-
location was performed to determine its 
affect on rigid body and flexible modes
Advanced Air Vehicles Program
Advanced Air Transport Technology Project
Varying FEM Bungee Pivot Point Results
Pivot Forward Pivot Neutral Pivot Aft
Rigid Body 
“Plunge” Mode
Bungee Pivot 
Location
SW1B
0.37 RB plunge mode
1.08 SW1B Mode
0.38 Hz RB “plunge” mode 
1.03 Hz SW1B Mode
0.41 RB “plunge mode”
1.05 SW1B mode
0.35 RB plunge mode 
0.99 SW1B Mode
Full FuelEmpty Fuel
* All frequencies are normalized with respect to measured SW1B empty fuel
SW1B frequencies approach each other when changing pivot location to account for CG shift 
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Advanced Air Vehicles Program
Advanced Air Transport Technology Project
Examining other flexible modes
• Only the SW1B and rigid body plunge mode changes when fore-aft (X) location of 
pivot changes.
• Pivot location negligibly affects other primary flexible modes
Negligible 
Change
Approaches same 
frequency for SW1B
Advanced Air Vehicles Program
Advanced Air Transport Technology Project
Moment of Inertia Test
Configuration Description Pendulum length
Fuel 
condition
A Lifting hardware only Long N/A
B Lifting hardware only Short N/A
C X-56A + lifting hardware Short Empty
D X-56A + lifting hardware Long Empty
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MOI Test Configuration
Objective: Measure pitch MOI using 
compound pendulum method
Calculated pitch moment of inertia was within 2.5% 
between short and long pendulum configurations
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𝑤ଵ𝑇ଵଶ𝐿ଵ
4𝜋ଶ
−
𝑤ଶ𝑇ଶଶ𝐿ଶ
4𝜋ଶ
−
𝑤ଷ𝐿ଷଶ
𝑔
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Advanced Air Vehicles Program
Advanced Air Transport Technology Project
Lessons Learned
Wing-only Strongback Ground Vibration Test:
• Install additional tri-axial accelerometers at the winglet for better resolution of mode shapes.
• Power-on actuators to prevent drooping from excitations.
Full-Aircraft Ground Vibration Test:
• Additional scrutiny is required when using multiple bungees because of the increased risk of 
coupling between bungees or with the rigid-body modes of the flexible vehicle.
• Eliminate all potential degrees of freedom (that is, the metal sling rotation around the bungee) that 
could interfere with the rigid-body structural modes.
• Perform pre-test analysis with various boundary conditions to identify potential boundary-condition 
sensitivities for obtaining quality data.
• When possible, instrument the soft-support system (bungees and hardware) to verify their 
independence from the structural modes and to assist with any required troubleshooting.
• Ensure that the bungees are sufficiently flexible, and minimize interference in all degrees of freedom 
of interest.
Aircraft Pitch Moment of Inertia Test:
• When using knife-edges, curved-out V-channels further reduce friction.
• Use multiple sources for period measurement as a sanity check for accurate data.
Advanced Air Vehicles Program
Advanced Air Transport Technology Project
Acknowledgements
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• NASA Aeronautics Advanced Air Vehicles Program and the Advanced 
Air Transport Technology Project
• Air Force Research Lab 
• Lockheed Martin Skunkworks
• Armstrong Flight Loads Lab
• ATA Engineering
Advanced Air Vehicles Program
Advanced Air Transport Technology Project
Questions
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Advanced Air Vehicles Program
Advanced Air Transport Technology Project
Backup Reference Slides
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Advanced Air Vehicles Program
Advanced Air Transport Technology Project
X-56A Aircraft GVT Excitation Cases
• Shaker
 Left & Right wings (45° & 90°)
 Fuselage Fwd (Vert & Lat)
 Fuselage Aft (Vert)
• Impact Hammer
 Nose boom (Vert & Lat)
 Nose landing gear (Fwd/Aft & Lat)
 Main landing gear (Fwd/Aft & Lat)
 Engines (Lat)
 Left & Right wings (Vert)
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Shaker 
Impact Hammer
Advanced Air Vehicles Program
Advanced Air Transport Technology Project
• Primary Modes:
• Symmetric Wing 1st Bending (SW1B)
• Antisymmetric Wing 1st Bending (AW1B)
• Symmetric Wing 1st Torsion (SW1T)
• Antisymmetric Wing 1st Torsion (AW1T)
• Secondary Modes
• Symmetric Wing 1st Bending and Symmetry Main Landing Gear Lateral (SW1B & S MLG Lat)
• Antisymmetric Wing 1st Bending Lateral and Antisymmetric Winglets (A MLG Lat & AWL)
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X-56A Structural Dynamics Ground Testing Overview and Lessons Learned

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