This presentation presents the results from a wing load test in the FLL (Flight Loads Lab). Load calibration techniques for strain gages and fiber optics were compared and contrasted. This presentation has the potential to lead to more research testing at NASA Armstrong in the areas of load calibration technology

Hudson, Larry

Jordan, Ashante

Lokos, William

Miller, Eric J.

Pena, Frank

NASA Technical Reports Server

Armstrong Flight Research Center
Evaluation of Wing Load Calibration and Sensing Methods Using 
Conventional Strain Gages and a Fiber Optic Sensing System 
Installed on a Straight Tapered Wing
Eric J. Miller, Frank Pena, Ashante Jordan, Larry Hudson
NASA Armstrong Flight Research Center, Edwards, California, 93523
William Lokos
Jacobs Engineering, NASA Armstrong Flight Research Center, Edwards Air Force Base, California 93523
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https://ntrs.nasa.gov/search.jsp?R=20190000062 2019-08-30T04:54:02+00:00Z
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Outline
• Introduction 
• Historical Examples
• Structural Sensing Methods
• Linear Regression Methods
• Operational Loads Estimation Algorithm  
• Instrumentation
• Conventional Strain Gages
• Fiber Optic Strain Sensors
• Load Cases
• Test Results
• Conclusions
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Introduction
• Reasons for monitoring structural loads
1. Airworthiness Clearance – Confidence in the loads for envelope clearance, do the loads 
match predictions?
2. Health Monitoring – Insight into the operational loads environment allows for more informed 
inspections and maintenance decisions
3. Structural Load Alleviation – Reducing the internal loading during maneuvers or gust 
encounters provide protection against structural overload
4. Structural Optimization / Model Validation – New structures are being developed and the 
models need to be validated in relevant environments
5. Aerodynamic Model Validation – The measured structural loads with inertial correction can be 
used to validate computational fluid dynamic (CFD) models
• It is important to understand what information is required before selecting the sensors and load 
monitoring methodology 
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Introduction
• New instrumentation techniques have been developed that allow 
a much greater number of sensors to be monitored thus allowing 
greater insight into the structural response of structures in-flight 
• A straight tapered wing with 30ft semi span was instrumented with 
both conventional strain gages and fiber optic strain sensors
• A conventional loads calibration was conducted on the wing, 
known loads were applied to the wing and strain gage and fiber 
optic strain sensor data was recorded 
• The loads test program was named the Calibration Research 
Wing or CREW Loads Test, it also served as a pathfinder for the 
Passive Aeroelastic Tailored Wing Testing
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Historical Examples
X-29 Forward Swept Wing Aircraft
F-18 Active Aeroelastic Wing (AAW) 
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Structural Sensing Methods
• Methods discussed in this presentation:
• Linear regression methods
• Operational Loads Estimation Algorithm  
• Other Methods:
• Photogrammetry
• Flight calibration methods
• Finite element methods
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Structural Load Sensing Scheme
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Methods – Linear Regression
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• Example: 3-gage equation that uses gages A,C, and D and 
shear load V
• The least squares solution for the coefficients 𝛽 can then be 
found
• Strain gage, fiber optic strain sensing, and load data is collected during three load cycles
• The linear portion of cycle 2 or 3 (green arrow) is taken from the dataset for calibration 
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Methods – Operational Loads Estimation Algorithm 
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ሻ𝑑2𝑧(𝑦
𝑑𝑦2
=
ሻ𝑀(𝑦
𝐸(𝑦ሻ𝐼(𝑦ሻ
• Moment-strain relationship of the classical beam
𝜎𝑢𝑝𝑝𝑒𝑟 𝑦 =
𝑀(𝑦ሻ ⋅ 𝑐upper(𝑦ሻ
𝐼(𝑦ሻ
= 𝐸(𝑦ሻ ⋅ 𝜀𝑢𝑝𝑝𝑒𝑟 𝑦
• Bending stress and normal stress strain relationship
𝑀 𝑦
𝐸 𝑦 𝐼 𝑦
=
𝜀upper 𝑦
𝑐upper 𝑦
𝑐𝑢𝑝𝑝𝑒𝑟 𝑦 =
ℎ 𝑦 ⋅ 𝜀upper 𝑦
𝜀𝑙𝑜𝑤𝑒𝑟 𝑦 − 𝜀upper 𝑦
𝐸 𝑦 𝐼 𝑦 = 𝑀 𝑦 ⋅
ℎ 𝑦
𝜀𝑙𝑜𝑤𝑒𝑟 𝑦 − 𝜀upper 𝑦
• Distance to the neutral axis
• Section properties of the beam structure
• A single load case is applied to the wing to calculate the section EI 
properties
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• Integrating moment strain relationship to 
calculate slope
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𝜕𝑧
𝜕𝑦
= tan 𝜙 ≈ 𝜙𝑛 = 𝜙0 +෍
𝑖=1
𝑛−1
𝑀
𝐸𝐼 𝑖
+
𝑀
𝐸𝐼 𝑖+1
⋅
Δ𝐿
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Methods – Operational Loads Estimation Algorithm 
• The bending moment can be calculated by 
knowing the strains and section properties 𝑀 𝑦 = 𝐸 𝑦 𝐼 𝑦 ⋅
𝜀𝑙𝑜𝑤𝑒𝑟 𝑦 − 𝜀𝑢𝑝𝑝𝑒𝑟 𝑦
ℎ 𝑦
𝑍𝑛 = 𝑍0 +෍
𝑖=1
𝑛
𝑠𝑖𝑛 𝜙𝑖 ⋅ Δ𝐿𝑖• The displacement can be determined by 
integrating the slope
𝑍𝑂𝐿𝐸𝐴 = 𝑍0 +෍
𝑖=1
𝑛
𝜙𝑖 ⋅ Δ𝐿𝑖
• Using small angle approximation sin 𝜙 ≈ 𝜙, the 
calculation for vertical displacement becomes
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Methods - Operational Loads Estimation Algorithm 
• Initial Values:
• The wing displacement ZOLEA is 
calculated based on section properties and 
strains
• Initial guess for shear load (VModel) is 
provided
• The shear load (VModel) is integrated 3 times to 
get a wing displacement (ZModel)
• The ZModel is compared to the ZOLEA
• If ZModel is not within an error threshold, the 
VModel is multiplied by a correction factor 
and the model integration loop is repeated
• If ZModel is within the error threshold
• VOLEA and MOLEA are then calculated 
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Test Article – Instrumentation
• Foil strain gages
• 5 spanwise stations with eight full-bridges per station
• Eight internal full-bridge gages
• 14 quarter-bridge gages co-located along optical fibers
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Test Article – Instrumentation
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Test Article – Instrumentation
• Fiber Optic Strain Sensing
• Eight optical fibers installed on 
upper and lower surfaces
• Each spar has a fiber on the 
upper and lower surfaces
• Fiber along the 40% chord on 
upper and lower surfaces
• Saw-tooth pattern on the upper 
and lower surfaces
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Upper Surface
Lower Surface
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Test Article – Instrumentation
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Locating Instrumentation
• Laser scan of instrumentation locations was conducted and resulting points were transferred into a 
FEM model
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Load Cases – Bending Torque Plot
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Load Cases – Shear, Bending and Torque
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Test Results
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What target errors should we expect for our 
high aspect ratio straight tapered wing?
• Shear: <5%
• Bending: <5%
• Torque: <20%
• Result Cases:
• Error Analysis:
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Comparison of Methods
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Comparison of Methods
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Comparison of Methods
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Comparison of Methods
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Summary
• The project requirements for load monitoring play a key role in determining which sensor and 
calibration method should be used
• Fiber optic measurement techniques allow for a greater number of strain sensors to be installed 
• One conventional approach and two new approaches were presented for deriving sensing loads on a 
straight wing
• Linear regression using metallic foil strain gages
• Linear regression using FOSS
• Operational Loads Estimation Algorithm using FOSS
• Linear regression techniques can work with FOSS sensors for deriving a distributed load along the 
wing span resulting in greater number of load measurement stations
• New methods such as the OLEA can provide an efficient method for monitoring distributed wing shear 
loads using only external surface strains 
• Method only requires one calibration load case, thus simplifying the load calibration test
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Backup
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Load Sensing Recommendations for a High Aspect Ratio Wing
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Discussion – Strain Gage vs FOSS Installation
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• Metallic Foil Strain Gages
• A lot of previous heritage from past projects
• Full bridge provides built-in temperature compensation
• A requirement for only a small number of strain sensors are most likely best handled by strain gages
• Fiber Optic Strain Sensors
• Capability to be multiplexed serially allowing for multiple spanwise measurements on one fiber
• Lightweight for number of strain sensors compared to weight of similar number of strain gages
• Hermetically coated glass is chemically inert, not susceptible to corrosion, and does not have potential for 
ground loops, electrical faults, sparking, or Joule heating. These sensors also are not negatively impacted 
like common aircraft avionics systems with reactions to Electro-Magnetic Interference (EMI) or Electro-
Magnetic Pulses (EMP).
• Fiber can be installed much quicker than strain gages for equal number of sensors
• Fiber is much better suited to open areas and may be much better suited to observing large global effects 
than strain gages
• Fiber is much better suited for buckling than strain gages


Evaluation of Wing Load Calibration and Sensing Methods Using Conventional Strain Gages and a Fiber Optic Sensing System Installed on a Straight Tapered Wing

Evaluation of Wing Load Calibration and Sensing Methods Using Conventional Strain Gages and a Fiber Optic Sensing System Installed on a Straight Tapered Wing

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