November 2012

NASA/TM2012-217784

NESC-RP-10-00632

 

Icing Research Tunnel (IRT) 

Force Measurement System (FMS)

Paul W. Roberts/NESC

Langley Research Center, Hampton, Virginia
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November 2012

NASA/TM2012-217784

NESC-RP-10-00632

 

Icing Research Tunnel (IRT) 

Force Measurement System (FMS)

Paul W. Roberts/NESC

Langley Research Center, Hampton, Virginia
Available from:

NASA Center for AeroSpace Information

7115 Standard Drive

Hanover, MD 21076-1320

443-757-5802

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Icing Research Tunnel (IRT) Force Measurement System (FMS)

September 20, 2012
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Approval and Document Revision History 

NOTE: This document was approved at the September 20, 2012, NRB. This document was 

submitted to the NESC Director on October 1, 2012, for configuration control. 

Approved: Original Signature on File 10/2/12 

 NESC Director Date 

Version Description of Revision Office of Primary 

Responsibility 

Effective Date 

1.0 Initial Release Mr. Paul Roberts, NESC 

Associate Principal Engineer 

9/20/12 
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Table of Contents 

Technical Assessment Report 

1.0 Notification and Authorization .........................................................................................6

2.0 Signature Page ....................................................................................................................7

3.0 Team List ............................................................................................................................8

4.0 Executive Summary ...........................................................................................................9

5.0 Assessment Plan ...............................................................................................................10

6.0 Problem Description and Proposed Solutions ...............................................................10

6.1 Problem Description/Current System Challenges ................................................ 10

6.2 IRT System Update Requests and Improvement Priorities .................................. 13

6.3 Overall Current System Accuracy from the Uncertainty Analysis ....................... 14

6.4 Concept Development and Concept Selection ...................................................... 15

6.4.1 Concept Number One: Multi-piece Balance Grounded to the Wind Tunnel 

Structure ................................................................................................................ 16

6.4.2 Concept Number Two: Multi-piece Balance Grounded to the Floor ................... 17

6.4.3 Concept Number Three: Monolithic Balance Grounded to the Floor .................. 18

6.4.4 Concept Number Four: Monolithic Balance Grounded to the Wind Tunnel 

Structure ................................................................................................................ 19

6.5 Proposed In-Situ Check-Load/Calibration Method .............................................. 20

6.5.1 Introduction ........................................................................................................... 20

6.5.2 Calibration and Verification ................................................................................. 21

6.5.3 Calibration Load Schedule and Hardware Design ................................................ 22

6.5.4 Full-Span Configuration: Check-Loads/Calibration ............................................. 25

7.0 Data Analysis ....................................................................................................................26

7.1 Current System Information ................................................................................. 26

7.2 Turntable Measurement Data ................................................................................ 26

7.3 Uncertainty Analysis ............................................................................................. 29

7.3.1 Analysis of the Existing IRT Lower Balance System .......................................... 29

7.3.2 Analysis of the Proposed New IRT Lower Balance ............................................. 35

7.3.3 Conclusions ........................................................................................................... 39

7.4 Structural Review.................................................................................................. 40

7.4.1 Summary ............................................................................................................... 41

7.4.2 Background/Observations ..................................................................................... 41

8.0 Findings, Observations, and NESC Recommendations ...............................................44

8.1 Findings................................................................................................................. 44

8.2 Observations ......................................................................................................... 45

8.3 NESC Recommendations...................................................................................... 45

9.0 Alternate Viewpoints .......................................................................................................48

10.0 Other Deliverables ...........................................................................................................48
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11.0 Lessons to Learn ..............................................................................................................48

12.0 Recommendations for NASA Standards and Specifications .......................................48

13.0 Definition of Terms ..........................................................................................................49

14.0 Acronyms List ..................................................................................................................49

15.0 References .........................................................................................................................50

16.0 Appendices ........................................................................................................................50

List of Figures 

Figure 6.1-1. Schematic of the IRT FMS ................................................................................... 11

Figure 6.1-2. Lower Balance and Turntable System Directly Below the IRT Test Section ...... 12

Figure 6.1-3. IRT Test Section with the Full-span Calibration Fixture Installed ....................... 12

Figure 6.4-1. Upper Balance System .......................................................................................... 15

Figure 6.4-2. Multi-piece Balance and Turntable that is Mounted on the Tunnel Structure ...... 16

Figure 6.4-3. Multi-piece Balance and Turntable that is Floor-Mounted ................................... 17

Figure 6.4-4. Monolithic Balance and Turntable that is Floor-Mounted ................................... 18

Figure 6.4-5. Monolithic Balance and Turntable that is Mounted on the Tunnel Structure ....... 19

Figure 6.5-1. Check-Load/Calibration Hardware Required to Execute the Loading ................. 24

Figure 6.5-2. Actuator Positions Required for Check-Load/Calibration Loadings .................... 25

Figure 7.2-1. Overhead Sketch of the Turntable Angle Measurement System .......................... 27

Figure 7.2-2. Photograph of the Turntable Measurement Setup ................................................ 27

Figure 7.2-3. Set Point Error versus Turntable Angle ................................................................ 28

Figure 7.3-1. Relationship between Two Error Sources ............................................................. 29

Figure 7.3-2. Current IRT-FMS Schematic Diagram ................................................................. 31

Figure 7.3-3. Forces and Moments on the model and Aerodynamic Coefficients ..................... 31

Figure 7.3-4. Proposed Lower Monolithic Balance Layout ....................................................... 36

Figure 7.3-5. Forces and Moments on the model and Aerodynamic Coefficients ..................... 37

Figure 7.4-1. Load Path Termination in Tunnel. The photo shows the former cylindrical balance 

columns now mounted to blocks of concrete. This is directly beneath the floor 

area that is below the test section/turntable/balance system. ................................ 42

Figure 7.4-2. Turntable Cruciform Base and Support Tube ....................................................... 43

List of Tables 

Table 6.3-1. Current System Aerodynamic Coefficient Systematic Errors .............................. 14

Table 6.5-1. Force Balance Calibration 2010, Summary Worksheet ........................................ 21

Table 6.5-2. DOE Calibration Load Schedule .......................................................................... 23

Table 7.1-1. System Evaluation Metrics ................................................................................... 26

Table 7.3-1. Coefficients and Estimated Uncertainty of the Current FMS ............................... 34

Table 7.3-2. Coefficients and Estimated Uncertainty of the Current FMS with Updated 

Load Cells ............................................................................................................. 35

Table 7.3-3. Coefficients and Estimated Uncertainties of Proposed Single Piece Lower 

Balance FMS ......................................................................................................... 39
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Table 7.3-4. Estimated Uncertainties of Proposed FMS ........................................................... 40

Table 8.3-1. Cost Range Estimates for FMS Upgrade Recommendations ............................... 47

 
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Technical Assessment Report 

1.0 Notification and Authorization 

Mr. Mark Woike, Electronics Engineer at the Glenn Research Center (GRC), requested that the 

NASA Engineering and Safety Center (NESC) provide technical support for an evaluation of the 

existing force measurement system (FMS) at the GRC’s Icing Research Tunnel (IRT) with the 

intent of developing conceptual designs to improve the tunnel’s force measurement capability in 

order to better meet test customer needs. 

An NESC initial evaluation was approved on October 1, 2010. Mr. Paul Roberts, NESC 

Associate Principal Engineer at Langley Research Center (LaRC), was assigned to lead the 

assessment. An Assessment Plan was approved by the NESC Review Board (NRB) on 

July 29, 2010. 

The key stakeholder for this assessment was Mr. Mark Woike at GRC. 
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2.0 Signature Page 

Submitted by: 

Team Signature Page on File – 10/4/12 

Mr. Paul W. Roberts Date 

 

 

Significant Contributors: 

Mr. Ray D. Rhew Date Mr. Jared A. Dervan Date 

Dr. Drew Landman Date Mr. K. Christopher Lynn Date 

Mr. Nans Kunz Date Mr. David King Date 

Mr. Naresh Patel Date 

Signatories declare the findings, observations, and NESC recommendations compiled in the 

report are factually based from data extracted from Program/Project documents, contractor 

reports, and open literature, and/or generated from independently conducted tests, analyses, and 

inspections. 
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3.0 Team List 

Name Discipline Organization/Location 

Core Team 

Paul Roberts 

NESC Team Lead, Associate Principal 

Engineer LaRC 

Ray Rhew FMS Engineer – Deputy LaRC 

Jared Dervan Resident Engineer MSFC 

David King FMS Engineer Triumph Aerospace 

Nans Kunz NESC Chief Engineer ARC 

Drew Landman Statistics Old Dominion University 

Chris Lynn FMS Engineer LaRC 

Naresh Patel FMS Engineer Modern Machine & Tool 

Roy Savage MTSO Program Analyst LaRC 

Mark Woike Electronics Engineer GRC 

Administrative Support 

Linda Burgess Planning and Control Analyst LaRC/AMA 

Tina Dunn-Pittman Project Coordinator LaRC/AMA 

Christina Williams Technical Writer LaRC/AMA 
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4.0 Executive Summary 

The Glenn Research Center (GRC) Icing Research Tunnel (IRT) is one of NASA’s unique 

aerodynamic test facilities. It is one of the largest icing wind tunnels in the world. Because of 

this unique capability, the facility is constantly in high demand. Some of the tunnel’s main 

features are a large external balance and turntable system. The IRT is considering upgrades to 

these systems to increase the force and moment measurement accuracy and to improve facility 

test throughput. This force measurement system (FMS) was designed in the early 1970s by civil 

servants who have since retired. The IRT operations crew is considering options to improve 

their facility product quality, reliability, and test throughput. To support this effort, the NASA 

Engineering and Safety Center (NESC) was requested to assess the current system, evaluate 

operational needs, and develop new concepts and/or upgrade options including the associated 

cost estimates. 

The NESC created a team of experts from across NASA and industry to perform this assessment. 

This involved becoming familiar with the existing facility and interviewing the operations staff 

to gain an understanding of current operations and facility weaknesses and to solicit their 

thoughts on requirements, needs, goals, and desires. The NESC team then developed multiple 

design configuration upgrade options that would meet these requirements/needs/goals. The pros 

and cons of each system were first discussed within the team, and then the first round of down 

selected concepts was discussed with the facility personnel. The pros and cons, expected 

accuracy, benefits, etc., for each of the final down selected options are discussed in this report. 

As part of this assessment, the NESC team performed a system uncertainty analysis to compare 

the current system accuracies with predicted accuracies of various upgrade concepts. A 

structural evaluation was included as well to gain a high confidence in the ability to upgrade the 

existing facility. 

Since it is unknown how much funding would be ultimately available to upgrade this facility, the 

resulting recommendations include upgrade design concepts/options at different cost levels. The 

NESC recommendations to the IRT range from lower cost options including using the current 

hardware and changing the current calibration method to higher cost options that include 

replacing the entire turntable and the FMS. The benefits for each of these recommended options 

should help the facility determine which upgrades will have the most impact relative to their 

available funding. The specifications/descriptions in this report can also be used to help develop 

the statement of work, specifications, and other documentation that will be needed to begin the 

IRT modification/upgrade project. 
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5.0 Assessment Plan 

The IRT at GRC has a complex multi-piece force measurement and turntable system that is 

expected to need upgrades in the near future. There are few force measurement experts in the 

United States or in the world that have experience with these types of systems. The scope of this 

task included performing an assessment of the current IRT FMS, developing concepts for 

improvements or a new design, and generating a cost estimate and specifications that will be 

used to procure a new FMS. 

The plan required the team to review the existing literature and other documentation of the 

current IRT FMS, inspect the facility, and interview the operations staff. The NESC team relied 

on the broad expertise of the facility personnel to become familiar with operational issues and to 

identify areas where the greatest potential for system improvements exists. From these 

interviews, a list of facility needs was developed and priorities were ranked and listed, as shown 

in Table 7.1-1. Following the accomplishment of these tasks, an uncertainty analysis was 

performed for the FMS options. Tests such as measurement of the turntable system accuracies 

were performed to generate the required data to complete the uncertainty analysis. Clear 

definition of the requirements and the development of a facility prioritization list was the next 

step in the process, followed by the development of multiple new system concepts. Computeraided design models were developed, and predicted system uncertainties were calculated. The 

pros and cons of each concept were then discussed with the IRT facility personnel, and their 

thoughts were recorded. All of this information was used to down select a final recommended 

new system that best met all of the facility requirements. Finally, the team broke down the 

upgrade into possible steps and developed cost estimates and specifications such that the facility 

could upgrade their current system as funding becomes available. 

Disengagement will occur upon the delivery of a final report describing the current FMS analysis 

and conceptual design improvements for upgrading the balance system. This report will include 

technical specifications that would allow GRC to develop a request for proposal for developing 

an updated FMS for the IRT. 

6.0 Problem Description and Proposed Solutions 

6.1 Problem Description/Current System Challenges 

The GRC IRT is one of NASA’s unique aerodynamic test facilities and is one of the largest icing 

wind tunnels in the world. Because of this unique capability, the facility is constantly in high 

demand. Two of the tunnel’s main features that are the primary focus of this assessment are a 

large external balance/FMS and a turntable system. 

A good representation of the entire system can be seen in the schematic of the turntable and the 

FMS in Figure 6.1-1. 
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Figure 6.1-1. Schematic of the IRT FMS 

Figure 6.1-2 shows some of the turntable drive system and two of the lower balance system load 

cell/flexure linkages. Figure 6.1-3 shows the IRT test section with the full-span calibration 

hardware in place. A significant amount of time and effort is required to install and remove this 

hardware prior to performing a calibration. As a result, the team was told that many IRT 

customers chose not to have the FMS in place during their tests because of the impact on 

productivity. 
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Figure 6.1-2. Lower Balance and Turntable System Directly Below the IRT Test Section 

Figure 6.1-3. IRT Test Section with the Full-span Calibration Fixture Installed 
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A more detailed description of the current IRT force balance system can be acquired from the 

document, which is included in Appendix B. This system needs to be updated to increase the 

force and moment measurement accuracy and to improve facility throughput. From the 

documentation, it appears that the current FMS was last modified/updated/designed in the early 

1970s by civil servants who have since retired. The NESC was requested to form a team of 

experts to assess the current system, develop new concepts if required, and develop cost 

estimates and specifications for any suggested upgrades. 

6.2 IRT System Update Requests and Improvement Priorities 

Through multiple meetings with the IRT team, a list of requirements, needs, requests, goals, and 

various background statements was generated. The list is as follows: 

1. The lower balance system shall be integrated into the model turntable that will be in the 

IRT floor and shall replace the existing turntable system. 

2. The range of the turntable shall be +/–30 degrees for model angle of attack (AOA). 

3. The turntable measurement system of the AOA shall be within +/–0.05 degrees. 

4. The force balance system shall measure model lift, drag, and pitching moment. 

5. The maximum load measurement capability for semi-span or full-span testing shall be: 

a. 20,000 pounds lift 

b. 5,000 pounds drag 

c. 20,000 foot-pounds pitching moment 

6. The loads shall be measured within +/–1 percent of full scale. 

7. The force balance system should consider including an internal calibration system so no 

loads are required to be applied inside the test section. 

8. The upper measurement system shall be integrated into the ceiling hatch with a 2-inch 

diameter access for instrumentation. The upper balance system can be a permanent 

fixture of the ceiling hatch or can be removable. The removable system would be 

preferred, yielding less chance for damage when not in use. 

9. The upper balance system shall be used for full-span models that are attached to the 

turntable measurement system. 

10. The design of the turntable and upper balance system must allow for thermal contraction 

and expansion to accommodate model length changes. 

11. The turntable measurement system shall be able to withstand moisture from the cleaning 

of models and warm-up of the facility; water drips onto the turntable system. 

12. The access area up through the balance and turntable will be 12 inches wide by 

24 inches in the flow direction; the centered vertical axis will be in the center of the area 

(this area is for instrumentation and heated air line supply). 

It is important to note that initial discussions clearly indicate that the throughput and test 

efficiency are potentially more important than increasing the accuracy of the FMS. Complex 

models that include multiple pass-through lines (e.g., high pressure air, hydraulics) are often 

installed in this facility. The current balance and turntable system has limited clearances for 

model access, which makes it challenging to install pass-through lines and keep them from 
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fouling. Fouling lines carry loads that can introduce significant errors into the FMS. When 

incorporating a model pass-through, it is best to incorporate long “soft” lines paralleling the 

balance system. This way, the balance system carries the majority of the loads as it is designed 

to do. Also, it is good practice to apply the maximum loads on the model that are expected in a 

wind tunnel test to gain a high confidence of the system uncertainties and to check any passthrough for unacceptable fouling. Developing a system with adequate pass-through clearances 

and easy application of check loads will significantly increase wind tunnel productivity. Testing 

efficiency is a critical aspect of meeting the heavy demand for test time in this specialized wind 

tunnel. 

6.3 Overall Current System Accuracy from the Uncertainty Analysis 

During this assessment, an uncertainty analysis of the calculated aerodynamic coefficients was 

performed to understand the level of impact that force measurement improvements would have 

on the wind tunnel output data. A detailed explanation of this analysis is provided in Section 7.3. 

The percent error is determined by comparing the nominal coefficient value as determined by a 

typical semi-span icing model to the calculated systematic uncertainty. The systematic error in 

the aerodynamic coefficients is a function of load cell readings, the dynamic pressure, and the 

AOA as related through the balance data reduction equation. The equation is derived in Sections 

7.3.1.4 and 7.3.1.5. The existing systematic error estimates for these parameters were researched 

and used in the uncertainty estimate. Existing systematic uncertainty data could be found for all 

sources except the turntable positioning accuracy. A test to measure the unloaded positioning 

accuracy was developed and performed to acquire the required data and include it in the 

uncertainty analysis; see Appendix C, Tables C-1 through C-3. The overall current system 

accuracy for a typical semi-span wing test was determined to be as follows (see Table 6.3-1). 

Table 6.3-1. Current System Aerodynamic Coefficient Systematic Errors 

CD CL Cm

Total Systematic 0.005 0.023 0.024 

Nominal Coefficient 

Value 

0.147 1.02 –0.083 

% of Nominal 3.54 2.32 28.3 

These values do not include random error and are calculated at a 95-percent confidence level. It 

should be noted that a complete uncertainty analysis would include random error, which is best 

determined by using replicate wind tunnel test data. Unfortunately, the data were unavailable to 

the team. 

In addition, it should be noted that the individual load cell calibration sheets supplied appear to 

report error well in excess of what can be inferred from the supplied whole-system calibration 

results. Since the pitching moment is derived from multiple load cells, it is the most affected 

measurement. 
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6.4 Concept Development and Concept Selection 

Following the data-gathering phase of this assessment, the team held brainstorming sessions to 

develop various concepts that would improve the wind tunnel throughput and the system force 

measurement accuracies. It was found that the vast majority of the IRT tests are run in a semispan configuration. Also, the team found that the upper balance system, which is used for fullspan testing, was a straightforward monolithic design. The upper balance system, shown in 

Figure 6.4-1, is a single-piece construction balance that incorporates flexures and a spline 

mounting that allows for thermal expansion and contraction in the system. 

Figure 6.4-1. Upper Balance System 

Three upper balance systems with different load ranges currently exist so the load ranges can be 

set for each test by changing between the various balances. Therefore, the team’s focus was 

applied to generating concepts that would be highly efficient for semi-span testing. These 

concepts could be used in conjunction with the existing upper balance system when full-span 

testing was required. The team developed four key concepts that involved complete system 

upgrades using new turntables and new force measurement balance systems. The main 

differences in the concepts involved was whether the FMSs were grounded to the wind tunnel or 

grounded to the basement floor and whether a monolithic or multi-piece constructed FMS was 

used. Conceptual diagrams of these four systems are shown in Figures 6.4-2 through 6.4-5. 

Concept key advantages and key disadvantages are discussed separately. 
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The four concepts that were generated for the lower balance system include combinations of 

systems that are grounded to the wind tunnel structure or systems that are grounded directly to 

the floor/earth. The main advantage of grounding the system to the tunnel structure, as shown in 

Figures 6.4-2 and 6.4-5, is that the balance system moves with the thermal expansion of the 

tunnel. This reduces the possibility of fouling and changes in the relative position of the model 

to the test section. This is the method that most aerodynamic tests use when they have stingmounted model systems. The main disadvantage to grounding the system to the tunnel is that 

often these types of systems experience more dynamics than systems that are tied to solid 

ground. The reverse is true when a balance system is grounded to the floor, as shown in Figures 

6.4-3 and 6.4-4. Floor-grounded systems tend to have less dynamic inputs; however, thermal 

movement of the tunnel system relative to the balance can cause fouling and can result in 

changes in the relative position of the model in the test section. 

6.4.1 Concept Number One: Multi-piece Balance Grounded to the Wind Tunnel 

Structure 

The first lower balance system concept, shown in Figure 6.4-2, is an improvement on the 

existing system concept. It incorporates a multi-piece construction external balance system that 

is composed of linkages and load cells to measure the forces and moments. The balance system 

would be grounded to the wind tunnel test section structure. 

Figure 6.4-2. Multi-piece Balance and Turntable that is Mounted on the Tunnel Structure 
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Advantages of this system are its relatively low cost and the ability to change load ranges by 

changing out load cells. While changing out load cells is appealing, the ability to change load 

cells in a multi-piece balance to adjust the load range is often not practical. Depending on the 

external balance design, the logistics of changing out load cells while the balance is installed can 

be challenging. Also, the balance characteristics change so much that a new full calibration is 

required. Disadvantages include the large physical size, which would hinder model installation 

and would also make the initial system installation difficult, possibly causing significant tunnel 

downtime. The initial calibration would be performed at the manufacturing site, but subsequent 

calibrations would need to be performed in place, requiring tunnel shutdown. 

6.4.2 Concept Number Two: Multi-piece Balance Grounded to the Floor 

The second concept, shown in Figure 6.4-3, uses the same type of multi-piece balance 

construction except it would be grounded to the floor instead of grounded to the tunnel. 

Figure 6.4-3. Multi-piece Balance and Turntable that is Floor-Mounted 

Advantages for this system are similar to those of the previous concept; however, this system 

would be easier to install since it would not have to be adapted to the existing tunnel structure. 

An added disadvantage would be that the grounding stand would be an added obstacle to the 

tunnel technicians as they install models in the tunnel. Also, by having the tunnel grounded at a 

different place than the balance, it is critical that a thorough system analysis be performed to 
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ensure that the tunnel movement due to vibration and thermal affects would not cause issues with 

the model position relative to the flow. Also, the clearance gaps between the metric balance and 

the non-metric tunnel floor would have to be analyzed to ensure that acceptable dimensions 

would be maintained. 

6.4.3 Concept Number Three: Monolithic Balance Grounded to the Floor 

A third concept, shown in Figure 6.4-4, includes using a monolithic balance and grounding it on 

a stand through the floor below the tunnel. 

Figure 6.4-4. Monolithic Balance and Turntable that is Floor-Mounted 

In this concept, the monolithic balance would typically cost more than the previously mentioned 

multi-piece balances. However, the single-piece balance would be able to be easily removed for 

calibration away from the facility. This would allow the facility to replace the live balance with 

a dummy balance and run tests that do not require force measurements while the balance 

calibration is being updated at another lab. The disadvantages would be similar to those of the 

previous concept. The grounding stand would be an obstacle to the technicians installing models 

in the tunnel. 
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6.4.4 Concept Number Four: Monolithic Balance Grounded to the Wind Tunnel 

Structure 

The final concept included the use of a monolithic balance that is grounded to the wind tunnel 

test section and is shown in Figure 6.4-5. This is the concept that the team would recommend if 

a complete new system was developed; more details of the advantages and disadvantages of this 

system are discussed. 

Figure 6.4-5. Monolithic Balance and Turntable that is Mounted on the Tunnel Structure 

This concept optimizes the tunnel efficiency for the operators. A significant advantage for this 

concept is that the single-piece balance is robust and rarely needs to be calibrated and would be 

able to be easily removed for calibration away from the facility when it does require calibration. 

Similar to the third concept, this would allow the facility to replace the live balance with a 

dummy balance and run tests that do not require force measurements while the balance 

calibration is being updated at another lab. When used for semi-span testing, the manufacturer’s 

calibration coefficients could be used and, with only a few check loads applied, most 

aerodynamic tests could be performed. This would reduce the facility downtime by eliminating 

most of the pre-test system calibration effort that is currently required. Also, since this balance is 

grounded to the tunnel test section, it operates in a similar manner to balances that are internally 
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mounted in models used in typical wind tunnel tests. By having the same ground, the balance 

moves with the tunnel, and there is less concern with changes in flow alignment with the 

measuring system and less concern about maintaining the metric and non-metric gaps. Mounting 

the balance to the tunnel also allows technicians full access under the balance and turntable 

system, which improves the ability to install models in the test section. 

The main disadvantage of the single balance concept is the initial cost of the monolithic balance. 

These balances are typically expensive since they require complex machining to create multiple 

measuring elements from a single piece of material. However, when looking through NASA’s 

balance inventory, the team found a blank (i.e., a partially constructed balance) that could 

possibly be used in this case to defer costs. This balance could then be part of a set of existing 

semi-span balances that could possibly be borrowed from other NASA facilities to change the 

load ranges when necessary for a test. One advantage, relative to the IRT for this type of 

balance, is that for the IRT only two forces and one moment need to be measured. This greatly 

simplifies the balance design so that a new, custom three-component balance would not be as 

expensive as a six-component balance of this size. Of course, if balances from other facilities are 

not available to be borrowed, then a related disadvantage of the monolithic balance approach is 

that a second monolithic balance would be required for cases where changing the balance is 

necessary to optimize accuracy. Another consideration for the monolithic balance upgrade 

approach is that even with a single monolithic balance designed to operate over the entire load 

range of the IRT, it will likely be able to meet the 1-percent accuracy specification and be more 

accurate than the current multi-balance FMS. 

6.5 Proposed In-Situ Check-Load/Calibration Method 

6.5.1 Introduction 

This section on a proposed in-situ check-load/calibration method is included within this report to 

address three of the requests listed in Section 6.2: 

• The loads will be measured within +/–1 percent of full scale. 

• The force balance system shall have an internal calibration system so no loads are 

required to be applied inside of the test section. 

• It is important to note that initial discussions clearly indicated that the throughput and test 

efficiency were potentially more important than increasing the accuracy of the FMS. 

Testing efficiency is critical for meeting the heavy demand for test time in this 

specialized wind tunnel. 

While the request to have an internal calibration system is not included in any of the concepts, 

the proposed method addresses the accuracy and efficiency requirements. Additionally, a 

minimum set of check loads is recommended for the FMS for verifications that result in loads 

needing to be applied in a test section. 
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6.5.2 Calibration and Verification 

Verification and calibration should be based on the requirements of the system. The 

requirements for the force balance system are listed in Section 6.2. The accuracy requirement is 

stated as follows: “The loads shall be measured within +/–1 percent of full scale.” A review of 

selected previous calibrations was conducted. The results of a calibration from 2010 are shown 

in Table 6.5-1. 

Table 6.5-1. Force Balance Calibration 2010, Summary Worksheet 

 Standard Deviation of Back-Computed Residuals* 

Angle of 

Attack Lift Lift Drag Drag Moment Moment 

Alpha 

Initial 

Constants 

Final 

Constants 

Initial 

Constants 

Final 

Constants 

Initial 

Constants 

Final 

Constants 

Degrees 

Std Dev 

% FS 

Std Dev 

% FS 

Std Dev %FS Std Dev % 

FS 

Std Dev % 

FS 

Std Dev % 

FS 

–20.0 0.2 0.1 0.8 0.1 1.3 1.7

 

–10.0 0.2 0.1 0.7 0.0 1.0 1.0

 

0.0 0.2 0.1 1.5 0.4 1.3 1.2

10.0 0.2 0.1 0.7 0.1 1.3 1.1

20.0 0.2 0.1 0.7 0.1 1.4 1.1

 

All 0.2 0.1 0.9 0.1 1.3 1.2

*Standard Deviation of Back-Computed Residuals: The standard deviation of the computed residuals from the loads 

applied during the calibration. These calibration loads were also used to develop the calibration matrix. 

 

The initial constants columns are the residuals computed using the original load-cell-only 

calibrations, while the final constants are the residuals computed using the in-situ calibration. 

While there is improvement in the residuals for drag using the final constants, lift and pitch are 

essentially the same. Also, the requirement of +/–1 percent is satisfied for lift and drag using the 

initial constants and slightly missed for pitch. Observing that the initial constants nearly meet 

the accuracy requirement leads to the following proposed calibration (i.e., verification) 

methodology: 

• Apply check loads to verify the initial calibration constants. 

• Develop check-load/calibration hardware to reduce the check-load/calibration/ 

verification time to less than one shift (if possible). 

• If the verification is not satisfactory (i.e., does not meet the 1-percent requirement), then 

apply additional loads, if necessary, to compute a new set of calibration constants that 

account for in-situ effects, such as interactions. 
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The proposed approach will apply for all four scenarios listed below: 

1) Existing balance in semi-span mode. 

2) Existing balances in full-span mode**. 

3) New balance in semi-span mode. 

4) New balances in full-span mode**. 

** See Section 6.5.3 for full-span check-loads/calibration discussion. 

As noted in Section 6.5.3, the full-span model is an exception. When both the lower and upper 

balance systems are used, there is a higher probability of requiring a full in-situ calibration due to 

the model-dependent stiffness effects. 

6.5.3 Calibration Load Schedule and Hardware Design 

Based on the proposed approach and the objectives of the assessment, the most efficient method 

to develop the load schedule is to use design of experiments (DOE) [ref. 4]. DOE is a 

statistically based approach to maximize the amount of information obtained for a given amount 

of resources and provide objective/quantifiable knowledge. Additionally, to take full advantage 

of DOE and the requirement to reduce the loading to one shift, new calibration hardware is 

proposed that enables the most efficient execution of the load schedule. 

A principle of DOE is to make each data point as information rich as possible. Therefore, 

applying this to force measurement calibration equates to applying as many load components as 

is practical per load point. Table 6.5-2 presents the proposed load schedule utilizing a factorial 

design approach while taking into account the hardware capabilities (the load magnitudes are 

based on the 2010 calibration referenced previously). The calibration design can be scaled as 

needed to adjust for different full-scale load requirements. A subset of this load schedule can be 

utilized for check loads initially. If the computed residuals (i.e., the difference between the 

computed and applied loads) are within the required accuracy, then the verification is complete. 

However, if the residuals are not acceptable, then the remainder of the schedule can be executed 

to develop a new in-situ matrix. 

 
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Table 6.5-2. DOE Calibration Load Schedule 

Actuator 

Load 

Position 

Full 

Load 

Points Lift Drag Pitch 

Applied 

Load Load Angle 

Pitch 

Distance 

(lbs) (lbs) (ft-lbs) (lbs) (degrees) (inches) 

1 1 –3000 1000 2000 3162 –18.4 –8

10 2 3000 0 2000 3000 0.0 8

3 3 –3000 1000 –2000 3162 –18.4 8

6 4 3000 1000 –2000 3162 18.4 –8

8 5 3000 1000 2000 3162 18.4 8

zero 6 0 0 0 0

5 7 –3000 0 –2000 3000 0.0 8

zero 8 0 0 0 0

7 9 3000 0 –2000 3000 0.0 –8

zero 10 0 0 0 0

2 11 –3000 0 2000 3000 0.0 –8

Mid Point Loads (same positions as above, and can be easily applied after setup) 

1 12 –1500 500 1000 1581 –18.4 –8

10 13 1500 0 1000 1500 0.0 8

3 14 –1500 500 –1000 1581 –18.4 8

6 15 1500 500 –1000 1581 18.4 –8

8 16 1500 500 1000 1581 18.4 8

5 17 –1500 0 –1000 1500 0.0 8

7 18 1500 0 –1000 1500 0.0 –8

2 19 –1500 0 1000 1500 0.0 –8

Note: actuator load positions 4 and 9 apply only a lift force. This is not required for the calibration but is easy to 

apply and provides additional information if needed for assessment or troubleshooting. 

The load schedule will enable computation of the coefficients for the following model: 

ܴ௜ ൌ ܽ௜ ൅෍ܾ௜,௝

௡

௝ୀଵ

௝ܨ෍෍൅ ௝ܨ

௡

௞ୀଵ

௡

௝ୀଵ

௞ܨ

where ݅ ൌ 1	݋ݐ	3 and ്݆݇. 

This is more than required for verifying the existing system calibration, but it enables 

interactions to be included if needed. 

The calibration hardware required to execute the loadings is shown in Figure 6.5-1. 

Figure 6.5-2 shows all of the planned actuator positions; however, only one actuator is required 

per loading. The positions are shown for tension loadings only. Typically, tension loads are 

more easily aligned. However, if compression loads are possible with the hardware and can be 
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shown to maintain accuracy, utilizing compression loads will further cut the number of 

configurations in half. 

Figure 6.5-1. Check-Load/Calibration Hardware Required to Execute the Loading 
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Figure 6.5-2. Actuator Positions Required for Check-Load/Calibration Loadings 

The hardware consists of attach points to the tunnel wall in the form of rails bolted in place with 

pre-determined actuator pin locations. The system uses the same actuator and load cells that are 

currently used for applying loads augmented with additional length rods as needed. A new piece 

of hardware with actuator attach points is needed to provide the necessary locations on the center 

post. 

6.5.4 Full-Span Configuration: Check-Loads/Calibration 

The full-span configuration presents a unique scenario for the FMS. The uniqueness is due to 

the influence of the test article on the measurement performance. When the upper and lower 

balance systems are connected through the test article, the amount of load each will measure is 

dependent on the stiffness of the test article. Therefore, it is recommended that each full-span 

test article have a full set of loads applied to develop an in-situ calibration matrix. It is 

understood that it will be difficult to apply loads to the test articles in certain configurations and 

the check-load approach can be used. This is a typical approach that is used when a full 

calibration is not feasible; however, accuracies of less than 0.25 percent are typically the goal in 

this situation. 

An assessment of the test article stiffness impact on the calibration can be executed using 

dummy test articles with different spring constants as calibration hardware. Performing a 

minimum of two calibrations to bound varying model system stiffness can provide a quantitative 

assessment of the impact by comparing the calibration residuals and the coefficient differences. 
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7.0 Data Analysis 

7.1 Current System Information 

Literature detailing the structural design, calibration rationale, and total system accuracies is 

almost nonexistent, so the team held multiple technical interchanges with the facility personnel 

to gather as much data as possible. An initial list of requirements was supplied by the facility 

personnel and is listed in Section 6.2. It is extremely important to reiterate that initial 

discussions with facility personnel clearly indicated that the throughput and test efficiency was 

potentially more important than just increasing the accuracy of the FMS. Testing efficiency was 

a critical aspect of meeting the heavy demand for test time in this specialized wind tunnel. Since 

this throughput and test efficiency was not directly listed in the requirements, the NESC team 

developed the following list of evaluation metrics shown in Table 7.1-1. 

Table 7.1-1. System Evaluation Metrics 

Order of 

Importance Evaluation Metrics

1 Productivity (test throughput, ease of use/calibration, training for new 

system) 

2 Reliability 

3 Facility Impacts (downtime, facility modification) 

4 Capability (higher loads, AOA) 

5 Cost 

6 Accuracy Improvement (AOA, loads) 

These metrics were used to judge the various FMS concepts and select the most appropriate 

system for recommendation to the IRT. 

7.2 Turntable Measurement Data 

During the initial site visit, observations of the turntable operation and visual inspections of the 

mechanisms led to the belief that that the turntable setting angle, and thus the model AOA, 

would be a significant source of error in the delivered aerodynamic coefficients. The existing 

turntable motion mechanism is an old system that uses multiple bolt shanks as gear teeth for the 

ring gear. The control system has been updated to slowly move to command positions. This 

helps reduce positioning error in the gear system. The slow positioning is an area where 

throughput can be improved with a newer turntable system with a faster rotational speed. A plan 

was developed to quantify the angle setting accuracy of the turntable; the measurements were 

completed on a subsequent site visit. 

To measure the absolute angle of rotation of the turntable, a Theodolite1 was placed on the center 

of rotation of the turntable. The center mark was scribed in the floor plates of the turntable. An 

alignment laser was mounted off the turntable and aligned with the center mark; the turntable 

 

1

http://en.wikipedia.org/wiki/Theodolite (Accessed on September 24, 2012)
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was rotated through its range of motion and the center mark stayed within the laser dot, 

confirming the center location. Also, spirit levels were placed on the turntable as it was rotated 

through its range of motion. The turntable rotation plane was level within +/–20 seconds (based 

on one division of the spirit levels). 

The Theodolite was placed such that its rotation axis was coincident with the turntable axis, and 

the Theodolite was leveled. There are optic components in the Theodolite that assist with this 

position adjustment. A target, comprised of finely printed vertical lines, was taped to the 

downstream turning vanes. The target was approximately 111 feet from the Theodolite; see 

Figure 7.2-1 and Figure 7.2-2. 

Figure 7.2-1. Overhead Sketch of the Turntable Angle Measurement System 

Figure 7.2-2. Photograph of the Turntable Measurement Setup 
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The turntable was commanded to the start position. The Theodolite was set on the target, and the 

readout was zeroed. The turntable was then commanded to a new position. Once the motion 

stopped, the Theodolite was used to reacquire the target, and the absolute angle was read and 

recorded. This was repeated through several yaw runs. 

The turntable was rotated in a unidirectional manner from positive to negative; see Table C-1, 

Appendix C. The turntable was then rotated in a unidirectional manner from negative to 

positive, as shown in Table C-2, Appendix C. This should be the best-case measurements as the 

slop or backlash in the drive system is removed. Also, a random run was performed such that the 

slop or backlash of the drive system was not minimized; see Table C-3, Appendix C. 

Several observations were noted while acquiring the data. There was a 0.08- to 0.10-degree 

offset between the set point angle and the feedback angle. This could probably be resolved with 

a setting in the control software. The average error for all runs was –0.08 degrees, and the 

standard deviation in the error for all runs was approximately 0.03 degrees. The angle errors are 

highest at the highest angles, as shown in Figure 7.2-3. 

Figure 7.2-3. Set Point Error versus Turntable Angle 

The average of the error when zero was commanded was 0.005 degrees. The turntable angle 

accuracy appears to be much better than initially anticipated and was not a significant contributor 

to the uncertainty in the aerodynamic coefficient calculations, at least when tested at ambient 

room temperature conditions without a temperature gradient or cold test section. 

Setpoint error

-0.1000

-0.0800

-0.0600

-0.0400

-0.0200

0.0000

0.0200

0.0400

0.0600

0.0800

0.1000

-25.00 -20.00 -15.00 -10.00 -5.00 0.00 5.00 10.00 15.00 20.00 25.00

Setpoint error
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7.3 Uncertainty Analysis 

The total error estimate in a measurement is the combination of systematic (β) and random (ε) 

errors. Systematic error is that portion of the total error that remains constant in repeated 

measurements throughout the test. Random error is that portion of the total error that varies 

randomly in repeated measurements throughout the test and adds “scatter” to the data. The 

relationship between the two error sources is depicted in Figure 7.3-1. The random error 

component is seen as a distribution centered about the mean of the measurement. The systematic 

error is an offset of this mean from the true (unknown) value. In this analysis, the random error 

distribution is assumed to be approximately normal (Gaussian). 

Figure 7.3-1. Relationship between Two Error Sources 

The uncertainty analysis of the existing and proposed new external lower balance system was 

based on a representative semi-span wing test case. To calculate system uncertainties, force and 

moment estimates were chosen based on historical data from a semi-span wing model geometry 

with ice shapes consistent with IRT testing. The model has a chosen wing area (S) of 5.8 square 

feet, semi-span (b/2) of 4.5 feet, and AOA (α) of 12 degrees. The lift coefficient (CL) is 

approximately unity with a drag coefficient (CD) of approximately 0.14. The chosen operating 

dynamic pressure (q) is approximately 100 psf. Load cell component loads were calculated 

using the representative aerodynamic forces and moments, balance geometry, and load cell 

configuration. 

7.3.1 Analysis of the Existing IRT Lower Balance System 

A simplified uncertainty analysis was performed for the existing external lower balance system. 

The uncertainty in force and moment coefficients was estimated. This estimate is later compared 

with a similar set of estimates for the recommended concept number four. This comparison 
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allows the IRT personnel to generate an understanding of the impact an upgrade will have on 

their future aerodynamic test data. 

7.3.1.1 Overview and Assumptions 

Full-span model force measurement testing in the IRT requires the simultaneous use of an upper 

and lower balance system. Using both of these balance systems for full-span model testing will 

generate unique uncertainties for specific models. These uncertainties must be developed for 

each full-span model. It was decided that using only the floor balance in the uncertainty analysis 

would generate enough information to gain confidence in system upgrade decisions. Therefore, 

the existing upper balance system was not included in this analysis. The following assumptions 

were used to perform the uncertainty analysis for the external lower balance system. 

• Force and moment coefficients were functions of the independent variables AOA (α), 

dynamic pressure (q), and the three primary load cells (RA, RB, and RC) only. 

• Sources of uncertainty associated with the data acquisition and signal conditioning 

system were assumed to be small compared with the aforementioned sources and 

negated. 

• Thermal effects were not considered in this analysis and may be significant. 

• The Taylor-Series Method (TSM) of combining multiple sources of systematic error with 

the assumption of negligible correlations is adequate for comparison purposes 

[refs. 1 and 3]. 

• Elemental systematic error source contributions are estimated by manufacturer’s 

specifications and/or available calibrations. 

• A 95-percent confidence level was chosen in keeping with engineering practice and as 

recommended in the references. 

• A single semi-span test case was chosen as representative for this analysis. 

• Actual wind tunnel test data were not made available. Since random error estimates are 

best computed with repeated measurements using the entire FMS (end to end), they are 

not included in this analysis but could be accounted for later if/when data become 

available. The method for inclusion of random uncertainty contributions to overall 

uncertainty is provided. 

7.3.1.2 Layout of IRT Lower Balance System 

The lower external balance metric frame is supported vertically by three flexures. Force on the 

frame is reacted in the axial direction through the axial load cell RA. Load cells RB and RC are 

located 1.25 feet west (xB) and east (xC) of the balance center and measure normal force (N) and 

pitching moment (PM). The schematic diagram shown in Figure 7.3-2 shows the location of the 

load cells and links with respect to the metric frame. A dashed outline shows the location of a 

semi-span wing model. The metric frame rotates with the turntable through the AOA. 
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Figure 7.3-2. Current IRT-FMS Schematic Diagram 

7.3.1.3 Data Reduction Equations 

Force component resolution is shown in the vector diagram in Figure 7.3-3. Forces and 

moments from load cells are calculated assuming that tension in the load cell link causes a 

positive force. 

Figure 7.3-3. Forces and Moments on the Model and Aerodynamic Coefficients 

RA

RB RC

Flow Direction

North

South

East

+ α

Balance Center

Metric Frame
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The resulting data reduction equations (DREs) are summarized below. The normal (N) and axial 

(A) forces are combined in resultant (R). R is resolved into component forces lift (L) and drag 

(D). 

cos( ) sin( ) ( )cos( ) sin( )

sin( ) cos( ) ( )sin( ) cos( )

B C

A

BC A

BC A

BB CC

NR R

A R

LN A R R R

DN A R R R

PM R x R x

αα α α

αα α α

= +

=

= − =+ −

= + =+ +

= −

The CD, CL, and pitching moment coefficient (Cm) are calculated. S is the wing area; q, the 

dynamic pressure; and c, the chord length. 

( )sin( ) cos( ) BC A

D

D RR R C

qS qS

+ + α α = =

( ) cos( ) sin( ) BC A

L

L RR R C

qS qS

+ − α α = =

BB CC

m

PM Rx Rx C

qSc qSc

− = =

7.3.1.4 TSM Uncertainty Analysis 

Consider a general case where an experimental result r is a function of j measured variables Xj, 

called the DRE. 

r = r(X1, X2, X3,..., Xj) 

If the measured variables Xj may be assumed independent, then the overall uncertainty Ur in the 

result r may be expressed as: 

2 2 2

r r r U = b + s

where the overall uncertainty is partitioned into systematic uncertainty contributions (br) and 

random uncertainty contributions (sr). When they are defined at the standard deviation level, 

they are called the standard values and are combined using a root sum square as shown above. 

The systematic standard uncertainty contribution may now be written as the sum of contributions 

from the elemental sources. 

2

1

2

2

Xi

j

i i

r b

X

r b = 















∂

∂ =
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The overall systematic standard uncertainty may be found best through estimating the individual 

elemental source contributions (b) using calibration results and manufacturer’s specifications and 

combining them. Using the TSM and the DRE for the existing balance, the systematic standard 

uncertainty in a given force coefficient (CF) may be found as: 

1

2 2 2 2 2 2

FA B C

F F F FF

CR R R q

ABC

C C C CC bb b b bb

RRR q α α

     ∂ ∂ ∂ ∂∂      

= + + ++                 ∂ ∂ ∂ ∂∂        

All elemental source contributions Xi b are stated at the standard deviation level. 

The random error contribution to the overall measurement is best estimated experimentally. For 

instance, replicate CD measurements will allow calculation of a standard deviation. 

The general methodology outlined here is applicable to all of the measured forces and moments. 

Individual source values and partial derivative terms are tabulated in Appendix A. The 

uncertainty calculations for CD are given below as a sample. 

Starting with the DRE for CD: 

( )sin( ) cos( ) BC A

D

D RR R C

qS qS

+ + α α = =

Partial derivative terms are calculated next: 

2

cos

sin

cos cos sin

sin sin cos

D

A

D D

B C

D CBA

D CBA

C

R qS

C C

R R qS

C RRR

qS

C RRR

q qS

α

α

α αα

α

αα α

∂ = ∂

∂ ∂ = = ∂ ∂

∂ + − = ∂

∂ + + = − ∂

The total systematic standard uncertainty in CD may be found as: 

1

2 2 2 2 2 2

DA B C

D D D DD

CR R R q

ABC

C C C CC bb b b bb

RRR q α α

     ∂ ∂ ∂ ∂∂      

= + + ++                 ∂ ∂ ∂ ∂∂        
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The random and systematic uncertainty estimates are combined for an estimate of the total 

standard uncertainty. At this time, sufficient data were not available for random uncertainty 

estimates. At such a time that data are available, the random and systematic standard uncertainty 

contributions may be combined as shown below. 

2 2 2

CD CD CD u = b + s

A confidence interval about the sample mean value of CD is formed using the expanded 

uncertainty U. Expanded uncertainty values are found using the product of a coverage factor and 

the total standard uncertainty (which by definition is at the standard deviation level). The 

recommended coverage factor is the student’s t statistic for the appropriate degrees of freedom 

(DOF). 

CD U = t ⋅u

Using a 95-percent confidence interval and assuming that the large sample assumption is 

satisfied (DOF > 30), the coverage factor may be assumed as 2. For adjustment of the coverage 

factor for small samples, see reference 1. 

x U95 = 2 ⋅u

Individual CD results may now be reported with a confidence interval using the mean value 

bounded by the confidence interval +/–U. Using the method described here under the 

assumptions outlined allows the user to report a bound on a mean drag coefficient (CD ). This is 

the interval within which the true result resides with the chosen confidence of 95 percent. 

CD = CD ±U95

7.3.1.5 Overall IRT Existing Lower Balance System Uncertainty Results 

The systematic uncertainty expressed as an expanded value (e.g., CD 2b ) for each of the measured 

coefficients is tabulated in Table 7.3-1. Detailed calculations for all tables can be found in 

Appendix A. The percent of nominal value expresses the expanded systematic uncertainty as a 

percent of the nominal coefficient at the given representative conditions. 

 

Table 7.3-1. Coefficients and Estimated Uncertainty of the Current FMS 

CD CL Cm

Expanded Systematic Uncertainty 0.005 0.024 0.024 

Nominal Coefficient Value 0.147 1.02 –0.083 

% of Nominal 3.54 2.32 28.3 
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Using the available system information, an analysis revealed a system uncertainty value for the 

Cm to be 28 percent. While there were no clear uncertainty requirements for system 

acceptability, it is evident that a 28-percent error is not desired. However, indications from the 

available system calibration led the team to believe that an updated set of data could reduce this 

uncertainty. 

7.3.1.6 Possible Improvements for the IRT Existing Lower Balance System Uncertainty 

Results 

Using the available data, an uncertainty analysis revealed the major sources of error in the 

existing IRT balance as the component normal force load cell calibration accuracies. The 

performance of the existing balance could be improved by choosing load cells with improved 

accuracy. Commercially available load cells of the same capacity can offer 0.05 percent of fullscale accuracy and better versus the 0.2 percent currently in use. If the higher accuracy normal 

force load cells (0.05 percent) were used with the same semi-span test case of this analysis, the 

following uncertainties would result (see Table 7.3-2). To further consider component 

contributions to overall uncertainty, see Appendix A, where details are presented. 

Table 7.3-2. Coefficients and Estimated Uncertainty of the Current FMS with Updated Load Cells 

CD CL Cm

Expanded Systematic Uncertainty 0.002 0.006 0.006 

Nominal Coefficient Value 0.147 1.02 –0.083 

% of Nominal 1.22 0.582 7.07 

7.3.2 Analysis of the Proposed New IRT Lower Balance 

A simplified uncertainty analysis was performed for the recommended FMS, concept number 

four, and is described in Section 6.4.4. The uncertainty in the force and moment coefficients was 

estimated. This estimate is later compared with the previously generated set of estimates for the 

existing IRT lower balance system. This comparison allows the IRT personnel to generate an 

understanding of the impact an upgrade will have on their future aerodynamic test data. 

7.3.2.1 Overview and Assumptions 

The uncertainty in the force and moment coefficients was estimated under the following 

assumptions. 

• Force and moment coefficients were functions of the independent variables AOA (α) and 

dynamic pressure (q) and the measured components normal (N), axial (A), and pitching 

moment(PM) only. 

• Sources of uncertainty associated with the data acquisition and signal conditioning 

system were assumed to be small compared with the aforementioned sources and 

negated. 

• Thermal effects were not considered in this analysis. 
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• The TSM of combining multiple sources of systematic error with the assumption of 

negligible correlations is adequate for comparison purposes. 

• Elemental systematic error source contributions are estimated by manufacturer’s 

specifications and/or available calibrations of similar existing balance designs. 

• A 95-percent confidence level was chosen in keeping with engineering practice and as 

recommended in the references. 

• A single semi-span test case was chosen as representative for this analysis. 

7.3.2.2 Layout of Proposed Semi-span Style Monolithic Lower Balance System in IRT 

The monolithic lower balance system is mounted such that it rotates with the turntable through 

the AOA. The schematic diagram in Figure 7.3-4 shows the location of the balance as a shaded 

circle. A dashed outline shows the location of a semi-span wing model. 

Figure 7.3-4. Proposed Lower Monolithic Balance Layout 

7.3.2.3 Data Reduction Equations 

Force component resolution is shown in the vector diagram in Figure 7.3-5. The proposed 

monolithic balance design measures the normal (N), axial (A), and pitching moment(PM) 

components directly (i.e., forces/moments are not resolved through multiple load cells). 

RA

Flow Direction

South

North

West East

+ α

Balance Center
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Figure 7.3-5. Forces and Moments on the Model and Aerodynamic Coefficients 

The resulting DREs are summarized below. The normal (N) and axial (A) forces are combined 

in resultant (R). R is resolved into component forces lift (L) and drag (D). 

cos( ) sin( )

sin( ) cos( )

LN A

DN A

α α

α α

= −

= +

CD, CL, and Cm are calculated in terms of balance force components. S is the wing area; q, the 

dynamic pressure; and c, the chord length. 

sin( ) cos( ) DDN A C

qS qS

α α + = =

cos( ) sin( ) LLN A C

qS qS

α α − = =

m

PM C

qSc =
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7.3.2.4 TSM Uncertainty Analysis 

The TSM approach is identical to that discussed in Section 7.3.1.4. Using the TSM and the DRE 

for the new balance, the systematic standard uncertainty in the CD may be found as: 

The general methodology is applicable to all of the measured forces and moments. Individual 

source uncertainty values and partial derivative terms are tabulated in Appendix A. The 

uncertainty calculations for the CD are given below as a sample. 

Starting with the DRE for CD: 

Partial derivative terms are calculated next. 

The systematic standard uncertainty in CD may be found as: 

 

The random and systematic uncertainty estimates are combined for an estimate of the total 

standard uncertainty. At this time, sufficient data were not available for random uncertainty 

2

1

2 2 2 2

































∂

∂  + 

 





∂

∂  + 

 





∂

∂  + 

 





∂

∂ = q

D D

A

D

N

D

C b

q

C b C b

A

C b

N

C b D α α

qS

N A

qS

D CD

sin(α) + cos(α) = =

q S

N A

q

C

qS

C N A

A qS

C

N qS

C

D

D

D

D

2

sin cos

cos sin

cos

sin

α α

α α

α

α

α

+ = − ∂∂

− = ∂∂

= ∂

∂

= ∂

∂

2

1

2 2 2 2

































∂

∂  + 

 





∂

∂  + 

 





∂

∂  + 

 





∂

∂ = q

D D

A

D

N

D

C b

q

C b C b

A

C b

N

C b D α α
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estimates. At such a time that data are available, the random and systematic standard uncertainty 

contributions may be combined as: 

2 2 2

CD CD CD u = b + s

A confidence interval about the sample mean value of CD is formed using the expanded 

uncertainty U. Expanded uncertainty values are found using the product of a coverage factor and 

the total standard uncertainty (which by definition is at the standard deviation level). The 

recommended coverage factor is the student’s t statistic for the appropriate DOF. 

CD U = t ⋅u

Using a 95-percent confidence interval and assuming the large sample assumption is satisfied 

(DOF > 30), the coverage factor may be assumed as 2. For adjustment of the coverage factor for 

small samples, see reference 1. 

x U95 = 2 ⋅u

Individual CD results may now be reported with a confidence interval using the mean value 

bounded by the confidence interval +/–U. Using the method described here under the 

assumptions outlined allows the user to report a bound on a mean drag coefficient (CD ). This is 

the 95-percent confidence interval where the true result resides. 

CD = CD ±U95

7.3.2.5 Overall Proposed New IRT Floor Balance Uncertainty Results 

The systematic uncertainty expressed as an expanded value (e.g., CD 2b ) for each of the measured 

coefficients is tabulated in Table 7.3-3. The percent of nominal value expresses the expanded 

systematic uncertainty as a percent of the nominal coefficient at the given representative 

conditions. 

Table 7.3-3. Coefficients and Estimated Uncertainties of Proposed Single Piece Lower Balance FMS 

CD CL Cm

Expanded Systematic Uncertainty 0.002 0.005 0.005 

Nominal Coefficient Value 0.147 1.02 –0.083 

% of Nominal 1.45 0.495 6.00 

7.3.3 Conclusions 

In conclusion, the estimated uncertainties of various recommended FMS upgrades can be 

compared in Table 7.3-4. From the comparison chart, it can be seen that the greatest overall 
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uncertainty improvement will be generated by the team’s primary recommendation to replace the 

lower balance system with a new monolithic balance. By updating the normal force load cells, 

large uncertainty improvements can be made; however, this will not generate any throughput 

efficiencies that will be incurred using a monolithic balance system. 

Table 7.3-4. Estimated Uncertainties of Proposed FMS 

CD CL Cm

Current FMS Estimated 

Uncertainty % of Nominal 

3.54 2.32 28.3 

Updated FMS with Updated NF 

Load Cells % of Nominal

1.22 0.5817 7.07

Updated FMS with Single-Piece 

Balance % of Nominal 

(Similar accuracies are expected 

for concepts 1 through 4) 

1.45 0.495 6.00

Upgrading the FMS with a single-piece balance and a new turntable system will yield 

uncertainty estimates similar to upgrading with a single-piece balance. The main advantage of 

upgrading the turntable would be a more efficient test setup and increased test throughput 

efficiency. 

It should be noted that no data were provided for the measurement of dynamic pressure and the 

analysis uses an estimated value based on typical pressure transducer accuracies. The current 

axial load cell of the existing balance has an uncertainty value, which is expressed as a 

percentage of reading compared with the proposed balance, which is stated as a percentage of 

full scale. Therefore, the uncertainty in CD for higher drag test articles will increase for the 

existing balance as a percentage of nominal aerodynamic coefficient but decrease for the 

proposed new monolithic balance. While a thermal analysis was not possible, the monolithic 

design is expected to be more robust to thermal effects than a multi-piece balance design. 

7.4 Structural Review 

As part of the design trade and configuration evaluation, the existing structure was investigated 

to determine whether the load path of a new turntable or load balance should be tied to the tunnel 

structure or taken directly to ground. The team also assessed the breadth of structural 

modifications that would be required to accommodate the recommended design changes. Note 

that a structural stress analysis was not conducted as part of this assessment. The observations 

and recommendations in this section are only based on visual inspections of the existing 

structure, review of the drawings and existing analyses, and interviews with those familiar with 

the IRT operations. 
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7.4.1 Summary 

Recommendation 2, installing a new monolithic balance and a new turntable system, will yield 

the highest productivity and the most reliable approach. Structural analyses and modifications 

would be recommended to remove all unnecessary redundant structures and to start with the 

cleanest/simplest interface for a new modern model support system/turntable. The existing 

structure appears to be adequate so the modifications would primarily be cleaning up/simplifying 

and verifying the new load paths to ground. If Recommendation 3 is pursued, namely to install a 

modern monolithic balance on the top of the existing turntable, the upgrade would utilize the 

existing load path. Therefore, the upgrade should be adequate for the full established capacity of 

the wind tunnel unless it is limited by the capacity of the particular monolithic balance that is 

installed. Since there are no known problems with the existing structure, it would appear that the 

upgrade could be performed without structural modifications, except where necessary, to 

properly provide a mounting interface for the monolithic balance on top of the turntable 

cruciform. Also, since it utilizes the existing load path, a full stress analysis to ground would not 

be required. Recommendation 4 involves replacing the load cells within the existing balance 

system with more accurate loads cells. Load cell replacement does not involve a structural 

modification; therefore, no structural calculations or analysis should be required for this option. 

7.4.2 Background/Observations 

Primarily from inspecting the existing structural loads paths, it can be seen that there have been 

several past modifications to the load path and structure since the tunnel was originally 

constructed. In the original structural arrangement, metric loads were beamed through the tunnel 

structure and down through the floor of the area beneath the test section into an actual balance 

room, as shown in Figure 7.4-1. 
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Figure 7.4-1. Load Path Termination in Tunnel. The photo shows the former cylindrical balance 

columns now mounted to blocks of concrete. This is directly beneath the floor area that is below 

the test section/turntable/balance system. 

There appears to be evidence of several generations of modifications and changes since that 

time, including the complete removal of the balance room. However, the original vertical load 

paths are still present with the flexures at the top; these appear to be anchored to the floor area. 

It appears that with most of these layered updates and modifications, previous structural load 

paths and fixtures were not removed unless necessary, so the current structure appears to be 

redundant and, as a result, indeterminate. As stated above, because no issues with this existing 

load path are evident, a minimal cost upgrade could be implemented using this same philosophy 

without modifying the existing load path. However, it is recommended that the stress analyses 

be reviewed and perhaps a new stress analysis be initiated, including a finite element model 

(FEM) of the complete load path to ground. Due to the redundancy of the structural load path, it 

may also be desirable to add strain gauges to the existing structure in key locations to validate 

the results of the FEM. If an upgrade to the turntable is desired, then a full structural 

analysis/FEM would also be required since a new turntable would not be designed to attach to 

the current indeterminate, redundant load paths. 

The Recommendation 3 option to upgrade the existing system by installing a monolithic balance 

onto the existing turntable could require modification to both the turntable plate and supporting 

cruciform. In all, the modifications required appear to be relatively minor and should not 

diminish the load capability or significantly alter the existing load path of the system. Select 

support beams welded to the underside of the turntable plate could require removal to 

accommodate overhead installation of the monolithic balance. The tube spanning the distance 

from the cruciform to the turntable plate would also need to be removed. Further modification of 
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the cruciform would also be needed to provide a sufficient mounting surface for the balance. 

The portion of the supporting cruciform and its interface to the tube can be seen in Figure 7.4-2. 

Figure 7.4-2. Turntable Cruciform Base and Support Tube 

Installing the monolithic balance on top of the existing turntable cruciform would eliminate the 

labor- and time-intensive need to install the existing FMS balance assembly. This would 

significantly increase the accessibility to the interior of the test models; however, the IRT 

operations staff indicated the desire to remove more of the existing structure for even better 

access to the interiors of models mounted in the test section. Based only on the visual inspection 

of the existing structure and the highly redundant loads paths, this does not appear possible 

without a full structural analysis. A complete FEM could be exercised to completely identity 

many of the old structural remnants from past configurations that could be removed. This would 

most likely include the three vertical posts that originally transferred the loads to the long 

removed balance room, but still tie to the floor beneath the test section. The resulting 

structure/design could be optimized for maximum access to the test section or for other 

operational needs. A detailed FEM would also be required to pursue the Recommendation 2 
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option to mount a new monolithic balance on a new turntable. For this upgrade option, most if 

not all of the hardware/structure for the existing turntable and FMS would be removed, and the 

new system would be optimized for maximum model access and reliability. 

8.0 Findings, Observations, and NESC Recommendations 

8.1 Findings 

The following findings were identified: 

F-1. For the IRT, throughput/efficiency is equally or more important than force measurement 

accuracy. The focus on throughput and efficiency greatly increases the importance of 

certain aspects, such as model access during buildup, calibration setup and performance, 

installation and removal of the lower balance system, etc. 

F-2. As stated in Appendix B, the current calibration loads are not applied near the model 

center of pressure. Loading at the center of pressure generates calibration data that are 

similar to what will be experienced in wind tunnel testing. For best practices, one would 

“calibrate as you test.” 

F-3. Calibration loads for full-span and semi-span models are applied to a fixture that is 

removed and replaced by a model for wind tunnel testing, and no on-model check-loads 

are typically applied. 

F-4. Upper balance systems (multiple ranges) that are used for full-span model testing have 

monolithic construction and were designed to allow thermal expansion and contraction. 

F-5. The current calibration procedure has been inherited over the years, and the logic of why 

loads are applied as listed in the current load schedule has been lost. 

F-6. The largest source of uncertainty in measured aerodynamic force and moment 

coefficients when using the current FMS was found to be the normal force load cells 

stated calibration accuracies. 

F-7. The uncertainty analysis of the existing FMS for a typical semi-span wing test article 

reveals the following uncertainty values of the computed aerodynamic coefficients: 

CD: 3.5 percent 

CL: 2.3 percent 

Cm: 28 percent 

F-8. The existing turntable positioning accuracy is +/–0.03 degrees, measured at room 

temperature with no model loads. This accuracy is more than adequate to meet the 

facility needs of 0.05 degrees. However, no measurements or estimates were made to 

determine how this accuracy may change with loads, cold temperatures, or temperature 

gradients that likely exist during operation. 
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F-9. Based on interviews with the tunnel operations crew, the existing turntable and FMS 

structure makes it difficult to build up complex models with multiple pass-through lines. 

8.2 Observations 

The following observations were identified: 

O-1. The current FMS and turntable system contain a conglomeration of many generations of 

updates and modifications that have been implemented over the life of this facility, 

including partial removals, abandoned-in-place structures, and mechanisms from the 

previous updates and modifications. Although the current system is working and appears 

to have adequate positioning accuracy for the turntable, if a problem were to occur within 

this old system, it may be difficult to diagnose or repair, especially since those familiar 

with the last modification/update (about 20 years ago) have since retired. 

O-2. The existing turntable motion mechanism is an old system that actually uses multiple bolt 

shanks as the gear teeth for the ring gear and, although it appears to provide the desired 

accuracy, when measured at uniform ambient room temperature, it is most likely not as 

robust as more modern turntable mechanisms would be. 

O-3. Testing productivity would likely be increased with a faster rotating turntable that could 

reposition the model to the desired AOA more quickly. 

8.3 NESC Recommendations 

The following NESC recommendations are directed toward the IRT and Aeronautics Research 

Mission Directorate (ARMD) and are listed in the order of assessed priority. Recommendation 1 

is the minimum suggested action the IRT and ARMD should implement. Recommendation 1 

will improve both the throughput and the productivity during the calibration/check-load 

procedures for the current FMS and for any of the following FMS recommendations that may be 

implemented. Recommendation 2 results in the highest productivity, the highest accuracy, and 

the most reliable FMS, but it has the highest estimated cost. Recommendations 3 and 4 offer 

improvement, reliability, and lower estimated costs. However, the efficiency and test throughput 

will be less than for Recommendation 2. Table 8.3-1 is a comparison of the recommendations 

and includes cost estimates and benefit statements. It should be noted that the cost estimates 

include only design, manufacturing, and installation costs; no other criteria were incorporated 

into the estimates. 

R-1. Modify the calibration hardware to enable the ability to apply loads about wind tunnel 

model centers of pressure, which would require changing the check load/calibration 

procedures used at the facility, entail developing and documenting a new calibration 

schedule, and potentially involve developing new calibration spreadsheet software. 

 Modifying calibration procedures for the current FMS would include: 

a) Apply check-loads about the center of pressure of the wind tunnel model to ensure 

that the uncertainty of the FMS meets the test requirements. If the check-load 
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uncertainties in this step meet the test requirements, then steps b, c, and d are not 

required. 

b) For semi-span testing, perform an in-situ calibration applying loads about the center 

of pressure. 

c) Perform in-situ model calibrations when testing a full-span model. 

d) Document any new calibration methodology, including the logic of why loads are 

applied in specific circumstances. (F-2, F-3, F-4, F-5) 

R-2. Incorporate a new model support and turntable system with a new monolithic balance 

with continued use of the existing upper balance systems for full-span testing to attain the 

maximum desired results of force measurement accuracy and throughput. (F-1, F-4, F-5 

F-6, F-7, F-9, O-1, O-2) 

R-3. If R-2 is not implemented, then develop a new monolithic balance mounted on the 

existing turntable with continued use of the existing upper balance systems for testing 

full-span models to improve in the calibration throughput, increase system force 

measurement accuracies, and improve productivity. (F-1, F-4, F-5 F-6, F-7, F-8, F-9) 

R-4. If neither R-2 or R-3 are implemented, then replace the normal force load cells with 

higher accuracy (0.05 percent or better full-scale) models with continued use of the 

existing upper balance systems for full-span testing. This improvement represents the 

most cost-effective method to reduce the uncertainty of the aerodynamic coefficients to 

CD, 1.22 percent; CL, 0.582 percent; and Cm, 7.07 percent. (F-6, F-7, F-8)
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Table 8.3-1. Cost Range Estimates for FMS Upgrade Recommendations 

Recommendation # Design Cost Range ($) Manufacturing and 

Installation Cost 

Range ($) 

Benefit 

R-1 40K to 50K 60K to 170K Significant increase in 

calibration knowledge at 

low cost (calibrate as 

you test), and significant 

increase of end-to-end 

system confidence in 

uncertainties. 

R-2 185K to 200K 640K to 735K Significant increase in 

accuracy and most 

significant increase in 

throughput. The system 

can be designed with a 

focus on minimizing 

model change effort. 

R-3 105K to 135K 300K to 400K Significant 

improvement in 

accuracy and throughput 

with medium 

expenditure. 

R-4 10K to 15K 25K to 40K Significant increase in 

accuracy with low cost 

(no throughput 

benefits) 

Two examples of how to interpret/combine the cost estimates follow: 

1. If the IRT were to implement R-1, the total cost estimates would be generated by adding 

the Design Cost Range with the Manufacturing and Installation Range to get a total 

estimated cost range of $100K to $220K (low cost equals $40K + $60K; high cost equals 

$50K + $170K). 

2. If the IRT were to implement R-1 and R-2, the total cost would be generated by adding 

the total cost ranges of R-1 with those of R-2. R-2 would have a total estimated cost 

range of $825K to $935K (low cost equals $185K + $640K; high cost equals 

$200K + $735K). Therefore, the total estimated cost range to implement both R-1 and 

R-3 would be $925K to $1,155K (low cost equals $100K (from example 1 

above) + $825K; high cost equals $220K (from example 1 above) + $935K). 

A similar methodology would be used to calculate the estimated cost ranges for a combination of 

R-1 and R-3 or for a combination of R-1 and R-4. 
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9.0 Alternate Viewpoints

There were no alternate viewpoints identified within the NESC team or the NRB quorum during 

the course of this assessment. 

10.0 Other Deliverables 

There are no other deliverables for this assessment. 

11.0 Lessons to Learn 

LL-1. A thorough analysis of perceived component uncertainties is always prudent. While the 

group consensus centered on poor turntable positioning performance, actual surveys 

showed otherwise (supported by F-8). 

LL-2. When operating and calibrating a test and measurement system such as this IRT and the 

associated force balance system, it is important to include an expert in the type of systems 

being used either on a regular basis as part of the operations team or, as a minimum, to 

occasionally review and optimize the related procedures. In this case, the experts on the 

NESC assessment team have presented Recommendation 1, which highlights several 

improvements in the calibration process that both simplify the process and improve the 

overall calibration accuracy. These include both applying the calibration loads to as near 

the center of pressure as possible and recognizing that the full-span model support 

load/force distribution is dependent on the model stiffness and, therefore, should be 

calibrated on the actual model or something that simulates the model stiffness instead of 

the rigid calibration beam that is currently used. 

LL-3. When incorporating a model pass-through (e.g., air lines, wiring, pressure measurement 

lines), it is best to incorporate long “soft” lines paralleling the balance system. This way, 

the balance system carries the majority of the loads as it is designed to do. 

LL-4. It is good practice to apply the maximum loads on the model that are expected in a wind 

tunnel test to get a high confidence of the system uncertainties and to check any passthrough for unacceptable fouling. Note that this is often overlooked during test 

preparation. 

LL-5. It is wise to find instrumentation experts to act either as advisors or as smart buyers for 

program and facility major instrumentation purchases and upgrades. Purchasing 

instrumentation systems without expert advice can be costly to NASA and can result in 

delivery of a less-than-optimal system or one that does not actually meet program needs. 

12.0 Recommendations for NASA Standards and Specifications 

No recommendations for NASA standards and specifications were identified as a result of this 

assessment. 
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13.0 Definition of Terms 

Corrective Actions Changes to design processes, work instructions, workmanship practices, 

training, inspections, tests, procedures, specifications, drawings, tools, 

equipment, facilities, resources, or material that result in preventing, 

minimizing, or limiting the potential for recurrence of a problem. 

Finding A conclusion based on facts established by the investigating authority. 

Lessons Learned Knowledge or understanding gained by experience. The experience may 

be positive, as in a successful test or mission, or negative, as in a mishap 

or failure. A lesson must be significant in that it has real or assumed 

impact on operations; valid in that it is factually and technically correct; 

and applicable in that it identifies a specific design, process, or decision 

that reduces or limits the potential for failures and mishaps, or reinforces a 

positive result. 

Observation A factor, event, or circumstance identified during the assessment that did 

not contribute to the problem, but if left uncorrected has the potential to 

cause a mishap, injury, or increase the severity should a mishap occur. 

Alternatively, an observation could be a positive acknowledgement of a 

Center/Program/Project/Organization’s operational structure, tools, and/or 

support provided. 

Problem The subject of the independent technical assessment. 

Proximate Cause The event(s) that occurred, including any condition(s) that existed 

immediately before the undesired outcome, directly resulted in its 

occurrence and, if eliminated or modified, would have prevented the 

undesired outcome. 

Recommendation An action identified by the NESC to correct a root cause or deficiency 

identified during the investigation. The recommendations may be used by 

the responsible Center/Program/Project/Organization in the preparation of 

a corrective action plan. 

Root Cause One of multiple factors (events, conditions, or organizational factors) that 

contributed to or created the proximate cause and subsequent undesired 

outcome and, if eliminated or modified, would have prevented the 

undesired outcome. Typically, multiple root causes contribute to an 

undesired outcome. 

14.0 Acronyms List 

A Axial 

AIAA American Institute of Aeronautics and Astronautics 

AOA Angle of Attack 
NASA Engineering and Safety Center 

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CD Drag Coefficient 

CF Force Coefficient 

CL Lift Coefficient 

Cm Pitching Moment Coefficient 

D Drag 

DOE Design of Experiment 

DOF Degree of Freedom 

DRE Data Reduction Equation 

FEM Finite Element Model 

FMS Force Measurement System 

GRC Glenn Research Center 

GSFC Goddard Space Flight Center 

IRT Icing Research Tunnel 

JSC Johnson Space Center 

KSC Kennedy Space Center 

L Lift 

LaRC Langley Research Center 

MSFC Marshall Space Flight Center 

MTSO Management Technical Support Office 

N Normal 

NESC NASA Engineering and Safety Center 

NRB NESC Review Board 

PM Pitching Moment 

q Dynamic Pressure 

R Resultant 

TIM Technical Interface Meeting 

TSM Taylor-Series Method 

15.0 References 

1. Coleman H. W. and Steele, G. W.; Experimentation, Validation, and Uncertainty Analysis 

for Engineers, 3rd ed., 2009. 

2. “Assessment of Wind Tunnel Data Uncertainty,” AIAA S-071A, 1999. 

3. “Test Uncertainty,” ASME PTC 19.1-2005. 

4. Montgomery, D. C.; Design and Analysis of Experiments, Eighth Edition, 2012. 

16.0 Appendices

Appendix A. Simplified Uncertainty Analysis Calculations 

Appendix B. Calibration and Operation of the Force Balance System in the NASA-Glenn 6-ft 

by 9-ft Icing Research Tunnel 

Appendix C. Turntable Measurement Data 
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Appendix A. Simplified Uncertainty Analysis Calculations 

A.1 Existing IRT Lower Balance System 

Uncertainty in CL, starting with the DREs for CL: 

( ) cos( ) sin( ) BC A

L

L RR R C

qS qS

+ − α α = =

Partial derivative terms are calculated: 

2

sin

cos

( )sin cos

( ) cos( ) sin( )

L

A

L L

B C

L BC A

L BC A

C

R qS

C C

R R qS

C RR R

qS

C RR R

q qS

α

α

α α

α

α α

∂ − = ∂

∂ ∂ = = ∂ ∂

∂ −+ − = ∂

∂ −+ + = ∂

The absolute standard systematic uncertainty in CL may be found as: 

1

2 2 2 2 2 2

LA B C

L L L LL

CR R R q

ABC

C C C CC bb b b bb

RRR q α α

     ∂ ∂ ∂ ∂∂      

= + + ++                 ∂ ∂ ∂ ∂∂        

Uncertainty in Cm, starting with the DREs for Cm: 

BB CC

m

PM Rx Rx C

qSc qSc

− = =

 
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Partial derivative terms: 

2

m B

B

m C

c

m BB CC

C x

R qSc

C x

R qSc

C Rx Rx

q q Sc

∂ = ∂

∂ − = ∂

∂ −+ = ∂

The systematic standard uncertainty in Cm may be found as: 

1

2 2 2 2

mB C

mmm

CR R q

B C

CCC bb b b

RR q

    ∂∂∂    

= ++             ∂∂ ∂      

Table A-1. Systematic Standard Uncertainty Estimates (b) for Existing Balance using 

Representative Semi-span Icing Model 

Description Symbol Nominal Units

b (1 Std 

Dev) Comments

Load cell for axial RA –40 lbs –0.08

Stated at 0.4% of reading by 

calibration supplied by IRT.

Load cell for normal RB 275 lbs 5

Stated at 0.2% of FS = 5000

by calibration supplied by 

IRT.

Load cell for normal RC 325 lbs 5

Stated at 0.2% of FS = 5000

by calibration supplied by 

IRT.

AOA α 12 deg 0.035

From turntable survey 

performed in this study.

Dynamic pressure q 100.8 psf 0.0101

No uncertainty given, chosen 

as representative. 

Wing planform area S 5.8 ft

2 const

Arm B xB 1.25 ft const

Arm C xC 1.25 ft const

Chord c 1.29 ft const

(b*PD)^2 (b*PD)^2 (b*PD)^2

dCD/dRA = 0.0016731 1.79148E-08 dCL/dRA = -0.0003556 8.09395E-10 - - -

dCD/dRB = 0.0003556 3.1617E-06 dCL/dRB = 0.0016731 6.99796E-05 dCm/dRB = 0.0016588 6.87942E-05

dCD/dRC = 0.0003556 3.1617E-06 dCL/dRC = 0.0016731 6.99796E-05 dCm/dRC = -0.0016588 6.87942E-05

dCD/dalpha = 1.0180710 3.86765E-07 dCL/dalpha = -0.1464510 8.00342E-09 - - -

dCD/dq = -0.0014529 2.14479E-10 dCL/dq = -0.0100999 1.03647E-08 dCm/dq = 0.0008228 6.87942E-11

Drag Lift Pitching Moment

Partial Derivative (PD) Partial Derivative (PD) Partial Derivative (PD)
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A.2 Proposed New Semi-span IRT Floor Balance 

Uncertainty in CL, starting with the DREs for CL: 

Partial derivative terms: 

2

cos

sin

sin cos

cos( ) sin( )

L

L

L

L

C

N qS

C

A qS

C N A

qS

CN A

q qS

α

α

α α

α

α α

∂ = ∂

∂ − = ∂

∂ − − = ∂

∂− + = ∂

The systematic standard uncertainty in CL may be found as: 

1

222 2 2

L

L LLL

CN A q

CCCC bb bbb

NA q α α

      ∂∂∂∂ 

= +++             ∂ ∂∂ ∂    

Uncertainty in Cm, starting with the DREs for Cm: 

m

PM C

qSc =

Partial derivative terms: 

2

1 m

m

C

PM qSc

C PM

q q Sc

∂ = ∂

∂ − = ∂

The systematic standard uncertainty in Cm may be found as: 

1

2 2 2

m

m m

C PM q

C C bb b

PM q

    ∂ ∂ 

= +         ∂ ∂    
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Table A-2. Systematic Standard Uncertainty Estimates (b) for New Semi-span Balance 

using Representative Semi-span Icing Model

Description Symbol Nominal Units 

b (1 Std 

Dev) Comments 

Axial A –40 lbs 0.4 Stated at 0.08% of FS = 1000* 

Normal N 600 lbs 1.5 

Stated at 0.03% of FS = 

10000* 

Pitching moment PM –62.5 ft-lbs 3.75 Stated at 0.05% of FS = 7500* 

AOA α 12 deg 0.035 

*From similar NASA LaRC 

balance calibrations 

from turntable survey, 

performed in this study 

Dynamic pressure q 100.8 psf 0.0101 

No uncertainty given, chosen 

as representative 

Wing planform area S 5.8 ft2 const 

Chord c 1.28889 ft const 

 

 

(b*PD)^2 (b*PD)^2 (b*PD)^2

dCD/dA = 0.0016731 4.4787E-07 dCL/dA = -0.0003556 2.02349E-08 - - -

dCD/dN = 0.0003556 2.84553E-07 dCL/dN = 0.0016731 6.29817E-06 - - -

- - - - dCm/dPM = 0.0013271 6.19148E-06

dCD/dalpha = 1.0180710 3.86765E-07 dCL/dalpha = -0.1464510 8.00342E-09 - - -

dCD/dq = -0.0014529 2.14479E-10 dCL/dq = -0.0100999 1.03647E-08 dCm/dq = 0.0008228 6.87942E-11

Drag Lift Pitching Moment

Partial Deriv Partial Deriv Partial Deriv
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Appendix B. Calibration and Operation of the Force Balance System 

in the NASA-Glenn 6-ft by 9-ft Icing Research Tunnel 
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Appendix C. Turntable Measurement Data 

Table C-1. Unidirectional Positive to Negative2

True Angle Set Point Set Point Error Feedback Feedback Error 

18.43 18.50 0.07 18.41 –0.02 

16.94 17.00 0.06 16.91 –0.03 

14.95 15.00 0.05 14.91 –0.04 

12.97 13.00 0.03 12.91 –0.06 

10.97 11.00 0.03 10.91 –0.06 

9.00 9.00 0.00 8.92 –0.08 

6.99 7.00 0.01 6.92 –0.07 

4.99 5.00 0.01 4.91 –0.08 

3.99 4.00 0.01 3.91 –0.08 

3.00 3.00 0.00 2.91 –0.09 

2.00 2.00 0.00 1.91 –0.09 

1.50 1.50 0.00 1.41 –0.09 

1.01 1.00 –0.01 0.92 –0.09 

0.50 0.50 0.00 0.40 –0.10 

0.00 0.00 0.00 –0.08 –0.08 

–0.49 –0.50 –0.01 –0.58 –0.09 

–0.98 –1.00 –0.02 –1.08 –0.10 

–1.49 –1.50 –0.01 –1.58 –0.09 

–1.99 –2.00 –0.01 –2.08 –0.09 

–2.98 –3.00 –0.02 –3.08 –0.10 

–3.98 –4.00 –0.03 –4.08 –0.11 

–4.98 –5.00 –0.02 –5.08 –0.10 

–6.97 –7.00 –0.03 –7.08 –0.11 

–8.95 –9.00 –0.05 –9.08 –0.13 

–10.94 –11.00 –0.06 –11.08 –0.14 

–12.95 –13.00 –0.05 –13.08 –0.13 

–14.93 –15.00 –0.07 –15.08 –0.15 

–16.93 –17.00 –0.07 –17.08 –0.15 

–18.91 –19.00 –0.09 –19.08 –0.17 

 

2

 True Angle - the true angle of the turntable as read by the Theodolite. 

Set Point - Position angle commanded by the wind tunnel control system. 

Set Point Error - difference between the set point and true angle. 

Feedback - the encoder reading of the position of the turntable. 

Feedback error - the difference between the encoder reading and true angle. 
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Table C-2. Unidirectional Negative to Positive3

True Angle Set Point Set Point Error Feedback Feedback Error 

–18.43 –18.50 –0.07 –18.58 –0.15 

–16.94 –17.00 –0.06 –17.09 –0.15 

–14.95 –15.00 –0.05 –15.09 –0.14 

–12.98 –13.00 –0.02 –13.10 –0.12 

–10.99 –11.00 –0.01 –11.10 –0.11 

–9.00 –9.00 0.00 –9.09 –0.09 

–7.00 –7.00 0.00 –7.09 –0.09 

–5.02 –5.00 0.02 –5.09 –0.08 

–4.02 –4.00 0.02 –4.09 –0.07 

–3.01 –3.00 0.01 –3.10 –0.09 

–2.01 –2.00 0.01 –2.10 –0.09 

–1.52 –1.50 0.02 –1.59 –0.07 

–1.02 –1.00 0.02 –1.10 –0.08 

–0.51 –0.50 0.01 –0.59 –0.08 

0.00 0.00 0.00 –0.08 –0.08 

0.48 0.50 0.02 0.41 –0.07 

0.98 1.00 0.02 0.90 –0.08 

1.49 1.50 0.01 1.40 –0.09 

1.98 2.00 0.02 1.90 –0.08 

2.99 3.00 0.02 2.90 –0.09 

3.98 4.00 0.02 3.90 –0.08 

4.98 5.00 0.02 4.91 –0.07 

6.97 7.00 0.03 6.91 –0.06 

8.96 9.00 0.04 8.90 –0.06 

10.95 11.00 0.05 10.90 –0.05 

12.95 13.00 0.05 12.90 –0.05 

14.94 15.00 0.06 14.90 –0.04 

16.93 17.00 0.07 16.90 –0.03 

18.91 19.00 0.09 18.90 –0.01 

 

 

3

True Angle - the true angle of the turntable as read by the Theodolite. 

Set Point - Position angle commanded by the wind tunnel control system. 

Set Point Error - difference between the set point and true angle. 

Feedback - the encoder reading of the position of the turntable. 

Feedback error - the difference between the encoder reading and true angle. 
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Table C-3. Random Run4

True Angle Set Point Set Point Error Feedback Feedback Error 

0.00 0.00 0.00 –0.09 –0.09 

–18.91 –19.00 –0.09 –19.09 –0.18 

0.00 0.00 0.00 –0.08 –0.08 

18.91 19.00 0.09 18.90 –0.01 

0.00 0.00 0.00 –0.08 –0.08 

–14.95 –15.00 –0.05 –15.09 –0.14 

–0.01 0.00 0.01 –0.08 –0.07 

14.94 15.00 0.06 14.90 –0.04 

0.00 0.00 0.00 –0.08 –0.08 

–9.96 –10.00 –0.04 –10.08 –0.12 

–0.01 0.00 0.01 –0.09 –0.08 

9.96 10.00 0.04 9.91 –0.05 

–0.01 0.00 0.01 –0.08 –0.07 

–6.98 –7.00 –0.02 –7.08 –0.10 

–0.01 0.00 0.01 –0.09 –0.08 

6.97 7.00 0.03 6.91 –0.06 

–0.01 0.00 0.01 –0.09 –0.08 

–4.99 –5.00 –0.01 –5.09 –0.10 

–0.01 0.00 0.01 –0.09 –0.08 

4.98 5.00 0.02 4.91 –0.07 

–0.01 0.00 0.01 –0.08 –0.07 

–2.00 –2.00 0.00 –2.08 –0.08 

0.00 0.00 0.00 –0.07 –0.07 

1.99 2.00 0.01 1.90 –0.09 

–0.01 0.00 0.01 –0.09 –0.08 

–1.01 –1.00 0.01 –1.09 –0.08 

0.00 0.00 0.00 –0.07 –0.07 

0.98 1.00 0.02 0.91 –0.07 

–0.01 0.00 0.01 –0.08 –0.07 

1.98 2.00 0.02 1.90 –0.08 

–2.00 –2.00 0.00 –2.08 –0.08 

4.98 5.00 0.02 4.90 –0.08 

–4.99 –5.00 –0.01 –5.09 –0.10 

9.96 10.00 0.04 9.91 –0.05 

–9.96 –10.00 –0.04 –10.08 –0.12 

17.93 18.00 0.07 17.91 –0.02 

–17.94 –18.00 –0.06 –18.08 –0.14 

–0.01 0.00 0.01 –0.09 –0.08 

 

4

 True Angle - the true angle of the turntable as read by the Theodolite. 

Set Point - Position angle commanded by the wind tunnel control system. 

Set Point Error - difference between the set point and true angle. 

Feedback - the encoder reading of the position of the turntable. 

Feedback error - the difference between the encoder reading and true angle. 
REPORT DOCUMENTATION PAGE Form Approved

OMB No. 0704-0188

2. REPORT TYPE 

Technical Memorandum

 4. TITLE AND SUBTITLE

Icing Research Tunnel (IRT) Force Measurement System (FMS)

5a. CONTRACT NUMBER

 6. AUTHOR(S)

Roberts, Paul W.

 7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES)

NASA Langley Research Center

Hampton, VA 23681-2199

 9. SPONSORING/MONITORING AGENCY NAME(S) AND ADDRESS(ES)

National Aeronautics and Space Administration

Washington, DC 20546-0001

 8. PERFORMING ORGANIZATION

 REPORT NUMBER

L-20207 NESC-RP-10-00632

10. SPONSOR/MONITOR'S ACRONYM(S)

NASA

13. SUPPLEMENTARY NOTES

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Unclassified - Unlimited

Subject Category - 09 Research and Support Facilities (Air)

Availability: NASA CASI (443) 757-5802

19a. NAME OF RESPONSIBLE PERSON

STI Help Desk (email: help@sti.nasa.gov)

14. ABSTRACT

An Electronics Engineer at the Glenn Research Center (GRC), requested the NASA Engineering and Safety Center (NESC) 

provide technical support for an evaluation of the existing force measurement system (FMS) at the GRC's Icing Research 

Tunnel (IRT) with the intent of developing conceptual designs to improve the tunnel's force measurement capability in order to 

better meet test customer needs. This report contains the outcome of the NESC technical review.

15. SUBJECT TERMS

NASA Engineering and Safety Center; Force measurement system; Icing Research Tunnel; Wind tunnel

18. NUMBER

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73

19b. TELEPHONE NUMBER (Include area code)

(443) 757-5802

a. REPORT

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b. ABSTRACT

U

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3. DATES COVERED (From - To)

July 2010 - October 2012

5b. GRANT NUMBER

5c. PROGRAM ELEMENT NUMBER

5d. PROJECT NUMBER

5e. TASK NUMBER

5f. WORK UNIT NUMBER

869021.05.07.08.01

11. SPONSOR/MONITOR'S REPORT

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NASA/TM-2012-217784

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The public reporting burden for this collection of information is estimated to average 1 hour per response, including the time for reviewing instructions, searching existing data sources, 

gathering and maintaining the data needed, and completing and reviewing the collection of information. Send comments regarding this burden estimate or any other aspect of this 

collection of information, including suggestions for reducing this burden, to Department of Defense, Washington Headquarters Services, Directorate for Information Operations and 

Reports (0704-0188), 1215 Jefferson Davis Highway, Suite 1204, Arlington, VA 22202-4302. Respondents should be aware that notwithstanding any other provision of law, no person 

shall be subject to any penalty for failing to comply with a collection of information if it does not display a currently valid OMB control number.

PLEASE DO NOT RETURN YOUR FORM TO THE ABOVE ADDRESS.

1. REPORT DATE (DD-MM-YYYY)

01- 11 - 2012