Aerospace and aircraft structures research at the National Research Council of Canada (NRC)

Article Type: Technical Paper From: International Journal of Structural Integrity, Volume 1, Issue 3

Introduction

National Research Council of Canada (NRC) Aerospace is pleased to be hosting the International Committee on Aeronautical Fatigue (ICAF) Conference and Symposium from May 29 to June 3, 2011, in Montreal, Canada. The theme for this conference will be structural integrity: influence of efficiency and green imperatives. As a lead up to this event, NRC Aerospace has prepared a brief overview of aerospace in Canada, and aerospace and aircraft structures research at the NRC, including a detailed look at NRC’s notable full-scale fatigue test of the F/A-18 wing. For more information on ICAF 2011, visit the web site: www.icaf2011.org

Aerospace in Canada

The aerospace industry is one of Canada’s major industrial success stories, growing from near obscurity 30 years ago to a position among world leaders today. Canada is now among the top five nations in the world for aerospace, behind the USA, the UK, France and Germany, with an aerospace sector that employs more than 80,000 Canadians and earns more than $23 billion a year. Canada’s aerospace industry is also one of only a handful of fully integrated aerospace industries in the world, with complete capability from design and development through to manufacture and after-sales service.

Canada’s huge size, rugged landscape and sometimes punishing weather have all pushed the aerospace industry to overcome these challenges and produced some great Canadian success stories. The first-ever crash position indicator was developed in Canada, as were the popular Dash 8 commuter transports and the niche-market regional jet, both of which were conceived to meet a geographical need.

These breakthroughs were also developed with the help of researchers from the NRC Institute for Aerospace Research in its various incarnations. NRC’s aerospace laboratories offer unique facilities and expertise that the Canadian aerospace industry can use for the development and testing of new innovations and products. It is a system that works well, with research partners making excellent use of NRC facilities and equipment, including nine research aircraft(fixed wing and rotary), eight wind tunnels, engine test cells, a full-scale structural test rig and a manufacturing technology centre (Plate 1).

Plate 1

In fact, since its inception as the National Aeronautical Establishment (NAE)in 1951, the NRC Institute for Aerospace Research has been a focal point for aerospace research excellence in Canada. NRC has developed innovative technologies, supported the needs of Canada’s aerospace industry and helped Canada gain world leadership in niche aerospace markets, such as regional and commuter aircraft, civilian helicopters and small gas turbine engines, for more than 50 years.

NRC Aerospace research and technology development

The NRC Institute for Aerospace Research (and now called NRC Aerospace to encompass aerospace expertise at NRC beyond the institute) today is truly Canada’s National Aerospace Laboratory, undertaking and promoting research and technology development to support the Canadian aerospace community in matters affecting the design, manufacture, performance, use and safety of aerospace vehicles and related products.

With laboratories primarily in Ottawa and Montreal, augmented by NRC research and business facilities across Canada, NRC Aerospace maintains an internal research program and undertakes research and technology development contracts,consulting, fee-for-service testing, and calibrations for hundreds of companies and organizations from around the world. It develops and transfers technology through collaborative research contracts and licensing arrangements. NRC Aerospace actively seeks out clients, collaborators and licensees for its services, technologies and expertise.

NRC Aerospace maintains expertise in and operates national facilities in five areas of interest:

• 1.

  Aerodynamics. NRC Aerospace engages in research and provides services to clients in fixed- and rotary-wing aerodynamics, and in the aerodynamics of surface vehicles and ground-based structures. Sophisticated computational fluid dynamics tools coupled with high-quality data from NRC’s eight productive wind tunnels provide cost-effective support for aerodynamic research and product development. A significant capability also exists to study the effects of icing on the performance of aircraft, helicopters and cables(Figure 1).

• 2.

  Aerospace manufacturing technology. While maintaining high levels of quality, reliability and performance, NRC Aerospace investigates and develops aerospace manufacturing technologies that are expected to yield significant cost savings. These technologies include metal and composite forming, joining,machining and finishing technologies for both aircraft and engine components, as well as automation and robotics for assembly and surface treatments.

• 3.

  Flight research. Using NRC’s nine research aircraft, NRC Aerospace performs airborne research and flight tests, explores flight mechanics and avionics technologies, and provides expertise in aircraft safety related systems and accident investigations. Its Flight Recorder Playback Centre provides complete capabilities for the playback and analysis of cockpit voice recorders, digital and analogue flight data recorders, air traffic control tapes and videotapes.

• 4.

  Gas turbine research. In compliance with increasingly stringent environmental, safety and operational requirements, NRC Aerospace has unique facilities and expertise to assist industry in developing and evaluating the performance of gas turbine engines and components, including icing conditions(Plate 2).

• 5.

  Structures and materials performance. NRC Aerospace conducts research on issues affecting the design, strength, durability, structural integrity and performance of new and legacy aircraft structures and components. It develops new design, analysis and manufacturing technologies for structures and materials, as well as for noise and vibration control. NRC Aerospace also researches technologies to support existing aircraft fleets, such as non-destructive inspection (NDI) and better coatings.

Plate 2

In addition, as the dividing lines between disciplines disappear more and more, NRC Aerospace is drawing on complementary expertise within the rest of NRC to advance aerospace R&TD. This expertise includes physicists with expertise in nanotechnology to develop nanocomposites,biologists with expertise in marine biology to develop algal biofuels, and chemical engineers with expertise in battery technology to develop aircraft fuel cells, among others.

This exchange of expertise and ideas between disciplines within the same organization allows rapid technological advancement in new and exciting areas. It also breaks down barriers to communication and lowers the cost of cross-disciplinary research. No other single organization in Canada offers industry coordinated access to this level of collaboration and wide-ranging research infrastructure.

At the same time, NRC Aerospace offers training, coaching, mentoring, access to state-of-the-art facilities, and opportunities for students at all levels to work alongside top-notch researchers and research teams. NRC Aerospace also has access to a number of NRC-sponsored programs to provide students the opportunity to develop their skills and expertise, while at the same time, gaining valuable experience working with research groups or leaders in Canadian government laboratories and research institutions.

For more information on NRC Aerospace, visit the web site: www.NRCaerospace.gc.ca. For more information on student employment programs, visit the web site: www.nrc-cnrc.gc.ca/eng/careers/programs/employment-programs.html

Structures research at the NRC

Full-scale aircraft structural testing was already underway in the Structures Section of the Department of Mechanical Engineering when the original NAE was created in 1951, and this technology has continued to be a fundamental part of the program. However, the original structures laboratory gradually expanded into other areas, including numerical structural analysis, repair technologies and life assessment methodologies. Laboratory staff was at the forefront of developments in finite element (FE) techniques in the early 1960s and provided support to Canada’s Department of National Defence and Canadian Industry. The laboratory still maintains extensive FE capabilities, but has significantly decreased its involvement in FE development (Plate 3).

Plate 3

The laboratory also developed methods for inferring aircraft trajectories from debris fields in response to a tragic crash of a DC-8 aircraft in Ste. Therese, Quebec, in 1963. When an aircraft has broken up in flight, the trajectories of the various pieces can provide valuable clues about the cause of the crash.

In addition, the laboratory was one of the first in the world to recognize the value of fracture surface analysis, which relies on the fine features of a fracture surface to diagnose the mechanisms of crack growth and the causes of component failure. It developed an international reputation for its work in failure analysis using transmission electron microscopy.

Over time, the laboratory expanded further into materials and recruited materials-science specialists to conduct detailed studies of the microstructure of metallic material and metallic composites, and achieved international prominence for work on nickel-based super alloys. These studies found application in corrosion and fatigue work, in development of structurally tailored materials, and in bonding and repair of composite and metallic structures.

Structural integrity

The current NRC Aerospace Structures and Materials Performance Laboratory(SMPL) demonstrates compliance with applicable structural integrity regulations including validation of crack growth models for primary metallic structure. Crack growth rates can be measured by several techniques including the use of crack growth gauges, ladder gauges, potential drop, visual inspection, video monitoring and the use of state-of-the-art non-destructive testing (NDT) tools. These measurements can be further supported by post-test fractographic studies using techniques such as striation counting. By measuring crack propagation rates in critical areas and correlating these against both calculated and measured cyclic stress levels, it is possible to validate the predictive capability of these models.

Demonstration of regulatory requirements is based upon obtaining sufficient data for certification of the aircraft, component, detail or element through an acceptable level of analytical correlations. The airframe and, in particular,critical or novel details are typically instrumented with sensors to provide detailed information about the stresses imparted into these areas. This information can then be correlated with the performance of these areas throughout the test. This knowledge is used to predict the performance of such locations in derivatives with modifications and greatly enhances confidence in associated inspection regimes, providing a sound basis for the certification of derivative airframes or aircraft modifications (Plate 4).

Plate 4

A full-scale test often involves demonstrated compliance with regulations for typical repairs and allowable damage limits. Damage reporting and disposition procedures for aircraft structures generate reports documenting cracks or damage upon their discovery. In some instances, the discovered cracks and damage are monitored until they have extended to a size/level to demonstrate adequate damage tolerance and to ensure an adequate interval between detection and criticality. Repairs may then be introduced through disposition reports that are carried out by qualified airframe technicians, consistent with maintenance practices planned for fleet operation.

In order to demonstrate that there is no widespread fatigue damage (WFD),post-test teardown inspections supported by fractography are tools that help identify occurrences of WFD. These inspections are supported with robust NDI techniques capable of identifying the small cracks characteristic of this type of damage. Any teardown of areas where WFD might occur uses best shop practices to minimize the likelihood of the effects of disassembly masking the presence of small cracks.

In parallel with any full-scale test program is the development of NDT procedures for specific aircraft locations. NRC has qualified inspectors and researchers to develop NDT techniques for the inspection of critical or damage prone locations of aircraft, including metallic parts covered by composite wing skins and all composite parts. NRC can develop test inspection reference pieces simulating cracked or corroded metallic areas on an as required basis and carries out probability of detection evaluations.

At the end of durability and damage tolerance testing, NRC demonstrates the residual strength of the structure. The Structures Group of SMPL conducts static residual strength testing to determine whether the airframe is able to meet and exceed limit load requirements after natural damage has developed and grown during repeated load lifetime testing and/or the introduction of artificial damage in vulnerable locations of critical structure.

The Structures Group has experienced researchers, engineers and technologists to develop test plans, document results and carry out experimental stress analysis, FE analysis and lifting evaluations of critical areas. They also demonstrate the functionality and reliability of structural health monitoring(SHM) technologies for damage detection, loads monitoring and component monitoring and sensing designed for in-service applications. The full-scale fatigue test facility is also used to validate SHM techniques. NRC has been involved in a major multi-faceted program, Holistic structural integrity process integrating SHM and DTA for several years and is currently evaluating a number of sensors for potential use in SHM systems on NRC-developed SHM test beds. These structural test beds are available to companies interested in evaluating their SHM system and sensor capabilities.

Composites

In 1984, a composites facility was created, quickly establishing an international reputation. Early fundamental work concentrated on certification methodologies, influences of moisture and thermal exposure on structural performance, and the effect of impact damage. During this early work, NRC Aerospace partnered with Bombardier (then Canadair) and Transport Canada to conduct a major investigation of the technologies and manufacturing methods needed to use composite materials for a business jet wing.

The program later expanded to include the use of composite patches for repair, advanced manufacturing techniques based on closed-loop autoclave control systems, and out-of-autoclave processes such as RTM and its variants. With the use of advanced sensors, cure modelling remains among the structures laboratory’s fundamental thrusts.

In 2004, the composite research expanded with the creation of the NRC Aerospace Manufacturing Technology Centre in Montreal. The centre’s aim was to develop core competencies and demonstrate modern methods of aerospace manufacturing technologies that have the potential for significant cost savings,while also maintaining high levels of quality, reliability and performance.

Working with Bell Helicopter, Bombardier Aerospace and a small Canadian company, Composites Atlantic Limited, NRC Aerospace researchers from the manufacturing technology centre and SMPL developed an advanced manufacturing process using automated fiber placement and unique bonding procedure to create an all-composite helicopter tail boom that is simpler to construct, less costly and more durable than its predecessors. NRC structures researchers in Ottawa have statically tested the helicopter tail boom at elevated temperature beyond ultimate load and carried out design concept static and fatigue element testing and composite fuselage joining prototyping investigations. Currently, the Structures Group is fatigue testing a prototype tail boom at elevated temperatures to determine the durability of the fiber placement tail boom.

Life extension

Over the last 15 years, NRC Aerospace has become a world-recognized expert in life extension methodologies and technologies for both airframe and gas turbine engines. On the airframe side, because of the economic pressures to extend both civil and military aircraft beyond their original design lives, NRC Aerospace recognized the necessity to understand a number of phenomena that affected the integrity of these airframes that had accumulated flight times and cycles to unprecedented levels.

NRC Aerospace has established itself as a world leader in enhanced visual inspection technology and in the quantification of both fatigue and corrosion damage. Through partnerships and contracts with military organizations and US original equipment manufacturers (OEMs), progress is being made that will increase flight safety and reduce cost of ownership. Novel repair methods, such as composite patching and retrogression and re-aging, are examples of this progress.

SMPL also maintains a unique capability to evaluate engine degradation processes through examination of failed parts and through accelerated testing. This understanding has been used to develop effective repair processes. Examples of success include advanced coatings for erosion prevention and a highly successful blade repair and certification program done in partnership with Orenda, Inc. and the Canadian Department of National Defence. The scope of this program is expanding to include thermal barrier coatings and nano-layered coatings. Work is also accelerating in the development and repair of advanced single crystal alloys.

More than 50 years of experience in the structures group culminated in the International Follow-On Structural Test Program (IFOSTP) from 2001 to 2005(described in detail below). When the Canadian forces acquired the CF-18 aircraft in the early l980s, it was immediately used in roles outside the original design specifications. The Hornet aircraft manufactured by McDonnell Douglas (now Boeing), was designed to meet US Navy fatigue design and test requirements through fatigue testing representative of two severe lifetimes. Owing to differences between what was tested and the operational service missions flown by the Canadian forces, the Department of National Defence was concerned with the fatigue mechanisms involved, and commissioned NRC Aerospace researchers to work with their Australian counterparts, the Royal Australian Air Force (RAAF) and the Australian Defence Science and Technology Organization, to ensure that the aircraft’s useful life was not compromised. Canada undertook additional full-scale testing of the aircraft fuselage and wing while Australia was evaluating the aft fuselage and empennage.

Subsequently, testing of the CF18 aircraft was carried out to three repeated load lifetimes during the IFOSTP to ensure safe operation of the major structural components to the originally projected cumulative flight hours at aircraft procurement. Since the original basis of certification used a safe life approach the aircraft structure is primarily managed using this philosophy. However, at the discretion of the Technical Airworthiness Authority some structural locations may be managed using inspection-based methods based on proven damage tolerance of the structure. The result, IFOSTP, was one of the longest running international programs, with high payoff for the air forces that use F-18 aircraft (Canadian Aeronautics and Space Journal, 2001).

For more information on NRC Aerospace structures and materials performance research, contact: aerospace@nrc.gc.ca

Project success story: full-scale fatigue test of the F/A-18 wing

From 2001 to 2005, the NRC Institute for Aerospace Research conducted active testing of FT-245, a full-scale fatigue test of the F/A-18 aircraft’s wing. The objective of this four-year test was to determine the economic life of the inner and outer wing box under representative loading that reflected typically more severe Canadian forces and RAAF usage.

Plate 5

As the most complex test undertaken by NRC-IAR to date, FT-245 posed a series of formidable challenges, which resulted in an array of innovative approaches to full-scale fatigue testing. These included:

• development of a service-based spectrum for an actively controlled aircraft,including buffet;

• aggressive spectrum editing techniques to reduce the number of load lines to a workable quantity; and

• a new method for calculating actuator loads more suitable for large numbers of actuators (Plate 5).

The team also worked closely with its test equipment supplier, MTS Systems Corporation, to extend the limits of the test equipment and software available at the time, including:

• incorporating optimally sized hydraulic components and developing low-mass fixturing to maximize test speed;

• developing a sophisticated trend monitoring system to automatically detect structural changes during testing; and

• developing an automated load checking system to document any missed end levels directly rather than having to store data at nominal end points for subsequent verification.

Service-based usage spectrum

A significant contributor to the complexity of the test was its extremely large usage spectrum. FT-245 was the first test in the world of an actively controlled aircraft based on in-service data rather than on simplistic design load assumptions. This in-service data were captured with F/A-18 aircraft’s Maintenance Signal Data Recording System, an on-board data-recording device that monitors and automatically records flight parameters, engine data, stores data,weapons data and seven channels of strain data at specified frequencies for specified events. Therefore, a time history of these parameters was available for every flight of every aircraft in the fleet.

Usage for FT-245 was defined as the average usage experienced by the squadron determined to be the “most severe.” A time history representative of one year of average flying for this most severe squadron was then developed with flights ordered to give the correct mission distribution with time. The maneuver loads spectrum was derived using an empirical parametric loads formulation (PLF)process developed at Bombardier Aerospace Defence Services (Hewitt et al.,1996). This process was based on knowledge of the aerodynamic loading actions and an analysis of measured flight loads data, and provides section loads as a function of flight parameters and control surface deflections. Since the control surface deflections are measured only once every five seconds, intermediate values were derived using the once-per-second flight parameter data and the flight computer control laws. Calculated loads were verified by comparing them against flight measured data for typical missions. Since the PLF method was able to predict loads at 10 Hz, this maneuver loads spectrum contained more than ten million lines.

Wings are also subjected to aerodynamic buffet, which adds large numbers of relatively low amplitude cycles to the maneuver loads. This was addressed (Weiss and Kroese, 2004) by characterizing the buffet in terms of angle of attack (AOA)and dynamic pressure (Q) and collecting time segments of flight test data for relevant wing loads in each of the AOA/Q bins. The flight test buffet data were separated from the maneuver data by filtering at 2 Hz. Buffet loads were reconstructed for the test spectrum based on the AOA/Q trace of the spectrum flights by selecting a length of buffet data from the database for the appropriate AOA/Q bin and adding it to the previously developed maneuver loads. The dynamic loads were captured at 483 Hz, so the addition of buffet to the spectrum increased the number of lines in the spectrum by an order of magnitude.

Turbulence loads were developed in a similar manner to buffet loads, with the exception that they were limited to very low AOAs and low altitude.

Aggressive spectrum editing

Comprising maneuver, buffet and turbulence loads, the raw FT-245 spectrum contained nearly 25 million load lines – all of which were different. Since it would have been impractical to apply such a large spectrum in a full-scale fatigue test, aggressive spectrum truncation was required to reduce the number of load lines (Hewitt et al., 2003).

Truncation of the spectrum was based on damage calculations for 12 control point loads that drive the critical area stresses. Initially, control points loads were selected based only on maneuver loads. However, when it was found that dynamic effects on the F/A-18 wing were significant, additional control points were added. Of the 12 control points, five were maneuver loads and seven were a combination of both dynamic and maneuver loads.

Ideally, the truncation level would have been set such that the increase in life for any load due to the truncation would be minimal. However, with such a large spectrum this was not entirely possible. While it was possible to keep the life increases for the primary maneuver-dominated loads to less than about 5 percent, life increases of up to about 50 percent had to be accepted for some of the buffet-dominated loads such as the trailing edge flap hinge moments. However, if buffet were not included, the life increase would be about 400 percent. Thus, even a 50 percent life increase provided a much more representative test than previous tests that did not include buffet.

Even with this relatively severe truncation, the sequence still contained nearly 150,000 end points, all of which were independent, while the maximum number of load conditions allowed at the time was only 50,000. Considering that the accuracy with which aerodynamic loads can be calculated or measured in a flight test article (no better than 5 percent), and the accuracy and repeatability with which they can be applied (perhaps 1 percent), it appeared that many of the load conditions would be essentially the same. Therefore, it made sense to group the load conditions into bins and then to apply an average load condition for all conditions in the bin.

This binning was performed based on the 12 control point loads used for truncation. The bin size for each load was initially set based on the accuracy of the calculated load, and then all bins were equally scaled until the number of occupied bins was less than 50,000. One load case from each bin was selected for application to the test, generally with near mid-bin loads. Fatigue calculations for each of the loads showed minimal changes in calculated fatigue lives.

Deriving actuator loads from panel loads

Achieving accurate loading on the large number of actuators required to meet the volume of load conditions demanded by the FT-245 spectrum required the NRC Aerospace engineering team to devise a new method for calculating actuator loads(Hiscocks et al., 2001) (Plate 6).

Plate 6

Loads matching is typically accomplished on a wing by matching shear, moment and torque at a number of stations along the wing. This works well as long as there are only two actuators in the chordwise direction. However, the high fidelity and large number of load conditions that must be matched for this test required the deployment of more actuators in the chordwise direction. In this more complex configuration, torque matching can be achieved through multiple combinations of actuator loads, thus giving rise to unrepresentative loading.

The shear, moment and torque diagrams used for loads matching are derived from the integration of aerodynamic and inertia panel loads. For FT-245, it made more sense to match these panel loads directly rather than using the shear,moment and torque quantities derived from them, which ultimately contain less information. To accomplish this, an optimization process was developed to minimize the errors in panel loading while constraining 22 resulting interface loads, such as wing root and wing fold bending moment, to within very close tolerances of the calculated values. The resultant actuator loads derived from this process are more sensitive to load distribution, ultimately providing a good match to the panel loads and excellent matches to interface loads.

High-fidelity, low-mass test rig

The general philosophy in designing the FT-245 test arrangement was that wherever loads were being introduced into the test specimen itself, other than through loading pads, they must be via the F/A-18 structure. This ensured that loads were being introduced in a representative manner. The test article itself comprised a right-hand inner and outer wing box. However, to ensure representative loading, the wing was mounted on a representative fuselage and featured representative flaps, ailerons and pylons. A left-hand inner wing was used to react the right-hand wing loads.

Reliable servohydraulic actuation was achieved through the deployment of optimally sized, high performance, equal-area actuators, load cells and hydraulic pumps supplied by MTS. A series of ten actuators was used to provide a representative bending moment distribution in the fuselage, particularly in the area of the wing lug attachments, in order to ensure both representative loading in the wing attachments and to minimize premature failure of the transition fuselage. The left-hand inner wing was loaded by six actuators via three contour boards at the inboard and outboard pylon stations and the wing fold. A further 47 actuators were used to load the inner and outer right-hand wing, control surfaces and stores for a total of 63 actuators.

Attachments to the wing and control surfaces utilized thin (50 mm diameter)aluminum disks bonded directly to the surface with either an acrylic or epoxy adhesive, depending upon the substrate. Lower in mass than typical steel backed neoprene pads, these aluminum disks effectively reduced fixturing weight without introducing significant stiffness in the specimen. This allowed increased testing speeds without the risk of introducing dynamic strain errors (Hewitt,2000). The placement of these pads also avoided installed fasteners to allow for increased access for fastener removal and inspection.

Sophisticated end level verification and trend monitoring

To address the enormous data storage and analysis demands of FT-245, the NRC Aerospace team collaborated closely with MTS to develop end level verification and sophisticated, new trend monitoring software (Hewitt and Nelson, 2002;Hewitt and Hiscocks, 2004).

This software was deployed on an MTS Aero-90 test system with fully integrated data acquisition and control, and enabled the automatic documentation of exceedances of channel peaks or valleys beyond user defined limits. This was far more efficient than recording the feedback on every channel at some nominal end point. Capable of continually monitoring the strain data from over 600 gauges on specific end levels and comparing them with individual limits for every channel for each load condition, the trend monitoring system notified an operator whenever any strains fell outside of defined limits based on previous history, indicating a change in the structure, test rig or loading system.

Ultimately, this capability resulted in earlier detection of structural degradation during FT-245 and helped reduce the amount of stored data by several orders of magnitude.

Test performance

The FT-245 test system described above has proved to be extremely reliable– no pad or linkage failures or major system problems have been experienced. The system has performed at an average speed of 1 block of 155,000 end levels about every four days – operating 24 hours/day – and yielded an average cycling rate of about 0.2 Hz.

The system also proved to be highly accurate. FT-245 faithfully replicated wing, control surface and fuselage problems experienced in-service by the Canadian forces and RAAF fleets, some of which were not found in the original F/A-18 testing. These new findings were due to the more severe, service-based FT-245 usage spectrum that was applied, which included more dynamic buffet loading than the OEM’s testing. Many of these problems were discovered with the aid of the new trend monitoring system.

Faster testing

Extended wing testing followed the completion of the FT-245 CF-18 wing test after revisions were made to the Canadian forces lifting policy for the aircraft. This testing targeted component areas that were subject to dynamic loads on the aircraft wing, and the new policy required fatigue testing to be carried out for five lifetimes to ensure these unmonitored dynamically loaded areas were certified. Therefore, the equivalent of five repeated load lifetimes was required to simulate the required flight hours of CF-18 usage (Plate 7).

Plate 7

These were applied during two separate tests that targeted the trailing edge surface attachments of the wing and the wing tip, where a missile launcher and store are mounted in about half the flights of the aircraft. The first test,FT-193, targeted the outer wing trailing edge surface aileron and actuator attachments, aft and rear spars, wing tip to wing tip launcher shear and torsion with constraints that required wing fold bending moment, shear and torsion to be accurately applied. The second test targeted the wing-side trailing edge flap outboard half-hinge and localized back-up structure of the inner wing in a separate test designated FT-169.

Owing to the nature of the loading, each of these tests had long spectra approaching one million lines. These were generated from the original 25 million line spectrum data block and truncated to eliminate small loads while retaining 90 percent of the damage content. In order to complete these tests in the desired time frame, NRC used a new control technology called cross-coupling compensation (C³) and MTS AeroST. This segment optimized feed-forward technology was developed by NRC under a collaborative research program with MTS Systems, and demonstrated a significantly increased test speed over the FT-245. Eight times the number of end point load cases was applied in FT193 as compared with FT-245, and testing was completed in 15 months versus four years. Note,however, that increased speed for full-scale testing is dependent on several factors, including the stiffness of the aircraft, the structural moving masses,and accuracy tolerances required on the target loads, which make optimum speeds difficult to predict prior to test tuning. Based on proof of concept testing during FT-193 testing, MTS has since incorporated this C³ technology into their newest control systems for use on current and future full-scale tests.

As analysts demand more and more data and load histories increase in length,test programs will undoubtedly continue to grow in complexity and sophistication. Given this, the NRC Aerospace innovative approaches to FT-245 full-scale structural testing should be instructive to all test laboratories facing extensive, service-based usage spectrums, the deployment of large numbers of actuators, and the enormous volumes of test data that the future of full-scale structural testing surely holds.

For more information on the NRC Aerospace full-scale fatigue test of the F/A-18 wing, contact: aerospace@nrc.gc.ca

Acknowledgements

The authors of this paper acknowledge that excerpts from this paper were published previously in the March 2005 issue of Aerospace Testing International that was jointly authored by Dr R. Hewitt and Mr R. Rutledge of the Structures and Materials Performance Laboratory at the Institute for Aerospace Research.

Jerzy KomorowskiNational Research Council, Ottawa, Canada