May 2008, Vol. 5, Issue 59

Integrated Systems

SectorEngineering

INSIDETechnology 1Heritage 3Processes, Tools& Training 4People 6

Gunfire Loads Are Simulated in A-10 Fatigue TestingBy BRYAN DUTTON (ISWR), SEBASTIAN GRASSO (ISER), KEN GRUBE (ISER), and JAMES VAUGHAN (ISWR)

If you walked by or worked near the El Segundobuilding 202 Structures Test Lab in January orFebruary, you couldn’t help hearing what soundedlike a Gatling gun firing at a very high speed. AirForce representatives who witnessed the testingsaid it sounded like the real thing.

A full scale fatigue test of the USAF A-10A air-craft is currently being performed by NorthropGrumman in order to certify the aircraft for anadditional lifetime of service. The GAU-8 30 mmGatling gun mounted in the fuselage nose fires3900 rounds per minute and is an integral part ofthis weapon system. In addition to flight maneu-ver spectrum testing, requirements for additionallife certification in-cluded a simulation ofgun loads in the fullscale test under condi-tions that accountedfor the long termeffects of gunfirerecoil and counterrecoil loads and actualdynamic response ofthe structure. A spec-trum of simultaneous-ly applied axial andlateral loads derivedfrom previous flighttest measurementswas applied for nearlyone million cycles.Dynamic replicationof the gunfire recoilloads on a full scaleairframe test articlewas a technical chal-

The Leading Edge is published for the employeesof Engineering inIntegrated Systems.

Editorial submissions are subject to edit andapproval by NorthropGrumman Corp.

The Leading Edgepromptly corrects errors of fact. Please submit corrections to: nichole.cabral@ngc.com

EXECUTIVE EDITORSGiorgio Accolti-GilJohn CaskoBob DevoeFrank FloresShannon JonesBob MartinGlenn Masukawa

EDITORSector Engineering Staff

CONTRIBUTORS Bryan DuttonSebastian GrassoKen Grube Diane HensleyAmanda Muller

PRODUCTIONGraphic Media Design310-332-9222

Please direct comments or story ideas to:nichole.cabral@ngc.com

lenge that was met and successfully achieved byNorthrop Grumman A-10 IPT team members fromISWR El Segundo and ISER Bethpage.

Unique load application and reaction test fixtureswere designed and installed to execute the gun fir-ing fatigue load breakout test. The reaction fixturingincluded longitudinal fixed load reaction linkslocated at the aircraft pylons at the wing root and alateral load reaction fixture attached to the lowerfuselage skin in close proximity to the lateral loadintroduction location. The test article rested onthree fixed vertical supports located where the air-craft is balanced during flight spectrum testing. Theflight spectrum load cylinders were left attached but

Figure 1. Gun Fire Test Fixture – General Arrangement

see A-10 FATIGUE TESTING, Page 2

MAY 20082

A-10 Fatigue Testing (from page 1)

hydraulically isolated, preventing inadvertent load applica-tion. Both longitudinal and lateral loads were applied simul-taneously to the gun attach points at 60 hertz to simulate therecoil and counter-recoil loads imparted during actual gun fir-ing. The load introduction equipment and test fixtures areshown in Figure 1.

The complexity of performing a dynamic load applicationtest at the high frequencies representative of actual gun firerates with synchronized multiple load inputs (longitudinal andlateral) required an in-depth detailed dynamic analysis be per-formed to specify unique load application equipment and acontrol system capable of meeting the test requirements.

NGC engineers worked with the application engineeringstaff at Team Corporation to procure two purpose designedservo-actuators to apply the axial and side loads. The actua-tors have close coupled accumulators and an integral high flowservo-valve. Figure 2 shows the actuator setup for the lateralload application; the longitudinal load is similar. The 3 stageflapper nozzle valve arrangement shown was needed to sup-port the required loads application rates of 60 hertz with

T E C H N O L O G Y

load was isolated and applied in only one axis. Applied loadswere measured and controlled via feedback from calibratedload cells installed at the points of load application to ensurethat the correct loads were input to the critical aircraft struc-ture. Applied actuator loads were reacted by counterbalancemasses suspended on air bearings on free standing supportframes, mitigating dynamic load transfer to the facility struc-tural load frame and reducing total system deflections.

Accurate control of applied test loads presented a significantengineering challenge due to the required test frequency andpotential dynamic response of the aircraft structure and loadreaction system. The solution was the utilization of TeamCorporation supplied high fidelity load introduction actuatorsand hydraulic system components and the implementation of aFCS/Moog control system that had the ability to select controlbetween force and position. The control system’s seamlesstransition between position and force allowed for primary con-trol feedback on position with the secondary outer loop con-trol closed on load to aid the stability and accuracy of theapplied loads. The FCS/Moog proprietary FasTEST® wave-form replication software contained complex closed loop algo-rithms that performed transfer function iterations to reduceerror signal and allow optimization of the desired waveform.

The applied load spectrum and profile was determinedfrom historical A-10 operational and flight test data. Therequired number of test cycles was determined from survey ofactual fielded aircraft gun usage data and was reduced torounds fired per flight hour. The loads were applied in“bursts” of 100 rounds with counter recoil loads applied in themiddle of each gun burst (the recoil load was performed in themiddle rather than the end of each burst so the support fix-tures would not have to “catch” the inertial mass after the ten-sion load was applied). A total of 982,730 load cycles wereapplied, demonstrating gun support structure strength for asecond service life. A typical representation of applied gun-fire loads is shown in Figure 3 indicating an average cyclicfrequency of 60.6 hertz. ■

Figure 2. Gun Fire Test – Lateral Load Test Actuator

Figure 3. Gun Fire Test – Applied Loads

potential displacements of +/-0.25 inches. The valves arecomprised of a Moog pilot valve and an industrial flapperservo valve capable of approximately 5 GPM. This flappervalve is the primary input to a Team Corporation slave stageservo valve capable of 40 GPM.

The longitudinal load was applied at the nose of the aircraftthrough a simulated gun barrel that accommodates two TeamCorporation hydrostatic spherical couplings. The specified12,620 peak load value was measured at the aft gun mount fit-ting. The peak lateral load of 4,340 pounds was applied at thesimulated aft gun mount through a hydrostatic spherical cou-pling and split fitting that ensured all applied lateral load wasreacted through the LHS of the fuselage frame at FS 238.00.The hydrostatic spherical couplings ensured that the dynamic