The first F-35 Joint Strike Fighter rolled off the assembly line in February at Lockheed Martin in Fort Worth, Texas, to usher in the first of twenty-two aircraft built for the current System Development and Demonstration phase of the multi-service fighter program. The aircraft, a conventional takeoff and landing variant, or F-35A, is formally designated 2AA-0001 (but shortened to AA-1 by most). Its primary purpose is to pave the way for subsequent F-35A Joint Strike Fighters.
This initial aircraft will also be the first F-35 to fly later in 2006. Before taking to the air, though, the aircraft must submit to a series of rigorous ground tests to show that components tested separately in laboratories can now operate as designed once integrated into an actual airframe. "Ground tests are essential to the success of a flight test program," notes Doug Pearson, Lockheed Martin vice president of the F-35 Integrated Test Force in Fort Worth and a former commander of the Air Force Flight Test Center at Edwards AFB, California. "We celebrate getting an aircraft off the ground for the first time. However, a thorough ground test program forms the foundation for that first flight."
Test Sequence
The ground test program begins once the AA-1 moves from the assembly line to the flight line where it will undergo a combination of aircraft and subsystem checkouts and flight readiness certification tests. The tests are logically sequenced to provide time for the results to be analyzed. "Most certification ground tests require a complete aircraft from a structural and onboard subsystems standpoint," explains John Korstian, site lead for the JSF Integrated Test Force in Fort Worth, "so it's difficult to perform the tests simultaneously. Documented results from the various tests will support a host of flight readiness activities as well."
Fuel System Checkout
Fuel is pumped into the aircraft for the first time during fuel system checkout. The tests, which take fourteen days to complete, are conducted in a fuel test facility designed specifically for the F-35. During the tests, the aircraft is parked on three large metal plates in the floor of the test facility with each of the three aircraft wheels resting on one plate. Each plate, which contains a scale, is attached to a separate hydraulic lift. The three independent lifting points allow the aircraft to be raised and lowered for measurements at various aircraft attitudes.
All functions of the fuel system are checked: fuel transfer, aerial refueling, and ground refueling and defueling. As the fuel moves through the system, laboratory engineers determine how much usable and unusable fuel the aircraft can hold in its wing and fuselage tanks as well as the gross weight and center of gravity of the aircraft at several deck angles. Measured results are compared to the cockpit fuel gauge readings. These tests will be done with internal fuel only because AA-1 will not be flown with external fuel tanks in the flight test program.
Every F-35 will go through a fuel system checkout as part of its flight preparations. Because AA-1 will be the first System Development and Demonstration F-35 aircraft to fly, its fuel system is tested more extensively. AA-1 ground testing involves more detailed verification of key performance characteristics, such as engine feed rates, fuel transfer rates, and refuel and defuel rates. The tests also include thorough cleaning and inspecting for foreign object debris by pumping fuel through the tanks, passing fuel through a series of check filters, and flushing any residual debris from the tanks. This fuel tank cleaning and inspecting process is repeated on every subsequent F-35.
Structural Coupling
Structural coupling, the interactions between the flight control system and the structural dynamics and aerodynamics of the airframe, is tested to validate that the F-35 flight control system does not interact with structural vibration modes of the aircraft. The phenomenon, induced by propagating sensors sending signals through the flight control system, can cause control surfaces to oscillate at dangerous and potentially damaging frequencies. These tests also require about fourteen days and are conducted in a run station on the flight line.
"Simply put, these tests involve observing how the flight control system reacts to aircraft motions," explains Tom Phillips, deputy director of the JSF Integrated Test Force in Fort Worth. "We generate motions on the aircraft with its own control surface actuators. As we start shaking the tail, for example, those motions are detected by the flight control system. Certain motions or combinations of motions can become amplified to the point that the airplane starts hopping about. We then develop filters within the flight control system to reduce the destabilizing effect of the structural modes on the motions sensed by the flight control sensors."
Structural coupling tests are performed on an aircraft that is as structurally complete as possible. External fasteners, panels, doors, and even the weight of a pilot are required to ensure a realistic mass distribution and center of gravity. The aircraft sits on its own landing gear as flight control surfaces are actuated by external means at various frequencies and amplitudes, or sweeps. The resulting output signals from the flight control sensors are compared to the input signals and then compared to predicted results.
Ground Vibration Tests
Ground vibration tests certify the airframe is resistant to flutter, a dynamic instability that can cause sudden, destructive vibration levels in an aircraft. Flutter characteristics of an aircraft design are established early in the development process using computer models of the airframe. These software-based predictions are then tested in wind tunnels with small-scale models. Ground vibration testing then validates this wind tunnel data. These tests, which require about twenty-five days, are also performed in a run station on the flight line. In addition to AA-1, ground vibration tests are accomplished on the test aircraft of each variant (CTOL, STOVL, and CV) that are instrumented to verify flutter resistance.
"An airframe behaves like a tuning fork," explains Phillips. "It can resonate at certain frequencies. Since these resonations can be destructive, we want to make sure that the airplane does not have a tuning fork mode in flight. We test for resonating modes for all the structures of the airplane: wings, tails, and fuselage. We design airplanes so they don't have these modes. Ground vibration tests validate those designs."
Mechanical shakers generate the vibrations. Each device resembles a large paint can with a small tube protruding from one end. The tube goes in and out of the can at prescribed frequencies. The testing planned for AA-1 requires up to six shakers, each attached to different places on the aircraft, wings, tails, and empennage. The aircraft is suspended from the ground during these tests. Up to 300 accelerometers are used to measure how the aircraft responds to the induced vibrations. The engine is slowly rotated during these tests to protect bearings, which can be deformed by the vibrations if left in one position. Measurements are taken with the aircraft full of fuel, empty of fuel, and at a flutter-sensitive mid-range fuel load. Flight control surfaces are actuated during these tests as well.
Integrated Power Package Runs
The integrated power package, which combines an auxiliary power unit, emergency power unit, and environmental control system, is tested for its ability to provide electrical power and cooling to the airplane during ground maintenance without the engine running. A test pilot starts the power package from the cockpit before the first engine start. These tests are performed at a run station on the flight line over a six-day period in which all operating modes are evaluated.
Engine Runs
The initial engine runs evaluate the start system, engine operations at power settings up to maximum afterburner, propulsion system integration with all aircraft subsystems, general operation of the aircraft on internal electrical and hydraulic power, and the thermal management system. The first engine start, called a green run, generates a white cloud of smoke from the nozzle of the F135 turbofan engine as preservation fluids are purged. This sight is normal, but startles many who have never seen it.
The aircraft is tied down to the ground for all operations above idle power because forces generated by high-power runs are well above the capability of the wheel brakes to hold. A holdback fitting is used to attach the aircraft's arresting hook to a metal tie-down embedded in the floor of the run station. A mobile mission control facility is used to monitor engine and aircraft systems operations through measurements telemetered from the aircraft. More than 500 measurements are relayed to the mobile facility from the aircraft instrumentation system during the tests.
To date, Pratt & Whitney's nine F135 ground test engines have accumulated more than 4,500 test hours. The testing has included engines for the F-35A conventional takeoff and landing, or CTOL, and F-35C carrier-based, or CV, variants, as well as the F-35B short takeoff/vertical landing, or STOVL, variant with its shaft-driven lift fan propulsion system. The first flight test F135 engine was delivered to Lockheed Martin in Fort Worth in December 2005 and was fit tested successfully in AA-1 in January 2006.
Proof Load And Flight Loads Calibration Tests
Proof load tests apply a variety of known loads to the airframe and verify that the structure can withstand the stresses. These tests validate the overall strength of the airframe to 100 percent of its design limit, or to the maximum load the aircraft would ever experience in flight. Flight loads calibration tests are performed along with the proof tests. These tests are used to calibrate strain gauges imbedded in the aircraft structure. Before these tests are completed, the airframe is cleared by structural analysis only, allowing it to be flown to forty percent of its design limit load. The proof load and flight loads calibration tests take about sixty days to complete and are performed in a structural testing facility in Fort Worth.
"The test is called a proof load test because a full-scale static test is not needed for the AA-1 airframe since it is structurally different from subsequent airframes," Korstian explains. "Subsequent F-35 airframes have been structurally optimized to reduce overall aircraft weight, so AA-1 is unique in this respect. AA-1 is eventually loaded to 100 percent of its design limit load to allow us to take the airframe to eighty percent of its design limit load during flight tests. Eighty percent is all we need to satisfy all flight test objectives planned for the first airframe."
The proof test fixture itself is an impressive structure. The thirty-foot-tall fixture is constructed of steel I-beams more common in skyscraper construction. A crane loads the aircraft into the fixture — minus engine and landing gear — and onto a raised platform at the center of the structure. The aircraft is held in place with specially designed fittings and a large structural plug in place of an engine. Hydraulically actuated rams and vacuum pads are attached to the aircraft at hundreds of locations, creating a multitude of beams, wires, and hoses. The forces applied to the airframe and the resulting stresses and strains are controlled and monitored through several banks of computers positioned in the fixture and in a nearby control room.
This fixture is used for the flight loads calibration tests of all F-35 variants, so it is specifically designed for adaptability. Its fifty-foot span allows the fixture to accommodate the increased wing size of the F-35C. The hydraulic control equipment surrounds the fixture. Hydraulic, vacuum, and electrical lines run into troughs embedded in the floor. Other features, specialized alignment tools, and the vacuum pad attachment system (which replaces time-consuming epoxy-based methods) provide a minimal amount of aircraft downtime.
Six full-scale test articles will later be tested using the same methods and facility used for proof load testing. "Full-scale static and durability test articles are planned for each variant," notes Keith Carlson, lead for F-35 flight loads calibration and proof testing. "These airframes, which will never fly, are tested to the full 150 percent design limit load, allowing subsequent development F-35s to fly to 100 percent of their design loads."
Electromagnetic Interference Safety-Of-Flight Test
The electromagnetic interference safety of flight, or EMI SOF, test is a relatively short certification test during which the electromagnetic interaction between the various electrical systems on the aircraft is evaluated. Electromagnetic signals from one system can interfere with the operation of other systems in unexpected and unpredictable ways. An extreme example would be interference from a UHF/VHF radio generating uncommanded motions in the flight control system. These tests require approximately four days and are conducted in a run station on the flight line. An EMI SOF test is done on every test aircraft in the current phase of the program. The tests, however, are not repeated on production aircraft.
Electrical systems evaluated in these tests include flight test instrumentation and telemetry systems, radios, navigation equipment, flight control system, environmental controls, and propulsion controls. A complex test matrix is used to evaluate electromagnetic compatibility of the systems and to identify electromagnetic interferences. Any interferences are evaluated and eliminated before flight. This test is sequenced as the last step in the ground test schedule to avoid having to repeat it for updated software loads.
Taxi Tests
Taxi tests begin with a slow walk and end with a fast run — up to 110 knots. The first test, the ramp test, involves the aircraft moving under its own power for the first time. The test, done at walking speeds, is used to check the steering and brakes. After the ramp test, the aircraft is taxied onto the runway at Naval Air Station Joint Reserve Base Fort Worth for runway taxi tests, planned for speeds of thirty, sixty-five, eighty-five, and 110 knots.
Objectives for these tests include symmetric and differential braking, steering with and without use of the nosewheel steering system, runway centerline tracking with the nosewheel steering system with brakes only and then with rudder only, brake anti-skid system operation, tail hook deployment, and maximum symmetrical braking from higher speeds. The tail hook is tested during these tests, too, should it be needed in an emergency during the flight test program.
Flight Clearance And Flight Authorization
After taxi tests, the aircraft is prepared for its first flight. The flight clearance provides approval for the aircraft to fly within a certain envelope for specified aircraft and software configurations. The F-35 Joint Program Office grants this flight clearance based on results from analysis, qualification tests, lab tests, and on-aircraft ground tests, including taxi tests. The clearance itself is a formal signed document that arrives after the successful completion of the taxi tests.
The Lockheed Martin chief engineer for F-35 then provides final notification of readiness to fly to the JSF Program Office. The test team receives a final clearance letter from the program executive officer for JSF, Navy RAdm. Steven L. Enewold. "The letter is the admiral's approval to us that he has reviewed all the data provided by his experts and they have all agreed the first F-35 is ready to fly," says Pearson. "The first flight of the F-35 will take place soon after we receive the reply. Then all the attention shifts to the flight test program."
Pearson emphasizes first flight is a beginning, not an end. "Thousands of extraordinary people have worked extraordinarily hard to get us to this point," he says. "The initial flight is but one of nearly 7,000 planned test sorties we expect to conduct over the next six years. Our work is just starting. Our objective is to field as quickly as possible an affordable, world-class weapon system that the United States and its allies can use for decades to come."
Kathleen Pai is in the communications leadership development program at Lockheed Martin.
