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Testing for combat

DEMONSTRATING THAT he F-22 is at least twice as effective in combat as the F-15, as required by contract, will require a combination of wargaming, simulation and flight testing. Computer modelling is being used to develop a statistical basis for the comparison, with almost 1 million simulated engagements already run. Wargaming provides unlimited threat densities, but involves no pilots or real equipment, so some 1,000 full-mission simulations will be run, involving up to six pilots versus 80 enemy aircraft and 80 ground threats. The final element will be flight-testing, which will include "several hundred" warlike scenarios.

Like every other aspect of the F-22 programme, flight-testing is integrated. The Combined Test Force (CTF) at Edwards AFB, California, brings industry and the Air Force together into a single operating team. The CTF director is Air Force, and has deputy directors from the contractor team and the Air Force Operational Test and Evaluation Center (AFOTEC). Under them are functional groupings of contractor and Air Force engineers.

"On previous programmes, industry and Air Force worked in parallel," explains John Peiper, F-22 flight-test manager and CTF deputy director. He cites an "outstanding effort" which has resulted in the agreement of joint procedures, under which testers will work with one set of instructions and one set of data. "Pratt & Whitney is integrated into everything. It's one of the team," Peiper adds.

 

Requirements defined

Flight-test requirement working groups for each discipline, made up of engineers from the contractor and Air Force air-vehicle teams, are defining their requirements with participation from the testers and Air Combat Command. "Working groups take discipline test requirements and convert them into flight-test instructions, real-time data requirements and post-flight analysis specifications," says Peiper.

Detailed flight-test plans are drawn up on test information sheets. Previous flight-test programmes have been researched to measure the "overhead" - the time added to that required to perform a test manoeuvre to allow for taxiing, climb, etc. "We try to make good use of lost time," says Peiper. Test-planning software developed by G&C of San Juan Capistrano, California, is being used to determine what other test points can be covered on a flight, by climbing to the test area in a particular manner, for example, to check aircraft performance or perform acoustic-vibration surveys.

 

AVIONICS TESTING

Requirements for flight testing of the mission avionics are now being drawn up, although the first avionics-test F-22, aircraft 4004, will not be flown until 1999. Peiper says the majority of flight profiles will involve one F-22 versus four targets, but scenarios with up to four F-22 versus 12 adversaries will be flown.

A review of the F-22 programme extended the development phase to allow more time for flight-testing of the integrated avionics. The number of hours to be flown was actually reduced, but the calendar span was increased by nine months . "We can't just test the radar," Peiper explains. "The avionics are 100% integrated all the time. When performing the primary test objective, we get other data. It totally changes the way we do flight test," he says.

The team has developed tables indicating what data would be gathered concurrently while performing the primary test mission. "This actually reduced the number of hours required to get the same data," Peiper says. Closer to the end of engineering and manufacturing development, however, test-point dependency increases. "What we do tomorrow increasingly depends on what we did yesterday. If we get an anomaly, we have to stop and figure it out," he says. As a result, the time for complete testing was increased.

The data-processing task has been divided up, with the Air Force handling real-time data and the contractor team performing post-flight analysis. "The Edwards network will run batch processing on canned analysis programs while still allowing interaction for quick-look analysis," Peiper says. Data will be processed sufficiently to check that the aircraft is meeting predictions, then sent over the network for detailed analysis at each company site.

Instrumentation has been designed into the aircraft using the CATIA and COMOK tools. Installation on the first aircraft has gone "amazingly smoothly", Peiper says. Instrumentation was checked out as the aircraft was been assembled, with both Boeing and Lockheed Martin Tactical Aircraft Systems delivering their sections with instrumentation already installed and tested. During assembly, telemetry from the integrated data-acquisition package in the main weapons-bay has been sent to the Marietta test centre to exercise the system before flight.

Aircraft 4001, the first F-22, is assigned to envelope expansion, flutter testing, loads measurement and handling-qualities evaluation. Aircraft 4002 is the performance aircraft, and will be used for propulsion and high angle-off-attack testing. Aircraft 4003 is the first Block 2 F-22 and will take over envelope-expansion testing from the first aircraft. Aircraft 4004-4009 will be used for avionics testing. The last two development aircraft and first two low-rate initial production F-22s will be used for initial operational test and evaluation by the Air Force.

First flight is scheduled for 29 May. Aircraft 4001 will be cleared to a ferry-flight envelope "...roughly equivalent to that of a commercial airliner, enough to get the aircraft checked out, shaken down and ferried to Edwards," says Peiper. After the first flight from Marietta, the aircraft will be grounded there for strain-gauge calibration, structural testing, final coatings application and ground-vibration testing. It is to be flown again in October for four to six flights, including limited aerial-refuelling qualification so that it can be ferried non-stop to Edwards.

Training for the first flight began in November 1996, for pilots, maintainers and mission-control staff, says Peiper. In January, the first-flight team began training in Fort Worth in a replica control room linked to the piloted vehicle-systems simulator. In April, training shifted to the real control room at Marietta, with a first-flight dress rehearsal using an F-15 as a surrogate for the F-22. A final training session is planned just before first flight.

 

F-16 simulator

In an "unprecedented" move, the F-22's flight-control software was flight-tested a full year ahead of the first flight, using the US Air Force's F-16 variable-stability in-flight simulator aircraft (VISTA). The VISTA/F-16's digital computers were programmed with the flight-control laws for the F-22's initial envelope, to evaluate handling qualities.

Supporting development and testing of the F-22 is an array of ground rigs. Lockheed Martin's Fort Worth plant hosts the vehicle-system simulator (VSS) and fuel-system simulator. The VSS is the F-22 "iron bird" and is used to test the hydraulic and electric systems with their associated hardware and software. The simulator will be used during flight-testing to troubleshoot anomalies uncovered on the aircraft. The fuel system simulator, complete with four 2,280litre underwing tanks, is used to test fuel transfer and the software for centre-of-gravity control. The rig can be tilted upwards 60í, and can simulate aerial-refuelling. Fort Worth is also responsible for the handling-qualities laboratory, which is used to develop the F-22's flight-control laws.

Boeing is responsible for testing the avionics suite. "It's basically a building block approach," says Avionics Integrated Laboratory (AIL) manager Bruce Ammerman. Block one began in February 1997 with development testing, integration and verification of requirements. Sensors and a radar will be mounted in the AIL tower, located on the west side of Boeing Field, Seattle. "A mini-AIL is being established at the B-1B/F-16 test building at Edwards AFB, which will support flight testing. Initial testing will also take place at Marietta on aircraft 4004, the first F-22 to be fitted with a full avionics suite. Flight tests are due to begin in August 1999.

The F-22 AIL is similar to the nearby Integrated Aircraft Systems Laboratory (IASL) which was developed to support the Boeing 777 airliner programme. "There is a very close relationship with the IASL, but we've leveraged off the 777, and we prototyped the AIL concept during dem/val, which preceeded the 777 programme," adds Ammerman.

A 757 flying test bed (FTB) was flown to Boeing's Wichita, Kansas, site in November 1996 to be modified to accept the F-22 forward fuselage. Following completion of structural changes around August 1997, it will be returned to Boeing for system installation before being flown flying to Baltimore in November to have the Northrop Grumman APG-77 radar fitted. Installation of the sensor wing, which forms the second FTB modification phase, is scheduled for August 1998. Mounted on the crown of the fuselage behind the flight deck, the 8.5m-span wing will replicate the position of electronic-warfare (EW) and communication/ navigation/identification (CNI) apertures on the F-22.

"The fact that the sensors are embedded in the F-22 gave us something to scratch our heads over," comments FTB aircraft-modification team leader Tom Skelly. "One of our hurdles was to analyse the aerodynamics correctly. After windtunnel tests, we found the right angle of incidence of the sensor wing to produce a zero angle-of-attack in cruise."

 

Further tests

After installation of the sensor wing, the FTB will begin further tests in late 1998, including flights against combat aircraft. On board will be 25 technicians who will be able to make system and software changes in flight as the data is analysed.

Eleven APG-77 radars are to be delivered to support the FTB, AIL and flight tests, the sixth being the first radar to actually be flown on the F-22. Three mission-software blocks are in development. Block one, containing core search and track mission-software and basic radar functionality, is due for completion in October 1998 to support the first FTB tests. Block two, providing basic air-combat capability, electronic warfare, stores management and limited weapons functionality, will be completed by June 1999. Block three, with complete sensor fusion, CNI and EW functionality and full weapons capability, is due to be wrapped up in April 2000.

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