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Aviation History
1998
1998 - 2819.PDF
WSr PILOTS REPORT Raptor approaches supersonic tests SEEK EAGLE EXPANDS F-16 LOAD MODELLING THE FIRST supersonic tests of the Lockheed Martin Boeing F-22A Raptor air superiority fighter are expected to take place later this month at Edwards AFB, California. The tests will be conducted with the Raptor 1 (aircraft 4001), which is now dedicated to expanding the high and fast part of the envelope following die arrival of the Raptor 2, which will be used forslow-speed handling and high angle- of-attack testing. The supersonic flight will mark the fastest speed yet achieved by the engi neering and manufacturing development configuration F-22 A, which to date has flown at 423kt (780km/h) and a maximum altitude of 40,000ft (12,200m). "So far, it has demonstrated excellent perfor mance and flying qualities at all altitudes, including those where other aircraft become sluggish," says Lt Col Steve Rainey, operations officer of the 411th Tactical Test Squadron of the F-22 Combined Test Force (CTF). The test team plans to complete 183 flight hours by the end of December 1998 and, by the end of September, to have flown 43 sorties for a total of 71.6h. The CTF plans in December to begin tests of a software update for the aircraft's triplex digital fly-by-wire flight control system, being designed to counter a "nose kick" experienced during manoeuvres at Mach 0.8 and 35,000ft. The motion, sometimes also referred to as "roll jerk", or "nose wander", has been seen "by all Raptor pilots to date", says Rainey, who adds that die "...correction will be made with the December OFP [operations flight programme] update". Nose kick occurs when the aircraft is rolled to the right with "full check" at this specific height and speed condition. Other changes are also being made to several systems nominally controlled by the integrated vehicle subsystem controller (IVSC). The FVSC co-ordinates operations between various subsystems as well as directly controlling oth ers. IVSC systems include the hydraulics, fuel management and electrical power areas, as well as the aircraft's brake-by-wire system. Changes are planned following the discovery of significant differential braking between the left and right main gears during taxiing and land ing. The alterations include revised brake sys tem hardware and updated brake deceleration command software. Odier changes are also in hand for the envi ronmental control system, which ".. .continues to concern us the most", says the CTF. One potential solution being studied is the addition of flow balance devices to the system, which could be redesigned to measure correctly the pressure differential within the ECS. The CTF says that the programme is on schedule for the start of initial operational test and evaluation in 2002 and initial operational clearance in 2004. • F-16s will have expanded air-to-ground roles after Seek Eagle load modelling efforts THE US AIR FORCE'S Seek Eagle office, which specialises in testing new and existing stores in new configura tions, is developing a full span test model of the Lockheed Martin F-16, following experiences gained on a test effort to expand the aircraft's air-to- ground role. The drawbacks of the current half- span model were revealed during tests for limited cycle oscillation (LCO) on F-16s equipped with AGM-65 Maverick and AGM-88 HARM missiles for sup pression of enemy air defence missions. Although not as potentially destructive as flutter, LCO is an aeroelastic instabil ity which appears with certain loads on the F-16 wing as a limited amplitude, self-sustaining oscillation. The Seek Eagle programme, based at Eglin AFB, Florida, performed two half- span analyses to try to predict LCO with asymmetric loads of HARMs, Mavericks, air-to-air missiles and other stores. The half-span model could not reproduce the potential problem of asymmetric cou pling, however, and flight tests were con ducted on a modified F-16C Block 25 aircraft, based at Eglin, to verify some of the findings. The results showed that, while asym metric Maverick and HARM loads with wingtip missiles were benign, other combinations produced LCO within the F-16's normal operational flight enve lope. Symmetric Mavericks without wingtip missiles produced some LCO within the envelope, while symmetric HARMs with wingtip missiles generated "heavy" LCO. Although details of the resulting restrictions are classified, the team says that "new envelopes" are being recommended to the USAF as a result. The F-22 Raptor is due to fly even faster in tests this month FLIGHT INTERNATIONAL 7 - 13 October 1998 31
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