Testing of the rival Joint Strike Fighter (JSF) concept demonstrator aircraft is intensifying as Boeing and Lockheed Martin expand flight envelopes and begin crucial pre-flight test work on the short take-off and vertical landing (STOVL) variants.


Boeing's X-32A conventional take-off and landing (CTOL) version will begin performance tests around 8 December, pending the installation of a hydraulic system modification. "Procedural problems" with the system led to an emergency lakebed landing due to brake failure, and have limited Boeing to slow speed, aircraft carrier-approach handling qualities assessment flying. The modification adds a cross-over valve that will allow the accumulators in the aircraft's hydraulically-driven undercarriage and brake system, to recharge normally with the auxiliary power unit running.

Boeing JSF programme manager Frank Statkus says the change will "allow us to do the up and away points that the Boeing developmental aircraft is capable of doing. We will do supersonic flight and aerial refuelling among other requirements that Boeing has set itself over and above the main programme mandates. However, our objective is to get these done first." Boeing expects the X-32A to complete its share of the CDA (concept demonstrator aircraft) test phase in early February, the aircraft having accumulated around 14h on 22 flights by 20 November.

Meanwhile, X-32B engine runs have started at Palmdale, California, and the aircraft has been fitted with Pratt & Whitney's final integrated flight propulsion control software. Taxi tests are due to start "around the end of the first week of December", says Statkus. First flight is the end of February.

Lockheed Martin delivered the X-35A back to Palmdale on 22 November for conversion to the X-35B STOVL variant, having made the first supersonic flight the day before. The aircraft reached Mach 1.05 at 25,000ft (7,600m).

Lockheed Martin JSF programme manager Tom Burbage says the estimated time from the start of conversion - including fitting of the lift fan, drive shaft, a three bearing swivel duct and wing mounted roll posts - to the aircraft's maiden flight will be "between four and five months". Tie down tests over a hover pit will start at the end of January and data will be fed to Pratt & Whitney's West Palm Beach plant where an engine and lift fan is running. First flight of the X-35B is expected around April.

Part of the delay is due to control system changes mandated in recent months by the JSF Programme Office (JPO). In place of the originally planned light augmented control system, "the customer now wants a fully automated closed loop system," explains Harry Blot, JSF deputy programme manager.

While this has forced Boeing and Lockheed Martin to delay their respective X-32B and X-35B test flights, the net benefit will be to further reduce risk ahead of engineering and manufacturing development (EMD). STOVL flight testing, however, will not be completed before the submission of EMD proposals.

Tests of the revised X-35B lift fan, which has now been modified with a steel stabilising shaft to improve endurance, began on 20 November at Rolls-Royce's Indianapolis site.

Up to arrival at Palmdale for modification, the X-35A had completed 27 sorties. As well as the supersonic flight, it had been flown to a maximum altitude of 34,000ft, plus 5g and a maximum angle of attack of 20°. It also performed four practise carrier landings in preparation for X-35C trials.

Final pre-flight tests of the carrier version X-35C, with larger wing and control surfaces, are underway at Palmdale with first flight expected to take place in "early December", says Burbage. The aircraft is scheduled to complete around 20h over 20 flights before being ferried, in late January, to the US Naval test site at Patuxent River, Maryland, for the remainder of its simulated carrier approach handling qualities work, which Blot estimates will be concluded by the end of February.

The recently released final call for improvements requires both teams to submit their EMD proposals by February. "The government is having to make provision for us to verify what we put in the proposal with STOVL data," says Blot. If it is not subsequently verified, risk will go up and, with it, a price penalty will be imposed by the JPO, he adds.

Lockheed Martin plans to work from both ends of the STOVL flight envelope with the first flight being a hover. It will then perform a conventional take-off and landing, finally transitioning to a vertical landing. As the X-35A's outer mould lines are virtually identical to that of the X-35B, much of the up and away flight testing has already been accomplished.

Lockheed Martin estimates that by the end of the concept demonstration flight test, the X-35A/B will have clocked up 40h of flying and the X-35C around 60h. Both were designed to fly up to 300h and the company is pushing for a continuation of flight testing ahead of a downselect and EMD start.

• Lockheed Martin's F-16 power-by-wire testbed (Flight International, 31 October-6 November) went supersonic for the first time on 9 November, reaching Mach 1.3 on its third flight. The aircraft's electric flight-control actuation system is being tested at high dynamic pressure and in high-g manoeuvres.

Source: Flight International