By Graham Warwick

Lockheed Martin continues to struggle to gain momentum in flight testing of the Joint Strike Fighter, with the first F-35 again grounded by component failure and while the second aircraft heads toward a hiatus in flying that will last into early next year.

The issues will not affect the overall schedule, says Lockheed, but they are preventing key risks being retired early. These include additional noise tests funded by the Australians at a time when the F-35's environmental impact has become an issue for some international partners.

The first F-35, aircraft AA-1, is grounded awaiting the repair of nacelle vent fans designed to keep the engine bay cool on the ground. The situation has echoes of the overheating problems that dogged the F-22, but Lockheed says it is unique to AA-1.

"The issues we are dealing with are independent of the thermal management system," says deputy program manager Bobby Williams. "Nothing is on the critical path, but the earlier we get the data the better."

The problem involves repeated failures of the nacelle vent fans that force air through the space between engine and airframe to prevent heat from damaging the structure.

While the fan failures may be unrelated, thermal management is the "biggest challenge" in the F-35, says Daniel Kunec, JSF program office director, air system integration. "It is the most limiting feature, and there are still some challenges to be overcome," he told an AIAA propulsion conference in July.

Originally designed for the canceled Boeing Sikorsky RAH-66 Comanche, the vent fans are unique to AA-1. "They are one-off fans that were never fully qualified because AA-1 is a one-off," says Williams. AA-1 was built before a redesign to reduce the F-35's weight and is not production standard.

The fans are surrounded by fuel, which is colder than the ambient air, and in the heat and humidity of Fort Worth, Tex., condensation is coating circuit cards inside the units and causing corrosion. The fans are being repaired by supplier Hamilton Sundstrand, but the "long pole" delaying a return to flight is applying a new conformal coating to the cards.

Williams says the problem does not affect the second F-35, production-standard aircraft BF-1, as the vent fans were relocated during the redesign to make them more accessible, avoiding the condensation issue. He expects the fans to be back in AA-1 by the end of August, after which "a couple more flights" are needed at Fort Worth before the aircraft can make its delayed ferry flight to Edwards AFB, Calif., for testing.

Williams plays down concerns about overheating in the high desert of Edwards, although AA-1 does not incorporate design changes made to improve the F-35's thermal management. These include larger engine fuel pumps, which will be introduced during early low-rate initial production to provide the full thermal management capability.

Thermal management is a particular issue for the stealthy F-35. "There are very few areas to reject heat. We can dump it or burn it," says Kunec. "Every component has a heat budget and everything is cooled by fuel." The heated fuel is either burned in the engine or cooled by heat exchangers in the engine fan duct. "The massive fuel/air heat exchanger is our saving grace for coming close to specification."

Based on lessons learned with the F-22, Williams says, the F-35 has a requirement to operate on the ground for 60 min. on a 120F day, with fully heat-soaked fuel and a full solar load. At the end of 2005, the short takeoff and vertical landing (Stovl) F-35B could not meet the requirement, but after redesign now meets the specification, says Kunec.

Meeting specification at the end of a mission remains a challenge for the Stovl variant. "The lift fan is a huge heat source and at the end of the mission there is little fuel left to use as a heat sink, so there is a strong peak in temperature," says Kunec. The requirement is to operate for 30 min. after landing. "We meet that with no margin," says Williams.

Already delayed more than a month, AA-1's deployment to Edwards is to perform inflight engine shutdown testing over the dry lakebed. The deployment will also be used to collect additional noise data requested by Australia. Several countries face public concerns over the noise produced by the F-35, which with 40,000 lb. of thrust is considerably more powerful than the F-16 it will replace. In Norway, there are concerns noise issues could force construction of a new base to avoid local opposition at existing facilities.

"We are working the environmental issue, but we are not doing anything from a design standpoint," says Kunec, adding, "There is a lot of misinformation on F-35 noise." Near-field noise level is comparable to legacy fighters, he says. Lockheed has said the noise footprint that reaches the base perimeter is less severe, but Kunec says far-field or community noise "is at or close to the highest level." Flyover noise data collected with AA-1 at 1,000 ft. "is right at the top, but then everything over 90-100 dB. is a problem," he says.

On the emissions front, "NOx [nitrogen oxides] levels are way beyond any previous aircraft because of the higher combustor temperatures," says Kunec. But while NOx will be significantly higher, carbon monoxide and unburned hydrocarbon levels will be significantly lower.

While AA-1 remains grounded since completing its 45th flight on July 23, aircraft BF-1, the first Stovl F-35B, has logged nine flights since its June 11 debut and has about 15 more flights in conventional takeoff and landing mode before it too ceases flying, for scheduled upgrades. A decision to delay hover pit testing to January means the aircraft will not fly again until the second quarter of next year. A second F-35B, BF-2, will roll out on Aug. 17, but not fly until early next year.

The plan was to conduct pit testing of the Stovl propulsion system in late October and return to flight in November, but program officials have decided to delay the tests until after a redesigned Pratt & Whitney F135 engine is installed. This has pushed full-power hover pit work back to January, but will avoid the need to repeat tests, says Williams.

To minimize the impact, initial tests opening the Stovl propulsion system doors in flight will now be conducted before BF-1 is grounded, instead of after. These will involve opening the doors one at a time and checking handling qualities, says Williams. Electromagnetic-effects tests planned for later in the program have also been brought forward and will be performed while BF-1 is on the ground for upgrades.

Lockheed now plans to begin "build-down" flight tests in Stovl mode in the second quarter of 2009, but a date for the first full vertical landing has yet to be agreed. The original plan was to ferry BF-1 to the U.S. Navy's Patuxent River, Md., test center to perform the first vertical landing. "We are having ongoing discussions with the JPO on how we do the first vertical landing and we have not finalized a plan," says Williams.

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STOVL is the biggest problem, but all versions have the noise issue.