Sunday, January 02, 2011

Lockheed Sneaks Another F-35 Under the Wire

on the eve of the New Year, Lockheed Martin got another F-35 test aircraft flying, with AF-4 - a CTOL F-35A - making the program's 410th and last flight on 2010 on Dec. 30. That means nine development-test aircraft are flying, but leaves three still to get airborne.

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Photos: Lockheed Martin

On paper, the F-35 test program looks more successful than expected in 2010, with 410 flights versus a plan of 394 - but dig down and look at test points rather than test flights and the results were a mixed bag. Although the program ended the year close to its plan of acheiving almost 3,800 test points, they did not add up quite as expected.

The CTOL F-35A ended the year 50% ahead of plan on test points because aircraft AF-1 and -2 at Edwards AFB kept on flying, averaging 10 flights a month from June onwards. The F-35C carrier variant ended the year 125% ahead of plan, which was modest anyway because only one aircraft, CF-1, is flying.

The STOVL F-35B, however, ended the year 18% behind plan on test points because mechanical realibility issues prevented the four aircraft at NAS Patuxent River achieving the same flight rate as the F-35As. More critically, the program achieved less than half the test points required for two key objectives: ready for training (RFT) flight clearance and initial ship trails.

Both objectives were planned to be accomplished in 2010, but will now slip to the middle of 2011. Initial ship trials, orginally scheduled for March, are now planned between late August and November. That window is based on when the LHD-class amphibious assault ship USS Wasp can be modified with instrumentation to measure the ship environment during STOVL operations.

To achieve clearance for ship trails, the F-35B must complete 40 vertical landings in a range of conditions. The program has only done 10 since March 2010, seven of which count towards the total required. STOVL-mode testing was suspended in September, when premature wear on auxiliary-inlet door hinges was discovered. Vertical landings are expected to resume in January.

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Auxiliary doors are aft of the lift-fan door (here open to 65deg)

Hinge wear has been traced to higher-than-predicted airloads on the auxiliary doors. Components have been redesigned, but the main fix is to change the operation of the large lift-fan door forward of the auxiliary-inlet doors. Flight tests have shown that, when the lift-fan door is fully open, loads on the auxiliary doors are reduced.

Originally, the lift-fan door was scheduled to open to 65deg below 120kt in semi-jet-borne flight, and to 35deg above that airspeed. Now the door will stay fully open to 165kt to reduce the loads on the auxiliary-inlet doors. Lockheed's JD McFarlan, who is now in change of the test program, says the change does not significantly impact short take-off performance.

Investigation of the hinge-wear problem also revealed a lot of variation of the loads on the auxiliary doors caused by aircraft sideslip, so McFarlan says the flight-control software has been adjusted to tailor the slideslip characteristics in semi-jet-borne flight.

Following these changes, aircraft BF-2 is expected to make its first vertical landing early in the New Year. Along with BF-1, the original STOVL-mode test aircraft, BF-2 will then take up the task of clearing the F-35B test fleet for initial ship trails. This is now expected to be completed by the summer.

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