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  • JPO on STOVL F-35 Design Issues
    Posted by Graham Warwick 3:26 PM on Jan 27, 2011

    UPDATE: I just got some additional info on the design issues and fixes from Lockheed Martin's deputy general manager for the F-35 program, Eric Branyan. I've added them in italics.

    The Joint Strike Fighter program office has provided the detail behind defense secretary Robert Gates' Jan. 6 comment that issues with the STOVL F-35 "may lead to a redesign of the aircraft's structure and propulsion". (You can read AMy Butler's story here.)

    There are no surprises on the list. The issues detailed by the JPO have been reported on before, and in most cases fixes are in design or in test. They are: lift-fan clutch heating, driveshaft thermal expansion, roll-post heating, lift-fan doors, bulkhead cracking and pilot-vehicle interface issues.

    blog post photo
    Photo: US Navy

    Following the latest replan of the F-35 program, which adds $4.6 billion to development, the JPO tells Amy there is money "to address known discrete improvements" and additional reserves "to address unknown items that may be discovered in developmental flight test". The program office describes the known issues as "readily solvable through engineering adjustments."

    Lift-fan clutch heating has been addressed by adding a passive cooling circuit to provide cooling air to the clutch in up-and-away flight when the forced-cooling fan used in STOVL mode is turned off.

    Lift-system prime contractor Pratt & Whitney says the lift-fan clutch and roll-post actuators can get too warm in certain flight conditions "because the environment surrounding the hardware is more demanding than in the original design". Sensors are being installed to monitor temperatures.

    Branyan says the problem is heating of the roll-post actuator caused by leakage of hot engine air as the roll nozzle seals age. Insulation is being added. For now, the amount of time the aircraft spends in STOVL Mode 4 (jetborne flight) is being limited.

    Lift-fan clutch heating is caused by drag between plates when disengaged (the driveshaft turns in up-and-away flight). When alerted to high temperatures by the new sensor, the pilot will climb back to 10,000ft until the clutch cools down [see note below].

    The driveshaft has been redesigned with a new bellows coupling to accommodate greater-than-predicted variations in length resulting from airframe and propulsion system build tolerances, thermal and pressure growth and maneuver deflection. For the development jets, spacers will match the driveshafts to individual aircraft.

    Auxiliary-inlet door hinges have been redesigned to increase durability, and scheduling of the large lift-fan door adjusted - and sideslip flight-control laws refined - to reduce airloads on the doors. Allowable slideslip is now limited above 150kt in semi-jetborne flight.

    Cracking of the fuselage bulkhead - switched from titanium to aluminum in the STOVL F-35B to save weight - is being addressed by thickening the bulkhead for production aircraft and by "local blending" on assembled aircraft (all three variants). Lockheed says blending involves machining smoother curves on bends and corners in two small areas of the bulkhead to eliminate stress concentrations and prevent cracks starting.

    Branyan says blending to provide incremental additional life (beyond the 1,500h at which cracks appeared) can be done on the flight line, while a modification to restore full life will involve installing doublers and take more time.

    Issues with helmet-mounted display resolution (the HMD is the primary flight display on the F-35 as there is no HUD) are being tackled with supplier VSI, "and while there are no current plans to change suppliers, options are being considered in parallel that mitigate the most stressing issues," says Lockheed, adding there are no safety-of-flight issues or flight-test delays due to the HMD.

    Performance in day VFR missions is acceptable, says Branyan, but the visor-projected HMD is not meeting its night-vision performance goals. Fixes being worked on involve higher-sensitivity helmet-mounted cameras, higher-resolution display projectors and improved optics.

    It's a longish list, but in many ways the issues look less challenging than those already overcome during design and development of the lift system. I want to address those, too, but rather than drone on and on here, I'll do a follow-up post looking back over the issues faced during development of the STOVL propulsion system.

    NOTE: This was corrected from the original post (see comment below).

    Tags: ar99, tacair, F-35, JSF, STOVL

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