This post is one in a series on the development of the F-35. Click here for a Flash graphic with system cutaways. Click on Read More to read the entire post on that system and check out next week's special coverage of the program in the Dec 12 edition of AW&ST.
Cooling and wear have been the main design issues with the lift-fan clutch, which has to engage in 9-12sec - and transmit up to 6,000shp before a mechanical lock engages - yet last 1,500 engagements.
The dry clutch carries torque two ways: friction for initial engagement and spool-up of the lift fan; then a mechanical lock for acceleration to the full 29,000shp. The five clutch plates use aircraft braking material and are lasting "well over 1,000 engagements", says Rolls, but have not yet completed a 1,500-cycle run. Heating occurs only about once in every 500 flights, and not all clutches experience plate drag. Pratt & Whitney says the “trigger mechanism” is close to being identified and the solution may not require a redesign, simply an adjustment of tolerances.
The clutch generates a lot of heat - 12,000 BTU of thermal energy during the time it takes to engage. The majority of this heat is removed via oil, which is pumped through the clutch at 40gal/min in STOVL mode. But flight tests have shown the clutch generates heat even when not engaged, as the driveshaft is always turning.
To remove heat generated by drag between the clutch plates when disengaged, a passive cooling circuit has been added to provide cooling air in up-and-away flight, when the forced-cooling fan used in STOVL mode is turned off. An overtemperature sensor has also been added, to alert the pilot to climb above 10,000ft until the clutch has cooled.
Back to F-35B Graphic or on to Lift Fan Door.