‘BOYLE’ING POINT


KENNEDY SPACE CENTER


The failure of the Scaled Composites GlobalFlyer fuel system design to properly cater to the 350-year-old Boyle’s Law principles of pressure, temperature and volume nearly caused the 21st -century aircraft to run out of fuel on its initial round-the-world flight. But the aircraft’s team believe they have now introduced a fix that would please Robert Boyle, the 17th -century Irish chemist who originally docu- mented the kind of trouble GlobalFlyer could run into. But first they have to test it.


Eighty-three percent of the aircraft’s takeoff weight is fuel, carried in 13 tanks— four in each outboard wing, two more in each tail boom and a single 31-gal. header tank in the aft fuselage that directly supplies the Williams turbofan. The four outboard wing tanks vent into the wing’s trailing edge while the booms vent out the engine pylon.


The GlobalFlyer team found that on its first global circuit, 775 lb. of fuel was lost out of each boom tank, but that this loss occurred only during the 30,000-40,000-ft. climb, 0.8-2.8 hr. after takeoff. The loss began suddenly and ended slowly.


Calculations critical for the current flight indicate fuel loss on the previous flight occurred due to the following sequence: •The oddly shaped boom tanks have two separated high points that are connected with an internal vent line. When the GlobalFlyer started to climb with its 1 8,000-lb. fuel load that connect separated high points on the boom, these lines were filled with fuel and did not allow air to enter or exit the boom. But air already in the tanks began to expand as altitude increased. •Because they were full of fuel, the previous vent design did not relieve this pressure as intended. This increased pressure of the air expanding forced the fuel down in the boom tanks, displacing JP-4 out different vents at a rate of 1,550 lb/hr—as much as 4 gal/mm. The simple fix, the team hopes, is the addition of new vents sticking upward from the booms to relieve the pressure created when air in the tanks expands during climbout. This obviates the need for internal vents. There is really no simple way of testing the success of the modifications without flying . . . at near-gross weight according to Scaled Composites project documents. And “a test program is needed to be sure of the success of the mods’ That test will be made during the first 3-4 hr. after takeoff.


SOURCE:

AVIATION WEEK & SPACE TECHNOLOGY

January 30, 2006. (Pg. 39)

www.AviationNow.com/awst



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