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Aviation History
1964
1964 - 1993.PDF
B-70 THE STATE-OF-THE-ART IMPROVER BY IAIN PIKE Part 2 IN last week's issue we presented Part 1 of this detailed account of the heaviest and most powerful, and potentially one of the fastest, aircraft ever built; the North American XB-70A. Planned as an intercontinental bomber, the B-70 has been cut down to a programme of two research aircraft only; but they remain aero- planes (or, in current US parlance, "air vehicles") of unrivalled technical interest. The story is concluded here with an account of the airborne systems. Hydraulic System The XB-70A has more hydraulic equipment, and uses more fluid power, than any other aircraft ever built. There are four independent 4,000lb/sq in constant-pressure systems pres- surized by 12 engine-driven, positive-displacement, variable-output pumps made by Vickers Division of Sperry-Rand Corp. There are also 85 linear actuators, 44 hydraulic motors, 50 mechanical valves and about 400 electrically controlled solenoid valves. The main services are supplied by more than a mile of pipe containing approximately 3,300 brazed connections and 600 mechanical joints. Fluid capacity totals 220 US gal at room temperature and 260 at the maximum operating temperature. Seeking reliability in the difficult environment at minimum weight led to several departures from previous practice. Design of the system actually started with selection of the fluid. A non- flammable fluid would have been preferred, but none was available that would work at the -65°F to 450°F temperature range speci- fied (275°F is a conventional limit). Viscosity, bulk modulus, thermal stability, lubricating qualities and weight were very care- fully studied; somewhat less attention was given to foaming and oxidation, since the system is purged free of oxygen and closed. North American and Vickers first selected Oronite 8200 hydraulic fluid, but continued development produced Oronite 70 with better characteristics. Even this is considered a compromise, since it does not meet all requirements and led to problems in designing hardware. Oronite 70 has a high affinity for air and nitrogen, and, as the B-70 hydraulic reservoirs are pressurized with dry nitrogen, a little of the gas becomes dissolved in the fluid, creating compressibility problems. If the pressure is lowered, the nitrogen comes out of solution, leaving the fluid aerated; and this in turn can cause pump cavitation. Vickers admit that work is continuing on improving the fluid, and an endurance test programme using new variations is continuing. The four basic hydraulic systems provide hydraulic power to seven aircraft subsystems. These are: primary flight-control, vertical stabilizers, elevons, and augmentation; secondary flight- control, wing fold, canard trimmer and flaps; landing gear, legs and doors, nosewheel steering and wheel brakes; armament, door platforms; environmental drive, compressors, heat-loop pumps, blower fans and flood-flow system; utility, emergency generator drive, drag-chute system and windshield-ramp system; and pro- pulsion, air induction, fuel transfer and fuel boost. Of the four basic systems, two are primary and two utility. Each is powered by three pumps in a master/slave arrangement. One master pump supplies continuous hydraulic power for normal operations. The
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