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Aviation History
1979
1979 - 4705.PDF
W$-^~ •••'••••'-A MARINES :;::i::i::g::a:-^^^^^ Bl^::; ; .'; i .: ••Cft B Advanced Harrier 1AM WARWICK reports from St Loui AIR SUPPORT is vital to the US Marine Corps. Conventional car rier-based aircraft can provide that support but expensive supercarriers have to sit within range of the enemy. First priority must therefore be to capture an airfield, allowing airpower to come ashore and the ships to escape. V/Stol air support steps from ship to shore with the amphibious forces. US Marine Corps AV-8A Harriers do not require large carriers or captured concrete and are ideally suited to the Corps' special mode of operation. Harriers fly from compact helicopter carriers and, as no runway is required, allow selection of more advantageous beach-heads. The AV-8A Harrier— which entered service in April 1971— has convinced the service that its future lies with an all-V/Stol air arm. Achieving thds goal is not easy. The first step—the joint UK-US AV-16— was cancelled in 1974. Development of the 24,500'lb-thrust Pegasus 15 proved too expensive—an estimated $500 million. McDonnell Douglas—armed with a licence from Hawker Siddeley —then proposed a minimum-change AV-8A which would meet any AV-16 goals but would not require a new engine. This is the AV-8B and the Marine Corps has announced its intention to buy 336, to replace AV-8As and A-4s. The aircraft offers twice the AV-8A payload/range performance. Changes include a new wing giving reduced cruise drag and increased STO lift, a redesigned intake for more V/Stol thrust and less cruise drag, under- fuselage devices to increase VTO lift and thrust, a raised cockpit for better visibility and new avionics offering better reliability and maintainability. Although a minimum-change de velopment of the Harrier, the AV-.8B has more carbonnfibre in its structure than the F-18. More than 23 per cent of the airframe—1,1901b—is graphite epoxy. Composite items are the wing, front fuselage, tailplane, outrigger fairings, engine access panels and lift improvement strakes. Composites have no known fatigue life, do not corrode and are lighter than aluminium. Composite wing Weight saving is essential to achieve AV-16-like performance with the standard, 21,5001b-thrust Pegasus 11. The wing has a nominal 230ft2 gross area, almost 30ft2 up on the Harrier. Span is increased by 5ft. Apart from leading edges, tips, pylon and undercarriage attachments and centreline rib, the wing is totally com posite, saving 3301b. Multiple sine-wave spars provide reserve strength through redundant load paths and the wing is designed to withstand severe battle damage. One-piece upper and lower wingskins are mechanically attached, the lower permanently. The upper skin is re movable for inspection. Mechanical fasteners are used extensively in pref erence to bonding for ease of repair. A supercritical aerofoil section is used to improve cruise performance. The section gives lower transonic drag by delaying dragrise and increases manoeuvring lift-to-drag ratio by 1:7. Large single-slotted flaps are linked to nozzle deflection in STO (short take-off) to increase flow over the wing. Rear nozzle efflux entrains air over the deflected flap, increasing the velocity differential between upper and lower surfaces. This supercircula- tion generates more than 6,7001b of extra STO lift. Three wings have been built for the prototype programme, two flight-test and one structural test. The latter was tested to 150 per cent of YAV-8B loads for flight clearance and sub sequently underwent 500hr of testing to clear the YAV-8Bs for 250hr flying. The wing has since withstood 150 per cent AV-8B loads without failure, clearing the design for production. All three wings are built to produc tion standards. Design fatigue life of the aircraft is 6,000hr, with testing to Heading and below AV-8B is an evolutionary development of the AV-8A Harrier, so the aerodynamic improvements could be tested on modified Harriers—the YAV-8B prototypes
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