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Aviation History
1963
1963 - 1383.PDF
FLIGHT International, 8 August 1963 205 BELL X-22A FIRST DUAL TANDEM DUCTED- PROPELLER VTOL AIRCRAFT Bell X-22A model in hovering configuration V/STOL aircraft have now been evolved with a remarkable variety of configurations, and one of the most distinctive —indeed, grotesque—is the X-22A now being built for US Tri- Service research into V/STOL transport possibilities. Contractor is Bell Aerosystems Co, of Buffalo 5, NY. Despite a TFX-style dispute (a powerful lobby claims that Douglas submitted a tech nically superior, and cheaper, proposal), Bell are well on the way to completing two flight aircraft against a contract awarded last November. Purpose of the programme is "to explore the mechanical and aerodynamic problems associated with the dual-tandem, ducted- propeller design; secondary areas of interest include military operational capability, human-factors research and compatibility with civil airways." This quotation comes from an IAS paper by Bell's V. B. Paxhia and E. Y. Sing, respectively X-22A technical director and assistant technical director. Their account of the background to the design begins with the company's J44-lifted VTOL of 1954. In the same year Bell began to study ducted>*>ropeller designs, and in 1955-6 conducted an assault-transport study for the Navy. Gradually the company evolved the dual-tandem layout, with four ducted propellers all interconnected mechanically and pivoting in unison through some 90° to provide lift or thrust. This arrangement offers a good com promise between cruising speed and hovering efficiency, and it also provides very large hovering pitch trim and control forces without any significant loss in lift, merely by differential variation in propeller pitch.* Bell have found "the tandem four-duct configuration results in the minimum gross weight and most compact vehicle for a number of military and commercial missions." In 1959-60 the company prepared design studies for the Marine Corps and Army, the latter study being the D2022 powered by four 2,200 h.p. T55s and being able to carry 30 troops at a VTOL gross weight of 28,5001b. In * It is undoubtedly inferior in cruising flight to more conventional aero planes like the other Tri-Service V/STOL transport, the LTV XC-142A; but it is probably superior in hovering efficiency and control. 1961 the D2064 Tri-Service design followed, conceived jointly by Bell and Lockheed. The next stage was the X-22A. The requirement for engine-out hover eliminated single-engine designs and demanded a T/W ratio of 1.59 for a twin and 1.31, 1.21 and 1.16 for three, four and five engines. The choice was four GE T58 engines, which currently have a TBO of "about 400hr in Navy use and about l,200hr in civil use." There are many possible installations, and the last of six arrangements illustrated in Fig 1 proved the best compromise in: avoiding hot-gas and debris ingestion; minimizing complexity, weight and jet-impinge ment problems; providing clean, low-loss inlet and exhaust systems; and facilitating powerplant access and removal. Complexity and weight depend chiefly upon the transmission; the second scheme is the lightest, because normally each engine drives its adjacent propeller, but the 2801b penalty of the last three schemes is com pensated in the last example by excellent performance in other respects. Despite the fact that optimum duct performance for VTOL and high-speed cruise calls for a variable duct inlet, the X-22A has a fixed shape "with minimum lip area contraction ratio necessary to achieve maximum thrust for maximum endurance without a signi ficant loss in top speed." Duct profile is cylindrical around the squared-off propeller blades and a simple cone downstream, and plots of range, endurance and speed against duct diameter (diffuser) ratio indicated a value between 1.15 and 1.25. Variable-camber propellers [now well advanced in development (Right) Three-view show ing cruise configuration Fig I (left) Six possible powerplant layouts
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