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Aviation History
1944
1944 - 1634.PDF
144 FLIGHT AUGUST IOTH, 1944 THE B-29 almost instant approval, and on June 1—twenty days after Hitler had invaded the Low Countries—Boeing was author ised to construct wind tunnel models -and a mock-up ot the slightly revised model known as the XB-29. This was identical to the Model 345 except that gross weight had again been increased. Wind tunnel tests of the XB-29, conducted at the University of Washington, the California Institute of Tech nology and the National Advisory Committee for Aero nautics wind tunnels, so impressed the Air Forces that on August 24th, 1940, Boeing was authorised to build three full-sized XB-29 prototypes. On May 17th, 1941, before the first of these experimental models had been completed in Seattle, the Army had be come convinced that the Boeing design would fill a vital role in the nation's aerial offensive strategy and ordered the B-29 into quantity production at the Boeing Wichita plant. The first flight test was on September 21st with the aircraft in the air for one hour and 15 minutes. Testing continued throughout the rest of 1942, through 1943 anc* into 1944. ROOM FOR STOWAWAYS : The huge fin of the B-29. It was flight-tested on a reduced scale on a Fortress. of refinements in drag characteristics The Army, how ever, was uncertain of the wing because of its high loading. This, they feared, would reduce 'ceiling and affect hand ling characteristics during take-offs and landings.. Boe ing argued, and justified its argument, that the desired ceilings could be obtained, and that excellent landing and take-off characteristics would be provided through the use of the Boeing-created flap which would counteract high wing-loading. Wind tunnel tests verified this position to the satisfac tion of the Army representatives, and construction of the three experimental models was ordered. Wing Construction The Boeing "117" wing is of web-type construction rather than the tubular spar construction of the Flying Fortress wing. A heavy extruded duralumin flange was selected for the web spar. Weighing 255 pounds when machined, this flange is said to be the largest extrusion ever used in a production aircraft. The improved features of the Boeing " 117 " wings are: Decreased drag per pound of lift; increased thickness cf the inter-spar area, permitting greater strength of the primary structure and greater volume in which to carry fuel; better stall warning and more gradual stalling charSc-W teristics. As finally developed, the wing has a span of 141.2ft. with an aspect ratio of 11.5. The high aspect ratio results in high wing efficiency because a lower percentage of air loss occurs at the wing tips, and so induced drag is kept low. The trailing edge is approximately straight except for the portion between the inboard nacelles and the fuselagsj. The trailing edge of the flap in this portion hooks down ward, decreasing aerodynamic interference between the wing and the body, and minimising tail buffeting during climb. The 4^-degree dihedral angle of the wing, the 7-degree sweep-back and the large vertical fin make the machine very stable, both in direction and in roll, suppressing any tendency to slip or skid. Any tendency to fall off on one wing during a stall is immediately converted into a bank and then into a glide, straight ahead, a desirable characteristic. High wing loading requires large wing flap area to reduce the speed of landings and distance of take-offs. The Boe ing flap achieves for the B-29 a landing speed the same as that of the Boeing B-17. To decrease the take-off distance and speed, the flaps are extended 25 degrees at take-ofi. * The flaps roll back and down, increasing the wing area and the downwash angle of the air leaving the wing. When the flaps are extended the total wing area is in creased 19 per cent. The Controls Although, as indicated above, the demand for aerody namic cleanness was paramount in the development of the Boeing B-29, control and stability were given equal atten tion. When necessary, sacrifice in performance was made to achieve greater stability, resulting in an aircraft easy to fly and with no undesirable ground or flight charac teristics. Control forces are even lighter than in the B-17. Problems of developing control surfaces that would give adequate control and maximum stability for a machine of this size were many. At first it was thought that hydraulic boost of the flight controls would be necessary. How ever, by continued study and by wind-tunnel tests, and .by use of Boeing control tabs, control surfaces were devel oped that require no more control effort than those on much smaller machines. The result is a simpler, less expen sive, less vulnerable and more easily maintained control system than would have been the case with a hydraulic " boost" system, and at the same time one which retains the important pilot "feel" of the controls. Operation cf_ the rudder actually requires less effort than that of tHe" Flying Fortress. The r.ewly developed ailerons not only are aerodynamic- ally and statically balanced, but also have combination
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