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Aviation History
1947
1947 - 0916.PDF
.53" FLIGHT JUNE 5TH, 1947 have practical application to very largeaircraft where the pilot's skill and strength arc largely supplanted bymechanical means of one sort or another, and wherein the pilot controls the mech-anism which in turn places the aircraft at the desired attitude. If the C.G. islocated aft of the aerodynamic centre, the aircraft will trim at a high angle ofattack with the flaps or elevator sur- faces deflected downward rather thanupward from their normal position, thereby increasing the camber andrendering the whole aerofoil surface a high-lift device. It is possible thattrimmed lift coefficients in the order of 2.0 may be achieved by this method, andexperiments completed to date with such a device on conventional aircraftshow that the C.G. may be displaced at least 10 per cent of the mean aero-dynamic chord aft of a normal position without any uncomfortable rqeults inthe flying characteristics of the Machine t-i ALL-WINC AIRCRAFT crease in gross weight of 16 per centwithout increase to weight empty, with a corresponding increase in range up to30 per cent. " It is fairly obvious that the all-wingtype provides comparative structural simplicity, plus the possibility of struc-tural material distribution in a most effective way at maximum distancesfrom the neutral axis, plus an oppor- tunity to stow power plant, fuel andpay-load at desirable intervals along the span of the wing, which cannot beequalled in conventional types. These matters are rather intangible and diffi-cult to illustrate by numerical relation- ships. They depend to a large extent onthe type and size of the aircraft, what it is designed to carry, and what thedesired high speed may be." and control surface arrangement. Fig. 3shows two extremes of arrangement in which this machine was found satisfac-tory in flight. The principal early troubles related to cooling of the smallair-cooled engines, which were buried in the wing. This machine had made morethan 200 flights, and the control which gave the most trouble was the rudder.Many rudder configurations were tried, but in the end the best and most prac-tical was found to be the plain split flap used as a drag-producing device. It waslater combined with the trimming surface needed to counteract the diving momentof the landing flaps, forming movable control surfaces at the wing tips of theXB-35, as shown in Fig. 4. The N9M Four machines of the next type, theN9M, were built and tested. They were flying models of the XB-35, but were,of course, single-seaters with twin pusher engines. The first was completed andtested on December 27th, 1942. After completing some 30 hours' testing iteventually crashed and the pilot was killed. It was assumed that it got intoa spin. Subsequent models, over hun- dreds of hours, gave no trouble, and thelow-speed stall and spin tests with rear C.G. position were accomplished withoutfurther difficulty. Wind-tunnel model tests indicated that"tumbling" characteristics (tumbling is rotation about the pitch axis—ED.) were Fig. 6. (Below) The Northrop XP-79B turbo-jet flying scale model. Fig. 5. A guder version of the XP-79 with fixed landing gear shown during one of the early rocket-powered flights. "When these improvements in CLmaxand C Dmin can be realized, further start-ling gains in performance will accrue. It would seem, however, that the presentaccomplishments offer sufficient incen- tive to warrant all they have cost intime, effort and money/ and that the question, ' why bother with all-wing air-craft? ' is already well answered. Interference Elimination There are other major advantagesof the all-wing type which cannot be so definitely evaluated but which can anddo contribute appreciably to improve- ment in efficiency and range. Two of these,namely, the elimination of jet-tail surface interference and the possible eliminationof wing-tail surface shock wave inter- ference, have already been mentioned.The third, and the most immediately applicable to designs of the near future.is the improved adaptability of all-wing types to the distribution of major itemsof weight empty and useful load over the span of the wing. While such dis-tribution can be made to a limited extent in conventional aircraft, it can be muchmore fully accomplished in the all-wing type. Such weight distribution resultsin substantial savings in structural weight which have important effects onthe ratio of gross weight at take-off to landing weight. An analysis of the rangeformula indicates that this ratio is one of the most important range parameters.Competent authority has shown that dis- tribution of fuel in the wings insteadof the fuselage of a large conventional modern transport would allow an in- Mr. Northrop next turned his atten-tion to some of the problems his company laced when it began to tackle all-wingaircraft development. The first wind- tunnel model tested showed very erraticelevator effect. The trouble was traced to separation along the trailing edge,apparently due to the plan form con- figuration. A simple addition of ten percent to the chord length almost com- pletely eliminated the difficulty. It was soon determined that dataapplicable to conventional wings were unreliable for the degree of sweepbackrequired, and a whole new technique had to be developed to determine the limitswithin which taper ratio, sweepback and thickness ratio could be combined forsatisfactory results. All were explored on a series of wind-tunnel models, andwhen a reasonably satisfactory group of configurations had been determined, itwas decided to build the first piloted flying wing, the NiM (Northrop Model 1Mockup). This machine provided for changes in plan form, sweepback, di-hedral, tip configuration, C.G. location, greatly affected by minor differences inelevon and C.G. positions, but the machine would never tumble from anynormal flight condition such as a stall, spin or other to-be-expected manoeuvre. The next step, conceived in September,1942, was to design piloted glider models of a high-speed fighter using liquid-; ktj^(.motors. The small size made it neces- sary for the pilot to lie prone. Threesuch gliders were built. The first had skids for car tow but was too heavv toenable a car to get it up to flying speed. The second had a detachable trolley,which was not a success. The third had a fixed undercarriage, which spoilt theclean lines and added weight, but did enable the glider to be towed off, and itmade a number of successful flights. As the airframe of the XP-79 was con-sidered suitable for the rocket motors long before these had been sufficientlydeveloped, it was decided to build a modified version, the XP-79B, which hadtwo Westinghouse B-19 turbo jets. The pilot flew this machine successfully for15 minutes on September 12th, 1945, and
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