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Aviation History
1963
1963 - 0704.PDF
676 FLIGHT International, 9 May 1963 BOEING 727 ... At the outset a maximum CL of the order of 1.8 was being dis cussed, but as 1957 and 1958 went by Boeing set their sights pro gressively higher. The company have superb facilities for a pro gramme of this nature; their own 54,000 h.p. tunnel with an 8ft x 12ft working section was ideal, and three other 12ft tunnels were available in Seattle. In addition, really promising schemes could be turned into full-scale hardware and tested in the air on the "Dash 80"—the company-owned chocolate-and-yellow predecessor of the whole 707 family. Leading-edge systems investigated included slats, various types of Kriiger flaps (sections of the leading underside of the wing arranged to hinge downwards about the leading-edge) and even blown leading-edge flaps. Trailing-edge schemes initially com prised a variety of types of "fillet flap" close in against the fuselage, blown by high-pressure engine bleed air. By mid-1960 refined versions of this flap had been evolved, and a full-scale specimen with blowing over both the surface and its track fairings was flying on the Dash 80. But when the 727 was publicly announced on December 5 of that year the decision had been taken to adopt unblown triple-slotted flaps. This increased the weight and the number of flap segments, but mechanical complexity has in some respects actually been reduced (Table 1). Table I: Boeing transport comparison Gross weight Wing area Typical seats Cabin length Installed thrust CL fUps/Ci. clean TE flap segments TE flap tracks TE flap drive gearboxes LE flap segments LE flap power units 727 - 39 (or 35)% -32% -20% -25% -42% 2.24 + 20% -33% -22% -13% -50% 720B Ref (235,000) Ref (2,433) Ref (108) Ref (96' 4") Ref 1.7 Ref Ref Ref Ref Ref 707-320B + 40% -23% + 19% + 16% Ref 1.7 Ref -17% + 28% J 50% + 29% The 727 is the first aircraft in the world to have triple-slotted flaps, and the complete wing has a maximum lift coefficient of at least 2.8. This figure, which is more than half as high again as the original goal when 727 project studies began, is 2.24 times the maximum lift coefficient attainable in the clean condition. Both this ratio and the absolute value are believed to exceed the results achieved with any other high-subsonic wing. At a future date it would be logical to expect Boeing to introduce a closely similar high-lift system to the 707 Intercontinental, and possibly also to the 720B. It should also be noted that in the large diagram en page 677 the plot showing the evolution of the 727 high-lift system does not stop at the max/clean lift ratio of 2.24. A value of nearly 2.4 has been achieved during model testing of a wing with a triple-slotted flap and a blown leading-edge flap. Although this has not been adopted for the 727, it may point the way to future development. While one team at Seattle strove to extend the 727 speed range downward, another burnt the midnight oil in a search for the opti mum configuration. The simplest solution would have been merely to have produced a smaller 707. Douglas suggested this answer to the short-haul problem: the original DC-9 (Model 1925) had four Pratt & Whitney JTF-10 turbofans. and looked like a small DC-8. Boeing spent a lot of time on this sort of aeroplane, harden ing their interest into a study known as the 720 Junior. But • preliminary calculations repeatedly showed that a twin-engined aircraft would have lower direct operating costs, and in this market d.o.c. was considered to be even more important than usual. The project studies therefore began to show a preponderance of twins. For example, the 727-264C was the third sub-variant of the 64th 727 study with two engines (it happened to resemble the Caravelle), while the 727-265 was the 65th twin study (this one was basically laid out like a 707 but having only two JT3D or Conway turbofans). A particularly novel line of approach was to place two engines in pods above the wing trailing edge. The idea was to make the nacelles serve as shock bodies, but model testing showed that they were equally effective at inducing flutter. The idea of having three engines crept in gradually. Some engineers at Renton may have been aware of the fact that the origi nal de Havilland jet-airliner study of 1945 was based on three aft- mounted engines, and that the original SE.210 (Caravelle) was based on three similarly located Atars; but the airline industry— their customers—had grown up with a strong, if illogical, belief that all proper transport aircraft had an even number of engines. It is interesting to speculate upon whether Boeing would have decided to "go it alone" as the odd man out if nobody else had shown any interest in three-engine configurations. As it was, however, Avro, Bristol and de Havilland arrived at the formula of three aft-mounted engines in their fight to produce a short-haul jet for BEA, and this undoubtedly made such a novel configuration more acceptable at Renton. The winning de Havilland design, the D.H.121, started off remarkably similar to the final 727; but at BEA's insistence it was later cut down in size and turned into the Trident—described in last week's issue. By 1958, therefore, Boeing were examining 727 studies with two, three and four engines, without any prejudice for or against any given number or location. By this time an important new factor had become appreciated. For the first time in the United States it was possible for a company like Boeing to have a civil transport engine built to their own specification. The technology of gas turbines, and of turbofans in particular, had reached the point at which the development of a wholly civil engine was an acceptable risk; and the steady falling away of military business was making such a move increasingly agreeable to the engine builders. It is now no secret that Boeing held discussions with two engine companies, Pratt & Whitney and Rolls-Royce (and with Allison, acting as Rolls-Royce agents and potential licensees). Soon attractive paper turbofans began to arrive at Renton, and Boeing set about choosing the right engine, the right number and the right location. Provided one has enough experienced engineers, such a task is no hit-and-miss affair; and the answer which emerges is likely to be the correct one. The only real degree of uncertainty is introduced by the customers: some are not sufficiently skilled to know what they want, and others are prone to change their minds. In such matters Boeing are fortunate, for their mai-n customers are the US domestic trunk operators, and no more com petent set of airline engineers exist anywhere. Table 2: US customer specifications Airline AA EAL PAA TWA UAL How many engines? What body width? (in) How many first-ciass pas sengers? What landing field-length? (ft) What approach speed? (kt) What max. range (n.m.) 2 (or 3) * 136 (or 148) 64-68 4,700 115 1.000 3 148 72 117 1,500 3 (or 4) 148 64 i.600 2 (or 3) 148 (or 136) 65 4,500 III 1,000 4 (or 3) 148 68 5,200 125 1,500 * Figures in parentheses are acceptable second choices Boeing asked each of these airlines to define the main parameters of the ideal short-haul jet, and their answers are listed in Table 2. But long before these replies were in, Boeing's Transport Division was engaged in a parametric study of unparalleled breadth and depth. This study attempted to take into account not only all the major factors affecting the design of the 727, but also the effect of possible future changes in such matters as the length of given runways, the performance of existing or projected engines, the weather minima laid down by the FAA or the operators, the basic laws governing certification and the formula; used in the deter mination of d.o.c. As an example of the sort of calculations involved, Boeing examined the weather in New York for a whole year, correlated i t against the 11,041 departures scheduled from New York by a chosen airline at the exact times at which each flight was due to leave, and determined the precise difference in schedule reliability between a twin-engined 727 and a 727 with more than two engines. As a legacy from piston-engined days, the FAA virtually restrict twin-engined take-offs to landing minima—say, 300ft and one mile. Boeing ascertained that, in this respect, three-engined aircraft would be treated no worse than existing four-engined machines, and assumed take-off minima of 200ft and half a mile. Table 3: Weather reliability with two or three engines (New York) Departures delayed Ihr or more Annual delays of Ihr or ! more Departures delayed over j 2hr Annual delays over 2hr Two engines 3.1% 342 2% 221 Th ree engines 1.14% 126 0.34% 38 Difference 1.96% 216 1.66% 183 Continued on page 683, after fold-out illustrations of Boeing 727
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