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Aviation History
1958
1958 - 0258.PDF
272 FLIGHT, 28 February 1958 VISCOUNT 810 ... Continental Airlines have expressed dieir intention of convertingto "400 m.p.h. Viscounts" (with their obvious sales appeal) when the engines become available, but many other customers may beexpected to take factory deliveries of new 840s in die 1960s. With re-engining to 2,100 shaft power for take-off in prospect,it is tempting to ask why the 1,910 shaft horse power engine should need to be de-rated for take-off to 1,750 s.h.p. The shortanswer is that the maximum available power from the Dart 525s is not required, and trie engine is therefore physically restrictedthrough the fuel control unit so that unnecessary fuel is not used. It does not always follow that additional power will result in agreater margin of safety over the WAT curve limitation; for example, if more power is used for take-off, the aircraft's asym-metric yawing moment increases and the take-off safety speed is correspondingly raised. In any case, the important thing is to have the necessary powerunder the critical condition, not merely to have an excess under the non-critical sea-level conditions. This is precisely what the Dartde-rating achieves. Complementary to the instrument panel photograph below is this picture of the roof radio panel of Continental Airlines' 812. There is an optimum power for the aircraft weight, and this is set on the Viscount 810 at 1,800 s.h.p. Other advantages follow in train: the limitation on maximum power enables a rather lighter reduction gearing to be used than would otherwise be the case, along with a smaller propeller shaft, lighter propeller design and much reduced water/methanol consumption. Power-limiting of the engines in this way is illustrated on theaccompanying three-dimensional graph, the effect will be familiar to those who recall the "rated altitude" of piston engines. On theengines installed in the Viscount 810 (four Rolls-Royce RDa.7/1 Mk 525B) the "B" suffix denotes that a capsule has been removedfrom the water /methanol injection unit so that it has become insensitive to altitude; injection consequently restores power at anyaltitude (within the confines of the altitude/power curve) to 1,750 shaft horse power. The dotted triangle on the graph illustrates the1,750 s.h.p. boundary of this effect. With the RDa.ll engine, the parameters of the curve are modified; the maximum shaft horsepower of 2,400 is limited to 2,100 for take-off. Even so, water/ methanol injection will not be required at temperatures belowI.S.A. + 28 deg C—it will be needed, in fact, on very few occasions. The aircraft described in detail in this article is the Viscount812, 15 of which have been ordered by Continental Airlines of Denver, Colorado, who refer to the aircraft as the Viscount 2.The specification hence includes many items of American equip- ment, but the structure is naturally common to all 810/840 air-frames. The Continental aircraft do, however, provide a good example of the additional equipment that is required to offercompetitive service in the U.S.A. » Structure. The dimensional envelope of die 810 is almost TOentical to that of the 800; the fuselage is 46in longer than the700 and the pressure bulkhead has been moved 65in further aft. Some adjustment has been made to the fuselage jigging to allowthe back-to-back channel-section frames which carry the spar loads into the fuselage to be opened .040in to accommodate thedimensionally-increased top and bottom spar booms. Weadier radar, familiar in die nose of 700 series Viscounts, has again beenfitted. The forward entrance door is slightly smaller man die special-purpose door of the 800 series Viscounts, and for Con-tinental Airlines this revised door includes a built-in Airstep. In order that die modified floor in the region of the Airsteps may befully utilized when the steps are not fitted, a light alloy fairing covers the recessed floor. The hydraulic jack and airstep linkageis mounted under the floor, but the controls are situated on die bulkhead. An additional 11 cu ft of underfloor freight space isavailable when airsteps are not fitted. An emergency button releases (Right) A three-dimensional explanation of the effect of de-rating the powerplants to 1J50 s.h.p. for take-off. Fall-off in power with altitude and temperature is shown separately in the lower drawing, and the upper corves are combined above to show the effect of water/methanol injection. Centered in the 812 cockpit layout is the scope of the R.C.A. AVQ-10 weather radar; to accommodate it the torquemeters hare been paired. Note that 4in-diameter instruments are used on the Collins FD-105 flight system, which is combined with a Beadix Pfi-IQ autopilot iato which radio information can be coupled. 20,000 ISA I.S.A.»3O*C 10,000 POWER CURVE: ALTITUDE SENSITIVE WITH FUEL FLOW LIMITATION TO 1,750 sKp.MAX. TOWER RESTORATION BY WATER /Mf l(MAX.ALTITUC ABOUT 1OPOC POWER RESTOR/ BYWATE METHAN RESTRIC 1,750 s.h WATER /METHANOL INJECTION POWER LIMIT SENSITIVE TO TORQUE 1,750 POWER WATE METHA POW RESTO ISA t75O POWER
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