SLENDER AIRCRAFT FOR FLIGHT AT SUBSONIC. SPEEDS OVER SHORT RANGES
7.1 Gates1 concept of an aerobus. Most of the design problems discussed so far have been concerned with aircraft to fly over medium or long ranges, like the transatlantic range. Yet we have argued in Chapter 1 (see Fig. 1.7) that there is also a short-haul transportation gap to be filled by aircraft, when these take over from road and rail transport at distances beyond 300km or 500km. A typical short-haul operation may be a sortie of 2 x 400km without refuelling at the stop in between.
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Short-haul transport makes special demands: above all it must be cheap; and high technical standards as well as reliability, leading to high utilisation and low maintenance costs, can contribute to making it cheap, (see e. g.
A L Courtney (1965) and R H Whitby (1965), H Ziegler (1972)). Further, short – haul as compared with long-haul transport is special in that the actual door – to-door travel time may be substantially longer than the flight time owing to surface travel and airport processing, apart from any airborne or ground delays (see e. g. F R Steven (1973)). For these reasons, various attempts are made to design specialised aircraft for the purpose and to devise some integrated transport system. Technical solutions include helicopters and aircraft that can take-off and land vertically. We do not discuss these here but concentrate on one possible concept which has been proposed by S В Gates
(1960) , (1964) and (1965) (see also H H В M Thomas & D Rttchemann (1974)).
Gates had the social aspects of long-term developments in aviation foremost in mind. He wanted air transport expanded from a special service for only a small number of people to a utility service for everybody: his aerobus was to provide cheap wraps for air travellers. Although Gates’s ideas caught the imagination of many so that ‘airbus’ is now a household word, the present aircraft to which the name airbus has been given do not strictly conform to what Gates had in mind. His much more radical proposals still remain to be developed and put into practice. For our purpose here, Gates’s aerobus provides an instructive example of the possibilities of aerodynamic design.
Gates argued that Cayley’s concept does not necessarily lead to the only possible layout within a given set of aerodynamics. He proposed to discard the fuselage, as a non-lifting parasite, whose structural virtues are often cancelled in a conventional layout anyway, and consider once again allwing aircraft. An "aeroplane consisting of one wing, which would house all components, engines, crew, passengers, fuel and framework" was, in fact, patented as early as 1910, by H Junkers, and other notable attempts to design tailless allwing aircraft have been made by G T R Hill (1926) with his Pterodactyl, by A Lippisch (1931), by J К Northrop (1940), and by Armstrong Whitworth (AW 52, 1947). All these retained unswept or swept wings of fairly high aspect ratio and none was really successful. Another more suitable layout had to be found, and a strong contender turned out to be the slender wing.
If a classical aircraft is to be designed for very short ranges, its layout will be much the same as that of a long-range aircraft, as has been shown in section 4.1. In some ways, the design may be more demanding: a wing whose aspect ratio is still high must be combined efficiently with a relatively bigger fuselage, since the payload fraction will be higher (see Figs.1.3 and 4.4); and high-lift devices may have to be more effective, if shorter runways are to be used, i. e. the problems indicated in Fig. 4.10 may be more severe.
On the other hand, the potentially high aerodynamic efficiency of such an aircraft may not be fully used: the aircraft may cruise well below the maximum value (L/D)m of the lift-to-drag ratio because the fuel fraction is relatively small and the engine weight matters more. To illustrate this point, the L/D-curve of a typical swept aircraft from Fig. 4.1 has been redrawn in Fig.
7.1, but with the cruise point somewhere well below (L/D)n. This L/D-value is assumed to be sufficient to achieve the required short range. This is where the slender wing comes in: such an L/D-Value can be achieved also by a slender wing at subsonic speeds. To illustrate this, the appropriate L/D – curve of a typical slender aircraft from Fig. 6.2 has been reproduced in Fig. 7.1. The slender aircraft will cruise at a lower CL~value than the classical aircraft, i. e. it will have a larger wing and a lower wing loading.
A lower wing loading was one of Gates’ early, design aims: it should make takeoff and landing easier and safer. But it must be shown that the low wing loading is compatible with the other design aspects, especially with the structure and engine weights. This will be considered in section 7.2.
To fix our ideas, we may think of a slender allwing aircraft for short ranges having a compact shape like that sketched in Fig. 7.2, by comparison with a corresponding swept-winged layout, where the shading indicates the inhabited areas. The span of the allwing aircraft is significantly smaller than that of the swept aircraft, but its semispan-to-length ratio (about 0.35) may be larger than that of a slender aircraft to fly at supersonic speeds (about 0.25). The wing will have to be thick enough to house the flat3 noncylindrical, passenger cabin, which in turn implies that the aircraft must be large enough, having, perhaps, 150 seats or more (although 100-seaters have
Fig. 7.2 Comparison between classical and allwing layouts
been studied and found to be not impossible). The gradual evolution of allwing aircraft from swept wings to compact slender layouts has been described by G H Lee (1965) and supported by project studies. It was during this work, done at Handley-Page, that J В Edwards realised that, for an allwing slender aerobus, the engines could be mounted above the wing so that the engine noise would be shielded very effectively by the wing, as has already been described in section 5.9.