Currently, there is great emphasis, worldwide, on environmental issues. This will have an impact on civil aircraft design, manufacture and operation.Since the advent of the jet engine and swept wing aircraft, the trends have naturally tended towards greater productivity through increasing speed and payload. The cruise speed of conventional civil aircraft is unlikely to increase beyond current levels. Further increases in productivity are achieved by increasing payloads. This has led towards larger aircraft with the capability for increased ranges. It is shown that designing aircraft for longer ranges increases fuel burn significantly.A series of aircraft operational parameters have been analysed. Selected data and established trends for current and future aircraft are presented. The data has been interpreted into efficiency terms, relating payload, range, fuel consumed and a measure of unit costs. It is shown that 'value' (cost) and noise effective efficiencies decrease dramatically with increasing range.Environmental and economic considerations, in the future, may well demand greater efficiency in preference to productivity. One solution for long-range services is to use short-range hops. Another is via air-to-air refuelling. This will be addressed, in more detail, in a future paper. NOMENCLATUREc1, c2 technology constants (see Section 4) CFRP carbon fibre reinforced plastic DOC direct operating costs EXP exponential HBPR high by-pass ratio L/D aircraft lift/drag ratio M Mach number OEW operating empty weight PRE = WP *R/WFB, payload range efficiency R range (nm or km) SFC specific fuel consumption TOW take-off weight (MTOW, maximum) V aircraft velocity VEM = PRE/MTOW, value efficiency per MTOW unit VEO = PRE/OEW, value efficiency per OEW unit VEMPX = VEM*WP/X, non-dimensional value efficiency, Section 6 VEOPX = VEO*WP/X, non-dimensional value efficiency, Section 6 WFB block fuel load WFB/WP fuel payload fraction WFRes or WFR, reserve fuel load WFT total fuel load WP payload THE AERONAUTICAL JOURNAL

Summary As civil aviation expands, environmental aspects and fuel savings are becoming increasingly important. Amongst technologies proposed for more efficient flight, air-to-air refuelling (AAR), ‘hopping’ and flying in close formation (drag reduction), all have significant possibilities. It will be interesting to know also how these technologies may co-exist e.g. AAR and formation flying. In military use, AAR is virtually indispensable. Its benefits are real and largely proven in hostile and demanding scenarios. We present a case for applying AAR in a civil context to show that substantial reductions in fuel burn for long-range missions are achievable. Overall savings, including the fuel used during the tanker missions, would be of the order of 30-40% fuel and 35-40% financial. These are very significant in terms of the impact on aviation’s contribution to reducing atmospheric pollution. AAR allows smaller, efficient (greener) aircraft optimised for about 3,000nm range to fulfil long-range route requirements. This implies greater usage of smaller airports, relieving congestion and ATC demands on Hub airports. Problems due to shed vortices and wakes at airports are reduced. Smaller engines will be needed. Integrated (accepted) AAR could lead to further benefits. Aircraft could take-off ‘light’, with minimum fuel and reserves and a planned AAR a few minutes into the flight. The ‘light’ aircraft would not require over-rating of the engines during take-off and would therefore be less noisy during take-off and climb-out, permitting more acceptable night operations. The availability of civil AAR will enable opportunities for hitherto borderline technologies to be utilised in future aircraft. Laminar flow will provide fuel savings and increased efficiency in its own right but could be significantly enhanced within a civil AAR environment. Similarly, supersonic transport may become an acceptable economic option. AAR affords the possibility of a complete widening of the design space and this should appeal to the imagination of current and future designers.

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