Air to Air Refuelling in Civil Aviation, An Opportunity & A Vision Dr ...

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Fig. 32 shows possible options. Reverse tanking (tanker behind the receiver) with boom and receptacle may well be the preferred option. The tanker aircraft will do all the positioning and control work whilst the receiver aircraft continues on its way. There have been many recent advances in AAR systems e.g. in differential GPS and autonomous refuelling, RARO, night vision etc. as mentioned in previous papers Fig. 33 shows various tanker capabilities (fuel off-load and radius) currently envisaged by the military. The civil interest will be over a range of about 1000nm (radius 500nm). 4.6. Phasing in AAR This is related to the time and finances available. A worldwide network of bases for tanker operations can be envisaged. This would serve airlines using 3000 nm range aircraft equipped with AAR receptors. At present, operators tend to fall into various categories: Cargo, Passenger, Passenger and Cargo, Domestic (short haul), International (short and long haul), National (“Flag carriers”) and privately financed. Some diversity will exist with the introduction of civil AAR. A small carrier, equipped with short to medium range aircraft, could buy-in AAR services and offer long-haul flights. The AAR services may operate along the same economic principles as airport and ATC services. Based on figures already introduced, each tanker, in a day, can undertake 8 to 12 flights and service 20 to 30 aircraft. ICAO world airline data indicated 1,672,000,000 passenger movements worldwide in 2000, together with 33,500,000 tonnes of cargo. We assume that 25% of the passenger and 50% of the cargo departures were long haul (6000 nm). Using our 3000 nm aircraft with AAR for these long haul operations we would need 3,260 AAR operations daily. This equates to a global fleet of about 160 tankers. Such estimates can be further refined in due course. The civil AAR operations will be phased in over a period of time. It would be prudent to commence with cargo aircraft to build up public confidence for accepting the new procedures. During this introductory phase, AAR service suppliers would begin to generate finances, thereby encouraging financial interest. It is possible that a cargo carrier would seize the opportunity of providing the tanker services, especially if they already had operational / maintenance bases, strategically placed along the long-haul routes. The United States Transportation Command (USTRANSCOM) coordinates the movement of materials and supplies worldwide for the US military. Aircraft used for these operations include the C-5 and the C-17. Frequent use is made of AAR for the longer ranges. A proof of concept and initial evaluation of AAR in civil aviation to reduce fuel consumption could be undertaken by the military. Market Forecasts Using the latest Boeing Studies (Ref.15), one can conjecture with AAR, a change in the new deliveries at least, See Table 1. There is always 6 need for more detailed work including fuel savings and costs! In the above forecasts for 2027, 3,890 Freighters are included. These comprise 35% standard body (80 tonnes). With AAR, the Twin-Aisle aircraft needed will be the smaller range versions, either modified or new (more fuel efficient with increased payloads, pax + cargo). These will affect the Single-aisle numbers also. About 200 Tankers will be added. _ 5. CASE STUDIES AND COST IMPLICATIONS We consider a set of scenarios as illustrated in Fig.33. Depicted are possible routes (a, b and c) and tanker base locations. (a) Based at Anchorage North America to Asia could utilise AAR operations based at Anchorage. This is one of the busiest International air cargo airports, with 2.15 Million Tonnes handled in 2005. This gives an idea of the scale of operations passing through / over Anchorage. Typical distances are Tokyo to Anchorage, about 3000 nm and Anchorage to New York, 2935 nm. (b) Europe to Australia This would allow an airliner to depart, for example, from London with a full complement of passengers and freight and fly non-stop to Sydney. It could take off using de-rated thrust, climb quickly to best cruising altitude and use AAR to achieve a direct flight. Avoiding a technical stop would help in potentially halving the life-cycle costs and extending its service life. This case follows the example of the 9000nm aircraft potentially saving 40% fuel. (c) US West Coast to Australia West coast USA to Australia could use the old “Flying Clipper” routes of PAA - Honolulu, Guam, Nadi, etc as tanker bases. Typical distances in this region are Los Angeles to Honolulu, 2220 nm and Nadi to Sydney 1710 nm. The early “Flying Clippers” pioneered such routes. An Example with Freighters, B747F & B767F The B747-400F, operating close to Pt B (MTOW), can carry a payload of 123,000 lb over 7,200 nm. At reduced TOW, it can carry twice that payload over 3600. The B767-300F, at Pt A, carries 123,000 lb over 3,300 nm. Using various combinations of aircraft and payloads we note, in Table 2, fuel savings afforded by AAR. Tanker fuel has been taken into account, RT = 4. Refuelling the B767-300F with 88,451 lb and later ‘topping-up’ with 16,082 lb is not the most efficient scenario or use of tankers. Greater savings could be achieved with alternative arrangements, e.g. taking off light and refuelling to maximum capacity twice, or naturally, more efficient aircraft. In addition to the DOC estimates we currently employ for civil passenger aircraft, it would be useful to have some idea of two associated tanker costs. The first of these is the tanker operating cost per hour. This could be derived from military figures with a conversion to civil operations. The
second is the basic cost of converting say, an A330 or B767 into a civil MRTT. The cost will be appreciable but not restrictive. The possibility of converting an existing airliner to a pure freighter is factored in to the life cycle of most types. The additional cost of including AAR and tanking facilities is actually quite a small percentage of the total conversion cost. Freight doors and floor strengthening are the costly structural items. If, by utilising AAR, a technical stop can be avoided then a valuable ATC slot can be traded or sold. At London, Heathrow (LHR), at peak time, a paired departure and arrival slot would be worth about £30 million. 6. AN UNCONVENTIONAL CHALLENGING CONFIGURATION We have shown that maximum fuel efficiency is achieved by aircraft designed for approximately 3000 nm range. Using AAR, this high efficiency can be maintained over longer ranges. Currently, the B757 is a fuel-efficient aircraft. It carries 200 to 220 pax (single-aisle) over 3000 nm. It may not offer adequate comfort for long-range flights. It is opportune to consider possible challenging unconventional designs, exploiting the 3000nm range and the advantages afforded by AAR. A current interest is the design of moderate range aircraft, broadly in the class of the Airbus A320 series. We aim at 220 seats (as in A321) in an economy twin-aisle cabin. This implies a shorter, stubbier, fuselage than the A321. With emphasis on fuel-efficient flight, the Range parameter (X = V. (L/D)/SFC) needs to be increased. For conventional layouts, although near maturing technology levels, respectable gains are still feasible with lighter structures etc. It is always difficult to include radical changes in an established line! It may be useful to consider unconventional types such as a Joined-Wing layout enabling a high aspect ratio (AR) wing to improve L/D. Many related structural issues exist as shown in Fig.35, Ref.25. The join is at 0.6 y/s. The power-plant location offers several choices. The immediate benefit of reduced cabin noise afforded by rearmounted engines is evident. Structural integrity of fuselage or tail-mounted engines requires further thought. Similarly, the effects of thrust-line on high mounted engines compared with those at the fuselage side will have a bearing on wing-tail location. Design considerations (structural and aero-elastics) need to include the height of the rear wing and its join on the fuselage or vertical fin. An alternative proposal is with forward swept tips with a different set of compromises. All this will be an interesting future topic. It may be useful to think also in terms of a strut-braced version. The main sizing driver is the integration of a fuselage in a joined-wing layout coupled with the requirement for adequate fuel storage. The payload - range performance demands, therefore, lead to reasonably thick aerofoils (t/c normal to the LE, between 12 and 15%) operating at moderately high CL values, near 0.6. Depending on wing sweep, 7 thicker aerofoils can be adapted for hybrid laminar flow. Coupled with efficient propulsion, there is the prospect of 10-20% increase in X. This in turn implies weight reductions / fuel efficiency gains. 7. SUMMARISING, LIST FOR FIRST- LEVEL ANSWERS FROM DIFFERENT VIEWPOINTS It is useful to begin a list emphasising the key features / benefits using AAR from different viewpoints. Some questions have been answered at a first level. Now is the time for initiating serious studies. Most features are inter-related. It is hoped that these will provide insights into understanding AAR operations / feasibility. No doubt, readers will contribute, with a positive outlook. AAR is a "multiplier" and Enabler - All efficiency improvements currently envisaged for future designs and upgrades will be enhanced - Co-exists with Close Flying Formations (CFF) ideas - Long range fuel consumption down 30% - Should enable Laminar (natural or hybrid) flow achievement (+15% say) with lesser penalties - Works naturally with Close Formation Flying. L/D up 10% - Long range, direct flights enabled with full load, pax + cargo. Fuel cost savings: $175/pax for 6000nm with AAR, $320/pax for 9000nm with AAR (Assuming fuel at $4.7/USG). Extending the sums to 250 seaters, fuel cost savings are $43,750 and $80,000 for 6000nm and 9000nm flights respectively. Over a year, the savings for 6000nm and 9000nm aircraft are $16 & $29 million respectively. - Life cycle costs halved - Service life extended - Costs of technical stops eliminated - Aircraft operate at max. payload, max. efficiency Airframe and engine manufacturers modify design requirements - More, smaller aircraft required (3000 nm range) - More, lower thrust and smaller engines - Alternative, more suitable airframe shapes may evolve (Joined-wing) Smaller (and more uniform) aircraft imply - Reduced wake strength (function of weight and span or cav, given CL) - Reduced separation time, Increased frequency of aircraft movements - Reduced lateral translation of wake (ground effect) - Improved utilisation of adjacent runways - Reduced thrust requirements, climb quickly to requisite best altitude - Less engine noise, increase night flying capability - More movements from a given airport (up by 30%) - Fire hazard less with smaller aircraft, easier on Equipment (Aviation Week, 2008) Operators modify procedures - Reduced investment needed (smaller airframes cost less) - Operate at maximum payload for maximum efficiency
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