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.

Operations and aircraft design towards greener civil aviation using air-to-air refuelling

The interactions of wingtip vortices in close-formation flight of two wings were examined experimentally. Both wings consisting of a NACA 23012 airfoil section and a rectangular planform of an aspect ratio of 5 were set in echelon formation in a wind tunnel. The lift-to-drag ratio increased the most effectively when the two wings were overlapped with each other by 5% of the wingspan with zero vertical offset for the present wing configuration. Flow visualizations and particle image velocimetry measurements showed that the trailing vortex of the lead wing collided at a slightly inboard point of the trail wing and was divided into two streamwise vortices in the most effective overlap condition. It was also observed that the tip vortices of the lead and trail wings interacted closely with each other to form a pair of counter-rotating vortices for smaller overlap conditions less than 5 % with zero vertical offset. For such small overlap cases, circulation of the trailing vortex of the trail wing was reduced markedly by influences of the trailing vortex of the lead wing.

Detailed Observations of Interactions of Wingtip Vortices in Close-Formation Flight

Inspired by the energy savings that migrating birds achieve when flying in formations, of interest is achieving a similar level of savings with aircraft flying in the National Airspace System (NAS). In particular, with typically more than 50,000 flights in the NAS on any given day, there is much clustering of aircraft so there is much potential for creating formations. Most previous research in this area has focused on analyzing aircraft in a single formation ‐ developing computational models, performing wind tunnel tests, or developing controllers for maintaining a formation ‐ but little research has been done on investigating a NAS-wide implementation of formation flight for commercial aircraft. This work leverages the corridor-in-the-sky concept from on-going aviation research to develop a model for simulating formation flight with these corridors. The results show that for the top ten corridors there are approximately 1,200 flights which can benefit from formation flight. Compared with actual flight plans the resulting annual savings is approximately $600 million and compared against flying optimal, great circle trajectories, the resulting annual savings is approximately $320 million. Finally, Monte Carlo simulation shows that these savings are robust to uncertainty and fluctuated flight schedules.

An Analysis of the Potential Savings from Using Formation Flight in the NAS

Over the last 70 years, the civil aviation industry has dominated world transport. Growth has been upwards bigger, farther and faster on an economic productivity basis. With awareness of environmental issues, noise, emissions and energy / fossil fuel reserves, changes will happen and possibly in an accelerating fashion to improve the carbon balances. The NASA and ACARE (Europe) objectives are to reduce Aviation’s environmental impact by 50% or more. This paper reviews the current work towards meeting such challenging objectives. A new set of Efficiency metrics of Civil Aviation allow development of a “unified” consistent efficiency theme, relating Payload, Range, Fuel consumed and a measure of Unit Costs. The “value” (cost) and noise effective efficiencies decrease dramatically with increasing Range. An operational strategy leads to a way forward for Fuel-efficient Commercial Aviation using smaller aircraft, adopting Air-to-Air Refuelling (AAR) and Close Formation Flying (CFF). For longer ranges, AAR and CFF, in concert, go most of the way toward satisfying NASA / ACARE objectives. Several avenues of further work arise.

/pdf/highly-efficient-and-greener-civil-aviation-organising-a-443zob0pwi.pdf

Highly Efficient and Greener Civil Aviation - Organising a Step Jump towards ACARE Goals An Opportunity for the Present & a Vision for Future

Currently there is great emphasis on achieving efficient and optimised flight and operations. The planned budgets and future constraints may reflect this. The need for overall energy savings is being felt in all spheres of defence and commercial aviation. The military scene includes many types of aircraft fulfilling diverse roles during training and in action. An appreciation of Efficiency parameters leads to the consideration of speed (V), lift-drag ratio (L/D) and engine efficiency (SFC) with Range (R). A range parameter X = V L/D/SFC follows. Operational aspects e.g. Air to Air Refuelling (AAR) and Close Formation Flying (CFF) also contribute. Interplaying all these leads to new designs or morphing technologies for flight optimisation. We can think in terms of sub-systems or in a more global sense, incorporating several technologies optimally. We consider here the efficiency aspects of heavy-lift, military jet transports. Currently, there are only a few types in service and limited consistent data is on weight fractions available. Consistent data-sets on Russian aircraft are rare. We use the various weight fractions and payload range for the dimensional / non-dimensional parameters. Efficiency trends, established for civil passenger aircraft and later expanded to include civil freighter aircraft, have been described. These provide the background, a foundation and nature of comparative aspects for building a quantitative assessment of the military jet transports (even with data limitations). The non-dimensional parameters introduced (e.g. PRE/X and Z = R/X) allow use of data from current and older types and implied technology levels, with relative ease. This is an important feature of the work. Several similarities are noted in the weight ratios for the military transports and freighters. However, a surprising notable trend was that military transports use less Thrust to Weight ratio for “normal” 2.5 g flights.

http://enu.kz/repository/2009/AIAA-2009-44.pdf

Operating Efficiency of Military Jet Transports

The idea of flying commercial aircraft in formation to reduce fuel usage, has been around for some time. There are many results available using idealized approaches e.g. vortex lattice formulations. In view of the greater importance being attached to environmental aspects, the need has arisen to evaluate the possible advantages and disadvantages. A previous paper was concerned with a limited number of flight formation configurations comprising identical wings. Even then the predicted induced drag reductions of near 30%, affording overall drag reductions of the order of 15-20%. This paper steps up the analysis level exploiting a recently developed design method that allows span loading and camber control on wings without and with winglets. The method has been adapted to assess the aerodynamics of wings in formation and then to re-design them to eliminate induced roll effects. We have extended the analysis by enlarging the formation size, varying the spacing parameters (x, y and z) and varying the relative sizes of the aircraft within the formation The technique has proved to be easy and robust in use. It is enlightening as it gives, at every stage, a feel for what is happening in terms of camber development, pressure distributions and Centre of Pressure location. We have been able to define areas of specific interest that are worthy of further analysis in terms of formation geometry. Conversely, formation geometries that are not beneficial (drag penalties or the need for prohibitive control surface deflections) can be avoided. Lift induced drag reductions of up to 60% may be achieved on a Trail aircraft following a larger Lead wing. A limited number of results using an Euler solver reflect benefits of the same order for equi- and varying sized aircraft in formation. Several avenues of further work and development have arisen.

R. K. Nangia

Papers

Formation Flying of Commercial Aircraft, Variations in Relative Size / Spacing - Induced Effects & Control