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| Fig. 1: Boeing 787 in flight. This is one of the most efficient commercial aircraft currently in revenue service. (Wikimedia Commons) |
Aviation is a critical part of today's connected world. Commercial aviation allows people and goods rapid transportation to all points of the globe, including many trips that would be impractical by other means.
There is, however, intense pressure to make flying more efficient. Fossil fuels are a limited resource; extrapolating from global consumption data suggests that oil and gas will run out in about fifty years, and coal in two hundred. [1] While more reserves will likely be discovered, it is still fundamental that there is some finite reserve, and at some point all of it will be consumed. There is also the environmental aspect to consider. Aviation consumes approximately 2-3% of all fossil fuel use, and about 12% of transportation fuel use. [2] However, the emissions from aviation occur at altitude, and this results in a disproportionate effect on climate. [3] This means that improvements in aviation efficiency are more valuable than equal reductions elsewhere. Finally, there is financial pressure on the airlines; recent estimates have fuel accounting for 30-40% of airline's operating costs. [4,5] Futhermore, the aviation industry is still growing, at about 5% annually. [6]
Together, these call for an effort to make large improvements in aviation energy efficiency.
Today's airplanes are spectacularly efficient; current airliners are about 80% more efficient than the first jetliners in the 1960s, as result of improvements in all areas of the aircraft. [7]
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| Fig. 2: Operation of a high-bypass turbofan engine. Most air is passed around the core, reducing fuel consumption. (Wikimedia Commons) |
Much of the improvement has come from improvements in the jet engines themselves. The shift to high-bypass turbofan engines resulted in about a 40% improvement in jet engine fuel consumption. [6] Fig. 2 shows a high-bypass turbofan. Note that a large fraction of the air is accelerated by the fan around the core of the engine. By accelerating a large volume of air, high-bypass turbofans can produce more thrust with significantly lower fuel use.
Improvements in aerodynamics have also increased aircraft efficiency. An aircraft wing produces lift by pushing air downwards. This results in a lift force upwards, and also a drag force backward. The ratio of these is a measure of how aerodynamically efficient a wing is, and is called the lift-to-drag ratio (L/D). Since 1980, there has been a 15% improvement in this number. [6]
Because air that flows over the wing is turned downward, the wing tips make the air swirl into vortices. This swirling motion of the air takes energy that would otherwise have gone into flying, and therefore reduces the efficiency of the airplane. While this effect can't be avoided while also generating lift, it can be reduced significantly. This is the reason for the winglets seen on many commercial airliners seen today. First introduced on the Boeing 747-400, winglets are designed to interact with the air at the wingtips, reducing the intensity of the vortices, and therefore reducing drag. [8] On average, these devices reduce fuel use on a typical flight by about 4%. [8,9]
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| Fig. 3: Boeing 737-800 equipped with the split scimitar winglets. (Wikimedia Commons) |
Boeing recently made "split-scimitar" winglets available for their 737 series of airplanes. These winglets add a second fin below the wing. By splitting the winglet into two, the wingtip vortices can be further reduced without excessive loads on the wing. These devices are expected to cut fuel burn an additional 2% beyond a standard winglet. [10]
Jet engines are also being improved. Pratt and Whitney's new geared turbofan engine is expected to be approximately 15% more efficient than current engines. [11] This is largely because the gearing system allows each stage of the engine to operate closer to their most efficient speeds. The GTF will have minimal impact initially, as these gains won't be fully realized until these engines are used on a large fraction of airplanes.
Future improvements must be made if commercial aviation is to exist as fuel supplies eventually wane and climate concerns continue to grow. Much of the large improvements in engines and aerodynamics have been made, and will likely continue incrementally in future. However, there are several operational changes that can be made, without replacing the entire commercial aviation fleet, to achieve large improvements in efficiency.
Single-engine taxiing as part of normal operations on the ground has the potential to drastically cut fuel use and emissions for a portion of the flight that isn't doing useful work. Single-engine taxiing uses only one of the engines on the aircraft to taxi around the airport. Since the engines are at idle for most of this time, this save a significant amount of fuel. Data from Table 4 of Deonandan and Balakrishnan indicates airplanes at an average US airport use up to 250 kg of fuel per flight simply taxiing around the airport. [12] A study done at MIT indicates that simply switching to single-engine taxiing can save up to 40% of taxiing fuel. [12] This could save nearly 261 tonnes of fuel annually at just the top 20 US airports. Note that data was available for only half of all flights, and so this is a conservative estimate.
Current air traffic control (ATC) is based on a radar and transponder system that is decades old, and implementation of next generation ATC procedures will have large benefits. Using GPS for more precise guidance, flexible routing to take advantage of winds, and continuous descents into airports to minimize fuel burn on descent all will reduce overall fuel consumption significantly. It is estimated that 8% of all aviation fuel use is spent on inefficiencies in the flight path that could be avoided with improved ATC systems. [5] Preliminary trials of many of these technologies has yielded good results.
It is inevitable that at some point fossil fuel use must be eliminated. In the meantime, and to prepare for a possible transition to synthetic fuels, aviation must become much more efficient. There have been continual improvements in commercial jets for decades. There is still room for large improvements; making the operational changes suggested above impose few technical challenges. While implementation will be gradual to verify safety procedures, these are gains that are relatively easy to achieve, as the commercial airliner fleet need not be totally replaced, but only upgraded with some new instrumentation. This and other improvements on the horizon have the potential to effect percent savings in the double- digits, putting commercial aviation on an efficient path to the future.
© Arul Suresh. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
[1] "BP Statistical Review of World Energy," June 2014.
[2] B. Owen, D. S. Lee, and L. Lim, "Flying into the Future: Aviation Emissions Scenarios to 2050," Environ. Sci. Technol. 44, 2255 (2010).
[3] J. E. Penner et al., eds., Aviation and the Global Atmosphere: A Special Report of Intergovernmental Panel on Climate Change (Cambridge U. Press, 1999).
[4] M. Maynard, "To Save Fuel, Airlines Find No Speck Too Small," New York Times, 11 Jun 08.
[5] "Revolutionising Air Traffic Management: Practical Steps to Accelerating Airspace Efficiency in Your Region," Air Transport Action Group, November 2012.
[6] J. J. Lee et al. "Historical and Future Trends in Aircraft Performance, Cost, and Emissions," Annu. Rev. Energy Environ. 26, 167 (2001).
[7] "Beginner's Guide to Aviation Efficiency," Air Transport Action Group, June 2010.
[8] W. Freitag and E. T. Schulze, "Blended Winglets Improve Performance," AERO Magazine, Boeing Commercial Airplanes, Quarter 3, 2009, p. 9.
[9] Assessment of Wingtip Modifications to Increase the Fuel Efficiency of Air Force Aircraft (National Academies Press, 2007).
[10] G. Karp "Tipping Scales in Airlines' Favor," Chicago Tribune, 4 Mar 14.
[11] "PurePower Engine Family Specs Chart," Pratt and Whitney, September 2012.
[12] I. Deonandan and H. Balakrishnan, "Evaluation of Strategies for Reducing Taxi-Out Emissions at Airports," Am. Inst. Aeronautics Astronautics, AIAA 2010-9370, 13 Sep 10.
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