Fuel economy in aircraft

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Fuel economy in aircraft
Aircraft.in.cruise.arp.jpg
A Boeing 747 in cruise flight. Aircraft like this burn jet fuel

Aircraft must consume fuel to supply the energy needed to move the vehicles and their passengers. Fuel economy is a measure of how much fuel an aircraft needs to operate, and it can be expressed in several ways, for example by the liters of fuel consumed per passenger per kilometer. Aerodynamic drag, which exerts a force on the aircraft in the opposite direction from the velocity, is a principal determinant of energy consumption in aircraft because they operate at such high speeds.

Factors in aircraft fuel economy[edit]

Each model of aircraft has a maximum range speed for a given total load (fuel plus payload), which is the speed at which it is most fuel efficient.[1] Flying slower or faster than this optimimum speed increases fuel consumption per mile flown. There is an optimum speed for efficiency because the component of drag resulting from airframe skin friction against the air increases at a square function of air speed, but the drag resulting from generating lift decreases with air speed. (These are technically called parasitic drag and induced drag, respectively.)[1] The desirability of a low maximum range speed to reduce environmental and climate impacts is at odds in aircraft design with the benefit to revenue streams of making that design speed higher, to increase the passenger miles flown per day.[2]

Aircraft weight is also a factor in fuel economy, because more lift-generating drag (induced drag) results as weight increases. If airframe weight is reduced, engines that are smaller and lighter can be used, and for a given range the fuel capacity can be reduced. Thus some weight savings can be compounded for an increase in fuel efficiency. A rule-of-thumb being that a 1% weight reduction corresponds to around a 0.75% reduction in fuel consumption.[3]

Flight altitude affects engine efficiency. Jet-engine efficiency increases at altitude up to the tropopause, the temperature minimum of the atmosphere; at lower temperatures, the engine efficiency is higher.[1] Jet engine efficiency is also increased at high speeds, but above about Mach 0.85 the aerodynamic drag on the airframe overwhelms this effect.

This is because above that speed air begins to become incompressible, causing shockwaves form that greatly increase drag. For supersonic flight (Mach 1.0 and higher), fuel consumption is increased tremendously.

Changes in commercial aircraft fuel economy since the 1950s[edit]

Although modern jet aircraft have twice the fuel efficient of the earliest jet airliners, [4] they are only slightly more fuel efficient than the latest piston engine airliners of the late 1950s such as the Lockheed L-1649 Starliner and Douglas DC-7.[2] Nonetheless, jets have about twice the cruise speed. The early jet airliners were designed at a time when air crew labor costs were higher relative to fuel costs than today. Despite the high fuel consumption, because fuel was inexpensive in that era the higher speed resulted in favorable economics since crew costs and amortization of capital investment in the aircraft could be spread over more seat miles flown per day.[5]

Today's turboprop airliners have better fuel efficiency than current jet airliners, in part because of their lower cruising speed and propellers that are more efficient than those of the 1950s-era piston-powered airlines.[6] Among major airlines, those which have turboprop equipped regional carrier subsidiaries typically rank high in overall fleet fuel efficiency. For example, although Alaska Airlines scored at the top of a 2011-2012 fuel efficiency ranking, if its regional carrier—turbo-prop equipped Horizon Air—were dropped from the consideration, the airline's ranking would be lower.

Jet aircraft efficiency[edit]

Jet aircraft efficiencies are improving: Between 1960 and 2000 there was a 55% overall fuel efficiency gain (if one were to exclude the inefficient and limited fleet of the De Havilland Comet 4 and to consider the Boeing 707 as the base case).[2] Most of the improvements in efficiency were gained in the first decade when jet craft first came into widespread commercial use. Between 1971 and 1998 the fleet-average annual improvement per available seat-kilometre was estimated at 2.4%. Concorde the supersonic transport managed about 17 passenger-miles to the Imperial gallon; similar to a business jet, but much worse than a subsonic turbofan aircraft. Airbus states a fuel rate consumption of their A380 at less than 3 L/100 km per passenger (78 passenger-miles per US gallon).[7]

Weight effect[edit]

As over 80% of the fully laden take-off weight of a modern aircraft such as the Airbus A380 is craft and fuel, there remains considerable room for future improvements in fuel efficiency. The weight of an aircraft can be reduced by using light-weight materials such as titanium, carbon fiber and other composite plastics. Expensive materials may be used, if the reduction of mass justifies the price of materials through improved fuel efficiency. The improvements achieved in fuel efficiency by mass reduction, reduces the amount of fuel that needs to be carried. This further reduces the mass of the aircraft and therefore enables further gains in fuel efficiency. For example, the Airbus A380 design includes multiple light-weight materials.

Aerodynamics[edit]

Airbus has showcased wingtip devices (sharklets or winglets) that can achieve 3.5 percent reduction in fuel consumption.[8][9] There are wingtip devices on the Airbus A380. Further developed Minix winglets have been said to offer 6 percent reduction in fuel consumption.[10] Winglets at the tip of an aircraft wing, can be retrofitted to any airplane, and smooths out the wing-tip vortex, reducing the aircraft's wing drag.[10]

NASA and Boeing are conducting tests on a 500 lb (230 kg) "blended wing" aircraft. This design allows for greater fuel efficiency since the whole craft produces lift, not just the wings.[11] The blended wing body (BWB) concept offers advantages in structural, aerodynamic and operating efficiencies over today's more conventional fuselage-and-wing designs. These features translate into greater range, fuel economy, reliability and life cycle savings, as well as lower manufacturing costs.[12][13] NASA has created a cruise efficient STOL (CESTOL) concept.

Fraunhofer Institute for Manufacturing Engineering and Applied Materials Research (IFAM) have researched a shark skin imitating paint that would reduce drag through a riblet effect.[14] Aircraft are a major potential application for new technologies such as aluminium metal foam and nanotechnology such as the shark skin imitating paint.

Fuel consumption factors[edit]

To save fuel, Simon Weselby presented the following measures while flying, in his example of an A330 flying 4,600 kilometres (2,900 mi):[15]

  • direct routing: 40 km (25 mi) less distance saves 190 kg (419 lb) fuel
  • vertical flight profile optimization: fly 600 m (1,969 ft) below optimum altitude, 600 kg (1,323 lb) more fuel consumed
  • cruising speed: 0.01 mach above optimum, 800 kg (1,764 lb) more fuel consumed
  • aircraft weight: 1,000 kg (2,205 lb) more fuel on board, 150 kg (331 lb) more fuel consumed. 100 litres (22 imp gal; 26 US gal) of unused potable water, 15 kg (33 lb) more fuel consumed.

Also operational procedures can save fuel:[15]

  • 10 minutes less APU use, 35 kg (77 lb) fuel saved
  • reduced flap approach, 15 kg (33 lb) fuel saved
  • reduced thrust reversal, 30 kg (66 lb) fuel saved

Maintenance saves fuel as well:[15]

  • no engine wash schedule: 100 kg (220 lb) more fuel consumed
  • slat rigging, 5 mm (0.20 in) gap, 50 kg (110 lb) more fuel consumed
  • spoiler rigging, 10 mm (0.39 in) gap, 40 kg (88 lb) more fuel consumed
  • damaged door seal, 15 kg (33 lb) more fuel consumed

Propellers versus jets[edit]

Propfans are a more fuel efficient technology than jets or turboprops, but turboprops have an optimum speed below about 450 mph (700 km/h).[16] This speed is less than used with jets by major airlines today. However, the decrease in speed reduces drag. With the current [dated info] high price for jet fuel and the emphasis on engine/airframe efficiency to reduce emissions, there is renewed interest in the propfan concept for jetliners that might come into service beyond the Boeing 787 and Airbus A350XWB. For instance, Airbus has patented aircraft designs with twin rear-mounted counter-rotating propfans.[17] NASA has conducted an Advanced Turboprop Project (ATP), where they researched a variable pitch propfan that produced less noise and achieved high speeds.

Related to fuel efficiency is the impact of aviation emissions on climate.

Example Values[edit]

Turboprop[edit]

300 nautical miles (560 km) sector

model first flight seats Fuel efficiency per seat
ATR 72-600 2009 70 2.69 litres per 100 kilometres (87 mpg-US) [18]
Bombardier Dash 8 Q400 1998 78 3.38 litres per 100 kilometres (70 mpg-US) [19]

Regional[edit]

500 nautical miles (930 km) sector (except 737-300 : 507 nautical miles (939 km))

model first flight seats Fuel efficiency per seat
Boeing 737-300 1984 126 3.46 litres per 100 kilometres (68.0 mpg-US)[20]
Boeing 737-900ER 2006 180 2.59 litres per 100 kilometres (91 mpg-US)[21]
Bombardier CRJ1000 2009 100 3.33 litres per 100 kilometres (71 mpg-US) [22]

Short haul[edit]

1,000 nautical miles (1,900 km) sector

model first flight seats Fuel efficiency per seat
Airbus A319 1995 124 2.95 litres per 100 kilometres (80 mpg-US)[23]
Airbus A320 1987 150 2.61 litres per 100 kilometres (90 mpg-US)[23]
Airbus A321-200 1996 180 2.5 litres per 100 kilometres (94 mpg-US)[23]
Airbus A330-200 1997 293 2.37 litres per 100 kilometres (99 mpg-US)[23]
Boeing 737-700 1997 128 2.71 litres per 100 kilometres (87 mpg-US)[23]
Boeing 737-800 1997 160 2.68 litres per 100 kilometres (88 mpg-US)[23]
Boeing 737-900ER 2006 180 2.32 litres per 100 kilometres (101 mpg-US)[21]
Boeing 757-200 1982 190 3.02 litres per 100 kilometres (78 mpg-US)[23]
Boeing 787-3 2009 289 1.98 litres per 100 kilometres (119 mpg-US)[23]

Medium haul[edit]

3,000 nautical miles (5,600 km) sector (except 767-400ER : 3,265 nautical miles (6,047 km))

model first flight seats Fuel efficiency per seat
Airbus A330-200 1997 241 3.11 litres per 100 kilometres (76 mpg-US)[24]
Airbus A330-300 1992 262 2.98 litres per 100 kilometres (79 mpg-US)[24]
Airbus A340-300 1992 262 3.25 litres per 100 kilometres (72 mpg-US)[24]
Boeing 747-8 2011 467 2.59 litres per 100 kilometres (91 mpg-US)[25]
Boeing 767-400ER 1999 304 2.43 litres per 100 kilometres (97 mpg-US)[20]
Boeing 777-200ER 1996 301 2.89 litres per 100 kilometres (81 mpg-US)[24]
Boeing 777-300 1997 368 2.61 litres per 100 kilometres (90 mpg-US)[26]

Long haul[edit]

6,000 nautical miles (11,000 km) sector (except A380 : average)

model first flight seats Fuel efficiency per seat
Airbus A330-200 1997 241 3.32 litres per 100 kilometres (71 mpg-US)[24]
Airbus A340-300 1992 262 3.49 litres per 100 kilometres (67.4 mpg-US)[24]
Airbus A380 2005 555 2.9 litres per 100 kilometres (81 mpg-US)[27]
Boeing 747-400 1988 416 3.26 litres per 100 kilometres (72 mpg-US)[28]
Boeing 747-8 2011 467 2.75 litres per 100 kilometres (86 mpg-US)[25]
Boeing 777-200ER 1996 301 3.08 litres per 100 kilometres (76 mpg-US)[24]
Boeing 777-300ER 2003 365 2.84 litres per 100 kilometres (83 mpg-US)[26]

For comparison, a Volvo Buses 9700 averages 0.41 litres per 100 kilometres (570 mpg-US) per seat for 63 seats,.[29] In highway travel an average auto has the potential for 1.61 litres per 100 kilometres (146 mpg-US)[30] per seat (assuming 4 seats) and for a 5-seat 2014 Toyota Prius, 0.98 litres per 100 kilometres (240 mpg-US).[31] While this shows the capabilities of the vehicles, the load factors (percentage of seats occupied) may differ between personal use and societal averages for long-distance auto use, and among those of particular airlines.

References[edit]

  1. ^ a b "Drag". Skybrary. Retrieved June 8, 2014. 
  2. ^ a b c Peeters, A.; Middel; Hoolhorst (2005). "Fuel efficiency of commercial aircraft: An overview of historical and future trends". National Aerospace Laboratory. Retrieved July 6, 2014. 
  3. ^ Barney L. Capehart (2007). Encyclopedia of Energy Engineering and Technology 1. CRC Press. ISBN 978-0-8493-3653-9. 
  4. ^ "Efficiency Trends for New Commercial Jet Aircraft, 1960 to 2008". The International Council on Clean Transportation. November 2009. 
  5. ^ Brian M. Yutko and R. John Hansman (May 2011). "Approaches to Representing Aircraft Fuel Efficiency. Performance for the Purpose of a Commercial Aircraft". MIT International Center for Air Transportation. 
  6. ^ Irene Kwan (3 June 2014). "Back to the Future: Return of the Turboprop?". The International Council on Clean Transport. 
  7. ^ "The A380: The future of flying". Airbus. Archived from the original on 2007-12-14. Retrieved 2008-03-22. 
  8. ^ "'Shark fin' wings give airline chiefs something to smile about". NZ Herald News. 2009-11-17. Retrieved 2012-09-29. 
  9. ^ "A320 plane shark fins small winglets successfully completed the first flight test". Ylcrafts.com. Retrieved 2012-09-29. 
  10. ^ a b "Minix wing tip device promises 6% gain in fuel efficiency for airliners". Gizmag.com. Retrieved 2012-09-29. 
  11. ^ Hank Green (October 8, 2007). "Fuel Efficient Plane Prototype Hits Skies". Ecogeek. Retrieved July 6, 2014. 
  12. ^ Tom Koehler (October 27, 2006). "Boeing to Begin Ground Testing of X-48B Blended Wing Body Concept" (Press release). Boeing. Retrieved April 10, 2012. 
  13. ^ Philip Lorenz III (July 3, 2007). "AEDC testing brings unique blended wing aircraft closer to flight.". Arnold Air Force Base. Retrieved April 10, 2012. 
  14. ^ Mahony, Melissa (2010-05-25). "A sharkskin coating for ships, planes and blades". SmartPlanet. Retrieved 2012-09-29. 
  15. ^ a b c Fuel Burn vs. Maintenance Costs, IATA maintenance cost conference, October 2012
  16. ^ Spakovszky, Zoltan (2009). "Unified Propulsion Lecture 1". Unified Engineering Lecture Notes. MIT. Retrieved 2009-04-03. 
  17. ^ US application 2009020643, Airbus & Christophe Cros, "Aircraft having reduced environmental impact", published 2009-01-22 
  18. ^ "ATR 72-600 : Quick View". ATR. June 2014. 
  19. ^ Mark Brouwer, Siddharth Srinivasan. "Proud to fly a Turboprop: Q400 vs ATR72". The Flying Engineer. 
  20. ^ a b Scott McCartney (Aug 12, 2010). "A Prius With Wings vs. a Guzzler in the Clouds". Wall Street Journal. 
  21. ^ a b "737 performance summary". Boeing. 2007. 
  22. ^ "Bombardier CRJ1000 Fuel Consumption". Sun Airlines. 20 August 2013. 
  23. ^ a b c d e f g h "Analysing the options for 757 replacement". Aircraft Commerce. aug-sept 2005. 
  24. ^ a b c d e f g "Boeing: 777 way much better than A330". Aspire aviation. 8 December 2010. 
  25. ^ a b "747-8 performance summary". Boeing. 2010. 
  26. ^ a b "777 performance summary". Boeing. 2009. 
  27. ^ Stefan Lovgren (January 18, 2005). "Airbus Unveils A380 "Superjumbo" Jet". National Geographic. 
  28. ^ "747 performance summary". Boeing. 2010. 
  29. ^ Edward Jobson (12 September 2013). "Coach fuel consumption". volvo buses. 
  30. ^ DEFRA (2008). 2008 Guidelines to Defra’s GHG Conversion Factors: Methodology Paper for Transport Emission Factors
  31. ^ "2014 Toyota Prius fuel economy". United States Environmental Protection Agency.