Assessment of Wingtip Modifications to Increase the Fuel Efficiency of Air Force Aircraft (2007) / Chapter Skim
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-- which are the aircraft covered in this study -- account for over half of all aviation fuel.2 One very visible action taken by commercial airframe manufacturers and operators to reduce fuel consumption is the modification of an aircraft's wingtip by installing, for example, near-vertical "winglets" to reduce aerodynamic drag. Experience shows that these tip devices reduce block fuel consumption (total fuel burn from engine start at the beginning of a flight to engine shutdown at the end of that flight)
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. Thus, in this report, the term "winglet" denotes the traditional, nearly vertical wingtip design, while "wingtip modifications" is used to refer to the more general set of wingtip designs, including winglets and wingtip extensions, aimed at reducing aerodynamic drag.
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FINDINgS AND RECOMMENDATION In this section, the committee presents two findings and a recommendation in response to the four tasks it was asked to perform. Feasibility and Cost Effectiveness of Modifying Air Force Aircraft Finding: The committee's analysis for a broad range of fuel prices and with the data available to it on potential improvements in block fuel savings, modification cost estimates, operational parameters for the aircraft, and so forth indicates that wingtip modifications offer significant potential for improved fuel economy in certain Air Force aircraft, particularly the KC-135R/T and the KC-10.
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Based on factors such as estimated fuel savings, cost of modification, operational flexibility, mission profiles, and remaining service life, the committee ranked these aircraft in order of their likely suitability for wingtip modifications, as shown in Table S-1. kC-0 The KC-10 airframe is based on the commercial DC-10 airframe, and early commercial DC-10 flight tests validated a 2-3 percent improvement in fuel efficiency at cruise conditions with winglets as compared with the original wing design.4 Not only was the DC-10 modified and tested with winglets, but its successor, the MD-11, was designed and certified with winglets.
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The wind tunnel test indicated that winglets would reduce KC-135 aircraft drag by 6-8 percent,5 and flight tests with a KC-135 modified with winglets indicated substantial benefits. The study also indicated that the structural modifications required to install winglets on the KC-135s are a reasonable-size work package.
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Intelligence, Sureillance, and Reconnaissance Aircraft While these aircraft are mentioned in the study's statement of task, the committee notes that they are not major fuel consumers, and their wings are already optimized for aerodynamic efficiency such that they would be expected to derive little benefit from wingtip modifications. other Air Force Aircraft Finding: Most of the aircraft in the Air Force inventory that derive from commercial aircraft now operating with winglets already have winglets, or the decision has been made to install winglets.
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The easiest decisions on whether to install winglets obviously involve aircraft in the Air Force inventory that derive from commercial aircraft now operating with winglets. In each case, the aircraft structure has already been studied and determined to be appropriate, the engineering design has been done, the modifications have been prototyped, tested, and certified, modification kits developed, flight manuals revised as required, and so on.
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Preliminary Net Present Value Analysis The committee followed up the qualitative analysis described above with a preliminary NPV analysis based on a simple spreadsheet model that considered a range of assumed modification costs and fuel savings for the most promising aircraft identified above. These preliminary NPV calculations confirm that wingtip modifications should be seriously considered for the KC-135R/T and KC-10 (see "Fuel Price Analysis," below)
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These numbers are illustrative only, and more accurate estimates of breakeven fuel prices would require engineering analysis to determine actual modification costs and the fuel savings potential for each aircraft. Impacts on Aircraft Maintenance and Flight Operations Commercial experience with aircraft that have installed winglets has shown that there have been no significant impacts on aircraft maintenance, flight operations, or ground operations (gate space, taxiways, hangars, etc.)
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For the KC-135R/T undergoing programmed depot maintenance at contractor facilities, the Air Force should consider adding any proposed wingtip modifications to the existing overhaul contract. This would minimize training and allow returning the aircraft to the Air Force in the shortest possible time.
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The Air Force should support the analysis required and make the appropriate modifications as quickly as possible. There are also other ways to reduce fuel consumption, many of which have already been adopted by the commercial airlines.
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