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Assessment of Wingtip Modifications to Increase the Fuel Efficiency of Air Force Aircraft Summary Since the 1970s, when the price of aviation fuel began to spiral upward, airlines and aircraft manufacturers have explored many ways to reduce fuel consumption by improving the operating efficiency of their aircraft. Fuel economy concerns have been particularly keen for operators of commercial aircraft, which typically fly many hours per day in competitive markets, but they have been growing for military aircraft as well. The fuel consumed by the U.S. Air Force is in excess of 3 billion gallons per year, which is over 8 million gallons per day.1 Aviation fuel accounts for much of this total, and the aircraft used by the Air Force for airlift, aerial refueling, and intelligence, surveillance, and reconnaissance (ISR)—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) of the modified aircraft by 1 Ron Sega, 2006, “Air Force energy strategy,” Worldwide Energy Conference and Trade Show, Arlington, Va., April 19. 2 Data provided by Defense Energy Support Center (DESC) on fuel usage by mission design series for FY05.
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Assessment of Wingtip Modifications to Increase the Fuel Efficiency of Air Force Aircraft 3-5 percent, depending on the trip length.3 These wingtip modifications are offered as options to the original design of many newer commercial jetliners but are also available for retrofit to selected older aircraft. To date, however, only one military-unique aircraft (the C-17 transport) features winglets, though some studies have been conducted on the feasibility of retrofitting tip modification devices on other military aircraft. In light of its growing concerns about rising fuel costs, the Air Force asked the National Research Council (NRC) to evaluate its aircraft inventory and to identify those aircraft that may be good candidates for winglet modifications. Specifically, the Air Force asked the NRC to perform the following four tasks: Examine the feasibility of modifying Air Force airlift, aerial refueling, and ISR aircraft with winglets, to include a cost-effectiveness analysis of the feasible winglet modifications in net present value (NPV) terms. Determine the market price of aviation fuel at which incorporating winglets would be beneficial for each platform. Consider impacts to aircraft maintenance and flight operations (including ground operations). Offer investment strategies the Air Force could implement with commercial partners to minimize Air Force capital investment and maximize investment return. Although the statement of task above refers specifically to “winglets,” the Committee on Assessment of Aircraft Winglets for Large Aircraft Fuel Efficiency chose to broaden the scope of its deliberations slightly by including a variety of possible modifications to the wingtip (e.g., wingtip span extensions). 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. These tasks call for a quantitative assessment of the costs and benefits of winglet modifications on a variety of platforms. In a comprehensive analysis, one would need to include the nonrecurring engineering costs of 3 This range of 3-5 percent block fuel savings, derived from commercial experience, is lower than the 5-7 percent cited by the U.S. House of Representatives Armed Services Committee in Report 109-452, which may reflect fuel savings under cruise conditions.
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Assessment of Wingtip Modifications to Increase the Fuel Efficiency of Air Force Aircraft wing analysis and wingtip design, the costs of materials, manpower, and out-of-service time to accomplish the modification, financial implications, training costs, potential impacts on maintenance docks and hangar space, costs associated with software and technical manual revisions, and any impacts on maintenance, operations, or mission accomplishment. Benefits to be considered would include not only improved fuel economy but also improved payload-range capability, improved takeoff performance, and less takeoff noise. In most cases, quantitative data on these costs and benefits were not known or not available. However, the committee did use preliminary NPV calculations to estimate payback periods for wingtip modification investments on various platforms by treating fuel costs, savings, and wing modification costs parametrically. These calculations supplemented the committee’s expert judgment on which platforms appear to be the best candidates for wingtip modification. Besides wingtip devices, there are other methods to reduce aircraft fuel consumption, but since they were not mentioned in the statement of task, the committee did not examine them in detail, nor did it examine the extent to which the Air Force might already be using some of these methods. Likewise, it did not make any formal recommendations concerning them. However, the committee suggests this is an area that should be considered as potentially providing significant fuel savings to the Air Force. 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. To assess the feasibility and cost-effectiveness of wingtip modifications, the committee began by investigating those aircraft in the Air Force inventory that burn the most fuel. In decreasing order of annual fuel burn (by
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Assessment of Wingtip Modifications to Increase the Fuel Efficiency of Air Force Aircraft TABLE S-1 Potential for Wingtip Modifications to Benefit Air Force Aircraft Aircraft Priority/Potential Benefit KC-10 High KC-135R/T High C-5 Medium C-17 Medium/low C-130H/J Low fleet), they are the C-17, KC-135R/T, C-5, KC-10, and C-130H/J. 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-10 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. With the computational fluid dynamics (CFD) tools of today, moreover, a winglet or other wingtip modification designed for the KC-10 aircraft might well achieve greater fuel savings than were demonstrated on the DC-10 fitted with winglets some 25 years ago. In addition, recent winglet design experience using high Reynolds number (RN) wind tunnels could have applicability for winglet designs that may be more effective on the KC-10 and other government transport aircraft. As a result of all of this past work, the KC-10 fleet would require much less development time and effort to determine the effectiveness and suitability of various aerodynamic improvements. KC-135R/T The KC-135 airframe is closely related to the commercial Boeing 707. In the late 1970s and early 1980s, a joint National Aeronautics and Space 4 A.B. Taylor, 1983, “DC-10 winglet flight evaluation summary report,” NASA-CR-3748, December.
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Assessment of Wingtip Modifications to Increase the Fuel Efficiency of Air Force Aircraft Administration (NASA)/Air Force program was conducted to evaluate the benefits that could be achieved from retrofitting winglets on the KC-135 aircraft. 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. Additional study would now be required to establish that the wings of these aging aircraft still meet the requirements of winglet installation. C-5 Given that the C-5 is one of the largest contributors to the Air Force’s fuel consumption and that its missions are long range, a study to quantify the potential gains and the effects of integrating winglets is warranted. Unfortunately, unlike the KC-10 and the KC-135, on whose derivative commercial aircraft there has been a comprehensive winglet development effort, a C-5 fleet retrofit program would add a measurable nonrecurring cost and require a longer time to recover the investment. C-17 A number of design considerations led to the final winglet configuration on the C-17. One such consideration was that the wingspan was limited to that of the C-141 in order to maintain compatibility with facility infrastructure. The C-17 winglet was shown in wind tunnel testing to provide approximately 2.5 percent reduction in drag under cruise conditions. Also, flight testing showed no additional buffeting for takeoff or landing.6 However, while these benefits are considered to be substantial, the C-17 winglet was developed in a low-RN wind tunnel. The low-RN environment can give misleading results with regard to drag, buffet, pitching moment, and loads because the much higher RN of the full-scale flight vehicle exhibits different flow phenomena. Also, the C-17 configuration was developed in the 1980s, before the full-scale wind tunnel at the National Transonic 5 NASA,1982, “KC-135 winglet program review,” NASA Conference Publication 2111, January. 6 Robb Gregg, Senior Manager for Aircraft Programs, Boeing Phantom Works, “Drag improvement: A study of the DC-10/MD-11/C-17 winglet programs,” Presentation to the committee on December 13, 2006.
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Assessment of Wingtip Modifications to Increase the Fuel Efficiency of Air Force Aircraft Facility became available and before modern Navier-Stokes CFD tools had been developed. With these new capabilities, a more accurate assessment of the current C-17 winglet design could be obtained. In addition, with these new tools and lessons learned from other winglet designs, it may be possible to improve the C-17 winglet design to reduce cruise drag another 1 percent or more.7 C-130H/J Compared with the gains realized for commercial airline applications, the performance benefits provided by wingtip modifications on the C-130 would be less. For one thing, the C-130’s wing is already very efficient because its aspect ratio of 10 is relatively high. Another reason for the lower gain in expected winglet efficiency is the C-130’s unswept wing with its lower tip loading. In addition, since winglets are more effective for longer ranges and with the higher wingtip loading realized at higher altitudes, the potential benefit of winglets for the C-130 is limited. Intelligence, Surveillance, 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. The remaining Air Force aircraft that are derivatives of commercial aircraft do not appear to be good candidates for wingtip modifications. 7 Robb Gregg, Senior Manager for Aircraft Programs. Boeing Phantom Works, “Drag improvement: A study of the DC-10/MD-11/C-17 winglet programs,” Presentation to the committee on December 13, 2006.
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Assessment of Wingtip Modifications to Increase the Fuel Efficiency of Air Force Aircraft TABLE S-2 Winglet Status of Air Force Aircraft Derived from Commercial Airframes Air Force Aircraft Commercial Equivalent Inventory Winglets C-9 Douglas DC-9-30 3 No C-20B Gulfstream GIII 5 Yes C-20H Gulfstream GIV, GIVSP 2 Yes C-37 Gulfstream GV 9 Yes C-21 Learjet 35A 59 No C-40B Boeing 737-700 4 Yes C-40C Boeing 737-700 3 Yes VC-25 Boeing 747-200 (-300 wings) 2 No E-4 Boeing 747-200 4 No C-32 Boeing 757-200 4 Yes SOURCE: Data provided by USAF. 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. However, the committee’s review of all such Air Force aircraft revealed that most of them already have winglets, or the decision has been made to incorporate winglets, as shown in Table S-2. All of these aircraft have winglets except for the C-9s, the C-21s, the VC-25s, and the E-4s. The three C-9s, derivatives of the DC-9, are scheduled to retire in FY11 and should not be considered for wingtip modifications. Also, past work on winglets for the DC-9, as discussed in Chapter 3, did not prove to be favorable. The C-21s, derivatives of the Learjet 35A, are small aircraft, so the entire fleet uses approximately 8 million gallons of fuel per year and would not be a priority for modification. Furthermore, they have tip tanks, and wingtip modifications would require the removal of these tanks, severely limiting the range of these aircraft even with a more efficient wing. Lastly, the VC-25s and the E-4s are derivatives of the Boeing 747-200, with the VC-25s having 747-300 wings. The 747-200 has not been produced since the late 1980s, so the commercial fleet is aging and retiring from service. As a result, the entire cost of winglets designed for 747-200/300 wings would have to be borne by the government. All of the Boeing 747s in the commercial world that have winglets are 747-400s,
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Assessment of Wingtip Modifications to Increase the Fuel Efficiency of Air Force Aircraft which have a structurally modified wing. The structural modification to allow installing the 747-400 wingtip on the VC-25s or the E-4s would be very expensive and impractical. 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). However, a detailed engineering and economic analysis would be required for each aircraft type before a final decision could be made to proceed with the installation of winglets or other aerodynamic modifications. Recommendation: The Air Force should initiate an engineering analysis with the original equipment manufacturers (OEMs) to determine (1) the extent and cost of modifications needed for the KC-135R/T and the KC-10 to enable the installation of wingtip devices and (2) the fuel savings that could be achieved by this modification for each aircraft type. It should then perform an NPV analysis with these data to calculate the net savings. The Air Force should also analyze the C-5 and C-17 for potential wingtip modifications. The OEMs have the detailed knowledge of wing designs and previous modifications that is necessary for carrying out these analyses. The results should be shared with the other Services operating similar aircraft. Fuel Price Analysis To illustrate the types of costs and benefits that might be realized through wingtip modifications (e.g., winglets) that would produce a reduction in fuel burn, the committee performed its own preliminary NPV analysis for the KC-135R/T and the KC-10. The analysis was used to determine whether wingtip modifications for selected aircraft would pay for themselves well before the aircraft are due to retire. Since it is not possible to know the modification costs and fuel savings without performing a detailed engineering analysis, these were treated as parameters in the model.
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Assessment of Wingtip Modifications to Increase the Fuel Efficiency of Air Force Aircraft The range for modification costs was chosen from list prices and committee estimates. For fuel savings, the calculations were done for block fuel savings of 3 percent and 5 percent, consistent with commercial airline experience and the findings of this report. Results were calculated for the worst-case (highest modification cost and lowest fuel savings) and best-case (lowest modification cost and highest fuel savings) payback periods at a fuel cost of $2.50 per gallon. The committee assumed an annual fuel cost escalation rate of 3 percent and a discount rate of 3 percent. In the KC-135R/T best case, net savings become positive 9 years after starting the modification program. All 417 aircraft in the inventory are modified. Total net savings to the Air Force are approximately $400 million (FY07 $). In the KC-135R/T worst case, net savings become positive 24 years after starting the modification program. Only 217 of the 417 aircraft in the inventory are modified (the others are not modified because they are expected to be retired from the inventory before reaching the end of their payback periods). Total net savings to the Air Force are approximately $36 million (FY07 $). In the KC-10 best case, net savings become positive 8 years after starting the modification program. All 59 aircraft in the inventory are modified. Total net savings to the Air Force are approximately $221 million (FY07 $). In the KC-10 worst case, net savings become positive 23 years after starting the modification program. Only 53 of the 59 aircraft in the inventory are modified (the others are not modified because they are expected to be retired from the inventory before reaching the ends of their payback periods). Total net savings to the Air Force are approximately $12 million (FY07 $). The price per gallon of fuel was also parameterized at $2.50, $5.00, $10.00, and $20.00 to account for the fully burdened cost of fuel. In constant dollars, when the cost of fuel is doubled, the payback period is cut in half. Total net savings to the Air Force rise significantly. 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.). Similarly, the Air Force has not experienced any significant impacts on
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Assessment of Wingtip Modifications to Increase the Fuel Efficiency of Air Force Aircraft aircraft maintenance or flight operations for aircraft it currently operates with winglets, and the committee does not expect any major problems with modifications to other aircraft under consideration. Investment Strategies Implementing the Modifications Should the decision be to proceed with wingtip modification on the KC-10, the committee recommends the work be done while the aircraft are in normal scheduled overhaul. Since the KC-10 is maintained on contract with industry engineers who have intimate knowledge of commercial DC-10s, it is possible that wingtip modification could be added to the work specification with little or no added downtime or loss of operational availability. Much of the same applies to the KC-135R/T aircraft fleet, except that unlike the KC-10, many of these aircraft are maintained by Air Force personnel in-house. The committee therefore believes that the wingtips could be retrofitted while the aircraft are undergoing their 5-year cycle of programmed depot maintenance. Rather than divert Air Force mechanics from other tasks, however, it might be wiser to partner with industry and have an experienced contract field team work with the Air Force mechanics to accomplish the modification. 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. Financing Options Wingtip modification programs and other fuel economy investments are examples of long-term investments that may require a significant initial investment that provides returns over time. Securing financing for such long-term investments is always a challenge given the current military acquisition practices and congressional appropriation processes. In a previ-
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Assessment of Wingtip Modifications to Increase the Fuel Efficiency of Air Force Aircraft ous report on engine fuel economy in military aircraft,8 the NRC discussed innovative financing mechanisms that might be pursued. The statement of task for that study included a request to “develop implementation strategies to include conventional, as well as innovative, acquisition, financing, and support concepts.”9 The committee believes that three of the mechanisms discussed in that report—specifically, creating a line item in the defense budget, implementing an “energy savings performance contract” strategy, and competing airframe maintenance contracts—could be applicable in implementing wingtip modifications. Those mechanisms are discussed in some detail in the earlier report. CONCLUDING REMARKS It is clear that aerodynamic improvements, including winglets, can make significant contributions to the efficiency of aircraft and should be considered for the military fleets discussed in this report. In each case, however, the appropriateness of such structural modifications must be determined fleet by fleet. These decisions are very complex and will depend on many factors, including the design of the aircraft structures, design margin within those structures, the condition of the structures, mission profiles, utilization rates, fuel consumption rates, fuel prices, and the remaining life of the aircraft. 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. The committee believes it is important for these other strategies to be considered, and while they were not the focus of this study and the extent to which the Air Force may already be using some of these strategies was not examined, examples are provided in Appendix B for the reader’s benefit. 8 NRC, 2007, Improving the Efficiency of Engines for Large Nonfighter Aircraft, Washington, D.C.: The National Academies Press. 9 Ibid.
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