4
Assessment of Wingtip Modifications for Various Air Force Aircraft and Potential Investment Strategies

This chapter provides the committee’s evaluation of steps the Air Force could take to improve the fuel economy of aircraft in its inventory, in particular by modifying the wingtips. It begins with a checklist of factors that must be considered to determine if these modifications make sense. This is followed by a discussion of specific aircraft in the Air Force inventory, including those that are responsible for the greatest fuel consumption as well as those that are derived from commercial aircraft. The committee then identifies those aircraft that appear most promising for wingtip modification. For these selected aircraft, a simple spreadsheet model is used to estimate payback periods for modification investments, treating modification costs and fuel prices as parameters. These calculations are combined with the committee’s expert judgment to prioritize various aircraft for their suitability for wingtip modifications. Finally, the chapter concludes with a discussion of strategies by which the Air Force can optimize its investment in fuel economy programs.

CHECKLIST FOR MAKING WINGTIP MODIFICATION DECISIONS

The investment in winglets for a particular aircraft type depends on a number of factors, including the potential fuel burn efficiency improvements provided, the size of the statement of work required for the installation, the utilization rate of the aircraft fleet, and the expected lifespan of that



The National Academies | 500 Fifth St. N.W. | Washington, D.C. 20001
Copyright © National Academy of Sciences. All rights reserved.
Terms of Use and Privacy Statement



Below are the first 10 and last 10 pages of uncorrected machine-read text (when available) of this chapter, followed by the top 30 algorithmically extracted key phrases from the chapter as a whole.
Intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text on the opening pages of each chapter. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.

Do not use for reproduction, copying, pasting, or reading; exclusively for search engines.

OCR for page 53
Assessment of Wingtip Modifications to Increase the Fuel Efficiency of Air Force Aircraft 4 Assessment of Wingtip Modifications for Various Air Force Aircraft and Potential Investment Strategies This chapter provides the committee’s evaluation of steps the Air Force could take to improve the fuel economy of aircraft in its inventory, in particular by modifying the wingtips. It begins with a checklist of factors that must be considered to determine if these modifications make sense. This is followed by a discussion of specific aircraft in the Air Force inventory, including those that are responsible for the greatest fuel consumption as well as those that are derived from commercial aircraft. The committee then identifies those aircraft that appear most promising for wingtip modification. For these selected aircraft, a simple spreadsheet model is used to estimate payback periods for modification investments, treating modification costs and fuel prices as parameters. These calculations are combined with the committee’s expert judgment to prioritize various aircraft for their suitability for wingtip modifications. Finally, the chapter concludes with a discussion of strategies by which the Air Force can optimize its investment in fuel economy programs. CHECKLIST FOR MAKING WINGTIP MODIFICATION DECISIONS The investment in winglets for a particular aircraft type depends on a number of factors, including the potential fuel burn efficiency improvements provided, the size of the statement of work required for the installation, the utilization rate of the aircraft fleet, and the expected lifespan of that

OCR for page 53
Assessment of Wingtip Modifications to Increase the Fuel Efficiency of Air Force Aircraft particular fleet. An extensive engineering and economic analysis would be required for each aircraft type in order to determine the appropriateness of installing winglets. The following elements are necessary in order to make a balanced decision for each aircraft fleet. Technical Issues Cruise Fuel Burn Efficiency Improvement The primary reason for installing winglets (or other tip devices) is to improve the efficiency of the fuel burn at cruise conditions of the aircraft. The two most important components of fuel burn efficiency affected by winglets are the aerodynamic efficiency of the configuration, measured in terms of lift-to-drag ratio (L/D), and the empty weight of the aircraft, which will increase when the winglets are installed. The viability of a winglet installation is different for each aircraft configuration, and sophisticated design studies are required to achieve the proper balance between aerodynamic efficiency and weight efficiency. There are numerous design parameters involved in selecting the optimum winglet configuration, including winglet span, area, sweep, taper, cant angle (inclination), twist, thickness, sweep, juncture flow, etc. The selection of materials for winglet construction will affect the empty weight. Moreover, the additional loads and moments imparted to the wing when the winglet is installed may require that the wing be strengthened, adding more weight. A sophisticated dynamic aeroelastic analysis of the wing/winglet structure is required for this assessment. Collateral Impact of the Winglet Installation on Airplane Design In addition to the aerodynamic and structural effects of the installed winglet, ancillary issues related to the winglets must also be considered, including the need to revise flight control systems, brought about by the changed stability and control characteristics. These include changed longitudinal, lateral, and directional stability characteristics and altered control system effectiveness, particularly with regard to the effectiveness of outboard ailerons and spoilers. Winglets also can affect the configuration of tip lighting systems and the lightning strike protection systems for the wing.

OCR for page 53
Assessment of Wingtip Modifications to Increase the Fuel Efficiency of Air Force Aircraft Collateral Impact of the Winglet Installation on the Infrastructure The interaction of the airplane with its infrastructure must also be factored into a winglet decision. Typically the physical span of the aircraft increases with the installation of winglets, but not as much as with a conventional tip extension. Nevertheless, consideration must be given to issues related to ground handling, parking, and maintenance (depot and field) and associated facilities such as gates, ramps, hangars, runways, taxiways, etc. This is particularly important when analyzing the economic life cycle of winglets. Design Information Availability Developing a winglet design for an existing aircraft requires a deep understanding of the characteristics of the original aircraft design. Generally that detailed design knowledge resides primarily with the original equipment manufacturer (OEM). However, there have been successful retrofit designs of winglets that were originated by third-party companies. For older aircraft, the existing design data may be scarce and not compatible with current design tools. In addition, there may be few, if any, engineering personnel with a working knowledge of that particular aircraft design. These factors must be considered in developing a financial estimate for the cost and risk of developing a winglet retrofit design. Economic Issues In addition to the formidable technical challenges of developing a winglet retrofit configuration, there are significant economic factors that come into play when making a life-cycle business case. Among the factors that must be considered are the following: Cost of installation. A contractor will need to charge a reasonable price to establish a positive business case for proceeding. The fixed cost to the contractor will consist of engineering and tooling costs required to design, test, and validate the winglet configuration. That cost is nearly independent of the size of the fleet of airplanes, so the larger the subject fleet, the more units that the fixed design costs can be amortized over, making the business case for winglets more likely to be favorable for a large fleet of aircraft.

OCR for page 53
Assessment of Wingtip Modifications to Increase the Fuel Efficiency of Air Force Aircraft Life span of the fleet. A retrofit design solution will have a potentially longer payback period for a younger fleet of airplanes with a longer economic life than for an aging fleet that is soon to be retired. This economic factor can also be influenced by the rate at which the retrofit is conducted. A slow retrofit program eats into the payback on the initial investment, while a rapid fleet retrofit accelerates the payback period. Utilization rate of the fleet. Winglets reduce the fuel burn per flying hour of an aircraft. The more the aircraft is used, the faster the investment will be paid back. This favors installing winglets on heavily used fleets. Cost of fuel. Since the means of paying back the initial investment is a reduction in the amount of fuel consumed, costlier fuel means that the payback is quicker and more likely. Less costly fuel requires a longer payback period. Cost of capital. As with any up-front investment, there is a cost for the capital that is expended before payback can occur. Assuming that the capital investment is made with borrowed money, the economic environment in terms of interest rates and inflation must be considered to understand the business case. High interest rates and low inflation will adversely affect the business case, while low interest rates and high inflation will make the cost of borrowing less. Putting It All Together A business case model can be created to establish the viability of a winglet retrofit program for a fleet of airplanes. Independent variables in the assessment include the following: Winglet unit price ($/airplane), Fuel burn reduction (%), Cost of fuel ($/gallon or lb), Interest rate (%), Inflation rate (%), Fleet size (number of airplanes), Fleet utilization (hours/year), Retrofit rate (airplanes/year), and Life span remaining (years).

OCR for page 53
Assessment of Wingtip Modifications to Increase the Fuel Efficiency of Air Force Aircraft These variables can be used in a business case model to determine the cash flow profile. The profile will be negative during the development and early retrofit years and should become positive during the lifetime of the program. If there is not a positive outcome, winglets should not be installed. If the outcome is positive but requires a long period to break even, the decision is not clear cut. If the outcome is positive over a short period, winglets should clearly be installed. CANDIDATE AIRCRAFT IN THE AIR FORCE INVENTORY Given the emphasis on fuel economy in the study’s statement of task, the committee began by considering those aircraft that consume the greatest amount of fuel, as shown in Figure 4-1. The five that stand out most clearly are, in order of annual fuel usage by fleet, the C-17, KC-135R/T, C-5, KC-10, and C-130H/J. As noted in Chapter 3, the C-17 already has winglets, and the KC-135 and KC-10, which are closely related to the Boeing 707 and DC-10 commercial airframes, respectively, have been studied previously for wingtip modifications. The aircraft are discussed further below. FIGURE 4-1 Fuel usage of selected Air Force aircraft (by fleet) in FY05. SOURCE: DESC.

OCR for page 53
Assessment of Wingtip Modifications to Increase the Fuel Efficiency of Air Force Aircraft C-17 The C-17 is the most current freighter aircraft and one that has some of the latest structural and aerodynamic improvements installed. As described in Chapter 3, the C-17 aircraft is already equipped with a winglet that was incorporated into the original design. While a newly designed winglet for the C-17 might result in somewhat improved cruise fuel efficiency, the magnitude of that improvement is likely to be in the 1 to 2 percent range,1 and it would only make sense if combined with other efficiency improvement modifications. The considerable data already developed for the C-17 could also be considered for further wing upgrades beyond winglets. These should be reviewed and considered for possible installation since much of the research has already been accomplished. KC-135R/T As noted in Chapter 3, there has been some testing with winglets and other improvements on the KC-135/707 wings. These studies should be reviewed for applicability. The issue with these airframes is primarily their age and the limited remaining useful life. The current fleet of approximately 417 KC-135R/T aircraft would be good candidates for winglet installation. Some of these aircraft are expected to be in service until approximately 2040. The fleet of TF-33/JT-3D equipped KC-135s (D/Es) is potentially subject to retirement, so they may not have sufficient payback life remaining. The RC-135V/W/S/TC fleet may be candidates for winglets as well. However, the installation of the overwing high-frequency antenna and wingtip pitot-static probes would probably create another problem for the addition of winglets. Therefore, the committee believes the focus within the KC-135 fleet should be on the R/T models. Three related military fleets derived from the Boeing 707 commercial aircraft are the E-3 AWACS, the E-6 TACAMO, and the E-8 JSTARS. While the wings of these aircraft are closely related to those of the KC-135 fleet, any such winglet would have to be further investigated to confirm aerodynamic and structural compatibility. In addition, there would need to be consideration of winglet interference with the AWACS antenna function. The TACAMO has an extended wingtip that houses antenna pods, so it is 1 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.

OCR for page 53
Assessment of Wingtip Modifications to Increase the Fuel Efficiency of Air Force Aircraft very unlikely that those aircraft could be modified for winglets. Moreover, less is known about the structural suitability of these 707-based platforms than is currently known about the KC-135 fleet. The KC-135/Boeing 707 aircraft are similar in gross weight to the Boeing 757 commercial aircraft, with a maximum takeoff weight (MTOW) of approximately 250,000 lb. The Boeing 737-NG winglet solution has been installed on the Boeing 757 using a 17.5-in. transition wingtip that accommodates the tip airfoil differences between the 737-NG and the 757. This experience may provide a solution for the KC-135/Boeing 707 military fleet as well. C-5 The Lockheed Martin C-5 is a global strategic airlift system capable of carrying outsized cargo. A total of 111 C-5s are in service with the Air Force. A portion of the fleet is being modified with modern commercial turbofan engines, improving range by up to 11 percent. Aerodynamic range improvement efforts have focused on airframe housekeeping such as orphan weight removal and airframe cleanup. Given that the C-5 is one of the major contributors to fuel consumption by the Air Force and its missions are long range, a study to quantify the potential performance gains and integration effects of winglets is warranted. Unlike efforts devoted to winglets for the commercial transport, there has been no detailed C-5 winglet development effort, adding a sizable nonrecurring cost to a fleet retrofit program and extending the time required to recover the investment. Specific data for C-5 aerodynamic improvements have not been approved for release by the System Program Office. The committee believes that the C-5 has the potential for drag reduction with wingtip modifications because of its current large fuel consumption, its missions, wing design, etc. Aerodynamic improvements, combined with orphan weight and obsolete component removal, would contribute to operating efficiency increases for the aircraft. KC-10 The KC-10 is a military derivative of the McDonnell Douglas DC-10 commercial aircraft. As mentioned in Chapter 3, while there has not been a winglet retrofit program for the DC-10, winglets were successfully flight tested on the aircraft and were later successfully incorporated into the

OCR for page 53
Assessment of Wingtip Modifications to Increase the Fuel Efficiency of Air Force Aircraft derivative MD-11 aircraft. There are currently 59 KC-10 aircraft in the Air Force inventory, and they are flown extensively. The KC-10 fleet is quite young, with the oldest aircraft having approximately 22,000 flight hours and 14,000 flight cycles. For comparison, the older DC-10 still in service had over 131,000 flight hours and 45,000 flight cycles as of September 2006. There are also approximately 150 of the DC-10 family of aircraft still in commercial service, both passenger and cargo, so the combined potential market for retrofits may be large enough to motivate a retrofit program. The MD-11 experience indicates that a successful winglet can be incorporated into the DC-10-based wing design. The DC-10 flight test program that was conducted identified approximately 3 percent cruise efficiency improvement, which was later replicated on the MD-11 design.2 In addition, recent winglet design experience using modern CFD methods and high Reynolds number (RN) wind tunnels may provide lessons that could have applicability for winglet designs that may be more effective on the KC-10 and other government transport aircraft. With new multidisciplinary design and optimization methods available, it is likely that an even more efficient and simpler design could be feasible for this aircraft family. C-130H/J The Lockheed Martin C-130 is a tactical airlifter designed to operate from short, austere airfields. While the committee’s focus was on the C-130H/J, a total of 655 aircraft in 16 variations carry out a broad spectrum of missions, from intertheater airlift to electronic and psychological warfare. In evaluating the suitability of the C-130 for the application of winglets to increase cruise efficiency, several factors suggest that performance might improve less than seen on commercial aircraft. The C-130’s wing is already very efficient because its aspect ratio of 10 is relatively high, reducing the overall benefit expected from winglets. The wing design was driven by the need for short-field performance—a requirement not imposed on jet airliners—as well as cruise performance, resulting in the high-aspect-ratio geometry and associated high aerodynamic efficiency. A further reduction 2 Carl A. Shollenberger, John W. Humphreys, Frank S. Heiberger, and Robert M. Pearson, 1983. “Results of winglet development studies for DC-10 derivatives,” NASA-CR-3677, March.

OCR for page 53
Assessment of Wingtip Modifications to Increase the Fuel Efficiency of Air Force Aircraft in winglet effectiveness is attributed to the C-130’s unswept wing with its lower tip loading. Operational considerations also reduce the effectiveness of a winglet modification program. The C-130 missions tend to be short range and flown at lower altitudes. Since winglets are more effective for longer ranges and with higher wingtip loading (realized at higher altitudes), the potential benefit of winglets for the C-130 is limited. A development effort would be needed to optimize winglet geometry, determine integration effects, and evaluate system-level benefits. Other drag reduction approaches, such as aft body strakes and revised wing fillets, have been identified in other studies and should be considered in any fuel consumption reduction study. Intelligence, Surveillance, and Reconnaissance Aircraft The committee was asked explicitly to consider the suitability of ISR aircraft for wingtip modifications. While the U-2 and Global Hawk fall into the ISR category, their mission requirements (extremely high altitude and long endurance) result in wing designs that are already extremely efficient and would be expected to show little if any benefit from winglets. In fact, there might be performance penalties for integrating winglets on these platforms because performance at high altitudes is extremely sensitive to weight. Thus, these aircraft are not good candidates for wingtip modification. Other Air Force Aircraft The easiest decisions on whether to install winglets obviously pertain to 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 found 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 below in Table 4-1. 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, all 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

OCR for page 53
Assessment of Wingtip Modifications to Increase the Fuel Efficiency of Air Force Aircraft TABLE 4-1 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 courtesy of U.S. Air Force. Chapter 3, did not prove to be favorable. The C-21s, derivatives of the Learjet 35A, are small aircraft, and the entire fleet uses approximately 8 million gallons of fuel per year and would not be a priority to modify. Furthermore, they have tip tanks, and wingtip modifications would require the removal of these tanks, severely limiting the range of the 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 747s in the commercial world that have winglets are 747-400s, 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. This discussion leads to the following finding: 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.

OCR for page 53
Assessment of Wingtip Modifications to Increase the Fuel Efficiency of Air Force Aircraft TABLE 4-2 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 PRIORITY AIRCRAFT TO BE CONSIDERED FOR WINGTIP MODIFICATION Based on the committee’s judgment on a variety of factors, some of which are detailed in the following pages, five aircraft were ranked in the order of their suitability for wing modifications, as shown in Table 4-2. However, these judgments are based on minimal basic data, and 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. PRELIMINARY NET PRESENT VALUE ANALYSIS To illustrate the types of benefits and costs that might be realized through wingtip modifications (e.g., winglets) that would produce a reduction in fuel burn, the committee shows here, as examples, the results of its preliminary net present value (NPV) analysis for the KC-135R/T and the KC-10. Appendix A shows the sets of data values the committee used for both aircraft, including number of aircraft, fuel burn, flying hours, and projected retirement dates to calculate the NPV of savings. The mission profiles are inherent in the data used for each aircraft. In particular, the fuel consumption rates (pounds or gallons per hour) are the average over the various mission profiles actually flown. Since it is not possible to know the fuel savings and modification cost for a specific aircraft without performing a detailed engineering analysis, as described earlier in this chapter, the committee parameterized fuel savings and modification cost for each aircraft. The calculations were done for block fuel savings of 3 percent and 5 percent, consistent with commercial airline experience and the findings

OCR for page 53
Assessment of Wingtip Modifications to Increase the Fuel Efficiency of Air Force Aircraft TABLE 4-3 Estimated Aircraft Modification Costsa Aircraft Estimated Modification Cost Range (million $) KC-135R/T 0.5-1.0 KC-10 1.5-3.0 aIncludes nonrecurring development costs. of this report. The price per gallon of fuel was parameterized at $2.50, $5.00, $10.00, and $20.00 to represent the fully burdened cost of fuel. (All monetary values are in dollars of 2007 purchasing power.) The committee estimated one modification cost range for the KC-135R/T and one for the KC-10, as shown in Table 4-3. For the NPV calculations, the committee assumed an annual fuel cost escalation rate of 3 percent and a discount rate of 3 percent. Using the above costs and fuel saving and the data in Appendix A, the committee first calculated the time required for fuel savings to pay back the cost of modifying an individual aircraft. The results shown in Tables 4-4 and 4-5 suggest that modifying the KC-135R/T and KC-10 aircraft in its inventory might financially benefit the Air Force.3 Even in the worst case (highest modification cost, lowest fuel usage reduction, and fuel cost of $2.50 per gallon) for each, the payback periods are within the expected remaining service lives of the aircraft. The results also show how the payback period is affected by the cost of fuel. In constant dollars, if the cost of fuel were to double, the payback period would be cut in half. The NPV results are shown in Figure 4-2 for the KC-135R/T and in Figure 4-3 for the KC-10. The figures show the estimated cumulative fleet net savings over time from the start of aircraft modification to when the last aircraft is retired from service. Results are shown for the worst-case (highest modification cost and lowest fuel usage reduction) and best-case (lowest modification cost and highest fuel usage reduction) payback periods at a fuel cost of $2.50 per gallon. These calculations also take into account the modification cost, aircraft-specific information such as number of 3 The committee’s parametric analysis suggests—but does not prove—that financial benefits would accrue from modifying these aircraft. As stated earlier in the report, deeper aircraft-specific engineering analysis is required to support more precise and higher confidence estimates of the costs and benefits of making the modifications.

OCR for page 53
Assessment of Wingtip Modifications to Increase the Fuel Efficiency of Air Force Aircraft TABLE 4-4 Payback Period for a KC-135R/T Using 649,000 gal/yr Estimated Cost of Modification (FY07 $M) Fuel Usage Reduction from Modification (%) Fuel Saved (K gal/yr) Fuel Cost Saved (FY07 $K) Payback Period (years) Fuel at $2.50/gal 0.5 5 32 81 6.2 0.5 3 19 49 10.3 1.0 5 32 81 12.3 1.0 3 19 49 20.6 Fuel at $5.00/gal 0.5 5 32 162 3.1 0.5 3 19 97 5.1 1.0 5 32 162 6.2 1.0 3 19 97 10.3 Fuel at $10.00/gal 0.5 5 32 324 1.5 0.5 3 19 195 2.6 1.0 5 32 324 3.1 1.0 3 19 195 5.1 Fuel at $20.00/gal 0.5 5 32 649 0.8 0.5 3 19 389 1.3 1.0 5 32 649 1.5 1.0 3 19 389 2.6 aircraft, projected lifetime, flight hours per year, and fuel burn. For these illustrative calculations, it was assumed that the nonrecurring engineering would be done by FY08 and the modifications would begin in 2009. The modifications would be done while an aircraft is undergoing regular depot maintenance, so it would not be out of service for any additional time. The committee also assumed for these calculations that all of the aircraft in the fleet would undergo programmed depot maintenance at a uniform rate between FY09 and FY13 inclusively. As shown in Figure 4-2, 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 approxi-

OCR for page 53
Assessment of Wingtip Modifications to Increase the Fuel Efficiency of Air Force Aircraft TABLE 4-5 Payback Period for a KC-10 Using 2.057 million gal/yr Estimated Cost of Modification (FY07 $M) Fuel Usage Reduction from Modification (%) Fuel Saved (K gal/yr) Fuel Cost Saved (FY07 $K) Payback Period (years) Fuel at $2.50/gal 1.5 5 103 257 5.8 1.5 3 62 154 9.7 3.0 5 103 257 11.7 3.0 3 62 154 19.4 Fuel at $5.00/gal 1.5 5 103 514 2.9 1.5 3 62 309 4.9 3.0 5 103 514 5.8 3.0 3 62 309 9.7 Fuel at $10.00/gal 1.5 5 103 1,028 1.5 1.5 3 62 617 2.4 3.0 5 103 1,028 2.9 3.0 3 62 617 4.9 Fuel at $20.00/gal 1.5 5 103 2,057 0.7 1.5 3 62 1,234 1.2 3.0 5 103 2,057 1.5 3.0 3 62 1,234 2.4 mately $400 million (FY07$). 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 ends of their payback periods. Total net savings to the Air Force are approximately $36 million (FY07$). As shown in Figure 4-3, 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

OCR for page 53
Assessment of Wingtip Modifications to Increase the Fuel Efficiency of Air Force Aircraft FIGURE 4-2 KC-135R/T estimated cumulative inventory-level net savings. 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 $12 million (FY07$). Figure 4-4 illustrates how the cost of fuel affects net savings. The KC-135R/T worst-case payback periods are shown at fuel costs of $2.50, $5.00, $10.00, and $20.00 per gallon. 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. The committee’s analyses give only rough estimates of the costs and benefits of the modifications but, for reasonable projected values of the various factors, these rough estimates suggest that further analysis is warranted. 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

OCR for page 53
Assessment of Wingtip Modifications to Increase the Fuel Efficiency of Air Force Aircraft FIGURE 4-3 KC-10 estimated cumulative inventory-level net savings. 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. 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 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. Once these analyses have been performed, more precise values for the modification costs and fuel savings will be known. The NPV calculations will give an idea of how long it takes to recover the investment. Note that

OCR for page 53
Assessment of Wingtip Modifications to Increase the Fuel Efficiency of Air Force Aircraft FIGURE 4-4 KC-135R/T effect of cost of fuel on payback period. an important parameter in the NPV calculation is the price of fuel, which cannot be known in advance but instead must be hazarded. In any event, based on this preliminary analysis and the current price of fuel, these modifications are worthy of very serious consideration and analysis. INVESTMENT STRATEGIES The statement of task for this study asks for “investment strategies that the Air Force could implement with commercial partners to minimize Air Force capital investment and maximize investment return.” Based on the analysis presented in this and earlier chapters, the committee proposes that the Air Force (1) follow through on its recommendation to initiate detailed engineering analysis in collaboration with the OEMs, (2) implement the modifications, if deemed cost effective, while the aircraft are in depot and in collaboration with industry, and (3) use innovative financing mechanisms as needed. The committee also suggests that the Air Force evaluate the fuel economy practices of commercial aircraft operators, some of which are described in Appendix B, and implement those that are applicable and

OCR for page 53
Assessment of Wingtip Modifications to Increase the Fuel Efficiency of Air Force Aircraft not currently used by the Air Force. The strategies for investing in wingtip modifications are described in further detail below. Performing Retrofit Studies and Implementing Modifications Fuel economy has been a primary focus of commercial aircraft operators for a number of years. They have done an excellent job of working with the airframe manufacturers to perfect the aerodynamic design of aircraft to include wingtip modifications that will reduce drag, of implementing maintenance and operations procedures that save fuel, and of making fuel conservation a part of everyone’s job and a factor in every decision. As a result, it is not surprising that this committee believes the aircraft with highest priority for further analysis are the KC-10 and the KC-135, two derivatives of commercial aircraft. The fact that these aircraft are commercial derivatives means that there is extensive commercial knowledge and experience to complement the military knowledge and experience. It also means that aerodynamic modifications have been examined more carefully and that more experienced engineers and maintenance personnel exist in the commercial industry than would be the case for military-unique aircraft such as the C-5, making the engineering analysis somewhat easier and increasing the availability of information. In any case, the feasibility and cost effectiveness of wingtip modifications on all of the aircraft should be worked out in partnership with the OEMs, whose knowledge of the aircraft structures and load distributions will be critical. In each case, the feasibility studies should be initiated as soon as possible. Then, a high priority should be given to funding the installation of wingtip modifications where they have been determined to be justified from a cost/benefit perspective. The sooner the modifications are incorporated, the sooner they will begin to pay back the initial investment and the less dependent the United States will be on foreign sources of fuel. In addition, the KC-10 and the KC-135 constitute the aerial refueling capability of the Air Force and as such are force multipliers. As the fuel efficiency of these aircraft improves, they can either extend their range, carry more payload (i.e., offload more fuel to other aircraft), or do a combination of both things. KC-10 In the case of the KC-10, winglet design work and testing have already been done on its commercial counterpart, the DC-10, as noted in Chapter 3.

OCR for page 53
Assessment of Wingtip Modifications to Increase the Fuel Efficiency of Air Force Aircraft Although winglets were never incorporated on commercial DC-10 fleets, the knowledge gained from the engineering analysis, design work, and flight tests led to the installation of winglets on the MD-11. There is also the potential that commercial DC-10 operators such as FedEx could follow the Air Force lead and thus create a larger market for wingtip device modifications to the KC-10/DC-10s. Should the decision be to proceed with such a modification, the committee suggests that 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. KC-135R/T Much of the same applies to the KC-135R/T aircraft fleet, except that unlike the KC-10, these aircraft are predominately maintained by Air Force personnel in-house. Also, as noted in Chapter 3, aerodynamic studies of wingtip modifications were done in the 1970s, and a test aircraft was modified with winglets and flight tested. Since the analysis and tests were done so many years ago and there are some uncertainties surrounding the condition of the KC-135 wings and their ability to handle the load increases from wingtip modifications, a sample of the fleet would have to be inspected. The best opportunity to do such an inspection or condition analysis is while the aircraft is in depot maintenance. Most depot overhauls of the KC-135s are performed by the Air Force at the Oklahoma City Air Logistics Center. During maintenance, paint is removed, engines are removed, and the aircraft are opened up for inspection of structural integrity, providing an excellent opportunity to take a careful look at the wings with minimal impact on depot flow. Like the KC-10s, these aircraft are critical to the operational commands, and every effort should be made not to increase scheduled downtime in the maintenance shops. Should the modifications be justified, the committee believes the wingtips could be retrofitted while the aircraft are undergoing their 5-yearly 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 augment the Air Force workforce to accomplish the modification. This would minimize the training required and allow returning the aircraft to the operational forces in the shortest time. For the

OCR for page 53
Assessment of Wingtip Modifications to Increase the Fuel Efficiency of Air Force Aircraft KC-135R/T undergoing programmed depot maintenance at contractor facilities, the Air Force should consider adding wingtip modifications to the existing overhaul contract. Other Aircraft The next priority aircraft for consideration of wingtip modifications are the C-5 and the C-17. The same factors discussed in the investment strategies for the KC-10 and the KC-135R/T should be part of the planning process for these fleets as well. Financing Mechanisms 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 previous report on engine fuel economy in military aircraft,4 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.” 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. 4 NRC, 2007, Improving the Efficiency of Engines for Large Nonfighter Aircraft, Washington, D.C.: The National Academies Press.