Appendix B
Additional Methods for Improving Fuel Consumption

The statement of task for this study focuses on the fuel economy of military aircraft and the potential of wingtip devices to reduce fuel consumption. However, wingtip devices are just one method for reducing fuel consumption. Other methods include making other aerodynamic modifications to the aircraft, improving engine efficiency, changing maintenance and operation practices, and improving weight management. Many of these strategies have already been adopted by the commercial airlines, which operate in an intensely competitive environment,1 and others have been touched upon by several recent studies.2 The committee believes it is important for these strategies to be considered, and while they were not the focus of this study, nor was the extent to which the Air Force may already be using some of them examined, some examples are discussed below for the reader’s benefit.

Based on commercial experience, these other methods are expected to be relatively inexpensive, easy to implement, and could yield fuel consumption benefits comparable to wingtip devices. This appendix first explains

1

Joseph C. Anselmo, 2004, “Airline fuel crisis,” Aviation Week & Space Technology (December 6):54-56.

2

Past studies on fuel conservation measures in the Air Force and at DOD include Defense Science Board (DSB), 2001, More Capable Warfighting Through Reduced Fuel Burden; Air Force Scientific Advisory Board, 2006, Technology Options for Improved Air Vehicle Fuel Efficiency; and NRC, 2007, Improving the Efficiency of Engines for Large Nonfighter Aircraft. Each of these studies included at least some discussion on current commercial practices.



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Assessment of Wingtip Modifications to Increase the Fuel Efficiency of Air Force Aircraft Appendix B Additional Methods for Improving Fuel Consumption The statement of task for this study focuses on the fuel economy of military aircraft and the potential of wingtip devices to reduce fuel consumption. However, wingtip devices are just one method for reducing fuel consumption. Other methods include making other aerodynamic modifications to the aircraft, improving engine efficiency, changing maintenance and operation practices, and improving weight management. Many of these strategies have already been adopted by the commercial airlines, which operate in an intensely competitive environment,1 and others have been touched upon by several recent studies.2 The committee believes it is important for these strategies to be considered, and while they were not the focus of this study, nor was the extent to which the Air Force may already be using some of them examined, some examples are discussed below for the reader’s benefit. Based on commercial experience, these other methods are expected to be relatively inexpensive, easy to implement, and could yield fuel consumption benefits comparable to wingtip devices. This appendix first explains 1 Joseph C. Anselmo, 2004, “Airline fuel crisis,” Aviation Week & Space Technology (December 6):54-56. 2 Past studies on fuel conservation measures in the Air Force and at DOD include Defense Science Board (DSB), 2001, More Capable Warfighting Through Reduced Fuel Burden; Air Force Scientific Advisory Board, 2006, Technology Options for Improved Air Vehicle Fuel Efficiency; and NRC, 2007, Improving the Efficiency of Engines for Large Nonfighter Aircraft. Each of these studies included at least some discussion on current commercial practices.

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Assessment of Wingtip Modifications to Increase the Fuel Efficiency of Air Force Aircraft some of the challenges experienced by commercial aircraft and then discusses other strategies for improved fuel efficiency. Since the preceding NRC report dealt with improving engine efficiency, an important determinant of fuel consumption, that strategy is not covered here.3 CHALLENGES The aging and service use of commercial aircraft and jet engines take a toll, reducing aerodynamic and propulsion efficiency, as evidenced by increased fuel burn. As aircraft age and material wears, or suffers minor damage, fuel efficiency tends to decline because of external repairs, increased air leakage from the fuselage, weight gain from the entry of moisture and from years of modification programs, and engine deterioration. It is common for new commercial aircraft types to experience fuel burn increases over the specification (or “book”) level of 2-4 percent within 4 years of entry into service. The regulatory agencies and internal technical organizations that certify continued airworthiness set the allowable in-service expansion of the original by tight manufacturing tolerances to accommodate the effects of normal wear and tear on commercial machinery. Then, too, owners and operators of aircraft often push the performance limits of their equipment to achieve greater payload, range, endurance, or takeoff performance. Regardless of the specifications that prevailed when the aircraft were procured, political, regulatory, economic, or demographic influences open up prospects for new missions or markets that lie tauntingly just beyond the existing capabilities of existing in-service aircraft. Aircraft operators must then either seek new equipment with the required performance or attempt to improve the performance of existing equipment, through modification, to accommodate those new missions and markets. Specific strategies to take on these challenges are discussed below. AERODYNAMICS Lessons learned from the commercial airplane industry suggest that aerodynamic improvements using strategies other than wingtip modification are worth consideration for the Air Force’s fleet of aircraft. Many of the its transport aircraft were designed in the early days of swept-wing trans- 3 NRC, 2007, Improving the Efficiency of Engines for Large Nonfighter Aircraft, Washington, D.C.: The National Academies Press.

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Assessment of Wingtip Modifications to Increase the Fuel Efficiency of Air Force Aircraft port design and do not take advantage of some more recent technological advancements, such as supercritical aft-loaded wings; low-interference, podmounted engine installations; reduced static stability; and digital designs with low excrescence drag. Wing Modifications A number of common performance improvements have been incorporated into the commercial fleet, both by the original equipment manufacturers (OEMs) and by third-party aircraft modification firms. Obviously, winglets are the most visible sign of this activity. Another common modification of earlier generation aircraft is re-rigging of the high-lift devices for cruise flight, creating a pseudo-aft-cambered wing. This has been done for the Boeing 727, for example. Another modification is the addition of a small, trailing-edge wedge on the lower surface of the wing. This creates some aft-camber and can also be used to change the span loading of the wing. That strategy was implemented on the MD-11 derivative of the DC-10 wing and is being studied for use on other aircraft. These trailing-edge modifications can be worth a reduction in fuel burn of up to 2.5 percent, depending on factors such as wing flexibility, trim drag characteristics, the original wing airfoil design, etc.4 Engine Installation Pod-mounted engine installations of early-generation aircraft were crude by the standards of today, when high-powered computational fluid dynamics (CFD) methods have allowed very close coupling of engines with little or no interference drag. If a re-engine program is considered for a transport-category airplane, it is likely that a new engine installation can take advantage of this technology, resulting in a shorter pylon with less weight and wetted area and perhaps less interference drag as well. It is not likely that redesign of an existing engine installation to reduce drag or weight would pay off on its own, but if combined with a re-engine program, there could be a synergistic payoff of 1-2 percent. 4 R.D. Gregg, R.W. Hoch, and P.A. Henne, 1989, “Application of divergent trailing-edge airfoil technology to the design of a derivative wing,” SAE Technical Paper 892288, September; P.A. Henne and R.D. Gregg, 1991, “New airfoil concept,” AIAA Journal of Aircraft 28(5):300-311.

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Assessment of Wingtip Modifications to Increase the Fuel Efficiency of Air Force Aircraft Aerodynamic Cleanup Aerodynamic cleanup programs are common, both for in-production and in-service airplanes. This would include redesign of excrescences, such as door seals, high-lift system seals, rigging, antenna installations, protruding fasteners, air inlets and exhausts for external air exchange systems, and so on. It also might include redesign of aerodynamic fairings, including flap support fairings, wing-to-body fairings, and the like. Up to 4 percent of airplane drag has been saved on commercial aircraft, some having cleanup programs and others not. As an example, the MD-11 had a Cruise Performance Improvement Program, which resulted in approximately a 4 percent improvement to the fuel burn efficiency of the modified aircraft.5 Further investigation would be required to determine if any of these redesigned items, which were above and beyond the basic improvements made to the original MD-11 design by incorporation of the winglets and trailing-edge wedges, are applicable to the KC-10/DC-10 family. MAINTENANCE AND OPERATIONS The mechanical condition of an aircraft and the means by which it is operated are critical for maintaining original performance design characteristics and objectives. As stated earlier, commercial aircraft typically exhibit fuel burn 2-4 percent above the book value within 4 or so years of entering service. Airline experience demonstrates that it is difficult to determine the relative contribution of the airframe and the engine to this fuel burn deterioration. Over the years, the airlines and commercial aircraft and engine manufacturers have developed comprehensive maintenance and operational procedures to return aircraft to their certified fuel-burn performance. Collectively, these efforts can improve fuel burn by 1-3 percent. These procedures are effective and relatively easy to implement. Where these procedures make operational sense and are not currently used by the Air Force, military managers should consider implementing the practices that have merit. 5 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 Maintenance Initial efforts to improve performance generally rest with an attempt to regain the original tolerances and material conditions for in-service aircraft. These efforts are generally accomplished according to priorities that are jointly developed with the OEM. Based on individual airline operating experience, these maintenance activities or fine-tuning exercises to return an aircraft as close as practical to its original material condition and configuration will frequently reduce fuel burn by 1-3 percent (or possibly more). Effective maintenance programs require a comprehensive knowledge of the mechanical condition of the aircraft and its systems and the conditions that cause mechanical malfunctions. They require, as well, a detailed accounting of the maintenance actions conducted and the resulting effect on the malfunction. Most important, program success requires the development of measures and standards for efficient operation of the equipment. Maintenance programs must be developed to take into account some of the systems and elements that, if not operating properly, can have a major negative impact on fuel burn:6 Air data. Air data generally refer to the aircraft’s pitot-static system, which gives crew and system a reference for airspeed, altitude, and vertical velocity. Air data refer as well to some engine instrumentation such as engine pressure ratio, which gives crew and systems proper engine power information. Proper maintenance of these systems is essential to assure that the aircraft is operating at the airspeed/Mach number, altitude, and power that give the most efficient fuel burn. In addition, improper power setting can result in asymmetric thrust, which must be compensated for by trimming the control surfaces, increasing drag. The commercial industry recently went through an accuracy improvement in air data systems to support the worldwide Reduced Vertical Separation Minima program. This revealed system deficiencies that have resulted in system improvements to assure optimum operational and fuel burn performance. The technology is now available that would allow collecting more accurate airspeed data. 6 These are also discussed in Improving the Efficiency of Engines for Large Nonfighter Aircraft, Washington, D.C.: The National Academies Press, 2007. That report also discusses improvements to the maintenance programs for engines when they are in depot (rather than on-wing). That discussion is omitted here.

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Assessment of Wingtip Modifications to Increase the Fuel Efficiency of Air Force Aircraft Pneumatics. Pneumatic leakage through door cutouts, improper sealing, airframe damage, and fuselage attach fittings adversely affects fuel burn in two ways: (1) extra fuel is consumed because the air-cycle machines must work harder to compensate for the leakage and (2) the leakage of air from the fuselage disrupts the airflow around the aircraft, resulting in increased drag. Close monitoring of the airframe and engine pneumatic systems is encouraged to maintain optimum fuel burn. Seals. It is essential to assure that the aerodynamic seals between the lower and upper wing are in good condition, especially on the leading edges. Flight controls. Flight controls must be properly rigged. Floating spoilers, flaps that are not properly seated, and ailerons not properly rigged can all have a very large impact on fuel burn. Large surfaces such as rudders are especially critical and adversely impact fuel burn if out of rig or trimmed to offset asymmetric thrust conditions. Fuel indicators. To assure the best flight profiles for fuel efficiency, it is essential to have accurate references for fuel quantity and fuel flow. In order to achieve this objective it is essential that fuel quantity probes and indicating systems as well as flowmeters be calibrated periodically. Engine performance. Over time, the wear on engine blades adversely affects the gas path of turbine engines. The earliest sign of these effects is commonly the loss of exhaust gas temperature (EGT) margins. This loss is typically between 5­°C and 7­°C of EGT per 1,000 hours of flight time and ultimately impacts takeoff performance, especially at hot or high-altitude airports with relatively short runways. This deterioration can be mitigated by a rigorous on-wing engine wash program that initially returns between 5­°C and 10­°C of EGT. However, as the engine continues to deteriorate over time, this effect decreases as well. Housekeeping. Simple housekeeping actions can have benefits, such as maintaining leading edges so that they are clean and free of excessive dents, making sure the pitot-static lines are free of obstructions, and assuring the proper calibration and functioning of systems to measure air mass temperature. The removal of fittings and materials remaining from past modifications or temporary accoutrements that add unnecessary weight to the airframe is also important. The importance of reducing unnecessary weight is discussed elsewhere in this appendix.

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Assessment of Wingtip Modifications to Increase the Fuel Efficiency of Air Force Aircraft Operations A number of operational procedures and practices have been developed by the air transport industry to reduce fuel consumption. Their effectiveness is dependent on (1) the commitment of management and flight crews to their use and (2) standardization in their application throughout all functions of the organization. The following elements are fundamental to controlling excessive fuel burn. They are well known by all aircraft operators. To the extent that they are effectively managed to affect fuel burn depends on how ingrained they are into the thought processes of individual flight, maintenance, planning, and configuration control personnel—in other words, how well they are accepted into the culture of the organization. Fuel Burn Tracking Most airlines have strict fuel burn reduction plans that track individual aircraft and flight crews to isolate equipment or operational factors that contribute to excessive fuel burn. The plans, which are frequently developed in conjunction with the aircraft manufacturer, include the following: Develop flight-phase operational configurations and profiles—that is, takeoff and climb to cruise, cruise, descent/land profiles—to provide the optimum airspeed and power setting for targeted fuel burn and flight performance at the given gross weight and altitude of the aircraft. Report periodically while in flight on fuel burn, power settings, airspeed, and altitude. Determine block fuel use for specific aircraft and flight crews. Continuous monitoring of cruise performance can give aircraft operators the information they need to decide how and where to save fuel. Such monitoring allows the operators to do the following: Adjust the baseline performance levels they use for flight planning so that the correct amount of fuel is loaded on each and every flight. Increase flight crew confidence in flight plans and possibly decrease the amount of discretionary fuel requested. Identify airplanes that burn a lot of fuel for possible corrective actions.

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Assessment of Wingtip Modifications to Increase the Fuel Efficiency of Air Force Aircraft Match the airplanes and engines that perform best with respect to fuel burn to fly the longest range/endurance missions. If a specific aircraft is flagged as having excessive fuel burn, maintenance action is initiated to determine, and correct, the cause of that unnecessary burn (the preceding section on maintenance gives details). Airframe and engine manufacturers may be called on to assist if the corrective actions are not readily identifiable. If a particular flight crew, or flight crew member, consistently exceeds average block fuel usage for specific flight segments, the situation may be addressed with appropriate training. Wherever possible, the flight crew should assure that its fuel burn practices comply with the following guidelines: Use the manufacturer-recommended fuel burn procedures for wing tanks as appropriate to maintain wing structural integrity and stiffness. Maintain lateral balance during fuel burn. Maintain aft center of gravity (CG) with fuel burn. Trim One of the main reasons specific aircraft and/or flight crew members have excessive fuel burn is improper trim, which can come from a sub-optimal performance indicating system, fuel quantity system, or flight control rig or from poor flight crew performance. Airline experience has demonstrated that even pilots with thousands of flying hours and years of experience in the cockpit can fail to trim aircraft properly. A number of priorities must be observed to properly trim an aircraft. When the mission requires predominant use of the autopilot, the flight crew should assure that the aircraft is trimmed properly prior to connecting the autopilot and should then disconnect the autopilot periodically to retrim as necessary. Proper aircraft trim is achieved by the following means: Maintain lateral balance during fuel burn. Fly the aircraft manually to maintain straight and level flight. Balance the thrust using all of the engine performance indicators. Trim the elevator to eliminate elevator control force and maintain level flight.

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Assessment of Wingtip Modifications to Increase the Fuel Efficiency of Air Force Aircraft Trim the rudder to eliminate rudder control force and sideslip/ turning flight. Trim the aileron to eliminate control force. Verify control displacements (spoilers, ailerons, and rudder within manufacturer/service limits) for potential maintenance action (rigging). As mentioned in the maintenance section, it is important to verify control displacements (spoilers, ailerons, and rudder should be monitored within the manufacture’s service limits) for potential maintenance action (rigging). Also, it is obvious that failure to calibrate flight and performance instrumentation will prevent the flight crew from trimming the aircraft properly. Ground Operations Standard procedures exist for ground operations as well to minimize unnecessary use of engine power and the auxiliary power unit (APU). The following exemplify such procedures: Single-engine taxi is used for two-engine aircraft, and one- or two-engine shut-down taxi for three-engine and four-engine aircraft, whenever the airport and operational conditions and configurations allow. Engines are not started until the appropriate time in the departure sequence. The APU is not used until required for engine start or postflight operations unless external conditions require it (high temperatures, absence of ground power, etc.). WEIGHT MANAGEMENT The main goal of aircraft manufacturers is to design their aircraft to carry out the intended mission with the best possible performance. A common objective relative to that goal is to eliminate as much unnecessary weight and material as possible. This is true because every added pound of weight eats into aircraft performance margins by feeding the twin detriments of unnecessary fuel burn and reduced payload. Two facts are certain to apply to almost every commercial or military aircraft: (1) The basic air-

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Assessment of Wingtip Modifications to Increase the Fuel Efficiency of Air Force Aircraft craft empty weight will increase over the life of the aircraft (to the detriment of payload capability and fuel burn performance) and (2) mission demands will grow to push the operational limits of the aircraft. To address these realities, aircraft operators must work diligently and continuously to determine and control the actual weight and balance of their aircraft. This is accomplished by programs that allow the following: Periodic and accurate determination of individual aircraft weight and balance (CG). Controlling aircraft modification programs to minimize weight increases and maintaining allowable CG aft to reduce drag. Maintaining the external condition of the aircraft to maintain aerodynamic efficiency and minimize drag—for example, assure that dirt and other external contaminants such as grease build due to cleaning lubricants and the like do not add weight or affect the aerodynamics. Calibrating flight and performance instrumentation to assure proper criteria for weight, flight conditions, and performance. The following are examples of additional and relatively simple actions that can be taken to reduce fuel consumption: Establish a baseline of equipment and material routinely carried on the aircraft (pallets, tools, etc.). Obtain fleet aircraft weight samples to determine the spread in actual weights, including weighing some operational aircraft ready to go out on a mission and some empty aircraft. Weigh all the equipment that is put on aircraft, such as repair kits. Use actual rather than estimated weights for cargo. Load all materials so as to maintain the maximum allowable (or practical) aft CG. Revise operational practices to reduce unnecessary weight. For training and operational flights, eliminate any equipment that is not essential to the mission. Do not carry excess fuel since its weight increases fuel consumption. Review the need to carry remote station tools and equipment and accurately account for the weight of necessary tools and equipment. Weigh all cargo to verify that registered weights are accurate. Revise maintenance practices to reduce unnecessary weight. Ensure aircraft are clean and not carrying water, trash, or dirt in cavity and

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Assessment of Wingtip Modifications to Increase the Fuel Efficiency of Air Force Aircraft swamp areas. Check insulation blankets for condensation which can increase the weight of the blankets significantly—by, for example, more than 1,000 lb in the case of 707 blankets. Consider lighter weight replacement materials for nonstructural items such as floor panels (floors in KC-135s, for example, are plywood). Create a weight maintenance czar to keep aircraft weight as stable as possible over time. The commercial airline industry has also employed changes when designing new aircraft to improve CG management. Newer designs, such as the Boeing 777 and 787 and the MD-11, have used stability augmentation to allow smaller tail surfaces and to shift the CG aft, reducing trim drag. For an existing aircraft, it is probably not practical to change the design to improve stability or allow smaller tail surfaces. But, as mentioned above, by paying careful attention to payload loading position, an aircraft can be routinely flown near its aft CG limit, often saving a percent or more in trim drag. Commercial airlines have automated their loading processes to make aft loading more routine.