Winglet for older Military Aircraft - Save Fuel

HISTORY OF WINGTIP DEVICES

Within a few years of the first heavier-than-air flight, the idea of beneficial wingtip devices was introduced. Lanchester patented the concept of awing end plate in 1897 and suggested that it would reduce wing drag at lowspeeds. Theoretical studies of end plates by Munk in 1921 were followedby studies of von Karman and Burgers and Mangler in the 1930s, a patenton nonplanar wings was granted to Cone in 1962, and a paper on the topic was published by Lundry and Lissaman in 1968. This work was paralleled by many experimental studies (see, for example, National Advisory Committeefor Aeronautics (NACA) work from 1928 to 1950 ), most of which did not attain the potential savings suggested by the theory. This was partly due to simplistic design, which often included low-aspect-ratio, untwisted, flat-plate airfoils. Recognition of the importance of winglet location, twist, and aspect ratio was clear in the patent of Vogt in 1951 and in a variety ofother nonplanar wingtip geometries studied and patented by Cone. In the early 1970s, Whitcomb10 of the National Aeronautics and Space Administration(NASA) defined and tested high-aspect-ratio, carefully designed nonplanar wingtips, termed “winglets,” which were soon to appear on numerous aircraft, including Rutan’s VariEze in 1975 and the Learjet 28/29in 1977. The winglet of the Boeing 747-400 has a much lower dihedral angle than the Whitcomb winglet, and since that time, numerous vertical,canted, and horizontal wingtip extensions have been put into commercialand military service.

M.M. Munk, 1921, “The minimum induced drag of aerofoils,” NACA Report 121.T. von Karman and J.M. Burgers, “General aerodynamic theory—perfect fluids,” InAerodynamic Theory, W.F. Durand, ed., Berlin/Vienna: Julius Springer-Verlag, 1934-1936,and New York: Dover Publications, 1963, Div. , Vol. II, pp. 216-221.W. Mangler, 1938, “The lift distribution of wings with end plates,” NACA TM 856;transl. by J. Vanier from “Die Auftriebsverteilung am Tragflügel mit Endscheiben,” Luftfahrtforschung14:64-569.C.D. Cone, Minimum Induced Drag Airfoil Body, U.S. Patent 3,270,988, September1966.J.L. Lundry and P.B.S. Llssaman, 1968, “A numerical solution for the minimuminduced drag of nonplanar wings,” Journal of Aircraft 5(1).Paul E. Hemke, 1928, “Drag of wings with end plates,” NACA TR-267.John M. Riebe and James M. Watson, 1950, “The effect of end plates on swept wingsat low speed,” NACA TN-2229.

Introduction to wingtip aerodynamics

Much of the drag of an aircraft is related to the lift generated by its wing. To create this lift, the wing pushes downward on the air it encounters and leaves behind a wake with a complex field of velocities. This air behind the wing moves downward then outward, while the air outboard of the wing tips moves upward, then inward, forming two large vortices.

The energy required to create this wake is reflected in the airplane’s“induced” or “vortex” drag. For most aircraft, induced drag constitutes a large fraction, typically 40 percent, of cruise drag. During takeoff, induced drag is even more significant, typically accounting for 80-90 percent of the aircraft’s climb drag. And while takeoff constitutes only a short portion of the flight, changes in aircraft performance at these conditions influence the overall design and so have an indirect, but powerful, effect on the aircraft’scruise performance. Consequently, concepts that reduce induced drag canhave significant effects on fuel consumption. 11 Richard Vogt, Twisted Wing Tip Fin for Airplanes, U.S. Patent 2,576,981, December 1951.C.D.

Cone, Minimum Induced Drag Airfoil Body, U.S. Patent 3,270,988, September1966. 10 Richard T. Whitcomb, 1976, “A design approach and selected wind-tunnel results athigh subsonic speeds for wing-tip mounted winglets,” NASA TN D-8260. 11 Ilan Kroo, 2005, “Nonplanar wing concepts for increased aircraft efficiency,” VKILecture Series on Innovative Configurations and Advanced Concepts for Future Civil Aircraft, June 6-10.

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. 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.

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 Ron Sega, 2006, “Air Force energy strategy,” Worldwide Energy Conference and Trade Show, Arlington, Va., April 19.

Data provided by Defense Energy Support Center (DESC) on fuel usage by missiondesign series for FY05.

3-5 percent, depending on the trip length. 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:
1. 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.
2. Determine the market price of aviation fuel at which incorporating winglets would be beneficial for each platform.
3. Consider impacts to aircraft maintenance and flight operations(including ground operations).
4. 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 spanextensions). 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.

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 Committeein Report 109-452, which may reflect fuel savings under cruise conditions.

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.

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. 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. The C-17 already has winglets, and the KC-135 and KC-10, which are closely related tothe Boeing707 and DC-10 commercial airframes, respectively, have been studied previously for wingtip modifications.

Besides wingtip devices, there are other methods to reduce aircraftfuel consumption, but since they were not mentioned in the statement of task, the committee did not examine them in detail, nor did it examine theextent 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.

Feasibility and Cost Effectiveness of Modifying Air Force Aircraft

Finding: The committee’s analysis for a broad range of fuel prices andwith 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 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.

Potential for Wingtip Modifications to Benefit 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 AirForce aircraft that are derivatives of commercial aircraft do not appear to be good candidates for wingtip modifications.

Winglet Status of Air Force Aircraft Derived from Commercial Airframes

Air Force Aircraft Commercial Equivalent Inventory Winglets

COMMERCIAL EXPERIENCE

There are a number of very successful applications of winglets and wingtip extensions in the world’s commercial airplane fleet. These programs have been successful for a number of reasons, most notably because they have enhanced the economic value of the subject commercial airplanes. These wingtip device strategies have been employed both on new design aircraft and as post production retrofits on existing aircraft. The following is a summary of the strategies employed by the main commercial airframe manufacturers and two commercial airlines.

BENEFITS OF WINGTIP MODIFICATIONS

Reduced Fuel Burn

By reducing drag, wingtip devices help the aircraft operate more efficiently and, in turn, reduce fuel burn. The fuel savings benefits of wingtip modifications depend on the mission flight profile, particularly the range and time spent at cruise speed.

Commercial experience with winglet retrofits on the Boeing 737-300/700/800 indicate a 1.5 percent block fuel savings for trips of 250 nautical miles (nmi), increasing to 4 percent for trips of 2,000 nmi .15

For the Boeing 757-200 and 767-300, block fuel savings were 2 percent for 500 nmi trips and 6 percent for 6,000 nmi. On an annual basis, winglets were projected to result in savings to commercial operators of up to 130,000 gallons of fuel per aircraft on the 737-800 and up to 300,000 gallons per aircraft on the 757-200 .16 Reduced fuel consumption translates directly into a reduction in operating cost.

Increased Payload-Range Capability

If less fuel is required to accomplish a particular mission at a specific takeoff weight, then that credit can be realized in more than one way. For example, the aircraft can carry more weight (more payload) the same distance or it can carry the same payload farther (greater range). The increase in payload-range capability made possible by winglets on one commercial aircraft, the Boeing 737-800. The benefits begin to become apparent for ranges beyond 2,000 nmi. Between the 2,000 and 3,000 nmi range, winglets enable 80 nmi more range or 910 lb more payload. Beyond the 3,000 nmi range, winglets allow for 130 nmi more range or 5,800 lb more payload .17

In the commercial world, this capability translates into operational flexibility - for example, it offers a greater choice of aircraft along certain routes or the opening up of new routes and destinations that were not previously within range.

The increased payload-range capability is valued in military aircraft applications just as it is in commercial aircraft applications. Carrying more payload to the same distance could mean fewer sorties to accomplish a specific goal, or it could allow servicing more customers with the same number of operational aircraft.

Improved Takeoff Performance

The reduced drag associated with wingtip modifications reduces the thrust levels required for takeoff (reducing community noise at the sametime) and enables faster second-segment climb. This increased climb rate allows the use of airports having shorter runways and allows for operations from airports located at higher altitudes and in hotter climates. Alternatively, these advantages may be traded for carrying higher payloads or acombination of both.

Critical performance constraints for military aircraft can be dictated by either airfield constraints or a combat situation. For example, at an airfield in hostile territory, a steep climb out may be desired to reduce the time an aircraft is vulnerable to surface-to-air threat systems around the airfield.

Another example would be takeoff and landing constraints at a commercial airport where military tankers, airlift, or ISR platforms may also have tooperate.

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 reductionin 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 payfor themselves well before the aircraft are due to retire. Since it is not possibleto know the modification costs and fuel savings without performing adetailed engineering analysis, these were treated as parameters in the model.

The range for modification costs was chosen from list prices and committeeestimates. For fuel savings, the calculations were done for block fuel savingsof 3 percent and 5 percent, consistent with commercial airline experienceand 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 costof $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 aremodified. Total net savings to the Air Force are approximately $400 million(FY07 $). In the KC-135R/T worst case, net savings become positive24 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 $).

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 aircraft maintenance or flight operations for aircraft it currently operateswith winglets, and the committee does not expect any major problems withmodifications to other aircraft under consideration.

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.