Assessment of Wingtip Modifications to Increase the Fuel Efficiency of Air Force Aircraft (2007) / Chapter Skim
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3 Previous Analyses and Experience with Wingtip Modifications on Existing Aircraft
3 Previous Analyses and Experience with Wingtip Modifications on Existing Aircraft
Pages 31-52
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From page 31... ...
In some cases, military airplanes are closely related to commercial analogues (e.g., the C-32 is based on the Boeing 757-200) and can benefit from wingtip modification studies that have been conducted for the commercial aircraft.
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From page 32... ...
These wingtip device strategies have been employed both on new design aircraft and as postproduction retrofits on existing aircraft. The following is a summary of the strategies employed by the main commercial airframe manufacturers and two commercial airlines.
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From page 33... ...
. The purpose of the wingtip modification was to improve the cruise aerodynamic efficiency of the airplane and, to a certain extent, differentiate the -400 model of the 747 family from the earlier -100/200/300 models.
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From page 34... ...
, a small Seattle-based aircraft modification company. The API design was a relatively large winglet, 8 ft long, installed on a relatively small commercial airliner.
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From page 35... ...
The total 757 installation weighs 1,358 lb versus approximately 340 lb for the 737-NG. However, the 757 tip extension also increases the efficiency gains for the installation, resulting in a block fuel savings potential of up to 5 percent, depending on mission range.
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From page 36... ...
Both the 767-400ER and the 777-200LR/300ER incorporate a raked wingtip span extension design (Figure 3-4) .5 Similar in effect to winglets, the raked tips provide a reduction in cruise fuel burn and improved takeoff performance at the expense of longer wingspan.
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From page 37... ...
For reasons similar to the design solutions discussed for the 767-400ER and 777-200LR/300ER, Boeing selected a raked tip for the base 787-8 airplane. For growth versions of the 787, an even larger raked tip is envisioned, because the airplane will easily fit in the infrastructure that exists for the airplanes that it will replace.
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From page 38... ...
mcdonnell douglas Heritage Commercial Aircraft Aircraft produced by the McDonnell Douglas Corporation, now part of Boeing, continue to be a large segment of the installed fleet of commercial airplanes. The two main families include the small twinjet DC-9/MD80/MD-90/Boeing 717 models and the large trijet DC-10/MD-10/MD-11 models.
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From page 39... ...
DC-10 work in the early 1980s culminated in a flight test demonstration program that validated a 2-3 percent cruise efficiency improvement over the original DC-10 design, depending on the height of the winglet utilized.6 However, a production winglet design was not incorporated until the design of the MD-11, a derivative of the DC-10 (the military KC-10 is also a derivative of the DC-10)
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From page 40... ...
Those design studies have not been successful in creating a viable business case. Projected block fuel burn reductions of less than 2 percent are offset by substantial modification costs.
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From page 41... ...
. In 2006, Airbus flight tested several winglet designs on the A320 that appear to be similar to the 737-NG blended winglets.
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From page 42... ...
ASSESSmENt oF wINgtIP modIFICAtIoNS FIGURE 3-7 Airbus A320 with tip fence. SOURCE: Reproduced by permission of EADS North America.
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From page 43... ...
This improvement translated into a potential annual fuel savings of up to 110,000 gallons per aircraft for the modified fleet.12 With a projected payback period of 2 years for the first batch of modified 737 aircraft, the fuel burn savings satisfied financial and operational considerations for Southwest and the first order for winglets was placed in 2003. The modification process was coordinated through a Boeing Service Engineering team that operated as the single point of contact between 10Jim Sokol, Vice President of Maintenance and Engineering, Southwest Airlines, Conversation with the committee on December 14, 2006.
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From page 44... ...
The extra weight for this modification is estimated to be 783 to 801 lb. This proposal is based on a projected block fuel burn improvement of 2.6 percent for a 500 nmi stage and 4.4 percent for a 2,000 nmi stage and could save up to 100,000 gallons of fuel per aircraft per year.15 Because the wing structure must be modified, the out-of-service time to complete the winglet modification for the 737-300 aircraft is estimated to be 14 days.16 The results of the Southwest 737 winglet modification program are summarized in Table 3-1.
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From page 45... ...
American Airlines Following an extensive study, American Airlines decided in 2004 to add winglets to its long-range international fleet of 20 757-200ER aircraft.17 The decision was made subsequent to a detailed business planning effort. American considered the following costs and benefits in its winglets business case:18 The business case for installing winglets considers the following costs: • Materials (hardware, software, consumables)
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From page 46... ...
Prior to the modification program, American's 737-800 fleet was operating at an average +2.2 percent fuel burn over the specification, or book, level (worse than design level) .20 The modification program had the potential to return fuel burn to book level or better.
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From page 47... ...
This equated to 32 gal of fuel saved per flight hour.21 The modified 757-200ER fleet demonstrated a fuel savings of 3.3 percent in comparison with its unmodified 757-200ER fleet, a fuel savings of 40 gal per flight hour.22 Both findings are in line with reports of savings from other operators of similar aircraft.23 The results of American's 737-800 and 757-200ER winglet modification programs are summarized in Table 3-2. American concluded that there were no changes in flying qualities, and the Federal Aviation Administration (FAA)
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From page 48... ...
However, the lower winglet was eliminated after it was determined that the cruise performance goals could be met with a single upper winglet configuration; that ground clearance requirements would be problematic with the lower winglet; and that the lower winglet would result in higher manufacturing and maintenance costs. The planform and placement of the C-17 winglet were also driven by exterior lighting requirements.
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From page 49... ...
With these new capabilities, the current C-17 winglet design could be more accurately assessed. These new tools, together with lessons learned from other winglet designs, might make it possible to improve the C-17 winglet design, and thereby cruise drag, another 1 percent or more.26 25Robb 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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From page 50... ...
That work was followed by several Air Force contracts with Boeing to investigate the design space for winglets on the KC-135 and included extensive wind tunnel testing. That work, in turn, determined that winglets could greatly improve cruise efficiency.
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From page 51... ...
It actually has allowed using a smaller aircraft that was originally intended for domestic use, turning it into an efficient international aircraft. The commercial experience is that wingtip modifications make sense if one can achieve a 3-5 percent fuel burn improvement, if careful engineering analysis shows that the aircraft have sufficient structural integrity to easily accept wingtip extensions or winglets, and if the modifications are relatively easy to install.
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From page 52... ...
In Chapter 4, the committee reviews all the candidate aircraft in the Air Force inventory and recommends those that merit careful consideration for tip device retrofits.
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