Military


Boeing 797 Advanced Single-Aisle Transport (ASAT)

As aircraft manufacturers Boeing and Airbus continue to develop and mature new twin-aisle, wide body aircraft designs in the 210-3S0 seat class, for scheduled first deliveries in 2010 and 2013 respectively, it is anticipated that the next major development undertaking for both companies will be a new narrow body aircraft in the Boeing 737/Airbus A320 class. Southwest is one of the airlines pushing Boeing to develop such a plane. Southwest is the world's largest 737 operator. It is the only model the airline flys.

Between 1984 and 1988 when Airbus was moving beyond the A300 and A310 into the single aisle market to develop the A320, Boeing was creating an expanded 737 family with the -300 -400 and -500. The 1986 Northwest Airlines order for 100 A320 aircraft changed the face of aerospace, giving Airbus a significant early foothold in the US market. Boeing could have killed this upstart. If it had produced a clean sheet of paper the A320 never would have become Airbus's bread and butter. But coming off of the 757 and 767 development programs in the early 80s, the family of three major derivatives for the 737 was the only feasible course.

Narrow body aircraft dominated Boeing's 2009 deliveries. The 737 accounted for 372, or 77 percent, of the total of 481 aircraft delivered. Boeing's 2009 revenue from sales of LCA, $34.051 billion, represented an increase of slightly more than 20 percent compared to 2008 revenue of $28.263 billion. LCA sales revenue accounted for 49.9 percent of the company's total revenues in 2009.

At present, there is not much urgency for these new designs because of robust sales of their current offerings in this size class. However, current projections are that new designs will be introduced after 2015. Boeing and Airbus have been engaged in studies to investigate replacement designs for the 737 and A320, respectively, and published reports indicate that both manufacturers are depending on a next generation engine to power these new designs. The call to replace the 737 and the A320 are clear, though the gains of 20-30% efficiency sought by the airlines just aren't possible with today's engine technology. Blunting the introduction of new aircraft, either company could stop the CSeries CS100/300 and MS-21, both powered by the Pratt & Whitney PW1000G engine, and Comac C919 right in its tracks with a fresh design from the American or European airframers.

The 737/A320 class aircraft represent a significant portion of the global airline fleet. Sixty-five percent of the new aircraft produced over the 20 years 2010-2030 are projected to be in this class. Advances made to reduce the noise and emissions of these aircraft could provide a considerable positive contribution to the goal of minimizing the future environmental impact of aviation. What has not been determined, at least not external to the Boeing and Airbus in-house studies, is the most attractive advanced engine design for this class of aircraft in light of the current metrics of interest in the aviation industry.

The desire for higher engine efficiency has resulted in the evolution of aircraft gas turbine engines from turbojets, to low bypass ratio, first generation turbofans, to today's high bypass ratio turbofans. It is possible that future designs will continue this trend, leading to very-high or ultra-high bypass ratio (UHB) engines. Although increased bypass ratio has clear benefits in terms of propulsion system metrics such as specific fuel consumption, these benefits may not translate into aircraft system level benefits due to integration penalties. In this study, the design trade space for advanced turbofan engines applied to a single aisle transport (737/A320 class aircraft) is explored.

The benefits of increased bypass ratio and associated enabling technologies such as geared fan drive depend on the primary metrics of interest. For example, bypass ratios at which mission fuel consumption is minimized may not require geared fan technology. However, geared fan drive does enable higher bypass ratio designs which result in lower noise. Regardless of the engine architecture chosen, advanced aircraft can realize substantial improvements in fuel efficiency, emissions, and noise compared to the current vehicles in this size class.

There is a practical limit, however, to how much bypass ratio can be increased before significant penalties arise which begin to erode the benefits. Ultra-high bypass ratio engines have large, low speed fans. In a conventional turbofan engine, the relatively low rotational speed of this fan creates low-spool weight and performance issues because of a mismatch between the optimum fan speed and optimum low pressure turbine (LPT) speed. This mismatch can be avoided by connecting the fan and low-spool through a gearbox, which enables the fan and low-spool to operate at different rotational speeds. Use of a gear system does, however, introduce a separate set of concerns such as gearbox reliability, weight, and cost. In addition to issues which arise in the engine design itself, increasing bypass ratio at constant thrust increases engine and nacelle diameter. This increases engine installation penalties, such as nacelle weight and drag, and makes it more difficult to integrate the engine with the airframe. Integration is particularly difficult in the case of a conventional under-wing installation on a low wing aircraft. It is not readily apparent, therefore, whether the propulsion efficiency benefits of lower fan pressure ratio and higher bypass ratio lead to benefits at the aircraft system level.

Adjustment in landing gear length is necessary to accommodate larger engines. Nacelle ground clearance has been an issue for the Boeing 737 aircraft since introduction of the high bypass ratio CFM56-3 engines on the second generation 737-300,-400, and -500 models. The original 737-100 and -200 aircraft were equipped with low bypass ratio JT8D engines, having a diameter of ^42 inches. The minimum nacelle ground clearance on the 737-100 and -200 was only 20 inches. Integration of the higher bypass ratio CFM56-3 (BPR=6), which has a fan diameter of 60 inches, required side mounted engine accessories and a "squashed" nacelle shape to arrive at a minimum nacelle ground clearance of 18 inches. This arrangement was retained for integration of the newer CFM56-7B engine on the 737-600, -700, -800, -900 aircraft. Given the extreme measures required to integrate a 60 inch diameter engine on the 737, it is not likely that a larger engine could be retrofitted to a current 737-800 without other configuration changes.

The primary airframe technology advancement assumed in the Advanced Single-Aisle Transport (ASAT) was extensive use of composite materials for the airframe structure. For the Boeing 787 currently in development, as much as 50 percent of the primary structure is made of composite materials. This composite construction was assumed to result in a 15% reduction in weight of the wing, fuselage, and empennage compared to the metal construction of the 737-800. Other minor technology improvements based on the 787 included an increase in hydraulic pressure to 5000 psi, and a 1% reduction in drag. Changes were also made to the design mission to reflect performance enhancements projected for an advanced aircraft in this vehicle class. Cruise Mach was increased to 0.8 (typical cruise Mach for the 737-800 is 0.785) and design range (with 32,400 lb payload) was increased from 3060 nm to 3250 nm. The basic 737-800 geometry was not changed for the ASAT model, except for a slight increase in wing sweep to enable efficient cruise at Mach=0.8. (Increasing wing sweep increases the "drag-rise Mach number," the Mach number at which compressibility drag begins to greatly reduce aerodynamic efficiency).

Certain classes of internally pressurized aircraft fuselages, such as are found in passenger planes, can beneficially employ near-elliptical cross-sections. For example, an aircraft having a fuselage with a quasi-elliptical, or near-elliptical cross-section that is wider than it is tall, wherein the fuselage comprises a rigid, light weight shell 12 having respective opposite, closed nose and tail ends. This cross-section efficiently encloses a main deck cabin 18, typically provisioned as a spacious and comfortable twin-aisle, seven-abreast cabin, together with a cargo container (typically a LD-3-46W or similar, standardized type of container) in a pressurized lower deck hold. This twin-aisle fuselage cross-sectional shape has also been shown to provide a perimeter-per-seat ratio comparable to that of a corresponding single-aisle, six-abreast, conventional aircraft fuselage having a circular or "blended circular arc" cross-section, and consequently, can also provide a cross-section-parasite-drag-per-seat ratio and an empty-weight-per-seat ratio that, in a first-order analysis, are comparable to those of the corresponding single-aisle fuselage cross-section, while offering better passenger comfort and owner revenue options.

However, achieving an optimized, lightweight structure for such near-elliptical cross-section fuselages when they are constructed of composite materials, i.e., reinforcing fibers embedded in resin matrices, presents substantial engineering design challenges, not only because of the application of such materials to this relatively new application, but also because of the structural and weight penalties involved in moving from a fuselage design having a conventional circular cross-section to a fuselage design having a non-circular cross-section, especially those associated with the internal pressurization effects inherent in the design of high-altitude jet airliners.



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