The Boeing 757-200 - Research Paper Example

? Boeing 757-200 Assignment The Boeing 757-200 incorporated many improvements in aerodynamic and engine design. These, along with improvements in materials and avionics resulted in an aircraft with improved efficiency, performance and decreased costs through savings in maintenance and fuel. Changes included wing design and engines. Aim This essay will discuss the unique aerodynamic characteristics of the Boeing 757-200. Boeing 757-200 The Boeing 757-200 entered operational service in 1983 and featured numerous design enhancements in the areas of propulsion, aerodynamics, avionics and materials, aimed at meeting its medium haul clients requirements of increased fuel efficiency, decreased noise and increased operational performance (Boeing). The aircraft was available in passenger and freighter configurations, with a maximum takeoff weight of 255,000 pounds (lbs) providing increased payload or range (Boeing). To achieve these enhancements, modified or new design features included; Wing design, Engines, Improved light weight materials, and Modified flight station instrumentation. Wing Design The 757-200’s wing is swept at 250, has a longer span, higher upper surface camber and lower under surface camber, combined with sharper leading edges (Boeing). Relating these features to the lift equation: Lift= CL x (? p V2) x wing area (s), where CL is the coefficient of lift and p (rho) is density, (Dole and Lewis, 2000), as the wing area has increased, this, along with the increase in camber (increasing CL), means that lift is improved. Additionally, as wingspan increases, wingtips are farther apart which reduces the impact of trailing vortices on the wing and decreases induced drag (aerospaceweb). The 757-200 wing shape is a supercritical airfoil (Figure 1) (aerospaceweb). This is commonly used on aircraft that cruise at transonic (less than Mach 1 (Dole and Lewis, 2000)) speeds and is designed to reduce drag through delaying the speed at which the compressibility effect becomes significant (Aerospaceweb). Compressibility effect is the increase in density at an aerofoil due to forward motion (FAA, 2001). Figure 1: Boeing 757 wing airfoil section The differences in pressure distribution over the supercritical airfoil can be seen in comparing between the conventional and supercritical sections, as seen at Figures 2 and 3 (aerospaceweb). Figure 2: Pressure distribution over a conventional airfoil Figure 3: Pressure distribution over a supercritical airfoil Figure 4 (aerospaceweb) shows the coefficient of lift versus angle of attak for the 757-200 wing section. Figure 2: Boeing 757 wing airfoil section lift coefficient The aerodynamic benefits of a supercritical airfoil is related to critical Mach number. Accelerated airflow over an upper airfoil section due to wing camber can reach Mach 1 where the aircraft Mach number (speed) is lower. The speed at which the flow over the wing surface reaches Mach 1 is called the critical Mach number (FAA, 2001). As speed increases above the critical Mach number, areas of supersonic flow are created over the airfoil surfaces. This is accompanied by a shock wave which varies pressure and density. An adverse pressure gradient is created by slowed airflow, inducing higher pressure, which may result in a rapid separation of the airflow from the surface of the airfoil. This separation creates an area of turbulent wake resulting in rapid increases in drag. The Mach number at which this begins to occur is known as the drag divergence Mach number (FAA, 2001). The benefits of a super critical airfoil in combination with swept wings are that the critical Mach number is close to one thereby delaying and reducing the large increase in drag due to wave drag (aerospaceweb). In addition to the above, modified wing sweep enables the optimal combination between required cruising speed, CL and drag divergence (FAA, 2001), whilst the sharper leading edge reduces parasitic drag (Applied Aerodynamics, 2007). It is considered the sharper leading edges would induce an earlier onset of leading edge stall due to separation of flow at higher angles of attack and airflow may separate from the upper wing surface, thereby initiating wing stall at higher speeds (FAA, 2001). These effects would be countered by the 757-200’s slats. Additionally, extensive use of flaps, slats and spoilers, which, given the aircrafts’ increased lift characteristics, serve to slow the aircraft from cruise to landing speeds (Norriss and Wagner, 1998). These factors give a greater aerodynamic efficiency and lower fuel consumption. (Boeing, 2011), including less engine power being required for takeoff and landing and decreased noise. The design featured use of winglets. Winglets are small fences placed upward at the tip of the wing. These improve efficiency (6 to 9 percent according to NASA, by reducing lift-induced drag brought about by wing tip vortices (Chattot, 2005) (Figure 5 (aerospaceweb))). According to Chattot (2005), they also serve to redistribute the loads on the wing and decrease induced drag by lowering the root circulation and increasing it near the winglet junction. Figure 5: Wingtip Vortices Engine Design The engines fitted to the 757-200 included the new Rolls Royce RB211-535E4 series and the Pratt & Whitney PW 2037/2040 (Boeing). These develop between 36,000 and 43,500 lbs of thrust whilst reducing the weight of the engines by around 2000 lb compared to earlier engines (Boeing). These engines feature high-bypass-ratios. Were bypass’s not fitted to jet engines, then, due to the increase in velocity due to compression in the turbine stages, efficiency ((2/(1+c/v))) would be decreased. Bypassing and feeding air directly to successive stages of the turbines achieves increased burn rate due to greater oxygen content in the air and by mixing exhaust with bypass air, the final exhaust speed is decreased, thereby increasing efficiency and decreasing specific fuel consumption. (Dole and Lewis, 2000) (FAA, 2001). High-bypass fans also reduce engine exhaust noise as the flow of air surrounding the jet exhaust assists to buffer noise (Cumpsty, 2003). The engines incorporated use of wide-chord fan blades. These increase efficiency by eliminating mid-span snubbers on the fan blades that controlled vibration but impeded airflow and reduced aerodynamic efficiency (Hunecke, 2010). Deletion of snubbers provides a more aerodynamically efficient aerofoil and increased the blade chord for mechanical stability, reducing the number of blades by approximately one third (Hunecke,2010), thereby reducing weight. Noise containment is further aided by acoustic linings in the engine nacelles (Boeing). The engines feature Full Authority Digital Engine Control (FADEC), which ensures optimal power settings are maintained, thereby saving fuel (Boeing). Materials Lightweight materials contribute to the overall efficiency of the 757-200 through a weight saving of around 2000 lbs. Aluminium alloys are used in the wing skins, graphite/epoxy in control surfaces aerodynamic fairings, engine cowlings and landing gear doors and carbon brakes, which have a longer service life (Boeing). Through new methods of corrosion protection via application of special sealants and enamelling of the fuselage, structural maintenance time and costs are reduced (Boeing). Other Modifications Whilst not purely aerodynamic changes, the below enhanced further the important operational performance of the 757-200. The aircraft was designed for two-crew member operation (no third pilot (Flight Engineer)), thereby reducing labour costs and pioneered the use of digital electronics and advanced displays via a fully integrated flight management computer system (FMCS). Digital processors control navigation, guidance and engine thrust and the FMCS assures that the aircraft flies the most efficient route and flight profile for reduced fuel consumption, flight time and crew workload. The digital systems provided increased reliability and advanced features compared to older electro-mechanical instruments (Boeing). Operational performance of an aircraft includes operational availability (OA). That is time available for flight (not unserviceable or under maintenance). Through the use of digital flight instruments the mean time between failures (MTBF) was decreased, thus improving OA. The engine indicating and crew alerting system (EICAS) and built-in test equipment assists in decreasing turn-around times by providing maintenance information to ground crews. (Boeing). Flight safety was further improved through the provision of a wind shear detection system (Boeing). Conclusions The 757-200 was an advanced aircraft design for its day. Through the use of enhanced aerodynamic features such as winglets and increased camber, lift performance was increased with a corresponding reduction in fuel usage. Use of wide-chord hi bypass fan engines decreased weight and improved propulsion efficiency. References Aerospaceweb (n.d). Retrieved from: http://www.aerospaceweb.org/question/airfoils/q0073.shtml. Applied Aerodynamics. (2007) Applied Aerodynamics Digital Textbook. Stanford, CA: Desktop Aeronautics, Inc. Retrieved from: http://www.desktop.aero/appliedaero/preface/welcome.html. Boeing, (n.d), Retrieved from: http://www.boeing.com/commercial/757family/pf/pf_200back.html. Chattot, Jean-Jacques, (2005). Design and Analysis of Wing/Winglet Combinations Including Viscous Effects at Low Speeds. Davis California, USA: University of California. Retrieved from: http://mae.ucdavis.edu/~EAE127/C10725%5B001-005%5D%5B1%5D.pdf. Cumpsty, Nicholas. (2003). Jet Propulsion (2nd ed.). Cambridge, UK: Cambridge University Press. Dole, Charles. E. and Lewis. James. E. (2000). Flight Theory and Aerodynamics: A Practical Guide for Operational Safety (2nd ed.). New York: Wiley-Interscience. Federal Aviation Administration (FAA). (2001). Aerodynamics for Naval Aviators. Newcastle, WA: Aviation Supplies & Academics, Inc. Hunecke, Klaus. (2010). Jet Engines: Fundamentals of Theory, Design and Operation. Wiltshire, UK: Crowood Press. NASA, (n.d). Retrieved from: http://www.nasa.gov/centers/dryden/about/Organizations/Technology/Facts/TF-2004-15-DFRC.html. Norriss, Guy, Wagner, Mark. (1998). Boeing. Minneapolis, USA: Quayside Publishing Group. Mattingly, J., Heiser, W. and Pratt, D. (2003). Aircraft Engine Design. Reston, VA: AIAA Education. Read More