02-07-2013, 02:48 PM
DESIGN AND ANALYSIS OF AIR CRAFT WING
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ABSTRACT
An aircraft is a vehicle that is able to fly by gaining support from the air, or, in general, the atmosphere of a planet. It counters the force of gravity by using either static lift or by using the dynamic lift of an airfoil, or in a few cases the downward thrust from jet engines.
A fixed wing aircraft is an aircraft capable of flight using wings that generate lift due to the vehicle's forward airspeed and the shape of the wings. Fixed-wing aircraft are distinct from rotary- wing aircraft in which wings rotate about a fixed mass and ornithopters in which lift is generated by flapping wings.
A swept wing is a wing plan form favored for high subsonic jet speeds first investigated in Germany from 1935 onwards until the end of the second world war. Since the introduction of the mig-15 and north American f-86 which demonstrated a decisive superiority over the slower first generation of straight-wing jet fighters during the Korean war, swept wings have become almost universal on all but the slowest jets (such as the a-10).
Sweeping a wing forward has approximately the same effect as rearward in terms of drag reduction, but has other advantages in terms of low-speed handling where tip stall problems simply go away. In this case the low-speed air flows towards the fuselage, which acts as a very large wing fence. Additionally, wings are generally larger at the root anyway, which allows them to have better low-speed lift.
HISTORY
The first aircraft with swept wings were those designed by the British designer J.W.Dunne in the first decade of the 20th century. Dunne successfully employed severely swept wings in his tailless aircraft as a means of creating positive longitudinal static stability. Historically, many low-speed aircraft have had swept wings in order to avoid problems with their center of gravity, to move the wing spar into a more convenient location, or to improve the sideways view from the pilot's position. For instance, the Douglas DC-3 had a slight sweep to the leading edge of its wing. The wing sweep in low-speed aircraft was not intended to help with transonic performance, and although most have a small amount of wing sweep they are rarely described as swept wing aircraft. The Curtiss XP-55 was the first American swept wing airplane, although it was not considered successful. The swept wing had appeared before World War I, conceived as a means of permitting the design of safe, stable, and tailless flying wings. It imposed “self-damping” inherent stability upon the flying wing, and, as a result, many flying wing gliders and some powered aircraft appeared in the interwar years.
INTRODUCTION
A swept wing is a wing planform favored for high subsonic jet speeds first investigated in Germany from 1935 onwards until the end of the Second World War. Since the introduction of the MiG-15 and North American F-86 which demonstrated a decisive superiority over the slower first generation of straight-wing jet fighters during the Korean War, swept wings have become almost universal on all but the slowest jets (such as the A-10). Compared with straight wings common to propeller-powered aircraft, they have a "swept" wing root to wingtip direction angled beyond (usually aft ward) the span wise axis. This has the effect of delaying the drag rise caused by fluid compressibility near the speed of sound as swept wing fighters such as the F-86 were among the first to be able to exceed the speed of sound in a slight dive, and later in level flight.
TECHNOLOGY IMPACT
The Soviet Union was intrigued about the idea of swept wings on aircraft at the end of World War II in Europe, when their "captured aviation technology" counterparts to the western Allies spread out across the defeated Third Reich. Artem Mikoyan was asked by the Soviet government, principally by the government's TsAGI aviation research department, to develop a test-bed aircraft to research the swept wing idea—the result was the late 1945-flown, unusual MiG-8 Utka pusher canard layout aircraft, with its rearwards-located wings being swept back for this type of research. When applied to the jet-powered Mig-15, its maximum speed of 1,075 km/h (668 mph) outclassed the straight-winged American jets and piston-engine fighters first deployed to Korea.
Von Karman travelled to Germany near the end of the war as part of Operation Paperclip, and reached Braunschweig on May 7, discovering a number of swept wing models and a mass of technical data from the wind tunnels. One member of the US team was George S. Schairer, who was at that time working at the Boeing company. He immediately forwarded a letter to Ben Cohn at Boeing stating that they needed to investigate the concept. He also told Cohn to distribute the letter to other companies as well, although only Boeing and North American made immediate use of it.
SWEEP
The angle in plan view between a specified span wise line along an aerodynamic surface and the
normal to the plane of symmetry. For an aerodynamic surface as a whole, the quarter-chord line
is preferred, but any other specified line, such as the leading edge or trailing edge, may be taken
for a particular purpose.
DRAG DIVERGENCE MACH NUMBER
The Mach number for Drag Divergence, where the wave drag becomes significant, is more important: this is a little higher than Mcrit.
OVERVIEW
Adolf Busemann introduced the concept of the swept wing and presented this 1935 at the 5. Volta-Congress in Rome. Sweep theory in general was a subject of development and investigation throughout the 1930s and 1940s, but the breakthrough mathematical definition of sweep theory is generally credited to NACA's Robert T. Jones in 1945. Sweep theory builds on other wing lift theories. Lifting line theory describes lift generated by a straight wing (a wing in which the leading edge is perpendicular to the airflow). Weissinger theory describes the distribution of lift for a swept wing, but does not have the capability to include chord wise pressure distribution. There are other methods that do describe chord wise distributions, but they have other limitations. Jones' sweep theory provides a simple, comprehensive analysis of swept wing performance.
DESCRIPTION
To visualize the basic concept of simple sweep theory, consider a straight, non-swept wing of infinite length, which meets the airflow at a perpendicular angle. The resulting air pressure distribution is equivalent to the length of the wing's chord (the distance from the leading edge to the trailing edge). If we were to begin to slide the wing sideways (span wise), the sideways motion of the wing relative to the air would be added to the previously perpendicular airflow, resulting in airflow over the wing at an angle to the leading edge. This angle results in airflow traveling a greater distance from leading edge to trailing edge, and thus the air pressure is distributed over a greater distance (and consequently lessened at any particular point on the surface).
This scenario is identical to the airflow experienced by a swept wing as it travels through the air. The airflow over a swept wing encounters the wing at an angle. That angle can be broken down into two vectors, one perpendicular to the wing, and one parallel to the wing. The flow parallel to the wing has no effect on it, and since the perpendicular vector is shorter (meaning slower) than the actual airflow, it consequently exerts less pressure on the wing. In other words, the wing experiences airflow that is slower - and at lower pressures - than the actual speed of the aircraft.
One of the factors that must be taken into account when designing a high-speed wing is compressibility, which is the effect that acts upon a wing as it approaches and passes through the speed of sound. The significant negative effects of compressibility made it a prime issue with aeronautical engineers. Sweep theory helps mitigate the effects of compressibility in transonic and supersonic aircraft because of the reduced pressures. This allows the mach number of an aircraft to be higher than that actually experienced by the wing.
SUPERSONIC BEHAVIOR
Airflow at supersonic speeds generates lift through the formation of shock waves, as opposed to the patterns of airflow over and under the wing. These shock waves, as in the transonic case, generate large amounts of drag. One of these shock waves is created by the leading edge of the wing, but contributes little to the lift. In order to minimize the strength of this shock it needs to remain "attached" to the front of the wing, which demands a very sharp leading edge. To better shape the shocks that will contribute to lift, the rest of an ideal supersonic airfoil is roughly diamond-shaped in cross-section. For low-speed lift these same airfoils are very inefficient, leading to poor handling and very high landing speeds.
One way to avoid the need for a dedicated supersonic wing is to use a highly swept subsonic design. Airflow behind the shock waves of a moving body is reduced to subsonic speeds. This effect is used within the intakes of engines meant to operate in the supersonic, as jet engines are generally incapable of ingesting supersonic air directly. This can also be used to reduce the speed of the air as seen by the wing, using the shocks generated by the nose of the aircraft. As long as the wing lies behind the cone-shaped shock wave, it will "see" subsonic airflow and work as normal. The angle needed to lie behind the cone increases with increasing speed, at Mach 1.3 the angle is about 45 degrees, at Mach 2.0 it is 60 degrees. For instance, at Mach 1.3 the angle of the Mach cone formed off the body of the aircraft will be at about sin μ = 1/M (μ is the sweep angle of the Mach cone).
INWARD SPANWISE FLOW
Air flowing over any swept wing tends to move span wise towards the rearmost end of the wing. On a rearward-swept wing this is outwards towards the tip, while on a forward-swept wing it is inwards towards the root. As a result, the dangerous tip stall condition of a backwards-swept design becomes a safer and more controllable root stall on a forward swept design. This allows full aileron control despite loss of lift, and also means that drag-inducing leading edge slots or other devices are not required.
With the air flowing inwards, wingtip vortices and the accompanying drag are reduced, instead the fuselage acts as a very large wing fence and, since wings are generally larger at the root, this improves lift allowing a smaller wing.
As a result maneuverability is improved, especially at high angles of attack.
At transonic speeds, shockwaves build up first at the root rather than the tip, again helping to ensure effective aileron control.
YAW INSTABILITY
One problem with the forward-swept design is that when a swept wing yaws sideways, one wing moves rearwards. On a forward-swept design, this reduces the sweep of the rearward wing, increasing its drag and pushing it further back, increasing the amount of yaw and leading to directional instability. This can lead to a Dutch roll in reverse.
STALL CHARACTERISTICS
Any swept wing tends to be unstable in the stall, since the rearward end stalls first causing a pitch-up force worsening the stall and making recovery difficult. This effect is more significant with forward sweep because the rearward end is the root and carries greater lift.
However, if the aero elastic bending is sufficient, it can counteract this tendency by increasing the angle of attack at the wing tips to such an extent that the tips stall first and one of the main characteristics of the design is lost. Such a tip stall can be unpredictable, especially where one tip stalls before the other.
Composite materials allow aero elastic tailoring, so that as the wing approaches the stall it twists as it bends, so as to reduce the angle of attack at the tips. This ensures that the stall occurs at the wing root, making it more predictable and allowing the ailerons to retain full control.
CONCLUSION
The aim of this project was to design and analyze a Swept Wing for a medium transport aircraft. After identifying the potentials and advantages of the Swept wing the author managed to provide a detailed design of the wing structure and its various wing components. An estimation of the lift forces under various conditions was also conducted which were used in the structural analysis of the wing. A detailed Structural analysis of the wing was also conducted that provided the author with an understanding of the way the Swept wing behaves at various conditions. A structural analysis was also conducted which was able to verify the ability of an iso grid lattice structure to be used in the swept wing design. The detailed design of the swept wing and its components, including high lift devices, was conducted by referring to the aerodynamic requirements of a transport aircraft wing.