A plane designed for sub-Mach 1 speeds just be designed completely different than one built for speeds above Mach 1 due to significant changes in force load and the creation of supersonic showplaces. However the way an aircraft is constructed is Just as important as its design in the overall aerodynamics and structural integrity of the craft. Modern aircraft are constructed with thin sheets of a low weight, high strength composite material. The most common material used In the construction of aircraft Is aluminum (SAGES Module 2).
Aluminum alloys are preferred due to their high strength, low weight and relatively low cost of fabrication. A low weight material Is required to create an aircraft that cannot only lift Itself, but more Importantly the fuel It requires to operate and the cargo It must transport. Aircraft fuselages are built In the form of shells that must withstand the compression and forces applied to them without buckling (SAGES Module 2). An inherent problem with using large, thin sheets of aluminum alloy in the construction of the fuselage is its weakness to collapsing under high compression in areas without support.
This weakness is overcome by using longer 2 and stringers to reinforce the sheets of alloy. Another major problem associated with the materials used on aircraft is metal fatigue. Metal fatigue causes cracks and warping of the metal as stress Increases, and It Is a general rule all metal structures have a fatigue life (SAGES Module 2). One approach to stopping the formation of cracks is known as the fail-safe principle (SAGES Module 2). This method deliberately places joints near areas of high-stress in order to halt the growth of a crack so it may not spread and reduce the structural integrity of the entire aircraft.
A more modern method of increasing the strength and fatigue life of the aircraft components is a eternal called GLARE, which stands for glass-reinforced aluminum laminate (Gucci, and J. M. 72-9). This composite material is composed of sheets of aluminum bonded together by a glass fiber adhesive and a sandwich structure, as seen in Figure 1 . The advantages of this new material are increased fatigue resistance, impact resistance, corrosion resistance, and lighter weight. GLARE material Is also fast and simple to both repair and manufacture, making It a major asset In the production and maintenance of aircraft.
In most commercial aircraft, GLARE Is used In conjunction tit aluminum alloy, as shown In figure 2, to create a strengthened frame and more impact resistant fuselage (Gucci, and J. ;M. 72-9). In addition to the materials used in in creating a durable shell resistant to compression. There are two basic designs for the fuselage of an airplane, monochrome and semi-monochrome (“FAA Regulatory and Guidance Library’). The first method used in aircraft design was monochrome, which relies on the skin of the fuselage to withstand all primary stresses on the structure.
Depicted in figure 3, the skin of the craft is required to remain rigid without the use f anything more than the vertical support of bulkhead structures. This creates a problem where the skin of the aircraft must be built from heavier materials that can withstand the forces applied. To 3 alleviate this issue and allow for lightweight alloys to be used in the construction of aircraft, the semi-monochrome method of design was created. This newer method uses support structures called longer and stringers to give strength to the skin of the fuselage, which are compared to monochrome construction in figure 4.
Longer are large metal support strips that extend horizontally from the front of the fuselage o the back and provide primary load bearing support (“FAA Regulatory and Guidance Library’). Longer are supported by stringers, which are smaller metal strips attached directly to the skin of the fuselage and give shape and compression support (“FAA Regulatory and Guidance Library’). Longer, stringers, and bulkheads work in unison to create a rigid structure resistant to buckling caused by compression on the fuselage skin.
While the design of an aircraft is important in maintaining its structural integrity, the way an aircraft is constructed is fundamental in reducing metal fatigue and the mount of stress on individual components. There are three methods of aircraft constriction, differential, sentential, and integral (SAGES Module 2). In differential construction each of the airplane’s major components contain a large number of separate parts connected by riveting, bolting or welding, with riveting being the primary method of connection (SAGES Module 2).
This method of connection suffers from reduced structural integrity at the rivet holes. Stress builds up in the rivet holes of the aircraft and creates an area in which cracking and buckling could occur. This issue is overcome by the use of semi-integral construction. In semi-integral construction the components of the aircraft are bonded together by a high strength resin (SAGES Module 2). This method of construction is commonly used in the wings of an aircraft by creating a sandwich structure consisting of two sheets of aluminum with a foamed plastic core.
This creates a high strength, low weight structure that can be bonded with resin to allow for a more uniform stress 4 differentiation than riveting. However the most commonly used method of fabrication or wings and tail structures is integral construction. In integral construction each major assembly, such as a wing, is created from a solid cast or block (SAGES Module 2). This eliminates the structural weakness of a bonding Joint and allows stress to be distributed evenly throughout the part. This method is commonly used in wings and tail structures since they are the most stressed parts of an aircraft. Specially important in the fabrication of the wings. The wings of an aircraft must be built to reduce the amount of drag the aircraft experiences as well as increasing the mount of lift given to the structure. One method of decreasing the amount of drag is creating a greater length to width ratio of the wing. By using a longer wing with a narrower width, the amount of lift on the wing is greatly increased while simultaneously reducing the drag on the aircraft. Another method of improving the aerodynamics of an aircraft is the angle of the wings.
Instead of having wings expand straight out from the fuselage of the aircraft, they are fixed at an angle called sweep, shown in figure 5. In aircrafts operating below Mach 1 but greater than Mach . 5, pepperonis speeds will occur in some locations around the plane (SAGES Module 2). These supersonic air flows create shock waves that greatly increase the drag of the aircraft. The sweep of the wings increases the threshold at which shock waves occur on the wings, therefore reducing the frequency of showplaces.
The design of aircraft traveling at supersonic speeds is very different than aircraft built to fly below Mach 1 . In supersonic aircraft the wings require a much more pronounced sweep and the ratio of length to width of the wings is less important. The amount of lift generated at arioso angles of attack is much more important in supersonic aircraft. The angle of attack is the angle between the oncoming air and a reference point on the aircraft or 5 wing (“Boeing RARE Magazine” 13).
The added importance of angle of attack led to the creation of the delta winged aircraft, which only generate lift at high angles of attack. This aids the aircraft in takeoff and landing since it must do so at high velocity. At speeds above Mach 1. 5 a short, non-swept wing is most effective in reducing drag; however it is difficult to control in takeoff and landing (SAGES Module ). Modern military aircraft employ the swing-wing design, which allows the wings to be set at two positions as shown in figure 6.
The first position allows the wings to be expanded out to a low sweep for favorable takeoff and landing conditions, but can then fold the wings into a delta shape for supersonic flight (SAGES Module 2). The design of an aircraft is complemented by methods of construction to create an aerodynamic and robust structure. A well-constructed fuselage skin without the support of strategically places bulkheads and longer results in a product that appears stable, but will buckle under the compression applied to it during flight.
Similarly, a well-designed wing with the proper sweep and length will fall apart from the forces applied to it if not constructed to uniformly diffuse stress across the structure. The future of aerospace innovation, in both commercial and military use, will rely on the interconnectivity of construction and design in building the next generation of aircraft. 6 Figures Figure 1: GLARE Composite Material Sheets of Aluminum (darker) are sandwiched together by fiber glass adhesive (lighter) to create a stronger, low weight material. Glare is used on sections of the fuselage (shaded panels) in addition to aluminum alloy. Figure 3: Monochrome Fuselage Construction The skin is only supported by bulkheads and formers, most of the compression forces must be absorbed by the skin. Figure 4: Semi-monochrome Versus Monochrome Semi-monochrome gives the skin the added support of stringers to prevent buckling under compression forces. 8 Figure 5: Wing Sweep The wings are constructed at an angle in order to reduce the frequency of shock waves at high velocity. Figure 6: In position “a” the wings are positioned perpendicular to the aircraft for Geoff and landing.