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History Adolf Busemann, a German Aerodynamism was one of the first people to study the characteristics of high-speed flight in 1933. Busemann used a wind tunnel which could reach Mach 1.5 (1.5X the speed of sound), the findings clearly suggest the benefits of using an aerofoil with a thin leading edge at supersonic speeds. Busemann, also recommended the use of thin aerofoils to postpone subsonic drag rise. During the second world war, German scientist where far ahead of the world in deriving a possible solution to what they called the “limiting Mach Number” (Whitford 1998). The first official aircraft to travel at Mach speed was the Bell X-1 Rocket-powered aircraft in 1946. It was on the 13 th run of the aircraft when Capt. "Chuck broke the sound barrier by reaching a speed of Mach 1.06 and became the first pilot to fly at supersonic speed on the 14 th of October 1947. Ballistic study was a major influence in the design of the X-1. The body of the aircraft was shaped like a 0.5 caliber bullet to help reduce drag. While the wing was a mid-mounted, roughly elliptical platform with diagonally clipped tips as shown in Figure 1 below (Whitford 1998). Figure 1 : (Houghton et al., 2013, p.28), http://rahauav.com/Library/Aerodynamic/Aerodynamics%20for%20engineering %20students_6th_www.rahauav.com.pdf Purpose of design One of the most concerning problem form supersonic flight in the early stages was that of control and stability at high speeds. The majority of the work by Busemann and other German aerodynamicist was regarding that issue. After extensive study in that field aerodynamicists realised the major problem of the sudden drag increase when an aircraft

reaches Mach speeds. At just beyond Mach 1, it appeared that the rate of change of drag with speed becomes infinite. This meant that there was theoretically and maximum limit to the speed at which planes could fly, this was referred to as the “limiting Mach number” ( Whitford) . An Investigation conducted by the National Advisory committee for aeronautics in 1947, revealed an unusual occurrence of a flow phenomenon that occurs at the leading edge (shown in figure 1 above) of supersonic air-foils at higher Mach speeds. Through the elimination of an extensive separated flow condition over the leading edge of the aerofoil, caused a sudden increase in normal-force coefficient and in some cases showed a decrease in the drag coefficient ( Lindsey 1947). Flow separation is when streamlines detach from the surface of the object and forms eddies and vortices within the flow. This occurs when “the boundary layer of the fluid travels far enough against an adverse pressure gradient that the adverse pressure gradient that the speed of the boundary layer relative to the object falls almost to zero.” The Reynolds number is used to measure different flow phenomena including flow separation. Flow separation usually occurs at low Reynolds numbers and usually starts at Re 2.0 (Islam 2013). Early research in Laminar Flow Control (LFC) was in the 1950s and 1960s where suction was utilised to eliminate or reduce cross-flow instability that was created from swept wings. This research leads to the discovery that transition could be delayed for substantial distances with a large decrease in viscous drag. Turbulent drag reduction (TDR) is one of the main researched areas in aeronautics, as it decreases the turbulent streamlines on the wings and thus reduces overall drag. During the 1930s to the 1960s research in surface roughness reduction was one of the main TDR was achieved. Reducing surface friction might seem very simple, flow separation can provide negative surface friction, however, it creates a very large pressure drag that exceeds the original friction drag (Bushnell 2003). Thickness/chord (t/c) ratio is one of the biggest influences on the wave drag of a straight wing, as shown in figure 2 below. The reduction of the wings thickness causes the critical Mach number to increase. However, at supersonic speeds, thin wings provide a large reduction in wing drag, which could be considered the largest contributor to total drag. At supersonic, the wave drag is proportional to almost the square of t/c. For example, if the thickness of a wing is decreased by 50 per cent, the wave drag could be reduced by 75 per cent (Whitford 1998).

Figure 2: (Houghton et al., 2013, p.25), http://rahauav.com/Library/Aerodynamic/Aerodynamics%20for%20engineering %20students_6th_www.rahauav.com.pdf The Bell X-1 aircraft (first supersonic aircraft) had a biconvex, sharp-edged aerofoil, with a 4.9 percent thickness at the leading edge and 7.5 per cent thickness at the trailing edge. This incredibly thin design of the wing was necessary to decrease drag as much as possible since the available engines at the time were very inefficient and were lacking in thrust production (Whitford 1998). Materials Used Material selection for an aircraft is one of the most important procedures when designing the plane. Materials for each section must possess certain characteristics to ensure the flight goes according to plan especially at high speeds such as supersonic flight. Considerations such as specific strength, strength to weight ratio, tensile properties, fatigue strength, heat resistance, fabricability, and many others. Long-term operation at high speeds usually means high friction for a long period of time which results in heat build-up. For example, long-term operation at speeds above Mach 3.5 can cause the aircraft to increase its temperature up to 300 °C. Most Supersonic aircrafts designed are military based aircraft. They mainly use Lightweight carbon-fiber-reinforced polymer (CFRP). This material possesses high strength, fatigue strength, corrosion resistance, high creep strength, and most importantly its very lightweight. When designing both military and commercial supersonic aircrafts considerations in, temperatures, loads, moisture, radiation, maintenance, and environmental conditions must be considered. As shown in table 1 below the surface temperature increases as the speed increases, as a result of the increased friction (Huda and Edi 2013). Table 1: Aircraft skin temperatures at various speeds. Aluminum alloys are also other materials used in the construction of supersonic aircraft due to its light weight and relatively high strength, as well as very useful fabrication features.

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Material development and selection when designing a supersonic aircraft is very important, not only does it help reduce weight while maintaining strength and help reduce weight which reduces fuel consumption (for example, Boeing 787 Dreamliner uses of CRFP to help increase fuel efficiency) it also ensures that materials can be manufactured to the specific engineering requirement. As stated above, surface roughness is a major contributor to LFC reduction, therefore a material must be chosen which can be easily smoothed out to help achieve the desired effect. References Bushnell, D. M. 2003. "Aircraft Drag Reduction--a Review." Proceedings of the Institution of Mechanical Engineers 217 (1): 1. https://search-proquest- com.dbgw.lis.curtin.edu.au/docview/213200250?accountid=10382. Houghton, E., Carpenter, P., Collicott, S. and Valentine, D. (2013). Aerodynamics for engineering students . 6th ed. Elsevier, p.25-28. Hutchinson, John R. 1996. “Vertebrate Flight – The physics of flight.” UCMP. Huda, Zainul, and Prasetyo Edi. 2013. "Materials Selection In Design Of Structures And Engines Of Supersonic Aircrafts: A Review". Materials & Design 46: 552-560. doi: 10.1016/j.matdes.2012.10.001. Islam, Toukir, S. M. R. Hassan, and Mohammad Ali. 2013. "FLOW SEPARATION PHENOMENA FOR STEADY FLOW OVER A CIRCULAR CYLINDER AT LOW REYNOLDS NUMBER." International Journal of Automotive and Mechanical Engineering 8 (Jul): 1406-1415. doi: http://dx.doi.org.dbgw.lis.curtin.edu.au/10.15282/ijame.8.2013.28.0116. https://search- proquest-com.dbgw.lis.curtin.edu.au/docview/1774318736?accountid=10382. Lindsey, W., Daley, B. and Humphreys, M. (1947). The Flow and Force Characteristics of Supersonic Airfoils at High Subsonic Speeds. Reference Reviews , pp.40-41. https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19930081941.pdf Mechanical & Mechatronic Engg. 2005. "Separation Of Flow". Www-Mdp.Eng.Cam.Ac.Uk . http://www- mdp.eng.cam.ac.uk/web/library/enginfo/aerothermal_dvd_only/aero/fprops/introvisc/ node9.html. Whitford, Ray. 1998. "Supersonic Man is Fifty." Aircraft Engineering and Aerospace Technology 70 (1): 15. https://search-proquest- com.dbgw.lis.curtin.edu.au/docview/213778007?accountid=10382.