Aerodynamics of Supersonic Aircraft
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Aerodynamics of Supersonic Aircraft
The world over the past three decades has experienced manned aircraft travelling at supersonic speeds. Supersonic aircraft exhibit a much higher propulsion system as opposed to the previous aircraft, therefore, they are more efficient (Winchester, 2008). In this respect, the designer cannot allow this efficiency to drop below the theoretical optimum in spite of the increased complexity and weight. In addition, these types of aircraft have a greater interaction between the airframe and the engine than their previous counterparts. Apparently, it is no longer possible to rationalize between optimizing a propulsion package to a separately optimized airframe (Torenbeek, 2013). The two parts work in tandem and thus they should be fully integrated into all aspects by the designer.
Supersonic aircraft refer to those planes that travel at a relatively faster speed compared to that of sound (Gunston, 2008). These types of planes were developed in the mid-twentieth century and had been extensively deployed purely for research and military works. Two types of airlines, namely the Concorde and the A-11/SR-71 aircraft mark the development of a novel class of planes designed purposely for supersonic operation. The most typical example of a supersonic aircraft is the jet fighter, however, it does not travel at a speed that exceeds that of sound. Other examples include the Conair B-58 and XB-70 (Mindling & Boston, 2008).

Image of the Convair B-58 obtained from the US Air Force Museum website
The aircraft weighs 160,000 pounds, however, a weight of over 100,000 pounds was for the fuel alone. In order to achieve the desired aerodynamics performance, the wings were designed to be thin whereby the tip was 4.08% whereas the root was 3.46%. In 1962, it was awarded the Bleriot prize after becoming the first plane to set a record for flying from Los Angeles to New York in less than three hours. As time went by, another type the SR-71 covered the same trip in less than an hour. Nevertheless, the XB-70 was remarkable since it covered the same distance at Mach 3 for 33minites (Gunston, 2008).
Development of shockwaves and the sonic boom
Supersonic flights are characterized by aerodynamics called compressible flow since the compression is bound to shockwaves coupled with the sonic boom that is developed when an object is travelling at a speed that is greater than that of sound (Gunston, 2008). The challenge that supersonic airplanes face, noise is also a problem around both the airport and when the plane is in motion (the sonic boom). A sonic boom is a sound energy generated when an aircraft causes repeating pressure waves as it travels at supersonic speed. Since these waves travel at the speed of sound, an increase in velocity of an object in air compresses these waves thereby leading to an enormous shock wave (Gnani et al.,2016).
In this regards, the term hypersonic aircraft will be used to refer to those aircraft that travels at a speed above Mach 5. While moving at supersonic speed, a problem that tends to face most aircraft is shockwaves. According to the Bernoulli’s principle, as the velocity increases, the pressure around the tail region decreases causing turbulence that results to shockwaves and finally developing to form a drag (Gunston, 2008). Drag can be defined as a force that tends to resist the relative motion between to surfaces which in contact. In this case, it is the aircraft and air.
As the Mach of the aircraft increases, the flow near the thickest region of the plane approaches Mach 1. A shock will often develop at the tail edge of the supersonic region once the aircraft attains a velocity that is above the critical Mach. It will start at the supersonic region and continue to the thickest point of the aircraft until the tail edge is reached (Gunston, 2008). This will remain to be the case until it nearly the forefront. Once the shock enters the vanguard, the transonic speed regime will advance now to the supersonic regime.
Effects of shock waves on transonic region and the engine
When an aircraft is travelling, the air currents can either speed up or slow down from one point to another. In this regards, the airplane will experience a transonic flow characterized by supersonic and subsonic flow being experienced simultaneously on the aircraft surface. The interaction of these two flows coupled with the drag being encountered in the boundary layer yields a state that cannot be expressed mathematically. This phenomenon is often referred to as transonic flight. The transonic speed range begins when sonic flow formerly occurs over the surface of the airplane and ends when the flow is supersonic over the entire surface (Gunston, 2008). As the velocity will be increasing, pressure will decrease as air flows sub-sonically over the surface of the airplane. As aforementioned, shock waves thus will develop as a result of the discontinuity that will be created between the supersonic and subsonic follow as the plane will be moving. The variation of speed leads to changes in pressure which then affects the stability and controllability of the aircraft. Therefore, aerodynamic shockwaves would increase exponentially in the transonic region. The process would require more engine power to overcome the turbulence flow.
Additionally, the transonic region would experience a lot of heating that will be generated due to the friction between the air currents and the aircraft. Engineers often use aluminum alloys owing to their affordability, workability as well as easiness to conduct away excess heat. However, the majority of supersonic aircraft, such as those deployed for military purposes, have been designed to travel at subsonic speeds, and can only exceed the speed of sound for specific periods such as when encountering an enemy aircraft or when dropping a bombing some targets. Notably, some special cases exist such as the Lockheed SR-71 Blackbird military unique aircraft coupled with the Concorde supersonic civilian transport (Torenbeek, 2013). These two types of aircraft have been specially designed to travel continuously at speeds that surpass that of sound. Nonetheless, these kinds of aircraft experience severe problems associated with supersonic flight.
Supersonic Wing Designs
The effects of drag can be reduced by limiting the wing span which on the other end would reduce the efficiency of aerodynamic when travelling at a relatively slower speed. Supersonic aircraft need to take off and land at slow speed, therefore, it is imperative to its aerodynamic design to reflect speed range (Lyu & Martins, 2013). In order to address this issue, supersonic aircraft’s wings have been designed using a variable-geometry. Such arms are known as "swing-wings" since they can adjust depending on the velocity. For instance, the wings will widely spread when the aircraft is flying at a relatively low-speed and then drift backwards when the airplane is on supersonic flight.
Regressively, the swinging is detrimental to the longitudinal balance of the aircraft and adds weight and thus increasing the cost of travel (Lyu & Martins, 2013). This is the reason why it is not commonly used. A second technique that has been used is the delta-wing design. This has been used in supersonic aircraft such as the Concorde. The merit of this method is that the plane can attack promptly at low speeds. This is because it sets a vortex on the upper side that increases lift thereby offering a lower speed for landing. Other kinds of wings include the oblique wing, Sweep back wing and the swept forward wing (Lyu & Martins, 2013).
Commercial Supersonic Aircraft
Over the past few years, various aircraft have been deployed for transporting passengers from one place to another at speeds greater than that of sound. The first supersonic aircraft to achieve this purpose was the Tupolev Tu-144 developed by the Soviet in 1968 (Gunston, 2008). By 1997, the aircraft ceased to work. A second example is the Concorde; the plane worked from 1969 to 2003. However, since this version was expired no other supersonic aircraft has been in operation. An essential feature of this type of aircraft is the ability to sustain their supersonic identity indefinitely. This is because, the amount of fuel being used to overcome the shock waves is high therefore posing a severe economic challenge to such operations (Gunston, 2008).
In this respect, the airplanes have been designed such that the airframes are highly streamlined while the wings have a relatively short span. For a plane to take off or land at a relatively low speed, a vortex lift is used such that as the aircraft slows down, lift is restored by raising the plane’s nose to increase the penetration angle of the wing. The sharp edges of the wing will cause the air to deflect as it passes over the wing (Gunston, 2008). In this regards, the aircraft will primarily speed up the airflow and take off.
In conclusion, it is imperative for learners to understand the application of the supersonic theory. When aircraft are travelling at supersonic speed, they experience a drag that is often overcome by the various wing designs such as the delta wing type. Understanding of this concept is beneficial for flight test pilot and flight test engineer so that they can have the theoretical knowledge about the dynamics that supersonic aircraft undergo when in motion.

References
Gnani, F., Zare-Behtash, H., & Kontis, K. (2016). Pseudo-shock waves and their interactions in high-speed intakes. Progress in Aerospace Sciences, 82, 36-56.
Gunston, Bill (2008). Faster than Sound: The Story of Supersonic Flight . Somerset, UK: Haynes Publishing.
Lyu, Z., & Martins, J. R. R. A. (2013, January). Aerodynamic shape optimization of a blended-wing-body aircraft. In 51st AIAA Aerospace Sciences Meeting including the New Horizons Forum and Aerospace Exposition (pp. 2013-0283).
Mindling, G., & Bolton, R. (2008). US Air Force Tactical Missiles. Lulu. com.
Torenbeek, E. (2013). Advanced aircraft design: Conceptual design, technology and optimization of subsonic civil airplanes. John Wiley & Sons.
Winchester, J. (2008). World's Worst Aircraft. The Rosen Publishing Group.