Supersonic flight is one of the four types of the speeds involving compressible flows. Objects flying at supersonic speeds are typically traveling at speeds greater than the speed of sound. In such a manner, supersonic aircraft are aircraft that fly at speeds greater than the speed of sound. Most supersonic flights came were developed in the late twentieth century. Today, the use of supersonic aircraft is limited to the military and research purposes. The only known commercial airlines travelling at supersonic speeds are the Concorde and Tupolev planes. The aerodynamics of supersonic flights is different from that of subsonic flights. Consequently, supersonic flights require greater streamlined frames and a more powerful engine.
Theory of Supersonic Flights
Supersonic flights follow closely Bernoulli’s principle, which infers the pressure along streamlines as a function of the velocity. The law states that an increase in the speed produces a corresponding decrease in the pressure (Vos & Farokhi, 2015). The lift in supersonic flight is achieved majorly by the angle of attack. The angle of attack is significant in creating the pressure difference between the upper and lower sides of the airfoil. In most cases, the pressure difference occurs due to shock waves at the end of the airfoil’s leading edge (oblique shock). Consequently, the shock wave going down is greater than the one going up. The resulting pressure build-up under the airfoil creates an enormous lift, which puts the aircraft in the air. The diagram below shows the forces of drag and lift acting on an airfoil of the supersonic flight.
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Supersonic airfoils are designed with the view to creating a lift efficiently at supersonic speeds. Airfoils have sharp leading and trailing edges. Tapered ends are useful in preventing the creation of shock waves in front of the airfoil as it travels through the air. Sharp edges are in contrast to the airfoils in subsonic flights, which have rounded leading edges. Supersonic airfoil’s trailing edges have wing extensions for increasing the lift.
Shock Waves in Supersonic Flights
A shock wave is a form of the propagating disturbance. The wave appears when a disturbance moves at a greater speed than the speed of sound. Sandford (2012) argues that these waves are not purely sound waves; instead, they are a manifestation of sharp changes in the fluid properties. Additionally, these waves have properties of the abruptness and discontinuities in the pressure, density, and temperature. Nevertheless, the waves propagate large quantities of energy and have the capacity of passing through a medium, for example, the air or liquid. Shock waves are achieved in the supersonic flows by the use of expansion fans. Numerous measurements of disturbances in supersonic flights show values of 200nm; this figure characterizes shock waves as either a line in a two-dimensional flow field or a plane in a three-dimensional flow field (Houghton, 2013).
Shock waves appear when a medium moves at a greater speed than the sound. Usually, shock waves form when the Mach number is 1. The Mach number is the ratio of the speeds of an aircraft moving in the air to the sound velocity. High-pressure shock waves occur at a point where the sound waves cannot travel at the further speed. Shock waves adopt a characteristic a sound of a snap or noise in the air. The noise sound originates because, unlike in the subsonic flights where the air begins to move away from the incoming jet, the aircraft moving at the speeds of sound is able to catch up with its pressure waves thereby creating a characteristic noise sound. In such a manner, the snap sound is the only proof that the aircraft is moving at supersonic speeds. The diagrams below illustrate the shock waves that are created when an aircraft moves with the speed of sound and speeds greater the sound.
After losing a substantial energy, shock waves can degenerate into a standard thud sound. During the transition from the snap to thud sound, the shocks waves transform from nonlinear waves to linear waves. Simply put, thud sound waves are direct propagating waves. The graphical representation below shows the pressure with the time variation for an aircraft that is propagating at the supersonic speed.
Shock Waves in Transonic Regions
Transonic is a condition when velocities around a flying object vary as the object propels through the air. Speeds are either above, at, or below the speed of sound. Therefore, transonic speeds range from 0.8 to 1 Mach numbers (Stamps, 2013). While moving in transonic regions, an aircraft experiences varying degrees of drag. Shock waves in transonic regions cause instabilities to a plane. The effect occurs because shock waves move at the speed of sound and make the air and particles vibrate on the aircraft’s airfoils. When an aircraft is moving at transonic speeds, the shock waves from it accumulate in front of it thereby forming an enormous shock wave. Nevertheless, a jet must pass through the huge shock wave that it creates. Large shock waves form areas of the high instability, which may tear the aircraft apart.
The instability originates from the air that has to pass over an airfoil at speeds lower than the speed of sound at some points while moving at a speed greater than the speed of sound on other points in the same airframe. The effect may not be different from helicopters; transonic speeds may cause enormous stresses to the rotor blades and lead to instability. As a result, accidents may occur in such cases. An aircraft travelling at supersonic speeds in the transonic region creates an area of the low pressure and temperature on a jet frame. Consequently, a thick cloud is likely to form as the airframe temperature drops below the dew point. The clouds are likely to remain with the jet as it moves through transonic regions. In addition, as the jet accelerates through these areas and at supersonic speeds, expansion fans are likely to grow and cause the formation of bow waves. At the point of formation of bow waves, shock waves will intensify to the infinity. The picture below shows the formation of clouds as an aircraft that is travelling at the supersonic speed crosses transonic regions.
Sonic Boom in Supersonic Flights
A sonic boom is a loud sound that is associated with shock waves and is formed by an object moving at a speed greater than that of the sound. According to Benson (2013), sonic booms emerge because of the compression of pressure waves that are created when the aircraft moves at the supersonic speed thereby producing explosion-like sounds due to the dissipation of massive amounts of energy. As the pressure waves merge, the result is a massive shockwave that is travelling at the speed of sound. The sound of the sonic boom depends on the distance between an observer and a military jet that produces the boom. In that case, the sonic boom is perceived as a double sound when the aircraft is far away from the observer. Sonic waves created by aircraft moving at supersonic speed create a nuisance; they are likely to cause windows to rattle after an intense vibration.
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Booms are usually brief although the intensity of the boom depends on the physical properties and the use of an aircraft. For instance, military aircraft are likely to produce more intensive sonic booms as compared to a civil aircraft. The area exposed to the boom depends on the altitude of a plane. In such a manner, jets at higher altitude are likely to produce more widespread booms. The carpet boom is a particular kind of a boom that is formed when an aircraft is travelling at supersonic speeds while maintaining a definite altitude. Most people will hear the sonic boom after a minute of the plane passing; nevertheless, not all sonic booms may be heard from the ground. A sonic boom is profoundly affected by the atmospheric composition at the given time. Therefore, the temperature, moisture content, atmospheric particles, and winds have an effect on sonic booms (Beckerman, 2014). In addition, the ground cover is likely to change the perception of the sound explosion. Hard surfaces, for example, concrete are likely to cause the amplification of booms.
Supersonic Wing Designs
Supersonic wing designs refer to the standard patterns of the aircraft’s airfoils. The wings are intended to allow an aircraft achieve a sufficient lift within the shortest possible period. The designs are essential for the plane to meet set objectives. Supersonic wings have sharp leading and trailing edges. The edges are formed by the intersecting planes at an angle. The design of wings aims at preventing the formation of bow shockwaves in front of the airfoil as the aircraft is moving through the air. The design of a supersonic wing is specific as it strives to increase the angle of attack and reduce the wing drag thereby increasing the lift. Wings also use leading and trailing flaps in order to help an aircraft achieve enough lift at low speeds. Below, there is an illustration of the supersonic wing design.
Concorde was a supersonic commercial jet powered by turbo engines. The commercial aircraft was launched in 1969; it had operated for 27 years. The commercial aircraft had a carrying capacity of up to 128 people and flew at speeds twice the speed of sound (Leney & Macdonald, 2010). In total, twenty planes operated under the name Concorde and flew transatlantic routes. Notably, a plane travelled at twice the speed of common jetliners thereby consuming half time required by other planes to move from London or Paris to New York.
General features of the aircraft included delta wings with four Olympus engines. The plane had an inbuilt fly-by-wire control system and was the first commercial jet to use hybrid circuits. In order to achieve the highest speeds, the jet utilized double-edged delta wings. Other specifications included the variable air intake engine, super cruise capability, and droop nose for safe landing. In order to ensure the aircraft made the Atlantic crossing safe, it utilized a combination of turbo engines that were effective at supersonic speeds. In addition, the aircraft had a slender fuselage and a complex wing shape with the view to maximizing the fuel capacity and avoiding any unnecessary drag.
Concorde flew at over 18km above the surface, with cruise speeds of up to 2000km/hr sound (Leney & Macdonald, 2010). Although the plane had an excellent performance, there were several accidents involving it. For instance, the crash of the Air France flight killed all the passengers on board. The cause of the accident was a metal strip that ruptured the plane’s tires. A piece of the tire then burst and opened the fuel tank. As a result, the plane could not achieve a sufficient lift. Concorde’s legacy ended in 2003 following the decision by the Airbus to halt the maintenance of the aircraft. Moreover, the September 11 attacks on the World Trade Center in the United States caused the growing concern about the possible use of the plane by terrorists. In addition, the economic recession at the time contributed to the woes of the airline as the proceeds could not support the maintenance and operations costs.
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Effects of Supersonic Flights on Engines
Supersonic speeds have significant effects on the engines of aircraft. In such a manner, high rates require highly efficient engines. Drivers need to have the capacity for a large intake of compressible air in order to facilitate the production of thrust. To achieve this aim, engines create thrust by increasing the temperature of the air before it reaches the engine. Therefore, the supersonic speeds require that the engine can take in an amount of air that is essential for burning the jet fuel and creating the necessary thrust. Due to the intake design, the air needs to be slowed down before it is sucked into the engine. Sucking the air directly at the speeds can damage the motor and result in slow speeds. Therefore, supersonic speeds necessitate the use of cones or ramps for slowing down the incoming air before shock waves can reach the engine. Slowing the air is essential because it removes the energy in the engine, which can cause drag.
From the discussion above, supersonic flights are flights at speeds greater than the speed of sound. The aircraft travelling at such speeds requires special modifications. As a rule, a supersonic aircraft has long, slender, and delta-shaped wings; it is an ideal form for the fuselage to adapt to supersonic travels. Due to the displacement of air particles by a fast moving aircraft, shock waves and sonic booms are standard effects of supersonic flights. The use of supersonic flights is presently limited to the military and research fields, as the only know commercial supersonic aircraft ceased operations back in 2003.