Aerodynamics of Supersonic Aircraft
Supersonic flight is among the four regimes of flight, which include subsonic, transonic, supersonic, and hypersonic flights. Objects that move at supersonic speeds fly faster than the speed of sound, at around 1236 km/h, a speed that is called a Mach number. A Mach number is the ratio of the speed of a flying object to the speed of sound, and this implies that an object that is supersonic is said to travel faster than Mach 1. The theory of supersonic flight has its drawbacks when compared to the subsonic regime of flight, and this is evident in the fact that supersonic speed requires a much greater thrust as it advances past the transonic regime hence more engine power and finer streamlining (Courant and Friedrichs 7). An illustration of supersonic flight is as follows:
Fig 1: Supersonic flight
There are several factors and designs to consider that affect the optimization and speed of the supersonic regime, one of the aspects being the wings. The wing span should be limited so as to reduce the aerodynamic efficiency when moving at a slower speed. The above principle helps during landing and takeoff and the approach used is known as the variable-geometry swing, which extends wide when at a lower speed. Another factor that is dependent on the supersonic movement is the forces of flight; the weight of the object, the thrust, the drag, and lift (Gunston 14). If the above factors are manipulated well speed above Mach 5 is possible and effective.
A shock wave is a small disturbance that moves at a supersonic speed which is caused by a drastic increase in density, pressure and speed. Such waves arise from incidents such as explosions, very large electric discharges and supersonic movements and violent alterations in pressure. If an object moves at a speed which is less than Mach 1, a shock wave cannot be produced. Shock waves are different from normal sound waves from the wave front where compression happens and variable changes in temperature, density and stress. The above factors are what make shock waves travel faster than acoustic waves although shock waves decrease in speed faster when compared to ordinary sound waves because most of the energy is discharged as heat (Hanabeth 15).
When an object moves faster than the produced waves, no wave will move ahead of the source but will heap pressure behind and pile up to compression. The protruding waves are trapped in a cone that narrows while the speed source increases and the waves pile up, forming regions of high pressure beyond the compressed waves. The region within and outside the border of the cone is the shock waves. The magnitude of a shockwave decreases with further displacement with the absorption of energy as discussed earlier. When enough energy is absorbed, the shockwaves become normal sound waves lift (Gunston 14).
To understand further the shock wave phenomenon, an object in a fluid and its effects can be illustrated. When an object is moving in a fluid, circular waves that move in all directions away from the source are produced. The waves will continuously be produced as the object moves, and will move like ripples. The speed these ripples move is as a result of the produced waves. If the object moves quicker than the speed of the produced waves, the shape of the waves will become more V-shaped than circular if shown in a 2D diagram. The front of the object will be the pointy end of the wave (Hanabeth 15). Supersonic flight shock wave formation is illustrated below:
Fig 2: Shock wave in supersonic flight
Effects of Shockwaves in Transonic Flights
The major factor that may arise when shockwaves affect a flying body is the alterations in drag. The Mach number could rise from the drag divergence, moving it closer to 1 when the drag is high. One disadvantage that may affect drag is the use of thick airfoils. The airflow speed will be more as compared to thinner airfoils hence shockwaves may destabilize the moving body. Shockwaves mostly affect the wings of a body during flight. In contrast to the thick wings discussed above, thin wings do also have their disadvantages; they are a major setback when the vessel is taking off and may fail to regulate the thrust of the body at a transonic speed (Courant and Friedrichs 12). The boom waves may ripple off the wings, making them unable to carry the structure of the body, as compared to thicker wings.
A sonic boom is a sound arising from shockwaves created by a vessel moving through the air at a speed faster than sound. These sonic booms produce large amounts of sound energy, which may sound like explosions and an example of a sonic boom is the roar of a supersonic cannon ball shooting overhead. When a vessel travels in the air, it forms a series of pressure waves in front of the vessel as well as behind, and these waves travel at Mach 1. As the velocity increases, the waves streamline along the body to the back, and these join and compress resulting into a single shockwave and this happens if the flight of the vessel is smooth.
Pressure usually arises at the front of the vessel and steadily decreases to a negative value at the tail, which is then followed by an abrupt return to normal pressure after the vessel passes, a phenomenon called an N wave because of its shape (Raizer 13). The double “boom” arises due to the abrupt change in pressure; hence, the N wave creates two booms, when the initial pressure hits the front of the vessel and when the tail passes returning the pressure to normal. If the traveling object is of a particular shape, can make the pressure around it decrease drastically, which eventually leads to a decrease in temperature too, leading to condensation. The vessel usually creates a white cone of condensed air. The vessel traveling at Mach 1 may travel fast enough to displace the air around it, making the displaced air accelerate further above Mach 1, making the area of condensation correspond to the supersonic flow.
Supersonic Wing Designs and Airfoils
A supersonic airfoil is a cross section image formulated to create ease in the lift at supersonic speed. Such designs arise when the vessel is required to consistently operate at such velocity. These airfoils have thin layers formed on angled aircraft, with sharp fronts and edges. The edges are created in that manner to prevent a detached bow shock from forming in the airfoil as it travels through the air. Rounded edges are not advised as in subsonic airfoils because they would be too blunt to streamline effectively and increase the wave drag. At a supersonic velocity, drag is caused by friction between the air and the body of the vessel, lift (during take-off or navigation) or thickness of the vessel.
The above-mentioned setbacks have led to research on supersonic movement and modifications on airfoil design. The major challenge was coming up with a way to counteract or neutralize the problem of pressure exerted on the body during movement and the effects of the Mach cones. It has resulted in the modification of the tips of the wings to make the flow more efficient within these Mach cones. The above modifications are what form the various supersonic wing designs. The other parts of the wing aren’t altered much because they have little effect. The best design ever came up by engineers are the Delta wings. These are triangular shaped wings which were first used on aircraft during the Second World War and proved to be most effective.
Supersonic Transport (SST) Aircrafts
There are very few commercial aircraft that applied this design principle and were able to travel at supersonic speed. One of the supersonic transport vessels was the Tupolev Tu-144, which was a Soviet aircraft flown and retired between 1968 and 1997. The aircraft made 55 trips in its history and its average altitude was 1600 meters above sea level with a speed of 2000 kilometers per hour. This was the first ever aircraft to fly past Mach 2 although it crashed in 1977 at the Paris Air Show which slowed down further modifications. It then became commercial from 1977 to 1983 and was converted into a cargo plane after a second crash.
The second SST ever created was the prestigious Anglo-French Concorde, an aircraft that was manufactured jointly by the British Aircraft Corporation and Aerospatiale of France. Twenty aircrafts were built of this design in 1969 and were retired in 2003 (Orbelar 6). The aircrafts were modified, and the slender delta concept was introduced and the general idea of this theory was that the delta wings can create strong vortexes on the higher parts at high angles of attack. Air pressure is lowered by the vortex, and this in turn, increases the lift. It also helped the aircraft in traveling at stable low speeds and the length of the wing increased lift from the vortex it was operating over, which encouraged the wing to be lengthened along the fuselage. The only setback of the modification was that the SST would have to take off at a very steep gradient from the ground so as to create the required vortex lift. The landing gear would also have needed to be larger and longer so as to create the necessary angle for takeoff and landing.
The supersonic transport aircraft with the delta wings had several contributions and setbacks. The major advantages included the fast departures and landings, which took a span of around thirty minutes for the commercial and military vessels, improved safety and comfort during turbulence because of the fast speed hence; there were no delays during departure. The modifications that were installed in the war crafts enabled flexibility and ease in maneuvering and easy attack of enemy targets. It also enabled ease in the interception of enemy crafts.
As much as commercial supersonic travel had its merits; there are a few drawbacks that affected the industry, and these include high ticketing of the flights and noise pollution caused by the sonic boom (Gray and Beckerman 21).
Courant, Richard and K.O. Friedrichs. Supersonic Flow And Shockwaves. Springer, 2012.
Gray, Tyler and Joel Beckerman. The Sonic Boom: How Sound Transforms The Way We Think, Feel And Buy. Houghton Harcourt, 2014.
Gunston, Bill. Faster Than Sound: The Story Of Supersonic Flight. Haynes Publishing, 2008.
Hanabeth, Luke. Shockwaves. Hanabeth, 2012.
Orbelar, Christopher. The Concorde Story. Oxford: Osprey Publishing, 2004.
Raizer, Yu P. Physics of shock waves and high-temperature hydrodynamic phenomena. Courier Corporation, 2002.