Longing for high speed accompanies the whole history of the aviation technology. The emergence of jet engines and the subsequent fast increase of their power in the 40-s opened the prospect of a substantial increase in the speed and altitude of machines for various purposes, as well as creating a new class of aircraft known as supersonic aircraft. Nowadays, many tourists, especially wealthy ones, express a demand for the reduction of the flight time; in particular, taking into account how much time is necessary to spend in the airport before traveling. Thus, in order to satisfy this demand, still quite young but perspective market has appeared. A number of the aircraft industry companies are going to enter the market of high-speed jet planes. The secret of the achievement of supersonic speed has been known for a long time. What is needed is both the powerful engine and effective fuel, and the plane will be ready to break a sound barrier. However, it is necessary to solve a number of the accompanying problems related to design and economic issues. Therefore, it is necessary to study in detail the main nuances of the flight at a supersonic speed and address the history in order to define the problems arising when flying at a supersonic speed, and ways or their solution at present and in the future.
Supersonic speed is the speed of particles of substance, which is higher than a sound speed of this substance, or the speed of the body moving in the substance with a higher speed than the sound speed in terms of this environment. In aerodynamics, the speed is often characterized by Mach number (Smith, 2014). When moving in the environment at a supersonic speed, the body creates a sound wave following it. At the uniform rectilinear motion, the front of a sound wave has a cone-shaped form over a moving body. Radiation of a sound wave causes additional loss of energy of a moving body in addition to the energy losses due to friction and other forces. Under normal conditions in the atmosphere, the speed of a sound makes about 331 m/s. Higher speeds are sometimes expressed in Mach number and correspond to supersonic speeds. Spacecrafts and their carriers along with the majority of modern fighter aircraft reach supersonic speeds. Besides, some supersonic passenger planes such as Tu-144 and the Concorde have been developed (Smith, 2014).
Subsonic Flight vs. Supersonic Flight
The speed of vehicles flying below the sound speed refers to subsonic flight, while objects flying faster than the sound speed perform a supersonic flight. Commonly, supersonic flights pertain to military aircrafts used for reconnaissance or combat operations; the Mach number is more than 1. On the contrary, subsonic speeds, whose Mach number is less than 1, are peculiar to civil aircrafts used to travel and transport.
Aircrafts performing subsonic flights have propellers driven piston engines, high bypass turbofan engines, or turboprop engines in their construction. In turn, a supersonic flight can be achieved if aircrafts are equipped with low bypass turbofan engines. Finally, outside characteristics of the aircrafts are rather important. For example, a big sweep angle of wings helps to achieve a supersonic speed due to the improvement of aerodynamic features of the plane, while straight wings commonly used in civil aircrafts impede realization of supersonic flights (Hans-Reichel, 2012).
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Transonic region, which is sometimes called the region of “the mixed flow,” starts with the Mach number, at which at some point on the surface the flow velocity becomes sonic, and extends to the Mach number, at which the flow becomes supersonic everywhere. A distinctive feature of such flows is an existence of subsonic and supersonic areas of a flow. In other words, if the speed of the running flow is only a little less subsonic, there are flow regions with supersonic speeds around the flying object, and if the running flow is slightly supersonic, there are flow regions with subsonic speeds. Such mixed nature of flows poses essential difficulties for their theoretical research and systematization of data on the aerodynamic characteristics of bodies in this speed region (Mason, 2006).
Shock waves create rather big wave resistance. For this reason and also because transonic speeds often lead to dangerous fluctuations of some elements of the plane, pilots prefer flying either at subsonic, or supersonic speed. Experimental studies of the transonic region are complicated due to the fact that in this range of speeds only few changes of the Mach numbers have a considerable impact on the aerodynamic characteristics (Mason, 2006).
It is necessary examine a flight problem in transonic region, when the Mach number is close to 1 and the change of the pressure extends with a speed close to a flight speed. However, the motion from a standstill before reaching supersonic conditions passes through transonic speeds. When designing supersonic planes, taking off and landing requirements complicate the task since it is necessary to create a vehicle suitable for the three regions of flight, namely subsonic, transonic, and supersonic ones (Mason, 2006). In addition, along with modern requirements of stability and controllability, there are obvious difficulties, which should be overcome at the stage of final coordination of all conditions imposed on the supersonic aircraft.
Separation of a liquid or gas flow, which is one of many characteristic properties of a viscous flow, is a very complicated phenomenon. Separation of a flow leads to the energy losses. At a subsonic speed of an external flow, for example, flows around the aircraft and the line of a flow deviate, while resistance grows and carrying power falls, so that the contra flow and a stagnant zone are formed. In the region of transonic speeds, a problem of controllability and durability become complicated because of a flow separation. In case of an internal flow, the flow separation can cause deterioration of efficiency (Hans-Reichel, 2012).
The additional problems connected with the separation include the control of supersonic aircrafts and restrictions of some characteristics of these vehicles. For example, the transient is located somewhere between the leading and trailing edges of a plane wing, and the separation caused by the consolidation transient influences the distribution of pressure on a wing. In the transonic region, the separation often turns smooth and gradual increase of pressure on a wing into the extremely indignant distribution with considerable pulsations causing jolting of the device or strong changes in its stability and controllability (Mason, 2006).
The Supersonic Aircraft Designs
It is known that the main ways of the aircraft development were defined mainly by the progress of aircraft of military application the development of which requires much effort and special means to be used. Thus, civil aviation requiring reliability and convenience of operation is usually based on the principles of the military planes’ design. As a result of theoretical and experimental studies, it has been established that the most effective remedy of the decrease in the wave resistance and mitigation of the crisis phenomena at transonic speeds is the use of swept wing and tail. Implementation of the sweep idea in the plane glider allowed surpassing sound speed with no additional efforts. The use of a triangular wing, which combines the features of a big sweep, small lengthening, and relatively small thickness of a profile at the demanded rigidity, turned to be even more effective (Szondy, 2014).
Over the previous 10-20 years, the supersonic planes, fuselage of which is used for the creation of the carrying power, have appeared. Such fuselage is not shaped like a body of rotation (cone-cylinder-cone), but rather like a parallelepiped. Another equally important characteristic feature of supersonic planes is the application of fuselages to the nasal part considerably pushed forward. Since the use of the consolidation transient for the creation of additional carrying power appears to be most effective only at a constant high supersonic speed of flight, i.e. when the tilt angle of consolidation transient corresponds to the provision of a forward edge of a wing, it is especially expedient in passenger planes. Therefore, the Tu-144 and Concorde planes were equipped with the appropriate mutual position of engine nacelles and wing leading edge for the use of transients (Hans-Reichel, 2012).
A certain progress in aircraft construction is connected with the application of a modular design of a glider. Such approach allows performing the modernization of the let-out model by replacing all of the knots with more perfect ones for the production process. Among aviation materials, an important role is played by aluminum alloys. Much attention is paid to the development of new alloys and the composites reinforced by fibers. The scheme with the engines placed in a fuselage nearby each other is classical for two-engine planes. In the construction of modern supersonic aircrafts, there are used mainly central (frontal) air intakes, i.e. placed on the axis of symmetry of the aircraft, or side air intakes (on the sides of the fuselage) (Szondy, 2014).
The Power Plant Limitations
At supersonic speeds, traveling stability considerably worsens with the growth of the Mach numbers, especially at large angles of attack. Therefore, the Mach numbers have to be limited to the size, at which traveling stability is still sufficient. The tanks, bombs, blocks, rockets, etc. suspended under the plane, often worsen the traveling stability of the aircraft. Therefore, in the presence of external suspension brackets, the maximum permissible Mach number, as a rule, decreases. The most admissible Mach number can be limited according to the reliability of operation of the power plant (the stability of operation of the input device, compressor, combustion chamber and admissible temperature when entering the engine, operability of the cooling system of the engine, etc.) (Siouris, 2006).
During the flight of the aircraft at a sound speed, there is a shock wave that leads to a sharp decrease of the stream speed, and the pressure together with the density and temperature increase. Thus, there can be observed a release of a significant amount of energy into the environment surrounding the plane that leads to the intensive fluctuations of particles of air, manifested in the form of a thunderous sound. The sonic boom is short-term, but in certain cases it can be long. Its adverse effect is connected with a big intensity and suddenness of the sonic boom emergence. This phenomenon is amazingly similar to artillery volley, harmfully influencing the hearing organs and at the corresponding intensity being the cause of their damage (Conway, 2008).
The Past, Present, and the Future
Bell X-1 was an experimental plane, constructed in the USA. It was aimed at investigating the possibility of supersonic flights. This flying object was equipped with the rocket engine. On October 14, 1947, Bell X-1 broke a sound barrier for the first time (see Fig. 2).
Figure 1. Bell X-1, the first aircraft achieved the supersonic speed (Hallion, 2011).
On December 31, 1968, the legendary Soviet plane Tu-144, which became a first-ever passenger supersonic airliner, made the first flight. It have transported passengers for less than a year since two resonance accidents made constructors seriously doubt the reliability of the vehicle, and the profitability of operations was heavily negative. The destiny of the French plane “Concorde” was more successful. This supersonic airliner made the first flight on March 2, 1969, and operated on passenger airlines from 1976 to 2003. The reason of Concorde’s removal from operation was the same – loudness and unprofitability (Hallion, 2011).
However, the history of supersonic passenger aircraft has not ended after the removal of the Concorde. Today, it is expected that in 2017, the QSST plane will make the first flight. This airliner is designed for only twelve passengers and intended for charter business transportations. Recently, the idea of hypersonic passenger aircraft has been increasingly popular. This means the creation of planes able to rise to a suborbital orbit and fly there with a speed inconceivable in the atmosphere (5 Mach and above) (O’Ceallaigh, 2014).
When creating a small commercial supersonic plane of new generation, it is planned to use new technologies, in particular, passive management of laminar and turbulent transition in an interface by means of the micro-roughness distributed on the surface of an arrow-shaped wing near a leading edge. Besides, the amount of seats is enough to guarantee profitability, while consumption of fuel is minimized due to new stator-reactor, and the noise produced by these planes is identical to the noise produced by a big subsonic plane (O’Ceallaigh, 2014). Airlines count on the corporate clients making distant international flights and also on the high-ranking government officials. The studies conducted by the Aerion Corporation show sufficient market capacity: in 20 years, it is planned to sell more than 500 supersonic planes (Szondy, 2014).
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To sum up, it should be noted that overcoming a barrier of a sound speed is a considerable event in the history of mankind. Initially, such transition was very dangerous to aircraft, but now the problem is already eliminated. Nowadays, to characterize the airspeed, it becomes necessary and convenient to use such parameter as the Mach number. Such phenomena as a wave crisis, transonic region, and sonic booms deserve significant attention. Besides, controllability and characteristics of the aircraft stability worsen because of shift back of a point of the aerodynamic forces application. To mitigate these negative consequences of flights at supersonic speeds, planes are now equipped with arrow-shaped or triangular wings. The same characteristics were also embodied in supersonic civil aviation. However, the supersonic flight experience of the previous years was negative. All current developments take place only as projects fruitful to a varying extent. Nevertheless, the demand for superfast commercial planes of the future is currently increasing.