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Aircraft
Published in Suzanne K. Kearns, Fundamentals of International Aviation, 2021
The speed of sound is not constant, as it changes with air temperature, but on a standard day at sea level, it is 662 knots (1,225 kilometers per hour; 761 miles per hour). Therefore, as the speed of sound is not a single velocity, it is expressed as a ratio between an aircraft’s speed and the speed of sound. This ratio is called the Mach number, after Austrian philosopher Ernst Mach, and it is used to categorize aircraft types based on their maximum speed: Subsonic—Mach = < 1. This is the range that is generally used by international air transport aircraft. Aircraft generally operate at less than 0.8 Mach, as this speed range results in all airflow over the aircraft remaining lower than Mach 1.Transonic—Mach = 1. These aircraft are approaching Mach, and some points on the airframe will be less than the speed of sound, while other points will exceed it. In general, aircraft do not cruise near this speed due to high drag associated with the compressibility effect. A sonic boom is created when an aircraft attains Mach 1 (the aircraft is moving faster than the sound waves it creates, so the sound waves form a cone of sound behind the aircraft, which creates an extremely loud sound, like an explosion or gunshot). Aircraft operating through this range almost always have swept wings, meaning when viewed from above, the wings angle back in an inverted V shape.Supersonic—1Mach = > 1 through < 5. Aircraft aerodynamics beyond Mach are quite different from those in subsonic flight. As drag hits a maximum just before the speed of sound, once an aircraft accelerates past Mach 1, drag is reduced and fuel economy improves. However, because the lift-to-drag ratio is reduced, these aircraft almost always have a similar shape: a long and thin fuselage with large delta-shaped wings (that form a solid-triangle shape). High-supersonic speeds are often classified as Mach 3 - 5.Hypersonic—Mach > 5. In hypersonic flight, aerodynamic heating occurs, and aircraft must be designed to withstand extremely high temperatures. High-hypersonic speeds are often classified as 10 < Mach > 25, with space vehicle reentry speeds beyond Mach 25.
A novel relationship-oriented clustering approach for extracting relational patterns from the traffic tangled data
Published in Transportation Letters, 2023
Somayeh Akhavan Darabi, Vahid Baradaran
The regression method has been used to model the relations between the velocity and density variables. The velocity variable is interpreted as the motion rate of a vehicle. This variable is usually represented as distance per unit of time such as kilometers per hour (). Multiple velocity parameters exist in the literature that can be utilized for a traffic stream such as Time-mean-speed (), Space-mean-speed (), Free-flow-speed (), etc. In this research, the space-mean-speed () has been used for the collected data. The is defined as a statistical term which represents an average velocity according to the average travel time of the observations that pass a given segment of a route in a specified duration. The average travel time is divided by the measured distance in order to compute the space-mean-speed value (Soriguera and Robusté 2011). The density variable represents the intensity of the vehicles that occupy a given length of a route in a specified instant (Yu et al. 2016). This variable, as a key macroscopic factor of traffic flow, is usually used in surveillance systems by traffic experts. The density variable is a metric which also indicates: (1) the vicinity of vehicles to one another, (2) the easiness to maneuver, (3) the comfort of drivers in terms of psychological elements (Transportation Research Board (TRB). 2015; Al-Sobky and Mousa 2016). In this study, the average of the density over time is computed and it is represented as vehicles per kilometer (). To compute the density variable, several techniques such as videotaping, photographing, or observing predefined extents of a route are needed (TRB, 2015). The velocity and density variables are linearly related with a negative slope. Therefore, it can be concluded that the velocity of the passing vehicles on a route will decrease as their plurality (density) increases. The relation between these two variables can be modeled by , where the variable represents the number of vehicles which pass a predefined point per unit of time (Coifman 2015). The relation between the velocity and density variables with a negative slope can be observed for the collected data of this research in Figure 1. As illustrated in Figure 1, the velocity of the observations passing the Tehran-Saveh highway decreases as their plurality increases.