Basic Aerodynamic

Subsonic And Transonic

1 A). Explain subsonic and transonic.

The speed of sound is defined by distance travelled of a sound wave over a period of time through certain medium. The speed of sound is depending on temperature. International Standard Atmosphere (ISA) stated that standard temperature condition is 15 °C. The speed of sound under this temperature at sea level is 661 knots. Temperature also varies with altitude. The speed of sound is directly proportional to the temperature of the medium. If the altitude increase, the temperature will decrease and this will affects the speed of sound. When aircraft is flying at 40,000 feet, the speed of sound will decrease to 574 knots. This is because the temperature has drop to -55 °C. Mach number is a term to expressed the measurement of speed in high speed flight or high altitude flight. Aircraft that flying at speed of sound is travelling at Mach 1. Subsonic and transonic is a speed of sound that is categorized in high-speed flight.

Subsonic is a speed of aircraft that travelling below the speed of sound which is slower than Mach 1. Speed of subsonic is Mach number below 0.75 which is slower than Mach 1. When aircraft is travelling at subsonic speed, there are no shockwaves present. The figure below shows the design of the wing of the aircraft that usually travelling at subsonic speed.

Figure [ 1 ] Subsonic airfoil

From the figure shown above, the upper surface of the airfoil will compress the flow of air than the lower surface. This will cause the speed of the airflow at the upper surface increase more than the airflow at the lower surface. Based on Bernoulli’s principle, pressure at the upper surface is lower than the lower surface because airflow at the upper surface is travelling at high speed. Difference of pressure between upper and lower surface will increase the lift of the wing.

Transonic is a termed that usually to describe high subsonic speed. As the aircraft speed is...