When an airplane moves forward, the airflow continues to flow over the aircraft wings. When this airflow is fast enough, it flows over the wings and helps lift the aircraft off the ground.
In flight, this airflow continues to support the aircraft, provided it’s moving fast enough, but when the aircraft flies closer to, at or faster than the speed of sound, things start to get complicated.
The speed of sound is about 768 miles per hour (1,236 kilometers per hour) at sea level.
This is the reason why high speed aircrafts need to be designed differently, than those operating at lower speeds.
What is a Mach number?
In high-speed flight or high-altitude flight, the measurement of speed is expressed in terms of a “Mach number”, the ratio of the true airspeed of the aircraft to the speed of sound in the same atmospheric conditions. An aircraft traveling at the speed of sound is traveling at Mach 1.0.
MACH NUMBER = TAS ( TRUE AIRSPEED OF THE AIRCRAFT) LSS ( LOCAL SPEED OF SOUND)
For the study of high speed flight we are interested in the speed at which the infinitesimally small pressure disturbances (waves) travel through the atmosphere. The speed of propagation of small pressure waves depends upon the temperature of the air only.
The lower the temperature, the lower the speed of propagation.
How IAS affects Mach Number?
- It is known that temperature decreases with increasing altitude, so the speed of sound will decrease as altitude is increased.
- It is known that if altitude is increased at a constant IAS, the TAS increases.
- Hence, Mach no. increases if altitude is increased at a constant IAS.
- ISA (International Standard Atmosphere) states that temperature decreases from 15°c at sea level to -56.5°c at 36,090 ft., then remains constant.
- The speed of sound will therefore decrease with altitude up till tropopause and then remain constant.
What is Crossover altitude?
We know when the altitude decreases the temperature increases, LSS (Local Speed of Sound) will increase and TAS will increase.
- Climbing at a constant IAS, there is a risk of exceeding MMO.
- Descending at a constant Mach, there is a risk of exceeding VMO.
The point at which the aircraft changes to fly at a constant Mach number from a constant IAS climb to avoid exceeding MMO (maximum operating limit speed), is known as crossover altitude. It depends on the OAT. The lower the OAT, the lower the crossover altitude.
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