velocity of sound- Mach Number- critical mach no- drag divergence mach number

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What is the velocity of sound?
First we will know what sound is. Sound is the resulting effect of molecular collisions. Speed of sound is the speed of transmission of a disturbance (Molecular collision) through a medium air. Mathematically,

a= sqrt (ß* R* T)  m/s

ß- Ratio of specific heats Cp/Cv
R- Gas constant
T- Temperature

As the altitude increases, the temperature changes hence speed of sound too.

What is Mach number?
Mach number came into picture of aerodynamics by engineers in order to make their analysis ease. Mach number (M) is defined as the ratio of Speed of aircraft to the speed of sound. It has no unit.

M < 1    Subsonic
M=1      Sonic
M> 1     Supersonic (Upto 5)
M>5      Hypersonic

What is critical Mach number?
Though the free stream Mach number is subsonic, the aerofoil upper surface accelerates it to attain sonic at some point. When sonic speed reaches a shock wave occurs. Shock wave is a one having ability to increase pressure behind it. Because of the formation of shock at upper surface, upper side pressure increases than the down. So lift gets vanished. The free stream Mach number which is able to create a shock on upper side of wing is known as critical mach no.




What is drag divergence Mach number?

Beyond critical mach number, if we accelerate our aircraft, we will feel that a large amount of rise in aircraft drag. Reason is strong shock wave formation. 


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