Before discussing the drag divergence mach number, it is necessary to understand the meaning of the following terms,

## What is Mach number and drag force?

**1) Mach number:** The Mach number is the ratio of the speed of the aircraft to the speed of sound in that medium.

`M=\frac{V_{Aircraft}}{V_{Sound}}`

**2) Drag force:** The drag force is the force that opposes the motion of aircraft into the air. At the drag divergence Mach number, the value of drag force rapidly starts to increase. This drag force lowers the movement of aircraft.

Therefore aircraft required extra effort for moving at this condition.

## What is drag divergence mach number?

Drag divergence Mach number is defined as the Mach number at which the drag force on aerofoil structure starts to increase drastically.

The increase in drag coefficient is up to or more than 10 times of the drag coefficient at a lower speed of airfoil.

The value of drag divergence Mach number is always slightly greater than the critical Mach number.

The Drag divergence Mach number is generally denoted by the symbol `M_{DD}`.

## Drag divergence mach number graph:

The above figure shows the behavior of the drag coefficient for an increase in Mach number.

**When M ≤ Mcr:** When the value of mach number is up to the critical value of mach number, the change in drag coefficient for airfoil is negligible.

**When Mcr < M < MDD:** When the value of mach number increases from critical mach number up to the drag divergence mach number, the rate of change of coefficient of drag is lower.

**When M > Mcr:** When the value of Mach number is greater than `M_{DD}` then the drag force on aerofoil structure starts to increase drastically.

**When M = 1:** The value for drag coefficient is maximum at M=1.

**When M > 1:** After M = 1, the value of drag force decreases with an increase in Mach number.

## Reason behind drastic increase in drag coefficient at `M_{DD}`:

The drastic increase in drag coefficient has the following reasons:-

1) The generation of shock waves in the flow region.

2) Fow separation: The increase in pressure across the shock wave causes the generation of the pressure gradient. This pressure gradient leads to the separation of flow.