# Wave drag

## Explanation

Wave drag is a type of drag experienced by an aircraft during its transonic and supersonic flights. The drag is caused by the formation of shock waves as the aircraft flies at a speed closer to the speed of sound, usually referred to as the transonic region. As an aircraft approaches the transonic region, the local airflow around the airplane’s wing or other structures could easily accelerate to supersonic speeds. So when the shockwaves form at these regions, there will be a sudden increase in drag. As the airspeed increases beyond the critical Mach number of the aircraft, the shockwaves moves further back, covering larger wing area. At that point, the wave drag will lead to the boundary layer separation and would cause stall or other undesirable conditions such as a Mach tuck.

### How can wave drag be minimized?

One of the very first solution to minimize wave drag is the conception of sweeping the wings backwards so that the shockwaves are delayed. Then came the application of Whitcomb area rule that reduced the wave drag by indenting the fuselage at the point where the wing is attached. Other methods like the super-critical airfoil and anti-shock bodies also helps reducing the wave drag.

### How swept back wings help reduce the wave drag?

As seen in the explanation part above, the wave drag is caused as the local flow over the wing reaches supersonic. In a normal straight wing, the airflow is parallel to the chord like of the wing. And it’s same throughout from the root to tip of the wing. By sweeping the wings at a certain angle, the relative airflow breaks into two components (towards the span and along the chord), as the airflow is not parallel to the chord line like in the straight wing. Hence, the formation of shockwave is delayed and the wave drag is reduced.