Aircraft Motion
Physics of Aircraft
Weight and Thrust
Secondary Controls
Straight and Level
Aircraft Design Features
The Stall
Practice Exam

Induced Drag

Drag is the air resistance that all aircraft experience in flight and it acts parallel to the relative airflow. Drag is broken down into two categories – induced drag and parasite drag. We’ll start by looking at induced drag.

Induced Drag

We’ve learnt the aerofoil creates a total reaction by altering the speed and direction of the air flowing over it, and the total reaction has two components – lift and drag. The drag component is known as induced drag.

Since the lift and induced drag are components of the total reaction, it makes sense that whenever we increase the total reaction by increasing the angle of attack, we also increase induced drag.

Wingtip Vorticies

As angle of attack increases, the pressure difference between the top and bottom of the wing also increases. At the end of the wing, there is no barrier to prevent the higher pressure below the wing from spilling around the wingtip and over the top of the wing.

This creates a spiraling airflow trailing behind the wingtips – known as a wingtip vortex.

The energy that is used in creating these wingtip vortices comes from the forward movement of the aircraft and is experienced as drag. So as angle of attack increases, the wingtip vortices cause induced drag to increase significantly.

Since wingtip vortices have the biggest influence on induced drag, the best way for an aircraft designer to minimise induced drag is to improve the design of the wingtips

Induced Drag at Various Airspeeds

As speed increases, the angle of attack is reduced to maintain level flight and the pressure difference between the top and bottom of the wing reduces.

This reduces the strength of the wingtip vortices and so induced drag decreases as speed increases. Conversely, a decrease in speed results in an increase in induced drag.

The relationship between airspeed and induced drag is shown on the graph above. We can see that induced drag is inversely proportional to airspeed.