Airplanes generate thrust to overcome drag. Drag is the air resistance that opposes flight; it acts parallel and in the same direction as the relative airflow and is a relentless force that deserves a thorough Flight Club treatment and your full attention. Total drag consists of drag forces that are linked to lift production, known as induced drag, and those that are not linked to lift production, known as parasite drag.

Let's have a look at induced drag. At high angles of attack, the high-pressure air below the wing likes to swirl around the wingtip towards the low-pressure air above the wing. A twisting vortex of air forms behind the wing, deflecting the airflow downwards. An inclined local airflow is created, which is the average of relative airflow and the deflected airflow, resulting in the lift vector tilting backwards and contributing to total drag.

Now for parasite drag, which consists of form drag, skin friction drag, and interference drag. Form drag is caused by disturbed airflow that separated from the surface and spawned into turbulent wake. The more streamlined an object is, the less form drag it creates, so any obstruction to smooth airflow, such as dangling wheels, will produce form drag. If we flatten a spherical object completely, the only drag we get now is the skin friction. As the name suggests, skin friction depends on the quality of the skin surface the airflow passes over. A laminar flow results when the airflow passes over smooth surfaces, so drag is small. But introduce wing ice or exposed rivets, and a turbulent boundary layer forms, resulting in more skin friction drag.

The airflow around the wing may flow faster than the airflow around the fuselage, so where these different airflows meet, interference drag is born because they clash within the space they share. These graphs represent induced and parasite drag against airspeed. Induced drag is most significant at low airspeeds and high angles of attack, where the pressure differential between the top and bottom of the wing is the greatest. On the other hand, an increase in airspeed increases parasite drag by a factor of the square of airspeed. So, if you double the airspeed, for example, you get four times the parasite drag.