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What is Drag? All Explained Here! Part I

What is Drag?

When an object moves through the air it has to displace a certain amount of air mass and this particular air exerts a force which tries to retard the movement of the object. There are different types of drag and they can be classified according to their origin.

Lift Induced Drag

Lift-induced drag is the drag generates due to the generation of wingtip vortices. As we all know a pressure difference exists on two sides of the wing that creates the lift. High pressure at the bottom side of the wing tries to move towards the low-pressure top side creating a curl. In large aircrafts, this wingtip vortex is strong enough to jeopardize the safety of a small aircraft. Due to this reason, there is a minimum distance that should be maintained in between the two aircraft. In all infinite wings, there is a spanwise velocity component that creates the vortex. Lift-induced drag is a function of lift and when the lift increases pressure difference also increase creating much more strong vortices.


a= geometric angle of attack

ai= induced angle of attack

ae= effective angle of attack

 As shown in the above diagram a downwash component is created due to the vortices and it reduces the angle of attack(a) of the wing by changing the relative airflow direction.  The angle of attack of this aerofoil section reduced to the effective angle of attack(ae). Lift is always perpendicular to the relative airflow and with the changing of airflow direction lift inclined backward creating a component to opposite direction of aircraft movement and this component is called the drag.

Wingtip vortices contain a huge amount of transnational and kinetic energy. Where these energies came from?  They are from the engines! So it is a prime importance to reduce the lift-induced drag, if not engine should operate at high power to overcome the drag by burning more fuel and being fuel inefficient. At low speeds and at high angles of attack lift-induced drag is great and it will be greater near the stalling angle of attack.

Introduction of wingtips reduce the formation of wingtip vortices and reduce the lift-induced drag to a greater extent.

 Parasite Drag

Parasite drag/profile drag can be divided into two main drag types called skin friction drag and foam drag. There are other two types of parasite drag called interference drag and pressure drag. This is a function of aircraft speed and when the aircraft speed increases drag to increases. When the aircraft speed doubled the parasite drag is increased by four times. Parasite drag is not much significant at low speeds because lift-induced drag predominates at low speeds. At high speeds, most of the drag is coming from the parasite drag.This drag is independent of the amount of lift generated and parasite drag can exist even the lift is zero. In an aircraft half of the parasite drag is created by the aircraft wings and by keeping the wings cleaner parasite drag can be kept the minimum. Increasing of air density, aircraft speed, and aircraft size increase the parasite drag while being more streamliner reduces parasite drag.

Drag= ½ ɋv2scD

 Rho: density of air

V: velocity

V: surface area of the aerofoil

CD: represents angle of attack and shape of the aerofoil


Skin friction, Foam, Pressure, Interference and Total drag will be discussed in the next article on the same topic.

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