In aerodynamics, lift-induced drag, induced drag, vortex drag, or sometimes drag due to lift, is a drag force that occurs whenever a moving object redirects the airflow coming at it. This drag force occurs in airplanes due to wings or a lifting body redirecting air to cause lift and also in cars with airfoil wings that redirect air to cause a downforce. With other parameters remaining the same, as the angle of attack increases, induced drag increases.
It is not possible to have a wing of infinite span. However, the characteristics of such a wing can be measured on a section of wing spanning the width of a wind tunnel, since the walls block spanwise flow and create what is effectively a two-dimensional flow. The aerodynamic force is resolved into two components. By definition, the component parallel to the vector representing the relative velocity between the wing and the air is the drag; and the component normal to that vector is the lift. At practical angles of attack the lift greatly exceeds the drag.
A wing produces lift by turning the airflow around the wing in a downwards direction. On a wing of finite span some air leaks around the wingtip from the lower surface to the upper surface producing a wingtip vortex. These vortices trail behind the wing for a great distance and can be powerful, producing hazardous conditions for following aircraft flying into them. The deflection of the airflow by the wing produces a down flow or 'downwash' behind the wing. The tip vortices also modify the airflow around the wing, relative to that on a wing of infinite span, reducing the effectiveness of the wing to generate lift, thus requiring a higher angle of attack to compensate, and tilting the total aerodynamic force rearwards. The angular deflection is small and has little effect on the lift. However, there is an increase in the drag equal to the product of the lift force and the angle through which it is deflected. Since the deflection is itself a function of the lift the additional drag is proportional to the square of the lift.
Unlike parasitic drag on an object (which is directly proportional to the square of the airspeed), for a given lift, induced drag on an airfoil is inversely proportional to the square of the airspeed. In straight and level flight of an aircraft, lift varies only slowly because it is approximately equal to the weight of the aircraft. Consequently in straight and level flight, the induced drag is inversely proportional to the square of the airspeed. At the speed for minimum drag, induced drag is equal to parasitic drag.
Induced drag can be minimized by the following means:
For a wing with an elliptical lift distribution, induced drag is calculated as follows:
The speed for best endurance, i.e. time in the air, is the speed for minimum fuel flow rate. The fuel flow rate is calculated as the product of the drag or power required and the engine specific fuel consumption. The engine specific fuel consumption will be expressed in units of fuel flow rate per unit of thrust or per unit of power depending on whether the engine output is measured in thrust, as for a jet engine, or power, as for a turbo-prop engine.
The speed for best range, i.e. distance travelled, occurs at the speed at which a tangent from the origin touches the fuel flow rate curve. The curve of range versus airspeed is normally very flat and it is customary to operate at the speed for 99% best range since this gives about 5% greater speed for only 1% less range.