# Why is a reduction in lift called 'drag'?

Induced drag is high pressure air 'leaking' to the upper part of the wing (low pressure zone) creating a vortex which means the area of the wing tip with the vortex can't create lift. Drag on the other hand is the opposition to the flow of air. Why is induced drag called a drag when it doesn't oppose oncoming air but instead reduces wing lift?

Same thing applies to interference drag, high pressure air from the top wing of a biplane leaking to the low pressure of the wing underneath it hence reducing lift, yet it's called drag when it actually reduces lift.

So my question again is why is interference drag called a drag when in reality, it reduces lift and does not oppose oncoming air?

• The terms lift and drag are arbitrary, to some extent. There are lots of forces acting on an aerodynamic shape, and we label those that oppose the motion as drag, while those that are pendendicular to it as lift. They are convenient inventions of terms. A wing doesn’t feel lift or drag. It just feels pressure and shear. – MikeY Jul 5 '19 at 20:31
• @MikeY: No, they are well defined. All what is acting rearwards and parallel to the airflow is drag and all perpendicular to it is lift. Per definition. – Peter Kämpf Jul 5 '19 at 21:49
• @Peter, your definition for lift and drag matches mine. – MikeY Jul 5 '19 at 22:36
• @Jessica Ham. It might help to consider lift as a subset of drag. In in order to have lift you must have motion, all motion creates drag. Interference drag, as in biplanes, is a way of saying the energy I put into the first wing to create lift is moving the air in such a way that it degrades lift in the second wing, for example if I pitch up and the airflow from the top wing affects the lift creation of the bottom wing. This can also happen if engine pylons are too close to the upper wing, but they do not have nearly as bad an effect if below the wing. – Robert DiGiovanni Jul 5 '19 at 23:38
• Induced drag to me is the drag penalty that you get from accelerating air downwards. The more you have to accelerate the air the higher the drag. This doesn't reduce the lift it just creates a resistance in moving forward, a.k.a. drag. – Jan Jul 6 '19 at 6:07

Induced drag is unrelated to leaking and vortex creation. It exists even in a wind tunnel where the wingtips touch the walls and thus prevent leaking and vortices.

Then what is induced drag? To create lift, the wing needs to be at some angle with respect to airflow (or be asymmetric), so the force vector points mostly upwards but also somewhat aft. Forces that point aft are generally called drag. Induced drag is thus a 'real' drag, not a reduction in lift.

Induced drag is indeed related to vortices. However, I would not characterize the effect as:

a vortex which means the area of the wing tip with the vortex can't create lift

Instead, imagine that each section of the wing generates a vortex, which induces downwash/upwash (downward or upward flow of air) on another section of the wing. Because of the finite span of the wing and, as you put it, vortex rollover at the tip, there is a net downwash generated at each section of the wing.

Another way to visualize downwash is to think of it as an additional flow incidence on the wing, or an induced angle of attack ($$\alpha_i$$). Since the induced flow acts downward, it reduces the effective AOA on each section:

Not only does the induced AOA decrease lift, it also points the lift vector rearward from the reference flow. The sum of that component from every wing section is the induced drag.

You also mentioned interference drag. This drag is separate from the classical induced drag and is related to vortices/eddies created by the joining of two surfaces.

• Very well written with a good reference. The reduction in lift at wingtips (effective AOA), requiring higher AOA of entire wing to maintain lift, is PART of induced drag, and leads to an explanation of ground effect (in the reference). However, pitching the entire airfoil to create lift, as Carlo explained, creates induced drag (and vorticies). – Robert DiGiovanni Jul 6 '19 at 13:04

Induced drag is the drag caused by producing lift. All lift production causes drag, and induced drag is the blanket term for this. It's not specific to the wingtip vortex. It doesn't necessarily "oppose oncoming air", but it is a force that acts rearwards on the aircraft, so it's drag.

Interference drag is a question of perspective. It's like induced drag, but it's not producing lift. You're seeing that from the point-of-view of the lift it's not producing: "if this were productive, induced drag, it would be generating this much lift, so it must be a reduction in lift". But it's not producing a downward force on the aircraft, it's producing a rearward force on the aircraft.

You could choose to see all parasitic drag in the same way: "if this were a lifting-body airframe, and all this parasitic drag were induced drag with the same L/D ratio, it would be generating this much lift," but it's not very helpful to think that way. It might lead you to think that you shouldn't get any parasitic drag (or any interference drag) when the wings aren't producing any lift (e.g. when stalled), and then you'd be wrong.

• Actually, interference drag is a mixture of induced (due to distortions in the spanwise lift distribution) and pressure drag (from early flow separation). This term is not really helpful in describing what is going on in detail. – Peter Kämpf Jul 5 '19 at 21:52

No induced drag is a horizontal vector component of the lift line, which itself is a resultant force vector from altering the momentum of air flowing around an airfoil or other lifting surface. You’re on an interesting track there as both lift and drag are resultant forces from altering the momentum of air moving around an object. Lift is just described as the sum of all forces in the direction opposing gravity While drag is the sum of all forces opposing the force of thrust on an aircraft.

• Induced drag is a 3D finite-span effect and has nothing to do with airfoil. – JZYL Jul 6 '19 at 6:53
• Negative. Induced drag is ALWAYS present when an airfoil is generating lift. While wingtip vortices are one component of induced drag, it is by no means the only source of said drag. – Carlo Felicione Jul 6 '19 at 13:01
• This probably deserves its own discussion. Induced drag is a term discovered from lifting line theory developed from 3D potential flow. In 2D potential flow, induce drag is completely absent. In fact, a rotating circle can generate lift in 2D potential flow with zero drag (see Anderson, Fundamentals of Aerodynamics), which is called D'Alembert's Paradox. For a 2D airfoil, there is some small drag in potential flow, but I would call it form drag since it has do with the shape and so forth. – JZYL Jul 7 '19 at 4:06

The reduction of lift is not called drag. Lift is the sum of the forces acting to oppose the (apparent) force of gravity. Drag is the sum of forces acting to reduce momentum. Induced drag is anything that has the consequence of causing more drag (such as vortices coming off the wing-tips).