A similar image can be found in the Wikipedia entry on "Induced drag"-- reproduced below:1
The accompanying text reads (bolding and italics added):
"Induced drag is related to the amount of induced downwash in the vicinity of the wing. The grey vertical line labeled "L" is perpendicular to the free stream and indicates the orientation of the lift on the wing. The red vector labeled "Leff" is perpendicular to the actual airflow in the vicinity of the wing; it represents the lift on the airfoil section in two–dimensional flow at the same angle of attack. The lift generated by the wing has been tilted rearwards through an angle equal to the angle of the downwash in three-dimensional flow. The component of "Leff" parallel to the free stream is the induced drag on the wing."
Sources cited for the illustration and accompanying text include:
Hurt, H. H. (1965) Aerodynamics for Naval Aviators, Figure 1.30, NAVWEPS 00-80T-80
Clancy, L.J. (1975) Aerodynamics. Pitman Publishing Limited, London. ISBN 0-273-01120-0
Kermode, A.C. (1972). Mechanics of Flight, Figure 3.29, Ninth edition. Longman Scientific & Technical, England. ISBN 0-582-42254-X
McLean, Doug (2005). Wingtip Devices: What They Do and How They Do It (PDF). 2005 Boeing Performance and Flight Operations Engineering Conference.
Note that only the vector that is perpendicular to the free-stream relative wind-- i.e. the grey arrow marked "L" -- truly meets the strict definition of a Lift vector. The free-stream relative wind is the direction of the relative wind far in front of the aircraft, beyond any influence of the aircraft on the direction of the local relative wind.
Note also that the concept of "effective relative airflow" is only a theoretical one-- at nearly every point near the airfoil, the direction of the actual relative airflow is different than shown by the dashed red line. The actual relative airflow curves upward as it approaches the airfoil, curves downward behind the airfoil, etc in a complex pattern.
The explanation is apparently suggesting that this net change in the overall direction of the "effective relative airflow" would not exist in 2-D flow, e.g. if the airfoil fully spanned the test section of the a wind tunnel, meeting the wall on each side. It's far from obvious why this should be so. Certainly in such a case, there would still be some drag associated with the production of lift, but perhaps by some definitions this drag would not include any component called "induced drag".
This ASE answer is not intended to represent that this explanation is, or is not, a fully valid explanation of the origin of induced drag!
At any rate, the question seems to be trying to understand which of the vectors in the diagram can be seen as the "decomposed" constituents of other vectors. In the diagram in the original question, the heavy black arrow-- which is the vector labelled "Leff" in this answer-- is the vector sum of the "L" (Lift) vector and the "Di" (Induced Drag) vector. Therefore Induced Drag can be viewed as resulting directly from the fact that the Leff vector is tilted aft, due to the effective relative airflow being inclined downward due to downwash. In the diagram attached to this answer, it appears that the intention is the same, though a close look shows that the vectors as actually drawn do not quite add up this way.
For more on this concept, it may be very enlightening to read the "Induced Drag" section of the "Aerodynamics for Naval Aviators" text by H.H. Hurt Jr, revised 1965. A pdf is downloadable from the FAA website here, and the relevant content starts on page 66. Note that this source specifically addresses the upwash in front of the airfoil, as well as the downwash behind the airfoil. Differences between 2-D and 3-D flow, and the significance of the "bound vortex system" that develops in 3-D flow, are addressed in detail (see especially pages 61-66.) Here's a "teaser" from page 66 that may inspire some to explore further: "Hence, the sections of the wing operate in an average relative wind which is inclined downward one-half the final downwash angle."
If you are interested in understanding the flow of air around airfoils, including the concept of "circulation", you may also find it worthwhile to read the "Airfoils and Airflow" section of John S Denker's excellent "See How It Flies" website.
BUT If this is resultant force why is always drawn perpendicular to
effective airflow (which is impossible in real fluid)
These diagrams are not attempting to make any representation that Leff is the net resultant force arising from the interaction between the wing and the airflow. In real fluid, it goes without saying that there is an additional Drag component, separate from the Induced Drag component, which could be drawn perpendicular to the Leff arrow, or perpendicular to the L arrow, according to whichever better suited our purposes. (Only the latter would truly meet the strict definition of a Drag component.) Therefore there is no valid objection that the situation depicted by these diagrams "is impossible in real fluid." The remaining Drag component has simply been omitted for clarity, just as the Weight vector has been. The intent of the diagrams is only to explain the origin of the Induced Drag component, not to show all the forces, or even all the aerodynamic forces, acting on the wing.
- Accessed 11/17/2022