Im trying to understand why does L/D MAX, (the top of the polar curve that computes CL & CD ratio for any airfoil) is also the lowest point of the total drag curve. Im guessing the reason is that CD curve is not just about induced drag as I thought.
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2 Answers
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Airfoil drag is "parasitic" (or better: everything but induced) drag.
It consists of shear drag and pressure drag, the latter mostly from local flow separation. Both are only present when viscous flow is assumed. Airfoil drag is for the wing section without taking tip effects into account, presuming an infinitely wide wing. This kind of theoretical wing has no induced drag (d'Alembert's paradox).
L/D max is the point on the polar curve where the angle to the origin of the coordinate system is steepest. This is not the point of the lowest drag coefficient! Maybe you mean the point of lowest drag for a full airplane – but that is another story, with more than just the airfoil polar. Now induced drag is part of overall drag and due to its inverse relationship with speed, a distinct minimum can be found when lift is held constant.
Drag polar of the NACA 23012 airfoil (picture source, colored comments own work)
answered Oct 30, 2020 at 13:09
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$\begingroup$ I don´t know how to add the graph´s picture I was making reference to (RAF 32 airfoil), sorry about that... but in that one there are two Y axis, (one for CL,the other one for CD and the X axis is for AOA (I thought Cd was all about induced drag because of that). And L/D max is the AOA which correspond to the top of the polar curve (Cl/Cd). What Im trying to understand is how can that airfoil´s L/D max can also be found at the "point of lowest drag for a full airplane" where total drag is at its minimum? $\endgroup$ Oct 31, 2020 at 2:18
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1$\begingroup$ @Cristóbal: When in edit mode, there should be a symbol of a landscape in the toolbar. This lets you add either a link to a picture or upload a picture to add to your text. Did you try the help center yet (symbol with question mark in the top right)? $\endgroup$ Oct 31, 2020 at 10:53
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$\begingroup$ @ Peter Kampf: I don´t see any landscape symbol when in edit mode. I also cheked help center and I found same instructions about the symbol or link. Unfortunately there is no link to directly see the picture, so Im gonna paste the link of the site where the graph is ,even though I think the graph is not really important to answer my question. Thanks for your help. rcplanes.online/design.htm (Characteristic curves of RAF 32) $\endgroup$ Nov 1, 2020 at 5:33
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I'm trying to understand why does L/D MAX, (the top of the polar curve that computes CL & CD ratio for any airfoil) is also the lowest point of the total drag curve.
The graph in another answer shows how to find the max ratio of Cl / Cd, which is arithmetically equal to the max ratio of L/D.
The concept of minimum Drag (as opposed to minimum Drag coefficient) can be confusing. If we are talking about a Cl vs Cd graph obtained in a wind tunnel with constant airspeed, whether for an airfoil or for a whole airplane, we could say that Drag is minimized when Cd is minimized. This is obviously not the point where the Cl / Cd ratio is maximized.
But the situation is completely different in actual flight, where Lift is constrained to be equal to Weight (or in the case of a steady-state glide, constrained to be equal to Weight * cos ((arctan (D/L))), which is nearly equal to Weight), and the airspeed varies accordingly. In this case, we can show that Drag is minimized when the L/D (and Cl / Cd ) ratio is maximized, even though this is not the point where the Drag coefficient is smallest.
In the context of actual flight, we need to talk about the L/D or Cl / Cd ratio of the whole aircraft. Therefore there's really no context in which it makes sense to say that Drag is minimized when the Cl / Cd ratio of the airfoil is maximized.
Related ASE links--
answered Oct 30, 2020 at 16:36
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$\begingroup$ Airfol´s graph LDmax shows an AOA, and total drag curve´s LDmax shows a TAS. And I was thinking that flying leveled at that shown TAS would adjust that AOA for LDmax. But ...isn´t there a relationship between these two? Is L/D max just a concept that can be used for describing the best Lift-drag ratio of different things without any relationship? $\endgroup$ Oct 31, 2020 at 2:35