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I was reading Aerodynamics for Engineering Students by Houghton and saw the following graph:

enter image description here

A lot of other sources say that increasing camber increases CLmax, like the graph below.

enter image description here

I am a little confused by what camber actually does to CLmax? An explanation which focuses on the physics would be very helpful!

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    $\begingroup$ I think the bottom one is correct from reading papers which measure, or at least calculate it. but I don't know why the textbook would be wrong. Papers such as this: arc.aiaa.org/doi/pdf/10.2514/1.C034415 If the top were true, then only ailerson would have any use and flaps would be useless. $\endgroup$
    – DKNguyen
    Aug 6 '20 at 0:52
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Camber directly affects the amount of circulation an airfoil can generate, adding camber will always generate a higher zero alpha lift coefficient (intersection of the lift coefficient curve with the x=0 axis) up until a point that there is too much camber to sustain attachment.

This leads directly to the question asked - does camber increase CL_max? For any given AOA, there is an ideal suction side geometry that generates a pressure distribution that is always on the edge of separating. Liebeck defined this ideal shape but only for laminar flow, for which the boundary layer equations are closed form, this was all based on the work of Stratford who came up with the ideal pressure recovery. Having a surface that is about to seperate everywhere can even be true for a symmetrical section at some inclination, although it's not going to look like a NACA 0012/15 or anything familiar. The effect of the pressure surface on the amount of circulation generated, is much more insignificant than the suction surface, but it is not zero. Therefore, a cambered airfoil will make slightly more load at max CL than a symmetric section.

In reality, there is no magic button to increase, decrease camber. Airfoils are complex geometries that cannot be driven by one parameter without affecting everything else. Adding camber by 'bending' the airfoil somewhere near the front of the chord will give some more circulation, but also probably increase the departure angle, and create a risk of separation towards the trailing edge where the pressure recovery is too steep for the boundary layer to cope with.

Unfortunately there is no easy +camber = +max lift coefficient relationship, but for a cambered and non-cambered airfoil designed to operate on that separation limit, the cambered airfoil will generate slightly higher max lift.

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    $\begingroup$ Could you elaborate on your 2nd paragraph. I am still unclear on how you came to the conclusion that "camber will make slightly more load" based on what you said about the suction/pressure surface. $\endgroup$
    – Nick Hill
    Aug 6 '20 at 22:33
  • $\begingroup$ Sure @NickHill, I have added a bit more context about the "ideal suction surface". Once that concept is accepted, the question really becomes "can I add more circulation to the pressure side of the airfoil by changing it from symmetric", to which the answer is yes. But the answer to "can I add MUCH more circulation to the pressure side of the airfoil by changing it from symmetric" is unfortunately no. $\endgroup$ Aug 7 '20 at 13:38
  • $\begingroup$ @NickHill does this answer your question? $\endgroup$ Aug 10 '20 at 4:34
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In general, boundary layer development is key to understanding the role of airfoil geometry on performance metrics such as CLmax. In this case, with increasing AoA, positive camber creates a reduced adverse pressure gradient near the leading edge thus delaying the onset of massive boundary layer separation at stall angles relative to a symmetric airfoil. So, with positive camber, CLmax could be higher since massive flow separation occurs at a higher AoA.

Of course, you could use camber to reduce CLmax relative to symmetric airfoils.

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    $\begingroup$ Your answer seems to suggest that CLmax occur at a higher angle of attack. If you see the second graph in the figure above, camber actually decreases the stall angle $\endgroup$
    – Nick Hill
    Aug 6 '20 at 13:26
  • $\begingroup$ @Nick Hill lowering the leading edge decreases stall angle AOA as measured by the chord because the bottom camber is removed (as explained in paragraph 2 of my answer). But there is no magic here, the Clift/Cdrag is lower for the cambered wing, requiring more thrust for higher Lift. This is why the Fiesler Storch needed a 240 HP engine to do its legendary feats. $\endgroup$ Aug 6 '20 at 15:09
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Camber is measured by taking the midpoint between the top and bottom of the wing and comparing it to the chord line, which runs from the leading edge to the trailing edge.

The top graph simply shows that bending the leading edge down, as slats do, will begin to generate lift even if the wing is at zero AOA, because the top of the wing now has a greater curvature than the bottom. Notice a symmetrical wing (no camber) at zero AOA will have its upper and lower "lift" cancel out, and must be at a positive AOA to generate lift.

The undercambered wing in the second graph will produce a greater Coefficient of Lift because of it's ability to "trap" more air underneath the wing and force it downwards.

But one should be aware of the drag penalties of this type of wing, requiring much more thrust generate the same amount of lift as the symmetrical airfoil moving faster. This is why cambering (with slats and flaps) is generally not utilized in cruise.

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  • $\begingroup$ How does this explain why CLmax increases with camber? $\endgroup$
    – Nick Hill
    Aug 6 '20 at 13:26
  • $\begingroup$ See paragraph 2 and 3. $\endgroup$ Aug 6 '20 at 14:16
  • $\begingroup$ "trap" is a very dangerous term to use. All but a tiny amount of the load is generated from the suction side of the wing, especially in high lift configurations. Looking at chordwise pressure distributions of inclined sections vs cambered sections, you can see that the pressure side pressure distribution changes very little. $\endgroup$ Aug 6 '20 at 17:06
  • $\begingroup$ @Stuart Buckingham the most efficient lift comes from the suction side, but a significant portion comes from the pressure side by simple action/reaction of air being deflected downwards. Notice top lift on super critical wings is verboten because of Mach effects; there is still enough underneath to lift an airliner (if it is going fast enough). $\endgroup$ Aug 6 '20 at 20:43
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    $\begingroup$ @Stuart Buckingham the real eye opener for me was when I saw a lift vs AOA graph all the way out to 90 degrees. At 45 the inclined plane produces similar lift to just before "stall" AOA, albeit with hugely more drag. Agree with circulation and top lift, but think downwash from bottom inclined plane helps accelerate the air flow over the top within the nonstalled regime. $\endgroup$ Aug 7 '20 at 20:35

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