How does increasing the camber of an airfoil (like the NACA 0018) increase its coefficient of lift?
You're just curving the airfoil; I don't see how that increases lift for a given angle of attack?
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$\begingroup$ would this answer help? $\endgroup$– Peter KämpfMar 22 at 7:39
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3 Answers
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Think of an airfoil as a device for turning flow -- to create lift, the equal and opposite reaction F=ma is to accelerate air down.
Airfoil theory tells us that the combined effect of an airfoil can be considered as the superposition (addition) of certain effects.
- flat plate at angle of attack.
- thin camber line.
- symmetrical thickness form.
The thin camber line improves the airfoils ability to turn the flow -- and thereby create lift.
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you're just curving the airfoil, I don't see how that increases lift for a given angle of attack.
You know, you're right. Just curving the airfoil doesn't do a thing until the plane starts moving.
Up to a certain speed with a given chord length, defined as Reynolds number, the benefits of increasing thickness with large amounts of camber are non existent. From 1903 up until the beginning of the next decade, wings such as the NACA 0018 would have been laughed at.
At very slow airspeeds, early wings created most of their lift by raising pressure underneath the wings. Though some people cringe in horror over the use of the term "compression", this is what is going on$^1$. Higher pressure below. This is why early wings were thin with undercambered bottoms.
But, if you study the NACA 0018 at airfoil tools, as Reynolds number increases, the lift to drag ratios increase rather dramatically.
more lift is being generated by lowering pressure above the wing.
This comes at very little expense of increased drag. As planes got faster and faster, the issue become not making enough lift, but having too much! So airfoil designers began "filling in" the less L/D efficient lower "undercamber" and using the more L/D efficient top camber.
How the blunt nosed, fat wings performed so much better in 1918 was a marvel to the world, but is better understood by considering the shape of the resultant airflow at that speed, not the shape of the solid surface.
Variants of these wings served and continue to serve well up to speeds where higher Mach numbers force designers to trend back towards less thickness and camber.
$^1$ also known as higher density
answered Mar 22 at 9:17
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$\begingroup$ Hi, thanks for this, I think this explains it! So it’s all to do with pressure $\endgroup$– MichaelKMar 22 at 9:49
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$\begingroup$ Comments have been moved to chat; please do not continue the discussion here. Before posting a comment below this one, please review the purposes of comments. Comments that do not request clarification or suggest improvements usually belong as an answer, on Aviation Meta, or in Aviation Chat. Comments continuing discussion may be removed. $\endgroup$– Ralph J ♦Mar 26 at 17:54
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Quick & dirty: wing creates lift by bending downward the air flowing on it; by Newton's second law this downward air bending is compensated for an opposite upward force called lift.
You're just curving the airfoil; I don't see how that increases lift for a given angle of attack?
A higher camber doesn't curve only the airfoil but bends more air downward therefore creating more lift.
