If it is not asking too much, would you mind going over your explanation again, this time referring to "undercamber?"

From : Don Stackhouse

"Camber" and "undercamber" are closely related, although not quite exactly the same. However, the existence of undercamber generally implies that there is a positive amount of camber.

A symmetrical airfoil (i.e.: the bottom surface is an exact mirror image of the top surface) has zero camber. The term "camber" refers to how non-symmetrical an airfoil is. In the typical fashion used in the NACA airfoils and subsequently by almost everyone else, "camber" refers to the shape of the "mean line".

Imagine a line running through the exact middle of the thickness of the airfoil. Technically the correct way to plot this line is to draw a row of circles inside the airfoil, fitted so that they each just touch the top and bottom surfaces at their individual locations. The curve running smoothly through the centers of these circles is the "mean line". If the airfoil is symmetrical, the mean line will be a perfectly straight line. If the mean line is curved, then the airfoil has some non-zero amount of camber.

Now, draw a straight line from the leading edge to the trailing edge. This is the "chord line" of the airfoil, and the length of this line is of course the chord length. Next, measure how far it is from the chord line up to the camber line at the highest point on the camber line. Divide that height by the chord length, and multiply that result by 100%. That number is the airfoil's camber, expressed as a percent of the chord length.

The typical convention for describing airfoils, including most of the earlier NACA sections, describes the airfoil shape by starting with a symmetrical airfoil shape that describes the amount of thickness and how that thickness varies along the airfoil (how sharp is the leading edge, where along the chord is the thickest point located, how rapidly does the thickness change on either side of the max thickness point, etc.). Not surprisingly, this symmetrical shape is called the "thickness distribution".

The camber of the airfoil is described by a "camber line" or "mean line". This describes the shape of the camber line in terms of height above or below the chord line in percent.The airfoil shape is found by "wrapping" the thickness distribution on the camber line. Imagine you started with a symmetrical airfoil made of something soft, then bent it until its chord line was deformed to match the shape of the desired camber line, and you have the general idea.

Reference books such as "Theory of Wing Sections" by Abbott & Von Doenhoff include charts of coordinates for complete airfoils, as well as for individual thickness distributions and mean lines. New airfoils of different cambers and thicknesses can be made by scaling from those camber lines and thickness distributions, then combining the results.

A "single-surface" airfoil such as on an IFO essentially has a positive camber (because the covering material billows into a curved shape due to air loads when it starts making lift), with (for all practical purposes) a thickness of essentially zero.

As far as aerodynamic characteristics go, increasing the camber tends to increase the max lift coefficient that an airfoil can make, at least up to a point. Much of the lift characteristics of the airfoil are determined by the camber line, while much of the profile drag characteristics are determined by the thickness distribution, at least in the middle of that airfoil's operating range. At angles of attack close to stall, things get a little more complicated, but we'll save that discussion for another time.

OK, so what's this stuff called "undercamber" all about? Well, that's simply an airfoil where a portion of the underside is significantly concave.

Imagine we have a symmetrical airfoil we're going to use as our thickness distribution, and we begin bending it upward to give it some positive camber. As it bends, the top surfaces rises, and so does the bottom surface. At some point we've bent it far enough that the lower surface has become completely flat, and all of the curvature in the airfoil shows up in the upper surface. We now have a "flat-bottomed airfoil". It's nothing more than an airfoil with just the right combination of thickness and camber to make the bottom come out flat. Many folks will tell you that flat-bottomed airfoils are good for slow, trainer-like, "floaty" airplanes, but this is a dangerous oversimplification. Because there are many combinations of thickness and camber that can result in a flat bottom, it's possible to have a wide range of airfoil behavior within the family of flat-bottomed airfoils.

OK, so we bent our thickness distribution upward until the bottom was flat, and it became a flat-bottomed airfoil. What happens if we bend it up even further? The bottom surface in the middle of the airfoil will continue to rise as we bend in more and more camber. Since the bottom of the airfoil was flat before, the underside of the airfoil now forms into a concave shape. This concave underside is called "undercamber". Once again, there are a wide range of possible combinations of camber and thickness that will result in undercamber, so it's dangerous to generalize. However, the existence of undercamber does require that we have a lot of camber in comparison to the thickness, suggesting that we do have a fair amount of camber.

The tricky thing about both camber and thickness is that too much of either can give us an airfoil that has too much curvature on one or both surfaces for the airflow to follow smoothly, particularly at low Reynolds numbers (such as our slowfliers), and particularly on the portions of the airfoil aft of the highest and/or thickest points. If we have too much camber and/or thickness, the airflow over the aft portions of the upper surface can separate at higher angles of attack (limiting our slow speed performance and max lift), while too much undercamber can cause flow separations on the lower surface at lower angles of attack. With too much of both, we can get an airfoil that has significant amounts of separated flow on the top, bottom or both at ALL angles of attack. However, not enough of each can also cause problems. Designing really good airfoils, especially for very low Reynolds numbers, can become very tricky.

There's more on airfoil characteristics, as well as explanations of things like Reynolds numbers, what they are and how to use them, etc., in the "Ask Joe and Don" section of our website.

Don Stackhouse
DJ Aerotech