Subsonic airfoils should be round in the front and sharp in the back. A century of visual reminders should make this obvious. However, I see it violated often with regard to after-market wings people install on their cars. The wings are already not very effective for the speeds that most cars are driven, but they are -really- ineffective when mounted backwards. Remember: put the round end upstream and the sharp end downstream. That's really the big rule at the core of standard airfoil setup. Everything else is just tweaking and optimization. For our purposes, all airfoil diagrams shown in this primer series assume air movement from left to right.

Let's look at an example:

Take a symmetric airfoil and point it directly into the oncoming wind as shown in Figure 1. Since the airfoil is parallel with the wind, we can’t measure or feel any perpendicular forces (up or down in this case). The lift is zero. However, there is a slight tugging force from the friction of air dragging along the airfoil surface. We call this force drag.

You might wonder what use could come from a symmetric airfoil oriented parallel to the wind? It makes for a perfect streamlined fairing, a shield that hides some underlying non-streamlined structure like a wire, antenna, pipe, or landing gear strut.

Streamlining is nice, but we want lift. Let’s gently tip the nose up to some small angle as shown in Figure 2. Suddenly, there is a noticeable force upwards while the dragging force increases slightly. What you’ve just discovered is that an increase in angle between the chord line of an airfoil (an imaginary straight line stretching between leading edge and trailing edge) and the oncoming wind will increase the lifting force. This variable angle is called the Angle-Of-Attack, or AOA for short. What you need to know is that increasing the AOA will increase both the lift force and drag force up until about 15 degrees. After that, the lift force will start to fall off, but drag will continue to grow. We call this phenomenon stall. It is the result of the formerly smooth air over the wing breaking down and separating from the wing. One special note: if the airfoil has upward bow to the shape (camber), then increasing the angle-of-attack may actually decrease the drag force for a few degrees before it resumes its quick climb.

Angle-Of-Attack is the angular difference between where the wing is pointing and where it is moving. The first time I truly understood this was when, as a kid, I saw a Boeing jet climbing very slowly away from Columbus International Airport in Ohio. It appeared to be just plowing through the air nose-high. The airplane nose will not always point straight in the direction the airplane is flying, especially during landing and takeoff.

Figure 3 shows a typical airfoil geometry with the important components labeled. The Upper Surface is the wing section skin on top, from the leading edge to the trailing edge. The Lower Surface is the bottom wing section skin that goes from the leading edge to the trailing edge. Mentioned already is the chord line, which is an imaginary straight line between the leading edge and trailing edge; this is used for measuring/setting Angles-Of-Attack (see Figure 2).

Not to be confused with the chord line is the mean camber line, or meanline for short. The mean camber line is an imaginary line that divides the airfoil into equal (roughly) upper and lower halves. On a symmetrical airfoil, the mean camber line is the same as the chord line. However, if you bow the airfoil upwards, you are adding "camber" to the airfoil. A unique characteristic of airfoils with camber is that they produce lift even at zero degrees Angle-Of-Attack. The more camber, the more lift. Of course, there is an associated cost of more drag and more pitching moment (we'll get to that later) as well.

The perfect airfoil would allow you to change the mean camber during flight; providing ample camber for takeoff and very little during cruise. Fortunately, there is a simple method for doing this without resorting to bending or flexing the structure. Instead, we simply droop down the aft portion of the wing section using a hinge. This device is called a flap and temporarily adds camber to the wing section. Flaps allow our wing section to have lots of camber during takeoff and very little camber during cruise. If the flaps also extend backwards while they droop, then they also provide increased wing area, making for an amplified lifting effect.

This is a great place to stop as we have covered quite a bit so far. In the next part we will discuss lift and drag in more detail. We will discuss their respective coefficients which allow us to compare the performance of airfoils on a common scale.

We will also discuss Reynolds Numbers and how the air flowing over a wing creates boundary layers; a phenomenon which greatly affects the performance of our wing sections. Lack of this understanding on the part of the Wright Brothers during their scale model wind tunnel tests sent airfoil designers in the wrong direction and stifled airfoil development for about ten years. I can't fault the Wright Brothers though; they were blazing new trails as the first of a new breed of engineers: aeronautical engineers.

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Click HERE to go to Part 3 and learn about Predicting Lift Forces...