If you only remember one thing from this tutorial, it should be that subsonic airfoils are round in the front and sharp in the back. I see this violated all the time on the after-market wings kids are putting on their cars these days. Remember: round in front and sharp in back. That’s the big rule. Everything else is just tweaking and optimization. For our purposes, all diagrams and airfoil layouts will assume air movement from left to right; European textbooks sometimes assume the opposite.

Let’s take a symmetric airfoil and point it directly into the oncoming wind as shown in Figure 1. Right now, the oncoming air speed is about 60 miles per hour and 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.

What good would come from a symmetric airfoil oriented parallel to the wind? It makes for a perfect streamlined fairing. A cover of sorts that hides some underlying non-streamlined structure like a wire, antenna, pipe, or landing gear strut.

Now 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 discovered is that an increase in angle between the chordline (an imaginary straight line between leading edge and trailing edge) of the airfoil and the oncoming wind also increases 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 where the lift force will start to fall off and drag will grow quickly. Note that if the airfoil has upward bow (camber), then increasing the angle-of-attack may actually decrease the drag force for a few degrees before it continues 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 I was a kid and saw a Boeing jet climbing very slowing away from Columbus International Airport. It appeared to be just plowing through the air nose-high. The nose does not always point straight at the direction the airplane is flying. At high angles-of-attack, it appears to be pointing way up from the direction of travel.

Figure 3 shows a typical airfoil 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 line between the leading edge and trailing edge; this is used for 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 meanline is an imaginary line that divides the airfoil into roughly equal upper and lower halves. On a symmetrical airfoil, the 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.

The perfect airfoil would allow you to change the meanline during flight; lots of camber for takeoff and very little during cruise. Fortunately, we have developed a method for doing just 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 essentially adds camber to the wing section. Flaps allow our wing section to have lots of camber during takeoff and very little camber during cruise.

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 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.

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