Airshows are a great place to study crowd psychology. We stand patiently in long lines for hotdogs, bathrooms, and overpriced water. Yes, people do that at any large gathering, but the one group activity found only at airshows is the creation of the people-filled shadows as shown in Figure 1. As sunlight broils the crowd, they migrate under the protective wing shadows of a C-130 or a B-52 bomber. Without thinking, we have discovered yet another practical use for airplane wings.

This primer is about why we have wings at all. As Jack Moran says, wings are a thrust amplifier. Their magic is in their ability to defy gravity without using raw thrust from a fuel-guzzling rocket. Instead, wings use the air flowing past to create a vertical force called lift which seems to defy gravity. This tutorial is about how lift is created, how to estimate it, and how to make it happen.

The origin of lift is very simple: it is the result of having higher air pressure below the wing than you have above it. Very unlike a hammer, air can only impart forces to solid objects via pressure and friction. Those are the only two methods. I will repeat: lift is the result of having higher pressure below the wing than you have above it. Pretty simple eh?

Of course, there are many theories as to what causes the pressure difference. That's where people get all bent out of shape. Blame it on Bernoulli? Blame it on momentum transfer? The devil is in the details.

Streamlined wings aren't the only things that can create lift; a sheet of plywood could also generate lots of lift. Of course, a sheet of plywood is aerodynamically very inefficient. The secret to making this pressure-difference-maker more efficient is to use a cross sectional shape that won’t cause separation at the nose. Plywood has a sharp leading edge which generates oodles of drag; the retarding force that keeps us from moving forward as fast as we’d like to. Historically, good wings use special cross-sectional shapes that are round in the front and sharp in the back. We call this shape an Airfoil (Figure 2). Europeans and some other parts of the world call it a Profile.

Airfoils are flat two dimensional shapes and can’t produce any lift at all; great for textbooks, but lousy for lift. You have to extrude an airfoil spanwise to create an object that will make lift. We call this extruded shape a wing section (see Figure 3). Welcome to the third dimension.

So now you have a device that creates a pressure difference resulting in a slight down-deflection of air behind it. Who cares? Millions of airline passengers care!

Nature will direct the airflow around a wing section so that the air obeys the conservation laws of mass & momentum. It involves a lot of fancy math, so just believe me on this one. If the realworld physics are obeyed, you end up with about half the oncoming air going over the wing section and the other half going under the wing section. The point on the leading edge where the oncoming flow splits is called the stagnation point. Strangely enough, the velocity of air at that very point is zero! There's another stagnation point at the trailing edge where these two travelling air masses come back together. Figure 4 illustrates these stagnation points.

The air pressure along both the upper and lower surfaces can vary wildly, usually dropping much lower than ambient pressure, especially on top if the wing section is angled up at all. For a lifting airfoil, the air above is typically accelerated higher than the air below. Think of it as the air up front racing to fill the void of all that air you just pushed down behind the wing. From Bernoulli's famous effect, we know that when you speed up air, the air pressure drops. The end result is that the pressure difference between the lower and upper surface literally sucks the wing upward!

To conclude the idea, lift comes from a combined effort of the wing being sucked upwards and the wing deflecting some of the air beneath it. The effects are so intrinsically linked together that we can calculate the lift force by simply measuring surface pressures around the wing section/airfoil. That's one method which wind tunnels use to measure lift forces and pitching moments on a wing section model; many advanced wind tunnels use another techique for drag which measures how much momentum the model "steals" from the oncoming airflow via the boundary layer; we'll get into that later.

One last note about lift. A wing section exposed to an oncoming wind generates a single united force, usually pointing up vertically and slightly backwards. We call this the Resultant force. Lift is the portion of that force that is perpendicular to the direction of travel, not the direction the airfoil is pointing. Drag is the portion that is parallel to the direction of travel. See Figure 5 for an illustration.

There you have it. You know where airfoils, wing sections, and lift come from. Let’s get on with learning the practical stuff.


Now let's move onto BASIC TERMS & GEOMETRY....