Airshows are a great place to study airplanes and crowd psychology. We wait patiently in long lines for hotdogs, bathrooms and overpriced water. Of course, people do that at all large gatherings, but the one group activity found only at airshows is the creation of the people-filled shadows, as shown in Figure 1. As the hot Sun cooks the crowd, they migrate under the protective wing shadows of huge airplanes, preferably a C-130 or a B-52 bomber; something with a huge wing area. And just like that, we’ve discovered yet another practical use for airplane wings!

This primer talks about why we have wings at all. As the aerodynamicist Jack Moran said, wings are simply a thrust amplifier. Sure, we could use rockets to get from point A to point B, but that would be incredibly inefficient as far as fuel usage goes. That’s where wings come in. They provide a similar ability to defy gravity, but at a fraction of the fuel usage compared to rockets.

Rather than use directed raw force, wings have a unique characteristic; they generate a force that is perpendicular to the direction of movement. Airplanes move horizontally and wings push up vertically (LIFT). This magic of physics is simply a result of how air flows over the wings. 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 lower air pressure above the wing than below it. Air cannot impart direct forces on a wing like a hammer can. Instead, it can only impart forces via two methods: pressure and friction. Those are the only two methods. I will repeat: lift is the result of having lower pressure above the wing than the pressure below it. Pretty simple eh?

No doubt, there are many theories as to what causes the required 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. Unlike a wing, 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 an Aerofoil or  Profile.

Technically, airfoils are flat two-dimensional shapes and can’t produce any lift at all; great for pictures on paper, but lousy for lift. You have to extrude an airfoil into the 3^rd dimension to create an object that will make lift. We call this extruded shape a wing section (see Figure 3). Welcome to the real world.

So now you have a device that generates a pressure difference, resulting in vertical lift forces and 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. Blah, blah, blah, it involves a lot of fancy math, so just believe me on this one. If the real world physics are obeyed, half of the oncoming air will go over the wing section and half will go 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 the upper and lower surfaces can vary wildly. It usually drops lower than ambient pressure, especially if the wing section is angled up at all. For a lifting airfoil, the airflow 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 downward. 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 technique 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.

 

NOTE: DesignFOIL Standard Edition is on sale for only $29 right now.

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