I wrote a couple articles awhile ago on the Four Forces of Flight and Parts of an Airplane, but I think it’s time to go a bit more in depth on that fancy thing that keeps us in the sky. LIFT. And if it’s lift we’re after, we need a lift generating device – also known as a wing.
Before we get into how this stuff works, lets briefly go over the anatomy of wings.
This isn’t really a part of the wing, but it’s important because without the relative wind, the wing won’t generate lift. As the airplane moves through the air, it’s generating “wind” over the wings. The relative wind can be drawn as a line parallel to the direction the airplane is moving through the air, and is one of the lines used in determining the angle of attack.
The front edge and back edge of the wing. Simple enough.
It’s a line drawn between the leading edge and the trailing edge. This is the other line used to determine angle of attack.
Angle of Attack
The angle between the chord line of the wing and the relative wind. If the angle of attack becomes too great, the wing will stall. The angle at which this happens varies from wing to wing – and is known as the “Critical Angle of Attack.” I’ll discuss this more later.
Upper Camber is the thickness of the wing above the chord line, the same is true in reverse of the Lower Camber. I’ll show the effect that this has on lift later.
Angle of Incidence
The Angle of Incidence is the angle at which the wing is bolted to the airframe. A positive Angle of Incidence means that the wing is slightly pitched up into the relative wind when the longitudinal axis of the airplane (see my Parts of An Airplane article for more about the Longitudinal Axis) is parallel to the horizon. This isn’t shown in the above image.
The part of the wing closest to the fuselage.
The end of the wing.
The length of the wing from tip to tip.
Wings are almost never completely straight across. If you look at a wing from the front, you’ll notice that it makes a very shallow V shape, with the fuselage in the middle, or more correctly, it’s the angle formed between the wing tip and a line extending from the wing root parallel to the lateral axis.
How a wing works (what the FAA wants you to know)
The wing has a curved top and a (mostly) flat bottom – air molecules like each other and want to stay together, so when the wing moves through the air, the molecules on top move faster to meet up with the molecules on the bottom. Since they’re moving faster, the pressure on top is lower and the wing is drawn up into this lower pressure area (see Bernoulli’s Principle).
Wing Performance Factors
Angle of Attack
There are two ways to increase the amount of lift a wing produces while in flight: Changing the Angle of Attack, and Increasing Speed. Increasing the speed of the aircraft is simple enough – go faster, more air passes over the wings, more lift gets generated.
Changing the angle of attack, however, comes with some limitations. Sure, bringing the nose up makes the air going over the top of the wing have a longer way to go and it has to move faster, decreasing the pressure on top of the wing and increasing the pressure on the bottom because the wing is actually deflecting air downward as it plows through the sky. But this works up to a point – the wing stops generating lift after about 18º of pitch (depending on load factors). This is known as the Critical Angle of Attack.
Here’s how that works:
As you can see, the flow over the top of the wing becomes more and more turbulent as the Angle of Attack increases. When laminar flow over the top of the wing stops, the wing is generating more drag than lift and stalls. Still don’t believe me?
Notice how in the second video, the wing root stalls before the wing tip. This is intentional. Most modern wings are designed with a twist in them, giving the wing tips a lower angle of incidence than the wing root. This allows for greater roll control on the edge of stall since there is still lift being generated at the wing tip where the ailerons are attached.