The sight of a sixty thousand pound hunk of metal rising into the air and jetting off over the horizon is indeed inspiring, almost magical. It is even more impressive to see a giant Boeing 747 take off or land in a high crosswind strafing toward the runway almost sideways. But what allows these large heavy machines to seemingly defy gravity? It is an intriguing question and one that inspired the author to pursue a degree in Aerospace Engineering.
Reading through other articles on this topic from magazines and the Internet, one may still be left asking, “But how does it lift? What really gets these ponderous birds off of the ground?” There are many incorrect explanations and many poorly explained correct ones. There are also some explanations that try to hold a middle ground, trying to explain difficult concepts with clever, easy to take in interpretations of difficult concepts. These attempts usually fail in that they still do not explain what really happens in the phenomenon called “lift”. These articles usually state it is an aerodynamic force created by a difference in pressure. This is correct, but what causes this difference in pressure? For that matter, what is pressure? This article will attempt to answer these questions in an easy to understand fashion.
First one must understand what air actually is. Other explanations often tend to give air some kind of mystical status that somehow pulls or pushes on a wing or creates artificial wind or even a “pocket of air” on which the wing can rest. In reality, air is simply gas, a bunch of molecules moving about randomly. That’s it, nothing more and nothing less.
What then is pressure? Pressure is simply air molecules bouncing into an object. When someone blows air into a balloon, what stretches it out? It is the multitude of air molecules forced into the balloon that bounce against the inside. It may seem silly at first; a little air molecule bouncing against something could hardly produce any kind of force. But when thousands upon thousands of air molecules are striking a surface, the effect multiplies drastically. What then does a higher pressure mean? It means that more air molecules are striking a surface with more force.
What does this have to do with airplanes and wings? One more thing must be understood first. To illustrate the next important concept, imagine a pipe with air flowing through it. Now imagine that at a certain point, the pipe narrows. Does the air flow at the same speed in the narrow section as in the wide section? No it doesn’t, but why? It has to do with the mass flow rate, but what does that mean? Basically, all the air in the wide section is suddenly pushed into a smaller area. It has to get out of the way quick because more air is coming in. The air flows through the narrower section faster to keep the rate of flow constant. From a more academic viewpoint, air tends to remain the same density at a constant temperature. In order to remain the same density, the air occupying a certain length in the wide section of tube would have to occupy a longer section of narrower pipe. The derivative of the longer length means a higher velocity. Of course there are certain factors that apply that are beyond the scope of this article such as compressibility.
So what does a faster airflow have to do with anything? Pressure is simply air molecules bouncing off of something. The faster the molecules are going, the less chance they have to bounce against the side of the pipe. What does that mean? It means lower pressure! Of course, it also means that when they run into something directly in the airflow, the molecules hit it harder. This is called dynamic pressure. When the molecules bounce off of the side of the pipe, it is called static pressure. The pressure in a balloon is purely static pressure because the gas inside is not moving anywhere. Bernoulli’s Principle basically states that the total pressure of a system is equal to the sum of the static pressure and the dynamic pressure. There is more to it than that, but anything more will be confusing for now.
So how does this factor into an airplane flying? The typical airplane wing has a curved upper side and a relatively flat underside (the wing is often called an airfoil). The bottom of the wing is kind of like the wider section of the pipe. The air flows slower so the molecules strike the underside of the wing more. The top of the wing is like the narrower section of the pipe. The air must flow faster so the molecules have less chance to strike the wing. Since more air molecules are striking the bottom of the wing than the top, an upward force on the wing is naturally created. This is what an aerodynamic force and a difference in pressure is. In addition, the faster the flow of air, the greater the difference in pressure and the greater the aerodynamic force.
This is why an airplane must move to fly. By moving forward, the airplane creates a faster air flow over the wings. At a certain speed, the aerodynamic force is greater than gravity and the airplane can fly. A helicopter uses the same idea; the rotors are actually airfoils. The faster they spin the faster the airflow is and the more lift they create.
The concept of lift is really quite simple when broken down to its basics. Too many times, individuals focus on advanced concepts or phenomena without a proper understanding of the fundamentals. This article has attempted to provide an easy to read explanation of the reality and basics of lift. The Illustrated Guide to Aerodynamics by H. C. “Skip” Smith is recommended for further reading, or for a more academic treatment, Introduction to Flight by John D. Anderson, Jr.