Let’s talk about airplanes. Airplanes are cool, just in general, but they are REALLY cool if you think about them in terms of technology. The first controlled, sustained flight of a powered airplane was only in 1903, 121 years ago. Wilbur Wright flew their wonky-looking airplane at a speed of about 8 miles an hour, at an altitude of about ten feet off the ground, most of the time. Just 44 years later, in 1947, Chuck Yeager broke the sound barrier, exceeding Mach 1 (mach number is the speed of sound, about 760 mph [1220 kilometers per hour]) in the Bell X-1 over the desert in California. Just 17 years later, in 1964, the SR-71 reconnaissance plane was flown by the US Air Force. It could achieve speeds of about Mach 3.5 or about 2,200 mph (3500 km/h). It is still the fastest airplane ever, or at least that anyone is willing to talk about. Other jet-powered aircraft of the same era could reach altitudes of over 120,000 feet (38,000 meters). That’s quite a lot of progress in 60 years, or so. If you add the consideration that it was only 66 years from when the Wright Brothers stood on a windy beach in North Carolina to when Neil Armstrong stepped off of a spacecraft onto the surface of the Moon, the story of manned flight is one of the most amazing technological journeys of all time.
Source: National Air and Space Museum. The 1903 Wright Flyer
People have been watching birds fly, wishing they could join them, probably forever. Who doesn’t feel at least a bit of envy while watching a bird soaring through the air, seemingly free from the shackles of gravity? Well, people being the marvelously curious and inventive sorts of creatures that we are, having been experimenting with and thinking about ways to join the birds in the sky for a very long time. Icarus, flew through the sky on wings made from feathers and wax in Greek myths written over 2000 years ago. Leonardo da Vinci drew up some pretty good designs for flying machines about 500 years ago. The Montgolfier brothers were the first humans to actually “slip the surly bonds of Earth” (check out the poem “High Flight” https://www.poetryfoundation.org/poems/157986/high-flight-627d3cfb1e9b7 ) and fly through the air in their hot-air balloon in 1783. Following the Montgolfiers, people really started to experiment with flight, in balloons, and then in gliders. The step to the gliders was a big one, and drew greatly from people observing birds in flight. What was it about their wings that allowed them to fly? It wasn’t a simple question, and the answer wasn’t simple, either. Part of the problem is that bird wings work for birds, but the design of a bird wing is different in many ways from the design of a wing that can carry a person. Bird wings work, not only because of their shape, but because of their feathers, which play a very important role in trapping air beneath their wings and creating a pocket of dense air that “pushes” the bird upward. Bird wings change their shape as the bird flaps and the feathers move around to make flapping efficient. So, simply adopting a bird wing to try to build a flying machine, a la Icarus, didn’t work, although many, many people tried it. It took an understanding of Physics to build wings that could work on a glider and, particularly, on a machine that would allow for powered flight.
Source: US Air Force. Captain, later General Chuck Yeager with the Bell X-1 in which he broke the sound barrier in 1947.
The Wright Brothers were not the first to develop the familiar “airfoil” shape of aircraft wings as we know them today. Their genius was in building wings and experimenting with their designs until they built machines that could not only lift into the air, but be controlled. Incidentally, the control mechanisms the Wrights incorporated into their first gliders and their first powered “Wright Flyers” was very similar to how birds control their movement. The Wrights built wings that they could bend or “warp”, in order to change direction. That advancement was huge, because it allowed the Flyer to turn. Airplanes cannot turn if the wings cannot bank. The Wright Brothers are famous because they were smart, observant, meticulous in their craftsmanship, methodical in their testing, and very dedicated to make things work, no matter how many times they failed.
Airplanes fly because the wings produce enough upward force, called lift, to overcome the downward force of gravity. They can do this because of their shape and the forces that act on the wing as it moves through the air. It’s important to understand that it is the movement of the wing through the air that produces lift. A wing that isn’t moving, or that isn’t moving fast enough to generate lift (this condition is called a stall) will just fall from the sky, just like a brick. The wing usually, but not always, has what is commonly called an airfoil shape, with the upper surface being curved and the lower surface being flat. It is the difference in how air flows over the upper and lower surfaces that generates lift and allows the plane to fly.
Source: NASA
How lift is actually generated involves some very serious fluid dynamics and physics, but what follows is a fairly accurate explanation. There are two main effects that generate lift. One of them is probably what you are familiar with, if you’ve ever taken a science class and learned how wings work. It’s called the Bernoulli effect. Bernoulli discovered that any fluid (air is a fluid, which is just a term for any substance with no fixed shape) will exert less pressure when it is accelerated. For instance, if water is flowing through a tube, it exerts a given pressure outward on the sides of the tube. If that water then moves into a narrower part of the tube, it will move faster, but it will also exert less pressure on the tube. If you have ever seen a water-powered suction device, like a swimming pool vacuum, this is the principle upon which it works. If you send water through a hose, and then through a restriction in the hose, you can use the decreased pressure to provide suction. The airplane wing works the same way. As the fluid (air, in this case) moves across the wing, the wing splits the air into two streams, one moving under the wing and one above. Because the stream going over the curved upper surface of the wing has to move faster than the air going under the wing (the curved surface is longer than the flat surface, so the air has to “speed up” in order to get around the wing and meet back up with the slower air coming underneath), the air above will exert less pressure downward on the top surface of the wing than the slower air exerts upward on the bottom surface of the wing. This will, in effect, suck the wing upwards, as it will move toward the area of lower pressure.
Now, for those of you who have always heard that the wing works because the shape causes the air to push it upwards (as opposed to its being “sucked” upwards by the Bernoulli effect), Isaac Newton is about to rescue you. I mentioned that there are two main contributors to lift. The first is the Bernoulli effect we just described. The second is Newton’s Third Law of Motion. In previous articles, we’ve touched on Newton’s Laws of Thermodynamics. Now we’ll talk about one of his Laws of Motion. By the way, if this makes you think something like “Wow. Newton did a lot of stuff!”, you would be correct. He was very possibly the smartest, most influential scientist in history, which is truly saying something since he wasted a huge amount of his time on stuff like Alchemy. Anyway, back to the Third Law. It is the one about “for every action, there is an equal and opposite reaction”. In a nutshell, it means that if you push on something, is pushes back. For our airplane wing, this explains the second component of lift. As the wing moves through the air, it is at an angle, with the front of the wing somewhat higher than the back of the wing. The movement of the tilted wing through the air deflects air downward. At the same time the wing is pushing the air downward, the air is also pushing the wing upward. So, in effect, Bernoulli is pulling up on the top of the wing while Newton is pushing up on the bottom of the wing. If you look at a small, general aviation airplane, like the familiar Cessna SkyHawk or Piper Cherokee, you will see that the wing is tilted slightly upward, even when the plane is sitting flat on the ground. The wings are oriented like that because those small planes are not very powerful or very fast, so they need the extra lift provided by the tilt of the wing.
Changing how air flows around the wing allows for the pilot to control the plane. By moving parts of the wing, usually located along the back edge of the wing, the pilot can control the air flow and, therefore, the lift produced by the wing. For instance, if the pilot wants to turn left, he can use the controls in the cockpit to cause the control surfaces on the back edges of the wings (called ailerons) to move. In the left turn, the aileron on the right wing will tilt down a little and the aileron on the left wing will tilt up. When they do, it changes the airflow over the wings. Air will move a little faster over the right wing and a little slower over the left, causing the plane to bank to the left (right wing higher). Now, because the wings are banked, as opposed to level, some of the lift forces generated by the wing are pushing the plane to the left, instead of just pushing it up in the air. There are other controls, like the rudder and the elevator on the tail of the plane that also play a role, but turning is done mostly by the wings, not the rudder. If you try to turn a plane just by using the rudder without banking the wings, the plane will just sort of slide a little sideways while still travelling more or less the same direction.
If you are ever in a plane trying to land in a crosswind, you will see this in action. The pilot will use the rudder to try to point the plane in the general direction of the wind blowing across the runway while using the ailerons to keep the plane flying straight down the runway, even as the nose is pointed somewhat toward the direction of the wind. Right at the moment the first wheel touches down, the pilot will adjust the ailerons and the rudder to bring the wings back to level so the plane ends up (if all goes well) pointed down the runway. This one factor is why flying a small plane in high winds is so dangerous. Taking off is relatively easy, even when it’s windy, if you are careful and aware of the wind. Just the flying part, once you are up in the air is also relatively easy in high winds. It can affect your speed, slowing you down if it’s a headwind or speeding you up if it’s a tailwind. If you are flying in a strong crosswind, you have to use the rudder to orient the plane so the nose is pointed slightly into the wind to compensate. You end up going the direction you want to go, but the plane is sliding through the air a little sideways. Landing is the really hard part when it’s windy because the runway is where the runway is and if the wind is blowing across the runway it’s trying to push you away from the runway. You have to point the nose into the wind to keep the plane going straight along the runway. However, the landing gear can’t roll sideways. So, the pilot has to control the plane very precisely, to get the nose pointed in the right direction with the wheels lined up with the runway just as the tires hit the runway. Lots of opportunity for things to go badly during the process.
Figuring out how to turn is why the Wright Brothers learned to fly, if not exactly like the birds, then certainly with the birds. So next time you fly, give a thought to the Wright Brothers, Bernoulli, and Newton. You’re actually riding on their genius, not just on an airplane.