How Do Planes Fly? Nobody Knows Its Exact Mechanism!

 Since There Is Gravity, How Do These Planes Fly? No, we are serious!

We are so used to seeing things as "obvious"; because if we can manage to deceive ourselves that we understand how or why something is the way it is, we will be curbing our curiosity. However, one of the basic assumptions of science is that nothing is obvious in the Universe. If you think it's jet engines or wings that make planes fly, you're wrong. At least partially... Especially if you think that scientists know exactly how airplanes fly, you are wrong again.

 

Classic Explanation of Airplane Flight: Equal Path Theory

As every time a group of scientists and science lovers get on a plane, this time the talk is about how planes fly. With the power of the Mechanical Engineering education, those classic statements flew in the air:

The principle of flying an airplane is really exciting! Everyone thinks it's the jet engines that make the plane fly; however, the jet engine can only provide reverse thrust. So it can only push the plane forward; can't push it into the air! The only thing that makes it fly into the air is the wings. What allows wings to achieve this is the interesting wing shape. As the jet engine propels the airplane forward, the wings split the air coming over them in half, with some flowing over the wing and the other under it.

But if you look at the cross section of the wing, it looks like an asymmetrical droplet: The upper side is more cambered; the underside is flatter. This is called "erfoil". Because of this shape, the air flowing from the top has to accelerate in order to travel the same amount of distance in the same time period. In the framework of Bernoulli's Principle, the pressure of air decreases with increasing speed. This causes the pressure on the wing to drop. The underside is flat or less curved. Therefore the pressure stays the same (or does not drop by an equal amount). This pushes the wing up and the plane takes off! So the event is not in the engines, but in the form of wings!

Simetrik ve Asimetrik Airfoyil

What a great explanation, isn't it? This is called the Equal Path (Tracking) Theory. This is how most engineers would explain it; because that's how they learned. Specialized engineers such as Aeronautical Engineers may have learned some nuances and can transfer them to you; but that's how the average engineer or science buff will almost certainly explain the flight of an airplane. So much so that some engineers defend the absolute accuracy of their explanations with intense passion! In fact, if you search for foreign sites, you can see that almost all popular science sites describe the principle of flying an airplane like this. Some may offer "alternative explanations", such as "reduced density" of overhead air; But in essence, the principle they all describe is the same.

The 3 Big Problems of Classical Explanation

But there is a problem. There are even 3 problems. . As you may have noticed, this explanation is problematic in 3 important points:

1. In the classical explanation, the air flowing over and under the wing is said to travel at different speeds to reach the rear end of the wing at the same time. That is, the air at the top is accelerating "because it has to travel longer" and therefore its pressure drops. good but... Why would the splitting air molecules have to reach the end of the wing at the same time? Well, the air molecules above the wing could arrive much faster and earlier than those below (and vice versa)!
2. If this is the principle of flight of airplanes, how can aerobatic planes and jet planes fly upside down? All that aside, how can a cargo plane fly upside down? So much so that airplanes like the Boeing 747 could theoretically fly upside down. You can watch a wind tunnel experiment demonstrating this here. But of course, if a passenger plane did this in real life, there would be problems with pilot comfort, hydraulic/fuel flow. But of course, if a passenger plane did this in real life, there would be problems with pilot comfort, hydraulic/fuel flow.
3. If this is the principle of airplanes flying, how can paper airplanes with no "bomb" on their wings fly? We're not talking about things you do in such a hurry and go 30 centimeters away. We are talking about paper plane flights, such as the 69.14-meter flight that entered the Guinness Book of Records!

Of course, the last question is a bit more rhetorical; because paper airplanes are rather "floating"; they don't fly. But if you put a thruster behind them, they could also fly with ease.

The second question creates a much bigger problem: Because jet planes can not only fly upside down; they can also rise in this way! How do we explain this? For example, in a video published by Seeker on September 29, 2017 about the ability of airplanes to fly upside down, it is claimed that the wings of airplanes that can fly upside down are designed "symmetrically" to achieve this, after saying that the main flying principle of airplanes is the shape of the wing. But this contradicts their initial statement; because if the main principle for the aircraft to fly is the "asymmetrical droplet" design, the planes with the symmetrical droplet design should not have been able to take off from the very beginning!

The first question hits the explanation straight in the heart: Indeed, why would air molecules have to arrive at the end of the wing at the same time? They could also arrive at separate times, and so there would be no need for any change in speed.

But coincidentally, this article, published in Scientific American on February 1, 2020, encouraged us to delve deeper into the subject.

Since There Is Gravity, How Do These Planes Fly?

This sub-title is, of course, a humorous reference to the famous question that is often asked by those who do not understand evolution. But the joke points to a real problem: We're not sure exactly how planes fly!

At this point, it should be said: Engineers know very clearly what characteristics (mass, aerodynamic design, etc.) aircraft can take off, within the framework of their theories and mathematical formulas for the flight of an aircraft.

The problem is not aircraft safety or aircraft engineering. The problem is: Although our formulas fit what we see in nature, they are insufficient to explain the mechanism. In other words, as long as we follow our theories, we can easily fly planes, helicopters and rockets; however, our mastery of the details of our theories is not sufficient to explain exactly what was the main factor that made the plane fly.

 4 forces acting on airplanes

Within the framework of our theories at hand, we know that there are 4 main forces on an airplane in 4 different directions:

1. Weight:Uçağın kütlesi ve Dünya'nın kütleçekiminden ötürü aşağı doğru uygulanan kuvvettir.
2. 2. Thrust: It is the forward force acting on the aircraft due to the air molecules that jet engines "throw" backwards.
3. 3. Drag : It is the force that slows down the aircraft (acting backwards) due to the friction of air molecules striking the cross-sectional area perpendicular to the direction in which the aircraft is advancing.
4. 4. Transport: It is the force that makes the plane move upwards. But why? Where does this force come from?

From these forces, thrust overcomes drag and the plane can go forward. Carrying overcomes the weight and the plane can take off. There is no problem at all. But the "Why?" in 4th force The question is the million dollar question. We know the mechanisms of the first three very (more) clearly and can explain their reasons. But the fourth is in the question "Why?" A definite answer to the question is not yet available. But there are 2 possible explanations:

The first is the classic explanation above. Proponents of that statement have some common responses to criticism: For example, they say that paper airplanes don't actually fly and glide. Or they say that airplanes flying upside down can only fly temporarily, while they do not fly, but glide for a while by taking advantage of concepts such as the speed and angle of attack they already have, that is, the angle of the wing with the air molecules falling on the wing. So they have answers for practical applications. But their explanation of why the molecules must arrive at the back of the wing at the same time is not very satisfactory. Holger Babinsky of Cambridge University says:

The popular explanation for lift is a common, quick, and logical explanation at first glance. In fact, it also gives the correct answer; but it also causes an erroneous perception. It has a physical argument that has no logic and erroneously uses the Bernoulli Equation.

At this point, for those who are wondering, the Bernoulli Equation is the equation that Daniel Bernoulli declared in 1738 in his work Hydrodynamica, and its modern form was given by the great mathematician Leonhard Euler:

P₁+1/2*ρ*v₁²+ρ*g*h₁=P₂+1/2*ρ*v₂²+ρ*g*h₂

Here P is the pressure (energy), ρ is the density, v is the velocity, h is the height, and g is the gravitational acceleration. In this framework, what the equation says is that the energy per unit volume (which is the sum of pressure, kinetic energy and potential energies) is constant. However, this equation has some limitations. For example, the equation as such is only valid for incompressible fluid behavior. Many liquids and gases behave this way at low Mach velocities. Further forms of the equation have also been developed and generalized for compressible fluid behavior at high Mach velocities.

But Dr. As Babinsky said, in the flight of an airplane, this equation may not come into play. But as he means by saying "he gives the right answer", it is ultimately about pressure differences and the forces arising from them.

Second Possibility: Downstream Theory

Molecules standing still in the air would normally have a linear tendency to move over the wing of a forward plane. In other words, if the wing were straight, the molecules would flow without moving much up or down. But the wing shape forces the overflowing molecules to have a larger volume; because it pushes them upwards, expanding the volume they occupy. Therefore, the pressure of this air decreases. As the pressure of the air decreases, the velocity increases. In other words, the focus is on the opposite of the classical theory that "velocity difference changes pressure": What causes the speed difference is the pressure difference that occurs in the first place. As the classical theory suggests, the molecules are first forced to change their velocity and then their pressure does not.

Air flowing under the wing is forced into a smaller volume; because the wing flies over the air under it and it almost compresses the air below, forcing it into a smaller volume. Therefore, the pressure of this air rises. As the pressure rises, the velocity of the air decreases. This pressure difference creates an upward force on the wing. We call this force the bearing force.

Obviously we know that the classical theory is wrong already: Because, as wind tunnel experiments clearly show, molecules flowing from the top and bottom of the wing do not reach the end of the wing at the same time; molecules flowing from the top arrive much faster and first.

Indeed, this latter theory can explain the behavior of air flowing over and under the blades tested in wind tunnels. It is possible to see this in NASA's simulations. The over-wing molecular velocity seen in the simulations is much higher than predicted by the Equal Path Theory.

The Downstream Theory has a testable and interesting consequence: This movement of the wing should create a downward airflow. Just like if you stand under the wing of a helicopter, you can feel a great air flowing towards you… On airplanes, however, this should be much less palpable; because the wing does not rotate around a center 450-3000 times per minute. But there must still be a downward current; because helicopter blade and airplane wing have the same shape. Studies have shown that, indeed, airplane wings create a downward airflow!

Another important aspect of this theory is that it is more plausible in terms of Newtonian Physics. According to Newton's 3rd Law, every action (action force) must have an equal and opposite reaction (reaction force). Therefore, if there is a lift on an airplane, there must be an equal and opposite reaction force. The downward current is caused by this reaction force. This is independent of the wing shape of the aircraft! As the air flows under the wing, it pushes the aircraft upwards and also creates a backward drag force.

An important point to realize here is that airplane wings are not exactly parallel to the direction of movement during flight. As you can see from the wind tunnel video above, airplane wings fly through the air at a certain angle, even during horizontal flight. This is called the angle of attack, as we mentioned above. Due to this angle, both the air flowing over the wing and the air flowing under the wing are forced downwards. However, since the air flowing over the wing is forced downwards, the reaction force will be greater and the lift force will be created.

 If the angle of attack is too high, a phenomenon called "stall" will occur and the plane may crash.

The Fight Is Not Over!

Airplanes today are built around a much broader theory called the Navier-Stokes Equations; however, this theory alone cannot explain what the factor that creates the bearing force is. Because the Computational Fluid Dynamics (CFD) simulations we have obtained thanks to modern technology show that the pressure zones around the wing are much more complex. Doug McLean, who spent decades as a CFD expert at Boeing and author of a 2012 book called Understanding Aerodynamics: Arguments from Real Physics, says:

The wing's droplet design affects a much larger region called the pressure field. When lift is created, there is always a dense low-pressure area above the wing. Beneath it, a dense high-pressure area is formed. The points where these pressure fields (or pressure clouds) touch the wing are the points where we see the lift force on the wing. In this case, 4 factors are required to create the bearing force: The airflow needs to turn down, the airflow needs to accelerate, a low pressure area is required, and a high pressure area is required.

These four factors interact with each other in different ways and have a mutual cause-effect relationship. Neither can exist without the others. Pressure differences cause the lift force on the wing, but two other factors ensure the continuity of these two pressure areas. There has to be a circularity between these four forces. These four factors seem to be created together and perpetuate each other. I call this circular causation.

While this may sound impossible at first, Newton's Second Law makes it possible. This law, which we all memorized as F=maF=maF=ma, says that the force acting on an object is proportional to its acceleration. Under this law, we know that when pressure exerts a force on a fluid, it must also produce a change in velocity and direction (or both). But in a cyclical way, this pressure difference is also affected by the acceleration of the fluid. So when the wing is not in motion, none of these four factors are created because they cannot support each other. But as the wing moves through the air, each factor begins to affect each other and they all coexist.

Flow separation analysis around the blade

Potential Problems of Downstream Theory

There may be a problem that draws your attention in the above sequence of explanations: Our reason for rejecting the classical theory was that it assumed that air flowing from both sides of the wing must arrive at the end of the wing at the same time. But the second theory makes a similar assumption: The air flowing over the wing is forced downwards due to the oblique shape of the wing. Well, why would air molecules have to follow the shape of an airplane wing? Why don't they bounce off in the direction they were pushed upwards?

There is no clear answer to this question yet. But according to MIT's Mark Drela, this is because of the concept of vacuum. If the air molecules were to fly away in the direction in which they were launched (tangent to the wing), a vacuum would be created below them. This vacuum would quickly pull the surrounding molecules onto it, and the molecules would have filled this vacuum again. Therefore, molecules repelled by the shape of the wing follow the shape of the wing, instantly filling the vacuum they create. But this explanation has not convinced everyone. Babinsky says:

I hate to contradict my dear colleague Mark Drela; but if vacuum were really an explanation, we wouldn't be able to explain the fact that in some cases we actually see air flowing over objects leaving the object surfaces. I think his whole approach is correct but that part is wrong. The problem is that there is no simple and quick explanation for why airplanes can fly.

Long story short, someone asks you, "How does a plane fly?" If he asks, don't expect to be able to explain the whole mechanism in a single sentence. Even experts can't do that! The safest answer would be: "There is a pressure difference on both sides of the wing, but the mechanism that creates this difference has no simple explanation."