Why do planes fly? Why planes can't take off in extreme heat
People have always wanted to fly. More than 100 years ago, the Wright brothers succeeded in making this dream come true. Flying on the first airplanes was indeed a dangerous undertaking, but over time, flight safety in the world has improved significantly from year to year.
Today, out of 48,000,000 commercial flights per year worldwide, about 28 (!) end in accidents. Only 5–10 plane crashes a year result in fatalities.
And this is out of 48,000,000 flights! Such amazing results were made possible thanks to the tireless work of hundreds of thousands of people and organizations: designers aircraft, aircraft manufacturers, test pilots, electronic navigation systems manufacturers, air traffic controllers, flight simulator developers and many others. Today, aviation is one of the safest life processes, although not absolutely safe.
To increase your level of peace of mind on board, I think it is necessary to convey to you the principle of airplane flight. Indeed, realizing that the plane flies in full accordance with the laws of nature, and not contrary to them, many people become much calmer. People who have a fear of flying tend to think: “I don’t understand how this thing flies. It doesn’t seem to me reliable and safe to “hang out” at an altitude of 10,000 meters, when there is emptiness below you, and depend on the slightest whim of the engine.” In fact, the plane flies not in spite of, but thanks to the laws of nature and in complete harmony with them.
Watch birds: even without flapping their wings, they can soar in the air for quite a long time, and do not fall like a stone, right? Then why, in your opinion, does the plane allegedly depend on the “whim” of the engine? Why, according to the common misconception of aerophobes, a plane must inevitably fall if, God forbid, something happens to the engine or some other system?
The principle by which an airplane stays in the air is this: when an airplane passes through a stream of air at speed, the pressure under the airplane will always be higher than the pressure above it. That is, a “cushion” of air, or rather gas under high pressure, is formed under the plane. The higher the speed, the more difference in pressure, the “thicker” our “cushion”.
During flight, an airplane is affected by 4 main forces that balance each other. These are thrust, resistance, weight and lift. The more weight, the more lift the aircraft needs.
The lift force is generated according to Bernoulli's theorem. Without the desire to introduce readers to the depths of physics and aerodynamics, we can simplify Bernoulli’s theorem and present it in the following form:
L = lift (or "cushion thickness")12 – air densityS – aerodynamic surface area
Lift is generated when there is air density, an airfoil (such as a wing), and speed. The more speed and the larger area wing, the more lift. This is why large, heavy airplanes have large wings and need greater speed to take off.
To better understand this law, try placing your hand out the open window while driving a car, palm down and at an angle of about 45 degrees up. What will happen? That's right, the air flow will lift your arm up, just like it lifts an airplane, since the pressure under your arm will be higher than the pressure above your arm. Be sure to carry out this experiment and within a few minutes, feel the “elasticity” of the air. Your hand is a “mini-wing”, or, to put it more scientifically, an aerodynamic surface.
Most parts of the aircraft are such aerodynamic surfaces: these are the wings, the fuselage, the elevators, and even... the extended landing gear.
Thus,as long as the plane has wings and speed, it can “soar” in the air just like a bird.
One of the most common misconceptions of people who are afraid of flying is that if the engine fails, the plane will fall like a stone! By understanding the basic principle of flight, you now know that this will not happen. Firstly, the plane has at least one more engine, using which the plane can fly to its destination and return back. Secondly, even if two engines fail at once (which happens in the entire huge system called “ world aviation" extremely rarely, on average once every 7–8 years) the aircraft has enough altitude and speed to glide for about 40–45 minutes from cruising altitude. During this time, the aircraft may well reach the nearest airfield and land. Even if the rarest failure of all engines happens far from airports, the plane will most likely be able to glide and splash down, land on a field or other flat area.
The plane simply physically cannot fall down from a cruise altitude.
An airplane in the air is comparable to a fly in a can of condensed milk - the condensed milk tightly holds the fly, preventing it from falling to the bottom.
Likewise, the difference in pressure keeps the plane securely in the air. The only way to make a plane fall uncontrollably is to put it into a so-called tailspin, and for this you need to put in no less effort than to jump from Ostankino Tower. A plane cannot end up in a spin by accident. And as proof, fact - in the last 30 years this has happened only once.
Looking ahead, I will say that most aircraft, when starting to descend from flight level before landing, use an engine mode called “idle throttle”, which does not create thrust. That is, almost every plane plans for about half an hour before landing without using engine thrust. The noise that passengers hear in the aircraft cabin is nothing more than the idle mode of their operation, comparable to the operation of a car engine in neutral gear.
Remember - flight is as natural for an airplane as gravity is for a person. Flight occurs in complete harmony with and thanks to the laws of nature. What SEEMS unnatural and unreliable to you only SEEMS to you. And you must admit that there is often a great distance between what seems and how things really are.
Often, watching a plane flying in the sky, we wonder how the plane gets into the air. How does it fly? After all, an airplane is much heavier than air.
Why does the airship rise
We know that balloons and airships are lifted into the air Archimedes' force . Archimedes' law for gases states: " Nand a body immersed in gas experiences a buoyancy force equal to the force of gravity of the gas displaced by this body.” . This force is opposite in direction to gravity. That is, Archimedes' force is directed upward.
If the force of gravity is equal to the force of Archimedes, then the body is in equilibrium. If the force of Archimedes is greater than the force of gravity, then the body rises in the air. Since the cylinders of balloons and airships are filled with gas, which is lighter than air, the Archimedes force pushes them upward. Thus, the Archimedes force is the lifting force for lighter-than-air aircraft.
But the gravity of the aircraft significantly exceeds the force of Archimedes. Therefore, she cannot lift the plane into the air. So why does it still take off?
Airplane wing lift
The occurrence of lift is often explained by the difference in static pressures of air flows on the upper and lower surfaces of the aircraft wing.
Let's consider a simplified version of the appearance of the lifting force of a wing, which is located parallel to the air flow. The design of the wing is such that the upper part of its profile has a convex shape. The air flow flowing around the wing is divided into two: upper and lower. The speed of the bottom flow remains almost unchanged. But the speed of the top one increases due to the fact that it must cover a greater distance in the same time. According to Bernoulli's law, the higher the flow speed, the lower the pressure in it. Consequently, the pressure above the wing becomes lower. Due to the difference in these pressures, lift, which pushes the wing up, and with it the plane rises. And the greater this difference, the greater the lifting force.
But in this case, it is impossible to explain why lift appears when the wing profile has a concave-convex or biconvex symmetrical shape. After all, here the air flows travel the same distance, and there is no pressure difference.
In practice, the profile of an airplane wing is located at an angle to the air flow. This angle is called angle of attack . And the air flow, colliding with the lower surface of such a wing, is beveled and begins to move downwards. According to law of conservation of momentum the wing will be acted upon by a force directed in the opposite direction, that is, upward.
But this model, which describes the occurrence of lift, does not take into account the flow around the upper surface of the wing profile. Therefore, in this case, the magnitude of the lifting force is underestimated.
In reality, everything is much more complicated. The lift of an airplane wing does not exist as an independent quantity. This is one of the aerodynamic forces.
The oncoming air flow acts on the wing with a force called total aerodynamic force . And lifting force is one of the components of this force. The second component is drag force. The total aerodynamic force vector is the sum of the lift and drag force vectors. The lift vector is directed perpendicular to the velocity vector of the incoming air flow. And the drag force vector is parallel.
The total aerodynamic force is defined as the integral of the pressure around the contour of the wing profile:
Y – lifting force
R – traction
dΩ – profile boundary
R – the amount of pressure around the contour of the wing profile
n – normal to profile
Zhukovsky's theorem
How the lifting force of a wing is formed was first explained by the Russian scientist Nikolai Egorovich Zhukovsky, who is called the father of Russian aviation. In 1904, he formulated a theorem on the lifting force of a body flowing around a plane-parallel flow of an ideal liquid or gas.
Zhukovsky introduced the concept of flow velocity circulation, which made it possible to take into account the flow slope and obtain a more accurate value of the lift force.
The lift of a wing of infinite span is equal to the product of gas (liquid) density, gas (liquid) velocity, circulation flow velocity and the length of a selected section of the wing. The direction of action of the lifting force is obtained by rotating the oncoming flow velocity vector at a right angle against the circulation.
Lifting force
Medium density
Flow velocity at infinity
Flow velocity circulation (the vector is directed perpendicular to the profile plane, the direction of the vector depends on the direction of circulation),
Length of the wing segment (perpendicular to the profile plane).
The amount of lift depends on many factors: angle of attack, air flow density and speed, wing geometry, etc.
Zhukovsky's theorem forms the basis of modern wing theory.
An airplane can only take off if the lift force is greater than its weight. It develops speed with the help of engines. As speed increases, lift also increases. And the plane rises up.
If the lift and weight of an airplane are equal, then it flies horizontally. Airplane engines create thrust - a force whose direction coincides with the direction of movement of the aircraft and is opposite to the direction of drag. Thrust pushes the plane through the air. In horizontal flight at a constant speed, thrust and drag are balanced. If you increase thrust, the plane will begin to accelerate. But drag will also increase. And soon they will balance again. And the plane will fly at a constant, but higher speed.
If the speed decreases, then the lift force becomes less, and the plane begins to descend.
Airplanes, especially up close, are impressive with their size and weight. It remains unclear how such a bulky and heavy object rises into the heavenly heights. Moreover, not even all adults can answer this, and children’s questions can often confuse them.
To clarify this issue, we must turn to physics. The wing of an airplane that takes off creates a force that pushes it upward, that is, into the sky. It is called the lift force of an aircraft. In places where air flow is high, the pressure will be lower, making it easier to move and stay in the air without succumbing to gravity.
What keeps a plane in the air?
In general, the laws of our atmosphere are studied by the science of aerodynamics, which explains this behavior of iron birds. The basis for developing the process of taking off an aircraft was the theorem of the Russian scientist Zhukovsky, which he formulated at the beginning of the last century. The wings of a modern aircraft are so voluminous that the lift force lifts tens of tons of the aircraft's weight into the air.
Besides, a good helper in this matter is the speed; for modern aircraft it ranges from 180 to 250 kilometers per hour. In addition, airplanes fly quite high, which means that the atmospheric pressure there is not so strong and noticeable, and, therefore, it is easier to balance. The air is more rarefied, therefore its resistance is lower. It is curious that at high altitudes, airplanes consume less fuel, which, by the way, is not cheap. That's why they fly so high. True, if they flew directly over the roofs of houses, it would not be particularly convenient for us and it would be very noisy. A passenger plane can fly at an altitude of twelve kilometers above the earth's surface.
The most detailed answer to why you need to turn off electronic devices and why the plane actually flies! This is a must see!
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Flight altitude is one of the most important aviation parameters. In particular, speed and fuel consumption depend on it. Sometimes flight safety depends on the choice of altitude. For example, pilots have to change altitude when there is a sudden change in weather conditions, due to thick fog, thick clouds, an extensive thunderstorm front or a turbulent zone.
What should the flight altitude be?
Unlike the speed of an airplane (where the faster the better), the flight altitude must be optimal. Moreover, each type of aircraft has its own. It would never occur to anyone to compare the altitudes at which, for example, sports, passenger or multi-role combat aircraft fly. And yet, here too there are record holders. 
The first flight altitude record was... three meters. It was to this height that the Wright Flyer aircraft of brothers Wilbur and Orville Wright first flew on December 17, 1903. 74 years later, on August 31, 1977, Soviet test pilot Alexander Fedotov set a world altitude record of 37,650 meters in a MiG-25 fighter. To this day, it remains the maximum flight altitude of a fighter.
At what altitude do passenger planes fly?
Civil airline aircraft rightfully constitute the largest group of modern aviation. As of 2015, there were 21.6 thousand multi-seat aircraft in the world, of which a third - 7.4 thousand - were large wide-body passenger airliners.When determining the optimal flight altitude (flight level), the dispatcher or crew commander is guided by the following. As is known, than more height, the thinner the air and the easier it is for the plane to fly - so it makes sense to fly higher. However, the wings of an airplane need support, and at the extreme high altitude(for example, in the stratosphere) it is clearly not enough, and the car will begin to “fall over” and the engines will stall.
The conclusion suggests itself: the commander (and today the on-board computer) chooses the “golden mean” - the ideal ratio of friction force and lift force. As a result, each type passenger airliners(subject to weather conditions, technical characteristics, duration and direction of flight) its own optimal altitude.
Why do planes fly at an altitude of 10,000 meters?
In general, the flight altitude of civil aircraft varies from 10 to 12 thousand meters when flying to the west and from 9 to 11 thousand meters when flying to the east. 12 thousand meters is maximum height For passenger aircraft, above which the engines begin to “suffocate” from lack of oxygen. Because of this, an altitude of 10,000 meters is considered the most optimal. 
At what altitude do fighter jets fly?
The altitude criteria of fighters are somewhat different, which is explained by their purpose: depending on the task at hand, combat operations have to be conducted at different altitudes. The technical equipment of modern fighters allows them to operate in a range from several tens of meters to tens of kilometers.
However, exorbitant heights for fighters are “out of fashion” these days. And there is an explanation for this. Modern air defense systems and air-to-air fighter missiles are capable of destroying targets at any altitude. Therefore, the main problem for a fighter is to detect and destroy the enemy earlier, while remaining unnoticed. The optimal flight altitude of a 5th generation fighter (service ceiling) is 20,000 meters.
Some researchers had crazy ideas - they wanted to fly, but why was the result so disastrous? For a long time there have been attempts to attach wings to oneself, and, flapping them, fly into the sky like birds. It turned out that human strength is not enough to lift oneself on flapping wings.
The first folk craftsmen were naturalists from China. Information about them was recorded in the Tsang-han-shu in the first century AD. Further history is replete with cases of this kind, which occurred in Europe, Asia, and Russia.
The first scientific basis for the process of flight was given by Leonardo da Vinci in 1505. He noticed that birds do not have to flap; they can stay in still air. From this, the scientist concluded that flight is possible when the wings move relative to the air, i.e. when they flap their wings in the absence of wind or when their wings are motionless.
Why is the plane flying?
Lifting force helps to keep it in the air, which acts only at high speeds. The special contraction of the wing allows for the creation of lift. The air that moves above and below the wing undergoes changes. Above the wing it is sparse, and below the wing it is sparse. Two air flows are created, directed vertically. The lower one raises the wings, i.e. plane, and the top one pushes up. Thus, it turns out that at high speeds the air is under aircraft becomes hard.
This is how vertical motion is realized, but what makes the plane move horizontally? - Engines! The propellers seem to be drilling a path into airspace overcoming air resistance.
Thus, the lift force overcomes the force of gravity, and the traction force overcomes the braking force, and the plane flies.
Physical phenomena underlying flight control
Everything is kept in balance by the lifting force and the force of gravity. The plane is flying straight. Increasing the flight speed will increase the lift force, the plane will begin to rise. To neutralize this effect, the pilot must lower the nose of the aircraft.
Reducing the speed will have the opposite effect, requiring the pilot to raise the nose of the aircraft. If this is not done, a crash will occur. Due to the above features, there is a risk of crashing when the aircraft loses altitude. If this happens close to the surface of the earth, the risk is almost 100%. If this happens high above the ground, the pilot will have time to increase speed and gain altitude.
