An airplane is an aircraft that has a mass greater than the mass of air, and a lift force created according to the aerodynamic principle (throwing down part of the air due to the flow around the wing). Lift is the answer to the question of why airplanes fly. It is created by load-bearing surfaces (mainly wings) when moving towards the air flow of an aircraft, which develops speed using a power plant or turbine. Due to the power plant that creates traction force, the aircraft is able to overcome air resistance.

Airplanes fly according to the laws of physics

Aerodynamics as a science is based on the theorem of Nikolai Egorovich Zhukovsky, an outstanding Russian scientist, the founder of aerodynamics, which was formulated back in 1904. A year later, in November 1905, Zhukovsky outlined his theory of creating the lifting force of an aircraft wing at a meeting of the Mathematical Society.

In order for the lifting force to lift a modern aircraft into the air, even weighing tens of tons, its wing must have sufficient area. The lift of a wing is influenced by many parameters, such as profile, area, wing planform, angle of attack, air speed and density. Each plane has its own minimum speed at which it can take off and fly without crashing. Thus, the minimum speed of modern passenger aircraft is in the range from 180 to 250 km/h.

Why do planes fly at different speeds?

The size of the aircraft also depends on the required speed. The wing area of ​​slow transport aircraft must be large enough, since the lift of the wing and the speed developed by the aircraft are directly proportional. The large wing area of ​​slow aircraft is due to the fact that at sufficiently low speeds the lift force is small.

High-speed aircraft, as a rule, have much smaller wings, while still providing sufficient lift. The lower the air density, the lower the lifting force of the wing becomes, so high altitude The aircraft's speed must be higher than when flying at low altitude.

Why do planes fly so high?

Flight altitude of modern jet aircraft lies between 5,000 and 10,000 meters above sea level. This can be explained very simply: at such an altitude, the air density is much lower, and, therefore, air resistance is lower. Airplanes fly at high altitudes because when flying at an altitude of 10 kilometers, the aircraft consumes 80% less fuel than when flying at an altitude of one kilometer. However, why then do they not fly even higher, in the upper layers of the atmosphere, where the air density is even less? The fact is that to create the necessary thrust by an aircraft engine, a certain minimum supply of air is required. Therefore, every aircraft has a maximum safe flight altitude, also called a “service ceiling.” For example, the service ceiling of the Tu-154 aircraft is about 12,100 meters.

The arrival of summer in some hot corners of our planet brings with it not only sweltering heat, but also flight delays at airports. For example, in Phoenix, Arizona, the air temperature recently reached +48°C and airlines were forced to cancel or reschedule over 40 flights. What is the reason? Don't planes fly when it's hot? They fly, but not at any temperature. According to media reports, heat poses a particular problem for Bombardier CRJ aircraft, which have a maximum takeoff operating temperature of +47.5°C. In the same time, large planes from Airbus and Boeing can fly at temperatures up to +52°С degrees or so. Let's figure out what causes these restrictions.

Lift principle

Before explaining why not every aircraft is able to take off at high air temperatures, it is necessary to understand the very principle of how airplanes fly. Of course, everyone remembers the answer from school: “It’s all about the lift of the wing.” Yes, this is true, but not very convincing. To really understand the laws of physics that are involved here, you need to pay attention to law of momentum. In classical mechanics, the momentum of a body is equal to the product of the mass m of this body and its speed v, the direction of the momentum coincides with the direction of the velocity vector.

At this point, you might think that we are talking about a change in the airplane's momentum. No, instead consider the change in air momentum, impinging on the plane of the wing. Imagine that each air molecule is a tiny ball that collides with an airplane. Below is a diagram that shows this process.

A moving wing collides with balloons(that is, air molecules). The balls change their momentum, which requires the application of force. Since action equals reaction, the force that the wing exerts on the air balloons is the same magnitude as the force that the balloons themselves exert on the wing. This leads to two results. Firstly, the lifting force of the wing is provided. Secondly, a reverse force appears - thrust. You can't achieve lifting without traction..

To generate lift, the plane must move, and to increase its speed, you need more thrust. To be more precise, you need just enough thrust to balance the force of air resistance - then you fly at the speed you want. Typically, this thrust is provided by a jet engine or propeller. Most likely, you could even use a rocket engine, but in any case, you need a thrust generator.

What does the temperature have to do with it?

If the wing hits just one ball of air (that is, a molecule), it will not produce much lift. To increase lift, you need a lot of collisions with air molecules. This can be achieved in two ways:

  • move faster, increasing the number of molecules that come into contact with the wing per unit time;
  • design wings with larger surface area, because in this case the wing will collide with a large number of molecules;
  • Another way to increase the contact surface area is to use greater angle of attack due to the tilt of the wings;
  • finally, it is possible to achieve a greater number of collisions between the wing and air molecules if the density of the air itself is higher, that is, the number of molecules themselves per unit volume is greater. In other words, increasing air density increases lift.

This conclusion brings us to air temperature. What is air? These are many microparticles, molecules that move right around us in different directions and at different speeds. And these particles collide with each other. As the temperature rises average speed the movement of molecules also increases. An increase in temperature leads to expansion of the gas, and at the same time - to a decrease in air density. Remember that heated air is lighter than cold air; the principle of hot air balloon aeronautics is based on this phenomenon.

So, for greater lift, you need either a higher speed, or a larger wing area, or a larger angle of attack of the molecules on the wing. Another condition: the higher the air density, the greater the lifting force. But the opposite is also true: the lower the air density, the lower the lift. And this is true for hot parts of the planet. Due to high temperatures, air density is too low for some aircraft, it is not enough for them to take off.

Of course, you can compensate for the decrease in air density by increasing the speed. But how can this be done in reality? In this case, it is necessary to install more powerful engines on the aircraft, or increase the length of the runway. Therefore, it is much easier for airlines to simply cancel some flights. Or at least move it to the evening, early morning, when the temperature environment will be below the maximum permissible limit.

There is probably no person who, watching a plane fly, has not wondered: “How does it do it?”

People have always dreamed of flying. Icarus can probably be considered the first aeronaut who tried to take off with the help of wings. Then, over the millennia, he had many followers, but the real success fell to the lot of the Wright brothers. They are considered the inventors of the airplane.

Seeing huge passenger airliners on the ground, double decker Boeings, for example, it is completely impossible to understand how this multi-ton metal colossus rises into the air, it seems so unnatural. Moreover, even people who have worked all their lives in industries related to aviation and, of course, know the theory of aeronautics, sometimes honestly admit that they do not understand how airplanes fly. But we will still try to figure it out.

The plane stays in the air thanks to the “lifting force” acting on it, which arises only in the movement provided by the engines mounted on the wings or fuselage.

  • Jet engines throw back a stream of combustion products of kerosene or other aviation fuel, pushing the plane forward.
  • The blades of the propeller engine seem to be screwed into the air and pull the plane along with them.

Lifting force

Lift occurs when the oncoming air flows around the wing. Due to the special shape of the wing section, part of the flow above the wing has a higher speed than the flow under the wing. This happens because the upper surface of the wing is convex, as opposed to the flat lower surface. As a result, the air flowing around the wing from above has to travel a longer distance and, accordingly, at a higher speed. And the higher the flow speed, the lower the pressure in it, and vice versa. The lower the speed, the greater the pressure.

In 1838, when aerodynamics as such did not yet exist, the Swiss physicist Daniel Bernoulli described this phenomenon, formulating a law named after him. Bernoulli, however, described the flow of fluid flows, but with the emergence and development of aviation, his discovery could not have come at a more opportune time. The pressure under the wing exceeds the pressure above and pushes the wing, and with it the plane, upward.

Another component of lift is the so-called “angle of attack”. The wing is located at an acute angle to the oncoming air flow, due to which the pressure under the wing is higher than above.

How fast do planes fly?

To generate lift, a certain, and quite high, speed is required. There is a minimum speed, which is necessary for lifting off the ground, a maximum speed, and a cruising speed, at which the plane flies most of the route, it is about 80% of the maximum. Cruising speed of modern passenger airliners 850-950 km per hour.

There is also the concept of ground speed, which consists of the aircraft’s own speed and the speed of the air currents that it has to overcome. It is on this basis that the flight duration is calculated.

The speed required for takeoff depends on the weight of the aircraft, and for modern passenger ships ranges from 180 to 280 km per hour. Landing is carried out at approximately the same speed.

Height

The flight altitude is also not chosen arbitrarily, but is determined by a large number of factors, fuel economy and safety considerations.

The air near the surface of the earth is more dense, and accordingly, it has greater resistance to movement, causing increased fuel consumption. As altitude increases, the air becomes more rarefied and resistance decreases. The optimal altitude for flight is considered to be about 10,000 meters. Fuel consumption is minimal.

Another significant advantage of flying at high altitudes is the absence of birds, collisions with which have more than once led to disasters.

Climb above 12,000-13,000 meters civil aircraft they cannot, since too much vacuum prevents the normal operation of the engines.

Airplane control

The aircraft is controlled by increasing or decreasing engine thrust. In this case, the speed changes, respectively, the lifting force and flight altitude. For more precise control of the processes of changing altitude and turning, wing mechanization devices and rudders located on the tail unit are used.

Takeoff and landing

In order for the lift to become sufficient to lift the aircraft off the ground, it must develop sufficient speed. This is what runways are used for. Heavy passenger or transport aircraft require long runways, 3-4 kilometers long.

The condition of the strips is carefully monitored airfield services, keeping them in perfectly clean condition, since foreign objects entering the engine can lead to an accident, and snow and ice on the runway pose a great danger during takeoff and landing.

As the plane takes off, there comes a moment after which it is no longer possible to cancel the takeoff, since the speed becomes so high that the plane will no longer be able to stop within the runway. This is called “speed of decision making”.

Landing is a very critical moment of flight; pilots gradually slow down, as a result of which lift decreases and the plane descends. Just before the ground, the speed is already so low that flaps are extended on the wings, which slightly increase the lift and allow the plane to land softly.

Thus, no matter how strange it may seem to us, airplanes fly, and in strict accordance with the laws of physics.



Most of us still sometimes ask ourselves how an aircraft weighing up to 600 tons or more can stay in the air.

From school textbooks it is clear that they rise, obeying the laws of physics, and all flying structures rise, from light sports airplanes to heavy transport aircraft or shapeless helicopters. This happens due to engine traction and lift.

Almost everyone knows the phrase “lifting force,” but not everyone can explain how it happens. But in fact, this action can be explained without getting into mathematical formulas and axioms.

The wing of an aircraft is the main load-bearing surface of the aircraft. Almost always having a certain profile, in which the upper part is convex and the lower part is flat. When the air flow passes under the lower part of the aircraft's profile, there is practically no change in its structure and shape. The air flow, passing over the upper part of the profile, narrows, since for the air flow the upper plane of the profile is like a concave wall in a pipe, it seems to flow through it.

To drive away certain time the same volume of air through this “pressed” pipe, it needs to be moved faster. According to Bernoulli's law, which is taught in the school physics curriculum, the higher the flow speed, the lower its pressure. It follows from this that the pressure above the entire wing, and therefore above the profile, is lower than the pressure below it.

A force is generated that wants to squeeze out the wing, and, consequently, the entire plane. This is called lift. If it becomes greater than the weight of the aircraft, it takes off. The higher the speed, the greater the lift. If the weight of the aircraft and the value of the lifting force are equal, then the aircraft will move to a horizontal position. Its aircraft engine gives it quite good speed, i.e. the traction force it creates.

Using the above principles, it is possible, in theory, to make any object with any mass and shape take off. Not a standard form, i.e. different from airplanes is a helicopter. It is strikingly different from an airplane, but it is lifted into the air for the same reason. A helicopter has a wing with an aerodynamic profile that is the blade of its main rotor.

The blade creates lift by moving in the air flow as the propeller rotates, which lifts it and moves the helicopter forward. This occurs when the rotation angle of the propeller changes, resulting in the appearance of a horizontal component of the lift force, which acts as the thrust force of the aircraft engine.

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 flow lifts the wings, i.e. plane, and the top one pushes up. Thus, it turns out that at high speeds the air under the aircraft becomes solid.

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

In an airplane, 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.