Due to which the plane rises upward. The force of rejection is ground training on a pilot and how an airplane flies. Physical phenomena underlying flight control

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. At 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 capable of taking 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.

The 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 increases, the average speed 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 or early morning, when the ambient temperature is below the maximum permissible limit.

It’s quite strange to watch how a multi-ton machine easily rises from the airfield runway and smoothly gains altitude. It would seem that lifting such a heavy structure into the air is an impossible task. But, as we see, this is not so. Why doesn't the plane fall, and why does it fly?

The answer to this question lies in the physical laws that make it possible to lift aircraft into the air. They are true not only for gliders and light sport aircraft, but also for multi-ton transport aircraft that are capable of carrying additional payload. And in general, the flight of a helicopter seems fantastic, which can not only move in a straight line, but also hover in one place.

Flight aircraft became possible thanks to the combined use of two forces - lifting and traction forces of the engines. And if everything is more or less clear with the traction force, then with the lifting force everything is somewhat more complicated. Despite the fact that we are all familiar with this expression, not everyone can explain it.

So, what is the nature of the appearance of lift?

Let's take a close look at the wing of the airplane, thanks to which it can stay in the air. From below it is completely flat, and from above it has a spherical shape, with a convexity outward. While the aircraft is moving, air flows calmly pass under the lower part of the wing without undergoing any changes. But in order to pass over the upper surface of the wings, the air flow must be compressed. As a result, we get the effect of a squeezed pipe through which air must pass.

To go around the spherical surface of the wing, the air will take longer than when passing under the lower, flat surface. For this reason, it moves faster over the wing, which in turn leads to a pressure difference. It is much larger under the wing than above the wing, which is what causes lift. In this case, Bernoulli’s law applies, with which each of us is familiar from school. The most important thing is that the pressure difference will be greater, the higher the speed of the object. So it turns out that lift can only occur when the aircraft is moving. She puts pressure on the wing, forcing it to rise.

As the plane accelerates through runway, the pressure difference also increases, which leads to the emergence of lift. As the speed increases, it gradually increases, becomes equal to the mass of the aircraft, and as soon as it exceeds it, it takes off. After gaining altitude, pilots reduce speed, the lift force is compared with the weight of the aircraft, which causes it to fly in a horizontal plane.

In order for the plane to move forward, it is equipped with powerful engines that drive air flow in the direction of the wings. With their help, you can regulate the intensity of the air flow, and, consequently, the traction force.

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 downward. 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 air environment. 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.

If you fly often or often watch planes on services like , you've probably asked yourself questions about why the plane flies the way it does and not otherwise. What's the logic? Let's try to figure it out.

Why does a plane fly not in a straight line, but in an arc?

If you look at the flight path on the display in the cabin or on the computer at home, it does not look straight, but arched, curved towards the nearest pole (north in the northern hemisphere, south in the southern hemisphere). In fact, throughout almost the entire route (and the longer it is, the fairer it is) it tries to fly in a straight line. It’s just that the displays are flat, and the Earth is round, and the projection of a three-dimensional map onto a flat one modifies its proportions: the closer to the poles, the more curved the “arc” will be. This is very easy to check: take a globe and stretch a thread across its surface between two cities. This will be the shortest route. If you now transfer the line of the thread onto the paper, you will get an arc.

That is, the plane always flies in a straight line?

The plane does not fly as it pleases, but along air routes that are laid, of course, in such a way as to minimize the distance. The routes consist of segments between control points: they can be used as radio beacons, or simply coordinates on the map, which are assigned five-letter designations, most often easy to pronounce and therefore memorable. Or rather, you need to pronounce them letter by letter, but, you see, remembering combinations like DOPIK or OKUDI is easier than GRDFT and UOIUA.

When plotting a route for each specific flight, various parameters are used, including the type of aircraft itself. So, for example, for twin-engine aircraft (and they are actively replacing three- and four-engine aircraft), ETOPS (Extended range twin engine operational performance standards) apply, which regulate route planning in such a way that the aircraft, crossing oceans, deserts or poles, is at the same time within a certain flight time to the nearest airfield capable of receiving this type of aircraft. Thanks to this, if one of the engines fails, it will be able to reliably reach the place where it was committed. emergency landing. Different planes and airlines are certified for different times flight, it can be 60, 120 and even 180 and in rare cases 240 (!) minutes. Meanwhile, it is planned to certify the Airbus A350XWB for 350 minutes, and the Boeing 787 for 330; this would eliminate the need for four-engine aircraft even on routes like Sydney-Santiago (the world's longest commercial route over sea).

By what principle do planes move around the airport?

Firstly, it all depends on which band you are from at the moment take off at the departure airport and land at the arrival airport. If there are several options, then for each of them there are several exit and entry schemes: if you explain it in words, then the plane must proceed to each of the points of the scheme at a certain altitude at a certain (within limits) speed. The choice of runway depends on the current load of the airport, as well as, first of all, the wind. The fact is that both during takeoff and landing the wind must be headwind (or blow from the side, but still from the front): if the wind blows from behind, then the plane, in order to maintain the required speed relative to the air, will have to have too high a speed relative to the ground - maybe the strip is not long enough for take-off or braking. Therefore, depending on the direction of the wind, the plane moves either in one direction or in the other during takeoff and landing, and the runway has two takeoff and landing courses, which, rounded to tens of degrees, are used to designate the runway. For example, if the course is 90 in one direction, then in the other it will be 270, and the strip will be called “09/27”. If, as often happens in major airports, there are two parallel stripes, they are designated as left and right. For example, in Sheremetyevo 07L/25R and 07R/25L, respectively, and in Pulkovo – 10L/28R and 10R/28L.

At some airports, the runways only work in one direction - for example, in Sochi there are mountains on one side, so you can only take off towards the sea and land only from the sea: in any direction the wind will blow from behind either during take-off or landing, so the pilots are guaranteed to experience a little extreme.

Flight patterns in the airport area take into account numerous restrictions - for example, a ban on aircraft flying directly over cities or special zones: these can be both sensitive objects and banal ones cottage villages Rublyovka, whose residents don’t really like the noise overhead.

Why does a plane fly faster in one direction than in the other?

This is a “holiday” question - perhaps more copies have been broken only around the problem with an airplane standing on a moving belt - “whether it will take off or not.” Indeed, the plane flies faster to the east than to the west, and if you get from Moscow to Los Angeles in 13 hours, then you can get back in 12.

That is, it is faster to fly from west to east than from east to west.

The humanist thinks that the Earth is spinning, and when you fly in one direction, the destination gets closer, because the planet manages to turn under you.

If you hear such an explanation, urgently give the person a geography textbook for the sixth grade, where they will explain to him that, firstly, the Earth rotates from west to east (i.e., according to this theory, everything should be the other way around), and secondly, the atmosphere rotates with the Earth. Otherwise it would be possible to rise into the air hot air balloon and hang in place, waiting for the turn to the place where you need to land: free travel!

The technician is trying to explain this phenomenon by the Coriolis force, which acts on the plane in the non-inertial reference frame “Earth-plane”: when moving in one direction, its weight becomes greater, and in the other, accordingly, less. The only trouble is that the difference in the weight of the aircraft created by the Coriolis force is very small even compared to the mass of the payload on board. But that’s not so bad: since when does mass affect speed? You can drive a car at 100 km/h, either alone or with five people. The only difference will be in fuel consumption.

The real reason that a plane flies faster to the east than to the west is that winds at an altitude of several kilometers most often blow from west to east, and so in one direction the wind turns out to be tailwind, increasing the speed relative to the Earth, and in the other - oncoming, slowing down. Why do the winds blow this way? Ask Coriolis, for example. By the way, the study of high-altitude jet streams (these are strong winds in the form of relatively narrow air currents in certain zones of the atmosphere) makes it possible to plot routes in such a way that, once “in the jet,” you can maximize speed and save fuel.

 

It might be useful to read: