Mankind has long been interested in the question of how it happens that a multi-ton aircraft easily rises to heaven. How does takeoff take place and how do planes fly? When an airliner moves at high speed along the runway, the wings develop lift and work from the bottom up.
When the aircraft moves, a pressure difference is generated between the lower and upper sides of the wing, which results in a lift force that keeps the aircraft in the air. Those. high air pressure from below pushes the wing up, while low air pressure from above pulls the wing towards itself. As a result, the wing rises.
To take off an airliner, it needs a sufficient takeoff run. Wing lift increases as speed increases., which should exceed the takeoff limit. Then the pilot increases the angle of takeoff, pulling the steering wheel towards you. The bow of the liner rises up, and the car rises into the air.
Then retractable landing gear and exhaust lights. In order to reduce the wing lift, the pilot gradually retracts the mechanization. When the airliner reaches the required level, the pilot sets standard pressure, and engines - nominal mode. To see how the plane takes off, we suggest watching the video at the end of the article.
The ship takes off at an angle. From a practical point of view, this can be explained as follows. The elevator is a movable surface, by controlling which you can cause the aircraft to deviate in pitch.
The elevator can control the pitch angle, i.e. change the rate of climb or loss of altitude. This is due to a change in the angle of attack and lift force. By increasing the speed of the engine, the propeller starts spinning faster and lifts the airliner up. Conversely, by directing the elevators down, the nose of the aircraft goes down, while the engine speed should be reduced.
The tail section of an airliner equipped with a rudder and brakes on both sides of the wheels.
How do airliners fly
When answering the question why planes fly, one should remember the law of physics. The pressure difference affects the lift force of the wing.
The flow rate will be greater if the air pressure is low and vice versa.
Therefore, if the speed of an airliner is high, then its wings acquire lift, which pushes the aircraft.
Some circumstances also influence the lifting force of an airliner's wing: the angle of attack, the speed and density of the air flow, the area, profile and shape of the wing.
Modern liners have minimum speed from 180 to 250 km/h, at which takeoff is carried out, plans in the sky and does not fall.
Flight altitude
What is the maximum and safe altitude of the aircraft.
Not all ships have the same flight altitude, "air ceiling" can fluctuate at height from 5000 to 12100 meters. At high altitudes, the air density is minimal, while the liner achieves the lowest air resistance.
The engine of the liner needs a fixed volume of air for combustion, because the engine will not create the necessary thrust. Also, when flying at high altitude, the aircraft saves fuel up to 80%, in contrast to the altitude up to a kilometer.
What keeps the plane in the air
To answer why airplanes fly, it is necessary to analyze in turn the principles of its movement in the air. A jet airliner with passengers on board reaches several tons, but at the same time, it easily takes off and carries out a thousand-kilometer flight.
The movement in the air is also affected by the dynamic properties of the apparatus, the design of the units that form the flight configuration.
Forces affecting the movement of an aircraft in the air
The operation of an airliner begins with the engine starting. Small craft are powered by piston engines that turn propellers to create thrust to help the aircraft move through the air.
Large airliners are powered by jet engines, which emit a lot of air during operation, while the jet force propels the aircraft forward.
Why does an airplane take off and stay in the air for a long time? Because the shape of the wings has a different configuration: rounded on top and flat on the bottom, then the air flow on both sides is not the same. On top of the wings, the air glides and becomes rarefied, and its pressure is less than the air below the wing. Therefore, through uneven air pressure and the shape of the wings, a force arises that leads to the takeoff of the aircraft upwards.
But in order for an airliner to easily take off from the ground, it needs to take off at high speed along the runway.
From this follows the conclusion that in order for an airliner to be unhindered in flight, it needs moving air, which cuts through the wings and creates lift.
Airplane takeoff and speed
Many passengers are interested in the question, what speed does the plane develop during takeoff? There is a misconception that the takeoff speed for each aircraft is the same. To answer the question, what is the speed of the aircraft during takeoff, you should pay attention to important factors.
- The airliner does not have a strictly fixed speed. The lifting force of an air liner depends on its mass and the length of the wings.. Takeoff is performed when a lift force is created in the oncoming flow, which is much greater than the mass of the aircraft. Therefore, the takeoff and speed of the aircraft depends on wind direction, atmospheric pressure, humidity, precipitation, runway length and condition.
- To create lift and successfully lift off the ground, the aircraft needs to gain maximum takeoff speed and sufficient takeoff run. This requires long runways. The larger the aircraft, the longer the runway required.
- Each aircraft has its own scale of takeoff speeds, because they all have their own purpose: passenger, sport, cargo. The lighter the aircraft, the lower the takeoff speed and vice versa.
Boeing 737 passenger jet takeoff
- The takeoff run of an airliner on the runway begins when the engine will reach 800 rpm per minute, the pilot slowly releases the brakes and holds the control stick at neutral. The aircraft then continues on three wheels;
- Before taking off from the ground the speed of the liner should reach 180 km per hour. Then the pilot pulls the lever, which leads to the deflection of the flaps - flaps and raising the nose of the aircraft. Further acceleration is carried out on two wheels;
- After, with a raised bow, the airliner accelerates on two wheels to 220 km per hour, and then take off from the ground.
Therefore, if you want to know in more detail how the plane takes off, to what height and at what speed, we offer you this information in our article. We hope you enjoy your air travel.
Man has always dreamed of flying in the sky. Remember the story of Icarus and his son? This, of course, is just a myth and we will never know how it really happened, but this story fully reveals the thirst to soar in the sky. The first attempts to fly into the sky were made with the help of a huge one, which is now more of a means for romantic walks in the sky, then the airship appeared, and with it, planes and helicopters later appeared. Now it is almost no news or something unusual for anyone that you can fly in 3 hours by plane to another continent. But how does it happen? Why do planes fly and don't crash?
The explanation from a physical point of view is quite simple, but it is more difficult to implement it in practice.
For many years, various experiments were carried out to create a flying machine, many prototypes were created. But to understand why airplanes fly, it is enough to know Newton's second law and be able to reproduce it in practice. Now people, or rather engineers and scientists, are already trying to create a machine that would fly at colossal speeds, several times higher than the speed of sound. That is, the question is no longer how airplanes fly, but how to make them fly faster.
Two things for an airplane to take off are powerful engines and proper wing design.
The engines create tremendous thrust that pushes forward. But this is not enough, because you also need to go up, and in this situation it turns out that so far we can only accelerate along the surface to great speed. The next important point is the shape of the wings and the body of the aircraft itself. It is they who create the uplifting force. The wings are made in such a way that the air below them becomes slower than above them, and as a result, it turns out that the air from below pushes the body up, and the air above the wing is unable to resist this effect when the aircraft reaches a certain speed. This phenomenon is called lift in physics, and to understand this in more detail, you need to have a little knowledge of aerodynamics and other related laws. But to understand why airplanes fly, this knowledge is enough.
Landing and takeoff - what is needed for this car?
An airplane needs a huge runway, or rather, a long runway. This is due to the fact that he first needs to gain a certain speed for takeoff. In order for the lifting force to begin to act, it is necessary to accelerate the aircraft to such a speed that the air from below the wings begins to lift the structure up. The question of why planes fly low concerns precisely this part when the car is taking off or landing. A low start makes it possible for an airplane to rise very high into the sky, and we often see this in clear weather - scheduled planes, leaving a white trail behind them, move people from one point to another much faster than can be done using land transport or sea.
Aircraft fuel
Also interested in why planes fly on kerosene. Yes, basically it is, but the fact is that some types of equipment use the usual gasoline and even diesel fuel as fuel.
But what is the advantage of kerosene? There are several of them.
The first, perhaps, can be called its cost. It is much cheaper than gasoline. The second reason can be called its lightness, in comparison with the same gasoline. Also, kerosene tends to burn, so to speak, smoothly. In cars - cars or trucks - we need the ability to abruptly turn on and off the engine when the aircraft is designed to start it and constantly keep the turbines moving at a given speed for a long time, if we talk about passenger aircraft. Light-engine aircraft, which is not designed to transport huge cargo, but for the most part is associated with the military industry, agriculture, etc. (such a car can only accommodate up to two people), is small and maneuverable, and therefore gasoline is suitable for this area. Its explosive combustion is suitable for the type of turbines that are installed in light aircraft.
Helicopter - a competitor or a friend of the aircraft?
An interesting invention of mankind associated with movement in airspace is a helicopter. He has the main advantage over the aircraft - vertical takeoff and landing. It does not require a huge space for acceleration, and why do planes fly only from seats equipped for this purpose? That's right, you need a sufficiently long and smooth surface. Otherwise, the outcome of the landing somewhere in the field may become fraught with the destruction of the machine, and even worse - human casualties. A helicopter landing can be made on the roof of a building, which is adapted, in a stadium, etc. This function is not available for an aircraft, although designers are already working to combine power with vertical takeoff.
Often, watching an airplane flying in the sky, we wonder how the plane rises into the air. How does he 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 strength of Archimedes . Archimedes' law for gases states: " Hand a body immersed in a gas is subjected to a buoyant force equal to the force of gravity of the gas displaced by this body. . This force is opposite in direction to gravity. That is, the force of Archimedes is directed upwards.
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 a gas that is lighter than air, the Archimedes force pushes them up. Thus, the Archimedes force is the lifting force for aircraft lighter than air.
But the gravity of the aircraft is much greater than the force of Archimedes. Therefore, she cannot lift the plane into the air. So why is he still flying?
Aircraft wing lift
The emergence of lift is often explained by the difference in static pressure of air flows on the upper and lower surfaces of the wing of the aircraft.
Consider a simplified version of the appearance of the lifting force of the 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 around the wing is divided into two: upper and lower. The bottom flow rate remains virtually unchanged. But the speed of the upper one increases due to the fact that it must overcome a greater distance in the same time. According to Bernoulli's law, the higher the flow rate, the lower the pressure in it. Consequently, the pressure over the wing becomes lower. Due to the difference in these pressures, lifting force, 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 the lift force appears when the wing profile has a concave-convex or biconvex symmetrical shape. After all, here the air flows pass the same distance, and there is no pressure difference.
In practice, the wing profile of an aircraft is at an angle to the airflow. This corner is called angle of attack . And the air flow, colliding with the lower surface of such a wing, is beveled and acquires a downward movement. 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 fact, everything is much more complicated. The lift force of an aircraft 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 full aerodynamic force . And the 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 vectors. The lift force 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 airfoil:
Y - lift force
R – traction
dΩ – profile boundary
R is the pressure value around the contour of the wing profile
n – profile normal
Zhukovsky's theorem
How the wing lift is formed was first explained by the Russian scientist Nikolai Yegorovich Zhukovsky, who is called the father of Russian aviation. In 1904, he formulated a theorem on the lifting force of a body in 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 lifting force.
The lift force of an infinite span wing is equal to the product of the density of the gas (liquid), the velocity of the gas (liquid), the circulation velocity of the flow, and the length of the selected segment of the wing. The direction of the lift force is obtained by turning the velocity vector of the oncoming flow at a right angle against the circulation.
lifting force
Medium density
Flow rate at infinity
Flow velocity circulation (the vector is directed perpendicular to the plane of the profile, the direction of the vector depends on the direction of circulation),
The length of the wing segment (perpendicular to the profile plane).
The amount of lift depends on many factors: the angle of attack, the density and speed of the air flow, the geometry of the wing, etc.
Zhukovsky's theorem is the basis of modern wing theory.
An aircraft can only take off if the lift force is greater than its weight. It develops speed with the help of engines. As the speed increases, the lift also increases. And the plane takes off.
If the lift and weight of the aircraft are equal, then it flies horizontally. Aircraft engines create thrust - a force whose direction coincides with the direction of movement of the aircraft and is opposite to the direction of drag. The thrust pushes the aircraft through the air. In level flight at a constant speed, thrust and drag are balanced. If you increase the thrust, the plane will begin to accelerate. But the frontal resistance 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 also decreases, and the aircraft begins to decline.
There is probably no person who, looking at how an airplane flies, did not ask himself the question: “How does he do it?”
People have always dreamed of flying. The first aeronaut who tried to take off with the help of wings can probably be considered Icarus. Then, for thousands of years, he had many followers, but the real success fell to the Wright brothers. They are considered the inventors of the aircraft.
Seeing huge passenger liners on the ground, double-decker Boeings, for example, it is absolutely 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, who know the theory of aeronautics, sometimes honestly admit that they do not understand how airplanes fly. But we'll still try to figure it out.
The aircraft is kept in the air thanks to the “lift force” acting on it, which occurs only in the movement, which is provided by engines mounted on the wings or fuselage.
- Jet engines throw back a stream of products of combustion of kerosene or other aviation fuel, pushing the aircraft forward.
- The blades of the propeller engine seem to be screwed into the air and pull the plane behind them.
lifting force
Lift is generated when air flows over the wing. Due to the special shape of the wing section, the part of the flow above the wing has a greater speed than the flow under the wing. This is because the top surface of the wing is convex, as opposed to the flat bottom. As a result, the air flowing around the wing from above has to travel a longer distance, correspondingly at a higher speed. And the higher the flow rate, the lower the pressure in it, and vice versa. The slower the speed, the greater the pressure.
In 1838, when aerodynamics as such did not yet exist, the Swiss physicist Daniel Bernoulli described this phenomenon by formulating the law named after him. Bernoulli, however, described the flow of fluid flows, but with the emergence and development of aviation, his discovery turned out to be most welcome. The pressure under the wing exceeds the pressure from above and pushes the wing, and with it the aircraft, up.
Another term for 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 on top.
How fast do planes fly
For the emergence of lifting force, a certain, and rather high, speed of movement is necessary. There is a minimum speed, it is necessary to lift off the ground, maximum, and cruising, at which the aircraft flies most of the route, it is about 80% of the maximum. The cruising speed of modern passenger liners is 850-950 km per hour.
There is also the concept of ground speed, which is the sum of the own speed of the aircraft and the speed of the air currents that it has to overcome. It is on the basis of it that the duration of the flight is calculated.
The speed required for takeoff depends on the mass of the aircraft, and for modern passenger ships it ranges from 180 to 280 km per hour. Approximately at the same speed landing is made.
Height
The flight altitude is also not chosen arbitrarily, but is determined by a large number of factors, fuel economy and safety considerations.
At the surface of the earth, the air is denser, respectively, it has a large resistance to movement, causing increased fuel consumption. As altitude increases, the air becomes thinner and drag decreases. The optimal height for flight is considered to be about 10,000 meters. The fuel consumption is minimal.
Another significant advantage of flying at high altitudes is the absence of birds here, collisions with which have repeatedly led to disasters.
Civil aircraft cannot rise above 12,000-13,000 meters, since too much vacuum interferes with the normal operation of the engines.
Aircraft control
Aircraft control is carried out by increasing or decreasing engine thrust. This changes the speed, respectively, the lifting force and flight altitude. For finer control of the processes of changing the height and turns, the means of mechanization of the wing and the rudders located on the tail unit are used.
Takeoff and landing
In order for the lift force to become sufficient to lift the aircraft off the ground, it must develop sufficient speed. This is what airstrips are for. For heavy passenger or transport aircraft, long runways are needed, 3-4 kilometers long.
Airfield services carefully monitor the condition of the runways, maintaining them in a 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.
When the aircraft is taking off, there comes a moment after which it is no longer possible to cancel the takeoff, since the speed becomes so high that the aircraft will no longer be able to stop within the runway. This is what is called - "the speed of decision-making."
Landing is a very crucial moment of the flight, the pilots gradually slow down, as a result of which the lift decreases and the aircraft decreases. Just before the ground, the speed is already so low that flaps are released on the wings, which somewhat increase the lift and allow the aircraft to land softly.
Thus, no matter how strange it may seem to us, planes fly, and in strict accordance with the laws of physics.
The arrival of summer in some hot corners of our planet brings with it not only exhausting 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 in the heat? They fly, but not at any temperature. According to media reports, the heat is a particular problem for Bombardier CRJ aircraft, which have a maximum take-off operating temperature of +47.5°C. In the same time, large aircraft from Airbus and Boeing can fly at temperatures up to +52°С degrees or so. Let's take a look at the reasons for these restrictions.
lift principle
Before explaining why not every plane 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 stage, you might think that we are talking about changing the momentum of the aircraft. No, instead consider the change in air momentum incident on the wing plane. Imagine that each air molecule is a tiny ball that hits 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 the action is equal to the reaction, the force that the wing exerts on the air balloons is of the same magnitude as the force that the balloons themselves exert on the wing. This leads to two results. First, the lifting force of the wing is provided. Secondly, there is a reverse force - thrust. You can't reach the lift without pulling.
In order to generate lift, the plane must move, and in order to increase its speed, you need more thrust. To be more precise, you need exactly the right amount of 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 a propeller. Most likely, you could even use a rocket engine, but in any case - you need a thrust generator.
What is the temperature here?
If the wing collides with just one ball of air (i.e. a molecule), this will not lead to much lift. It takes a lot of collisions with air molecules to increase lift. 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 the wings more 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 a larger angle of attack due to the tilt of the wings;
- finally, a greater number of collisions of the wing with air molecules can be achieved if air density is higher, that is, the number of molecules themselves per unit volume is greater. In other words, an increase in air density increases lift.
This conclusion brings us to air temperature. What is air? This is a lot of 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, the average speed of the 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, it is on this phenomenon that the principle of aeronautics of hot air balloons is built.
So, for more 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 value, the greater the lifting force. But the reverse is also true: the lower the air density, the lower the lift. And this is true for hot corners of the planet. Due to the high temperature, the 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 ambient temperature is below the maximum allowable limit.
