Moments of forces acting on the aircraft. Forces acting on an aircraft when the aircraft descends. Forces acting on an aircraft during gliding
An airplane is an aircraft that is many times heavier than air. In order for it to fly, a combination of several conditions is needed. It is important to combine the correct angle of attack with many different factors.
Why does he fly
Basically, flight aircraft is the result of the action of several forces on the aircraft. The forces acting on the aircraft arise when air currents move towards the wings. They are rotated at a certain angle. In addition, they always have a special streamlined shape. Thanks to this, they "get up in the air."
The process is affected by the altitude of the aircraft, and its engines accelerate. Burning, kerosene provokes the release of gas, which breaks out with great force. Screw engines lift the aircraft up.
About the corner
Back in the 19th century, researchers proved that a suitable angle of attack is an indicator of 2-9 degrees. If it turns out to be less, then there will be little resistance. At the same time, lift force calculations show that the figure will be small.
If the angle is steeper, then the resistance will become greater, and this will turn the wings into sails.
One of the most important criteria in an airplane is the ratio of lift to drag. quality, and the greater it is, the less energy the aircraft will need when flying.
About lift
The lift force is a component of the aerodynamic force, it is perpendicular to the aircraft motion vector in the flow and arises due to the fact that the flow around the vehicle is asymmetrical. The lift force formula looks like this.
How is lift generated?
In current aircraft, the wings are a static structure. It will not create lift by itself. Lifting a heavy machine up is possible due to the gradual acceleration to climb the aircraft. In this case, the wings, which are placed at an acute angle to the flow, form a different pressure. It becomes smaller above the structure and increases below it.
And thanks to the difference in pressure, in fact, the aerodynamic force arises, the height is gained. What indicators are represented in the lift force formula? An asymmetrical wing profile is used. On this moment the angle of attack does not exceed 3-5 degrees. And this is enough for modern aircraft to take off.
Since the creation of the first aircraft, their design has been significantly changed. At the moment, the wings have an asymmetrical profile, their upper metal sheet is convex.
The bottom sheets of the structure are even. This is done so that the air flows through without any obstacles. In fact, the lift force formula is implemented in practice in this way: the upper air flows pass long way due to the bulge of the wings compared to the lower ones. And the air behind the plate remains in the same amount. As a result, the upper air flow moves faster, and an area with lower pressure is formed there.
The difference in pressure above and below the wings, together with the operation of the engines, leads to a climb to the desired height. It is important that the angle of attack is normal. Otherwise, the lifting force will fall.
The higher the speed of the apparatus, the higher, according to the lift formula, the indicator of the latter. If the speed is equal to the mass, the aircraft goes into a horizontal direction. Speed is created by the operation of aircraft engines. And if the pressure over the wing has dropped, it can be seen immediately with the naked eye.
If the aircraft maneuvers suddenly, then a white jet appears above the wing. This is the condensate of water vapor, which is formed due to the fact that the pressure drops.
About coefficient
The lift coefficient is a dimensionless quantity. It directly depends on the shape of the wings. The angle of attack also matters. It is used when calculating the lifting force when the speed and air density are known. The dependence of the coefficient on the angle of attack is displayed clearly during flight tests.
About aerodynamic laws
When an aircraft moves, its speed, other characteristics of movement change, as well as the characteristics of the air currents that flow around it. At the same time, the flow spectra also change. This is an unsteady movement.
To better understand this, simplifications are needed. This will greatly simplify the conclusion, and the engineering value will remain the same.
First, it is best to consider steady motion. This means that they will not change over time.
Secondly, it is better to accept the hypothesis of the continuity of the medium. That is, the molecular motions of air are not taken into account. Air is considered as an inseparable medium with a constant density.
Thirdly, it is better to accept that the air is not viscous. In fact, its viscosity is zero, and there are no internal friction forces. That is, the boundary layer is removed from the flow spectrum, and drag is not taken into account.
Possession of the main aerodynamic laws allows you to build mathematical models of how an aircraft is flown around by air currents. It also allows you to calculate the indicator of the main forces, which depend on how the pressure is distributed over the aircraft.
How an airplane is flown
Of course, in order for the flight process to be safe and comfortable, wings and an engine alone will not be enough. It is important to manage a multi-ton machine. And the accuracy of taxiing during takeoff and landing is very important.
For pilots, landing is considered a controlled fall. In its process, there is a significant decrease in speed, and as a result, the car loses height. It is important that the speed is chosen as accurately as possible to ensure a smooth fall. This is what causes the chassis to touch the strip softly.
The control of an aircraft is fundamentally different from the control of a ground vehicle. The steering wheel is needed to tilt the car up and down, to create a roll. "Toward" means to climb, and "away" means to dive. To change course, you need to press the pedals, and then use the steering wheel to correct the slope. This maneuver in the language of pilots is called a "turn" or "turn".
In order for the car to turn around and stabilize the flight, there is a vertical keel in the tail of the device. Above it are "wings", which are horizontal stabilizers. It is thanks to them that the aircraft does not descend and does not gain altitude spontaneously.
Elevators are placed on the stabilizers. To make engine control possible, levers were placed at the pilots' seats. When the plane takes off, they are moved forward. Takeoff means maximum thrust. It is needed in order for the aircraft to gain takeoff speed.
If a heavy machine sits down, the levers are retracted. This is the minimum thrust mode.
You can watch how, before sitting down, the rear parts big wings descend down. They are called flaps and perform a number of tasks. As the plane descends, the extended flaps slow down the aircraft. This does not allow her to accelerate.
If the plane is landing, and the speed is not too high, the flaps perform the task of creating additional lift. Then the height is lost quite smoothly. When a car takes off, the flaps help keep the plane in the air.
Conclusion
Thus, modern aircraft are real airships. They are automated and reliable. Their trajectories of movement, the entire flight lends itself to a fairly detailed calculation.
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The mechanical effect of the oncoming flow on the aircraft is reduced to loads continuously distributed over its surface. For ease of study, these distributed loads lead to a resultant force applied at the center of mass of the aircraft, which is called the aerodynamic force and is denoted (see Fig. 22), as well as a moment around the center of mass, which is called the aerodynamic moment and is denoted by .
Rice. 22. Aerodynamic force and aerodynamic moment acting on an aircraft when it is flowed around by an oncoming flow
Theoretical and experimental studies have shown that the magnitude of the aerodynamic force is directly proportional to the velocity head of the oncoming flow and the characteristic area of the streamlined body S:
, (32)
Where C R- coefficient of proportionality, which is called the coefficient of aerodynamic force.
The aerodynamic moment is also directly proportional to the dynamic pressure, the characteristic area S and the characteristic linear size of the streamlined body l:
, (33)
Where m- coefficient of proportionality, which is called the coefficient of aerodynamic moment.
The characteristic area and characteristic size are taken, respectively, for the areas and dimensions of those parts of the aircraft that contribute the main share to the creation of the calculated force or moment.
Let us decompose the aerodynamic force into components along the axes of the coupled and velocity coordinate systems. In the associated coordinate system, these projections are denoted and named as follows:
– aerodynamic longitudinal force;
is the aerodynamic normal force;
is the aerodynamic transverse force.
In the velocity coordinate system:
is the drag force;
– aerodynamic lifting force;
is the aerodynamic side force.
On fig. 23 shows the projections of the aerodynamic force on the axes of the coupled and velocity coordinate systems in the absence of slip.
Rice. 23. Expansion of the aerodynamic force along the axes of the coupled and velocity coordinate systems for b = 0
In the future, we will deal mainly with the projections of the aerodynamic force on the axes of the velocity coordinate system. Using formula (32), we write down the expressions for these projections. In this case, as a characteristic, we will take the characteristic area of the element that plays the main role in creating this force.
Thus, the drag force of an aircraft is the sum of the drag forces of the fuselage, wing, empennage and other parts of the aircraft. For the characteristic area, you can take the area of the midsection of the fuselage S m.f:
, (34)
Where C xa is the drag coefficient.
The wing plays the main role in creating the lift force of the aircraft, therefore, the wing area is taken as characteristic S cr:
, (35)
Where C ya is the lift coefficient.
The aerodynamic side force is mainly determined by the vertical tail and fuselage, the wing, horizontal tail and other parts of the aircraft make a much smaller contribution to the creation of this force. Since the vertical tail is the main element in creating lateral force (it is intended for this), then its area S v.o and are taken as characteristic:
, (36)
Where Cza is the lateral force coefficient.
Since the aerodynamic moments acting on the aircraft are calculated mainly relative to the associated coordinate axes, we will find the projections of the moment on the axes of the associated coordinate system (see Fig. 24).
Rice. 24. Components of the aerodynamic moment
in the bound coordinate system
X is called the roll moment. It is determined mainly by the forces acting on the wing of the aircraft and, to a lesser extent, on the vertical and horizontal tail:
, (37)
Where mx is the roll moment coefficient.
Aerodynamic moment about axis 0 Y is called the yaw moment. It is created by forces acting mainly on the vertical tail and fuselage. This moment is calculated using the following formula:
, (38)
Where m y is the yaw moment coefficient;
L c.o - shoulder of the vertical tail (distance from the point of application of the aerodynamic force arising on the vertical tail to the center of mass of the aircraft).
Aerodynamic moment about axis 0 Z is called the pitching moment. It is created by forces acting on the wing, horizontal tail and fuselage. The vertical tail practically does not participate in the creation of the pitching moment. The pitching moment is calculated by the formula.
SCHEME OF MOMENTS AFFECTING AN AIRCRAFT IN A RELATED COORDINATE SYSTEM.
Moments acting on the aircraft. According to the origin and nature of the impact on the aircraft, the moments are divided into steering, static And rotational. Steering torques occur when the aircraft control surface of the elevator (stabilizer), ailerons, and rudder deviate. Steering moments also include moments that occur when the flaps, brake flaps, landing gear, brake parachutes, etc. are extended. Static moments are due to a change in the angle of attack or slip. A static moment is stabilizing if it tends to eliminate the change in angle or that caused it. If the moment tends to increase the angle or , then such a moment is called destabilizing. Rotational moments are directed against rotation and are damping moments. When the aircraft rotates around one axis, moments can occur relative to other axes. For example, when an aircraft rotates (around the axis), a moment arises about the axis. Such moments are called crossovers.
All forces applied to the aircraft can be transferred to the center of gravity O (Fig. 8). If point O is the origin of a rectangular coordinate system, then the axes of the coordinate system can be directed along the longitudinal axis of the aircraft - the axis, along the vertical axis - the axis, along the transverse axis - the axis. The moments acting on the aircraft can be represented by three components - 
, which are directed along the corresponding coordinate axes.
Rice. 8. Forces and moments acting on an aircraft in flight
Under the action of forces, the aircraft performs translational movements in the directions of the axes of the coordinate system with linear accelerations 
. The influence of moments creates rotational movements about the axes of the coordinate system with angular velocities. Thus, the aircraft has six degrees of freedom: three degrees of translational motion and three - rotational.
