The term "derivation" has many meanings in everyday life. It is formed by the Latin word derivative, which means "abduction", "deviation". The term in the general sense is understood as a deviation from the trajectory, a departure from fundamental values.
Military derivation
With regard to shooting from a firearm, derivation denotes the deviation of the trajectory of a bullet or projectile. It is caused by their rotation, which occurs due to rifling in the bore of a firearm. Derivation is also the deflection of a bullet caused by the gyroscopic effect and Magnus.
Forces acting on a bullet
Bullets while moving along the trajectory after exiting the barrel experience the action of gravity and air resistance. The first force is always downward, causing the thrown body to descend.
The force of air resistance, constantly acting on the bullet, slows down its forward movement and is always directed towards. She does everything possible to overturn the flying body, direct its head part back.
Due to the impact of theseforces, the movement of the bullet does not occur in accordance with the line of throw, but along an uneven, curved curve below the line of throw, which is called the trajectory.
The force of air resistance owes its occurrence to several factors, namely: friction, turbulence, ballistic wave.
Bullet and Friction
Air particles in direct contact with the bullet (projectile), due to contact with its surface, move with it. The layer following the first layer of air particles, due to the viscosity of the air medium, also begins to move. However, at a slower rate.
This layer transfers motion to the next layer and so on. As long as the air particles cease to be affected, their velocity relative to the flying bullet becomes zero. The air environment, starting from the one directly in contact with the bullet (projectile) and ending with the one in which the particle velocity becomes equal to 0, is called the boundary layer.
It generates "tangential stresses", in other words - friction. It reduces the distance of the bullet (projectile), slowing down its speed.
Processes in the boundary layer
The boundary layer surrounding the flying body breaks off when it reaches the bottom. In this case, a space of rarefaction arises. A pressure difference is formed that acts on the bullet head and its bottom. This process generates a force whose vector is directed in the opposite direction to the movement. Air particles rushing into the rarefied area create areas of swirl.
Ballistic wave
In flight, the bullet impacts with air particles, which, colliding, begin to oscillate. This results in air seals. They form sound waves. As a result, the flight of a bullet is accompanied by a characteristic sound. After the bullet starts moving at a speed that is less than sonic, the resulting compaction is ahead of it, running forward, without seriously affecting the flight.
But when flying, in which the speed of a bullet or projectile is higher than the sound, sound waves run into each other, form a compacted wave (ballistic), which slows down the bullet. Calculations show that at the front, the pressure of a ballistic wave on it is about 8-10 atmospheres. To overcome it, the main part of the energy of the flying body is expended.
Other factors affecting bullet flight
In addition to the forces of air resistance and gravity, the bullet is affected by: atmospheric pressure, temperature values of the environment, wind direction, air humidity.
Atmospheric pressure on the Earth's surface is uneven relative to sea level. With an increase of 100 meters, it decreases by approximately 10 mmHg. As a result, shooting at altitude is carried out under conditions of reduced resistance and air density. This leads to an increase in flight range.
Humidity also has an effect, but only slightly. It is usually not taken into account, except for long-range shooting. If the wind is fair when shooting, then the bullet will flygreater distance than in a no-wind condition. Head wind - the distance decreases. Side winds have a great influence on the bullet, deflect it in the direction they blow.
All of the above forces and factors act on the bullet at angles to it. Their influence is aimed at overturning a moving body. Therefore, to prevent the bullet (projectile) from tipping over in flight, they are given a rotational movement when leaving the bore. It is formed by the presence of rifling in the barrel.
A rotating bullet acquires gyroscopic properties that allow the flying body to maintain its position in space. In this case, the bullet gets the opportunity to resist the influence of external forces for a significant segment of its path, to maintain a given position of the axis. However, the rotating bullet in flight deviates from the straight direction of movement, which causes derivation.
Gyroscopic effect and Magnus effect
The gyroscopic effect is a phenomenon in which the direction of movement in space of a rapidly rotating body remains unchanged. It is inherent not only in bullets, shells, but also in numerous technical devices, such as turbine rotors, aircraft propellers, as well as all celestial bodies moving in orbits.
The Magnus effect is a physical phenomenon that occurs when air flows around a rotating bullet. A rotating body creates around itself a vortex motion and pressure differences, due to which a force arises that has a vector direction perpendicular toair flow.
With regard to the practical plane, this means that in the presence of a side wind from the left side, the bullet blows up, and from the right - down. But at short distances, the influence of the Magnus effect is insignificant. It should be taken into account when shooting at long distances. As a result, snipers are forced to use a special device - an anemometer, which measures wind speed. Moreover, in practice, 7, 62 tables taking into account bullet derivation are common.
Reasons for derivation and its meaning
The derivation of the bullet is always directed in the direction in which the barrel rifling runs. Due to the fact that all modern models of rifled weapons have rifling in the direction from the left - up - to the right (with the exception of small arms in Japan), the deviation of the bullet, the projectile is carried out to the right side.
Derivation grows disproportionately relative to the shooting distance. Along with the increase in the range of the bullet, the derivation tends to gradually increase. Therefore, the trajectory of a bullet, when viewed from above, is a line in which the curvature is constantly increasing.
When shooting at a distance of 1 km, derivation has a significant effect on bullet deflection. So in standard reference books, table 3 of a bullet 7, 62 x 39 shows the derivation in the amount of about 40-60 cm. However, numerous studies by specialists in the field of ballistics lead to the conclusion that the derivationshould only be taken into account at distances over 300 m.
Modern artillery takes into account derivational corrections automatically or by using firing tables. Separate samples of small arms are supplied with optical sights, in which it is taken into account constructively. The sights are mounted in such a way that when fired, the bullet automatically goes a little to the left. Upon reaching a distance of 300 m, she is on the aiming line.
Factors affecting derivation
Derivation is influenced by certain factors, namely:
- Rifled pitch in the bore. The steeper it is cut, the stronger the rotation, the derivation of the bullet becomes more significant.
- Weight characteristics of the bullet. A heavier object is less deflected by the derivation effect. With the same caliber, the deviation from the trajectory along the line of sight will be less if the weight of the bullet is greater.
- Throwing angle. This is the so-called elevation of the trunk. The larger this angle, the smaller the derivation. A bullet fired vertically upward (the angle is 90 degrees) is not affected by the overturning moment, as a result of which there is no derivation. Such features are taken into account when shooting at flying targets.
- Ambient temperature. The derivation of the bullet manifests itself more significantly if the air temperature drops.
- Counter currents of air. If the wind blows against the flying bullet, then the derivation increases.
In order to reduce the effect of bullet spin derivationin flight, special bullets have now been developed. They have a peculiar internal structure with selected centers of mass and gravity.
Bullets (shells) fired from smooth-bore weapons (no rifling), as well as those in which stabilization in flight is carried out by plumage, and that do not rotate, do not experience the phenomenon of derivation.
