The term "derivation" has many meanings in everyday life. It is formed by the Latin word derivative, which means "abduction", "rejection". The term in the general sense is understood as a deviation from the trajectory, a departure from fundamental values.
Derivation in the military field
With reference 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 barrel of a firearm. Derivation is also a bullet deflection caused by the gyroscopic and Magnus effects.
Forces acting on a bullet
Bullets moving along the trajectory after exiting the barrel are affected by gravity and air resistance. The first force is always directed downward, causing the abandoned body to decline.
The force of air resistance, constantly acting on the bullet, slows down its forward movement and is always directed towards. She does everything possible in order to overturn a flying body, to direct its head part back.
Due to the influence of these forces, the bullet does not move in accordance with the throw line, but along an uneven, curved curve below the cast line, which is called the trajectory.
The force of air resistance owes its origin 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 also begins to move due to the viscosity of the air. However, at a lower speed.
This layer transfers the motion to the next and so on. As long as the air particles cease to be affected, their speed relative to the flying bullet becomes equal to zero. The air environment, starting from directly in contact with a bullet (projectile) and ending with one in which the particle velocity becomes equal to 0, is called a boundary layer.
In it, "tangential stresses" are formed, in other words, friction. It reduces the distance of the bullet (projectile), slowing its speed.
Boundary layer processes
The boundary layer surrounding the flying body comes off when it reaches the bottom. This creates a vacuum space. A pressure difference is formed that acts on the head of the bullet and its bottom. This process generates a force whose vector is directed in the opposite direction to the movement. Air particles bursting into a rarefied region create regions of swirl.
Ballistic wave
In flight, a bullet acts with air particles, which, when encountered, 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 begins to move at a speed that is less than sonic, the resulting compaction is ahead of it, running forward, without seriously affecting the flight.
But during a flight in which the speed of a bullet or projectile is higher than sound, the sound waves run against each other, form a compacted wave (ballistic), which slows down the bullet. Calculations show that at the front, the pressure on it of a ballistic wave is about 8-10 atmospheres. To overcome it, the bulk of the energy of a flying body is expended.
Other factors affecting the flight of a bullet
In addition to the forces of air resistance and gravity, the bullet is affected by: atmospheric pressure, temperature values of the medium, wind direction, air humidity.
Atmospheric pressure on the Earth's surface is uneven with respect to sea level. With an increase of 100 meters, it decreases by about 10 mmHg. As a result of this, firing at altitude is carried out under conditions of reduced drag and air density. This leads to an increase in flight range.
Humidity also has an effect, but not significantly. It is usually not taken into account, with the exception of long-range shooting. If the wind is favorable during firing, then the bullet will fly a greater distance than in the condition of calm. Headwind - distance decreases. Lateral winds on the bullet have a great impact, deflect it in the direction where they blow.
All the above forces and factors act on the bullet at angles to it. Their influence is aimed at overturning a moving body. Therefore, in order to prevent the bullet (projectile) from tipping over during flight, they are given a rotational movement when exiting the barrel. It is formed by the presence of rifling in the trunk.
A rotating bullet acquires gyroscopic properties that allow a flying body to maintain its position in space. In this case, the bullet gets the opportunity to resist the influence of external forces on a significant segment of its path, to maintain a given position of the axis. However, a bullet rotating in flight deviates from the rectilinear direction of motion, which causes derivation.
Gyroscopic effect and Magnus effect
The gyroscopic effect is a phenomenon in which the direction of motion in the 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 an air stream flows around a rotating bullet. A rotating body creates a vortex motion and pressure differences around itself, due to which a force arises having a vector direction perpendicular to the air flow.
With regard to the practical plane, this means that in the presence of a crosswind, the bullet blows upward on the left side and downward on the right. But at short distances, the effect of the Magnus effect is negligible. It should be taken into account when shooting long distances. As a result, sniper shooters are forced to use a special device - an anemometer, which measures the speed of the wind. Moreover, in practice derivation-specific bullets 7.62 tables are common.
The causes of derivation and its significance
Derivation of a bullet is always directed in the direction in which the stem cuts go. 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 of Japan), the bullet and projectile are deflected to the right.
Derivation is growing disproportionately with respect to the firing distance. Together with an increase in the range of a bullet, derivation tends to a gradual increase. Therefore, the trajectory of a bullet, when viewed from above, is a line in which the curvature is constantly increasing.
When firing at a distance of 1 km, derivation has a significant effect on bullet deflection. So in standard reference books table 3 bullets 7.62 x 39 derivation shows in the order of 40-60 cm. However, numerous studies by specialists in the field of ballistics lead to the conclusion that derivation should be taken into account only at distances of more than 300 m.
Modern artillery takes into account derivational amendments automatically, or through the use of shooting tables. Separate samples of small arms are equipped 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. When reaching a distance of 300 m, she is on the target line.
Derivative Factors
Derivation is influenced by certain factors, namely:
- Threaded pitch in the bore. The steeper it is sliced, the stronger the rotation, the derivation of the bullet becomes more significant.
- Weight characteristics of the bullet. A heavier object with a derivative effect deviates less. With the same caliber, moving away from the trajectory along the aiming line will be less if the bullet weighs more.
- Throw angle. This is the so-called elevation of the trunk. The larger this angle, the less derivation. The tipping moment is not affected by a bullet launched upward vertically (angle is 90 degrees), as a result of which there is no derivation. Such features are taken into account when shooting at flying targets.
- Ambient temperature. Derivation of a bullet manifests itself more significantly if the air temperature drops.
- Counter flows of air. If the wind blows towards a flying bullet, then the derivation increases.
In order to reduce the effect of derivation by rotation of a bullet in flight, special bullets have now been developed. They have a peculiar internal structure with selected centers of mass and gravity.
Bullets (shells) fired from smoothbore weapons (no rifling), as well as those in which stabilization in flight is carried out by the plumage, and that do not rotate, do not experience the phenomenon of derivation.