MAGNUS EFFECT IN EXTERNAL BALLISTICS

 

Magnus Effect in External Ballistics

The Magnus effect is a phenomenon that occurs in external ballistics. External ballistics deals with the behavior of projectile such as a bullet or shell, after it has exited the barrel of a firearm and is traveling through the air towards its target. The Magnus effect specifically influences the trajectory of the projectile during this phase of its flight.

What is Magnus Effect?

The Magnus effect is a physical phenomenon that occurs when a spinning object moves through a fluid, such as air. It is named after Heinrich Gustav Magnus, a German Physicist who studied the effect in the 19the Century. In the context of firearms, the Magnus effect plays a significant role in the behavior of a projectile such as a bullet as it travels through the air, particularly in the presence of spin imparted by the rifling of the barrel.

How the Magnus Effect Works?

When a projectile spins, it creates a difference in air pressure around the surface of the projectile due to the interaction between the spin and the surrounding air flow.

A.    Spin and Rifling: In most modern firearms, the barrel is rifled, which means it has spiral grooves on the inside. These grooves impart a spin to the bullet as it is fired. The spin stabilizes the bullet, ensuring it flies straight and does not tumble.

B.     Air Flow Around the Bullet: As the bullet travels through the air, the surface of the bullet interacts with the surrounding air. The side of the bullet that is spinning in the direction of the airflow moves faster relative to the air, while the opposite side moves slower.

C.    Pressure Differential: Due to the difference in relative speeds between the air and the surface of the bullet, a pressure differential is created. On the side of the bullet where the surface is moving in the same direction as the airflow, the air pressure is lower. On the opposite side, where the surface is moving against the airflow, the pressure is higher.

D.    Relative Motion of Air and Surface:

When a bullet spins as it moves through the air, different parts of its surface move at different relatives’ speeds with respect to the surrounding air. This difference in relative speed is crucial to understanding how the pressure difference is created.

·        Side A (Same Direction as Spin): On the side of the bullet where the surface is moving in the same direction as the airflow (due to the spin), the relative speed of the air and the surface of the bullet is higher. Essentially, the bullets spin pulls the air along with it, causing the air to flow faster over this side.

·        Side B (Opposite Direction of Spin): On the opposite side of the bullet, where the surface is moving against the airflow, the relative speed of the air and the bullets surface is lower. Here the bullet spin pushes against the air, slowing the airflow down on this side.

E.     Bernoulli’s Principle:

The pressure difference can be explained by Bernoulli’s principle, which states that an increase in the speed of an air results in a decrease in pressure, and decrease in the air results in a high pressure.

·        Faster Air Flow (Low Pressure): On the side of the bullet where the air is moving faster (Side A), the pressure is lower. This is because, according to Bernoulli’s principle, faster-moving air exerts less pressure.

·        Slowe Airflow (Higher Pressure): On the side where the air is moving slower (Side B), the pressure is higher. The slower moving air exerts more pressure on this side.

 

F.     Creation of the Magnus Force:

The difference in pressure between side A (lower Pressure) and Side B (Higher Pressure) creates a net force perpendicular to the direction of the bullet’s motion. This force is what we call the Magnus Force.

·        Direction of the Magnus Force: The Magnus force acts from the side with higher pressure to the side with lower pressure. For example, if the bullet is spinning clockwise (as viewed from behind), the air pressure on the left side (side B) will be higher, and on the right side (side A), it will be lower. This creates a Magnus force that pushes the bullet to the right.

G.    Example in a Practical Context:

For instance, in a typical rifle, the rifling inside the barrel imparts a right-hand twist (Clock wise spin) to the bullet. As the bullet exits the barrel and travels downrange.

 

·        The air on the right side of the bullet (moving in the same direction as the spin) travels faster, reducing pressure on that side.

·        The air on the left side (moving against the spin) travels slower, increasing pressure on that side.

·        The result is a Magnus force that pushes the bullet to the right, causing what is known as “right-hand-drift”.

.