Airsoft Gun Reviews

Hop-up and Bernoulli’s Principle

Bernoulli’s principle as applied to an airsoft pellet is as follows. As a spinless spherical pellet flies along its trajectory through the air (the air being the “fluid” in this case) the pressures on all sides of the pellet are equal because the air is traveling the same velocity relative to the surface of the pellet. If a spin is applied to the pellet about an axis perpendicular to the velocity vector (i.e. a backspin) the air will be rushing faster (relative to the pellet surface) on the side that is spinning away from the velocity vector and slower on the side that is spinning towards the velocity vector. Bernoulli’s principle says this difference in fluid velocity implies a difference in pressures, which is a force that will cause the pellet to move in a direction perpendicular to the velocity vector.

Airsoft hop-up devices apply a backspin to the pellet so that the pressure force acts on the pellet opposite to the direction that gravity is pulling it. This causes the pellet to fall less over a given distance than it would without the spin applied to it.

In airsoft guns this is often implemented as a rubber piece at the rear of the barrel that is thicker at the top of the barrel than the bottom. As the pellet moves past this piece it tends to roll, inducing a backspin. This is usually adjustable so that the effect can be tuned.

Bernoulli’s principle is often mis-applied in the case of spinning balls. The real cause of the apparent lift on an airsoft pellet is The Magnus Effect. There is a layer of non-moving air on the surface of the pellet (boundary layer). This is why a golf ball has dimples; this layer acts like ball bearings. With the case of a spinning ball, this layer gets thrown off at an angle. Newton’s laws say that in order for air to be thrown in one direction, the ball has to move in the other direction. In the case of a pellet spinning with a backspin, Bernoulli would predict that the ball would move downward because the bottom of the pellet is moving in the same direction that the pellet is moving and therefore less pressure would be present on the bottom of the pellet and it would drop. We know that this is not true. Also, the Bernoulli explanation requires that the object be moving through the air as well as spinning. This is not true either. A spinning object that is not moving will also experience Magnus Forces. According to the Magnus explanation, the rotating ball would throw air downward and to the rear, thus giving lift. The air on the bottom of the ball is slowed down, so when the separated air comes back together, it is lower than the middle of the ball appearing like a comet’s tail pointing down. This can be verified in wind tunnels and is very well documented in fluid dynamics physics textbooks.

Posted by Airsoft Reviews in Airsoft Beginners Guide