Airsoft explained III - the Hop Up

In this episode of "Airsoft explained" we'll discuss the magic caused by the "Hop Up" system. HU is a part of most airsoft replicas, you would not want to buy one without it.

As you know, a stone thrown horizontally after a moment hits the ground because of gravity. The same goes to airsoft BBs. Hop Up was designed to somehow balance the gravity and allow the BBs to travel horizontally on a longer distance. This device uses the Bernoulli's principle.

What is the mysterious Hop Up system? It's a small device placed at the end of a barrel, next to the clip. It consists of a rubber 'gasket' and a mechanism to adjust it (which I'll describe later). This rubber 'gasket' has tubular shape, and in one place inside has relatively thicker spot. The rubber is placed on a barrel (the spot goes into the hole on a barrel). When a BB goes through the barrel it touches this thicker spot which causes it to rotate.

Before I write anything more I suggest opening the page airsoftclub.ru/FAQ/hopup in a separate browser window. I'll explain everything referring to images at the bottom of that article.

The first image shows the BB fired from a replica with HU. It translates and rotates at the same time. Because this rotation means different relative velocities between upper and lower semispheres of the BB, the slight pressure difference occurs. This pressure difference balances the gravity, as in airplanes. You can see how the HU is installed on the barrel (and the schematics of a gearbox) on the images no. 5 and 6.

Because BBs are manufactured in different weights, a good replica must have an adjustable hopup. By adjusting the depth the rubber spot goes into the barrel you can change the speed of rotation fo the BB fired. This way you can adjust HU to stabilise the flight of BBs with different weights.