How it Works The GH-2 hovercraft uses two ducts for propulsion - a lift duct in the front and a thrust duct in the back. The lift duct blows pressurized air underneath the craft creating an air cushion. The thrust duct blows its air out toward the rear of the craft propelling it forward. This air flows over the rudder(s) at the rear of the craft for steering. The GH-1 hovercraft uses one thrust duct unit. The duct blows 1/3 to 1/2 of its air underneath the craft to create an air cushion; the other half blows toward the rear sending the craft forward. Like the GH-2, thrust air is used for steering. GH-2 uses the "airbox" to support the duct and adjustable door. The adjustable airbox uses a trap door designed to allow experimentation with different lift / thrust combinations. As the name suggests, the trapdoor opens and closes. When the trapdoor is closed, all of the air from the rear duct is sent out the back to provide thrust. When the door is closed more air is diverted underneath the craft providing more lift. Since the GH-1 hovercraft comes with only one thrust duct unit, it always operates with its airbox trapdoor open. The GH-1's cabin and hull are optimized for single duct operation.