We’ve been fascinated by hovercraft since childhood. They look like a pressure vessel, a pillow, or a skirt, gliding across the water, spraying water. This article will explore the nature of hovercraft and how they work.
What is a hovercraft?
The hovercraft is one of the greatest inventions in the history of the world. Its operation and basic concept were first proposed by Swedish scientist Emanuel Swedenborg in 1716. Subsequent designs continued to emerge, and by the 20th century, modern hovercraft design and reliability standards had become essential.
British inventor Christopher Cockerill patented the first commercially approved hovercraft in 1952. A hovercraft is essentially an amphibious vehicle capable of traveling on land and sea. These vehicles rely on a simple principle: they generate lift to offset resistance (i.e., friction) on the land or water surface, keeping the vehicle slightly above the water surface, thereby gaining buoyancy.
In short, because they can float on the ground or water using the cushion of air they generate, they are called “aircraft,” a word derived from the word “hover.” While lift varies from aircraft to aircraft, aircraft typically achieve at least 15 centimeters (5.5 in) of lift, with some designs capable of even 15 or 20 centimeters more!
Now, without further ado, let’s dive into the mechanics of aircraft operation and its fundamental principles.
How Aircraft Work?
Aircraft utilize a simple principle of physics: air pressure differences are necessary to generate lift to counteract gravity. Aircraft systems draw in air at atmospheric pressure using a fan or blower. This air is directed downward onto an open air cushion or flexible skirt below, which temporarily traps or restricts the intake air.
This intake air gradually dissipates or is exhausted from the bottom of the skirt or air cushion; however, at some point, the amount of air trapped inside becomes significantly greater than the amount exhausted. Restricted by the air cushion or skirt surface, the deck, and the water or land itself, the compressed air cannot flow freely, resulting in a significant increase in net pressure, far exceeding normal atmospheric pressure. This causes the vessel to float slightly above the surface of the water, generating buoyancy.
It’s important to note that the principles of a hovercraft should not be confused with those of a planing craft. Planing craft are traditional watercraft that generate hydrodynamic lift, allowing them to partially float above the surface. Hovercraft, on the other hand, utilize the aerodynamic lift generated by a downward-moving air mass.
We now know that the bottom air cushion is a key feature of aircraft. Air is intentionally trapped and allowed to escape slowly, providing sufficient air pressure to lift the aircraft.
To this end, there are two main air cushion designs, or more precisely, two operating principles: open chamber and closed chamber, or momentum curtain theory.
The open chamber is the older and simpler design. In this case, air is drawn completely and unrestricted into the air cushion or shell at a high flow rate by a fan or suction device. A certain amount of air is trapped inside, while the rest escapes through the bottom of the chamber. Even if the fan capacity is increased, the lift will increase slightly, but at the same time, the amount of air discharged will also increase due to the increased flow rate.
On the other hand, a closed chamber, which utilizes the physics of a momentum curtain, is simply an air cushion or skirt with an internal barrier. This allows air to be drawn in at a controlled rate through concrete channels in the sides and, more importantly, significantly increases flow pressure.
Because of the controlled airflow, the leakage rate or flow rate of the air as it dissipates at the bottom is relatively low. According to the momentum curtain theory, the air is directed to the perimeter of the air cushion, acting as a curtain.
Thus, due to the instantaneous air pressure generated by these factors, the lift of a closed air chamber is higher than that of an open air chamber, making it more efficient.
Basic Aircraft Design and Operation
Aircraft fly on a partially enclosed air cushion beneath them, supported by high pressure within a flexible skirt or membrane, allowing them to float on water or land.
All aircraft have the same configuration and operate according to roughly the same operating sequence, which is roughly as follows:
Propellers: As we’ve already understood, the primary challenge for an airship is to continuously draw in air and push it downward toward the surrounding edge or air cushion. This requires one or more propellers or suction blowers of the required capacity. These draw air from the surrounding environment at a very high flow rate and direct it downward, inflating the air cushion to the desired pressure.
These propellers are also called lift propellers. They are mounted on a shaft or duct, with evenly spaced air intakes that are then directed toward the air cushion. Proper air distribution throughout the air cushion is crucial after suction. The propellers are connected to the motor via gears. While traditional twin propellers offer high suction efficiency, modern designs utilize single propellers or advanced suction systems.
Propellers or impellers: Also known as propellers, these propel the airship forward or backward. The lift propellers or suction blades generate compressed air, helping the airship to lift off the water and eliminate any contact drag or surface forces.
Basic Aircraft Design and Operation
Ultimately, however, a ship requires propulsion and direction to reach its desired destination. Therefore, the main propeller located at the stern is responsible for providing this power. However, the design and configuration of aircraft propellers differ from traditional marine propellers because they are completely unaffected by water forces, and the vessel is primarily above water. Therefore, they can be considered similar to marine propellers. Air is drawn in from the bow, generating forward thrust, and vice versa. The blades have variable pitch.
Furthermore, in most cases, the engine-driven propellers are directly connected to the lift propeller system. Therefore, they also operate the elevators. Therefore, the required amount of lift air and the capacity of the lift propellers, in turn, determine the engine and propeller speeds.
Hovercraft have much less drag than traditional vessels, and they can reach very high speeds for a given engine and propeller power. However, slight splash drag may occur due to the effects of high-speed surface spray.
Lining: The lining or curtain material is carefully selected to be both flexible and capable of absorbing maximum air at high pressure. It must also be durable, weather-resistant, and well-suited to harsh environments.
Rubber is the most commonly used material due to its excellent properties such as strength and durability. The hovercraft cover must be thick and strong. Modern hovercraft use advanced materials such as nylon, vinyl, or composite, durable synthetic materials.
Engine and Propulsion System: The engine provides the power required to drive the propellers and lift mechanism. The engine’s main shaft is connected to the lift propeller and propeller through a clutch and reduction gear. The engine configuration and type depend on the vessel’s needs and size.
After the lift propeller is activated, the remaining power is redirected to the internal propulsion system to generate thrust. The net thrust generated depends on the engine capacity, size, and propeller type. Modern designs use advanced ducted jet propellers instead of traditional propellers like those used in aircraft, generating greater power.
Steering: Aircraft are steered using wings or tail rudders, similar to rudders on airplanes. These rudders are mounted in the airflow path at the rear of the duct.
Aircraft come in a variety of sizes and passenger capacities. Passenger airships can carry up to several hundred people, while patrol boats, yachts, and naval vessels can carry fewer people.
