How it Operates



How it Operates

A hovercraft is an air cushion vehicle (ACV) that flies above the earth's surface on a cushion of air. An engine that provides both the lift cushion and the thrust for forward or reverse movement powers it. It is a true multi-terrain, year-round vehicle that can make the transition from land to water without touching the surface. A Thrust propeller provides forward propulsion and directional control is accomplished via rudders mounted aft of the Thrust Fan.

The Lift Fan provides air to inflate the "Skirt"¯ to create a semi-sealed area between the hull and the ground permitting the craft to "Hover"¯ in ground effect. On the 19XR, lift is controlled by the lift throttle handle on the left side of the handle bars. This regulates air pressure and volume in and under the skirt.

Steering is accomplished with moveable rudders mounted behind the thrust propeller. A horizontal wing acts as an in flight trim system.

To properly operate your hovercraft you must become familiar with the machine and all of its components. This helps to build a general understanding of the mechanics and limitations of the craft.

Principals of Operation

A Hovercraft is a vehicle which travels over any surface on a cushion of air which is trapped in a chamber under the vehicle. This chamber is supplied with air under pressure from an axial 4-blade lift fan. The top and bottom of the chamber is formed by the vehicle bottom and the surface over which the vehicle is traveling respectively. The sides of the chamber are formed by the flexible skirt. The simplest skirt is the "C"¯ skirt or the straight skirt shown in Fig. I.

FIGURE I




A vehicle using a "C"¯ skirt must have a round or nearly round platform shape in order for the skirt to inflate and contain the air properly. This skirt should have a maximum height of about 10-15% the diameter of the vehicle. Any greater height will result in less stability. There must be a sufficient volume of air supplied to the chamber so the air escaping from gaps between the skirt and the surface is replaced. Operating over a smooth surface requires less air supply than operating over a rough surface. Operating over grass, especially tall grass requires a much higher volume of air than over concrete or ice.

Figure II shows the peripheral jet type Hovercraft, which traps air by means of a curtain or skirt formed by a jet or stream of fast moving air. This jet of air is aimed down and inward at a 45 degree angle for best efficiency and highest lift.

FIGURE II



The peripheral jet hovercraft was one of the first full size machines built. It was built in the late 1950's by Sir Christopher Cockrell in Great Britain. This type of Hovercraft required a large amount of horsepower for the weight it lifted. Too much power was required to maintain the air jet. This type of craft was later fitted with a flexible extension to the air jet to increase the total height of the hull with a corresponding decrease in the air jet height and power required, as shown in Fig. III.

FIGURE III


Experimenting with flexible skirts showed that the lifting efficiency and stability of the craft could be increased by using a bag skirt system. (Fig. IV.) The vehicle could be built to almost any platform shape and is simpler to build because air does not have to be supplied to a jet system all the way around the perimeter of the vehicle. The air could be ducted directly into the bag and the chamber. The pressure in the bag could be equal to or greater than the pressure in the chamber. The greater the bag pressure, the harder the ride over rough surfaces.



Bag skirts generally give the best stability and the roughest ride. To get a smoother ride, Finger skirts may be added to (Fig. IV) the bag skirt or they may be used separately, with a decrease in the operating height of the hull. Finger skirts are difficult to build due to the large number of separate fingers that must be attached. The overall result of flexible skirts is to give greater operating height with less power required for lift.

With flexible skirts the power required for lift varies from 20 to over 200 lbs. per horsepower and depends on many factors. In general a vehicle that has about 100 to 150 lbs. weight for every horsepower of lift will operate well provided an efficient lift fan and duct are used.

Hovercraft Defined

There are different types of hovercraft being produced and used throughout the world. For many centuries man has been traveling over the world's seas at ever increasing speeds. New technologies were introduced in order to make the speed increase possible. Conventional displacement mono-hulls could no longer keep up and multi-hulls and planing hulls were introduced. Even higher speeds were achieved with hydrofoils and air cushion vehicles. The practical maximum speed of all marine craft mentioned so far lies around 100 km/h. The drawback of the recent trend for high-speed marine craft is the increased power requirement and fuel consumption. It is very unlikely that any "conventional" marine craft will be able to operate at much higher speeds with acceptable fuel efficiency. The excessive power requirement of high-speed marine craft is mainly caused by viscous drag; well over 50% of the drag is caused by water friction. The obvious solution is to minimize water contact. This approach works for hydrofoils and hovercraft. The speed of a hovercraft is bounded by the sea state and longitudinal stability considerations.


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