how hovercrafts work

12/20/13 How Hovercrafts Work

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What areHovercrafts?

Vehicles designed to travel close to but above ground or water. These vehicles aresupported in various ways. Some of them have a specially designed wing that will liftthem just off the surface over which they travel when they have reached a sufficienthorizontal speed (the ground effect). Hovercrafts are usually supported by fans that

force air down under the vehicle to create lift, Air propellers, water propellers, orwater jets usually provide forward propulsion. Air-cushion vehicles can attain higher

speeds than can either ships or most land vehicles and use much less power thanhelicopters of the same weight. Air-cushion suspension has also been applied to other

forms of transportation, in particular trains, such as the French Aero train and theBritish hover train.

Hovercraft History

The first recorded design for a hovercraft was in 1716 put forward by EmmanualSwedenborg, a Swedish designer and philosopher. The project was short-lived and acraft was never built. Swedenborg realized that to operate such a machine required a

source of energy far greater than any available at that time. In the mid-1870s, theBritish engineer Sir John Thornycroft built a number of model craft to check the air-

cushion effects and even filed patents involving air-lubricated hulls, although thetechnology required to implement the concept did not yet exist. From this time bothAmerican and European engineers continued work on the problems of designing a

practical craft. In the early 1950s the British inventor Christopher Cockerell began toexperiment with such vehicles, and in 1955 he obtained a patent for a vehicle that was

"neither an airplane, nor a boat, nor a wheeled land craft." He had a boat builderproduce a two-foot prototype, which he demonstrated to the military in 1956 withoutarousing interest. Cockerell persevered, and in 1959 a commercially built one-personHovercraft crossed the English Channel. In 1962 a British vehicle became the first to

go into active service on a 19-mi (31-km) ferry run.

Creation of Hovercrafts

When building a hovercraft it is imperative that you are sure you have a firmgrasp of the important concepts and principles involved. An elementary knowledge of

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physics is required, but higher level math, physics, fluid dynamics, etc. is notnecessary. Ease of use, cost, availability and safety are all significant considerations

when building a hovercraft. Care must be taken in selecting a motor and propeller forthe proper function and stability of the hovercraft and to meet your needs for thrustand lift. A good skirt design is essential for stability and of course, body designs mustbe well thought-out in order to meet your needs for speed and stability. Finally, therudders must be well weighed out in order to avoid weighing down your hovercraft

and also well shaped in order to move air as efficiently as possible.

How Does a Hovercraft work

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Hovercrafts work on the two main principles of lift and propulsion. When dealingwith a hovercraft, the existence of lift is imperative for the proper function of the

vehicle. lift is an essential factor because it is that which allows the craft to ride on acushion of air several inches off the ground. This process, the process of attaining liftbegins by directing airflow under the craft. In order to quarantine the air under the air

cushion, a skirt is required. This is done in order to create pressure under thehovercraft which forces the vehicle off the ground. Attaining the proper amount of

airflow is imperative for the maintenance of the craft’s stability. If too much airflow isdirected under the craft, it will then hover too high above the ground, resulting in thehovercraft to tip. Not enough lift will cause the craft to remain on the ground which

defeats the very purpose of the hovercraft altogether. The source of the airflow whichpropels the craft of the ground is a fan. The fan can be used for lift and thrust. It canbe dedicated to lift or thrust or even both simultaneously. In either case the passage

where the air flows through to reach the air cushion affects the stability of thehovercraft. This passage is a hole located on the base of the craft. Another vital

component is the motor. The motor is usually located in the rear of the vehicle and isthe heaviest of the components. Due to the weight of the motor, extra pressure isrequired under the area where the motor is positioned in order to attain hovering

capabilities.

That which makes hovercrafts so efficient and different from other vehicles ofits category is that very little force is required for it to move. Propulsion is that whichmakes the craft move. The source of this effect is the fan, which is used to move theair for propulsion. However odd as it may seem, the fan produces more than enough

force for the hovercraft to move. This is achieved through the existence of anothermajor factor:

friction, or better yet, the elimination of friction. Hovercrafts have no contact with theground, therefore any resistance the ground may produce under other circumstances

is now non-existent for the craft. As explained above, the propulsion of the craftrequires a fan but a normal fan is not sufficient. This is because a normal fan does not

blow air straight back. Instead it spins the air in a spiral shape. Therefore engineersdecided to use turbines or stationary blades, that un-spin the air. When air does not

spin more of its kinetic energy can be used for translation and less is required forrotation.

The shape of the body also affects the stability of the hovercraft. The larger thearea of the base, the more stable it will be. Wider base=greater stability. Longer and

narrower shapes increase speed but decrease stability. Most hovercrafts haverounded ends, and offer both stability and speed.

The skirt is another vital component. The common skirt is known as a bag skirt.It is comprised of a bag that covers the bottom of the base and has holes in it to allow

air to escape and push the craft off the ground. Each part of the skirt inflatesindependently which makes repairs much easier and improves stability.

Unfortunately, the more stable a skirt, the slower it will go.



When the hovercraft is finally able to move it will most definitely requiresteering capabilities. This is achieved through the use of rudders. These rudders can

be controlled by a variety of devices including computers. Rudders cannot be tooheavy otherwise they will weigh down the craft because they are located very close to

the motor. The shape of the rudder dictates how well it will be able to move air.

When riding a hovercraft the natural state of motion is easily seen to be constantvector velocity with a constant rate of rotation. A sloping floor will definitely changeyour velocity vector without changing your rate of rotation. In addition to Newton’s

three laws of motion it will become obvious that to avoid spinning or tilting thehovercraft you must apply the forces in line with the center of mass of the

combination of the craft and your body.

Lifting Fan

Firstly the volume of air needed is very large and a propeller is designed to bemost efficient in open air like on an aircraft. Also the fan needs to force air into

the chamber below the craft so creating a specific pressure under the craft.Propellers again are not efficient in applications when an air backpressure willbe applied to the propeller blades as they rotate. Because of this the lifting fan

on most Hovercraft uses what is known as a centrifugal fan. This is a fan inwhich two discs and fitted together and looks rather like a doughnut with

angled slats at their edges.

When the assembly is rotated at high-speed air is sucked into the center hole inthe fan and the slats force it out at theedges. The advantages of the fan are twofold. They operate efficiently in anenvironment when backpressure is highand they will move larger volumes of airfor a given rotation speed than apropeller with the same speed and powerinput. The lifting fan is coupled via a

gearbox to the engine. The engine also drives the propeller on the craft, whichprovides thrust for forward motion of the Hovercraft.



Thrust Propellers

The propeller used to drive the hovercraft along is usually an aircraft type withvariable pitch blades. Its speed of rotation must remain fixed to that of the

engine and the lift fan.

This is because the amount of lift air required dictates the engine speed todrives the lift fan. In turn the amount of propulsion, which the propellersprovide, must be obtained by varying the propeller pitch and not its rate of

rotation. This system is termed 'integrated lift/propulsion'. A Hovercraft havingmore than one lift fan and propeller generally has a separate engine for each

fan-and propeller unit.

The propellers used on hovercraft can vary from four-bladed versions andabout nine feet in diameter on the smaller craft to the four propellers on theSRN4 cross-Channel hovercraft. These are four-bladed and nineteen feet in

diameter! On the SRN 4 the pylons on which they are mounted can be rotatedto change the direction of thrust. On smaller craft, rudders like on aircraft, are

used for direction control.

Momentum Curtain

When early models were built and analysis was done on the airflow using theplenum chamber type of hovercraft it showed that there were problems withstability. In addition the craft would require enormous power to maintain a

reasonable hover height.



Stability of the hovercraft on its cushion of air remained a real problem despitesome design efforts and a new approach was needed. To solve these problems,

a plenum chamber with a momentum curtain was developed by Sir ChristopherCockerall.

Hovercraft Skirt

Despite the momentum curtain being very effective the hover height was stilltoo low unless great, and uneconomical, power was used. Simple obstacles

such as small waves, or tide-formed ridges of shingle on a beach, could prove tobe too much for the hover height of the craft. These problems led to the

development of the 'skirt'.

The skirt is a shaped, flexible strip fitted below the bottom edges of the plenumchamber slot. As the hovercraft lifts, the skirt extends below it to retain a much

deeper cushion of air. The development of the skirt enables a hovercraft tomaintain its normal operating speed through large waves and also allows it to

pass over rocks, ridges and gullies.

The skirt of a hovercraft is one of its most design sensitive parts. The designmust be just right, or an uncomfortable ride for passengers or damage to the

craft and the skirts results. Also, excessive wear of the skirt can occur if itsedges are flapping up and down on the surface of the water. The skirt material

has to be light flexible and durable all at the same time.

For the skirt to meet all of its requirements the design and use of new materialshas slowly evolved. The current skirts use ‘fingers at the lower edge of the skirtenvelope which can be unbolted and replaced. By doing this there is a quickand easy way to counter the effects of wear without having to replace the wholeskirt structure. A shocking example of the costs is the replacement of the skirtassembly on the SRN 4’s which used to cross the English Channel from the UKto France. The replacement cost for a set of skirts for this craft is over 5 millionUS Dollars.

The Engine



The SRN 1 andother early

hovercrafts usedpiston typeengines. As

models like theSRN 4 and SRN6 were brought

into service theytended to favorthe use of gasturbines. This

type of engine issmaller and

lighter for a given horsepower and has been used extensively in turbo propaircraft.

The engine has a mainshaft on which ismounted a compressorand a turbine. A startermotor is connected toone end of the shaft andthe other end isconnected to the lift fanand propellergearboxes. Bothcompressor and turbinelook like fans with alarge number of blades.

When the engine is started, the compressor compresses air from the engineintakes and pushes it into combustion chambers mounted around the engine.Fuel is squirted into the combustion chambers and ignited. The compressed airthen rapidly expands as it is heated and forces its way out through the turbineto the exhaust. As the gas pressure rises, the turbine speeds up, thereby drivingthe compressor faster. The engine speed increases until it reaches the engine'snormal operating speed.

However the use of these engines results in a very high level of engine noiseoutside the craft. In the SRN 6 this meant that it was possible to hear the crafttraveling across the Solent between Portsmouth and the Isle of Wight in the

UK several miles away. With the newer generation of craft close attention waspaid to engine noise and fuel efficiency. The current AP188craft that runs on



the old SRN6 routes has now moved back towards piston engines and usesmarine diesel engines that are much quieter and fuel efficient.

Airbox

The box-like structure at the rear of the hovercraft, right behind the propeller, Thebox-like structure is called an airbox. The airbox takes about 10% of the air being

pushed backward by the propeller and forces it downward, underneath the hovercraft.There are three small ducts cut into the base of the hovercraft, underneath the airbox.

Two of these ducts lead into the skirt, which is basically a bag that goes all the wayaround the perimeter of the craft, while the third duct leads directly underneath the

hovercraft.

Hovering Power

Take a hovercraft which, complete with crew, fuel and load, weighs 2,000pounds (lbs.), and is 15 feet (ft.) long and 7 ft. wide. Its area would be

15 ft x 7 ft. = 105 square (sq.) ft.

If the craft is to hover, the pressure of air forming the cushion must be 2,000pounds or greater. This represents 19 pounds. per sq. ft. Yes, only 19 pounds.per sq. ft.is required to lift the hovercraft which seems much smaller than you

might imagine!

From existing designs of Hovercraft that have been developed, it is possible tomake some simple estimate of the power needed to lift a Hovercraft. Using 19

pounds per square foot it is estimated 4 horsepower for each sq. ft. of curtain orskirt area can maintain that hover.

Curtain area is its length times its height. A hovercraft 15 ft. long by 7 ft. widewould have a curtain length of 44 ft.-twice the length plus twice the width.

If we want it to hover one foot high we would need sufficient power to providea curtain of 44 x 1 sq. ft. At 4 horsepower per sq, ft. we would need 176

horsepower Just to lift the craft up to hover one foot above the ground. Don'tforget we now need to push the craft along as well so that engine is the

minimum size we can use.

Rudders



Hovercraft Operation

Piloting a hovercraft is an interesting proposition. Since very little of it actuallytouches the ground, there isn't much friction, making it very difficult to steer and also

very susceptible to strong winds. Imagine trying to drive around on top of an air-hockey puck! We've discovered that the best way to drive it is treat it like a jetski, i.e.

leaning back and forth and steering very carefully. It is also possible to do a 360-degree turn without stopping, which is quite a sight!

Aerodynamics

Aerodynamics is defined as the branch of fluid physics that studies the forcesexerted by air or other gases in motion. Examples include the airflow around

bodies moving at speed through the atmosphere (such as land vehicles, bullets,rockets, and aircraft), the behavior of gas in engines and furnaces, air

conditioning of buildings, the deposition of snow, the operation of air-cushion vehicles (hovercraft), wind loads on buildings and bridges, birdand insect flight, musical wind instruments, and meteorology. For maximum

efficiency, the aim is usually to design the shape of an object to produce astreamlined flow, with a minimum of turbulence in the moving air. The

behavior of aerosols or the pollution of the atmosphere by foreign particles areother aspects of aerodynamics.

Conclusion

Hovercrafts are generally simple mechanisms in theory. Yet the process fromtheory to manifestation is not as easy as it may seem. A plethora of problems existand must be faced in order to attain a well functioning hovercraft. The plans anddesigns must be flawless. One must take under consideration the weight and the



shape of each component in order to avoid problems such as instability anddysfunction. One thing is certain; when building a hovercraft, be well aware of the

demands of construction. Be prepared and willing to embrace failure for it is the onlyway to success. Only after failed attempts will you be able to finally design an effective

hovercraft.

how hovercrafts work

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