Hovercraft 1

Working principle and some Design Concept of Hovercraft

Abstract

A hovercraft is an air-cushioned vehicle that is, it does not require wheels like other

automobiles, and hence surface friction is not a factor for a hovercraft. A hovercraft can

go over land as well as water without any difficulty and is majorly used in marshy areas

for patrolling and defense applications. A hovercraft is also used for tourism. A

hovercraft works on the principle of propulsion and is futuristic vehicle and can be used

in many defense and miscellaneous applications.

Department of Mechanical Engineering

Bangalore Institute Of Technology, v.v.Puram, Bangalore 1


PRINCIPLE OF WORKING

A Hovercraft or also called an air cushion vehicle is a vehicle that can drive on both land

and water. This vehicle differs from other vehicle in that way, it needs no surface contact

for traction. Obstacles such as gullies and waves can be taken very easily like it is a flat

surface. The reason for this is a generated air cushion between the hovercraft itself and

the ground surface.

The Hovercraft lifts due to the pressure generated by the air in the chamber {formed by

the bottom portion of the hull and the skirt]. We know that Pressure = Force /Area {P/A]

this relation gives the pressure required for the lift of the hovercraft.

So the Hovercraft lifts due to the pressure by the fan and hence floats above the ground.

Air leaks from below the skirt. This loss of air is taken care by axial flow fan, which is

continuously supplying air to the chamber. Flaps are provided to help uniform air leakage

from the bottom of the skirt. As the craft is above the ground there is no friction between

the ground and the craft, now the craft is propelled with the help of the propulsion fan.

The control of direction is done by using rudder and a steering mechanism.

In the hovercraft are placed two propellers both driven by a gasoline engine and one of

them is used to provide lift by keeping a low-pressure air cavity under the craft full of air.

As the air pressure is increased the air lifts the craft by filling the cavity. The cavity or

chamber in which the air is kept is called a ‘plenum ‘chamber. At the point when the air

pressure equals the weight of the hovercraft over the chambers surface area the hovercraft

lifts and the air starts to escape around the edge of the skirt. The escaped air creates an

air-lubricated layer between the hovercraft and the ground surface. This will lead to a




frictionless motion of the hovercraft, taken into account that the minimum contact

between skirt and the ground surface during the motion is negligible.

The amount of the total weight that a hovercraft can raise is equal to the pressure in the

plenum chamber multiplied by the area of the hovercraft. This plenum chamber principle

is visualized in fig. 1.

A constant feed of air is needed to lift the hovercraft and to compensate for the air being

lost through the holes in the bottom plate.[1] The flow must also be greater than the

amount of air that escapes through the holes in the bottom plate. The rate of air loss is not

constant, because there is no way of ensuring that any air escapes evenly all around the

hovercraft. To maintain also the lift, the engine and propeller have to be sufficiently

powerful enough to provide a high air flow rate into the chamber. A cylinder is placed

around the lift propeller to improve the efficiency, because it reduces the pressure loss

around the propeller tube. The second propeller is driven by another motor and is placed

on the back end of the hovercraft. This motor can only deliver a constant speed to the

propeller in contrast to the lift propeller and is used to generate a displacement forward.




Without a rudder the hovercraft is un-maneuverable. Rudders action not only creates

turning moment but also a drifting force and rolling moment, which leads to an outward

banked turn.

HOVERCRAFT TERMINOLOGY

AIR STRAIGHTENERS:

The stator blades which serve to form the air into a straight line thrust.

BAG SKIRT:

Older versions of hovercraft used continuous skirt which looks like an inner tube.

DRAG FLAP:

Flap under rear skirt which prevents skirts from scooping up water and increasing drag.

FAN DUCT:

The circular structure around the fan which assists its efficiency and houses the air

straighteners.

HOVER HEIGHT:

The height of the hard structure above a hard surface at operating speeds.

HUMP:

When starting on water the lift air creates a saucer in the water and climbing out of it is

called flying over hump.

ON CUSHION:

The position of the hovercraft when the hard structure is clear of the surface riding on its

cushion of air.




SEGMENTED SKIRT:

Skirt made up of individual pockets, segments of fingers, each separately supplied with

lift air.

SKIRT TIES:

Plastic ties which hold the skirt segments to the skirt tie anchor on the hull.

TRIM:

Posture of the hovercraft when flying or floating; this may be nose-up, nose-down or

leaning left or right. Ideally the craft should be level or slightly nose up.

The various parts are as shown in the figure 2.

Figure 2: Hovercraft parts




HULL

[1] The hovercraft hull is a solid structure which forms the base of the hovercraft which

supports the lift and thrust systems and also the driver.

For amateur hovercraft builders, plywood is the material of choice. Plywood is

affordable and easy to use. Plywood has a better “strength to weight” than steel and is

much lighter than most fiberglass hovercraft. When encapsulated with epoxy resin,

plywood is as durable and as low in maintenance as any other material. Plywood

hovercraft can be built with basic hand and wood working tools.

[2] The joining of the plywood is a “stitch and glue” method. Panels are cut to shape from

plans and joined together at the seams. A hovercraft can be assembled quickly after all the

panels are cut out.




SKIRT

The skirt is one the most important parts of a hovercraft as it is the part that allows the

hovercraft to clear obstacles. Generally speaking the higher the skirt, the larger the

obstacle that the craft will clear. However, if the skirt is too tall, the craft will ‘slides off

the cushion and the cushion will deflate or the craft will become extremely unstable.

The invention of the flexible skirt was a big step forward in the hovercraft history. Before

the flexible skirt had thought necessary, powerful lift engines were needed to create only

a few millimeters lift under the hull hard structure. The main idea behind the air cushion

introduction was to create air lubrication between the ground surface and the hovercraft,

in a way that leads to a significant of the lift power. The use of an inflated bag around the

hovercraft resulted in an efficient air distribution and a reduction of the lift power. Other

advantages of the flexible skirt development are a better stability and a better obstacle

clearance. The function of skirts for a hovercraft has proved to be as important to the air

cushion vehicles as that of the rubber tyres for an automobile. The material type and the

structure of the skirt are playing an important role in the skirt design step. The material

for a skirt bag should have high tearing and tension strength, but with as few as possible

abrasion. [3] The skirt bag is normally divided in several parts to improve the absorption

of obstacles.

Types of skirts

i. The bag skirt

ii. The segmented skirt

iii. The juped skirt

RUDDERS




Rudders are important for steering the craft. Besides their steering capability they

have a nice effect to increase usable thrust. As the propeller of fan turns the air will

generate swirls and the rudders will straighten them again. They actually act as flow

straighteners.This is why most hovercraft have more than one rudder (besides the higher

steering forces you can achieve covering more flow area).

[3] An important aspect of rudders is their drag. Build wrong they may consume more

thrust for drag then you can gain from straightening the flow so they should be made

keeping this in mind. The highest drag is from a flat plate. Round at least the corner

towards the propeller or fan and send the other side flat. A better solution would be to use

an airfoil shape. The rudders can actually made thicker with less drag than what you

would achieve with a flat plate. It doesn’t have to be a complicated airfoil, a NACA000x

(where x stands for the thickness in %) will do the job.

The pivot point of rudders is usually at 1/3 of the chord (the depth of the rudder) this

helps to lower the steering forces. The force on the forward third is compensated with the

third behind the pivot point so the force you have to hold with your steering is only the

one on the rearward third (.times the lever).

Unfortunately rudders have a less nice side effect; they also act as an elevator or push the

hull down on the outside of the curve. This can be either compensated with an elevator or

special rudder arrangements. A rudder setup, which compensates this negative effect and




even supports leaning into the right direction, is the one shown in the figure 3 below. The

upper rudder pivot point is set back behind the lower pivot point. What this does is that

the rudders are just straight up if you drive straight ahead. As soon as you turn them the

top part will fold a bit downwards and generate some lift. Set up right the craft will nicely

lean into the curve.

Figure 3: Normal and Angular Rudders

Another method, which combines rudders and elevator into one unit, are inverted V

rudders as shown in the drawing below. The disadvantage of this setup is that the linkage




gets complicated. For a turn the rudder pairs have to turn in opposite directions. If you

want to make a left turn the left rudder pair has to go up and the right pair has to go down.

The same for a right turn, just into the opposite direction. To use the rudders as an

elevator all rudders have to move into the same directions (All up to lift the bow, All

down to push the nose down.). To make the situation worse it may be required for fine

trim of the craft to have one rudder pair go up more than the other pair down for curves.

This may be the reason why this excellent rudder arrangement isn’t used for most

hovercrafts (Refer figure 4). It more or less demands that the rudders are actuated by a

fly-by-wire system have the mechanical design would be a nightmare.

Figure 4: Inverted V - Rudders

Very important for all rudders arrangements is to keep the play in the hinges as low as

possible to avoid that the pilot has to continuously play with the steering. The same

applies for the mechanical strength of all attachments. They should be made strong

enough not to flex which would also result in problems similar to the once with too much

play in the hinges.

POWER TO WEIGHT RATIO




Although an air cushion vehicle does not require the critical power-to-weight ratio

precision as does an airplane in order for it to still operate, it is nonetheless necessary to

consider the power-to-weight ratio at the design stage of an air cushion vehicle, rather

than find out later that there isn’t enough power to lift or move the craft.

The power-to-weight ratio determines in large part the amount of ground clearance

between the skirt and the ground surface. The greater this round clearance the more

efficiently the propulsion system operates. That is not to say that the higher the hovercraft

lifts into the air the better. Lifting it too high will cause instability.

Such is the power of the lift that even a severally overloaded and miscalculated power-to-

weight ration hovercraft construction will still work, but it is far from ideal.

Power-to-weight ratio = P / W

POWER TO WEIGHT TO STRENGTH RATIO

The next consideration for a properly functioning hovercraft is the power-to-weight-to-

strength ratio. This deals with the structural strength of the raft to be light enough to be

lifted by the air cushion created underneath, yet strong enough to carry the weight of the

engine, its passengers or payload.




Air cushion vehicle hull construction is more closely based on aviation rather than marine

construction for the simple reason that aviation hulls are a combination of strength and

lightness as opposed to strength as a priority.

Although wood and plywood are often used, many hovercraft hull structures are made of

aluminum skin, welded or riveted onto an aluminum web or frame. Enclosed spaces are

sealed to provide airtight compartments for natural buoyancy.

A hole in the raft can be made to feed air to the plenum chamber beneath the craft.

However, the use of the new skirt techniques makes peripheral jets, led in from the edge f

the raft through ducts, more sensible and more common.

Other crafts use aluminum-honeycombed paneling to provide the buoyancy, and fiber

glass and composite materials, such as PVC, are becoming more popular as they combine

strength, lightness and buoyancy in a single material.




Figure 5: Fan placement and the airflow in a Hovercraft

THE DESIGN PROCESS




In designing Hovercraft one of the first steps is to decide what is needed that is nothing

but specifications. Difficult as it always seems, the designer must try to write out a series

of requirements. Size, weight, performance, control, cost, noise, life expectancy,

appearance, and comfort are the contents in the specifications. Improper specifications

soon become apparent and consequently, specifications often needed a revision. Conflicts

arise and compromise becomes unavoidable. Probably the most common specification are

Size, Cost, Performance, Dependability / maintenance, appearance and capabilities.

Hovercraft lift calculator




Enter the required data in the following fields; use the metric or imperial boxes, not both.

When done, press the "Calculate" button. The answers will appear below. Press the

"Clear Input" button before starting a new calculation.

Top of Form

Enter hull length in meters: or feet:

Enter hull width in meters: or feet:

Enter Amount of air gap required in mm (i.e. 13): or inches (i.e. 0.5):

Enter max. gross weight of craft in kg: pounds:

.

(m) (ft)

Total hover gap area (m^2) (sq. ft.)

Total cushion area (m^2) (sq. ft.)

Cushion pressure (N/m^2 = Pa) (lbs/sq. ft, PSF)

Cushion pressure(mm of water @

4degC)

(inches of water @

60degF)

Expected actual air

velocity(m/sec) (ft/sec)

Lift air volume (m^3/sec) (CFS)

Estimated lift engine

power(kW) (HP)

Estimated fan diameter (m) (inches)




Results below are computed from the Input Data above and

are to be considered approximate values only.

Table 1: Hover calculator

(Readings)

7.4 BAG SKIRT:

Recently the most widely used skirt was the bag skirt, which is like a giant inner

tube fitted around the crafts perimeter. Additional pressure is needed to inflate. The bag

against air pressure under the craft in the cushion.

BAGS

The bag is popular as its manufacture uses the least amount of material compared to other

skirts types. There is usually little or no wastage. In the US this is still the most common

skirt found on home built machines. Fabrics for bags are usually the same as for jupes,

which is 5.21N/m2. Neoprene coated nylon.

The nylon-base fabric is usually about 2.33N/m2.

Bags are of two types, flow through and non- flow through.

Drape the material over the corner and mark the shape of the joint. Allow a 0.0254m glue

seam. Seams should always lay in the direction of travel as much as possible.

Now it is very important that the relative position and shape of each corner with respect to

each other corners be obtained before cutting the final skirt panels.

Making skirt is an art. The simpler the craft shape the easier fitting the bag becomes. Coat

the thread with epoxy or urethane to prevent fraying. Bags can be fitted to the curved




Hulls and other odd shapes but with more difficulties. The draw back of this approach is

that no two skirts will be alike and each hovercraft could have different characteristics

depending on the quality and actual shape of the skirt.

Bags assume different cross section shapes depending upon the attachment points; the

pressure ratio between the bag and the cushion, as well as the local skirt height. Creases

in the bag will cause drag, wear and uncomfortable operating characteristics.

For best results when bonding skirt seams, properly clean, glue, and rivet using solid

aluminum rivets with large flange aluminum backing washers placed on each side of the

seam and spaced about 0.0381 m apart. However, neoprene contact (weather stripping)

cement will, for most applications, be adequate.

There is a natural tendency for bags to bounce. This is especially bad over smooth hard

surfaces like concrete and ice. Diaphragms fitted internally will reduce bounce but this

solution is uncommon. Instead several strips of skirt material are sewn along each side of

the bag to help damp out oscillations and act as a spray deflector. Its is tough that bounce

is more slightly to occur when the bag cushion pressure ration lies between 1.2 and 2.0 of

course this is the ratio at which most bags operate.

Non- flow-through bags are usually inflated through a splitter plate connected by a small

duct placed directly under the lift pan in the fan duct. Its size is usually 10% of the lift

duct area. The remaining duct area provides the lift air for the cushion. For best overall

craft performance lift air should be introduced into the forward part of the cushion.




ENGINES:

The lack of engines specially developed for hovercraft use has been, and continues to be ,

a serious impediment to the production of low cost, reliable, high performance hovercraft.

Some types of engines are most suitable than others, but all have been adapted and thus

all types have problems when used in hovercrafts.

FAN

The requirements of the fan are that it must provide the required volume of air at the

required pressure.

PRESSURE REQUIREMENT

The definition of pressure is the force exerted per unit area.




Cushion pressure = Total weight of the craft / area of the cushion.

As the cushion pressure is selected to offset the craft and thereby provide lift, [3] any

change in craft weight or pay load, will require a change in cushion pressure. Any change

in hover gap will result in variation of the air volume required to maintain the cushion. In

practice, this variation occurs in the change in surfaces over which the craft travels. As a

consequence, it is necessary for the fan selected to supply sufficient pressure and volume

in different conditions specified.

Here,

.

Total pressure inside the skirt, Pt1 = 823 + Ps1 ------------------------ (1)

Total pressure outside the skirt, Pt2 = 823 + Ps2 ------------------------ (2)

We know that when the air leaves the system all the static pressure is converted into

velocity pressure.

Therefore total pressure as only one component viz.., velocity pressure.

We know velocity pressure = ½ ρV2

Where,

ρ = density of air

V= velocity of air

Flow rate,

Qnom = V * A

Qnom = √ (2 * Pc / ρ)* h* CPc * Dc.

The discharge coefficient ‘Dc’ depends on the angle formed between the skirt and the

ground and is found from Von Mises equation.

Dc= 0.5 + (0.4 * 10^-3) * θs + (0.109 * 10^-4) * θs2 - (0.494 * 10^-7) * θs3




+ (0.345*10^-9)* θs4.

For 45 degree skirt angle the Dc=0.537.

Design factor to account for non-ideal surface conditions (Ks).

Typical design factors

Ice = 1.

Hard mud = 1.2.

Very short or wet grass = 1.3.

Sand or 4" grass (pliable) = 1.4.

Short course grass = 1.5.

Wet or sticky mud or Long pliable grass = 1.6.

Very choppy water or long course grass = 1.7.

Very sticky mud or shallow shale 1.8+.

Assuming Ks =1.8

Therefore,

Cushion design flow rate,

Q =Qnom*Ks m3/ sec

Depending on this value we select appropriate fan.

ENVIRONMENTAL IMPACT OF HOVERCRAFT

For the majority of the people hovercraft is a new experience. Our knowledge of them

is usually gleaned from newspapers, television, magazines and the like, and

consequently misconceptions are ripe.

For any vehicle to operate economically the drag, or resistance to motion, must be

kept to a minimum. The majority of the drag arises from the vessels hull through the

water; therefore we can reduce the drag and consequently propulsive power by




minimizing hull contact. The hovercraft achieves this by using low-pressure air to

form air cushion beneath it, thus actually lifting the hull clear of water. The main

point here is that because the hovercraft does not pierce the surface over which it is

traveling the advantage is two-fold:

a) less friction = less fuel burnt to move,

b) Less friction or interruption to the surface = less disturbance of the environment.

Therefore hovercraft can be used to best advantage in shallow water and drying areas,

these areas often being remote and environmentally sensitive. Most of these areas are

wetlands, swamps, and river delta. They are not only the feeding ground for a lot of

different species of birds as well very often used for marine live for hiding from

predators. In these sensitive areas are nearly to transportation means available.

Hovercrafts are able to operate in these areas in an environmentally friendly way.

Being amphibious the hovercraft can use direct routes across sand banks, marshes and

flats, with no loss of speed and comfort, with no loss of speed or comfort. Channel

dredging becomes unnecessary, whilst rivers and tidal estuaries present no problem

for the passage of the vessel. Previously inaccessible areas may be accessed

Economically with little or no impact on their environment. If this area is a future

wetland where nobody could proof that it is a wetland or not-since it was not

accessible by traditional transportation means, or a very shallow bay where you need

to download your data from your instrument platform. With the Hovercraft you can

reach destinations which where before not accessible for research and studies.

Damage to the shore environment, such as beaches, mud flats and vegetarian is

virtually nil because of the hovercraft’s low pressure “footprint”. For example, the

average human being when standing on a beach exerts a pressure of some

0.021N/mm2 underfoot, rising locally to 0.0172 N/mm2 when walking. The average

hovercraft by comparison, exerts a pressure of only N/mm2 on the surface regardless

of speed. This “footprint” pressure is less then that of a seagull standing on one leg.




All advanced Hovercrafts are fully amphibious and create virtually no under water

noise, just atmospheric noise levels which would typical of truck or bus. The fact that

there are o underwater protrusions eliminates the usual thrashing noise signature

associated with conventional propeller driven craft, as well as negating any possible

sea bed erosion when operating in shallow waters. It therefore becomes obvious that

fish and other marine life are in no way affected. This has been confirmed by

independent scientific tests. The major noise factor with any hovercraft is the air

propeller noise, which in case is largely directional in characteristic and more

apparent to above surface creatures like humans. To minimize this effect advanced

hovercraft propulsion propellers have been designed with low tip speed to minimize

atmospheric noise. What must be attempted is to keep the noise within tolerable limits

and we believe that we have gone a long way towards this.

Another important factor is that, by necessity, the hovercraft hull is ‘sealed unit’. Any

accidental discharges and leaks are fully contained within the hull structure,

remaining there to be pumped out an appropriate shore facility. There is no exhaust

discharge into the water as with most conventional water craft, thus eliminating the

pollution of the marine environment by oil and fuel particles, particularly prevalent

with two stroke outboard motor usage. These engines operate at a mixing range of

1:50=1part oil:50 parts gasoline where the unburned oil and fuel particles get

discharged over the lower unit of engine-basically oil injecting the environment you

navigate in. Atmospheric pollution is also considerably is less because of the inherent

fuel efficiency of the modern four stroke machinery utilized, as opposed to very large

percentage of marine propulsion units which still utilize the stroke principle. In

addition, the hovercraft is in itself a fuel efficient mode of transport, thus lessening the

pollution of the atmosphere even more. For example a fully loaded six-person

hovercraft burns less fuel per hour then a typical Jet Ski; it does not pollute the water

and does not disturb the bottom in shallow areas which many fish rely upon for food

and breeding. The lower hull of a anti fouling coating like on all common boat hulls.




These anti fouling paints have just one purpose-slowly come off your hull with

whatever marine lives adhere to it. On an average of 30 boat hull you have to apply all

two years a new bottom coat-besides the expenses of haul out-you will apply between

0.00373m3 -0.00568m3 of bottom paint to your hull. Bottom paint which tries to have

a very high copper contend to work effectively (30 to 75%). Copper is just not the

best stuff for our marine environment, but we do our best to supply on a continuous

basis till we find a better solution……when? If the last fish is dead?

The wake created by the passage of the hovercraft is minimal, ensuring that river bank

erosion and damage to foreshore by the waves created is virtually nil. A study in the

United Kingdom concluded the passage of hovercraft over inter tidal areas caused no

damage to sea grasses or invertebrates. It was also noted that the bird life rapidly

adjusted to the presence of the hovercraft. This has been confirmed on the Gold Coast

(Australia) where a commercial operator passed over the same area of the beach many

times a day for more then three years without any effect to the Yabby population

( Yabbies live just below the wet sand) on the actual operating launch ramp. The air

pressure exerted by the hovercraft is considerably less then the normal pressure

variations due to natural tidal movements. Commercial operators in other areas

around Australia have had similar experiences.

Last not least – Hovercraft need minimum base requirements. Hovercrafts do not

require docks, piers or dredged channels and can operate off many beach sites. Road

access to the maintenance base is desirable but not essential. Hovercraft can work in

areas and leave without the need for environmentally damaging the construction

projects to support the operation.




APPLICATIONS OF HOVERCRAFTS

Due to their characteristics hovercrafts have a big variety of applications, some of

which are listed below.

Passenger transport and tourism related excursions.

Commercial freight transportation.

Exploration

Search and rescue

Patrol and security

Amphibious

Fast attack

Mine counter measures

Policing and Customs.




Logistics

Medical Evacuation

Crash Rescue

Hydrographic survey

Commando Missions

Range patrol

CONCLUSION

A hovercraft being an expensive piece of equipment is useful in many

applications like tourism, commercial transportation over land and water,

exploration of marshy areas, patrol and security of marshy areas and

border security in deserts, amphibious assaults, immediate military attacks,

carrying out counter measures for mining areas, policing and custom

applications, medical evacuations can be carried out by a hovercraft, crash

rescues, hydrographic surveys, commando missions and range patrols can

also be done by a hovercraft.




Through the capital cost and maintenance cost of the hovercraft is very

high. It is useful in many applications and can be useful in many more

futuristic applications due to ease of control.

References

Websites

http://4wings.com.phtemp.com/index.html

http://rchovercraft.com/index.html

http://www.geocities.com/ronaldb66/index.html



http://www.geocities.com/ronaldb66/index.html

http://rchovercraft.com/index.html

http://4wings.com.phtemp.com/index.html






















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Hovercraft 1