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Development of a Integrated Air Cushioned Vehicle (Hovercraft)  S.V. Uma Maheswara Rao1, V.S. Surya Prakash2 1, 2 (Professor, M.E Student, Department of Marine Engineering, AUCE (A), Visakhapatnam-5300003)
A  hovercraft  is  a  vehicle  that  hovers  just  above  the ground,  or  over  snow  or  water,  by  a  cushion of air trapped under the body creating lift. Air propellers, water propellers, or water jets usually provide propulsion. This type of vehicle is known as air cushion vehicle(ACV).it is a  craft  capable  of  travelling  over  land,  water  or  ice  and other  surfaces  both  at  moderate speeds,  and  even it coul
d hover at stationary condition.  It operates  by  creating  a  cushion  of  high  pressure  air between  the  hull  of  the  vessel  and  the  surface below. Fig1 illustrates the operational principles and basic components of a typical hovercraft. Specifically for our hovercraft, has three main design groups: the lift, thrust, and steering systems. The arrangement of the hovercraft is similar to that shown in Fig 1
Fig 1: Air Cushioned Vehicle (Hover craft)
The propeller shown must be designed for a vehicle as typically a fan for creating vortices to mix the air, reducing the ejected air’s translational kinetic energy to provide the necessary lift and thrust. Typically the cushioning effect is contained between a flexible skirt. Hovercrafts are hybrid vessels they typically hover at heights between 200mm and 600mm above any surface and can operate at speeds above 37km per hour.  They can clean gradient up to 20 degree. Locations  which  are  not  easily  accessible  by  landed vehicles  due  to
natural  phenomena  are  best  suited  for hovercrafts. Today they are commonly used as specialized transport  in  disaster  relief, coast ground military and survey applications as well as for sports and passenger services. Very large versions have been used to transport tanks, soldiers and large equipment in hostile environment and terrain. In riverine areas, there is great need for a transport syst
em that would be fast, efficient, safe and low in cost.  Time  is  spent  in  transferring  load  from  landed vehicle  to  a  boat.  With hovercraft there is no need for transfer of goods since it operates both on land and water. It  is  said  to  be  faster  than  a  boat  of  same  specifications which makes it deliver service on time.
II.Principle Of Operation
The  hovercraft  floats  above  the  ground  surface  on  a cushion  of  air  supplied  by  the  lift  fan.  The air cushion makes the hovercraft essentially frictionless. The hovercraft relies on a stable cushion of air to maintain sufficient lift. The air ejected from the propeller is separated by a horizontal divider into pressurized air utilized for the air cushion and momentum used for thrust. The weight distribution on top of the deck is arranged so that the air is distributed the air from the rear of the deck throughout the cushion volume in an approximately even fashion to provide the necessary support. The skirt extending below the deck provides containment, improves balance, and allows the craft to traverse more varied terrain. We maintain the rigidity of the skirt by filling the air-tight skirt with the same pressurized air diverted towards lift. The skirt inflates and the increasing air pressure acts on the base of the hull thereby pushing up (lifting) the unit.  Small  air gaps are left underneath  the  skirt  prevent  it from  bursting  and  provide  the  cushion  of  air  needed.  A little effort on the hovercraft propels it in th
e direction of the push [7]. Steering effect is achieved by mounting rudders in the airflow from the blower or propeller.  A change in direction of the rudders changes the direction of air flow thereby resulting in a change in direction of the vehicle.  This is achieved by connecting wire cables and pulleys to a handle. When the handle is pushed it changes the direction of the rudders.
III.Conceptual Design
Integrated lift and thrust hovercraft
Fig2: Integrated Hover Craft
This design proposes an integrated a single propeller is used for both the lift and thrust requirements. Here in this design the air from the shroud is split to cater to two for the lift and thrust. 40% of the air is split and directed towards the base which will fulfil the lift requirements and the rest of the air is used to propel the hovercraft thereby fulfilling the thrust requirements. The salient feature of this design is that it requires only one source of power as shown in Fig only one engine for thrust and lift [4] and therefore this design becomes an economically better option. However there are issues in this design such as air distribution that require further attention.
IV.Design Of Major Components
4.1 The hull, skirt calculations:
Our intention is to design a hovercraft for demonstration purpose. so a total weight of 200 kg is considered of this, 100 kg has been taken as passenger weight, and the remaining 100 kg as the hovercraft weight, which includes the weight of the base, the weight of the engine, impeller, shroud, the air box, steering mechanism, rudder system, the engine frame, the weight of the skirt, petrol tank etc.[3].
Fig3: Hull Dimensions  Total Weight  = 200 kg
Length = 7 feet
Breadth  = 4 feet
Area of Base, A = 28 sq.ft  = 2.5 m2
Cushion Pressure,
P c=
Weight
Areaofbase
=
200
2.5
=80 kg/m2
P c= 80 ×9.81=784.8 N/m2 (Pa) Escape Velocity,
V e=2 ×P c
ρ
=
2 ×80
1.16
=11.75 m/s
Perimeter of Skirt
5.25 + 5.25 + 2.5 + 2.5 = 15.5 ft = 4.72 m Hover Gap
50 cm = 0.05 m
4.2 Lift and thrust calculations Escape Velocity,
V e=2 ×P c
ρ
=
2 ×80
1.16
=11.75 m/s
Perimeter of Skirt = 5.25 + 5.25 + 2.5 + 2.5                        = 15.5 ft = 4.72 m
Hover Gap = 50 cm = 0.05 m
Escape Area, A e = 4.72 x 0.05 m = 0.236 m2pulleys
Volume of Air Lost = V e x A e = 11.75 x 0.236                          = 2.773 m3/s
This much volume of air is required to lift the hovercraft of the total airflow generated by the impeller, 33% is used to lift the hovercraft. This 33% corresponds to 2.773 m3/s. So the total volume of air that must be generated is three times this quantity.
Therefore,
Total Volume of Airflow required
3.33 x 2.773 = 9.23 m3/s
Hence, we need to select an impeller that can provide us with pressure of 785 Pa and airflow of 9.23
m3/s.
4.3 Splitter Area and Thrust Area
Fig 4: Splitter Area of Propeller
Area of the duct,a d=π
4 (.92−.22 )
= 0.6048 m2 For Splitter Height = 0.30 m
Splitter Area,
a sp= 1
2
(θ−sinθ )r2,θ= 1400
a sp=1
2
{(  140 ×
π
180
−sin140 )×0.452 }
= 0.2419 m2
Thrust Area, a tℎ=a d−a sp                              = 0.3629 m2
Thrust Ratio = a tℎ
a d =0.3629
0.6048
=60.0 %
Lift Ratio = a sp
a d =0.2419
0.6048
=39.99%
4.4. Fan Selection
The selection of a suitable impeller is a relatively tough task. In an integrated hovercraft, the impeller is used to provide both, the lift as well as the thrust. Usually, industrial fans are used for this purpose. Some of the most important factors that need to be considered while selecting an impeller are the size of the impeller, the number of blades, the pitch angle of the blade, and the power required. The power source (in our case – engine) should be able to provide enough power to run the impeller at the required working conditions [6].
Formulae:
For a change in speed:
The Rotational Frequency Ratio
(k) =  N2    ( NewSpeed  )
N1    (OldSpeed )
To change the parameters for the fan at the new speed we can apply the following laws,
Airflow (q),    q2  = q1 x k
Pressure (p),    p2 = p1 x k2
Power (P),      P2 = P1 x k3
In our case, we required a pressure of 785 Pa and airflow of 10m3/s. For selecting the fan we used sizing software called Multi-Wing Optimizer 7.0.1.144, provide by Multi-Wing India Pvt Ltd.
The specification of the impeller selected for our hovercraft is
900/3-6/31.5/PAG/5ZL
Breaking Down the Part Number Code:
Fig 5: Breaking down the code
In the sizing software we input conditions such as Pressure required and airflow.
Fig 6: Sizing Software
Based on the data obtained from the calculation’s the hovercraft has been modelled in the catia for understanding the 3-dimensonal design of how the hover craft looks like. The parts are modelled in individual workbenches and later on assembled.
Now the fabrication of the hovercraft (acv) is initiated. The materials availability according to the design is a big task.
V.Fabrication Of The Hover Craft
5.1 Hull:
Hull is made by sandwiching polystyrene sheets between plywood sheets; Industrial grade glue is used for this purpose. First plywood is cut according to the specifications given in Fig 7 glue is applied to a plywood sheet and 4 polystyrene sheets, and they are stuck together. Polystyrene sheets are kept
horizontal to the plywood. Then another layer of polystyrene is adhered to the existing one to get required thickness, this time sheet are arranged vertical to plywood. Then another plywood sheet is adhered at the top. Prepared hull is kept under pressure overnight.
Fig 7: Hull

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