dr. n ramachandran arun premraj b080375me joe t …joe t devassy b080111me josef sebastian b080053me...
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Submitted by
ARUN PREMRAJ B080375ME
JIJU B080236ME
JOE DAVIS B080137ME
JOE T DEVASSY B080111ME
JOSEF SEBASTIAN B080053ME
Name of the Project Guide: DR. N RAMACHANDRAN
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To design, fabricate and test a single engine hovercraft which uses a
single fan for both thrust and lift
WORK DONE IN S-7
Calculation of Power Requirements
Design of Hull
Design of fan duct system, belt drive and splitter plate calculations
Design of steering system
Selection of materials for various parts
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• Hull is made from marine plywood. Marine plywood has a better strength to weight ratio than steel. It is also light in weight compared to other materials.
• Skirt is made of Polyurethane-coated nylon fabric. Nylon fiber characteristics include light weight, high strength and softness with good durability. Even when wet, nylon's overall strength may decrease only about 15% .It also has high shearing and tearing strength.
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Bottom
Top
Side
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• Fans generate the air pressure which is used to inflate the cushion contained within the skirt beneath the Hovercraft
• Provides lift and thrust which propels the craft forward
• Flaps connected to the duct helps in guiding the direction of its movement.
• Flaps are connected to the steering system.
• The fan is mounted within a protective duct called shroud.
• Shroud reduces losses in thrust from the tip vortices of the fan.
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• Tip velocity
• It is preferred to keep the tip speed below 180 m/s
• High tip speed result in damage of the blades caused by dust particles present in the air stream
• Calculation
• The fan has a diameter of 0.6 m and its speed is 1500 RPM
D is the diameter of the fan
Circumference = πD
= π X 0.6 = 1.88 m
• 1500 RPM = 25 revolutions per second
• Therefore , Tip speed = 1.88 X 25 = 47 m/s
• Hence the tip speed is well below the recommended maximum of 180 m/s
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Thrust used for the forward motion of the hovercraft comes from fan. Thrust is the force applied by the volume of air passed at the discharge of the fan. Gross Thrust, Tg = Qd x Vd x ρ Where Qd = Quantity of air at discharge
Vd = Discharge velocity ρ = Density of air
A thrust fan works by taking still air from in front of it and using the fan blades to Increase it’s pressure and velocity. If the air at the inlet already has some momentum, the fan is unable to increase it’s velocity by the same amount, this difference is reffered to as ‘ momentum drag’ Dm. Dm = Qd x Vo x ρ Where Vo = Free stream velocity Quantity of air, Qd= Fan area(A) x Discharge velocity (Vd) Net Thrust, Tn = Qd x ρ (Vd- Vo) = Vd x A x ρ (Vd – Vo) [Newtons]
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On an integrated lift design, the discharge from the fan is divided. Part of the lower portion of the fan is used to provide lift and the rest provides thrust. To obtain the area under the splitter, calculate the area of the Quadrant AOBCA and subtract the area of the triangle AOBA. The angle O can be calculated from the formula Angle O = 2 X Sin-1 ( Splitter Length/Diameter of duct) For a .748 m long splitter and .8m dia duct, this angle is 138 degrees The area of the quadrant AOBCA = πr2 (Angle O/ 360) = π X .42 X(138/360) = 0.193m2
The distance from the duct to the splitter is 0.2m (h1 ) . h2 is therefore, 0.4 - 0.2= 0.2m The area of the triangle AOBA = (h2 X Splitter length)/2 = 0.2 X .748 X0.5 = 0.0748m2
Therefore area under the splitter = AOBCA – AOBA = 0.193 – 0.0748 = 0.1182m2
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• Rudders-Rudders operates by redirecting air from the fan thus imparting a turning motion to the craft. A rudder is a flat sheet of material attached with hinges to the craft. Rudders are shaped so as to minimize hydrodynamic or aero dynamic drag.
• Steering handle and steering shaft-When steering shaft is turned ,force applied is transmitted to rudder through cables. Rudder’s then rotates on a hinge and provide necessary direction controls .
• Cables- used to transmit mechanical force from steering to rudders by the movement of an inner cable relative to the outer cable housing.
• Pipes - Pipe bents, metal hoops or angled metal tubes -used when ropes or cables are to turn a sharp corner.
• Support mechanism
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• Steering mechanism of hovercraft consists of the following parts
1) Rudders-fibre glass is used. It is light, strong and can be easily cut into desired shape.
2) Steering handle and rod-material used is mild steel which is available in workshop.
3) Cables-consist of inner cable usually steel and protective outer cable housing usually plastic .
4) Support Mechanism –material is mild steel
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Selection of proper size for the base plate
Ribs were fitted at bottom of base plate to take care of impacts
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14 Cockpit which encloses driver, engine etc . . . was made
Fixing of slant plates for making air chamber
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Slant plates and supports were fixed for better strength
Holes for air vent made throughout the craft
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Primer coated throughout the craft for smoothening air flow
Top portion of air chamber covered
Completed air chamber
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• THEORETICAL LIFT HORSEPOWER NEEDED = CUSHION PRESSURE x VOLUME FLOW RATE
• VOLUME FLOW RATE= AIR VELOCITY x AREA OF HOVER GAP
• AREA OF HOVER GAP = HEIGHT OF AIR GAP x LIFT PERIMETER
• CUSHION PRESSURE = LIFTING FORCE CUSHION AREA
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• DYNAMIC PRESSURE = DENSITY × VELOCITY2 2
Power Calculations for a 2.21 x 1.42 m hull hovercraft
Total Mass = Mass of (Engine + Structure +Human ) = 20+ 100 + 80 = 200 kg Lifting Force = 200 kg x 9.81 m/s2 =1766 N Cushion Area = 2.21m x 1.42m = 3.138m2 , Considering triangular regions, actual area is 90% of it =.9 x3.138 = 2.824m2 Cushion Pressure = 1962 = 695 Pa 2.824
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According to Bernoulli’s equation, Dynamic pressure = 0.5 x 1.15 x V2 = 695 (Density of air = 1.15kg/m3 ) From the above relation ,Theoretical Air Velocity = V= 35 m/s But practically due to variable friction against the skirt and the surface, affect values of air velocity. So practically air velocity will be about 60% of theoretical air velocity Air velocity =0.6 x 35= 21 m/s Area of hover gap = 0.013m x 2(2.21+1.42)m =0.094 m2 Lift Volume Flow Rate =0.094m2 x 21 m/s = 1.98 m3/s Theoretical lift power required = 1.98 m3/s x 695 Pa = 1375 W = 1.8 HP If a single engine is used for lift and for thrust, we have to increase this theoretical horsepower by a different factor because of the much larger inefficiencies in turning the air around a corner, then confining it by forcing it to flow through internal ductwork until it is allowed to fill the skirts. Considering inefficiencies in turning the air around corner and friction efficiencies will be as low as 30% so a 1.8 HP/.3 = 6 HP power is required
(Courtesy: James Perozzo, Hovercrafting as a Hobby, 2001)
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Engine and Fan were procured from SSI, Coimbatore
Fan was made using reinforced fiberglass plastic
4 blade fan with diameter 0.9 m
Engine power approximately 6 HP at 3000 RPM
2-stroke Kerosene –Petrol engine
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WORK TO BE COMPLETED
Fixing of engine and fan
Steering system
Fixing of Skirt
Balancing and testing