During the last century the service speed of transport concept is dramatically increased, specially the marine transportation. Displacement ships moving at high speed through water causes wave making drag in proportion to square of their speed. Thus hydrodynamic resistance was the prime factor limiting their speed and performance. One way to avoid this is to isolate hull in contact with water surface.
This project attempts to design Air Cushion Vehicle which rides on bubble of air thus omitting the effect of hydrodynamic forces largely and providing to access to almost terrains.
This report covers the concept evaluation starting with a brief introduction, history, different parts of hovercraft and the science of hovercraft. This is followed by selections of lifts system, thrust system, skirt system and comparisons of different lift, thrust and transmission system. We tried to select the best available design according to requirement of the project.
TABLE OF CONTENT
CHAPTER 1. INTRODUCTION 7
1.1 What are Hovercrafts? 8
1.2 History 9
1.3 HOW DO HOVERCRAFTS WORK? 10
1.4 Parts of hover craft 11
1.5 Science of hovercraft 13
1.6 Propelling and steering 14
CHAPTER 2. LIFT SYSTEM DESIGN 15
2.1 Types of lift system 16
2.1.1 Plenum chamber system 16
2.1.2 Air jet 16
2.1.3 Distributed air supply 17
2.2 Choice of lift system 17
2.3 Fan selection for lift 17
2.3.1 Axial flow fan 18
2.3.2 Centrifugal fan 18
2.3.3 Selection of the fan 18
CHAPTER 3. THRUST SYSTEM 19
3.1 Types of thrust fans 20
3.1.1 Air Propellers 20
3.1.2 Ducted fans 21
3.1.3 Fan jets 21
3.2 Choice of the thrust system 21
CHAPTER 4. SKIRT SYSTEM 23
4.1 Advantages of flexible skirts 24
4.2 Bag skirt 25
4 .2.1 Skirt characteristic 25
4.3 Finger skirt 26
4 .3.1 Characteristic of the finger skirt 26
4.4 Bag and finger 26
4.5 Choice of skirt 27
CHAPTER 5. DIFFERENT LIFT, THRUST AND TRANSMISSION SYSTEM 28
5.1 Aerodynamically and mechanically separated 29
5.2 Aerodynamically separated but mechanically variable ratio 29
5.3 Aerodynamically separated but mechanically fixed ratio 29
5.4 Aerodynamically and mechanically integrated 29
5.5 Comparison 30
CHAPTER 6. CONCEPTS 33
6.1 Hovercraft A 34
6.1.1 Features 34
6.1.2 Advantages 34
6.1.3 Disadvantages 34
6.2 Hovercraft B 35
6.2.1 Features 35
6.2.2 Advantages 35
6.2.3 Disadvantages 35
6.3 Hovercraft C 36
6.3.1 Features 36
6.3.2 Advantages 36
6.3.3 Disadvantages 36
CHAPTER 7. CONCEPT EVALUATION 37
7.1 Introduction 38
7.2 Weighting matrix 38
7.3 Rating matrix 38
APPENDICES 41
APPENDIX-1 STATEMENT OF REQUIRMENT 42
APPENDIX-2 GAINTT CHART
LIST OF FIGURES
1 HOW DO HOVERCRAFTS WORK? 10
2 Parts of hovercraft 11
3 Working of hovercraft 13
4 Steering of hovercraft 14
5 Principle of lift 16
6 Axial fan 18
7 Centrifugal Fan 18
8 Air Propellers 20
9 Ducted fan 21
10 Schematic diagram of bag skirt 25
11 Bag skirt 25
12 Finger Skirt 26
13 Bag & Finger Skirt 27
13 Hovercraft A 34
14 Hovercraft B 35
15 Hovercraft C 36
CHAPTER # 1
INTRODUCTION
1.1 What are Hovercrafts?
Vehicles designed to travel close to but above ground or water. These vehicles are supported in various ways. Some of them have a specially designed wing that will lift them just off the surface over which they travel when they have reached a sufficient horizontal speed (the ground effect).
Hovercrafts are usually supported by fans that force air down under the vehicle to create lift, Air propellers, water propellers, or water 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 than helicopters of the same weight.
Hovercraft is a transportation vehicle that rides slightly above the earth's surface. The air is continuously forced under the vehicle by a fan, generating the cushion that greatly reduces friction between the moving vehicle and surface. The air is delivered through ducts and injected at the periphery of the vehicle in a downward and inward direction. This type of vehicle can equally ride over ice, water, marsh, or relatively level land.
1.2 HISTORY
The first recorded design for a hovercraft was in 1716 put forward by Emmanual Swedenborg, a Swedish designer and philosopher. The project was short-lived and a craft 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, the British 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 the technology required to implement the concept did not yet exist. From this time both American and European engineers continued work on the problems of designing a practical craft.
In the early 1950s the British inventor Christopher Cockerell began to experiment 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 builder produce a two-foot prototype, which he demonstrated to the military in 1956 without arousing interest. Cockerell persevered, and in 1959 a commercially built one-person Hovercraft 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.
1.3 HOW DO HOVERCRAFTS WORK?
Front_cutaway_view_of_a_flexible_skirt_model_hovercraft
Figure 1 How Do Hovercrafts Work
Hovercrafts work on the two main principles of lift and propulsion. how
When dealing with 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 a cushion of air several inches off the ground. This process, the process of attaining lift begins by directing airflow under the craft.
In order to trap the air under the air cushion, a skirt is required. This is done in order to create pressure under the hovercraft 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 is directed under the craft, it will then hover too high above the ground, resulting in the hovercraft 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 which propels the craft off the ground is a fan. The fan can be used for lift and thrust. It can be 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 the hovercraft. This passage is a hole located on the base of the craft.
1.4 PARTS OF HOVERCRAFT
Figure1.Parts of hovercraft
A: Passenger Cabin and Controls
B: Lift Propeller (Cage Blade)
C: Lift Air Inlet
D: Propulsion Motor
E: Propulsion Propeller
F: Rudder
G: Rudder Gearbox and Motor
H: Fuel Tank
I: Annular Rings + Air Outlet Nozzles
J: Lift Motor
K: Air Plenum
1.5 SCIENCE OF HOVERCRAFT
In basic hovercraft design large ducted fans, like enclosed propellers, were used to provide lift by keeping a low pressure air cavity under the craft. As air pressure is increased the air lifts the craft by filling the cavity. The cavity or chamber in which the air is kept was called a "plenum" chamber. This word plenum is taken from the Latin word, meaning full.
At the point when the air pressure equals the weight of the craft over the chamber's surface area the craft lifts and the air starts to escape around the edges. To maintain the lift the engine and propeller or fan has to be sufficiently powerful enough to provide a high air flow rate into the chamber. The flow has to be greater than the amount of air that escapes from around the edges of the plenum chamber.
Figure 2. Working of hovercraft
1.6 PROPELLING AND STEERING
Figure 3. Steering of hovercraft
A hovercraft is pushed forward by a propeller that is powered by a motor. The propeller on a hovercraft works the same way as a propeller on an airplane. Each blade on the propeller is shaped like an airplane wing. As the blades are spun by the motor a difference in air pressure occurs between the front and back side of the propeller. As a result a propulsive force is created and the craft can be pushed forward. A hovercraft can be steered left or right by deflecting the airflow produced by the propeller using a rudder. The rudder sits behind the propeller and is moved by a steering gear or motor of its own. By moving the rudder to the right the craft is pushed in a left direction. Steering a hovercraft is much less precise than steering a car, a boat, or a plane because each of these other craft rely on frictional contact with the ground, water, or air in order to move. A hovercraft floats on a cushion of air and so there is nothing to push against in order to gain precise control.
CHAPTER # 2
LIFT SYSTEM DESIGN
The main requirement of the lift system of light weight hovercraft is to be providing sufficient air flow to maintain the designed hover height with out excessive skirt drag and with the minimum expenditure of power and fuel.
The system must also have an over load pressure capacity to prevent fan stalling due to cushion pressure fluctuations as the craft traverse undulating ground or waves.
Figure 4. Principle of lift
2.1 TYPES OF LIFT SYSTEM
Plenum Chamber design
Distributed Air Supply
Air Jets
2.1.1 Plenum chamber system
This is the simplest of lift system, where the air is dumped directly into the cushion. Thus there is no obstruction between the pumping device and the crafts bottom. There are constructional advantages in this system because the simplicity of operation, however frictional losses, increase because of the impact of air directly on the ground. Besides, due to the direct pumping system, the stability of the craft is also jeopardized especially if only one fan is used.
2.1.2 Air Jet
This system is a great improvement in the lifting of the craft, where small amount of air is required at high pressures. This air forms a curtain to the cushion. For this purpose, high pressure compressors are required, further more curtains containing numerous nozzles is also required, and the need to construct the nozzles on the entire periphery of the craft, air jet system is rejected.
2.1.3 Distributed Air Supply
In this system air pressure is increased only to the extent, than after transmission losses it will be enough to raise the support of the craft. Because of this feature air is promptly supplied all around the craft ensuring the craft stability. Further more maximum advantage of the compressed air is achieved by supplying more to the bow section. Whereby, on operation it can be utilized along the entire length of the craft before it escapes to the atmosphere
In this context, construction of the distribution channels or ducts would not prove to be of great difficulty since they could be easily incorporated along the side wall of the hull; however it must be ensured that no sudden expansion or contraction of the duct should occur since it would invariably result in pressure drop. Furthermore the route of air supply must be smooth, for this guide vanes are installed when ever the air is supposed to turn a bend this would certainly decrease losses.
2.2 CHOICE OF LIFT SYSTEM
Thus because of the simplicity of design, easy in construction and stability in operation, the distributed air supply system is chosen.
2.3 FAN SELECTION FOR LIFT
The function of lift fan is to provide
• Sufficient pressurized air to support the craft weight on the cushion
• The required flow rate in order to maintain the design skirt tip air gap , and in addition to meet the requirement due to wave pumping , heave motion pumping and the changed of cushion air requirements as the hovercraft moves over differing terrain
• A suitable fan pressure/flow characteristic curve so that it will not stall during craft operation.
There are two generic types of fans which may be used.
1 Axial flow fans
2 Centrifugal fans
2.3.1 Axial Flow Fan
Axial flow fan moves air parallel to axis of rotation. Axial flow fans are very common and are used for large volumes of air at low pressures.
Figure 7. Axial fan
2.3.2 Centrifugal Fan
The centrifugal fans consist essentially of a stationary casing containing a rotating impeller which imparts a high velocity to the air and a number of fixed divergent passages in which air is decelerated with a consequent rise in static pressure. Centrifugal fans offer higher pressure but at lower air flow rates.
hoverc3
Figure 8. Centrifugal Fan
2.3.3 Selection Of The Fan
Axial flow fans have high potential for the both high pressure ratios and higher efficiencies, compared to centrifugal fan. They have much larger flow rates for a given frontal area. They have lesser weight for specified level of performance, which is particularly important for hovercrafts.
CHAPTER # 3
THRUST SYSTEM
The propulsion system of light hovercraft should be designed to give adequate thrust for acceptable performance both over land and water.
The thrust required for satisfactory operation over water is greater than required over land. A craft statically hovering over water will create a desperation on the surface beneath it .In order to overcome this depression and ride into a planning mode or as it is commonly termed as "to get over hump" a certain amount of thrust is required which is a function of craft weight and cushion pressure.
3.1 TYPE OF THRUST FANS
There are various types of thrust systems which can be availed to run the craft. However all of them are initiated from Newton's second law of motion. If it is desired to have a fully amphibious hovercraft then the only practical means of propulsion are air propellers, ducted fans and fan jets. However if the hovercraft is purely marine than water propulsion is preferred since it has the advantage of less power consumption and are also less power susceptible to head and other wind forces.
The main disadvantage of air propellers is that they have low efficiency at lower forward speed. The three types of propulsion systems for the amphibious hovercraft are discussed in detailed.
3.1.1 Air Propellers
Propellers or airscrew are designed to convert the rotational torque to the power plant into axial thrust of the fluid. The energy transfer occurs when the propeller "bites" into the distributed fluid pushes it backward and the law of action and reaction provides a forward force on the propeller shaft, so called "propeller thrust".
hoverc6
Figure 7. Air Propellers
3.1.2 Ducted Fans
Ducted fans are a form of air propellers where the propellers are installed with in circular duct which defines air inlet and outlet. The possibility of having a ducted fan is high in small hovercraft where the fan is installed at a lower height and the occupants of the craft are sitting in open air. By installing a proper duct around the fan , it makes propulsion system as effective as the un-ducted propeller of twice the area ,as well as keeping the fingers out of the fan and retaining the bits should it burst .Here too multi-bladed fans can be used for increasing efficiency.
Figure 8. Ducted fan
3.1.3 Fan Jets
Fan jets propulsion system is yet another form of providing driving thrust. This system is a refinement of the ducted fan where the cross section of the duct decreases with length, thus forming a nozzle .In this form of construction the pressure rise is decreased to ambient conditions consequently and the velocity increase. In this way a jet of air thrust is produced from a very small cross section area at the rear of the craft.
The improvisation has not only similar efficiency to a ducted fan but also aids the control of the craft. In this form of propulsion the reversing of the craft is possible by installing "buckets" which changes the direction of thrust .The system has by far the best optimum efficiency with maximum control effectiveness.
3.2 CHOICE OF THE THRUST SYSTEM
The best option in hand is the ducted fan. This system is as efficient as the air jet system for propulsion. In comparison to the propeller the ducted fan produce the same thrust when the area is half in size of the propeller .Thus a reduction in the diameter is initiated.
Besides the benefit of the efficiency , the duct itself proves to be a great safety for straying hands and fingers .Further more due to extreme stresses of fatigue if any part of the propeller breaks it is confined with in the duct, thus safety is achieved in both ways. Propellers must be replaced if damaged by the debris whereas only blade is replaceable on a ducted fan.
CHAPTER # 4
SKIRT SYSTEM
All modern hovercrafts- large or small use skirt of one sort or another for the suspension system so that the power required to lift the craft can be minimized. The flow of air leaving the hovercraft air cushion is directly proportional to the craft peripheral length and hovercraft height. In order to reduce the flow and the power required to maintain it, flexible skirts were developed, allowing the hovercraft to maintain a reasonable hover height with a few centimeters air escape gap and minimal "bleed".
Hovercrafts skirts are required to fulfill the following functions
• Contain the cushion of air beneath the craft to give required lift
• Give adequate stability
• To return to original shape after having been deformed
4.1 ADVANTAGES OF FLEXIBLE SKIRTS
• Significant reduction of lift power
• Practical obstacle clearance
• True amphibious capabilities
• Decreased calm water resistance, particularly at hump speed
• Improved maneuverability by use of skirt lifting and shifting
• Improves seaworthiness through wave-following capability of third generation designs such as low bag pressure responsive skirts
• Improved maintainability of ACVs since flexible skirts can be easily detached/attached and replaced
4.2 BAG SKIRT
The inflated loop consists essentially of a tube of material which is inflated at a slightly higher pressure than the air cushion beneath the craft.
Figure 9. Schematic diagram of bag skirt
4.2.1 Skirt Characteristics
The bag skirt is fairly simple to design and construct but gives a harder ride than the segmented type and has more limited obstacle clearance, depending upon the pressure differential between the loop and the air cushion. Usually it gives fairly high drag over undulating surfaces. Also inflated loop skirt is very stiff in roll and pitch.
Figure 10. Bag skirt
4.3 FINGER SKIRT
The finger skirt is comprised of a large number of separate segments which are able to slide and bellow individually to conform to the shape of the water surface or terrain over which the craft is traversing.
4.3.1 Characteristics of Finger Skirts
The straight finger skirt is easy to design and construct and repair work is also easy because of the small size of each segment. The extended finger however is a little more difficult to design. The finger skirt, straight or extended, gives a very smooth ride and has a low friction characteristic and so long as the cushion height is adequate, is ideal for traveling at high speed over waves and rough ground. Apart from the rear fingers which should have an anti-scoop flap, a hovercraft with a finger skirt does not trap water like a bag skirt at lift off. An extended finger uses a great deal more material than a straight finger or bag skirt and for this reason, it has become common practice for the expensive low friction neoprene coated fabric to be used only on the knuckle which constantly rubs across the on-coming waves and for less expensive non-coated fabric to be used on the bellow and sides of finger.
Figure 12. Finger skirt
4.4 BAG AND FINGER SKIRT
The integrated bag and finger skirt compromises between the flexibility of the finger skirt and the economy of the bag skirt. It is the most complex and sophisticated Hovercraft skirt - a combination of a pressurized bag skirt and finger skirt. This combination uses the positive aspects of both skirt designs while only implementing marginal disadvantages of the finger and bag skirt.
The advantages are a relatively smooth ride over most uneven surfaces with the finger section quickly adjusting to the surface contours. Low friction from the small cross section of the finger compared to the wide and relatively inflexible bag skirt. Damage to several fingers does not come at the same time and it is easier to repair in the field once damage occurs than a bag skirt which flattens off readily. It is easier and more inexpensive to replace some finger than a whole bag skirt of a craft. While a replaced finger has the same ground contact shape as the other entire finger and will not be worn off quicker than the surrounding finger.
Figure 14. Bag and finger skirt
4.5 CHOICE OF SKIRT
The simplest way to choose the skirt is to examine their development from the early type to present day arrangement. Bag type skirt has relatively high stiffness thus producing a very useful cushioning effect when the craft is operating on rough terrain, protecting the surface from severe impacts .On the other hand the advantages of finger type skirt is that we can obtain uniform cushion pressure by channeling the cushion air inwards, reducing escaping tendency of the air outwards. A compromise solution is that of using a bag with finger. This has the advantage of reducing drag and retaining the cushion effect of the bag too. The use of the finger on the outer skirt suggests a similar device for the keel and stability bags.
CHAPTER # 5
DIFFERENT LIFT, THRUST AND TRANSMISSION
SYSTEM
5.1 AERODYNAMICALLY AND MECHANICALLY SEPARATE
These are systems where the air handling devices (fans or propellers) are in no way connected together. No mechanical linkage and no aerodynamic linkage, each with separate power sources (engines). Each system can be operated completely without regard for the other.
5.2 AERODYNAMICALLY SEPARATE BUT MECHANICALLY 'VARIABLE RATIO'
These systems are partially integrated because they share a common power source that can be shared between them in a variable manner according to demand but other than that they operate separately to each other and are aerodynamically independent. The mechanical drive systems are organized so that the drive ratios are adjustable thereby allowing speed variation and the application of more or less power to either the lift or thrust systems as required. The power engine can change speed and the operating speed of either or both systems can optionally remain constant or vary as desired and controlled by the hovercraft operator.
5.3 AERODYNAMICALLY SEPARATE BUT MECHANICALLY 'FIXED RATIO'
These systems are mechanically integrated because they share a common power source and the drive ratios to each are fixed and non-variable. Output speed changes from the power source (engine) will increase the rotational speed of both the lift fans and the thrust propellers in the same ratio. Other than that, they are aerodynamically separate. Increase in engine speed will give more thrust and more lift simultaneously and vice versa.
5.4 AERODYNAMICALLY AND MECHANICALLY INTEGRATED
These systems typically have the fans (or propellers) providing both lift and thrust. There may be one or more fans (or propellers) but the common factor is that some air passing through the system will be used for thrust and some will be diverted for lifting the hovercraft. Variable ratio drive systems are pointless because the power source (engine) speed can be changed to alter the fan or propeller speed. Increase in engine speed will give more thrust and more lift simultaneously and vice versa.
5.5 COMPARISON
Table 1
Aerodynamically Separate
Aerodynamically Integrated
Separate design criteria, easy to get the best results for each system.
Compromised design criteria, hard to get the correct balance between system requirements. Thrust requirements are usually compromised by minimum lift requirements.
Ability to have a very flexible operating characteristic in the lift system making driving the hovercraft easier.
Operation is compromised. Usually over-ridden by minimum lift requirements so if too much thrust coincides then it has to be 'lost' by aerodynamic spoilers or skirt dragging, both of which result in lost power and possibly lost rubber from the skirt.
Less noise possible through targeted aerodynamic design.
Much noisier. The fan design usually operates according to the hovercraft lift requirements, which results in a very poor thrust performance.
Placement of lift and thrust systems can be separated to improve the lift system efficiency. It is preferred to introduce most of the lift air near the front of the cushion system for good dynamic response when operating at
high speed over rough water. Additionally the lift fans can be arranged to blow directly into the lift system with less bends or ducting and thereby suffer less losses.
Placement of both systems is usually at the rear of the hovercraft, which is not conductive to good lift system ducting efficiency since most of the lift air input to the skirt system is preferred at the front of the hovercraft. Usually the air is traveling at high velocity in a rearwards direction and it needs to be turned into a downward or forward direction to get to the skirt system. This result in considerable pressure losses from the bends and additional ducting required to redirect the air into a favorable position within the skirt system.
Because of the above, smaller engines are required. Performance can be achieved with less installed power that means smaller, lighter and more economical engines.
Larger engines required. More installed power required to achieve the same hovercraft payload result. Heavier engines reduce payload capability and require more fuel, which further reduces payload (or duration). It is a negative compound effect.
Table 2
Transmission Systems Comparison Table
Mechanically Separated
Mechanically Variable Drive Ratio
Mechanically Fixed Drive Ratio
Two independent power sources needed.
Can share power sources.
Can share power sources.
Very simple transmission
Systems possible because of direct coupling possibilities.
More complicated transmission systems required. Usually employing hydrostatic systems to the lift fan drive and mechanical systems to the thrust system. Sometimes variable mechanical systems are employed involving clutches and different drive ratios or gearboxes but this is even more complicated than and generally not as reliable as a well designed hydrostatic system.
Transmission systems are usually mechanical and not too complicated but duplicated separately for lift and thrust.
Very easy to control
separately. Lift system can be
operated without even starting the thrust engine.
Also full thrust can be achieved while the
hovercraft is floating.
With good design, separate control can be nearly as good as a mechanically separate system. Full lift is obtainable with zero thrust for easy craft handling. This system is ideally suited to middle size hovercraft with diesel engines and gives a big increase in overall hovercraft performance levels by allowing the most advantageous application of the limited power available from the diesel engine.
Not able to be controlled separately. If you need more lift then you also get more thrust, even if it is not wanted. Also if you need more thrust, for example when taking off or running into head wind then you automatically get more lift which may be a waste of power needed for thrust in difficult conditions.
Very good control available to the driver.
Very good control available to
the driver.
Not so good control available to the driver. However this system is still much better than the aerodynamically integrated system.
Efficient use of installed power results in smaller engines required.
Efficient use of installed power results in smaller engines required.
Not so efficient use of installed power results in larger engines required or a reduction in available performance.
Low operating cost.
Lowest operating cost.
High operating costs due to power wastage than ideal.
CHAPTER # 6
CONCEPTS
6.1 HOVERCRAFT A
6.1.1 Features
Skirt system
bag
Lift system
Air jet
Thrust system
Air propeller
Power Transmission Systems
Aerodynamically and mechanically separate
Fan
Axial flow fan
6.1.2 Advantages
• Bag type skirts are simple to design and construct.
• Due to air jet lifting system small amount of air is required at high pressures.
6.1.3 Disadvantages
• Bag type skirt gives harder ride and higher drag over undulating surfaces.
• Entire skirt will have to be replaced in case of puncture.
• High pressure compressors and numerous nozzles are required because of high pressure demand.
• Low efficiency at low speed due to air propeller thrust system.
Figure 15. Hovercraft A
6.2 HOVERCRAFT B
6.2.1 Features
Skirt system
finger
Lift system
plenum
Thrust system
Ducted Fan
Power Transmission Systems
Aerodynamically and mechanically integrated
Fan
Centrifugal flow fan
6.2.2 Advantages
• Finger type skirt gives uniform cushion pressure, reduces the escaping tendency of the air and has low friction characteristics.
• Plenum chamber is the simplest of all lift systems.
6.2.3 Disadvantages
• Finger type skirts utilize more skirt material thus elevating cost and weight.
• Frictional losses in this lift system increase because of the impact of air directly on the ground.
• Mechanical losses due to inclusion of gear box.
Figure 16. Hovercraft B
6.3 HOVERCRAFT C
6.3.1 Features
Skirt system
Bag and finger
Lift system
Distributed air supply
Thrust system
Ducted fans
Power Transmission Systems
Aerodynamically separate but mechanically 'variable ratio'.
Fan
Axial flow fan
6.3.2 Advantages
• It skirt compromises between flexibility of finger skirt and economy of bag skirt.
• Air is promptly supplied all around the craft ensuring craft stability and it is easy to construct.
• Twice as efficient as the unducted thrust systems.
6.3.3 Disadvantages
• Higher pressure losses.
• Difficult to troubleshoot problems after fabrication.
Figure 17. Hovercraft C
CHAPTER # 7
CONCEPT EVALUATION
7.1 INTRODUCTION
Each concept will be evaluated and a systematic concept evaluation process will be used to identify the most feasible concept for processor cooling. Following management tools will be used:
• Weighting Matrix
• Rating Matrix
In order to achieve the best design from the proposed concepts, each design will be evaluated against the statement of requirement (SOR) demands. Each demand has a different level of importance, therefore to determine the relative weight of each demand; they will be evaluated against each other by using the weighting matrix. From the results of concept evaluation, the group has concluded the final concept.
7.2 WEIGHTING MATRIX
The weighting matrix, Table-3 was used to assess the relative importance of the SOR sleeted criteria and produced a final justifiable weighting factor. The individual demands and wishes of the SOR were compared to each other .A value of 1 was assigned to the respective demand or wish of the SOR, if it proves to be more important than the one compared to.
The SOR features in each row were added the value entered as a weighting value. The weighting value column was then added. Each of the weighing value was then divided by this total to achieve a weighting factor, which was used by the Project Group to evaluate the conceptual designs.
7.3 RATING MATRIX
The rating matrix, Table-4 was used to asses the relative effectiveness of our concepts considered under the light of outlaid requirements. The relative numbers were assigned to the design proposals and multiplied by the weighting of the requirements prescribed in weighting matrix. The higher value corresponding to the one design was assigned as the best model; which was then selected.
Table 3 Weighting Matrix
D/W
CRITERIA
SOR
REF.
A
B
C
D
E
F
G
H
WEIGHT
VALUE
WEIGHT
FACTOR
A
D
Functional
characteristics
2.2
-
1
1
1
1
1
1
1
7
0.25
B
W(L)
Power
consumption
2.3.1.3
0
-
0
0
1
0
0
0
1
0.035
C
D
Weight
2.4.1
0
1
-
1
1
0
1
0.178
D
W(H)
Stability
2.5.6
0
1
0
-
1
0
1
1
4
0.143
E
W(L)
Operation and training
2.9
0
0
0
0
-
0
0
0
0
0
F
D
Safety
2.11.1
0
1
1
1
1
-
1
1
6
0.214
G
W(H)
Quality
3.1
0
1
0
0
1
0
-
0
2
0.07
H
Cost
5.0
0
1
0
0
1
0
1
-
3
0.107
TOTAL
0
6
2
3
7
1
5
4
28
APPENDICES
