ME 498-Senior Design

Portable Hovercraft

Jason Carlstrom, Nolan Davis, John Hamman, Matt Huff, Darren Popkins, Cory Roberts, Michael Rogers

Saint Martin’s UniversityHal and Inge Marcus School of Engineering

12/9/2013

Acknowledgements

Dr. Jung

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Dr. Kahn-Jetter

Dr. Slaboch

All our Engineering Faculty and Staff

Our Family and Friends

Table of Contents

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Acknowledgements 1

CHAPTER ONE: FALL DESIGN

First Design: Volkswagen Hovercraft

Chassis / Skirt 3

Lift / Propulsion 4

Weight 5

Second Design: Search and Rescue Hovercraft

Engine 6

Maneuverability/Body/Skirt 7

Design 8

Rescue 9

Final Design: Portable Hovercraft

Engine 10

Thrust and Lift 11

Performance 13

Static Analysis 14

Strength Test of Materials 15

Hull Design 16

Budget 20

Works Cited 22

CHAPTER TWO: SPRING CONSTRUCTION

Actual Costs

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Build Process

- Cowling- Body Panels- Steering

Obstacles

CHAPTER ONE: FALL DESIGN

Design One: VW Beetle Hover Craft

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This design will utilize the chassis and engine from a Volkswagen Beetle. The goals of this model are to be street legal and hover capable. We chose the VW as the base because of its simplicity, and light weight construction.

Chassis:

The craft will be constructed around the Volkswagen Beetle chassis. We chose the VW chassis because the major drive components are all contained in the chassis itself. The body is easily removed from the chassis as well making teardown very simple. Our plans include removing the body and possibly selling to recoup cost.

Once the body is removed we are left with the chassis itself. This is often called the “pan” and will be referred to that throughout this design. An illustration of a pan is shown below. The pan has a solid floor board integrated into the construction. This is a major design advantage as compared to a conventional vehicle chassis that uses only frame rails. This floor board will allow us to create an air tight seal underneath the craft without needing to fabricate massive patch panels saving on cost and fabrication time.

Skirt:

Material:

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Figure 2: VW Chassis

Figure 1: Chassis VW hovercraft design

Figure 3: VW Chassis hover craft design

There are multiple options available for skirt material. We have purchased two different types for testing thus far: Sport Nylon and PVC fabric. These materials will be tested for their permeability and strength. We also plan to test the skirt construction method. We are currently considering stitching and or adhesive/glues. These methods must also be tested for strength and permeability. We must ensure that our seams and seals can withstand the pressure we calculate for our air cushion ensuring blow outs will not occur.

Positions:

The skirt will need to have two positions; fully deployed and inflated for use in hover operations, and fully retracted for driving operations. The current design for a multi position skirt would have straps running under the craft and skirt. To deploy the skirt the straps will be loosened, to retract the skirt the straps are tightened.

To utilize the wheels during driving operations the skirt will need to leave adequate clearance for the rear wheels to spin and the front wheels to turn. Taking this into account we come up with a rough estimate of a skirt footprint of roughly 2700i n2 to 3125¿2.

Ride Height:

Further research must be conducted on skirt height. In a conventional hovercraft this would not be of major importance, but due to the wheels on our craft we need to ensure that the skirt provides enough lift for the wheels to clear the ground. The stock ground clearance for 1960’s era VW is approximately twelve inches. This however is for a suspension under full load, with body attached. The ride height will certainly be higher once the body is removed and all unnecessary weight is stripped. This means that our skirt will need to provide more than twelve inches of lift. If this amount of lift is not possible we may need to find another option. This could include lowering the ride height of the bug. This is easily achievable in a multitude of ways.

Commercially available lowering kit: We could fit one of the many aftermarket lowering kits available for VW Beetles. These kits allow for the ride height to be reduced dramatically. This method would be simple but not cost effective as kits run in the four hundred dollar range.

Smaller Diameter Tires: Another option would be fitting smaller diameter tires. The ride height improvement will not be as great as the lowering kit but this method would be much simpler and economical. Smaller tires would also help reduce weight of the craft. This would be simpler and cost effective.

Cut Springs: The last option would be to cut a certain amount of coils out of the springs. This would allow us to lower the ride height to our choosing, but would adversely affect the ride quality.

Lift:

The lift for our craft will come from 15 HP Briggs and Stratton lawn mower motor mounted where the passenger seat would originally be located. The motor will be will be perfect for this application due to its vertical shaft design. This will allow power to be transferred to our

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propeller without the need to construct a complicated transmission system. The propeller blade essentially replaces the lawn mower blade. A cowling will need to be built to surround the blade. This cowling will allow the air to be directed straight down through a hole cut into the pan, filling our skirt with air and creating the air cushion our craft will glide on.

Propulsion/Drive:

The goal of our craft is to be drivable on the street. This means that all of the original drive components must be retained in our final design. All suspension components are mounted to the front and rear of the pan. The motor and transmission are mounted off the rear of the pan. Retaining the drivability of our craft should be fairly simply. A mount will need to be fabricated to mount the steering column as well as pedal assembly once the body is removed. Throttle cables and other miscellaneous cables may also need to be rerouted.

The plan is to utilize the original VW engine for propulsion as well as drive. The VW Beetle came equipped with a differing range of engine sizes from 1200cc to 1600cc. These engines range from 45 to 65 horse power and should have more than enough power.

When in driving mode the engine and drive train will be used in original configuration. Once in hover mode the VW transmission will be placed in neutral ensuring that the rear wheels remain stationary. The propulsion fan will be mounted directly behind the original motor. A clutch mechanism will be designed to connect the propeller to one of the pulleys located on the rear of the engine. This could be a mechanical clutch or an additional pulley/belt system. Once the car is in neutral the propeller can be engaged connecting the prop to the engine. The engine can then be revved to spin the prop up to proper RPM.

Some of our concerns are:

The strength of the shafts at the rear of the engine. Will they be able to handle the extra load placed upon them by the prop?

The gearing, or lack thereof, on the rear shaft. Will we be able to reach the proper prop RPM without redlining our engine? Also will there be enough torque to spin the fan?

Weight:

The biggest concern of this design is the weight. Standard VW beetles vary on weight depending on the year and engine options. An average weight can be assumed to be 1900 lbs. This is very light for a production vehicle but very heavy for a hovercraft of our scale. The main weight savings will come from removing the body from the chassis. The table listed below lists the weights of various components. A minus indicates that the component will be removed. We come up with a final weight estimate of 1825 pounds. Using our estimated air cushion area we

can calculate the psi needed to lift our craft1825lbs2700 ¿2 ≈ .7 to

1825lbs3125 ¿2 ≈ .6 psi . converting these

values to pounds per square foot we end up with a pressure range of 87 psf to 101 psf. This pressure is too high for our skirt to contain.

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Conclusion:

The downfall of this design comes down to weight. We have concluded that starting with a Volkswagen Beetle chassis will be too heavy for the footprint the skirt will cover. Even after stripping the chassis of the body and extra parts we still estimate a weight of 1825lbs. This is just an estimate, as the weight could be much higher. We could compensate by increasing the lift fan power, but this will increase the weight and require a more robust skirt. We have decided to change directions and manufacture our own light weight chassis in hopes to decrease weight and increase speed and maneuverability.

Verdict:

Overall this project proved to be simply too expensive to be plausible. The sheer size and weight of the hovercraft demand a more powerful (and thus more expensive) engine.

Design Two: Search and Rescue Hovercraft

This design is intended to address the unique requirements of a hovercraft designated for quick response, search and rescue of a single non-ambulatory person in variable amphibious conditions.

Our goal for this project is to design and build a highly maneuverable hovercraft capable of speeds of over 35 miles per hour, with the ability to extract one person from water. The hovercraft will have a 600-800 pound payload to account for one driver, a two-person rescue crew, medical supplies, and the patient. Our estimated budget is $3200.

Engine:

We have incorporated a two-engine design, with one engine for lift and the other for thrust. The advantage of this system is that the thrust engine can be shut off without affecting the lift, which allows the hovercraft to be stationary on the water, which is a key characteristic for rescue. The hovercraft will need to remain in one spot while the rescue team enters the water and moves the patient onto the hovercraft.

Lift:

The lift fan will be powered by a Briggs & Stratton riding lawnmower engine with 15 horsepower. The engine will be positioned near the front of the hovercraft. The vertical shaft will work perfectly with our lift fan to power it without the need for much alteration. This is the ideal

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Component Weight (lbs)complete VW 1900VW Body -500Props 50Lift Mower 75Single Passenger 200misc 100Total 1825

engine because it is lighter than a more powerful engine, but has enough power to lift the hovercraft.

Thrust:

The thrust fan will be powered by either another lawnmower engine or a small VW Beetle 4 cylinder engine. Because our goal includes quick response and speeds of over 35 mph, we will need more power from the thrust engine than the one for lift. A slightly more powerful lawnmower engine might be enough, but our preferred engine would be a VW Beetle engine because it is air cooled, not terribly heavy and would produce plenty of horsepower. This decision will depend on what kind of budget we have to work with.

Handling / Maneuverability:

One of the main disadvantages of the typical hovercraft is that it is extremely difficult to maneuver and movements are not precise at all. This design incorporates several features that aim to increase both maneuverability and precision of our movements. Our initial design included side thrusters, which took a portion of the air from the lift fan and directed it out the sides through vents, which could be opened to provide lateral movement without turning the hovercraft. We tested this design in our small-scale model, but our tests revealed that the airflow was not sufficient to move the hovercraft from side to side. Our other idea to increase the precision of movement addresses the other main issue with a hovercraft, which is braking. By including a cowling around the thrust fan which can close and redirect the air flow, our fan creates a reverse thrust, which will slow the hovercraft considerably, allowing for much more responsive braking and stopping.

Body:

This design will use a foam core chassis with wood reinforcements for structural integrity. The chassis will be formed by layering sheets of foam in the shape of our hull, then applying fiberglass to the foam and wood. We will use several sheets of foam for the floor, and put in wood reinforcements underneath the lift fan, thrust fan, and the rest will be spaced evenly along where the crew will be seated.

General dimensions for the current design show the hovercraft roughly 9 feet wide by 14 feet long. The width needs to be adequate to hold a patient lying horizontally after being rescued, with enough room for the rescue tube and guardrails. As is discussed later, the surface area requires roughly 130 square feet, which drives our length dimension to be at least 14 feet. This allows plenty of room for the engines, all 4 passengers and the fan.

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Skirt:

Material:

We have purchased two different types for testing: Sport Nylon and PVC fabric. These materials will be tested for their permeability and strength. We also plan to test the skirt construction method. We are currently considering stitching and or adhesive/glues. These methods must also be tested for strength and permeability. We must ensure that our seams and seals can withstand the pressure we calculate for our air cushion ensuring blow outs will not occur.

Design:

Our initial plan for the skirt was a solid one-piece tube skirt. The advantage of this design is that it is much cheaper and easier to manufacture. It is also the most stable skirt design, which is important for such a large hovercraft, especially when loading a patient on board. An alternative option we considered was a “finger skirt”, which consists of many small individual sections that connect near the top. This type of skirt is much harder to make and is more expensive, but is much more durable than a tube skirt. This is because if one section or “finger” on the skirt rips the rest of the skirt remains intact. The downside of this style is that it is much less stable and more expensive to produce. After further research, we decided that the best option to help us achieve our goals would be a hybrid design featuring a finger skirt in the front and a tube skirt in the back. The advantage of this design is that the front of the hovercraft can withstand a small tear in the skirt without losing too much air pressure, but the back of the hovercraft will provide enough stability to keep the hovercraft stable while bringing the patient on board.

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Figure 4: Search and rescue craft design mock up Figure 5: An example of a finger skirt

Rescue

In order to save weight and reduce cost, our system for extracting a patient from the water will be as simple as possible. Our rescue system takes cues from lifeguard practices and applications. The craft will carry two dedicated rescue swimmers

and be equipped with; two lifeguard “rescue tubes”, a ring buoy and back board. This equipment will allow the craft to handle a multitude of rescue situations. The ring buoy can be utilized to rescue victims within tossing range while keeping the rescue swimmers safe inside the craft. The back board will allow possible head or spine injuries to be handled in the correct manner. The sides of our hovercraft would be angled down to allow for a smooth transfer of the back board from water to the hovercraft, and guide rails would help guide the board back onto the hovercraft for a gentle rescue without further aggravation of any injuries the patient may have suffered. There will be a dedicated seat for the victim once he is on board. In the case of a head or neck injury there will be tie downs to keep the back board safely secured to the deck.

Concerns

Our biggest concern for this project is our budget. Building a hovercraft capable of carrying four people (roughly 800 pounds) requires a large surface area and a skirt pressure high enough to support the craft. The average hovercraft has a skirt pressure of about 10 psf (pounds/square foot). Our initial design for the hovercraft produced an estimated total weight of 500-600 pounds. Adding the weight of 4 passengers, the hovercraft would need to lift a total of at least 1300 pounds. Assuming that we maintain the average skirt pressure for a hovercraft, ours would need a skirt surface area in contact with the ground of 130 square feet (just over 9 ft. by 14 ft.). This is a very large project to build and using lightweight (but cost effective) materials our budget came to an estimated $3200. Major components include our two engines, skirt materials, propeller, and foam sheets for our main body.

Verdict:

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Figure 6: Example of a Hybrid Design using both Finger Skirt (front) and solid tube (back).

Overall this project proved to be simply too expensive to be plausible. The sheer size and weight of the hovercraft demand a more powerful (and thus more expensive) engine, as well as large amounts of foam and skirt material. Unfortunately, it would not be reasonable to focus on a smaller rescue hovercraft because it takes at least two people to perform the physical rescue, and a hovercraft is not reliable enough to leave running without a driver. Therefore, 4 is the smallest number of people that the rescue hovercraft is practical for, and so it cannot be downsized to save on cost.

Final Design: Portable Collapsible Hovercraft

This design is designed to make a recreational hovercraft that can be folded up by a single driver and stored in the bed of a pickup truck easily.

Our goal for this project is to design and build a lightweight (under 200 lbs) hovercraft that’s capable of speeds of over 20+ miles per hour, with the ability to be folded flat along 2 hinge points, support the weight of an average sized person (200 lbs) and fit in the bed of a truck.

Engine

We have decided to go to a single engine design that will be able to provide both the lift and the thrust for the engine. The engine we have decided to go with is a Briggs & Stratton 1150 Snow Series engine with 6 horsepower. The engine will be positioned in the back of the craft and the horizontal shaft will work perfectly to provide the fan blades for the thrust. This is the ideal engine because it is only 15 pounds which is the heaviest individual component of the hovercraft.

Dimensions and General Craft Design

The craft will be made out of three panels, each three feet by three feet. The panels will be connected by hinges, so that the craft can be folded up and easily transported and stored. The panels will be constructed of foam core with wood support and fiberglass laminated skin. A seat will fold out of the center panel, as will control levers (one for throttle, the other for steering).Figure 8 shows a final design overview.

The foam we plan to use is expanded polystyrene foam; this is available at Lowes or Home Depot in a variety of sizes. We have discovered through testing that polyester resin will eat through our foam. After further research

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Figure 8: Final design mock up

Figure 7: Hull design mock up

we have found that epoxy resin will not eat the foam, thus our hull will be constructed of polystyrene foam coated with epoxy resin and fiberglass cloth.

Thrust and lift design

For the sake of easing calculations, all units have been switched to SI:Total engine output power E=6.5 h p=4849 WFan diameter D=30∈¿0.762 m

Skirt pressure Ps=12.12 lbff t 2=579 Pa

Standard atmospheric pressure Po=101kPaCraft weight W c=300lbf =9.32 slug=136 kg

Density of air at sea level, and room teature ρo=1.225 kgm3

Thrust Output

Research on fan thrust designs shows us that thrust fan/duct setups rarely exceed 50% efficiency (Brooks). So, for the sake of being conservative, we estimate our final fan will have about 30% efficiency. This should be a fairly good estimation, as we will do all we can to match the proper fan to our engine’s power and speed range, but we do not have access to the resources needed to fully design our own system. Because of this, much of these calculations are simply estimations, and the final design will depend on what engine we procure, and what fan is within our budget.

Thrust force is defined by the mass being moved times the velocity at exit, orT=m V d. In this case, the net thrust can be defined as the change in velocity from the front of the fan, to the back orT net=m (V d−V o ). The mass flow rate will also depend on the speed of the air, the density of the air and the area of flux. So, the final equation for thrust isT net=V d Ad ρd (V d−V o ). It should be noted that as the free stream airV o, reaches the same speed as the discharge air (such as when the craft nears its maximum speed) then the thrust force goes to zero. So, the highest thrust is when the fan is subjected to static conditions, such as when there is zero outside wind speed, and

the fan is not moving, or T net=V d2 Ad ρd. The power is defined asE=

m V d2

2. So, swapping in the

equation for thrust, we getE=T V d

2=

V d3 Ad ρd

2.

This equation is useful to allow us to find the discharge velocity, given fan power consumption.

Small engines typically exhibit fairly flat power curves. The lower end of the RPM range should only be about 22% less power than the peak

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output, as seen in Figure 8. So, using 2.6 kW as our low end of the power range, and 30% fan efficiency, the velocity of air at the discharge should be about 14.1 m/s. The high end of the engine’s power should give a discharge velocity of 17.3 m/s.

Estimating Lift Pressure and Required Airflow

The fan will be giving thrust, as well as lift power to the skirt. This means we will have to section off an area of the fan for lift air. Looking at Bernoulli’s equation, we can see that the velocity of air will create a certain pressure if the velocity is brought to zero. This is the ideal case for the craft, where there is zero air leakage from the skirt. Of course, there will be much air leakage, and some is actually required in order to keep the skirt off the ground. For the sake of determining available lifting pressure, we can use Bernoulli’s equation, coupled with ideal gas law. In the case of the hovercraft, we will assume negligible change in potential energy, so the

resulting equation comes to beV d

2

2+

Pd

ρd=

P s

ρ s. Using the ideal gas law,PV =mRT we can find

Pd V d

md=

Ps V s

ms, or

Pd

ρd=

P s

ρs. We are assuming standard atmospheric pressure and density of air, and

we know the skirt pressure should be 579 Pa, soρ s=1.232 kgm3 . Using these values we can find

that the required minimum velocity at the discharge should be 0.0055 m/s. This figure is very low compared to the velocity of our idealized fan at its low power setting, but keep in mind this is the velocity required to only lift the craft. It does not provide a layer of air between the skirt and the ground, nor does it take into account air leakage.

If we arbitrarily set the hover height (that is, the air gap between the skirt and the ground) to 2.5mm, then we can find a volumetric flow rate required to keep this gap. The perimeter of the skirt envelope was found using the dimensions of the craft, a skirt height of 8 inches (0.2 m), and a 45° skirt angle, the perimeter comes to 6.919 m. assuming the air inside the skirt envelope is

not moving initially, then Bernoulli’s equation simplifies toP s

ρs=

Pg

ρg+

V g2

2. So,V g the velocity at

the air gap comes to 0.055m/s. The area of flux is the gap times the perimeter, equaling 0.0346 m2 with a 2.5 mm gap. With these conditions we would require a volume flow rate of 0.00095 m3

/sec. So, with the air velocity flowing at the minimum speed for pressure buildup, we would require 0.173 m2 of fan area.

Lift Section of Fan

To find the area of the fan we must section off for lift, we can use simple geometry. Figure 10 shows the area of the fan output, the shaded region showing the area sectioned off for

lift. The area of the quadrant of the circle, with angle φ, is found by the equation Aq=

( φ360 )π D2

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Figure 9: Sample power curve for similar engine selected

Figure 10: Diagram of fan sections

. The area of the triangle ABC, is AABC=h2 S2

=D2

2sin2 φ

2. The angle φ, is simply φ=2 sin−1 s

D .

The area of the sectioned fan is the area of the quadrant minus the area of the triangle, thus

A s=Aq−A ABC. So the area of the lift section is A s=

( φ360 )π D2

4− D2

4sin φ

2cos φ

2. Using this

equation φcomes to 158 degrees., which gives h1=0.374 m, or about the radius of the fan.

It must be noted, all of the lift and thrust calculations are very tentative. These calculations are very simplified, and when actually building the craft, some tests must be conducted in order to maximize the system.

Craft Performance

Estimating the crafts acceleration can be accomplished using Newton’s Second Law

T net

mcraft=acceleration . The initial acceleration is easy enough to find, since V o, the free stream

velocity, is zero. The initial acceleration is 0.467 m/s/s. As the craft accelerates, the thrust decreases, so to find the velocity as a function of time, the following differential equation must

be solved dVdt =

(V d2−V d V ( t ) ) ρ Ad

mc

. A second order differential equation is formed as

d2 xd t 2 =

−V d ρ Ad

mc∗dx

dt+

V d2 ρ Ad

mc

. The differential equation was solved, and the velocity as a

function of time is found to be V (t )=V d(1−e

−V d A d ρtmc ).So, this equation limits the top speed of the

craft to that of the discharge speed. Using this function, it appears that it should take the craft about 19 seconds to accelerate to 50% of its max speed, or 8.65m/s (19 mph). It will take much longer to accelerate from 8.65 to 16 m/s (65.5 more seconds), but it is likely the craft will not be operated in this range often. Of course, these speeds neglect drag, but hopefully the low profile design does not slow it down too much.

Static analysis of the craft

The hull of the hovercraft is flat, and consists of three individual sections which are 3x3 feet. Because of the nearly uniform width of the craft, and the linear arrangement of the weights (the rider and engine/fan assembly), it can be modeled as a beam. To do this, first an estimate of

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the pressure it takes to lift the craft must be made. In this case, assuming a rider of 200 pounds and an engine weight of 100 pounds, with a lifting area of 24.75 ft2, the pressure to lift the craft

comes to 12.12 lb. / ft2. The engine’s weight was set at 1.5 feet from the aft edge, and to fully balance the forces, the rider’s center of mass must be set at about 3.5 feet from the front edge.

To find a linear distributed load, which accounts for the lift force,

the width of the craft can be multiplied by the pressure. This is relatively simple for most of the craft, as the distributed load becomes 12.12*3, or 36.36 lbs/ft. The very front section makes this rather difficult, however, since it is circular. To simplify the math required, the front was assumed to be triangular (see Figure 11). So for the first 1.5 feet of the craft, the distributed load equals 24.24*x. Integrating the distributed load functions gives the sheer force as a function of x. Integrating this once more gives the bending moment as a function of x. These functions were put into excel, and can be seen in Figure 12. So, with a 200 pound rider, and the configuration shown in Figure 11, the maximum sheer force comes to be 101.4 pounds, and the maximum bending moment is 145 ft.*lb.

0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 5.5 6 6.5 7 7.5 8 8.5 9

-100

-50

0

50

100

150

Sheer Force (lbs)Bending Moments (ft*lbs)

X (Feet)

Figure 12: Bending moment and sheer force diagram

Strength test of actual materials

We conducted a strength test on a piece of simulated structure from the hover craft. The piece was a sheet of foam, one inch thick, 15.375 inches long, and six inches wide. On one end was epoxied a piece of wood, used to simulate the hinge section on the folding hovercraft. On the two largest faces and around the wooden insert, fiberglass and epoxy resin was attached. The

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Figure 11: Side view of static forces on hover craft

goal of this experiment was two-fold. We wished to get a better sense of the strength of the materials we intend to use, as well as insuring that the design of the hinge section will be acceptable.

First, the test piece was clamped on the wooden section between two flat pieces of steel, in a bench vise. A pulley was set up across from it, at the same height as the predetermined attachment point of the piece. To test the deflection at the varying weights, an arrow shaft was attached to the base of the vise. As the piece flexed, an accurate measurement was taken from the straight arrow rod.

Results

Hinge strength

The wooden section used to simulate the hinge section on the final hover craft performed well. The test piece failed in the foam and fiberglass section about 4 inches above the wooden joint. This is a rather simplified set up, compared to what will actually be employed, as it has no holes for screws, and was clamped together. Even with the differences in the set up, it still

shows that the joint between the foam and wood should hold up.

Foam and laminate strength

The results of this strength test are somewhat inconclusive. Even still, much has been gleaned from the test. The first test failed at a force of 12.74 pounds, the face pointed towards the force buckled inward. The test clearly failed in compression, which was expected, as fiberglass is

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Figure 14: Shows compressive failure, but still intact wood structure

Figure 13: Materials test setup

known to be weak in compression. Since the side in tension was relatively undamaged, we decided to test the piece again, after turning it around. This, of course, was not intended to give any real data, but actually turned out quite useful. The failure limit for the second test was right at 12.5 pounds, nearly identical to the first. This shows that the fiberglass laminate, at least on a piece with only one layer thick, provides little strength in compression. This later determined how we calculated the configuration of our final design.

We also were able to find approximate values for E*I (the modulus of elasticity times the moment of inertia). The data gained for this value proved to be quite varied. For both tests, it decreased as the applied force increased. For example, the first test with the lowest applied weight (7.71 pounds) gives E*I as 37.1 lb.*ft2 , whereas the second weight of 12.74 gives E*I as 26.8 lb.*ft2. This is likely due to compressive damage, and the foam core being pushed beyond its elastic range. The second test ranged from E*I equaling 22.4 168.3 lb.*ft2. Because of the varied nature of the E*I values, we are not able to use the data gained for E*I.

Designing for hull thickness

Using the maximum bending moment from our test, we can get an accurate idea of how wide our hull should be. For both tests, the moment applied to the test piece was about 16 ft.*lbs. Since the moment of inertia varies linearly with the hull width, we can use this value to find the max bending moment that can be applied to the craft. With an identical setup of material configuration and foam thickness, our hull should be able to withstand six times the bending moment of the test piece, as the hull is six times as wide. The width was predetermined by size constraints we chose earlier. This means, our hull should be able to take a bending moment of about 100 ft.*lbs. The statically determined max bending moment is actually 140 ft.*lbs., so with one inch thick foam, we are still far below the required strength.

Another way we decided to look at the problem is by using the compressive limit of the foam. The manufacturer gives the foam we used in our test piece a maximum compressive limit of 25 psi (Foamular). If we assume the fiberglass in tension will exhibit negligible deformation, and be indestructible compared to the compressive strength of the foam (which it is certainly

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Figure 15: Compressive failure

Figure 16: Diagram for finding the thickness of the hull

many times stronger), then we can find the thickness of foam needed for our 36 inch wide craft. Put another way, the neutral axis would be located right at the surface where the foam and fiberglass meet (seen in Figure 16). The static calculations show that the maximum static bending moment is about 145 ft.*lbs. The equation for maximum stress in the given conditions is

σ= MyI , where M is the moment, y is the thickness, and I is the moment of inertia (in this case,

equal to by3 ). Simplifying, we getσ max=

3 Mb y2 , and solving for y yieldsy=√ 3 M

b σ max. Using this

equation, we find that the thickness of the craft needs to be 2.41 inches. Unfortunately, this thickness is rather impractical. The foam only comes in one or two inch thick sheets, so we would have to use four inch thick sheets, which raise cost and weight issues, and most importantly there is no safety factor. Another option could be to use multiple sheets of fiberglass, which would provide rigidity to the laminate and offer some compressive strength, but again, we run into the cost wall.

The Final Design

Since the strength of the foam and fiberglass was not strong enough on its own, we decided the most cost-effective method to remedy the situation was to add a wooden spine down the middle of each panel of the craft. Following is the strength calculations and factor of safety attained thereof.

The modulus of elasticity of a composite is calculated byEc=x Ef + (1−x ) Em, where x is the ratio of cross sectional area of the fiber to the matrix, Ecis the elastic modulus of the composite, E f is the elastic modulus of the fiber, and Em is the elastic modulus of the matrix (Kalpakjian). Using this equation, we found the elastic modulus of our fiberglass composite will be6.3∗106 psi, which assumes a 50/50 ratio of glass to resin. Using the same model, the tensile strength of the composite comes to 2.58∗106 psi.The elastic modulus for the wood we will likely use to build the craft (Douglas fir) will be about 1.95∗106 psi (WoodBin).For the sake of computation, we

assume that the modulus of elasticity of the foam core will be negligible, as it is far lower than either of the other two structural components. We also will assume that the fiberglass composite will offer no strength in

compression.

An excel program was created to be able to change the width of the wooden sections, in order to achive the desired safety factor. By using a total width of wood of 5 inches, there is a safety

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Figure 17: The final configuration and modulus of elasticity used for each material

factor of about 8 for the wooden sections. Of course, this is without considering the added strength of the foam core, and the top layer of fiberglass, so the safetly factor should be somehwat higher in actuality. The final configuration can be seen in Figure 17.

Table 1: Neutral axis, and strain calculationsComponent yi Ai Ai*yi Y ybar

Ai*y^2 I-x I-i Stress-i

Max Stress n

1 0.52 5 2.6 0.165 0.136 0.417 0.553501.3608 3940 psi

7.858612

2 0.01 2.39 0.0239 0.345 0.2847.757E-05 0.285

-6957.33

6.34*10^6 psi

370.832

Sum 7.39 2.6239 0.355

Re-adjustment of Bending and Sheer Forces

Now that a theoretical thrust for the engine has been found, we must make sure the moment caused by the thrust force is still within structural limits. Assuming half the fan is sectioned off for lift, which leaves the other half to supply 83.5 N, or 18.75 lbf. of thrust, at maximum output. The centroid of thrust would be located 18.2 inches above the deck of the craft. This adds an extra torque of 30ft*lbs at x=7.5 feet. The bending moment at this location was only 40 ft.*lbs., giving a total of 70 ft.*lbs., which is still far below the maximum already in place from static loading, of 140 ft.*lbs.

Hinge and bolt calculations

From earlier calculations, it is observed that the most shear force and moment a hinge or latch will face are 81.84 lbs. and 96.48 lb.-ft. respectively at the foremost joint. The chosen latch is capable of operating under forces up to 2000 lbs, which is greater than one order of magnitude of the calculated shear force assuming a 200 lb. rider. No failure is expected to occur through shear therefore as it is expected that the steel has homogeneous strength properties.

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Figure 19: Depiction of the expected forces and stresses on the joint section at a maximum

Figure 18: Diagram illustrating the values for the neutral axis location and strain calculations.

From the stress analysis shown below, which assume a static loading, it is clear that the moment at the intersection is sustained entirely by the hinges and latch system. We therefore get that the maximum shear sustained by all bolts in the front joint is 1157.76 lbs. We must then find whether this joint will fail by the wood beam or the steel bolt. From Mechanics of Materials, 8th edition by R.C. Hibbeler, we get the following equation:

τ max=VQ¿

, simplified for a regular bolt this becomes τ max=4

3πVr2 . Assuming a standard A36 steel

bolt, we can alter our earlier formula as follows; additionally we will add a safety factor of 2:

r=√ 83 π

Vτmax

As we assume that the stress will be sustained by at least 4 bolts per section, we get

that our bolts must be at least 0.1652 inches in diameter, of which bolts of this diameter are very practical and easy to obtain.

If we assume that the joint will fail by fracture of the wood beam, we must make the following assumptions: the bolts go through the full wood beam, the bolts will experience a maximum potential stress intensity factor of 3, and the stress is uniform in the axis of the bolt, furthermore the failure stress of pine wood from MATWEB.com was observed to be around 508 psi. The maximum stress the wood beam will experience is defined by the following equation:

σ max=VDt and comes to approximately 5266.6 psi, which would mean catastrophic failure. We

must therefore reinforce the area around the bolts, which is accomplished by the Plexiglas coating and small pieces of sheet metal around the bolt holes. As the Plexiglas coating is at least 0.2 inches and has a yield strength of around 14,504 psi, and any steel plate reinforcement will have a yield stress of approximately 36000 psi, we can safely assume that the bolts will not cause failure of the material and of the craft hull.

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Figure 20: Detailed view of hinge section with steel reinforcement

This design includes foam and wood sections, fiberglass overlay, and reinforcement steel which are bonded to the fiberglass.

The hinge used is a long piano hinge opposite the latch. As there will be significantly more screws used to hold this section and a much longer hinge, failure in the hinge is extremely improbable and can be ignored.

Budget

1) Snowblower 6 HP engine $100.002) Propulsion fan and hub $249.95

http://www.ebay.com/itm/45-Arrowprop-Air-Boat-Prop-w-Aluminum-Hub-No-45AB36-Use-It-or-Man-Cave-/171103051253?pt=Boat_Parts_Accessories_Gear&hash=item27d6895df5&vxp=mtr

3) Chicken wire, for prop cage $19.97http://www.amazon.com/308476B-48-Inch-50-Foot-Galvanized-Hexagonal/dp/B000XFX6TY/ref=sr_1_1?ie=UTF8&qid=1381036054&sr=8-1&keywords=chicken+wire

4) Plywood for lift fan, propulsion cowling. 2 sheets at $23.76 each $23.765) http://www.homedepot.com/p/Project-Panels-3-4-in-x-2-ft-x-4-ft-Pine-Plywood-2-Pack-

1502108/203444162#.Uo0d7OLOT8c6) 2x4x8 foot studs for frame structure, 10 total at $2.29 each $22.90

http://www.homedepot.com/p/Unbranded-2-x-4-x-8-Premium-Kiln-Dried-Whitewood-Stud-161640/202091220#.UlEBZBCWn8c

7) Sheet metal for creation of control fins $21.98http://www.homedepot.com/p/MD-Building-Products-3-ft-x-3-ft-Aluminum-Sheet-57000/100351161?N=c27v#.UlEDGhCWn8c

8) Piano wire for control systems $11.48http://www.amazon.com/Carbon-Smooth-Diameter-Precision-Tolerance/dp/B000VYNE3A/ref=sr_1_12?ie=UTF8&qid=1381042095&sr=8-12&keywords=piano+wire

9) Skirt material, $7.00 per yard, 11 yards $77.0010) 5 gallons unleaded gasoline, at $3.50 a gallon $17.5011) Foam sheets for platform, 4 at $20.37 each $81.48

http://www.homedepot.com/p/R-Tech-2-in-x-4-ft-x-8-ft-Foam-Insulation-310891/202532856?N=baxxZ1z0z6k1#.UlMZBRCWn8c

12) Skirt glue $9.95http://www.hovercraft.com/content/index.php?main_page=product_oversize_info&cPath=189_62&products_id=159

13) Fiberglass sheets, at $6.65 per yard, 10 yards $66.50http://www.uscomposites.com/cloth.html

14) Fiberglass epoxy for hull, 1 gallons at $39.00 each $39.00http://www.uscomposites.com/polyesters.html

15) Hinges for steering assembly. 5 sets at $2.77 each $13.8516) Aluminum tubing for fin assembly, 2 at $19.54 each $39.08

http://www.homedepot.com/p/Crown-Bolt-1-in-x-48-in-Aluminum-Square-Tube-with-1-16-in-Thick-40620/100337956#.Ul83uxDOT8c

17) Steel tube for engine mounts, load bearing structures. 4 at $11.22 each $44.88http://www.homedepot.com/p/Crown-Bolt-1-2-in-x-72-in-Plain-Steel-Square-Tube-with-1-16-in-Thick-42310/100338243#.Ul85gxDOT8c

18) Wood screws for hull, deck etc. 2 boxes at $8.47 each $16.94

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http://www.homedepot.com/p/Grip-Rite-8-x-2-1-2-in-Coarse-Polymer-Plated-Steel-Bugle-Head-Phillips-Wood-Screws-1-lb-per-Box-PTN212S1/100173447#.Ul86XRDOT8c

19) Heavy duty Loctite construction adhesive. 2 at $4.57 each $9.14http://www.homedepot.com/p/Loctite-9-fl-oz-Clear-Power-Grab-Heavy-Duty-Construction-Adhesive-1589157/203009262#.UosW5uLOT8c

20) 2 canisters paint primer, at $6.49 each $19.47http://www.oreillyauto.com/site/c/cat/Paint+%26+Body+Repair/C0171/C0014.oap?year=2010&make=Workhorse&model=W42&vi=5161417

21) 1 canisters red paint, $21.99http://www.amazon.com/Dupli-Color-BSP303-Candy-Finish-System/dp/B003TQEY4A/ref=sr_1_1?s=automotive&ie=UTF8&qid=1382123745&sr=1-1&keywords=red+car+paint

22) Conduit for control system cables, 2 at $9.99 each $19.98http://www.homedepot.com/p/AFC-Cable-Systems-1-2-in-x-25-ft-Non-Metallic-Liquidtight-Conduit-6002-22-00/202286718#.UmGMNhDOT8c

23) Electrical wire $6.99http://www.oreillyauto.com/site/c/detail/CTI0/85700/N1278.oap?ck=Search_N1278_1314373_3190&pt=N1278&ppt=C0335al

24) Electrical tape, 2 at $4.49 each $8.98http://www.oreillyauto.com/site/c/detail/MMM0/03799NA/N0226.oap?ck=Search_electrical+tape_1314373_3190&keyword=electrical+tape

25) SHIPPING COST AND TAXES. May be up to %20 or more of each products cost.Estimated to be around: $170.60TOTAL COST: $853.01 pretax/shippingTOTAL COST: $1023.612

Conclusion:

We fully expect this craft to be completely operational if built to the design specifications listed above and will be able to be built to be within a more realistic budget compared to the previous designs. The design should prove to be quite robust, unique, and reliable, owing to the use of lightweight composites and innovative design.

Works CitedBrooks, Ian. "Estimating Thrust and Lift Performance." build and calculation guide. n.d. Document.

efunda. n.d. web. 3 December 2013.

Foamular. n.d. web. 5 Dec 2013.

Hibbeler, R. C. . Mechanics of materials. 8th. Pearson Prentice Hall, print.

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Kalpakjian, Serope and Steven R. Schmid. Maufacturing Engineering and Technology. Upper Saddle River, NJ: Pearson Prentice Hall, 2010. print.

Resin Research. n.d. web. 5 Dec 2013.

WoodBin. n.d. web. 5 Dec 2013.

CHAPTER TWO: SPRING CONSTRUCTIONACTUAL COSTS

After tallying up our costs for building the portable hovercraft, our total cost is $1,150. This is $150 under our estimated budget from fall semester, so we came in 11.5% under our original estimation. We feel this is a significant achievement because there were a lot of unforeseen circumstances that resulted in higher costs. The main change that occurred was an increase in the width of our craft from 3 feet to 5 feet, which resulted in an increase in surface area from 27 square feet to 45 square feet. This greatly raised the price for some of our materials, including foam, fiberglass cloth and epoxy. A table of our costs is included below to show our most expensive items.

BUILD PROCESS

Our build process was determined by a few key pieces of information that needed to be tested before we could determine some of our other designs.

Cowling

The most important measurement for our hovercraft is the pressure that is being created by our fan and cowling. This meant that the first thing that had to be built was our cowling and fan, so that we could see what pressure we had to work with. The cowling proved difficult in

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construction because of its size and shape. In order to make the shape out of foam without having a lot of wasted foam, our team decided to cut each section in quarter circles out of 2-inch foam.

One quarter circle section example

Our cowling is approximately 16 inches in diameter at the front, and is 14 inches from intake toexit. This meant that our cowling had to be constructed in seven 2-inch layers, each with 4 quarters of a circle. In total, 28 quarter-circular pieces had to be traced and cut, and then glued together with epoxy.

Once the epoxy was dry, the blocky cowling could be sanded and shaped into a smooth curve.

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Cowling formed using foam and epoxy.

To make a round shape, with a smooth finish, a lathe was built to shape the cowling. The lathe proved to be a big project in itself. A bicycle wheel was mounted to a table. On the wheel a rack was made to hold the cowling. To turn the lathe, a skate board wheel was attached to an angle grinder. It was linked to the wheel by a belt made of rubber tubing. Once the lathe was turning, a knife was used to cut the foam down. This process was slow, and took quite a few hours of work. It ultimately left us with a nearly perfectly round cowling.

After the cowling was shaped, some wooden blocks were inserted, which would later be used for mounting the cowling. We then laid two base coats of resin down, and one layer of fiberglass. This resulted in a very sturdy, very well built cowling.

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Body

Because the pressure that our cowling was able to achieve was lower than we were expecting, the surface area of our craft needed to increase to lower the amount of pressure needed. The easiest way for us to do this is to increase the width of the craft, so that our original idea of it being collapsible would not have to be compromised. In order to ensure that our foam would be strong enough to withstand the new dimensions, two wood inserts would be added lengthwise to each section to add reinforcement. These would be spaced about 2 feet apart and would be held together by epoxy and fiberglass.

Skirt

The skirt was designed in Autodesk Inventor. It consists of eight straight tubular sections, with bounce webbing attached inside. The CAD files were very necessary to achieve proper skirt fit, size, and layout. The CAD files were printed out to create templates. The fabric was cut out, using a heavy nylon, with a rubberized backing. We were at a loss for a way to actually join the

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Completed cowling with fiberglass.

fabric, as sewing it seemed to be too time consuming. Luckily, the rubberized backing heat sealed together with a very strong bond. So, after all of the panels were cut out of the fabric, they were joined together by hot iron.

To attach the skirt to the hull of the craft, thin wooden strips were stapled to the inside of the skirt. These were then screwed to the craft using wood screws and fender washers (to ensure the screws did not sink into the foam of the hull).

A duct was needed to take the fan’s thrust air, and force it down into the skirt, and underside of the craft for lift. The first idea was to create a duct out of solely fabric, but unfortunately this was unable to hold up to the stresses involved. So a hybrid duct was made of extra foam, and fabric. It utilizes nearly half of the fan’s output, and connects directly to the skirt in the back of the craft. The fabric was heat sealed together, and attached to the foam sections via long screws.

Steering

For our steering setup, a bike was cannibalized for its handlebars and a brake handle. By simply turning the handlebars vertically, it became a steering level similar to an airplane, and the brake handle will be used to control our throttle. The steering assembly sits between the user’s legs and uses bike cables to turn the rudders. The cables will run along horizontally out to the sides of the craft, and run through bike cable sleeves.

The cables are pull only, so a circuit of cables runs from the lever to the rudder linkages. To keep slop out of the system, two springs are put in series with the cables.

OBSTACLES

The two biggest obstacles that we faced in this project were time and money. As discussed in the previous section, our build process was determined by our fan and cowling setup. We ordered the propeller in mid-February, and did not receive it for over a month. This really set us back

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because we couldn’t start construction on the rest of the craft until mid-to-late March. Once we finally got the propeller we could test the pressure.

Our other biggest concern was money.

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