Building a hovercraft with household objects.

Abstract

The coefficient of static and kinetic friction between a board and the ground is very high. Therefore trying to slide this board across the ground would be quite difficult. It is thought that if there is a pocket of air between the board and the ground, the coefficient of friction will go down, therefore the force due to friction also will, and the board will be moved easier.

Although the force of friction is essentially impossible to completely get rid of, it can be minimized substantially. With this pocket of air between the board and the ground, the board will glide above the floor with very little friction. This device would be known as a hovercraft, because it is “hovering” above the ground.

Purpose

To build a functioning hovercraft, efficient enough to carry the weight of a person using household objects.

Design

The hovercraft will be built using only materials that can be found in the typical Muskokan home. A leaf blower will be used to supply the airflow, a piece of plywood will serve as the base, and a tarp will be used to bladder the air, and to pressurize limit the airflow. Duct tape, industrial staples, and spray adhesive will be used to hold the tarp to the plywood.

Materials

16 square foot sheet of 3/8 inch thick plywood
Large industrial tarp
Leaf Blower
Frisbee
2 inch, 1/4-20 Bolt
1/4-20 Nut
1 Large washer, 2 small washers
Electric saber saw
Razor knife
Staple gun
Spray Adhesive
Silicon Caulking
Marker
2 Foot string
Hammer
Extension Cords
Stool
Measuring tape
Wrench
Ratchet
Drill

Procedure

Tie a permanent marker to the end of a 2’ long string, with a loop at the opposite end of the string.
Measure the exact center of the board, and drive a nail into it.
Loop the end of the string opposite of the marker around the nail. Pull the string tight and use this string to guide your marker in drawing a perfect circle. This will be much like using a compass (be careful not to tilt the marker, this will throw off your circle).
Use a saber saw to cut the out the circle you have drawn.
Drill a hole in the exact center of the circle. The bolt you have should snugly but easily fit through this hole.
Use the end of the leaf blower to trace a circle the exact size of the nozzle on the leaf blower onto the surface of your hovercraft. This circle should be halfway between the center and the perimeter, a foot from the center, and a foot from the perimeter.
Use a hammer and a chisel to make a hole in the board where the circle you traced is.
From this hole you punched, start using the saber saw again to cut out this hole.
Place the circular piece of plywood onto the tarp, and cut the tarp with a razor so that the edges of the tarp will fold up onto the top of your plywood about 6 inches.
Spray the tarp with adhesive as you fold it up onto the top, then use the staple gun to staple it down securely. Once the plywood is covered, flip it upside down, to apply pressure to the overlapped tarp to help it dry nicely.
Cut your Frisbee so that it is flat, and the lips are gone. Then drill a hole in it the same size as the one you drilled in the plywood.
While the plywood is upside down, place the contoured Frisbee in the exact center of the plywood, so that if not for the tarp in-between, you would be able to see clearly through the hole in the Frisbee and the hole in the plywood.
Place the large washer over the hole in the Frisbee, and one of the smaller ones on-top of that washer. Drill your bolt through the washers, the hole in the Frisbee, the tarp, and finally through the hole in the plywood. Flip the plywood over, put the other small washer on the screw on the other end, and using a ratchet and wrench, tighten the nut onto the bolt securely.
Again, flip the hovercraft so it is upside down. Using a razor cut six evenly spaced circles about an inch in diameter into the tarp. These circles, or holes should encircle the Frisbee you have secured to the plywood. Use duct tape to reinforce the areas of tarp around the holes, to prevent ripping.
Flip the soon-to-be hovercraft back over, so that the side where the wood is visible. Detach the extension pipe, or nozzle from the leaf blower, and place it into the hole you have cut for it. The pipe should fit snugly into the hole; use silicon caulking to fill in any space between the pipe and the edge of the hole in the wood. After this use duct tape to completely seal the space around the pipe.
Duct tape the edges of the tarp that are folded over onto the plywood down so that they are airtight. Then cover the entire top of the board with duct tape as to prevent air leaks.
Place the leaf blower on top of the pipe attachment. Turn the leaf blower on, let the bladder (tarp) fill up, look, listen and feel for any air leaks, if there is any, patch them up with some duct tape.

Observations

After building the hovercraft, although the original model was a success and worked properly and as planned, there were a few problems that would arise. The sliding of the craft on the ground, combined with the pressure the air put out, caused the bottom of the tarp to have scratches, holes and rips. These holes and rips caused the air that was being held in by the bladder or tarp to escape. The escape of the air reduced the pressure of the air coming out through the holes that were put in the bottom for the planned release of ...

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Observations

Another problem that was encountered was the insufficient adhesiveness of the duct tape. The duct tape was much strong enough to hold the tarp down onto the board with the staples and spray adhesive while the bladder was not inflated, however, when the hovercraft was turned on, it was a different story. The pressure put on the tarp by the air was so strong that the duct tape started to come loose and air was leaking out. This had the same effect as the rips in the tarp. The pressure of the planned escaped air was reduced, and the effectiveness of the hovercraft was compromised. To fix this problem, more duct tape was applied, along with silicon adhesive and industrial staples. Also the entire surface of the hovercraft was covered to prevent any further leaks.

It was discovered through use of the hovercraft that when it is being ridden, the weight must be distributed evenly in order for it to work effectively. This can be simply explained: when the weight on top of the hovercraft is distributed evenly, the board will be level, and perpendicular to the upward force being put on it by the air pressure, therefore there is only a vertical component of force, or acceleration. If the weight were to be unevenly distributed on the hovercraft, then the surface would not be completely parallel with the floor. Therefore the upward force acting on the hovercraft would not be perpendicular to the base of the unit. This would cause the force to have a horizontal component of force, and acceleration, and the hovercraft would lean to one side, causing the side of the craft to touch the ground, and thus defeating the purpose of the hovercraft.

Conclusion

A functioning hovercraft with enough power to support the weight of a person can be built using household items. Making this hovercraft was simple once the plans were made. It took just under six hours to build in its entirety. The plans for this hovercraft were devised after studying many different models of hovercrafts. Once the basic concept of a hovercraft was understood, the design began to come together. Modifications were made along the way and videos were used for reference.

The only complications that arose were due to minor oversights, and were easily fixed. For example the duct tape didn’t hold down as well as expected, so addition duct tape seals were made.

The circular design was used to maximize surface area, because the more surface area, the greater the force acting up on the object. The way it works al has to do with air pressure. The bladder underneath the base of the hovercraft fills up with air, as air escapes through the holes in the bladder. There are two ways that combine to make this device work. The air is entering the bladder a lot faster than the air is leaving; therefore the pressure of the air is increasing as time elapses. Since the air in the bladder has a higher pressure than the air outside the bladder, the air tries to even the pressure out by escaping through the holes at a fast rate. This will create a pocket of air underneath the bladder, therefore raisin the hovercraft off of the ground. The higher-pressure air rises, putting out an upward force on the hovercraft raising it up.

If this were to be done again, a stronger material would have been used in place of a tarp, as to prevent holes rips or scratches to the bladder. These holes reduce air pressure, making the hovercraft less effective. Perhaps some rubber bumpers would have been attached to save the bladder from being damaged in collisions. A more efficient way of keeping the bladder air tightly attached to the top of the hovercraft would have been used in place of duct tape, such as plastic window sealer, or silicon adhesive.

Analysis

What physics principles are involved in your project?

In building the hovercraft many principles studied this year came into effect. It was known that there would always be a constant force due to gravity working down on the hovercraft. To keep the hovercraft above the ground, it was known that a force acting upwards that was equal to or greater than the force due to gravity was needed. This is because if there is force acting on an object, it will accelerate in the direction of the force, if the forces balance each other out, there will be no net force, and no acceleration. The hovercraft will hover.

Fg = m(g) ↑ Fapp ≥ 743 N [up]

= 76(9.8) ┌┐

= 743 N [down] └┘

↓ Fg = 743 N [down]

Therefore the applied force must be equal

or greater than 743 N [up].

What criteria can be used to evaluate the success of your device?

Since the goal of this device is to reduce the force due to friction, and to try and lift as much mass as possible, perhaps the experiment can be graded by the amount of work it can save. Since work is found by multiplying the displacement by the force output, this would reduce work. The force required to move a mass would be a lot less because there is not as much friction acting against the applied force. If the applied force is lessened to more something the same distance, the work will also decrease.

Another way that the device can be evaluated by is the amount of mass the hovercraft can support before the weight is too much for the air pressure and it collapses.

What safety precautions were taken while building and testing this device?

In the building process many tools were used, such as a saber saw, a hammer, a razor blade, and even cast iron weights. For eye protection, tinted safety glasses were used, this shielded the eyes from the sawdust, or any other foreign object that may have came close to entering the eye. Sleeves were rolled up when using power tools, to prevent something from getting caught. Long pants were worn to protect legs from any objects that could cut or burn the builder. Hands were kept clear from the power tools while they were in use, to prevent any digits from being severed. Un-frayed extension cord was used and kept away from saw dust to prevent an electrical fire from taking place. Areas of the tarp were reinforced with duct tape to prevent the bladder from tearing, and releasing air, lessening the air pressure.

How did you test your device?

The hovercraft was tested through use. First the bladder was allowed to fill up, and then small weights were added, until it was certain that the mass of a person could be supported. After this was established, the builder mounted the hovercraft. The hovercraft was then inspected carefully while the airflow was on for any rips tares, or holes in tarp. Any found were patched with duct tape.

How exactly does your device work?

The main body of the hovercraft has a “bladder” underneath it that has one large hole leading into it, and six small ones on the bottom. The large hole on the top is where the leaf blower attaches; this is the input for the air. The six smaller holes on the bottom are to let air out at a regulated rate. The air enters the bladder at a much faster rate than the air exits through the holes on the bottom; this will cause the pressure of the air inside the bladder to increase over time. The increasing pressure of this air will cause the hovercraft to rise. The more pressure that is gained inside of the bladder, the higher the rate of air-diffusion will be, meaning the faster the air will leave to bladder through the holes. The air will try to even itself out by making the high-pressure air pass through the membrane (the bladder) into the low-pressure air. The air moving through these holes to the bottom of the hovercraft will create an air pocket between the base of the hovercraft and the ground. This will cause the hovercraft to float on air.

In order for the force to completely counter the force due to gravity, the force must be applied completely perpendicular to the surface of the hovercraft, and disperse itself evenly. In order for the force upwards to hit the hovercraft at a 90° angle, the hovercraft must be parallel to the ground. In order for the hovercraft to be parallel to the ground, the weight on top of the device must be in the center. Should the force hit the base at an angle, the force will have a horizontal component. If the force has a horizontal component, it will not hit the base evenly and will cause the craft to tilt. If the craft tilts then one side of the circle will touch the ground, causing the friction to be in effect again. Thus, defeating the purpose of the hovercraft.