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18-1 HOVERCRAFT Final Report Submitted to The Faculty of Operation Catapult LXXXI Rose-Hulman Institute of Technology Terre Haute, Indiana by Group 18 Christian Calyore Barron Collier High School Naples, Florida James Anaissie Little Rock Central High School Little Rock, Arkansas Alex Ray Walnut Hills High School Cincinnati, Ohio June 29, 2007 18-2 Introduction Imagine being able to float above the ground with a frictionless surface between you and the ground. Sounds impossible, right? Actually, this feat has been accomplished by the hovercraft. The hovercraft uses a cushion of air to “hover” frictionless above any terrain, ranging from flat ground to water. For many years people have been designing hovercrafts, trying to achieve the frictionless travel of the air cushion they travel on. The first commercial hovercraft, the Saunders Roe Number One or SRN1 for short, was developed by Sir Christopher Cockrell. Even though he had invented the first hovercraft, nobody believed that he had accomplished the seemingly impossible design of a hovercraft; therefore, he was forced to show his craft many times before it was accepted by society. Modern hovercrafts have evolved much from the original SRN1, and have become easier to produce. Also, modern hovercrafts are now not only being used for commercial purposes, but recreational purposes as well, such as racing. Now hovercrafts are being made smaller as toys for little children, and these miniature hovercrafts have the same capabilities of their larger counterparts. Hovercrafts use a lift system, a thrust system, and a system to control the craft. Most hovercrafts, with the exception of home-made crafts such as ours, use a separate fan for hover and thrust. The fan for hover is usually pointed downwards and diverted through a duct to the skirt, which then fills up with air, producing a cushion of air for the craft to hover on. There is also a fan or more than one fan for thrust, pushing the air backwards, and in turn pushing the craft forwards. The control system is either a rudder that redirects the air coming from the thrust fans, or the actual fans are pivoted from side to side to direct the air in the desirable direction. Method Seeming like a good way to start off the project, research was performed on how a hovercraft works and to brainstorm designs for the body and skirt of the hovercraft. The Internet was used to research the concept of hover and books were checked out from the library as well. Two days later the actual building procedure of the hovercraft began. First the base for the hovercraft was made by cutting out a block of one and a half inch hard styrofoam into a two by one foot rectangle using a box cutter. After that it was thought that a rounded front would be ideal for control of the hovercraft, and so the front was cut into a semi-circle with a six inch radius, six inches being half the width of the base. The distance desired clearance between the box and the propeller was calculated, and upon finding out that the propeller had a length of eight inches it was decided that nine and a half inches would be a sufficient dimension for clearance. Knowing this construction on the box was started using five square walls. In the rear wall a hole was cut for thrust with a two inch radius, hoping that would be enough for the force needed to move the hovercraft forward. The rest of the walls were all enclosed. In the front wall some way to mount the propeller facing backwards was needed, so the radius of the motor was measured. A hole was cut out so that a snug fit was made for the motor. Originally a seven inch square hole was going to be cut, but it was decided those dimensions were too large and the length of the hole was decreased to three inches. Once the 18-3 box was assembled a piece of cardboard was placed in a slanted position, in order to direct air from the propeller into the skirt. The piece of cardboard was placed so that the front end would be half way up the propeller and the back end was at the back of the hole in the base of the craft. Since most of the box was dead space, it would not direct the air flow well enough, so three pieces of cardboard were placed to direct all of the air through the hole for thrust. Once the interior of the box was finished, the box was attached to the base using duct tape and the final wall was attached with the propeller mounted in it. Using a trash bag material, a skirt was created that had ample room for inflation, and it was attached to the base. In order to vent air for lift, a plethora of holes were poked in the skirt using a thumbtack. The hovercraft was ready for the first test, which ended with failure. It was also realized that during this first test the propeller was not getting any air intake, so some holes were cut in the front wall, without risking structural integrity, to get as much air to the propeller as possible. Trying slight modifications before major ones, the holes in the skirt were enlarged. The hovercraft failed again. After all these failures, the problem was that either there was not enough air intake, or the hovercraft was too heavy. For the second trial design, the hovercraft was redone, except for the base, and it was decided that the next design would be a testing design. First a box was constructed. Three pieces of cardboard were cut out and a piece of cardboard was placed in the box to act as a duct to direct the air for hover. Then a piece of foam was cut to mount the propeller in with enough height to provide clearance between the propeller and the base. Both the box and the propeller mount were attached to the 12” base using duct tape. To enclose the rest of the box for safety without sacrificing air intake, the open spaces were covered with Base design for first two hovercraft. scraps of chicken wire. For 24” thrust air was not directed into a smaller opening, but instead the back was just left open. Another enclosed skirt was cut and attached to the base. Every skirt design previously tried was tested again to see if the problem was the skirt, air intake, or hovercraft’s weight. None of the soft skirt designs worked, so a hard skirt was thought to be the final skirt to test. A hard skirt is one in which a one inch strip of foam is attached to the whole perimeter of the base. Liquid Nails was used to attach the hard skirt to the base. Once this was finished the hard skirt was tested. The hovercraft still failed. Before starting the third design, ideas were formed on how to cut down on the hovercraft’s weight. The idea that seemed the most efficient was to cut the thickness of the board in half, and use half for the base and half for the skirt. This time the dimensions of the base had to be changed to nineteen by twelve inches. Again the front was rounded, so it was a semi circle. A rectangle was cut in the base with the same dimensions as the other two designs. 18-4 Unfortunately, the hovercraft had a bow in the base, so lead weights were applied to counteract the curve. For the walls, foam board was cut to the dimensions nine by five inches for both walls and ten and a quarter by five inches for the roof. After attaching the walls and ceiling, they were attached using hot glue. For the duct on this design, a flexible piece of cardboard was used to provide a more curved path to direct the air into the skirt. This duct was placed halfway up the propeller. Instead of chicken wire, mosquito netting was used because it weighed less than chicken wire. The mosquito netting was first applied to the rear. To maximize air intake from all degrees behind the propeller, the very front was a box made of mosquito netting. This box was attached, with a hole for the motor to go through. The motor was placed in a foam semi ellipse, and held up by attaching metal rods to the mount. Three rods were used on the bottom in a triangular design, and two rods were welded together to keep the top of the mount sturdy. The propeller was then reattached. The hovercraft was tested, and it hovered, but only without the battery sitting on it. This test showed that either the weight would have to be decreased, or there would have to be some way of producing more force for hover. With a greater surface area comes greater pressure, so to create more pressure a larger base was made, with the dimensions twenty-four by sixteen inches. This caused a thirty 16” percent increase in surface area. The base was created using a horizontal hot wire cutter, which made a 24” much cleaner cut than a vertical hot wire cutter. A skirt was made from a full sized foam board, in order to create a Final base design. greater pressure in the skirt. The skirt was then attached to the base using hot glue, which turned out to be much more effective than the liquid nails. The duct was this time cut to nine and a half by four and a quarter inches. After the duct was made, the box was constructed using nine and a half by four and a half inch walls. The roof was made to fit snugly between the two walls. All three of these pieces consisted of half the thickness of a foam board. Next the box was assembled using hot glue and duct tape, and attached to the base. A duct made from the bendable cardboard was placed halfway up the propeller. This time chicken wire was used instead of the mosquito netting because chicken wire allows for greater intake. The front was covered in chicken wire which went up an over the propeller to try and maximize air intake. The motor was again mounted on the same rods as on previous designs. The hovercraft was then tested, and did not hover. It was discovered that there was as much air bouncing back as was going into the duct and rear of the hovercraft. The problem was fixed by enclosing the propeller in a circular wall to prevent the air from bouncing back. When this idea was tested, the hovercraft hovered, but a new problem arose. The new problem was that the hovercraft spun in circles because more air was being pushed out one side than the other. First, the duct was straightened and then the propeller was 18-5 straightened, neither of which had an effect on the problem. The only other idea was to funnel the air to straighten the air flow. A funnel was made from two pieces of cardboard pointing towards the middle of the rudder. This idea worked when tested, and became the final design. The hover craft was thus controllable. Front View of Final Design Side View of Final Design Results and Analysis The first hovercraft design failed due to many factors. Fist of all, it was extremely heavy, which plays a big role in whether or not a hovercraft will push itself off the ground. Secondly, the wall holding the propeller was completely closed, which allowed no air for intake, resulting in absolutely no thrust or hover. Lastly, the skirt designs were faulty and poorly designed. For the second design, the problem was fixed by mainly concentrating on intake and weight. A box was constructed out of very thin cardboard, and chicken wire was used to provide a better intake. After trying the other skirt designs that failed on the first model with no success, it was found that a hard foam skirt helped trap air much better than a bag skirt. This happens because it is very flat on the ground, which traps air in the bottom and forces the hovercraft upwards and into a hovering state. The reason it didn’t hover is because there was still not enough intake and our air propulsion was lacking. The third model was a drastic change from the first and second. It was lightened up to an extreme by cutting the amount of foam used for the body, box, and skirt in the previous model in half, which resulted in much less weight. The mosquito netting was also extended past the blade, providing more air for intake. The method of using bars to hold up the propeller gave us maximum room for intake and was reasonably light as well, resulting in an extremely light hovercraft. After testing, it was found that the hovercraft finally hovered, but once the battery for the propeller was placed on the hovercraft, it immediately sunk back down to the ground. This exhibited that the hovercraft still weighed too much, as it ceased to hover once more weight was added. However, this slight hover was good progress and showed that more intake had helped give more propulsion and hover. In the last model, the surface area was increased, the skirt heightened, and the box shortened. This resulted in no hover, even with the battery on it. A lot of air was felt bouncing back out of the front, so the decision to fix that problem was to provide no room for the air to bounce back. A car was built using a circular frontal design, which resulted in hover, but uncontrollable spinning as well. To fix this, a single bladed rudder was constructed and was attached to a remote controlled servo. With no positive results, it was concluded that it wouldn’t 18-6 work because the rudder was not effective and needed more flaps. Two additional flaps were added in order to fix the problem, and when testing more air flow was found to be going one way than another. The best way to control air flow would be to create a funnel, and after the funnel was installed, the hovercraft was quite controllable. The reason the circular tunnel worked is because it allowed no air to escape, and the reason the funnel was so effective is that it channeled all the air into the rudder, and increased the velocity of the air. One of the main reasons for the successful hover of the hovercraft was the larger surface area of our body. The average pounds per square inch was much greater than our old models, and provided more than enough pressure to lift our hovercraft off the ground. The combination of a larger body and skirt, a circular tunnel, and a three sided rudder seemed to be the most effective technique in powering and handling a hovercraft. Discussion The project constructed by the team could be improved upon in a variety of ways given the materials and time. With the time and budget, a number of materials could be tested for the best strength-to-weight ratio if the foam insulation is not the best material. More time would allow for more applications of the different kinds of skirts which are used in order to see what provides the best hover for the craft. With more time, more base designs could be tested to find the ideal shape and size for speed and control. Time could also be used to fine tune the system to get a straighter flight and perhaps a manner of braking system could even be implemented. The application of the hovercraft design is growing, as improvements to design are steadily increasing. The most prominent uses for hovercrafts tend to be for recreational purposes, some on a small scale and some on a large enough scale to carry multiple people. Hovercrafts can also be used for civil commercial purposes, such as transportation. Their ability to hover over land and water with very little or no friction with the ground provides an ideal situation for turbulence and comfort. Also, with barely any friction and propellers as motors, the cost of gas is nearly negligible. The last major use for hovercrafts is for their military advantages. Hovercrafts provide an effective method for carrying heavy loads over difficult terrain, such as swamps. With so many practical and effective attributes, the future of hovercraft design looks bright.
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