Đang tải... (xem toàn văn)
Đây là tài liệu hay về Tàu đệm khi... giúp các bạn có đam mê về ngành tàu thủy có thể học tập và nghiên cứu. Tài liệu chi tiết rõ ràng giúp các bạn có thể dễ dàng tiêp thu nhanh chóng... chúc các bạn thành công
Air Cushion Vehicle (ACV) Final Report Submitted to The Faculty of Operation Catapult LXXXIV Rose-Hulman Institute of Technology Terre Haute, Indiana By Group 28 Brock McMullen Riverton Parke Junior – Senior High School Rosedale, Indiana Ty Wiggins Terre Haute North Vigo High School Terre Haute, Indiana Andrew Clayburn Carlisle High School Carlisle, Pennsylvania Chris Palermo Bronxville High School Bronxville, New York July 25, 2008 28-2 Introduction to the Hovercraft A hovercraft is a vehicle supported on a cushion of air, able to traverse many different types of sufficiently smooth terrain – including, in some cases, water. These are machines that slide along while balancing on top of an “air cushion” bubble. This bubble is generated by an air pump (fan) while a flexible “skirt” helps retain the bubble beneath the machine by limiting the air loss. A pocket of air is formed and the resulting pressure pushes the hull of the hovercraft up and away from the surface it is sitting on. Since the friction against the bottom of the craft has been significantly reduced because of this pocket of air, less energy is required to move it across a surface. This “air cushion” gives the object a much smoother ride compared to other vehicles across rough surfaces. Hovercraft have one or more separate engines - one engine drives the fan on the bottom of the hovercraft, (the impeller) which is responsible for lifting the vehicle by forcing high pressure air under the craft. The air then exits the apparatus through the "skirt", lifting the craft above the area on which the craft resides. One or more additional engines are used to provide thrust in order to propel the craft in the desired direction (these engines help push the hovercraft). A plethora of different directional utilities exist, but the most popular are thrust vectoring devices (such as rudders or differential thrusts), side thrust devices (such as puff ports or thrusters), and cushion tilt devices (skirt shifts, skirt lifts). Some hovercraft utilize ducting to allow one engine to perform both tasks by directing some of the air to the skirt, the rest of the air passing out of the back to push the craft forward. In preparing to design our own working hovercraft, we attempted to incorporate this one as well as many other proven designs in order to build a successful vehicle. History of the Hovercraft Hovercraft similar to those of today’s world started as an attempt in an experimental design to reduce the drag on boats and ships as they went through water. The first recorded design for an air cushion vehicle was from the work of Emmanual Swedenborg, the Swedish designer and philosopher, in 1716. The craft was similar in shape to that of an upturned dinghy with a cockpit at the center. Devices on either side of the model allowed the operator to raise or lower a pair of oar-like air scoops, which on downward strokes would force compressed air beneath the hull and therefore raise the vehicle above the surface. The project was short-lived and was never built, for Swedenborg soon realized that to operate such a machine required a source of energy far greater than that which could be supplied by a single human occupant. In later hovercraft history, Sir John Thornycroft built a number of model craft in the mid 1870s to check the ‘air cushion' effects and even filed patents involving air lubricated hulls. From this time, both American and European engineers continued work on the problems of designing a practical craft. Not until the early 20th century was a hovercraft possible, because only the internal combustion engine had the very high power to weight ratio suitable for hover flight. “Hovercraft” was the name coined for this air-cushion vehicle by its inventor, Sir Christopher Cockerell. Cockerell was born in Cambridge, United Kingdom, where his father, Sir Sydney Cockerell, was curator of the Fitzwilliam Museum, Christopher Cockerell was educated at Gresham's School. He then entered Cambridge University, England, as an undergraduate, where he studied engineering and was tutored by William Dobson Womersley. 28-3 In 1953, Cockerell tested his theories of an air-cushion device using an empty KiteKat cat food tin inside a coffee tin, an industrial air blower, and a pair of kitchen scales. His idea was to build a vehicle that would move over the water’s surface, floating on a layer of air. This would reduce friction between the water and vehicle. To test his hypothesis, he placed the smaller can inside the larger can and used a hairdryer to blow air into them. By 1955, he had built a working model from balsa wood and had taken out his first patent. Although there have been many variations (leading to the development of a typical hovercraft design, as seen below), Cockerell developed the first practical hovercraft designs – leading to the launch of the first hovercraft to be produced commercially, the SRN1, in 1959. 1. Propellers 2. Air 3. Fan 4. Flexible skirt Figure 1: An example of a common hovercraft design http://en.wikipedia.org/wiki/Image:Hovercraft_-_scheme.svg Objectives On our first day in groups, we were introduced to what had been expected of past hovercraft groups and how successful those groups were in accomplishing their objectives. When all hovercraft groups decided which objectives we would attempt to accomplish, the final objectives for all groups were the ability to lift (or hover), to move forward (thrust), to change directions, to stop, and (possibly) to hover over water. Since we were been given close to 3 weeks of time (62 hours of Group Work), we had a decent amount of time to build a functional model hovercraft and therefore exploited a multitude of different prototypes that should have fulfilled all of these objectives. Our craft was designed to be able to traverse both solid cement as well as many other surfaces, such as grass, sand, and possibly an obstacle course. 28-4 Design / Results On the second day of Operation Catapult and the first day of Group Work, we were introduced to the actual hovercraft project by Dr. Ferro. We were then asked to decide what our objectives would be, and determined them, as seen above under the title “Objectives”. We were then introduced to the design process, in which we were asked to identify the problem we are facing, analyze this problem, make decisions to solve the problem, build prototypes to execute these solutions, test the prototypes, and then document our results. This is the process we would utilize for the remainder of Catapult and served as the basis for all our actions during Group Work. In the first day of group work, we identified that the problem we were facing was the task of building a functional hovercraft capable of lifting, moving forward, changing directions, and stopping. The next task was to analyze this problem and produce a prototype that would “fix” this problem. We decided to accomplish this by creating different designs that we could use. In discussing a mass of varying designs, another task was to consider the materials necessary in order to build those vehicles given the objectives agreed upon by the group. Our original materials given to us before we considered other necessities were those essential to air cushion vehicles – these being two servos, and R/C components (Fireball modified 21d Engine). Along with these materials, we were given access to the machine shop and supply rooms. In our first prototype, we had a very basic design, including a single engine / fan, rudders, and bag skirt on top of a lightweight material: Styrofoam. We believed that we would start off with a simple design and would only use more elaborate designs if they became necessary. The Styrofoam base made creating a hovercraft easier because it needed less power in order to hover. Another principle of this design was that some air was redirected from thrust to lift so that our hovercraft would actually hover. When creating the vent for this initial craft, we placed two thin foam boards as semicircles on each side of the hole to be cut and proceeded to connect a very thin piece of sheet metal to the top of these two pieces. After this, we created a small duct opening below this vent in order to provide air to the skirt we planned to create for this craft, a “bag” skirt. When searching for how hovercrafts actually hover, we found that they utilize a skirt, and that there are a large amount of skirts – all having their own strengths and weaknesses. The bag skirt surrounds the craft and uses additional pressure in order to inflate the bag against air pressure under the craft in the cushion in order to create lift. The bag is popular as its manufacture uses the least amount of material compared to other skirt types. There is usually little or no wastage. In the bag skirt we would use in this design, bags are usually inflated through a splitter plate connected by a small duct placed directly under the lift fan in the fan duct. Another design, the jupe skirt, looks like the frustum of a cone resting upside down; it slopes approximately eight degrees and in this way, as the pressure builds, a vertical force is produced on the jupe which will cause its inflation. There are usually three skirts in this system directly below the lift force, and the weight distribution usually controls the direction of motion rather than rudders. An additional design, a segmented skirt (or finger skirt), functions by creating and attaching many bag skirts – this option makes it much easier to repair compared to the other skirts, yet requires much labor as well as its problems of poor durability and stability. Different skirts can be used in combination and it is not uncommon for segmented skirts and jupe skirts to be attached to the underside of bag skirts, as seen in Figure 2 below. Given our lightweight design, however, it was suggested by Dr. Onyancha (a professor at Rose-Hulman 28-5 who has had much previous experience when it comes to hovercrafts) that we use a solid skirt simply made out of the foam we had used previously to make the base – we thought that it was a possibility that this skirt would work and took the advice of our teacher, a huge modification to our original design. Figure 2: Combination of Segmented skirt and Jupe skirt http://4wings.com.phtemp.com/tip/image/bfdetail01.jpg Cost Labor Repairability Weight of skirt High Speed Complexity Bag 8 8 5 7 8 9 Jupe 8 6 5 7 7 7 Solid 9 9 3 5 8 8 Finger / Segmented 6 6 10 6 7 6 1 = Worst 10 = Best Figure 3: Decision Matrix for Skirt Choice Another change we made to our original design was a shroud so that air would not be wasted simply going off from the sides of the blades, which would have impaired both our lift and thrust. We fixed this problem by using two layers of foam boards that would fit perfectly around the fan but still manage to fit on the base board of the hovercraft in order to keep air on the inner side of the shroud, as seen in Figure 4. One alteration necessary to provide safety to the craft for those guiding it was a shroud covering both sides of the fan with chicken wire to prevent hands from being cut by the fast rotation of blades on our fan. However, when we tested this prototype, it barely hovered off the ground, causing us to consider our other designs that we came up with in the beginning of the project. The problems of this prototype were not numerous, but enough to prevent the craft from lifting, as we knew that there was simply not enough lift being produced in order for the thrust to become effective. After completing our testing of Prototype 1, we knew that there were two obvious problems with our hovercraft: there was not enough air going into the ducts that were releasing our air into the bottom of the craft and that the solid skirt was not working. 28-6 Figure 4: A typical shroud, as seen in the book Lightweight Hovercraft Design Taking these problems into account, we began to build our second prototype, aptly named Prototype 2. The main idea for this prototype was that more air could be used for lift as an alternative to the original design by cutting a base directly into the board and cutting a hole so that air goes directly into the bag instead of a large vent as seen before. There were many similarities between this design and that of the first prototype, but Prototype 2 fixed one of the two problems seen above in Prototype 1. To fix the amount of air used for lift, we decided we would use two vents on both side of the engine. We built two different sized pairs of vents, one being 3 ½” tall and the other set being 4” tall – both of them going the same distance in length. After testing both, we knew that we would get much more lift with the 4” ducts, so we used those ducts. When building our initial prototype, two solid skirts were created in order to ensure that if the craft did not float, then we could change the skirt and if it continued to not work, pinpoint the problem being the skirt. In doing so, we created a 2” thick skirt for our initial prototype, and a 1” thick skirt for the 2 nd prototype. Also, in the second skirt, we sanded down the skirt because of the results we saw in the first prototype – yet the bolts that held in the engine were too long so we decided to cut them down in order to provide enough lift. Yet when we added all the necessary equipment, including the battery, to the craft, this prototype merely vibrated and failed in actually hovering. When considering more designs, we knew that the 2 nd provided the most lift of all of our choices, and therefore decided that we should make some simple modifications to the design in order to allow this craft to hover. We knew that the problem with the solid skirt was a combination of both the screws protruding past the solid we had at the base as well as its inability to conform to the amount of air pushed to the bottom of the craft by the fan. Given these difficulties as well as our in-depth research of skirt designs, we decided that the most successful skirt in this design would be a bag skirt. The bag was considered a better skirt than the alternatives in our situation because of its low cost, low labor, ease of attachment, and speed. Similar to the previous models, we created a shroud that continued to allow most air to pass straight through the craft and benefit both lift and propulsion while not interfering with blades. The problems that arise from this component of the craft are that the shroud adds weight and that (if sanded incorrectly) it could possibly keep the machine unbalanced – yet we believed that the benefits of the shroud definitely outweigh its downfalls. Yet when we tested this model, given the multitude of things that could have possibly gone wrong, the vehicle travelled relatively straight (even though there was a very slight turn to the left) and thus we began to work on our rudders. 28-7 Once we created the functional rudders out of servos, wires, and the rudders themselves, we attached this directional device to the craft, yet we had much trouble getting this hovercraft to float. We soon began to spend the majority of our time testing our new skirts to work with the rudders, going through handfuls of different skirts each day. The rudders were the best they would ever get, and we decided to continue attempting a variety of different bag skirts and the way they would attach to the base of the craft. After multiple designs, we finally ended up with a functional hovercraft that accomplished all of the objectives we set out at the beginning of our experiment, as seen in Figure 5. Figure 5: The Final Product Analysis / Discussion After designing our multiple prototypes, many tests were conducted in order to determine whether or not each prototype would be successful in fulfilling our initial objectives. When the first hovercraft was completed (without the rudders, an essential part of the craft), we tested the craft to see whether or not it would hover. This craft had two major flaws: there was not enough air going into the ducts that were releasing our air into the bottom of the craft and that the solid skirt was not working. After making alterations to the second prototype in order to attend to these problems created by the original craft, and being successful, we then began attaching rudders to our craft. This was easily the most tedious and most frustrating aspect of the project, as we easily attempted a dozen different skirt designs to function cohesively with the vehicle. In the end, our final skirt design was different from other unsuccessful attempts due to how the holes in the bottom of the skirt corresponded to the area of the bag skirt in total. This was a problem in many models, as we either had too much or too little area with the bag skirt as well as too many or not enough holes in the bottom of the craft to actually allow the lift. However, when tested in both the land test as well as the water test, our craft performed well and satisfied all the objectives we wished to complete from the beginning. All in all, our group accomplished all of its objectives as defined on the first day of Group Work. Our many failures taught us more than our few successes, as it was during these difficulties that we actually found out what functions with a hovercraft and what doesn’t. If asked to do the project again, our group would most likely have stuck with the normal skirts instead of trying out a solid skirt which we had never heard of – the process of working on the solid skirt which ended up not working took a couple days that would have been spent in a better manner. The project was more fun than we could have imagined, and we would love to do it over again. In the course of this experiment, we found out how to build a functional hovercraft with varying components such as a skirt as well as rudders – all of which are good pieces of knowledge. Yet while we may have learned a lot about creating an actual air cushion vehicle, this project taught us more about persistence as well as determination to reach a common goal. 28-8 Bibliography “Combination of Hovercraft Skirts.” Google.com. 2007. http://4wings.com.phtemp.com/tip/image/bfdetail01.jpg (19 July 2008). Elsley, Gordon H. Hovercraft Design and Construction. Great Britain: David & Charles Ltd, 1968. Fitzgerald, Christopher and Robert Wilson. Light Hovercraft Design, 3 rd Edition. Alabama: The Hoverclub of America, Inc., 1995. “Hovercraft.” Wikipedia. 21 July 2008. http://en.wikipedia.org/wiki/Image:Hovercraft_-_scheme.svg (22 July 2008). “The History of Hovercraft and Air Cushion Vehicles.” Hovercraft. 13 August 2007. http://links999.net/hovercraft/h overcraft_history.html. (19 July 2008).