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The Piston Helicopter Engine

The Piston Engine/ Piston engines are the most common type of engine to be found in modern,ght helicopters. Normally there will be 4 or 6 cylinders in a horizontally opposed configuration. Pistons move back and fourth inside the cylinders. Inside each cylinder, fuel is mixed with air and ignited. The energy produced by the combustion of the fuel air mixture causes the gases to expand and drive the piston down into the cylinder. The piston is connected by a connecting rod con-rod to a drive-shaft which is forced to turn due to the movement of the piston. Piston engines may operate on either a two stroke cycle or a four stroke engine cycle. The Four Stroke Engine Cycle/ A complete cycle of the four stroke engine comprises four strokes of the piston moving within the cylinder. This cycle is also known as the Otto cycle after its inventor in 1876. The four strokes are: ol liInduction liCompression liCombustion (or Expansion) (or Power) liExhaust /ol Induction/ During induction, the fuel and air mixture is sucked into the cylinder through an open intake valve (on the right) as the piston moves from the top of the cylinder to the bottom of the cylinder. The exhaust valve (on the left) is closed. Compression/ Early in the compression stroke, the inlet valve closes and the fuel/air mixture is trapped in the cylinder. The piston then moves back up to the top of the cylinder. This compresses the mixture and causes the temperature and pressure of the fuel/air mixture to rise. As the piston reaches the top of the cylinder and completes its compression stroke, the fuel/air mixture is ignited by a spark from the spark plug. This causes combustion which causes the gases in the cylinder to expand. Power/ As the piston has passed the top of its stroke, the expanding gases force it back down the cylinder. This is called the power stroke as the heat energy provided by the combustion process is now converted into mechanical energy. Just before the piston reaches the bottom of its stroke, the exhaust valve will open. Exhaust/ As the piston returns to the top of the cylinder again, the burned gases are forced out of the cylinder and into the atmosphere through the exhaust manifold. As the piston nears the top of its stroke and while the last of the burned gases are being expelled, the inlet valve opens in preparation for the next induction stroke. In one complete Otto cycle, only one of the four strokes provides power - but the crank-shaft has rotated two times. Engine manufacturers increase the power of the engine by adding more cylinders. This has the added bonus of making the engine run smoother. Each cylinder will have the power stroke occurring at different positions during the rotation of the crankshaft to try to even out the power impulses. Compression Ratio/ This is the ratio of the total cylinder volume when the piston is at the bottom of its stroke (bottom dead centre BDC/em) compared to the top volume of the cylinder when the piston is at the top of its stroke (emtop dead centre TDC). The compression ratio is designed to suit the type of fuel used. If the compression ratio is too high, the fuel may ignite early and excessive wear will occur. Valves/ Both the inlet valves and the outlet valves must open and close at the correct times in relation to the movement of the piston. The timing of the valve operation is controlled by a camshaft. The camshaft only rotates at half the speed of the crankshaft. The camshaft operates rocker arms and pushrods that push to relevant valve open. When the camshaft releases the pressure, a spring returns the valve to the closed position. A typical helicopter piston engine speed in flight is 2700 revolutions per minute (RPM). Each inlet valve and exhaust valve opens once during the four strokes of the Otto cycle. I.e. once in every two revolutions of the crankshaft. This means that at 2700 RPM, each valve will open and close 1350 times per minute = 22 times per second. This is a very short time to get the fuel/air mixture into the cylinder and exhaust the burnt gases again. To increase the efficiency of the fuel/air mixture induction, the inlet valve opens before the piston reaches top dead centre (TDC). This allows maximum time to induce the fuel/air mixture into the cylinder. It is referred to as valve lead. Similarly the exhaust valve opens before the piston reaches bottom dead centre (BDC) on the power stroke. It is worth noting that for a very short time at the start of the induction stroke, the exhaust gases are still exiting through the open exhaust valve while the fuel/air mixture is being forced into the cylinder through the open inlet valve. This period of overlap when both the inlet valve and the exhaust valve are open at the same time is known as valve overlap. For information about turbine engines, visit the Turboshaft Engines Turbine Engines on my website. br class=clearp

Underwater Ecology for scuba divers

When you have finished reading this chapter, you must complete the following tasks: 1. identify what tasks may be performed by a novice diver on a dive involving the study of marinefe; 2. identify how to name a form of marinefe so as to avoid confusion; 3. list the most common forms offe in plankton; 4. list the benthic forms offe; 5. identify the sessile forms of benthicfe; 6. identify what phanerograms are; 7. name an item of equipment that is essential for observing marinefe; 8. identify the person who should coordinate the activity of marinefe observation; 9. describe the information to note on the dive slate when taking a photo for the study of marinefe; 10. describe the tasks that are an integral part of the underwater ecology dive of the PSS Advanced Open Water Diver course. The study of marinefe Naturalists are people who study nature and the environment. An underwater naturalist is therefore an expert on marinefe. As divers you are also underwater naturalists. Do you think this is attle exaggerated? Diving gives you the opportunity of observing strange and curious creatures live that most people only get to see on TV. After a few dives, you become familiar with how certain creatures behave, what they eat, where theyve and where they can usually be seen. Diving during the day among rocks you are surprised to see a creature that you usually associate with a sandy sea bed, for example a sole, or maybe a nocturnal creature. A non-diver, not as familiar with the underwater environment, would probably not realize how strange this was. Your experience has led you to getting to know the inhabitants of the underwater environment and also their different habitats. Your knowledge of the physical laws of water enables you to understand the influence this environment has on the organisms thatve there. Reduced gravity and relatively constant temperature enable gigantic creatures to exist under water. The greater density of water means that taperedfe forms have evolved, and the continual battle for survival has produced species that are masters of camouflage. By cultivating an interest in these incredible beings, you will learn more about the creatures you are already familiar with and discover new ones that you didn't even realize were there. However, this invitation to become more interested in marinefe is not intended as an invitation to touch, molest or interfere with the marinefe in any way. Your presence must be entirely passive: look, but don't touch, do not collect samples or change the natural behavior of an animal in any way. For example, feeding the fish causes them to behave un-naturally. Classifying forms of marinefe If you attend the PSS Underwater Ecology specialty course you will learn a lot about the different forms offe that populate the waters of the world and their parentage. You will also learn how man has classified the numerous species into homogeneous groups and has created a complex schema in which each different organism is inserted. Each organism has also been given a scientific name in Latin, which identifies its class and species. For example, Moby Dick, the white whale, was not actually a white whale (cetacea mysticeti) at all, but a sperm whale (cetacea odontoceti - meaning that it has teeth,ke orcas or dolphins). In French the sperm whale, is known as cachelot, in Spanish cachalote, in German pottwal. To avoid confusion,ke that of Moby Dick, in scientific publications naturalists call it by its scientific name of Physeter macrocephalus. However, let's leave this to the more specialized courses, for now we are only interested of getting an idea of the enormity of this world. Most of the underwater world is a pelagic (the surface waters of the sea) environment far from the coast. There are forms offe that stay there constantly and there are others that spend only a certain period of theirfe in this open water. Plankton, which consists of a multitude of drifting organisms that are not able to determine their position in the water, is transported by the current. In reality these organisms have amited ability to move which they use to ascend or descend, creating what is known as vertical migration. This may occur daily or according to the seasons. The most commonfe forms that make up plankton are microscopic and composed mainly of plant organisms (phytoplankton). Animal forms (zooplankton) make up the rest. As divers, we are generally only aware of their existence when we make safety stops and observe the more gigantic forms: the macroplankton or megaplankton. To see all the other types we would need a microscope. Then there are the nektonfe forms. These are able to determine their position in the sea and they also do this very efficiently: some fish of the tuna family travel up to 300 kilometers [160 nautical miles] a day! Generally, all these pelagic fish, except for some solitary predators, move in large shoals. Herring shoals can even reach up to fifteen kilometers [8 nautical miles] long and five kilometers [3 nautical miles] wide. Nearly all these types of fish have a blue back and a silvery-colored stomach so they camouflage with their environment (homochromatic) if seen from above or below. The larger fish are also part of the pelagic group: the whale shark and the basking shark, which can reach almost 20 meters [65 feet] in length. There is no reason to fear these gigantic creatures as they eat only plankton. Fish, cetaceans, some mollusks (squid, nautilus, argonaut), a few reptiles (turtles) and some crustaceans belong to the nekton class. To compliment the pelagic environment is the benthic group, that is, the organisms that dwell in or on the sea bed. Benthic organisms therefore populate all the sea beds, even those beyond the depth of 150-200 meters [500-650 feet]. This is known as the aphytal zone because there are no plant forms due to the lack of sunlight. In this zone, as well as animals, there are the Archea, a primitive domain only present in the abyssal zones and extreme environments (such as volcanoes, very salty or acid water), which make up the heredity of primordialfe forms that do not need oxygen. Above the aphytal zone is the phytal zone (plant organisms are present), which in turn is divided into four layers. The littoral fringe is the region of land wet only by the sea spray, therefore above the level of the high tide range. The eulittoral zone is the area covered and uncovered by the high and low tides. Beyond this is the sublittoral zone that extends to a depth (generally 35/40 meters [115-130 feet]) where phanerograms are present, that is, the superior plants with roots, stalk and fronds (posidonia, zostera, cymodocea, etc.). From this depth to the beginning of the afitale zone is the circalitorale zone, the typical environment for corals. The various forms of benthicfe have different relationships with the substrata on which theyve. The sessile forms are fixed in one point and do not move. These animals are usually filterers, such as mussels or gorgonia, and depend on the movement of the water for their nourishment. Then there are sedentary forms that move just a few meters [feet] during theirfe span, for example echinoderms (sea urchins and starfish). Some gastropods, such asmpets, also belong to this group. Instead, other benthic form move freely over greater distances even though their existence isnked to the sea floor. These are defined as vagile and include, among others, fish, crustaceans, mollusks (particularly cephalopods, such as the octopus). Finally, not to be forgotten are the forms offe that remain fixed in the substrata: the endofauna. These are diggers that, thanks to a mechanical or chemical action, burrow into the substrata, feeding off organic debris present in the silt or filtered from the water. As you have certainly understood, when you dive you see barely 5% of thefe forms surrounding you. Only experience and careful observation will increase this percentage, reaching the level of a good guide who is able to track down the most important examples. However, with further study of underwater ecology and the various forms of marinefe you will be able to increase this percentage even more significantly. By reaching just 10%, your usual dive sites that you have visited many times, will become an enchanted kingdom and a place where you will surely make fascinating discoveries. Relationships between the species If the classification of marinefe forms seems complex, the relationship the underwater creatures have with each other is even more so. Although study of this aspect can be difficult, it is also extremely interesting and rewarding. A diver experienced in diving on coral reefs will immediately recognize a cleaning station. This is where small cleaning wrasse or shrimps move over the body of the fish to remove parasites or dead scales. This action is appreciated so much by the fish that they even open their mouths so they can be cleaned inside, without doing any harm to the cleaners. It is an incredible sight to see the other fish waiting inne for their turn! The relationship between thefe forms may be very complex, going well beyond simple predation for food. For example there are animals that manage to survive only thanks to the other organisms they host. These may be animal or plantfe forms and they produce elements vital to thefe of the host (symbiosis). Instead, parasites occupy or colonize other organisms and feed off them. Somefe forms host, or are permanently accompanied by, other species that do not feed off their hosts but they obtain their food together (commensalism), both benefiting by this relationship, even for defensive purposes. As always the most feared species is man. We are the animals that cause the most damage in the water! Even while trying to understand the behavior of other species we are often influenced by a number of traditional beliefs. We believe that a moray eel is dangerous because it has its mouth open (in reality it is just breathing) or that the seaons that we see performing in circuses are cute friendly animals (on land you would probably get bitten if you went near them). Equipment and specific risks In some cases the only item of equipment you need to study marinefe is a dive slate, even though professionals often use sophisticated electronic apparatus. Therefore, you should have a fairly large dive slate. Better still is an underwater notebook that has waterproof pages as during the dive you will need to make a lot of notes. Some researchers attach or stick a ruler to the back of the dive slate so they can take measurements of the sessile or sedentary organisms. To identify and estimate the dimensions of vagile organisms from a distance some people draw different sized shapes on the dive slate that represent the species they are studying. This makes it easier to estimate the real dimensions of the specimens. Instead, if you have to study a particular habitat rather than a single species you can use plastic identification cards. However, only a few species are shown on these cards and it is a good idea to have a good publication about marinefe that you can consult later. The archives of marine fauna published on Internet by many universities can also be a helpful source of information. A compass and a diveght will also be useful on your dive. For more professional researchnes, grids, marker buoys and measured tape are also used. If you know how to use them, underwater cameras and video cameras also provide photographic evidence that can be studied later. A dive computer will give you the depth and the water temperature. Also note the composition of the sea bed and copy all the data you have gathered in a special notebook. Also make a photocopy of the notes you made on your dive slate during the dive. An underwater ecology dive made within the usualmits and applying the buddy system does not present any particular risks other than the possibility of causing creatures to act defensively if they are disturbed by divers. Planning the dive A dive involving the observation of marinefe must be planned very carefully, particularly when the research has a precise objective. Quantitative surveys, the gathering of data, and the comparisons of the various habitats are all fields of study that require accuracy if the material gathered is to be of use for further study. This means planning a well-organized, faultless dive. It is therefore recommended that all the various steps are coordinated and supervised by someone qualified in this field, ideally a marine biologist. If you want to further your study in this area, your local university will be happy to have an extra volunteer to help them in their research. In this case you may also have veryttle knowledge about biology, but you must be a good diver able to navigate perfectly. Survey methods Your task in the water will bemited to the observation of the various species and not collecting specimens (samples), which is regulated in almost every country in the world and often requires special authorization. This is also valid for collecting specimens for personal aquariums. In some countries it is not even permitted to take water out of the sea! Very different from the past when scientific research was done dry, using drag nets, dredgers, etc. Today, however, it is possible, at least for some scholars and researchers, to enter the water. When conducting a research in the water you must not damage the fragile ecosystem and your presence must not influence the outcome of the research in any way. Very often, before beginning a research project, you need to create a precise topograhic map of the area. Special techniques are used to do this which go way beyond those dealt with in this manual. The diver-naturalist must be well-trained and have experience in recognizing the species in the water and estimating the numbers of the subjects encountered and their size, which is distorted by the air in the mask. This is also true for those who take photographic evidence. The technique, in use since the 1970s, foresees that every photograph taken must be accompanied by a card containing all the necessary data. Other times it may be a quantitative photographic survey. In this case a photo-mosaic is made of an area by combining several photos for later examination. A stereo-photograhic process (two cameras set up together), often used in Sweden and Norway, provides three-dimensional images that make later examination of the subject easier. Volunteer divers can also make a visual census of the marinefe. This type of visual census began in Hawaii in the 1950s and has proven to be more accurate than video recordings, particularly if the divers are trained in recognizing particular underwater shapes. To make an estimate of the population of a given species you can apply thenear transept method. The transept is a layer of a known length and thickness (generally 2 or 4 meters [6 or 12 feet] wide and 25 or 50 meters [25 or 50 yards] long) that the diver moves along at a fixed distance from the sea bed counting the examples. At the end, dividing the number of the examples by the area (or the volume) covered gives the average density. This can then be multiplied by the surface area of the zone to be explored in order to obtain an estimate of the population. For vagile organisms, which move constantly, this method has certain disadvantages. The presence of the diver may frighten the creatures he is attempting to count and make them move away from the area before they can be counted. Also the speed that the specimens are counted is important: too fast and the number may be underestimated (in the rush some are missed), too slow and the number may be overestimated (specimens that move could be counted twice). Another method is a rapid visual census, also defined as species/times random count: the diver moves freely dividing the dive into periods of time (usually five periods of 10 minutes). The species encountered in the first period are given the highest mark (5), which decreases in the second period (4) and then decreases further in each successive period. The principle is based on the fact that the probability of immediate encounter is greater for the most numerous species. After numerous dives the sum of the score is made and this results in the estimated population. Another, more accurate, method (the visual fast count) is derived from this. This is not based on the period in which the species appears but on the frequency interval of the five periods. Making the dive An essential part of the underwater ecology dive is the briefing during which the instructor will explain the objective of the survey in detail. Some identification cards for the animals and plants found in the area will be made available, as will other publications that will help in identifying the various specimens. Each of the divers may also be asked to sketch a different example chosen from the identification cards or books. At the end of the dive the divers should exchange sketches and then identify, with the scientific name, the organism that has been drawn. After the general dive briefing you should get into the water at the established spot or in the area marked off by the teaching staff. You will be asked to look for unusual specimens that you are not familiar with and sketch them on your dive slate so you can identify them at the end of the dive. The task of sketching the specimen may be substituted by taking a photo or video if you have already attended the appropriate specialty course. In either case, as soon as you find an organism, you should try to understand what it is doing, what it eats, how itves, etc. Note your ideas on the dive slate and don't forget to add information about the depth, water temperature, type of habitat etc. If you note any interaction with other species make a note of this as well. After the dive you can use the identification cards and other publications provided by the instructor to help you identify what you have seen and find its scientific name. This will give you an opportunity to read about the creatures that interested you in the water. You will discover a fantastic world where there are often no distinct sexes and numerous organisms go through various phases during theirves. Some jellyfish, in just onefe, manage tove in all the different environments, even growing temporarily as benthonic forms. Welcome to the alien planet! Worldwide scuba schools