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The Turboshaft Engine


´╗┐Helicopter Engines/ One of the most common helicopter engines/strong used today is the strongturboshaft engine/strong. A strongturboshaft engine is effectively a jet engine and normally runs on a kerosene based fuel. There are many variants of turboshaft engines but they all follow basic concepts and principles. To keep things simple, I will be referring to the engine used by the Bell 206 JetRanger. The Rolls Royce Allison 250/c20J. The working cycle of the turbine engine is similar to that of the piston engine. There is induction, compression, combustion and exhaust. The major difference is that unlike the piston engine, the turbine engines cycles are continuous. Unlike airplane jet engines where the exhaust gases provide the thrust, the exhaust gases for the helicopter turbo shaft engine are intercepted by a turbine that transfers the energy from these gases to a gearbox that provides power for the helicopter. It is vital that you learn not to over-torque or over-temp the engine as this can have disastrous consequences for someone flying it, weeks or even months later. Misusing the engine will not necessarily cause a problem right away. Starting the helicopter engine causes more stress to the components than just about any other operation due to thermal shock and wear. For this reason, the start cycles are counted and recorded in the technical log after each flight. As helicopters tend to fly for short periods compared to airplanes, it is possible that the engine could reach its start countmit before it reaches its usefulfe hourmit. Start-ups are expensive and so it is better to leave the engine running for ten minutes while waiting for a passenger rather than shutting the engine down and then starting up again. The engine relies totally on the battery or Auxiliary Power Unit (APU) for startup. The engine has to be turning fast enough to keep enough cold air flowing through the turbine otherwise it will become very hot and damage the internal components. A weak battery could run out of power before the combustion process becomes self sustaining and this would be disastrous. A Hung Start/strong can happen if the engine fails to accelerate and the RPM stays constantly low. This uses up your battery power and if the battery runs out of power then the engine slows down, less and less cold air is drawn through the engine and subsequently the temperature in the combustion chamber becomes really hot within a few seconds causing a lot of damage. A Wet Start/strong is the equivalent of a flooded engine and the igniter has failed toght the fuel. Wait for at least 5 minutes and then vent the engine. Because the battery has already been partially drained by the failed start and the venting, it is probably a good idea to get an external start using an APU to make sure you do not run out of power on your next start attempt. If you follow the checkst exactly then you should have no problem starting the helicopter engine every time. The most common reason I have seen for hot starts is low time pilots starting the engine with the throttle already partially (or fully) open. Therefore it is vital for you to double check that the throttle is fully closed before pressing the starter button. Turbine engines take time to Spool up or Spool down when you make power changes. For this reason you should operate the collective or throttle very smoothly to prevent engine surging. Surging can happen when the airflow over the compressor blades becomes disturbed causing them to stall. This leads to loud banging noises (similar to a shotgun going off) and a very noticeable vibration from the engine. Shut down immediately if this happens during startup. The Air Inlet/ The air inlet is designed to stabilize the air before it enters the compressor. A particle separator may be fitted at this point to remove any foreign matter such as dust or sand which could cause erosion of the compressor blades. The Compressor/ The compressor is made up of a series of blades and an impellor (or centrifugal) compressor. It is designed to take large quantities of air and compress it before directing it to the combustion section. The engine in the JetRanger has a six stage axial flow compressor and a single impellor compressor. In the axial compressor, each stage is separated by stator vanes to make sure the air hits the following blades at the correct angle. As the air flows past these blades it becomes compressed more and more. The pressure rises and its velocity decreases. The centrifugal compressor diverts the air outwards into channels that lead to the combustion section while compressing the air further. During startup there is a bleed valve that opens and allows some of the air to escape from the compressor. This makes it easier to get the engine up to speed and takes less power from the battery. When the engine reaches a sustainable speed the valve closes automatically. Because of the high temperatures of the compressed air in the compressor section (up to 250 degrees celsius), this air is used to heat the cabin and for anti-icing. Anti-icing uses air from the rear of the compressor and directs it through the compressor casing and the inlet guide vanes to prevent ice forming there. When anti-icing is used there will be a small rise in Turbine Outlet Temperature (TOT). The Combustion Section/ In this section the fuel is mixed with the air and ignited. The air ducts are shaped in such a way that the flame never comes into contact with the metal casing but instead is contained within a shroud of cooler air. A large proportion of the air is used for cooling. Once the fuel ist and the engine is up to running speed, the combustion is self sustaining. The engine is turned initially by a starter. When there is enough airflow through the engine to keep everything cool, the fuel is ignited by the igniter plug. The fuel enters the combustion chamber through a fuel nozzle that atomizes the fuel. On ignition the gases expand and flow to the turbine section at an increased velocity. Approximately 60 to 80% of the air entering the combustion chamber is used to keep theners cool. The fuel nozzle is highly polished and engineers have to handle it very carefully as the tiniest scratch will disturb the spray pattern and cause hot spots which will eventually damage the turbine blades. It has holes for delivering fuel. The Turbine Section/ This section creates the power. The turbine inlet is the hottest part of the helicopter engine and it is too hot for temperature sensors to survive here. Temperature is therefore measured between the turbines by thermocouples (the readings are averaged and displayed in the cockpit instrument display) and is called the Turbine Outlet Temperature (TOT). The heat is kept to a manageable level by cold air extracted from the compressor which is driven through a connection by the turbine. The gases are directed through the compressor turbine blades (N1) thus ensuring that the compressor is continually powered. From there the gases pass through a two stage Free Turbine (N2). As the free turbine is not directly connected to the compressor, the engine is easier to turn during startup. The free turbine is connected to the Accessory gearbox which reduces the high speed of the turbine to a more manageable level. When more power is required, the compressor speed (N1) increases to supply more air. At the same time more fuel enters the combustion chamber and therefore N2 is maintained at a constant speed. The turbine blades are operating in a very hostile environment. As the temperatures are so high and the blades are spinning so very fast, centrifugal force causes the blades to stretch (blade creep). This is normal, however if the engine has had a hot start, the blade creep becomes much larger than normal and becomes permanent. The blades can make contact with the sides and expensive repairs will be required. Higher TOT temperatures are permitted during startup as the turbine is spinning relatively slowly. The N1 turbine is doing more work than the N2 turbine. It is also exposed to hotter gases. For these reasons the N1 turbine has only half of the servicefe of the N2 turbine. The Accessory Gearbox/ The accessory gearbox converts the high speed of the free turbine (N2) to a more manageable level. It has a drive-shaft powering the main rotor gearbox, a rear drive-shaft powering the tail rotor, a freewheel unit and attachment points for all the accessories such as fuel pump, tachometers, generator etc. Compressor Stall/ Compressor stall can occur on any turbine engine if the conditions present themselves. In order to meet the design requirements, the engine must have a relatively high power output, good fuel consumption and fast acceleration characteristics. For these reasons it is beneficial to operate as closely as possible to the stall angle of the compressor blades. Operating close to the stall angle has the following benefits: ul liThe volume of air passing through the engine is increased. liThe pressure ratio of the engine is increased thus increasing the power output. liTurbine temperatures can be increased because of the greater airflow. li The efficiency of the compressor and turbine sections are increased. /ul To reduce the risk of the compressor stalling during startup or acceleration, the fuel flow is carefully regulated. So what is compressor stall? Many pages could be devoted to explaining this but the following explanation should help answer the question. Compressor blades and vanes are aerofoils. The airflow over an aerofoil will separate and become turbulent if either of the following occurs: ul liThe velocity of the air passing over the aerofoil is too low. liThe angle of attack is too high. /ul If the airflow over an aerofoil separates then the aerofoil stalls. Approximately 80% of the air entering the engine is used for cooling. This means that much more air has to enter the engine than what is needed for cooling. The cooling air is used to control the length of the flame in the combustion chamber and prevent it from touching the sides of the container. The hot combustion gases are cooled by the cooling air and the cooling air is also heated by the combustion gases. This keeps the gases at an acceptable temperature as they mix and enter the turbine section. If too much fuel is supplied to the burner, there will be more than enough air to allow proper combustion. However as extra air is used during this combustion, there will be less air available for cooling and therefore the temperature inside the combustion chamber will rise. As the temperature rises, there will be more gases to be exhausted. It is possible that the volume of gases to be exhausted may exceed the capacity of the turbine and the turbine will choke. When this happens, the pressure inside the combustion chamber will rise rapidly and may equal or exceed the pressure that the compressor is producing. If the pressure in the combustion chamber is equal to the pressure of the compressor discharge air, then the compressor will stall. If the pressure in the combustion chamber exceeds the pressure of the compressor discharge air, then not only will the compressor stall but also the hot gases will flow from the combustion chamber into the compressor section. Both of these conditions will result in a loss of air into the combustion chamber. The flame will not have enough oxygen and will die, resulting in a rapid drop in temperature. As the temperature drops, the expansion is stopped (or greatly reduced). The turbine is no longer choked and the combustion chamber pressure drops to a very low value. The low pressure in the combustion chamber means that air can flow in the proper direction again. The compressor is no longer stalled and a Surge of air flows back into the combustion chamber. This extremely fast movement of air elongates the flame downstream and through the turbine causing another rapid expansion of the gases. The cycle repeats itself at approximately 120 times per second. Compressor stalls may or may not have an audible sound but there will often be a vibration. If the stall is severe a flame may emanate from the exhaust or a very loud backfire may be heard. Smoke may also be seen. If the proper corrective action is made immediately then it is unlikely that any damage will occur. The actions to be taken are: ul liReduce the throttle to flight idle. liIf the stall remains then close the throttle completely and shut down the engine. /ul Rapid throttle movements may induce stalling therefore it is a good idea to make smooth, slow throttle movements. Further information may be found at

Top Tourist Attractions in New York


´╗┐If you are planning to go on a vacation in the United States, you should make a point to visit New York City. Also known as the Big Apple, New York City is one of the most popular travel destinations in the country, attracting more than 45 million visitors every year. It is home to many world-famous attractions, as well as some of the finest hotels, restaurants, and shopping places in the world. You can rest assured that you will never run out of things to do in this fabulous city. Here is ast of the top tourist attractions in New York City. Statue of Liberty/strong Almost everybody who visits New York City will pay a visit to the Statue of Liberty. This magnificent statue is perhaps the most recognizable landmark in the United States, and it stands as a symbol of American freedom. During the mass immigration era, it was the first American landmark that was seen by immigrants who survived the hardships of the journey across the Atlantic. The Statue of Liberty is located on Liberty Island, off the coast of Manhattan, and it stands at a height of 151 feet, or 305 feet if it is measured with the pedestal. You can climb up a staircase inside the statue to reach the crown and take in a stunning view of New York City. If you want go to visit the Statue of Liberty, you have to take a ferry from Battery Park. strongCentral Park/strong Covering an area of 843 acres, Central Park is one of the largest urban parks in the world. It is situated in the heart of Manhattan, and it is a favorite recreational destination among residents and tourists in New York City. This wonderful park features several lakes, fountains, playgrounds, ice rinks, baseball fields, tennis courts, and other amenities. It is also home to well-known attractions such as the Metropolitan Museum of Art and the Central Park Zoo. strongEmpire State Building/strong The Empire State Building is probably the most famous skyscraper in New York City, and it has been featured in many popular films and postcards. When this amazing building was completed in the 1930s, it was called the 8th Wonder of the World. Although it no longer holds the title as the tallest building in the world, it still attracts about two million visitors every year. The building reaches to a height of 1,250 feet, and it occupies an entire block in the southern part of Midtown. You can take an elevator up to the 86th floor, where there is an observatory that offers a fantastic view of the city. strongTimes Square/strong Times Square is another attraction that you must visit when you are in New York City. This world-famous square is the most bustling square in the city, and it is known for its captivating billboard signs. It is one of the places that best reflect the Big Apple's reputation as “the city that never sleeps”. Covering the area between 6th and 9th Avenue and 40th and 53rd Street, Times Square provides excellent shopping and dining opportunities, as well as an extensive variety of entertainment. It is also the place where you will find the Broadway Theater district. strongMetropolitan Museum of Art Whether you are an art enthusiast or not, the Metropolitan Museum of Art is a place that you should visit when you are in New York City. With more than two million artworks, this museum is one of the largest art museums in the world. Its collection represents thousands of years of art history, and it consists of artworks from many places around the world, including North America, Europe, South America, Africa, Asia, and Oceania. You will find numerous paintings, drawings, and sculptures by famous artists in the museum, as well as crafts, decorative items, photographs, prints, arms and armor, musical instruments, and others. p left; Ellis Island/strong Ellis Island is another attraction in Manhattan that draws millions of visitors every year. This island served as the gateway to America during the mass immigration era, and it received over 12 million immigrants while it was operating as an immigration station between the years 1892 and 1954. It is believed that more than 50% of Americans are descendants of immigrants who passed through Ellis Island, and therefore, many of them have visited the island to trace their ancestral roots. In the main building of the Ellis Island museum, you will find artifacts, prints, photographs, interactive displays, videos, oral histories, and other exhibits that provide a comprehensive insight into the immigration experience on the island. You can get to Ellis Island by taking a ferry from Battery Park or Liberty State Park. strongAmerican Museum of Natural History/strong The American Museum of Natural History ranks among the largest and finest museums in the world, and it welcomes about four million visitors every year. Situated in Manhattan's Upper West Side, it consists of 46 permanent exhibition halls, a number of research laboratories, and a largebrary. The permanent collections of the museum comprise of more than 30 million specimens, ranging from dinosaur skeletons to meteorites. While you are touring the museum, you can learn about human origins, mammals, amphibians, birds, oceanfe, fossils, mineralogy, earth and space, and many other things that are related to nature and science. strongFifth Avenue If shopping is one of your favorite activities, you must not miss the opportunity to shop on Fifth Avenue when you are in New York City. This avenue is perhaps the most famous shopping street in the world, and it offers upscale retail stores, major department stores, jewelry stores, toy stores, gift stores, and others. Some of the famous brands that you can find on Fifth Avenue include Prada, Louis Vuitton, Versace, Gucci, Bergdorf-Goodman, Emanuel Ungaro, Ferragamo, Brooks Brothers, Harry Winston, Cartier, Bulgari, Van Cleef, Tiffany and Company, Fortunoff, and Arpels. If you feel tired after your shopping spree, you can relax in some of the excellent cafes, restaurants, and spas that can be found along the avenue.br class=clearp