Aircraft Propulsion - Research Paper Example

 Aircraft Propulsion Introduction It is just over a century since the first manned flight, achieved by the Wright brothers. Man has moved from the era of the horse drawn carriage to supersonic flight and even into space odysseys. The first problem the Wright brothers, among other aviation researchers, had to solve was the riddle of whether it was possible for man to achieve flight. After the brothers found a way to do it using a glider, their next challenge was the addition of power to the glider (Yu, 1998). The era of aircraft propulsion came into life when the brothers achieved this feat a few years after their first manned flight using a glider. Many researchers in the aviation industry continue research on better ways to power aircraft. In this era of green energy and sustainable development, it is imperative to find better ways of keeping aircraft in the air (Sabnis & Whitney, 2012). This paper explores the progress made in the area of aircraft propulsion. It looks at the different types of aircraft and their modes of propulsion. Fundamentals of Aircraft Propulsion The basic objectives that aircraft designers have are to ensure the craft can stay airborne and that the aviators can steer them in the required direction. This calls for proper take off and landing design as well as in-flight control and maneuverability. In order to attain these objectives, there are certain fundamental principles in physics that play an important role in the design and construction of the craft. The propulsion systems ensure that the craft remain airborne for as long as desired. The main principles that propulsion engineering takes into account include Newton laws of motion, the Bernoulli Effect, Boyle’s laws and gravitation (McClinton, 2008). The application of Newton’s laws of motion in propulsion design dictates the basic obstacles that every propulsion system needs to overcome. Newton’s first law, also called the law of inertia dictates that a body in motion or at rest remains in that state unless acted upon by an external force. In propulsion systems, external forces include gravity and air resistance. For an aircraft to remain in a state of motion there must be ways in which the system overcomes the retarding force of air resistance while at the same time it must allow for control over acceleration due to gravity. The second law talks about the change of acceleration a body experiences when an external force acts on it. This law is crucial in the design of aircraft controls in aspects such as increasing or reducing the speed of the aircraft, or changing its direction to correct for effects such as drag, in order to stick to its designated flight path. Newton’s third law states that for every action there is an equal and opposite reaction. As such, it is possible to design propulsion systems that depend on the action of one force on a body to produce motion in the opposite direction (Okai, 2008). The Bernoulli Effect describes the changes in pressure due to fluid motion. It is the principle behind aircraft wing and tail design. This principle describes the potential of a body rising, sinking, or changing direction due to a differential in the rate of fluid flow in different parts of the moving body (NASA, 2000). The potential to generate lift independent of another energy source made the first flights by gliders possible. It explains how kites fly and how birds that soar remain in flight without flapping their wings. It is still a very widely used concept in aircraft design. The final principle this paper tackles is Up-thrust. According to Archimedes, a body displaces an equal volume of fluid equivalent to its weight in a fluid. Up-thrust is the equivalent force that the fluid gives a body in resistance to its sinking. If Up-thrust exceeds the weight of the body, it floats. If it the weight of the body exceeds Up-thrust, then the body sinks. This principle is crucial in the design of drifters. The manipulation of these basic principles makes flight possible using several different methods. Aircraft design tends to employ several of them to attain lift and propulsion. In some cases, aircraft simply use the same principle to attain both lift and propulsion using different ways of manipulating the principles. In order to examine the principles of propulsion in aviation design, this paper groups aircraft into two major categories. The first category is the unpowered aircraft propulsion systems, which include gliders and drifters, while the second category looks at powered aircraft propulsion systems, which include propeller aircraft, jet propulsion systems, rocket propulsion and helicopters. Unpowered Aircraft Propulsion Systems Unpowered propulsion systems refer to those, which do not use any kind of external power to achieve flight and motion. These systems depend on other principles of flight to achieve flight. The two main types of propulsion systems are gliders and drifters. Gliders The earliest gliders were kites, built as toys for games and competitions. In fact, the gliders build by the Wright brothers came from a toy kite design. Kites had a harness connecting to a ground-mounted stake, or held by hand to enable someone to control it. The Wright brothers used this model to test their earliest flying machines. Glider design is an advanced field today, with complete fixed wing design that comes with all basic flight control implements. Gliders use the Bernoulli’s principle to generate lift, while it uses gravity to generate the forward motion (Yu, 1998). As gravity pulls on the glider, the wings of the plane use that force to cut through the air and in the process generates lift keeping the glider in flight. Most gliders require some kind of mechanical force to get airborne. They usually achieve this by taking off from a raised point such as a cliff or by taking off in tow of a powered craft and breaking off after the attainment of sufficient gliding altitude. Some of the limitations of gliders include their restrictive take off conditions and their limited range of operations. Since the gliders depend on other craft or on very specific points of take off, their operations cannot fit in areas where it is impossible to create necessary conditions. For a glider to take off there must be another aircraft that can tug it away or a way to take the glider up the cliff. In the issue of range, a glider can cover tens of kilometers but it is very sensitive to changes in the weather such as air pressure, rain, heat waves, strong wind and gushes among others (Yu, 1998). In rugged terrain such as mountain ranges, it is dangerous to fly gliders because of their limited maneuverability. Drifters A drifter is a large aircraft filled with a gas lighter than air. The use of drifters as flying machines came from the realization that it is possible to hoist heavy payloads using drifters filled with gases lighter than air. Drifters use Up-thrust to get airborne and they usually have a secondary means of navigation to control its flight path. Drifters without a secondary means of navigation simply rise until they find an air pocket drifting in the approximate desired direction of travel hence they use the power of the wind for propulsion. There are several types of drifters such as helium-filled airships and hot air balloons. Helium filled airships normally have a cabin located under the balloon which has a crew and the basic controls. Modern designs feature appropriately placed propellers for maneuverability. On the other hand, hot air balloons simply alter the density of air contained in the balloon by introducing heat in it. This makes the balloon float. This type of drifter also relies on the direction of the wind for flight. Hot air balloons have been in use for a very long time and are still popular with sightseers, researchers and advertisers. Powered Aircraft Propulsion Systems Powered aircraft propulsion systems refer to those that need some kind of power to take off, to remain airborne and to remain navigable. Aviation design normally makes tradeoffs between speed, payload capacity and range. These factors inform the design of aircraft because achieving any of them requires certain designs specifications. Propeller Aircraft Propeller aircraft depend on the aerofoil design of their wings to achieve lift. However, the wings must move at a certain speed to attain a sufficient amount of lift for the plane to take off (NASA, 2000). In this sense, there must be a means of propulsion that allows the plane to achieve the requisite speeds in order to sustain and maintain lift. For propeller aircraft, the propulsion of the plane comes from the propeller, usually mounted in front of either the aircraft or its wings. As the engine turns the propeller, the force generated pulls the plane forward and in the process the plane generates sufficient lift to move through the air. The propeller cuts the air in front of the plane in a screw fashion, which pulls the plane forward. The plane design determines how the pilot changes the speed of the aircraft. For fixed angle of attack propeller blades, the pilot changes the speed by increasing of reducing the speed of rotation of the blades. Faster rotation leads to greater airspeed of the plane. On the other hand, for movable propeller blades, changing the angle of attack changes the speed of the plane. Larger angles achieve greater speeds while smaller angles achieve lower speeds. Propeller planes tend to have capacity limitations because there is a limit to the amount of forward thrust a propeller can attain. In addition, the planes have a limitation in terms of the altitude it can attain simply because of the density characteristics of higher altitudes of the atmosphere. Jet Propulsion Jet propulsion systems work a bit like propellers but with the addition of a constraint, a fixed volume of space within which to generate thrust. Jet engines are designed to suck the air in front of the plane and to force in through the engine, where it mixes up with fuel (McClinton, 2008). After ignition, the air expands and is leaves the engine via the rear section of the engine. Jet engines achieve most of the motion for the plane by relying on both the first and the third laws of motion. The fins in front part of the engine suck the forward air creating an area of low pressure in front of the plane. This creates a bit of forward motion much like what a propeller would do. However, the significant power a plane uses for its propulsion comes from the release of a jet of compressed gasses through the engine. As the air passes through the engine, it mixes with fuel and undergoes compression, then combustion thereby expanding at an incredible pressure (Wittmer, Bieger, & Muller, 2011). This high-pressure exhaust from this expansion creates forward motion because of Newton’s third law. The reverse thrust from the jet creates a forward thrust, which moves the plane forward. Jet engines provide the power needed to propel a plane forward. Jet engines make long haul flights at very high altitudes possible. They are not as sensitive to air density differences in the upper parts of the atmosphere compared to propeller planes. In addition, jet engines make vary fast flights possible because of the potential to build higher capacity engines with reduced drag as compared to propellers. For the same engine radius, jet engines produce much more power compared to propellers. Rocket Aircraft Rockets provide the means of propulsion for specialized planes and spacecraft. A rocket is an engine that uses onboard liquid fuel only to release a jet at the back of the engine to generate motive force. It “is unique in that it starts off carrying all the materials that form the jet” (Okai, 2008, p. 3). Rockets use Newton’s third law of motion. The combustion of the fuel produces a high-pressure mixture which when released in the engines rear, produces a reaction force that moves the aircraft forward. Rockets do not rely on air or any other externalities to generate its motive force. This makes it possible for rocket-propelled planes to fly in space or near space conditions. It can “work in the vacuum of outer space” (Okai, 2008, p. 3). The added advantage at those altitudes is that because of the airlessness, the plane operates in a vacuum hence it does not experience air resistance. It operates at the near perfect first law conditions hence no more energy goes into keeping a vessel in flight. Such vessels typically operate by thrusting then gliding, after attaining requisite speeds. Rocket powered planes also do not need long runways to take off because they can achieve high speeds very quickly. Depending on the plane design, they can take off either horizontally or vertically. Rocket powered planes are not common because of high cost of construction and maintenance (Goedeking, 2010). The propulsion system is very promising provided sufficient amount of suitable fuel becomes available. Helicopters Helicopters are a special kind of aircraft with a vertical axis propeller that lifts the plane off the ground and provides the forward thrust required to fly the plane. Helicopters use a similar concept to the propeller plane in the fact that it cuts through the sit in a helical fashion. The main difference is that while a propeller plane uses aerodynamic foil to achieve lift after the forward propulsion by its propellers, a helicopter uses is blades to achieve both lift and propulsion. The general design of helicopters includes a horizontal axis rotor fixed at the tail end of the helicopter used to maintain stability in flight. Without this rotor, the helicopter will spin while airborne due to Newton’s third law. The rotor acts as a counter torque mechanism to keep the helicopter facing one direction. There is a limit to the altitude and the speed a helicopter can attain. The limitation in altitude relates to the safe air density within which a helicopter can operate. Higher altitudes present lower air pressure hence it alters the operating characteristics of the helicopter. The speed of a helicopter comes from the fact that one blade move forward, effectively faster than the helicopter, while the opposite blade moves in reverse during normal operation. Very fast speeds of the vessel will cause a significant loss of balance, which may cause the helicopter to roll over. A design feature meant to eliminate this phenomenon is the use of double vertical axis rotors. I. Conclusion Several researchers are developing models that can form part of the fuel for propulsion of aircraft in the future. This includes the use of Hydrogen and nuclear fuels (Okai, 2008). Some of the most difficult areas of engine research include noise reduction and ease of engine maintenance (NASA, 2000). There is a need to find more powerful propellants that use less fuel, or at least produce a reduced impact on the environment (Sabnis & Whitney, 2012). Most of these developments will depend on the degree of cooperation among players in the aviation industry. References Goedeking, P. (2010). Networks in Aviation: Strategies and Structures. Heidelberg: Springer. McClinton, C. R. (2008). High Speed/ Hypersonic Aircraft Propulsion Technology Developement. Advances on Propulsion Technology for Highj-SPeed Aircraft , 1-1 to 1-32. NASA. (2000). Small Aircraft Propulsion: The Future is Here, Glenn Propulsion Program Opens the Door to a New Era In General Aviation. Clevelend, OH: National Aeronautics and Space Administration. Okai, K. (2008). Hybrid Propulsion for Future Passenger Aircraft. Propulsion and Energy Systems (pp. 1-23). Tokyo: Unive. Sabnis, J., & Whitney, P. (2012). Green Aviation Propulsion System Challenges and Solutions. NASA Green Aviation Summit (pp. 1-23). Mountain View, CA: United Technologies Corporation. Wittmer, A., Bieger, T., & Muller, R. (2011). Aviation Systems: Management of the Intergrated Aviation Value Chain. Berlin: Springer. Yu, G. (1998). Operations Research in the Airline Industry. Heidelberg: Springer. Read More