Aircraft Propulsion System

 Introduction
An aircraft is any manmade object that moves through air and uses the air to generate the lift required for flight. This may include missiles, airplanes, helicopters, and any other vehicle that can sustain flight for a considerable time. Propulsion is an action of pushing forward. A propulsion system is therefore a machine that produces thrust through some action and pushes an object forward
. A propulsion system works on the principles of Newtons third law of motion, for every action, there is a reaction. The engine on the aircraft accelerates air and the action produces a force on the engine. The engine, which is attached to the aircraft, transfers the effect on the aircraft pushing it forward. The amount of thrust produced by the engine depends on the amount of air flowing through the engine per unit of time and the exit velocity of the air in contrast to the entry velocity. There are different propulsion systems each generating thrust in a slightly different way. This includes propellers, turbofans/jets, ramjets, and rockets. For Newtons first law of motion, it is clear that for the engine to produce motion on the aircraft; the thrust must at least equal the drag and any other resistance to motion the craft is experiencing. For the craft to accelerate, the thrust must exceed the drag. Most of the commercial airlines spend most of the flight time cruising in high altitudes therefore, engine efficiency is more important that acceleration ability. The turbofan engines are most fit for this task because they produce thrust by accelerating large volumes of air through the engines with low fuel usage. High bypass turbofan engines are the most common in commercial airlines and the CFM56-3 is perhaps the most popular one.
Aircraft Propulsion System
For the engine to operate efficiently at low speeds, the mass of air affected by the engine must increase. There is a challenge to increasing the amount of air affected by the engine because the fuel available in the engine mixes in a particular ration with the air for optimal performance. To overcome this challenge, engine designs have devised and innovative method of routing some of the air around the combustor and turbine. The modern engines are characterized by a bypass ratio (BPR). BPR is the ratio of the mass of air going around the combustor and turbine to the mass of air going through the core engine. Engines with a bypass ratio of zero are turbojets. Engines with a BPR of 1-2 are low BPR turbofans. Most of the commercial aircrafts have a BPR of 5-8. To enable optimal performance of the BPR engines there are gearings between the turbines and the fans (twin-spool engines). Only moderate BPR engines are used in commercial aircrafts because engines with higher BPR are larger, heavier, and more difficult to install, especially on the low wing configuration common in the modern aircrafts (Linke-Diesinger, 2008).
The CFM56-3 series is available in three variants (3-B1, 3B-2, & 3C-1) and was designed for the Boeing 737 second-generation aircraft. In is currently operational in the 300, 400, and 500 series. This compact workhorse entered service in 1984 and is one of the best selling engines ever. The engine has a cumulative time on wings of 38,736 hours and counting. The engine is proven in severe weather and meets a first run life of more than 16,000 engine flight hours. The CFM56-3 comprises of separately three major modules. These include the fan, core, and the low-pressure turbine (LPT) major modules. The module concept is based on the maintenance requirements of the engines and allows the maintenance professionals to work on the engine without dissembling other major modules (Kong, 2000). This module design not only makes the work of the design professionals easier but also provides the customer with low cost maintenance procedures. This purpose of this revolutionary design is to reduce the out of service time and to increase accessibility of components on-wings (Oates, 1989).
The CFM operates using a four stage integrated fan and a low-pressure compressor (LPC) booster, which is driven by a four stage low-pressure turbine (LPT). This dual rotor, high bypass, axial flow turbofan engine also features a nine stage high-pressure compressor (HPC) powered by a single-phase high-pressure turbine (HPT). Both of the turbines in the engine are driven by an annular combustor chamber, which enables the HPC to deliver high air velocity. The annular combustor feature smaller burner nozzles that provide a circular flame. This feature enables production of a hotter flame, which solves the hot-spot problems and increases engine power. The engine also features a pylon mounted thrust reverse that enables the craft to land on shorter runways by reducing the aircraft speed (Linke-Diesinger, 2008).
The engine has a short length and a solid structure because of the fan and turbine frames. The fan frame, a component of the fan major module, is located at the front of the engine next to the core module. At the other end of the engine is the turbine frame a component of the LPT major module. The engine has five major bearing connected to the lubrication system. The first bearing supports the radial and thrust loads. Another bearing support the radial load of the fan and booster assembly whereas the three support the HPC radial load and shaft thrust. This bearing is contained within the inlet gearbox assembly. The fourth bearing is located behind the LPT and supports the radial load of the HPT and LPT shafts. The final bearing is mounted within the turbine frame and supports the radial load of the LPT near the shafts end (Kong, 2000).
The CFM56-3 is a dual rotor engine consisting of a low-pressure system. The low-pressure system consists of a single stage fan, which is connected to a three-stage booster rotor assembly. Within the secondary airflow is an outlet guide vane (OGV) assembly. A four-stage booster stator assembly is housed within the primary airflow system. There are 12 VBV between the HPC and the booster for air cycling throughout the system. A four stage LPT drives the booster and fan. The engine also consists of a high-pressure system. The HPC is a nine-stage rotor with one variable inlet guide vane (IGV) assembly. A HPT nozzle and rotor assembly drives the HPC (Linke-Diesinger, 2008). The HPC houses 20 fuel nozzles in a short-machined ring construction annular combustor. Other components of the engine includes transfer gearbox (TGB), IGB, accessory gearbox (AGB), radial, and horizontal drive shafts.
The out casing of the engine comprises of external flanges. The flanges mating points are of a rabbet fit construction to reduce the air leakages and ensure appropriate alignment during assembly. The horizontal flanges are of a butt type and are identified as the front stators horizontal left and right flanges. The CFM56-3 engine utilizes a dry sump design in its oil system. This oil system consists of a lubrication unit and an external tank. The rate of oil application to the bearing is equal to or greater than the rate of supply. After the oil runs through the bearings, it goes through a cooling and filtering process before returning to the external oil tank. Labyrinth type seals secure the main engine bearing sumps with the airflow providing pressure across the seals. Using oil and air seal, a cooling and pressurization chamber is created around the bearing sump. This pressurization between the oil and air is supplied by the booster discharge and is vented through an air/oil separator to a center vent creating a low-pressure chamber. As the pressurized air attempts to escape through the least resist route, the air flows across the seals preventing them from leaking. In case oil leaks, it is collected in the air seal housing. The pressurized air forces the oil out through a drainage system, which drains overboard (Oates, 1989).
The high bypass structure of the CFM56-3 engine consists of two major paths. The primary and secondary paths are the avenues through which the engine discharges jet velocities. The primary airflow path follows the inner portion of the engine. Air moving through this path moves through the fans into the booster, through the core engine, and finally the LPT before exiting through the nacelle discharge duct. The secondary airflow paths through the first stage rotor and the OGVs before exiting through the nacelle discharge duct (Kong, 2000). The engine uses flow path aerodynamic stations to facilitate engine monitoring and performance assessments. Some of the flow path aerodynamics stations measure core, fan, speed, altitude correctness scheduling, and flight deck indicators (Oates, 1989).
The low-pressure turbine (LPT) is a major module consisting of three individual modules. These include the LPTs shaft, rotor, stator, and turbine frame modules. The LPT shaft serves several functions. These include connecting the fan booster with the LPT rotor. The fourth bearing within the engine system is part of the LPT shaft and provides support to the HPC. It also provides connections with the fan shaft at the tapering support and remnant shaft. The turbines rear frame provides structural support for the LPT rotor. It also facilitates the mounting of the engine on the airframe. The turbine rear frame also provides the exit point for the piping used to direct leaked oil out of the engine. The LPT also has a case and a shroud-supporting ring. This is an air seal segment that also serves as a thermal insulator. This module also houses instruments that read various parameters in the engine and relay the information to the cockpit (Linke-Diesinger, 2008).
Conclusion
The technology in the engines has improved unimaginably over a short duration. With improved efficiency and higher thrust, twin engines are now more common than ever. The high interest in large planes flying on twin engines comes from the expenditure associated with buying and maintaining aircraft engines. It is currently cheaper to acquire a large twin-engine plane for less than a four-engine plane. For many years, use of twin-engine aircrafts has been limited to short distances for obvious reasons. The FAA limits the use of twin engine planes to routes were the pilot can access an airport within 60 minutes in case of an engine failure. The limit is within 120 minutes for more modern plane designs because of some technical breakthroughs. For the twin-engine, aircraft to gain more acceptance, it must meet the climbing requirement of the flight with one engine out. The aircraft must be capable of operating with less than 50% thrust. The issue is compounded by the drag associated with the failed engine and the need to compensate with asymmetric thrust because propulsion is one-sided. Turbofan engines continue to prove their durability across the world in different operating conditions and demands.
 References
Kong, C. (2000). Propulsion system integration of turboprop aircraft for basic trainer. Aircraft Engineering and Aerospace Technology, 72(6), 524.
Linke-Diesinger, A. (2008). Systems of commercial turbofan engines: An introduction to systems functions. Berlin: Springer.
Oates, G. C. (1989). Aircraft propulsion systems technology and design. Washington, DC: American Institute of Aeronautics and Astronautics.