AERO ENGINES QUESTIONS PDF

66 CHAPTER XIII- INTRODUCTION AND TYPES OF AERO ENGINES 1. An engine is a device where-in energy in one form is converted into another form. Here the heat energy is convert...

66 CHAPTER XIII- INTRODUCTION AND TYPES OF AERO ENGINES 1. An engine is a device where-in energy in one form is converted into another form. Here the heat energy is converted into mechanical energy to produce required propulsion. The propulsion is achieved by imparting acceleration to a certain mass of gas as per Newton’s third law of motion. BASIC THEORY 2. Aero-engines are machines which transform the potential energy contained in fuel and air either into kinetic or mechanical energy. The gas energy is produced by the combustion of an air-fuel mixture. The forward thrust is produced as per Newton’s third law which states that ‘for every action, there is an equal and opposite reaction.’ The operating cycle (pressure / volume cycle) of a basic aero- engine is Brayton cycle. 3. Types of Propulsion (a) Direct Reaction Propulsion. (i) In the case of rockets and ram-jets, all the gas kinetic energy is used for propulsion. (ii) I n t h e c a s e o f t u r b o -jets, the gas kinetic energy is partially used for propulsion, the rest is transformed into mechanical energy. (b)_ Indirect Reaction Propulsion. In this case, the gas kinetic energy is almost transformed into mechanical energy. Eg: - Turbo shaft and Turbo prop engines. COMPONENTS OF AERO ENGINES 4. Operating Phases. There are basically five operating phases for any Aero- engine. They are as follows:- (a) Induction (b) Compression (c) Combustion (d) Expansion (e) Exhaust 5. In an Aero engine, the operating phases are achieved with the help of following components: (a) Air intake Assists in induction of air (b) Compressor Assists in compression of air (c) Combustion chamber : Assists in combustion of fuel and air (d) Turbine assembly : Assists in expansion of combustion gas (e) Exhaust assembly : Assists in exhaust of gas  67 6. Apart from above, aero-engine has mounted components on the engine such as Fuel pump, Oil pump, Vacuum pump, Booster pump, Generator etc. In case of four stroke engine used on small aircraft, magneto and carburetor would be fitted on the engine. 7. The figure below is the schematic diagram to illustrate the main operating phases of the aero- engine components ROTOR EXHAUST AIR INTAKE ASSY COMPRESSOR COMBUSTION TURBINE CHAMBER TYPES OF ENGINES 8. There are various types of engines in use today such as Heat engines, Electric motors, Generators, Hydroelectric turbines and Wind mills. However, in the field of aviation, heat engines are of great relevance. Heat engines are devices which convert heat energy into mechanical energy. PRINCIPLES OF PROPULSION 9. The propulsion of aircraft is achieved by imparting acceleration to a certain mass as per Newton’s third law of motion. The relation between the force ‘F’ and the acceleration ‘a’ imparted to the mass ‘m’ is F = ma 10. There are two types of propulsion. They are as follows:- (a) Propulsion By Action. It consists of rotating a propeller in the air so as to create aerodynamic forces and to accelerate the mass of air. In this type of propulsion, a great mass of air is expelled rearwards with a low increase of speed. This is what a propeller does on the aircraft. (b) Propulsion By Reaction. In this case, the forward force is produced by expelling a mass of gas with a certain speed. In this type of propulsion, a small mass of air is expelled rearwards with a great acceleration. This is the principle of je t propulsion. If ‘m’ is the mass flow of gas with ‘V1’ as inlet velocity and ‘V2’ as outlet velocity, then forward force ‘F’ is given by ‘F’ = m (V2- V1)  68 (c) Indirect Reaction Propulsion. In this case, the gas kinetic energy is almost transformed into mechanical energy. Eg: - Turbo shaft and Turbo prop engines. JET ENGINES 11. Gas turbine engines are divided into two main classes. They are Turbo jet engine and Turbo prop engine. Almost all the modern aircraft including military aircraft are powered by this class of engines. BASIC THEORY 12. The principle of operation of a jet engine is similar to a piston engine in that the processes such as induction,, compression, ignition and exhaust are the same. The main difference from piston engine is that in case of a jet engine, the processes are continuous and not intermittent which is the case with a piston engine. 13. The ambient air enters the engine through the air intake. This air is then compressed by a multi- stage axial compressor. The combustion is achieved in an annular chamber. Gases are expelled at a high velocity, which creates the required thrust. 14. As explained, whatever the propulsion mode may be, the operating phases are similar. In a gas turbine, these phases are achieved with the following elements:- (a) Air intake (b) Compressor (c) Combustion chamber (d) Turbine (e) Exhaust 15. The thrust produced in a jet engine can be mathematically shown. If ‘m’ is the mass flow of gas with ‘V1’ as inlet velocity and ‘V2’ as outlet velocity, then forward thrust ‘F’ is given by ‘F’ = m (V2-V1). A basic jet engine is shown below.  69 TURBO PROP ENGINE 16. Turbo prop engine is a gas turbine engine which supplies mechanical energy to a propeller/set of propellers for producing the required thrust. THEORY 17. The gas turbine engine that is used to drive a propeller as shown above is called a turbo prop engine.A turboprop engine is simply a turbine engine where a propeller is attached to the low-pressure rotor at the front, via a gearbox. The air that passes through the propeller near its inner diameter also passes through the compressor stages in the core of the engine and is further compressed and is processed through the engine cycle. The air that passes through the outer diameter of the propeller does not pass through the core of the engine ,but instead passes along the outside of the nacelle. The large volume of air pushed backward by the propeller provides airplane thrust in the same way as the smaller, high velocity air from the nozzle of a classic jet engine. TYPES OF TURBO PROP ENGINES 18. There are two types of turbo prop engines: (a) Single shaft engine (b) Free turbine engine 19. The main difference between single shaft and free turbine engine is in the transmission of power to the propeller. (a) Single Shaft. In a single-shaft engine, the propeller is driven by the same shaft (spool) that drives the compressor. Because the propeller needs to rotate at a lower RPM than the turbine, a Reduction gear box reduces the engine shaft rotational speed to accommodate the propeller through the propeller drive shaft. (b) Free T u r b i n e.In a free-turbine engine, the propeller is driven by a dedicated turbine. A different turbine drives the compressor; this turbine and its compressor run at near-constant RPM regardless of the propeller pitch and speed. Because the propeller needs to rotate at lower RPM than the turbine, a reduction gearbox converts the t u r b i n e RPM to an appropriate level for the propeller. SUMMARY 20. In any engine the basic working principle remains the same, Also the mode of accomplishing the operating phases and the gas flow through different aero-engines more or less remains the same. The thermodynamic cycle for the operation of a basic aero-engine is Brayton cycle. However, the performance characteristics vary with varying designs of its assemblies by different manufactures.  70 CHAPTER XIV-AIRCRAFT CONTROLS Note The Cadets should be demonstrated about the aircraft controls and other aircraft parts on the Microlight itself for better understanding. 1. A conventional wing aircraft flight control system consists of flight control surfaces, the respectiv e cockpit controls, connecting linkages, and th e necessary operating mechanisms to control an aircraft's direction in flight. Aircraft engine controls are also considered as flight controls as they change speed. Generally basic aircraft control can be classified as follows: (a) Primary controls (b) Secondary controls BASIC AIRCRAFT CONTROLS 2. The basic aircraft controls are classified in to following: (a) Primary Controls. Basically the primary aircraft controls are arranged as follows: (i) A control yoke (also known as a control column), centre stick or side-stick governs the aircraft's roll and pitch by moving the ailerons, when turned or deflected left and right, and moves the elevators when moved backwards or forwards (ii) Rudder pedals, to control yaw, which move the rudder; left foot forward will move the rudder left for instance. (iii) Throttle controls to control engine speed or thrust for powered aircraft. (b) Secondary Controls. The secondary controls are trim tab, flap (aircraft), Air brake (aircraft), Spoiler , Leading edge slats, and variable-sweep wing. (i) Trim Tabs. These are small control surfaces connected to the trailing edge of a larger control surface of aircraft, used to control the trim of the controls, i.e. to counteract aerodynamic forces and stabilise the aircraft in a particular desired attitude without the need for the operator to constantly apply a control force. This is done by  71 adjusting the angle of the tab relative to the larger surface. Changing the setting of a trim tab adjusts the neutral or resting positionof a control surface (such as an elevator or rudder). As the desired position of a control surface changes (corresponding mainly to different speeds), an adjustable trim tab will allow the operator to reduce the manual force required to maintain that position. (ii) Air brakes and Spoilers. Air Brakes or speed brakes are a type of flight control surface used on an aircraft to increase drag or increase the angle of approach during landing. Spoilers are designed to increase drag while making little change to lift. Thus, spoilers reduce the lift-to-drag ratio and require a higher angle of attack to maintain lift, resulting in a higher stall speed. Most gliders are equipped with spoilers on the wings in order to adjust their angle of descent during approach to landing. (iii) Slats. Slats are aerodynamic surfaces on the leading edge of the wings of fixed- wing aircraft which, when deployed, allow the wing to operate at a higher angle of attack. A higher coefficient of lift is produced as a result of angle of attack and speed, so by deploying slats an aircraft can fly at slower speeds, or take off and land in shorter distances. They are usually used while landing or performing maneuvers which take the aircraft close to the stall, but are usually retracted in normal flight to minimize drag. (iv) Variable - Sweep Wing. A variable- sweep w i n g , also known as"swing wing", is an aeroplane wing that may be swept back and then returned to its original position during flight. It allows the aircraft's plan form to be modified in flight, and is therefore an example of a variable- geometry aircraft.  72 (v) Flaps. Flaps are hinged surfaces mounted on the trailing edges of the wings of a fixed- wing aircraft to reduce the speed at which an aircraft can be safely flown and to increasethe angle of descent for landing. They shorten take-off and landing distances. Flaps do this by lowering the stall speed and increasing the drag. FUSELAGE 3. Fuselage is the main body of the aircraft to which all the other components are attached. It also contains the cockpit from where the pilot controls the aero-plane. It provides the space for the freight and passengers. BASIC DESIGN 4. The basic design of fuselage should satisfy the following:- (a) Smooth skin of the required aerodynamic form. (b) Sufficient strength to withstand aerodynamic loads, landing loads and handling loads. (c) Sufficient stiffness to retain its correct shape under all loads. (d) Mounting points for engine, armament, fuel tanks and equipment. (e) Protection of aircrew and passengers from ambient conditions. (f) Sufficient break down points for easy dismantling for transportation and port- holes accessible for inspection and servicing. (g) Design itself should be economical and easy for production and repairs. 5. A basic fuselage layout is shown below for easy understanding. As can be seen, it comprises fire wall, wing attachment points, landing gear attachment points, stringers, bulk head/formers and stressed skin. MATERIALS USED 6. Early aircraft were constructed of wood frames covered in fabric. As monoplanes became popular, metal frames improved the strength, which eventually led to all-metal aircraft with metal covering all surfaces. Some modern aircraft are constructed with composite materials for major control surfaces, wings, or the entire fuselage such as the Boeing 787. On the 787, it makes possible higher pressurization levels and larger windows for passenger comfort as well as lower weight to reduce operating costs. Hence the various types of materials can be classified as follows: (a) Wood (b) Metals (c) Composites  73 MAIN /TAIL PLANE AEROLONS ELEVATORS & RUDDERS (AA-7) 7. Fuselage is the main body of the aircraft to which all the other components are attached. It also contains the cockpit from where the pilot controls the aero-plane. It provides the space for the freight and passengers. MAIN PLANE 8. As shown in figure above, a wing is a type of fin with a surface that produces lift for flight or propulsion through the atmosphere, or through another gaseous or liquid fluid. As such, wings have an airfoil shape, a streamlined cross- sectional shape producing a useful lift to drag ratio. 9. There are various types of wings as shown in figure below. They are as follows: (a) Straight wing (b) Swept back wing (c) Delta wing (d) Tapered wing (e) Variable geometry wing AILERONS, ELEVATORS AND RUDDERS 10. The main control surfaces such as Aileron and Elevators of a fixed-wing aircraft are attached to the airframe on hinges or tracks so that they may move and thereby deflect the air stream passing over them. This redirection of the air stream generates an unbalanced force to rotate the plane  74 about the associated axis. The rudder is a fundamental control surface in order to provide means of controlling yaw of an airplane about its vertical axis. AILERONS 11. The figure below shows the position of Aileron and Elevator on an aircraft. Aileron Elevator 12. Ailerons are mounted on the trailing edge of each wing near the wingtips and move in opposite directions. When the pilot moves the stick left, or turns the wheel counter-clockwise, the left aileron goes up and the right aileron goes down. A raised aileron reduces lift on that wing and a lowered one increases lift, so moving the stick left causes the left wing to drop and the right wing to rise. This causes the aircraft to roll to the left and begin to turn to the left. Centering the stick returns the ailerons to neutral maintaining the bank angle. The aircraft will continue to turn until opposite aileron motion returns the bank angle to zero to fly straight. ELEVATORS 13. An elevator is mounted on the trailing edge of the horizontal stabilizer on each side of the fin in the tail, as shown in the figure above. They move up and down together. When the pilot pulls the stick backward, the elevators go up. Pushing the stick forward causes the elevators to go down. Raised elevators push down on the tail and cause the nose to pitch up. This makes the wings fly at a higher angle of attack, which generates more lift and more drag. Centering the stick returns the elevators to neutral and stops the change of pitch. Many aircraft use a stabilator — a moveable horizontal stabilizer — in place of an elevator. Some aircraft, such as an MD-80, use a servo tab within the elevator surface to aerodynamically move the main surface into position. Thedirection of travel of the control tab will thus be in a direction opposite to the main control surface. It is for this reason that an MD-80 tail looks like it has a 'split' elevator system. RUDDER 14. A typical view of Rudder is shown below.  75 15. The rudder is a fundamental control surface, typically controlled bypedals rather than at the stick. It is the primary means of controlling yaw-the rotation of an airplane about its vertical axis. The rudder may also be called upon to counter-act the adverse yaw produced by the roll-control surfaces. 16. On an aircraft, the rudder is a directional control surface. The rudder is usually attached to the fin (or vertical stabilizer) which allows the pilot to control yaw about the vertical axis, i.e. change the horizontal direction in which the nose is pointing. The rudder's direction in aircraft has been manipulated with the movement of a pair of foot pedals by the pilot. SUMMARY 17. Primary controls and secondary controls are the most essential control systems for all types of aircraft. Several technology research and development efforts exist to integrate the functions of flight control systems such as ailerons, elevators, elevens, flaps and flaperons into wings to perform the aerodynamic purpose with the advantages of less mass, lower cost, reduced drag and inertia (for faster, stronger control response).  76 CHAPTER XV- BASIC FLIGHT INSTRUMENTS 1. The best medium for flying an aircraft is the natural horizon. It is the place where the earth and the sky seem to meet. But during cloudy conditions and at night, the horizon is not visible. During such occasions, the instruments of an aircraft play a very vital role in aiding the pilot to fly the aircraft safely. As flying involves the third dimension, instruments become very important. The instruments also give out the health of the engine and re-assure the pilot that all vital parameters of flying are within the prescribed limits. AIR SPEED INDICATOR 2. The airspeed indicator is an instrument used in an aircraft to display the craft's airspeed to the pilot. The principle of an Air Speed Indicator is the measurement of two pressures called static and pitot pressures. 3. If an open ended tube is moved through the air, pressure will be exerted at the closed end of the tube. This pressure has two components- static and the dynamic. The static pressure is due to the pressure exerted by the atmosphere and the dynamic is due to the movement of the tube through the air. The total pressure is known as pitot pressure. the dynamic pressure is indicated in terms of speed of the aircraft. The dynamic pressure is calculated as: Dynamic = Pitot – Static Internal Mechanism of An Airspeed Indicator 4. This instrument uses an open ended capsule fixed inside an airtight case. The open end is connected to pitot pressure. Static pressure is fed inside the case. The static pressure remaining constant in the entire case, the variation is only in the pitot pressure due to the movement of the aircraft in air. The capsule accordingly expands or contracts and this variation is calibrated in terms of speed.  77 ALTIMETER 5. An altimeter is an instrument used to measure the altitude of an object above a fixed level usually the sea level. The altimeter shows the aircraft's altitude above mean sea- level. Altitude can be determined based on the measurement of atmospheric pressure. 6. The atmosphere has weight and this weight exerts pressure. This is known as static pressure. This pressure reduces with height at the rate of 1 millibar / hectapascal per 30 feet approximately. 7. An aneroid barometer is used to measure the atmospheric pressure. An aircraft altimeter is simply an aneroid barometer adapted to use in aircraft calibrated to read the atmospheric pressure in terms of height. This is done by measuring the difference between the pressure in a stack of aneroid capsules inside the altimeter and the atmospheric pressure obtained through the static system. As the aircraft ascends, the capsules expand and the static pressure drops, causing the altimeter to indicate a higher altitude. The opposite effect occurs when descending. 8. The altimeter has two or three capsules each having vacuum or partial vacuum in them. They are stacked together with one end fixed firmly down. There are three pointers to indicate height in 100s ,1000s and 10,000s feet. 9. The whole assembly is encased in a container having an inlet for the static pressure but otherwise is airtight. The movement of the capsules in response to the variation in the pressure due to variation of height is transmitted to the pointers which indicate the height on the dial. ARTIFICIAL HORIZON 10. The artificial horizon shows the aircraft's attitude relative to the horizon. From this, the pilot can tell whether the wings are level and if the aircraft nose is pointing above or below the horizon. This is a primary instrument for instrument flight and is useful in conditions of poor visibility. An artificial horizon is an instrument used in an aircraft to inform the pilot of the orientation of the aircraft relative to earth. It indicates pitch (fore and aft tilt) and bank or roll (side to side tilt). 11. The essential components of the indicator are “miniature wings", horizontal lines with a dot between them representing the actual wings and nose of the aircraft:- (a) The centre horizon bar separating the two halves of the display, with the top half usually blue in color to represent sky and the bottom half usually dark to represent earth.  78 (b) Degree marks representing the bank angle. They run along the rim of the dial. On a typical indicator, the first 3 marks on both sides of the center mark are10 degrees apart. The next is 60 degrees and the mark in the middle of the dial is 90 degrees. If the symbolic aircraft dot is above the horizon line (blue background) the aircraft is nose up. If the symbolic aircraft dot is below the horizon line (brown background) the aircraft is nose down. it is the horizon that moves up and down and turns, while the symbolic aircraft is fixed relative to the rest of the instrument panel. Artificial Horizon uses a vertical axis earth gyroscope having freedom in all three planes to indicate the aircraft’s attitude in pitch and roll. The gyroscope is geared to a display simultaneously displaying pitch and bank. The display is coloured to indicate the horizon as the division between the two coloured segments, blue for sky and brown for ground. SUMMARY 12. During this period, the basic three instruments have been covered to understand their use and functioning. One must remember that the instruments play a very vital part in helping the pilot to fly an aircraft. The height above the mean sea level, the condition of flight and the speed of the aircraft can thus be known by the pilot by monitoring the instruments.  79 CHAPTER XVI- INTRODUCTION TO RADARS 1. Radar was secretly developed by several nations before and during World War II. The term RADAR was coined in 1940 by the United States Navy as an acronym for Radio Detection and Ranging. The term radar has since entered English and other languages as the common noun radar, losing all capitalization. 2. The modern uses of radar are highly diverse, including air traffic control, radar astronomy, air- defence systems, antimissile systems; marine radars to locate landmarks and other ships; aircraft anti- collision systems; ocean surveillance systems, outer space surveillance systems; meteorological precipitation monitoring; and guided missile target locating systems. RADAR 3. Radar is a machine that uses radio waves to find other objects such as aircraft, ships, and rain.The basic parts of radar are:- (a) The transmitter creates the radio waves. (b) The antenna directs the radio waves. (c) The receiver measures the waves which are bounced back by the object that the radar is trying to find. (d) By doing this, the radar can find what place the object is at. 4. Radar is an object detection system which uses radio waves to determine the range, altitude, direction, or speed of objects. It can be used to detect aircraft, ships, spacecraft, guided missiles, motor vehicles, weather formations, and terrain. The radar dish or antenna transmits pulses of radio waves which bounce off any object in their path. The object returns a tiny part of the wave's energy to a dish or antenna which is usually located at the same site as the transmitter. 5. A radar system has a transmitter that emits radio waves called radar signals in predetermined directions. When these come into contact with an object they are usually reflected or scattered in many directions. Radar signals are reflected especially well by materials of considerable electrical conductivity—especially by most metals, by seawater and by wet lands. The radar signals that are reflected back towards the transmitter are the desirable ones that make radar work. 6. In aviation, aircraft are equipped with radar devices that warn of obstacles in or approaching their path and give accurate altitude readings.  80 TYPES OF RADARS PRIMARY RADAR 7. This radar uses the principle of pulse technique to determine range and bearing of an object. Working on echo and search light principle, a transmitter transmits pulses. All objects in the path of the pulses will reflect and scatter this energy. Some of the reflected energy reaches the receiver. The reflected energy is processed to give the required information. In this radar, the object’s cooperation is not required in the entire process. SECONDARY RADAR 8. In this system, a transmitter transmits a group of pulses. An aerial in the path of the pulses receives the signals and passes it to receiver. If the pulses are identified, then the transmitter gives out a reply. In this radar active cooperation of the other object is also required. CONTINUOUS WAVE RADAR 9. In this type of radar, both the transmission and the reception take place continuously. This requires set of two aerials, one for transmission and one for reception. SUMMARY 10. Understanding the basic functions of Radar would help cadets in better assimilation of facilities available in the ATC and their role in promoting safe flying.