Engine Performance (15.2) PDF

Engine Performance (15.2) Learning Objectives 15.2.1 Describe gross thrust, net thrust, choked nozzle thrust, thrust distribution, resultant thrust, thrust horsepower, equivalent shaft horsepower, speci c fuel consumption (Level 2). 15.2.2 Describe the classi cations of gas turbine en...

Engine Performance (15.2) Learning Objectives 15.2.1 Describe gross thrust, net thrust, choked nozzle thrust, thrust distribution, resultant thrust, thrust horsepower, equivalent shaft horsepower, speci c fuel consumption (Level 2). 15.2.2 Describe the classi cations of gas turbine engine ef ciency (Level 2). 15.2.3 Describe bypass ratio and engine pressure ratio (Level 2). 15.2.4 Describe variations of gas ow pressure, temperature and velocity at points along the gas path (Level 2). 15.2.5 Describe engine ratings, static thrust, in uence of speed, altitude and hot climate, at rating and engine limitations (Level 2). 2022-08-24 B1-15a Gas Turbine Engine Page 208 of 244 CASA Part 66 - Training Materials Only  Engine Thrust and Fuel Consumption Engine Thrust The terms used to describe the types of thrust produced by aircraft engines are: Gross thrust Net thrust Choked nozzle thrust Thrust distribution Resultant thrust. Some aircraft rely on engine-driven propellers to produce their thrust. An aircraft’s propeller gives small acceleration to a large mass of air. Propeller thrust is the thrust developed by the propeller, as illustrated below. Accelerated air mass (propellers) Jet aircraft produce thrust by accelerating air through an engine and ejecting it out a propelling nozzle. Unlike a turbo-propeller, a turbojet engine gives large acceleration to a small weight of air. Jet thrust is the thrust developed by a jet, as illustrated here. 2022-08-24 B1-15a Gas Turbine Engine Page 209 of 244 CASA Part 66 - Training Materials Only  Accelerated air mass - turbojet 2022-08-24 B1-15a Gas Turbine Engine Page 210 of 244 CASA Part 66 - Training Materials Only  Mass Air Flow and Thrust The basis of the thrust produced by a turbojet or turbofan engine is the change in momentum of air owing through the engine. Anything that increases the mass of air increases the thrust. Two factors that affect the mass of air are its density and the ram effect. Air density has a profound effect on the thrust produced. The volume of air owing through the engine is relatively xed for any particular rpm by the size and geometry of the inlet duct system. But since thrust is determined by mass, not the volume of air, any increase in its density increases its mass and thus the thrust. Let us look at two in uences on thrust the engine produces. First, pressure: At sea level we have the ISA value of 14.7 psi, and every climb away from sea level is a reduction in pressure (which is how we measure altitude). That drop in pressure cannot support the same temperature, and therefore the temperature drops. Both pressure and temperature affect density, so a reduction in pressure causes a reduction in density, which means less air results in less thrust. Pressure has a greater effect on performance than temperature. Second, temperature: An increase in temperature reduces performance (because the air is less dense), especially at altitudes close to sea level. However, at the tropopause, at 36 089 ft, the temperature remains at -56 °C. As the aircraft climbs, the engine performance continues to drop off due to pressure. So in summary, pressure reduces performance more than temperature; density reduces more because of pressure than temperature; thrust and mass air ow are greater at higher rpm; and the faster the speed, the greater the mass air ow and the greater the thrust. 2022-08-24 B1-15a Gas Turbine Engine Page 211 of 244 CASA Part 66 - Training Materials Only  The Reactive Force of Thrust The air discharge and the bullet leaving the gun do not create reactive power by exerting a pushing force on the outside air. Rather, their acting forces create a reacting force within the engine and the gun. In fact, if the air or bullet were to exit into a vacuum as rockets do in space, the exiting velocities would be greater and the resultant thrust would be greater. To create the acting force within a turbine engine, a continuous ow cycle is utilised. Gas turbine engines operate on a principle of continuous combustion, or one unit of mass air ow in and one unit of mass air ow out. Because the unit trying to exit has been increased in size (volume), it will have to accelerate greatly to leave the exhaust nozzle as the new unit enters the inlet. Thrust is transmitted to the aircraft through the engine mounts. The acting force is created within a turbine engine, not externally by pushing on the outside air. A simple explanation of a gas-turbine turbojet’s operation is that it is a device which increases potential energy and then converts to kinetic energy. Some of this energy performs work at the turbine, while the remainder exits the engine in the form of thrust. Newton’s Second Law states force is proportional to the product of mass times acceleration. Mass (m) and weight (W) are different types of quantity. However, when an object is in Earth’s gravitational eld, it is subjected to an attractive force we call weight. In Earth’s gravity, mass and weight can be treated as similar quantities. An object falls due to gravity, accelerating at 9.81 m/s/s (metres/second/second; SI units) or 32.2 feet/s/s (feet/second/second; British units). These constants are known as g, the acceleration due to Earth’s gravity. Weight is mass with gravity applied (an object on earth). Thus, weight (force) is simply the mass of an object multiplied by the acceleration of gravity. Turbojet and turbofan engines burn fuel to accelerate a mass of gas rearwards and create a forward reaction called thrust. Thrust creation inside a turbine engine 2022-08-24 B1-15a Gas Turbine Engine Page 212 of 244 CASA Part 66 - Training Materials Only  Considering acceleration to be: Then: This formula forms the foundation for calculating the maximum reactive thrust of a gas turbine engine. Static thrust is calculated as the maximum thrust developed by an engine on the test bed when V1 is 0. 2022-08-24 B1-15a Gas Turbine Engine Page 213 of 244 CASA Part 66 - Training Materials Only  Gross Thrust An engine develops its gross thrust when it is operating but not in motion. An aircraft’s gross thrust may be observed immediately prior to releasing the brakes for take-off. At this point, aircraft drag, inlet ram air and atmospheric changes do not affect the amount of thrust produced. The formula for gross thrust may be stated as: Where: Fg = Gross thrust in lb Wa = Weight of air ow in lb/s V2 = Exhaust velocity in ft/s V1 = Inlet velocity in ft/s g = Gravity acceleration, 32.2 ft/s2 The following animation shows thrust calculations under different conditions. Worked Example A turbojet engine is moving 122 lbs of air per second, and imparting to it, an acceleration of 1,600 ft/sec2. What is the thrust? 2022-08-24 B1-15a Gas Turbine Engine Page 214 of 244 CASA Part 66 - Training Materials Only  Net Thrust Net thrust is the effective thrust developed by the engine during ight. All engine, aircraft and atmospheric forces must be considered when calculating an engine’s net thrust. Net thrust may be stated basically as the gross thrust minus the effect of aircraft forward airspeed. Due to the fact that the engine does not have to force the air into the intake, net thrust initially decreases with aircraft acceleration until the engine inlet begins to experience the effect of ram recovery. This effect tends to actually increase net thrust over and above a predetermined airspeed. Ram effect generally commences around 160 mph. The formula for net thrust may be stated as: Where: Fn = Net thrust in lb Wa = Weight of air ow in lb/s V2 = Exhaust velocity in ft/s V1 = Inlet velocity in ft/s g = Gravity acceleration (32.2 ft/s²). 2022-08-24 B1-15a Gas Turbine Engine Page 215 of 244 CASA Part 66 - Training Materials Only  Choked Nozzle Thrust As a jet engine’s exhaust gases reach the speed of sound (Mach 1) through the propelling nozzle, the pressure differential across the nozzle is said to become choked. Choked Nozzle Thrust When a nozzle is choked, the pressure is such that the gases are travelling through it at the speed of sound and cannot be further accelerated. Any increase in internal engine pressure passes out of the nozzle still in the form of pressure. Even though this pressure energy cannot be turned into velocity energy, it is not lost. The pressure inside the nozzle is pushing in all directions, but when the neck is open, the air cannot push in the direction of the nozzle. The pressure in the other direction continues undiminished, and as a result the pressure of the gases pushes the engine forward. This extra pressure produces what is known as choked nozzle thrust, and is additional to the thrust produced by the exhaust gas velocity. The formula for choked nozzle thrust may be stated as: 2022-08-24 B1-15a Gas Turbine Engine Page 216 of 244 CASA Part 66 - Training Materials Only  Where: Fn = Net thrust in lb Wa = Weight of air ow in lb/s V2 = Exhaust velocity in ft/s V1 = Inlet velocity in ft/s2 g = Gravity acceleration (32.2 ft/s2) Aj = Area of jet nozzle in in2 Pj = Pressure at jet nozzle in psi Pam = Ambient pressure. Thrust Distribution and Resultant Thrust Jet thrust is not solely produced at the engine exhaust or propelling nozzle. It is developed throughout the engine as a reaction to the forces within the engine. As the mass air ow passes through an engine, changes in air ow velocity and pressures occur. For instance, in the diffuser section of an axial ow engine, air ow velocity (kinetic energy) is changed to pressure energy by the diffuser’s divergent shape. This change produces force in a forward direction. Conversely, at the turbine nozzle section, pressure energy is converted to velocity and produces force in a rearward direction. The diagram illustrates these principles. Thrust distribution is de ned as the forces resulting from the changes in the pressure and momentum of the gas stream reacting on the engine structures and rotating components. Pressure and momentum changes in gas stream (divergent ducts) 2022-08-24 B1-15a Gas Turbine Engine Page 217 of 244 CASA Part 66 - Training Materials Only  Pressure and momentum changes in gas stream (convergent ducts) Thrust distribution is, in effect, the reaction to the changes in the mass air ow pressure and velocity throughout the engine. An example of thrust distribution is shown below. Thrust distribution 2022-08-24 B1-15a Gas Turbine Engine Page 218 of 244 CASA Part 66 - Training Materials Only  Resultant Thrust In association with thrust distribution produced throughout an engine, it is possible to calculate the result, known as the resultant thrust. This is the Rated Thrust on the engine data plate and Engine Type Certi cate. Resultant thrust is the result of thrust forces felt in the rearward direction, deducted from the thrust forces felt in the forward direction. Note that in a typical turbojet engine, the combustion section produces the greatest forward thrust component. Thrust and Equivalent Shaft Horsepower The output of a turbojet or turbofan engine is measured in pounds of thrust. A pound of thrust is a unit of force, not power. Turboprop power is measured in shaft horsepower Thrust Horsepower It is possible to determine how much horsepower it would take to propel a turbojet powered aircraft at the same speeds. This is termed 'Thrust Horsepower'. Thrust horsepower also provides for comparison between gas turbine engines fuel consumption rates. Thrust horsepower is calculated only in ight. While the aircraft is stationary, no energy is expended for propulsion. An engine’s produced horsepower increases as airspeed increases. 2022-08-24 B1-15a Gas Turbine Engine Page 219 of 244 CASA Part 66 - Training Materials Only  Equivalent Shaft Horsepower Many turboprop engines are rated in Equivalent Shaft Horsepower (ESHP). ESHP is de ned as the sum of the power supplied to the propeller (SHP) and the jet thrust produced by the engine. Shaft horsepower (sometimes referred to as thermodynamic horsepower) is de ned as the total available horsepower of a xed turbine type turbo-propeller or turboshaft engine as measured on a dynamometer. Under static conditions (ground run-up), one shaft horsepower is equal to approximately 2.5 pounds of thrust. EHSP is calculated by the formula below: The illustration shows a typical xed turbine turbo-propeller engine. A turbo-propeller engine produces shaft horsepower and thrust from its exhaust section 2022-08-24 B1-15a Gas Turbine Engine Page 220 of 244 CASA Part 66 - Training Materials Only  Speci c Fuel Consumption Speci c Fuel Consumption (SFC) is a measure of thermal and propulsive ef ciency of a gas turbine engine. SFC is expressed as a ratio of rate of fuel consumption per hour per unit of power or thrust. This ratio is usually included in engine speci cations published by the manufacturer. SFC provides a means of comparing the fuel consumption or economy of operation of one engine to another, independent of its power or thrust rating. To move an aircraft through the air, a propulsion system is used to generate thrust. The amount of thrust an engine generates is important. But the rate of fuel consumption required to generate that thrust is also important because the aircraft must generate additional lift to carry the fuel throughout the ight. When considering an engine that produces most of its output via a shaft, e.g. a turbo-prop or turbo- shaft, the following formula applies: Thrust-Speci c Fuel Consumption (TSFC) This is a measure of fuel ef ciency of a thrust producing engine, e.g. a turbojet or bypass engine. The TSFC is a measure of the number of pounds of fuel burned per hour for each pound of thrust produced. 2022-08-24 B1-15a Gas Turbine Engine Page 221 of 244 CASA Part 66 - Training Materials Only  Turbine Engine Ef ciency Engine Ef ciencies Engine performance is not solely concerned with engine thrust. The ef cient conversion of fuel into kinetic energy to produce thrust is also important. Engine ef ciency is the output divided by the input. It can be de ned as the ratio of work accomplished by an engine to the energy consumed by the engine to produce that work. Ef ciency is usually expressed as a percentage. Due to mechanical friction, air leakage and other losses throughout an engine, the overall engine ef ciency is always less than 100%. Many factors are considered when the ef ciency of an engine is calculated. The following terms relate to engine ef ciency: Adiabatic ef ciency Thermal ef ciency Propulsive ef ciency Overall ef ciency. Adiabatic Ef ciency The adiabatic ef ciency of an engine is the ratio of work required to compress a gas, without gain or loss of heat, to the work actually accomplished by the turbine. Expressed more simply, adiabatic ef ciency is a ratio of actual work to theoretical work. A good adiabatic ef ciency is 90%. Thermal Ef ciency The thermal ef ciency of an engine is the ratio of net work produced by the engine to the theoretical heat energy that the combustion of fuel in the engine can produce. Expressed more simply, thermal ef ciency is a ratio of actual work to the work capability of the fuel. 2022-08-24 B1-15a Gas Turbine Engine Page 222 of 244 CASA Part 66 - Training Materials Only  Propulsive Ef ciency The propulsive ef ciency of an engine is a measure of the effectiveness with which energy in a power plant is converted to useful work for propelling the aircraft. In other words, it is the amount of thrust developed by the propelling nozzle compared with the energy supplied to it. The closer the aircraft speed comes to exhaust velocity, the higher the propulsive ef ciency. A comparison of propulsive ef ciencies with aircraft speed is shown in the graph below. Propulsive ef ciencies with aircraft speed Propulsive ef ciency is the ratio of forward aircraft speed to exhaust gas or propeller stream speed. The closer the aircraft speed comes to exhaust velocity, the higher the propulsive ef ciency Propulsive ef ciency is stated as a percentage, when an aircraft is stationary with the engines running, propulsive ef ciency is zero percent. 2022-08-24 B1-15a Gas Turbine Engine Page 223 of 244 CASA Part 66 - Training Materials Only  Overall Ef ciency The overall ef ciency of an engine is the product of its propulsive ef ciency times the engine’s thermal ef ciency. This is a measure of an engine’s ability to use: thrust fuel. The overall ef ciency of an engine directly relates to SFC and TSFC. Turbine Engine Performance Turbine engine performance is based on standards established by the Society of Automotive Engineers. These standards are known as the engine power ratings and are listed on an engine’s Type Certi cate Data Sheet. In most cases, engine power ratings are expressed in values of EPR, percent N1 or torque. Before carrying out the ground power check, you must calculate an EPR, N₁ or torque value which represents take-off thrust for the prevailing atmospheric conditions. In most cases, a take-off thrust setting curve is provided by the engine manufacturer to make these calculations. Ambient conditions have a dramatic effect on turbine performance; therefore, compressor inlet air temperature and pressure measurements must be accurate when computing take-off thrust. These measurements can be obtained from air traf c control or determined manually. Engine performance chart 2022-08-24 B1-15a Gas Turbine Engine Page 224 of 244 CASA Part 66 - Training Materials Only  How to Read the Chart In the chart above, the given ambient temperature is -20 °C and the airport barometric pressure is 30.50 inHg. To determine take-off EPR, locate -20 °C on the ambient temperature line (Item 1) and move down to intercept the barometric pressure scale at 30.50 inHg (Item 2). From this point, project a horizontal line to the right that intercepts the engine pressure ratio axis at 1.975 (item 3). To determine the turbine discharge pressure, project a horizontal line to the left from Item 2 that intercepts the second barometric scale (Item 4). From here, draw a vertical line down to the turbine discharge pressure line for a value of approximately 25.75 inHg (Item 5). This means the take-off EPR is 1.975, so advancing the throttle to the computed EPR and checking all other parameters (EGT, N₁, N₂, fuel ow, oil pressure and vibrations) are within maximum limits, take- off thrust is being produced. Relevant Youtube link: Turbine Engine Performance Video Turbofan Engine Bene ts A turbofan engine consists of a multi-bladed ducted propeller driven by a gas turbine engine. Turbofans were developed to provide a compromise between the best features of the turbojet and the turboprop. Turbofan engines have turbojet type cruise speed capability, yet achieve some of the short- eld take- off capabilities of a turboprop. Nearly all present-day airliners are powered by turbofan engines and are very ef cient when compared to turbojet engines. Turbofan engine 2022-08-24 B1-15a Gas Turbine Engine Page 225 of 244 CASA Part 66 - Training Materials Only  Bypass Ratio The path of air through a gas turbine engine varies according to the design of the engine. In turbojet applications, all the air is taken into the engine and passed through the compressor, then the combustion chamber, and exits via the exhaust. The principle of the bypass system involves a division in the air ow. In early bypass systems, the air was taken in and compressed by the low pressure compressor. A percentage was ducted to bypass the engine core and the remainder was delivered to the high pressure compressor. In modern turbofans, the bypass ratio is the percentage of fan duct air relative to the air ow through the core engine. Turbofans in civil aircraft are generally divided into four classi cations based on bypass ratio: Low bypass (1:1) Medium bypass (2:1 or 3:1) High bypass (4:1 to < 9:1) Ultra-high bypass (> 9:1). With regard to statement of the ratio, the '1' always represents the air ow through the core engine, which is the name used to describe the gas generator or high pressure compressor. The bypass ratio is the ratio of cool air that is bypassed through the duct to the ow of air passed through the gas generator. Bypass air ow 2022-08-24 B1-15a Gas Turbine Engine Page 226 of 244 CASA Part 66 - Training Materials Only  Low Bypass Generally, air ow mass in the fan section of a low-bypass engine is the same as air ow mass in the compressor. The fan discharge could be slightly higher or lower depending on the engine model, but bypass ratios are approximately 1:1. In some engines, the bypass air is ducted directly overboard through a short fan duct. However, in a ducted fan engine, the bypass air is ducted along the entire length of the engine, such as in the JT8 and RR RB183 Spey. Full fan ducts reduce aerodynamic drag and noise emissions. In either case, the end of the duct usually has a converging discharge nozzle that increases velocity and produces reactive thrust. Low bypass ratio designs 2022-08-24 B1-15a Gas Turbine Engine Page 227 of 244 CASA Part 66 - Training Materials Only  Medium Bypass Medium bypass engines have air ow bypass ratios ranging from 2:1 to 3:1. A medium bypass engine fan has a larger diameter than a fan used on a low-bypass engine of comparable power. Fan diameter determines the bypass ratio. Medium bypass turbofan engine 2022-08-24 B1-15a Gas Turbine Engine Page 228 of 244 CASA Part 66 - Training Materials Only  High Bypass High-bypass turbofan engines have bypass ratios of 4:1 to < 9:1 and use a larger diameter fan than those used on medium-bypass engines. High-bypass turbines offer higher propulsive ef ciencies and better fuel economy than low- or medium-bypass turbines. Consequently, they are the engines of choice on large airliners used for long ights. Some common large high-bypass turbofan engines include Pratt and Whitney’s JT9D and PW4000, the Rolls-Royce RB-211, and the General Electric CF6. High bypass turbofan - front view with fan blades removed 2022-08-24 B1-15a Gas Turbine Engine Page 229 of 244 CASA Part 66 - Training Materials Only  Ultra-High Bypass Ultra-high bypass turbo fans have a bypass ratio of 9:1 or greater and use very large fan diameters. They offer higher propulsive ef ciencies and better fuel economy than high-bypass turbofans. Some of these engines incorporate a geared fan to achieve optimum speed on the fan and LP compressor/turbine. Ultra-high-bypass engines include the CFM Leap (737 Max, 9:1), P&W 1000G (A320 Neo, 12.5:1), RR Trent 1000 (787, 10:1) and GE9X (777X, 10:1). Ultra-high bypass turbofan engine 2022-08-24 B1-15a Gas Turbine Engine Page 230 of 244 CASA Part 66 - Training Materials Only  Engine Pressure Ratio Engine Pressure Ratio (EPR) is measured as the ratio of the total pressure at the exit of the propelling nozzle divided by the total pressure at the entry to the compressor. In the diagram, the inlet pressure at the compressor is 10 psi and the turbine discharge pressure is 100 psi. The Engine Pressure Ratio is therefore 100:10 or, more simply put, 10:1. Engine Pressure Ratio - relative values of entry and exit air pressures 'Total Pressure' values are used to calculate EPR. That is, each sensed pressure value is the sum of static pressure at the sample area and the additional (dynamic) pressure caused by the velocity of the air ow. To sense total pressure, the pressure sensing ports must face forward into the rearwards gas ow - for the same reason that pitot tubes are forward facing. For example: EPR on a twin-spool gas turbine engine is calculated at Stations 2 and 7, and the pressures sampled are referred to as Pt2 and Pt7 respectively. Engine Pressure Ratio sensing schematic 2022-08-24 B1-15a Gas Turbine Engine Page 231 of 244 CASA Part 66 - Training Materials Only  Where: P represents pressure. t represents total pressure. The number (subscript) is the engine station at the zone being sampled. Turbine Engine Gas Flow In the Compressor Assembly Air pressure increases Temperature increases Velocity decreases. At the diffuser, just prior to combustion chamber entry, there is a nal dramatic increase in pressure and temperature and a decrease in velocity to aid in maintaining ame stabilisation. Pressure, temperature and velocity through a centrifugal compressor In the Combustion Area Air ow ignition causes a dramatic increase in temperature and velocity, but in line with the Brayton cycle principle, pressure remains relatively constant. 2022-08-24 B1-15a Gas Turbine Engine Page 232 of 244 CASA Part 66 - Training Materials Only  In the Turbine Assembly Velocity of air ow increases and decreases across the turbine stages. Due to the energy extraction by the turbine assembly, pressure and temperature gradually decrease. In the Exhaust Assembly Leaving the turbine, velocity decreases, and pressure and temperature increase until air ow reaches the propelling nozzle, where a dramatic increase in velocity takes place, with a resultant drop in pressure to ambient as it exits the nozzle. The diagram displays the behaviour of air ow through a gas turbine engine in relation to pressure, temperature and velocity. © RR The Jet Engine Pressure, temperature and velocity through an axial compressor 2022-08-24 B1-15a Gas Turbine Engine Page 233 of 244 CASA Part 66 - Training Materials Only  Engine Ratings and Limitations International Standard Atmosphere (ISA) To enable the performance of similar engines to be compared, it is necessary to standardise the variations of air temperature and pressure that occur with altitude and climatic conditions. Of the various de nitions of standard atmospheres, the one in most common use is International Standard Atmosphere (ISA). This is based on a temperature lapse rate of 1.98 °C per 1000 ft, resulting in a fall from 15 °C at sea level to -56.5°C at 36 089 ft (the tropopause). Above this altitude, the temperature is constant up to 65 617 ft. The ISA standard pressure at sea level is 1013.2 mbar, 29.92 inHg or 14.69 psi. This pressure drops to 0.023 mbar or 3.28 psi at the tropopause. The ICAO standard for measuring altitude is feet, and atmospheric pressure may be in pounds per square inch (psi), inches of mercury (inHg) or millibars (mbar). Engine Thrust in Flight In order to determine an engine’s net thrust, we need to consider in- ight variables, including: Mass air ow Altitude Air temperature Humidity Airspeed as well as; Ram effect Ram drag Engine rpm Engine conditions. The Effect of Mass Air ow on Thrust Thrust is produced by the reaction to accelerating the mass air ow through an engine. However, as the density of the air entering the inlet changes, the mass air ow changes. This is a result of more, or fewer, molecules per given volume of air being available to the engine. If the mass air ow through the engine is decreased, the thrust developed decreases. In fact, mass air ow and thrust are directly proportional. Control of engine rpm is the way to control thrust. 2022-08-24 B1-15a Gas Turbine Engine Page 234 of 244 CASA Part 66 - Training Materials Only  The Effect of Altitude on Thrust As altitude is increased, density decreases. Because air pressure decreases as altitude increases, the temperature also decreases, but cannot compensate for the loss of density caused by the increased altitude. Above 36 000 ft, the density decreases more rapidly because the temperature remains fairly constant above this altitude. For these reasons, thrust decreases with increased altitude. The graph below shows the effect of altitude on thrust. Thrust and altitude The altitude effect on thrust can also be discussed as a density and temperature effect. In this case, an increase in altitude causes a decrease in pressure and temperature. Since the temperature lapse rate is less than the pressure lapse rate as altitude is increased, the density is decreased. Although the decreased temperature increases thrust, the effect of decreased density more than offsets the effect of the colder temperature. The net result of increased altitude is a reduction in the thrust output. 2022-08-24 B1-15a Gas Turbine Engine Page 235 of 244 CASA Part 66 - Training Materials Only  The Effect of Air Temperature on Thrust Variations in the air temperature entering the engine will affect the air density and therefore engine performance. When the temperature is lowered, the density of the air is increased. Increased density increases the mass air ow through the engine and thus increases the thrust produced. The graph below shows the effect of air temperature on thrust. An engine operating under standard day conditions may produce 10 000 lb thrust, while on an extremely cold day the same engine may produce up to 12 000 lb thrust. Conversely, on an extremely hot day, the same engine may produce as low as 8000 lb thrust. Thrust and Temperature Thrust output improves rapidly with a reduction in outside air temperature (OAT) at constant altitude, rpm and airspeed. This increase occurs partly because the energy required per pound of air ow to drive the compressor varies directly with the temperature, thus leaving more energy to develop thrust. In addition, the thrust output increases since the air at reduced temperature has an increased density. The increase in density causes the mass ow through the engine to increase. The Effect of Humidity on Thrust Humidity is described as the percentage of water vapour in a given volume of air. Because water vapour displaces some of the air, the air is less dense; therefore, an increase in humidity reduces an engine’s mass air ow and lowers engine performance. In other words, as the humidity increases, the air density decreases with a corresponding decrease in thrust. 2022-08-24 B1-15a Gas Turbine Engine Page 236 of 244 CASA Part 66 - Training Materials Only  The Effect of Airspeed on Thrust As an aircraft’s forward speed increases, the velocity of the air at the engine inlet (V1) is increased. This results in less acceleration of the mass air ow through the engine, i.e. (V2 – V1), and therefore less thrust. The effect of increased airspeed may be summarised as follows: As airspeed increases, thrust decreases until ram effect restores thrust. The graph shows the effect of airspeed on thrust. Thrust and airspeed As airspeed is increased from static, the ram drag increases rapidly. The exhaust jet velocity (V2) remains relatively constant; therefore, the effect of the increase in airspeed is decreased speci c thrust. 2022-08-24 B1-15a Gas Turbine Engine Page 237 of 244 CASA Part 66 - Training Materials Only  Ram Effects and Engine RPM on Thrust Ram Effect A rise in pressure above existing outside atmospheric pressure at the engine inlet, as a result of the forward velocity of an aircraft, is referred to as ram. Since any ram effect increases compressor entrance pressure over atmospheric pressure, the resulting pressure rise increases the mass air ow and jet velocity, both of which tend to increase thrust. Although ram effect increases the engine thrust, the thrust being produced by the engine decreases for a given throttle setting as the aircraft gains airspeed. Therefore, two opposing trends occur when an aircraft’s speed is increased. What actually takes place is the net result of these two different effects. An engine’s thrust output temporarily decreases as aircraft speed increases from static, but soon ceases to decrease; towards the higher speeds, thrust output begins to increase again. Thrust initially decreases as airspeed increases due to a reduction in the acceleration (V1) of the mass air ow through the engine. However, as an aircraft’s airspeed increases, air is being rammed into the inlet, causing an increase in inlet pressure. This in turn increases the mass air ow into the engine and thrust is restored. In the graph, Curve A indicates the initial effect of airspeed, Curve B indicates the effect of ram pressure and Curve C indicates the resultant. Ram effect has the effect of increasing the thrust as the forward speed of the aircraft increases at a constant altitude. Ram effect 2022-08-24 B1-15a Gas Turbine Engine Page 238 of 244 CASA Part 66 - Training Materials Only  Ram Drag Ram drag, sometimes called inlet momentum drag, is the drag caused by the momentum of the air passing into the engine relative to the speed of the aircraft. This type of drag must be considered when determining an aircraft’s net thrust. In other words, the theoretical calculation of net thrust may require a reduction in the gure for forward airspeed to compensate for any inlet drag evident. Engine RPM For all engines, thrust increases rapidly as the rpm approaches its maximum design speed. The graph illustrates the effect of engine rpm on performance. RPM and Thrust 2022-08-24 B1-15a Gas Turbine Engine Page 239 of 244 CASA Part 66 - Training Materials Only  Engine Ratings Turbine engines, both turbojet and turbofan, are thrust-rated in terms of either engine pressure ratio or fan speed, and turboshaft turboprop engines are SHP-rated in the following categories: take-off, maximum continuous, maximum climb, maximum cruise and idle. For certi cation purposes, the manufacturer demonstrates to the regulatory authorities that the engine will perform at certain thrust or shaft horsepower levels for speci ed time intervals and still maintain its airworthiness and service life for the user. These ratings can usually be found on the engine Type Certi cate Data Sheets. The ratings are classi ed as follows: Take-Off Wet Thrust/SHP – This rating represents the maximum power available while in water injection and is time limited. It is used only during take-off operation. Engines are trimmed to this rating. Take-Off Dry Thrust/SHP – Limits on this rating are the same as Take-Off Wet, but without water injection. Engines are trimmed to this rating. Maximum Continuous Thrust/SHP – This rating has no time limit but is to be used only during emergency situations at the discretion of the pilot, for example, during a one-engine-out cruise operation. Maximum Climb Thrust/SHP – Maximum climb power settings are not time limited and are to be used for normal climb, when climbing to cruising altitude or when changing altitudes. This rating is sometimes the same as Maximum Continuous. Maximum Cruise Thrust/SHP – This rating is designed to be used for any time period during normal cruise at the discretion of the pilot. Idle Speed – This power setting is not actually a power rating, but the lowest usable thrust setting for either ground or in- ight operations. Thrust-producing turbine engines utilise either the EPR trim or the fan speed trim procedure. If the engine is con gured with an EPR system, the pilot uses a cockpit EPR gauge to set engine power and the engine is referred to as an EPR-rated engine. If the engine does not have an EPR system, it is trimmed in accordance with fan speed. In this case, the pilot uses a tachometer indicator to set engine power and the engine is referred to as a fan- speed-rated engine. 2022-08-24 B1-15a Gas Turbine Engine Page 240 of 244 CASA Part 66 - Training Materials Only  EPR gauges Some engines are at-rated to only 15 °C, others over 30 °C. This consideration depends largely on the needs of the aircraft manufacturer. Generally, at rating is believed to enable the engine to produce a constant rated thrust over a wide range of ambient temperatures without working the engine harder than necessary, in the interest of prolonging engine service life. For example, an engine rated at 3500 lb thrust at 15 °C might be re-rated to 3350 lb thrust at 32 °C. The aircraft user might not need to utilise 3500 lb thrust, nor the maximum gross weight of the aircraft, and would like to bene t from increased engine service life and lower fuel consumption by operating at 3350 lb thrust maximum. Flat-rating is an engine manufacturer’s way of re-rating an engine to a lower rated thrust than it would have at standard day temperatures. The engine can use that lower rated thrust over a wider ambient temperature range. Flat-rating is equally applicable to all types of gas turbine engines, both thrust-producing engines and torque-producing engines. The aircraft manufacturer typically uses the following process, or one very similar, when selecting the at rating that best suits its needs: The user decides the take-off power needed for their aircraft con guration, route requirements, runway lengths, runway altitudes, etc. The user calculates the highest ambient temperature at which required take-off power can be obtained. The engine and aircraft manufacturer print all of the ight manuals, operational instructions, etc. to re ect the selected take-off power as the maximum usable for normal operation. 2022-08-24 B1-15a Gas Turbine Engine Page 241 of 244 CASA Part 66 - Training Materials Only  EPR and Engine Total Inlet Temperature graph For a at-rated engine care must be taken when carrying out ground runs on a part-throttle engine to avoid advancing the throttle too far, exceeding take-off power limitations. On cold days this is especially true. An engine may be de-rated if it is installed in an aircraft that does not require the engine’s maximum rated power. 2022-08-24 B1-15a Gas Turbine Engine Page 242 of 244 CASA Part 66 - Training Materials Only  Engine Operating Limitations Damaged turbofan engine Engine operating limitations are determined by the turbine and nozzle materials used. If limits are exceeded, borescope inspections may need to be performed. Engine operating limitations are found in the engine Type Certi cate Data Sheet and the Aircraft Maintenance Manual and include (CF6 High-Bypass Turbofan Engine Series as an example): Maximum thrust settings for all conditions Fuel and oil pressures Engine oil temperatures Engine operating temperature limitations Engine maximum operating rpm. For a representative turboprop engine (Allison 250 Series), all of the above are included as well as: Torque Output shaft speed. For all gas turbine engines, vibration is also a limiting factor. 2022-08-24 B1-15a Gas Turbine Engine Page 243 of 244 CASA Part 66 - Training Materials Only  Engine operation limits 2022-08-24 B1-15a Gas Turbine Engine Page 244 of 244 CASA Part 66 - Training Materials Only