Aviation Australia PDF: Specific Fuel Consumption, Engine Performance, and Thrust (PDF)

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...

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  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  Accelerated air mass - turbojet 2022-08-24 B1-15a Gas Turbine Engine Page 210 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  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