tfe 731 chap 77 (1)

Garret TFE 731 Turbofan Engine (CAT C)

CHAPTER 77

of 12 FOR TRAINING PURPOSES ONLY © TFE 731 - ISSUE 2, 2010


CHAPTER 77


INTRODUCTION

0 TABLE OF CONTENTS

1 Air Density 3 2 TFE731 Thrust Ratings 4 3 Thrust Limitations 5 4 Max Speed (Mn) Schedule 6 5 Flat Rate (F/R) Schedule 7 6 F/R – Mn Schedule 8 7 F/R – Mn Schedule 9 8 Altitude Affects on F/R and N1 10


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ENGINE INDICATING

1 AIR DENSITY In the turbine theory part of Chapter 72, it was stated that the acceleration of the large mass of airflow by the fan creates thrust. It then can also be stated that for a given air density thrust is proportional to fan RPM, and using fan speed as an indication of thrust yields essentially the same accuracy as engine pressure ratio measurement. All engines are subjected to a test cell performance run where N1 RPM versus thrust is verified. This test assures obtainment of required thrust at lowest possible turbine temperatures. As fan RPM increases, the mass airflow increases and more thrust will be produced. As fan speed or air density changes, thrust changes. What causes a change in air density? Air density is changed by: Altitude Temperature Airspeed The ram pressure increases at the engine inlet as the aircraft accelerates. This has the effect of increasing density. Differences in inlet plenum shapes will also affect the air density at the engine inlet. Conversely, air density decreases with higher outside air temperatures (OAT) or altitude.


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2 TFE731 THRUST RATINGS This chart depicts the thrust ratings of selected TFE731 engines. The ratings shown are for uninstalled engines at standard day conditions. The OAT column indicates the maximum temperature at which the engine will produce 3230 pounds thrust up to 30°C outside air temperature. At temperatures above 30°C, less thrust will be produced. Some engines have an automatic performance reserve (APR) system that will increase thrust limits from one engine in the event of power loss in the other engine. This increased thrust mode is intended for short duration emergency use only, since use of APR power increases rotor stress. Additionally, some engine installations utilise a restricted performance reserve (RPR) system designed to enhance hot day-high altitude takeoff characteristics. The system provides an increase over normal takeoff thrust in a region determined by pressure altitude and OAT. The APR and RPR systems will be examined in other sections of this training guide.


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3 THRUST LIMITATIONS All turbine engines are limited in the amount of thrust produced by three factors rotor speeds, turbine temperature and a thrust limit placed on the engine by the airframe manufacturers. All compressor and turbine rotors, when subjected to extreme centrifugal forces at ultra-high speeds, will fail. The designed safe rotor speed is calculated and tested under actual operating conditions. The verified maximum RPM with conservative safety margins is then established as 100% RPM. While the actual RPM will vary due to rotating group size, 100% is considered maximum speed under most applications. The materials used within the turbine section determine the temperature limits. As temperature rises, more stress is placed on components and erosion of turbine rotors and nozzles is experienced. Limits are therefore placed on turbine temperatures based on the type materials used in the turbine. The limit is often referred to as the “thermodynamic thrust rating". This rating identifies the maximum thrust capability of that engine when operating at a maximum turbine temperature at standard sea level conditions. Aircraft are designed and certified based on a thrust limit. This thrust limit is referred to as the "flat rate". A conservative rating means that during normal conditions the engine operates under less stress and should last longer. It also means that as the engine hours accumulate, more of the rated power will remain as seals wear and turbine components erode.


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4 MAX SPEED (MN) SCHEDULE The maximum fan speed can be depicted on this illustration. The curve reveals that as the outside air temperature increases, the fan speed must decrease in order to keep the engine within its thermodynamic limit. Conversely, as the outside air temperature decreases, the fan speed (thrust) can increase. The increase in speed (thrust) under extremely cold conditions can continue until the wheel speed limit is reached. The maximum speed limit depicted here is often referred to as the "Mn" schedule.


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5 FLAT RATE (F/R) SCHEDULE An additional fan speed limit is shown here as the "flat rate" schedule. The fan speed limit decreases as OAT decreases. Since the air density increase at a colder OAT, the fan speed must decrease to maintain the same rated thrust. As the air thins with higher temperatures, fan speed must increase to maintain the same rated thrust. Fan speed within the limits of the flat rate schedule will satisfy all engine and thrust limitations. If the fan speed increases to satisfy the thrust requirements as OAT increases, it is obvious that at some point the engine thermodynamic thrust limits will be reached.


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6 F/R – MN SCHEDULE That limit is illustrated on this graph. The angled line depicts the FR limit and the curved line indicates the maximum speed (Mn) limit. Note that the FR and Mn schedule intersect at 22°C. That indicates that the engine will produce rated thrust at sea level up to 22°C. The fan speed required to produce that rated thrust will be determined by OAT. At temperatures above 22°C the fan speed will be limited by the Mn schedule and because of air density, rated thrust will not be developed. It can be said that the engine flat rate (EFR) depicted here is 22 °C. The FR and Mn schedules intersect at that temperature.


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7 F/R – MN SCHEDULE The figure shows the FR - Mn schedule in a different perspective. The chart shows that, as outside air temperature decreases, thrust will remain the same when on the FR schedule. As outside air temperature increases, thrust will decrease on the Mn schedule.


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8 ALTITUDE AFFECTS ON F/R AND N1 The affects of altitude on fan speed are displayed here. The diagonal line previously described as the FR limit was based on sea level conditions. Therefore, moving from sea level to 2000 feet pressure altitude would require an increase in fan speed to maintain the same thrust. As altitude increases, fan speed must be increased in order to maintain the same thrust level. Eventually the maximum speed (Mn) would be reached. From this point on to higher altitudes, however the engine would produce the thermodynamic thrust rating. Takeoff power settings are determined by examination of a flight manual chart very similar to this one. Using pressure altitude and OAT, the operator would determine the intersection between OAT and the FR or Mn schedule as applicable. The intersect point would indicate the maximum N1 that the engine could produce for takeoff. Understanding the terms flat rate (FR) and maximum speed (Mn) and the effects of air density will provide the basis of understanding for engine systems operation.


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tfe 731 chap 77 (1)

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