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Engine ratings
Safety & Ops
Vaughan Askue
Engine Ratings
One of the basic parameters used to calculate helicopter performance is the amount of power that the engines are capable of supplying. In the preliminary design phase, one of the designers’ primary tasks is to select a properly sized engine so the helicopter can achieve the performance that the marketing department requires. As the helicopter grows (they always do), the engineers are compelled to start hunting for larger engines to retain the required performance expectations. This ability of an engine to deliver power is defined in terms of ratings. In fact, rating is more than that—it is actually a guarantee, blessed by the certifying authority (the FAA in the United States), that the engine will deliver the rated power under a specific set of circumstances. This set of numbers becomes the foundation for all the performance capability calculated for that helicopter. The simple ability of an engine to deliver power normally is limited by the maximum temperature within the hottest portion of the internal gas flow: the higher the temperature, the higher the power. At some point the temperature approaches the limits of modern metallurgy. Spinning turbine blades operating under extreme centrifugal loads and extreme temperatures start to do bizarre things. The phenomenon that keeps the engine designers up at night most often is known as thermal creep. At very high temperatures, turbine blades start to elongate under centrifugal loads. These same turbine blades are spinning within a close-fit shroud or casing. If the blades grow too much, they will rub against the shroud and possibly fail. On the other hand, engine efficiency increases if the blades run as close as possible to the surface of the shroud. As you can see, the designer has a difficult compromise to work out. He wants the blade tips to run as close to the shroud as possible, so he can’t tolerate a temperature that allows the blades to grow (creep) too much. This situation is complicated by the fact that the rate of creep varies in a nonlinear fashion with temperature. One temperature results in little or no creep, whereas a slightly higher temperature leads to a dangerous amount of creep in a specified amount of time. The FAA (and all other regulatory authorities) recog8
nizes this problem by establishing a set of ratings that tend to allow higher powers for shorter periods. For example, civil turboshaft engines certified under FAA rules for single-engine applications normally use 2 ratings: a maximum continuous rating can be used for an unlimited period, and a takeoff rating normally is limited to 5 minutes per event. Because these ratings normally are determined by engine internal temperature, they are known as thermal ratings. An additional level of complexity is added for engines certified for twin-engine applications. In this case, the certifying agency has an additional set of ratings that can be used only by the remaining engine in the case of engine failure. These ratings are high enough that damage to the engine is likely and some kind of maintenance action, including returning the engine to the manufacturer, is usually required if they are used. Historically, single-engine ratings were specified for unlimited duration (called for maximum, continuous), 30-minute, and 2.5-minute time spans. With the advent of computercontrolled engines, the FAA allowed the manufacturers to split up the 2.5-minute rating to a very high power for 30 seconds and a slightly lower power for 2 minutes. Part of the rationale for this system was that the computer could limit the engine accurately while simultaneously counting down the time remaining for that rating much more reliably than a human pilot under extreme stress. The ability of an engine to deliver power also varies with both altitude and external temperature because as altitude and temperature increase, the density of the air (the number of air molecules per cubic foot of air) decreases. Simply put, the ability of the engine’s compressor to pull in enough air molecules to “feed the fire” decreases as altitude and temperature increase. Through a designer’s eyes, engine performance (and therefore the helicopter) starts degrading as soon as it rises above sea level on a standard day. This presents a problem because few helicopters spend their lives at or near sea level standard. If the manufacturer designs the drive train (transmissions, gearboxes, and shafting) for the thermal power that the engine can potentially deliver at sea level on a cool day, but the helicopter spends Air Medical Journal 23:4
chanical ratings in their brochures. If these ratings are specified at an altitude where the thermal rating is higher than the mechanical rating (sea level, for example), the engine clearly can’t actually deliver that power because it is limited by the mechanical limit. The rate of change of thermal power with altitude is fairly consistent between engines, so a knowledgeable observer could construct the power versus altitude relationship for a turboshaft engine just by knowing these 2 numbers.
Figure. Typical Engine Rating Structure
Thermal Rating
My thanks to Alan Todd at American Eurocopter for asking the question that led to this article. Vaughan Askue is the S-76 technical support manager for Sikorsky Aircraft Corp. in Stratford, Connecticut. He can be reached at [email protected].
Altitude Critical Altitude
Mechanical Rating
Power
1067-991X/2004/$30.00 Copyright 2004 by Air Medical Journal Associates doi:10.1016/j.amj.2004.04.007
Note: This column is written in an attempt to increase the understanding of medical personnel who work around helicopters and their pilots. I cannot do this alone. I need your comments to help me understand how well I am communicating and to find subjects that are interesting and helpful to you. I can be reached at [email protected] or by phone at (203) 386-6451.
most of its working life at high altitude and temperature, the drive train is overdesigned and therefore heavier than it needs to be. Helicopter designers are always trying to increase useful load by reducing the weight of the helicopter. One way to do this is to estimate where the helicopter will spend most of its working life and design the drive train for the power the engine can deliver under those conditions. This technique is called flat rating, and both airframe and engine manufacturers do it. An engine manufacturer flat rates an engine by designing its output gearbox and shafting for its mechanical rating. The figure shows a typical engine rating structure. The dashed lines are the thermal limits or the greatest power the engine can deliver within the FAA guidelines. The solid line shows the mechanical rating. The point where they intersect is called the critical altitude. As you can see, this engine is capable of delivering a fixed amount of power up to the critical altitude. Above this point, power falls off with increasing altitude. Below this point, the engine is operating well below its thermal limits, which tends to increase reliability. Helicopter designers are very sensitive to this whole issue and carefully match the engine rating structure to the desired characteristics of their helicopter. Sometimes airframe designers further limit (flat rate) the power available for performance by limiting the helicopter’s transmission ratings to a power lower than the engine mechanical rating. This is normally done for helicopters designed for high altitude operation. Some engine manufacturers specify both thermal and meJuly-August 2004
9
Vaughan Askue
Engine Ratings
One of the basic parameters used to calculate helicopter performance is the amount of power that the engines are capable of supplying. In the preliminary design phase, one of the designers’ primary tasks is to select a properly sized engine so the helicopter can achieve the performance that the marketing department requires. As the helicopter grows (they always do), the engineers are compelled to start hunting for larger engines to retain the required performance expectations. This ability of an engine to deliver power is defined in terms of ratings. In fact, rating is more than that—it is actually a guarantee, blessed by the certifying authority (the FAA in the United States), that the engine will deliver the rated power under a specific set of circumstances. This set of numbers becomes the foundation for all the performance capability calculated for that helicopter. The simple ability of an engine to deliver power normally is limited by the maximum temperature within the hottest portion of the internal gas flow: the higher the temperature, the higher the power. At some point the temperature approaches the limits of modern metallurgy. Spinning turbine blades operating under extreme centrifugal loads and extreme temperatures start to do bizarre things. The phenomenon that keeps the engine designers up at night most often is known as thermal creep. At very high temperatures, turbine blades start to elongate under centrifugal loads. These same turbine blades are spinning within a close-fit shroud or casing. If the blades grow too much, they will rub against the shroud and possibly fail. On the other hand, engine efficiency increases if the blades run as close as possible to the surface of the shroud. As you can see, the designer has a difficult compromise to work out. He wants the blade tips to run as close to the shroud as possible, so he can’t tolerate a temperature that allows the blades to grow (creep) too much. This situation is complicated by the fact that the rate of creep varies in a nonlinear fashion with temperature. One temperature results in little or no creep, whereas a slightly higher temperature leads to a dangerous amount of creep in a specified amount of time. The FAA (and all other regulatory authorities) recog8
nizes this problem by establishing a set of ratings that tend to allow higher powers for shorter periods. For example, civil turboshaft engines certified under FAA rules for single-engine applications normally use 2 ratings: a maximum continuous rating can be used for an unlimited period, and a takeoff rating normally is limited to 5 minutes per event. Because these ratings normally are determined by engine internal temperature, they are known as thermal ratings. An additional level of complexity is added for engines certified for twin-engine applications. In this case, the certifying agency has an additional set of ratings that can be used only by the remaining engine in the case of engine failure. These ratings are high enough that damage to the engine is likely and some kind of maintenance action, including returning the engine to the manufacturer, is usually required if they are used. Historically, single-engine ratings were specified for unlimited duration (called for maximum, continuous), 30-minute, and 2.5-minute time spans. With the advent of computercontrolled engines, the FAA allowed the manufacturers to split up the 2.5-minute rating to a very high power for 30 seconds and a slightly lower power for 2 minutes. Part of the rationale for this system was that the computer could limit the engine accurately while simultaneously counting down the time remaining for that rating much more reliably than a human pilot under extreme stress. The ability of an engine to deliver power also varies with both altitude and external temperature because as altitude and temperature increase, the density of the air (the number of air molecules per cubic foot of air) decreases. Simply put, the ability of the engine’s compressor to pull in enough air molecules to “feed the fire” decreases as altitude and temperature increase. Through a designer’s eyes, engine performance (and therefore the helicopter) starts degrading as soon as it rises above sea level on a standard day. This presents a problem because few helicopters spend their lives at or near sea level standard. If the manufacturer designs the drive train (transmissions, gearboxes, and shafting) for the thermal power that the engine can potentially deliver at sea level on a cool day, but the helicopter spends Air Medical Journal 23:4
chanical ratings in their brochures. If these ratings are specified at an altitude where the thermal rating is higher than the mechanical rating (sea level, for example), the engine clearly can’t actually deliver that power because it is limited by the mechanical limit. The rate of change of thermal power with altitude is fairly consistent between engines, so a knowledgeable observer could construct the power versus altitude relationship for a turboshaft engine just by knowing these 2 numbers.
Figure. Typical Engine Rating Structure
Thermal Rating
My thanks to Alan Todd at American Eurocopter for asking the question that led to this article. Vaughan Askue is the S-76 technical support manager for Sikorsky Aircraft Corp. in Stratford, Connecticut. He can be reached at [email protected].
Altitude Critical Altitude
Mechanical Rating
Power
1067-991X/2004/$30.00 Copyright 2004 by Air Medical Journal Associates doi:10.1016/j.amj.2004.04.007
Note: This column is written in an attempt to increase the understanding of medical personnel who work around helicopters and their pilots. I cannot do this alone. I need your comments to help me understand how well I am communicating and to find subjects that are interesting and helpful to you. I can be reached at [email protected] or by phone at (203) 386-6451.
most of its working life at high altitude and temperature, the drive train is overdesigned and therefore heavier than it needs to be. Helicopter designers are always trying to increase useful load by reducing the weight of the helicopter. One way to do this is to estimate where the helicopter will spend most of its working life and design the drive train for the power the engine can deliver under those conditions. This technique is called flat rating, and both airframe and engine manufacturers do it. An engine manufacturer flat rates an engine by designing its output gearbox and shafting for its mechanical rating. The figure shows a typical engine rating structure. The dashed lines are the thermal limits or the greatest power the engine can deliver within the FAA guidelines. The solid line shows the mechanical rating. The point where they intersect is called the critical altitude. As you can see, this engine is capable of delivering a fixed amount of power up to the critical altitude. Above this point, power falls off with increasing altitude. Below this point, the engine is operating well below its thermal limits, which tends to increase reliability. Helicopter designers are very sensitive to this whole issue and carefully match the engine rating structure to the desired characteristics of their helicopter. Sometimes airframe designers further limit (flat rate) the power available for performance by limiting the helicopter’s transmission ratings to a power lower than the engine mechanical rating. This is normally done for helicopters designed for high altitude operation. Some engine manufacturers specify both thermal and meJuly-August 2004
9