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#12
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In rec.aviation.owning wrote:
A gas turbine scales up easily and but is nearly impossible to scale down. The auto manuacturers found that out in the 1940s - remember the "car of the future" on the covers of Popular Science et al? Turbines for cars are further away now than they were 55 years ago. The turbine suffers from excessive fuel consumption at part throttle (the piston engine is incredibly flexible that way)and in smaller HP installations. So much of the useful load of an aircraft is fuel, that fuel efficiency is very important for overall mission performance. The problem of an engine is to find the most efficient way to expand a certain flow rate of compressed hot gas to atmospheric pressures. A turbine can do this with large mass flow rates, but as the flow rates become smaller, the turbine speeds (rpm) must increase enormously and the centrifugal accelerations get out of hand. On the other hand, a piston can process an expansion efficiently with small flow rates. Think of it this way - a model airplane engine can be made to run with 1/20 of a cubic inch (.049 cu inch to even .010 cu inch), but piston engine aircraft became impractical above a few thousand HP. That is the range of practicality for a piston concept. An engineering prof once said - if the gas turbine had been invented first, the piston engine would have been looked on an ingeneous solution to the turbine's material and speed and power range problems. Diesels may eventually make it. They have a weight problem that may be offset by a lower specific fuel consumption, but for a given operating condition, spark ignition engines can nearly approach the consumption of diesels by using turbo compounding and operation only at full throttle. While not quite a .049, here's a 3.7" in diameter, 2.6 lb turbine that produces 16.5 lb of thrust. http://jetcatusa.sitewavesonline.net/p70.html Their biggest turbine is 5.12", 5 lb, and produces 45 lb of thrust. Here's another outfit that sells a 3.5" diameter, 7.25" long, 1.9 lb turbine with 11.4 lb of thrust. http://www.swbturbines.com/model_turbines.htm Now granted these are turbojets, not turboprops, but it appears to me that making small turbines is possible... -- Jim Pennino Remove -spam-sux to reply. |
#13
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#14
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In rec.aviation.owning Stefan wrote:
wrote: Now granted these are turbojets, not turboprops, but it appears to me that making small turbines is possible... Nobody ever said that it's not possible. It's just not economical and will never be. Stefan Several people have said that but I've yet to see any analysis (with numbers) to back up that contention. Since (current) turbines are terribly inefficient at low throttle, I can see the problem with an aircraft that spends most of the time doing touch and goes. But where is the crossover point as dictated by the physics of turbines? C-182? C-209? Caravan? Whoops, that last one is already a turbine. -- Jim Pennino Remove -spam-sux to reply. |
#15
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wrote in message ... In rec.aviation.owning wrote: A gas turbine scales up easily and but is nearly impossible to scale down. The auto manuacturers found that out in the 1940s - remember the "car of the future" on the covers of Popular Science et al? Turbines for cars are further away now than they were 55 years ago. The turbine suffers from excessive fuel consumption at part throttle (the piston engine is incredibly flexible that way)and in smaller HP installations. So much of the useful load of an aircraft is fuel, that fuel efficiency is very important for overall mission performance. The problem of an engine is to find the most efficient way to expand a certain flow rate of compressed hot gas to atmospheric pressures. A turbine can do this with large mass flow rates, but as the flow rates become smaller, the turbine speeds (rpm) must increase enormously and the centrifugal accelerations get out of hand. On the other hand, a piston can process an expansion efficiently with small flow rates. Think of it this way - a model airplane engine can be made to run with 1/20 of a cubic inch (.049 cu inch to even .010 cu inch), but piston engine aircraft became impractical above a few thousand HP. That is the range of practicality for a piston concept. An engineering prof once said - if the gas turbine had been invented first, the piston engine would have been looked on an ingeneous solution to the turbine's material and speed and power range problems. Diesels may eventually make it. They have a weight problem that may be offset by a lower specific fuel consumption, but for a given operating condition, spark ignition engines can nearly approach the consumption of diesels by using turbo compounding and operation only at full throttle. While not quite a .049, here's a 3.7" in diameter, 2.6 lb turbine that produces 16.5 lb of thrust. http://jetcatusa.sitewavesonline.net/p70.html Their biggest turbine is 5.12", 5 lb, and produces 45 lb of thrust. Here's another outfit that sells a 3.5" diameter, 7.25" long, 1.9 lb turbine with 11.4 lb of thrust. http://www.swbturbines.com/model_turbines.htm Now granted these are turbojets, not turboprops, but it appears to me that making small turbines is possible... -- Jim Pennino You are missing the point. Everyone agrees that small turbines can be built, the issue is fuel consumption. What is the specific fuel consumption per lb of thrust? Mike MU-2 |
#16
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About the size of the Caravan 900hp+
Mike MU-2 wrote in message ... In rec.aviation.owning Stefan wrote: wrote: Now granted these are turbojets, not turboprops, but it appears to me that making small turbines is possible... Nobody ever said that it's not possible. It's just not economical and will never be. Stefan Several people have said that but I've yet to see any analysis (with numbers) to back up that contention. Since (current) turbines are terribly inefficient at low throttle, I can see the problem with an aircraft that spends most of the time doing touch and goes. But where is the crossover point as dictated by the physics of turbines? C-182? C-209? Caravan? Whoops, that last one is already a turbine. -- Jim Pennino Remove -spam-sux to reply. |
#17
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In rec.aviation.owning Mike Rapoport wrote:
wrote in message ... While not quite a .049, here's a 3.7" in diameter, 2.6 lb turbine that produces 16.5 lb of thrust. http://jetcatusa.sitewavesonline.net/p70.html Their biggest turbine is 5.12", 5 lb, and produces 45 lb of thrust. Here's another outfit that sells a 3.5" diameter, 7.25" long, 1.9 lb turbine with 11.4 lb of thrust. http://www.swbturbines.com/model_turbines.htm Now granted these are turbojets, not turboprops, but it appears to me that making small turbines is possible... -- Jim Pennino You are missing the point. Everyone agrees that small turbines can be built, the issue is fuel consumption. What is the specific fuel consumption per lb of thrust? Not quite "everyone" has signed on to that notion and you are one of few that has wanted to talk about numbers as opposed to making sweeping statements. For the 16.5 lb thrust engine it is 1.8 lb/hr-lb thrust, but I doubt fuel efficiency is a design criteria in a model airplane engine. The question remains, at what HP level, based on the physics of the engines, does the crossover from piston to turbine occur? As additional criteria, assume specific fuel consumption is the most important parameter and that the A/C spends the majority of its time in flight not doing touch and goes. -- Jim Pennino Remove -spam-sux to reply. |
#18
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In rec.aviation.owning Mike Rapoport wrote:
About the size of the Caravan 900hp+ Mike MU-2 According to the Cessna website, the current Caravan is 675hp. -- Jim Pennino Remove -spam-sux to reply. |
#19
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wrote in message ... In rec.aviation.owning Mike Rapoport wrote: wrote in message ... While not quite a .049, here's a 3.7" in diameter, 2.6 lb turbine that produces 16.5 lb of thrust. http://jetcatusa.sitewavesonline.net/p70.html Their biggest turbine is 5.12", 5 lb, and produces 45 lb of thrust. Here's another outfit that sells a 3.5" diameter, 7.25" long, 1.9 lb turbine with 11.4 lb of thrust. http://www.swbturbines.com/model_turbines.htm Now granted these are turbojets, not turboprops, but it appears to me that making small turbines is possible... -- Jim Pennino You are missing the point. Everyone agrees that small turbines can be built, the issue is fuel consumption. What is the specific fuel consumption per lb of thrust? Not quite "everyone" has signed on to that notion and you are one of few that has wanted to talk about numbers as opposed to making sweeping statements. For the 16.5 lb thrust engine it is 1.8 lb/hr-lb thrust, but I doubt fuel efficiency is a design criteria in a model airplane engine. The question remains, at what HP level, based on the physics of the engines, does the crossover from piston to turbine occur? As additional criteria, assume specific fuel consumption is the most important parameter and that the A/C spends the majority of its time in flight not doing touch and goes. -- Jim Pennino I think that you can look at the market to see where the crossover occurs. THere are currently no production piston aircraft engines over 450hp and there are no aircraft turbines under 400hp. Mike MU-2 |
#20
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The Caravan has a 940hp engine flat rated to 675hp. Turbines are typically
flat rated so that the engine can make rated power to reasonable altitudes and temperatures without having to design the gearbox for the full thermodynamic horsepower. To keep the comparison with piston engines apples to apples you need to use thermodynamic ratings. http://www.pwc.ca/en/3_0/3_0http://w.../3_0_2_1_2.asp To put some numbers on things, the engines in my MU-2 have a specific fuel consumption of .55lb/hp/hr and a piston engine is about .45 and diesels can be under .40. Huge (ship) diesels can be under .30. Compare your model aircraft engines with the TFE731-60 used on the Falcon 900EX which uses ..405lb/lb thrust/hr Mike MU-2 wrote in message ... In rec.aviation.owning Mike Rapoport wrote: About the size of the Caravan 900hp+ Mike MU-2 According to the Cessna website, the current Caravan is 675hp. -- Jim Pennino Remove -spam-sux to reply. |
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