If this is your first visit, be sure to check out the FAQ by clicking the link above. You may have to register before you can post: click the register link above to proceed. To start viewing messages, select the forum that you want to visit from the selection below. |
|
|
Thread Tools | Display Modes |
#21
|
|||
|
|||
|
#22
|
|||
|
|||
In rec.aviation.owning Mike Rapoport wrote:
wrote in message ... snip 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. 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. There's lots of ground turbines under 400hp so we know there's a market there; i.e. they must be practical and competive with pistons or they wouldn't sell. I thought the Chinese were still making a big radial, but I could be wrong on that one and it is a bit of a nit. If gasoline hadn't risen to twice the price of Jet-A (at least in parts of Europe), no one would be seriously discussing diesel engines for aircraft or actively developing them as several manufacturers are now. So put it this way, if it were the turbine makers instead of the diesel makers that jumped on this bandwagon, what would be their smallest engine? -- Jim Pennino Remove -spam-sux to reply. |
#23
|
|||
|
|||
In rec.aviation.owning Mike Rapoport wrote:
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 OK, that explains that. 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 Aha, numbers! So if one assumes the motivation to switch from a piston to a turbine is the price of gas is roughly twice Jet-A, the crossover point would be a turbine that did about .8 (to allow for the weight difference in the fuels). Any idea how small (in appropriate terms of hp) current technology can make a turbine with that consumption? -- Jim Pennino Remove -spam-sux to reply. |
#25
|
|||
|
|||
wrote in message ... In rec.aviation.owning Mike Rapoport wrote: wrote in message ... snip 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. 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. There's lots of ground turbines under 400hp so we know there's a market there; i.e. they must be practical and competive with pistons or they wouldn't sell. A lot of them are used to power natural gas compressors way out in the middle of nowhere and reliability is much more important than fuel efficiency and you have a large suitable fuel supply availible. So put it this way, if it were the turbine makers instead of the diesel makers that jumped on this bandwagon, what would be their smallest engine? Given the high initial cost of turbines and the hgiher fuel comsumption, I doubt that turbines would be competitive with gasoline engines given current price differentials between the two fuels. The beauty of a diesel aircraft engine is that it should cost the same as a gas engine, has fewer parts, uses less fuel and lasts longer. The turbine engine is more reliable but costs more and uses more fuel. The lower the hp the less competitive the turbine gets against the diesel. Mike MU-2 |
#26
|
|||
|
|||
wrote in message ... In rec.aviation.owning Mike Rapoport wrote: 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 OK, that explains that. 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 Aha, numbers! So if one assumes the motivation to switch from a piston to a turbine is the price of gas is roughly twice Jet-A, the crossover point would be a turbine that did about .8 (to allow for the weight difference in the fuels). Any idea how small (in appropriate terms of hp) current technology can make a turbine with that consumption? -- Jim Pennino That would be the economic crossover point if the engines cost the same. Of course a plane that needed twice the fuel (in lbs) to achieve the same performance wouldn't have much useful load or range. Mike MU-2 |
#27
|
|||
|
|||
In rec.aviation.owning Mike Rapoport wrote:
wrote in message ... In rec.aviation.owning Mike Rapoport wrote: 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 OK, that explains that. 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 Aha, numbers! So if one assumes the motivation to switch from a piston to a turbine is the price of gas is roughly twice Jet-A, the crossover point would be a turbine that did about .8 (to allow for the weight difference in the fuels). Any idea how small (in appropriate terms of hp) current technology can make a turbine with that consumption? -- Jim Pennino That would be the economic crossover point if the engines cost the same. Of course a plane that needed twice the fuel (in lbs) to achieve the same performance wouldn't have much useful load or range. Dropping a diesel in an airplane costs a bunch. The justification is the cost is recovered in lowered fuel costs. Your second point is certainly valid though and a minor problem with the diesels according to the AVweb article on them. -- Jim Pennino Remove -spam-sux to reply. |
#28
|
|||
|
|||
In rec.aviation.owning Mike Rapoport wrote:
wrote in message ... snip There's lots of ground turbines under 400hp so we know there's a market there; i.e. they must be practical and competive with pistons or they wouldn't sell. A lot of them are used to power natural gas compressors way out in the middle of nowhere and reliability is much more important than fuel efficiency and you have a large suitable fuel supply availible. True. So put it this way, if it were the turbine makers instead of the diesel makers that jumped on this bandwagon, what would be their smallest engine? Given the high initial cost of turbines and the hgiher fuel comsumption, I doubt that turbines would be competitive with gasoline engines given current price differentials between the two fuels. The beauty of a diesel aircraft engine is that it should cost the same as a gas engine, has fewer parts, uses less fuel and lasts longer. The turbine engine is more reliable but costs more and uses more fuel. The lower the hp the less competitive the turbine gets against the diesel. Your first sentence overlooks the fact that turbines are currently competitive at the Caravan level, but I pretty much agree with the rest. OK, let's say I buy into about 400hp as the "up to now" crossover point. Given the current fuel cost differential, where would you expect that point to move to assuming the engines were available? -- Jim Pennino Remove -spam-sux to reply. |
#29
|
|||
|
|||
|
#30
|
|||
|
|||
A diesel doesn't cost any more that a piston engine. A STC'd conversion
costs more but, in a new airplane the cost should be the same. Mike MU-2 wrote in message ... In rec.aviation.owning Mike Rapoport wrote: wrote in message ... In rec.aviation.owning Mike Rapoport wrote: 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 OK, that explains that. 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 Aha, numbers! So if one assumes the motivation to switch from a piston to a turbine is the price of gas is roughly twice Jet-A, the crossover point would be a turbine that did about .8 (to allow for the weight difference in the fuels). Any idea how small (in appropriate terms of hp) current technology can make a turbine with that consumption? -- Jim Pennino That would be the economic crossover point if the engines cost the same. Of course a plane that needed twice the fuel (in lbs) to achieve the same performance wouldn't have much useful load or range. Dropping a diesel in an airplane costs a bunch. The justification is the cost is recovered in lowered fuel costs. Your second point is certainly valid though and a minor problem with the diesels according to the AVweb article on them. -- Jim Pennino Remove -spam-sux to reply. |
Thread Tools | |
Display Modes | |
|
|