Compressor stall?? Help!
07-03-2006 | 08:32 PM
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Compressor stall?? Help!
I have heard on a couple of occasions a very unusual sound coming from an RC jet which I am not sure how to describe. Almost a violent air sucking sound that seems to resonate from the inlets. One occasion the turbine quit and ended in a deadstick landing. Some were calling it a !QUOT!Compressor stall!QUOT!. Does anyone know what I am referring to?
Until recently I have never experienced it on any of my turbines. First incident was a P-60 in a Maverick conversion ( with Bandit inlets) on start up it began to make that particular noise and I just shut it down. Second start attempt was just fine and it has never occurred while in the air (that I am aware of). It has done the same thing one other time.
Second incident is with a P-120 that I just got back from its 25 hour checkup. It was installed right back in the same airframe ( S. Bandit) that has had almost 90 flights and starts without a problem. I just did a test start after re-installing the P-120 and after idling for a minute or so I began to increase to full throttle. Once it just about hit full RPM I started to get a hint of this puzzling noise that I have never had occur with this setup. I didn't want to disturb the neighbors any further so I shut it down.
Is what I am hearing in fact a compressor stall, and if so is it dangerous? Can anything be changed to alleviate the problem?
Both have Bypass installations with FOD screens.
Thanks for any help you can shed on this,
Jim
Until recently I have never experienced it on any of my turbines. First incident was a P-60 in a Maverick conversion ( with Bandit inlets) on start up it began to make that particular noise and I just shut it down. Second start attempt was just fine and it has never occurred while in the air (that I am aware of). It has done the same thing one other time.
Second incident is with a P-120 that I just got back from its 25 hour checkup. It was installed right back in the same airframe ( S. Bandit) that has had almost 90 flights and starts without a problem. I just did a test start after re-installing the P-120 and after idling for a minute or so I began to increase to full throttle. Once it just about hit full RPM I started to get a hint of this puzzling noise that I have never had occur with this setup. I didn't want to disturb the neighbors any further so I shut it down.
Is what I am hearing in fact a compressor stall, and if so is it dangerous? Can anything be changed to alleviate the problem?
Both have Bypass installations with FOD screens.
Thanks for any help you can shed on this,
Jim
07-05-2006 | 02:56 AM
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RE: Compressor stall?? Help!
I spent my military time as a turbine engine mechanic and have witnessed compressor stalls in full scale aircraft(US ARMY Heli's). A turbine engine is a precarious balancing act of RPM/temperature/pressure that has to be maintained within tight parameters or you can get a compressor stall. In the case of the full scale aircraft this is done electro-mechanically with ECU and/or a mechanical fuel control, depending on the vintage and type of the engine we are talking about.
Essentially a compressor stall occurs when the combustor pressure rises beyond that being created by the compressor (cold)section of the engine. Since the internal pressure is always attempting to find it's way out of the engine and when everything is working properly it exits to the rear. If, for any reason, the pressure in the combustor(hot) section rises beyond that being created by the compressor you get a pressure reversion that flows backward toward the spinning compressor(rear to front of the engine). Since these engines are designed to flow in one direction only, inlet to exhaust nozzle, the reversed flow of air essentially acts like a brake and tries to stop(stall) the spinning compressor. Now when you consider that these compressors are spinning upward of 120,000 rpm you can imagine the catastrophic results that can take place when such things happen.
Stalls usually occur when throttling up quickly or rolling off the throttle quickly. In full scale aircraft the causes can be many but usually are the result of an improper rigging of the inlet or variable stator vane systems, cold section bleed bands that don't open at the right time, a mechanical or electrical fuel control unit that is damaged or out of adjustment or, in many cases, erosion of the compressor blades/vanes that leads to improper(too loose) tolerance of the parts which results in a drastic drop in efficiency(reduced airflow and pressure). Erosion is caused by dirt and debris ingestion(FOD) and can literally thin blades or in some severe cases literally roll the metal back on the leading edge of the blades destroying their aerodynamic properties. In some cases I have seen air density cause stalls when the engines was improperly rigged, ran fine in cold air but stalled in hot(thin) air.
In the case of model turbines these are basically smaller versions of the old 50's era centrifugal engines(think F-86 or Mig-15). These engines are essentially 2 impellers mounted back to back and connected together on the same shaft. The front impeller draws in air and spins it outward at a 90 degree angle into a duct called a scroll. It's the centrifugal force that is imparted to the air that results in a pressure rise. Next this compressed air is directed into the combustion chamber where the fuel is injected and ignited, once the burning process begins it is self sustaining and will burn as long as there is fuel to feed the flame, much like a blow torch. The gasses created by the burning process expand and are directed onto the aft impeller which causes it to rotate, think of a child's pinwheel and you will have the idea. The rotation of the aft (power turbine) drives the forward (compressor turbine). An interesting fact that most people are not aware of is that turbine engines use 70% of the power that they make simply to sustain their own operation, the remaining 30% is what we see as thrust or horsepower that turns a shaft in the case of turbo-shaft engines.
Things I witnessed when full scale turbines stalled were broken and burned internal parts in the compressor and power turbine sections, 20-30 foot long flames shooting out of the back(usually) or the front of the engine in some cases. In all cases the engines would make a loud banging noise, imagine the sound a machine gun makes but much louder and deeper in tone. In some cases the stalls were so severed that the engines failed completely and essentially came apart with catastrophic results. Have you ever noticed that in pictures of older full scale jets like the F-4 or F-105(among others) that there is a red stripe painted around the fuse about 2/3rd's of the way back on the airframe? What that line signifies is where the turbine wheels will leave the airframe should a catastrophic failure occur. All personnel are trained to never stand beside the aircraft on either side next to that line while the engines are running for up to a 100 feet(or more) from the aircraft. where turbines are concerned you are safer standing in front of or behind the aircraft than beside it.
I'm not sure this will answer your question but it should be informative to those who have heard the term compressor stall but had no idea what it meant.
Essentially a compressor stall occurs when the combustor pressure rises beyond that being created by the compressor (cold)section of the engine. Since the internal pressure is always attempting to find it's way out of the engine and when everything is working properly it exits to the rear. If, for any reason, the pressure in the combustor(hot) section rises beyond that being created by the compressor you get a pressure reversion that flows backward toward the spinning compressor(rear to front of the engine). Since these engines are designed to flow in one direction only, inlet to exhaust nozzle, the reversed flow of air essentially acts like a brake and tries to stop(stall) the spinning compressor. Now when you consider that these compressors are spinning upward of 120,000 rpm you can imagine the catastrophic results that can take place when such things happen.
Stalls usually occur when throttling up quickly or rolling off the throttle quickly. In full scale aircraft the causes can be many but usually are the result of an improper rigging of the inlet or variable stator vane systems, cold section bleed bands that don't open at the right time, a mechanical or electrical fuel control unit that is damaged or out of adjustment or, in many cases, erosion of the compressor blades/vanes that leads to improper(too loose) tolerance of the parts which results in a drastic drop in efficiency(reduced airflow and pressure). Erosion is caused by dirt and debris ingestion(FOD) and can literally thin blades or in some severe cases literally roll the metal back on the leading edge of the blades destroying their aerodynamic properties. In some cases I have seen air density cause stalls when the engines was improperly rigged, ran fine in cold air but stalled in hot(thin) air.
In the case of model turbines these are basically smaller versions of the old 50's era centrifugal engines(think F-86 or Mig-15). These engines are essentially 2 impellers mounted back to back and connected together on the same shaft. The front impeller draws in air and spins it outward at a 90 degree angle into a duct called a scroll. It's the centrifugal force that is imparted to the air that results in a pressure rise. Next this compressed air is directed into the combustion chamber where the fuel is injected and ignited, once the burning process begins it is self sustaining and will burn as long as there is fuel to feed the flame, much like a blow torch. The gasses created by the burning process expand and are directed onto the aft impeller which causes it to rotate, think of a child's pinwheel and you will have the idea. The rotation of the aft (power turbine) drives the forward (compressor turbine). An interesting fact that most people are not aware of is that turbine engines use 70% of the power that they make simply to sustain their own operation, the remaining 30% is what we see as thrust or horsepower that turns a shaft in the case of turbo-shaft engines.
Things I witnessed when full scale turbines stalled were broken and burned internal parts in the compressor and power turbine sections, 20-30 foot long flames shooting out of the back(usually) or the front of the engine in some cases. In all cases the engines would make a loud banging noise, imagine the sound a machine gun makes but much louder and deeper in tone. In some cases the stalls were so severed that the engines failed completely and essentially came apart with catastrophic results. Have you ever noticed that in pictures of older full scale jets like the F-4 or F-105(among others) that there is a red stripe painted around the fuse about 2/3rd's of the way back on the airframe? What that line signifies is where the turbine wheels will leave the airframe should a catastrophic failure occur. All personnel are trained to never stand beside the aircraft on either side next to that line while the engines are running for up to a 100 feet(or more) from the aircraft. where turbines are concerned you are safer standing in front of or behind the aircraft than beside it.
I'm not sure this will answer your question but it should be informative to those who have heard the term compressor stall but had no idea what it meant.
07-05-2006 | 07:05 AM
#4
RE: Compressor stall?? Help!
ORIGINAL: Shogun
I spent my military time as a turbine engine mechanic and have witnessed compressor stalls in full scale aircraft(US ARMY Heli's). A turbine engine is a precarious balancing act of RPM/temperature/pressure that has to be maintained within tight parameters or you can get a compressor stall. In the case of the full scale aircraft this is done electro-mechanically with ECU and/or a mechanical fuel control, depending on the vintage and type of the engine we are talking about.
Essentially a compressor stall occurs when the combustor pressure rises beyond that being created by the compressor (cold)section of the engine. Since the internal pressure is always attempting to find it's way out of the engine and when everything is working properly it exits to the rear. If, for any reason, the pressure in the combustor(hot) section rises beyond that being created by the compressor you get a pressure reversion that flows backward toward the spinning compressor(rear to front of the engine). Since these engines are designed to flow in one direction only, inlet to exhaust nozzle, the reversed flow of air essentially acts like a brake and tries to stop(stall) the spinning compressor. Now when you consider that these compressors are spinning upward of 120,000 rpm you can imagine the catastrophic results that can take place when such things happen.
Stalls usually occur when throttling up quickly or rolling off the throttle quickly. In full scale aircraft the causes can be many but usually are the result of an improper rigging of the inlet or variable stator vane systems, cold section bleed bands that don't open at the right time, a mechanical or electrical fuel control unit that is damaged or out of adjustment or, in many cases, erosion of the compressor blades/vanes that leads to improper(too loose) tolerance of the parts which results in a drastic drop in efficiency(reduced airflow and pressure). Erosion is caused by dirt and debris ingestion(FOD) and can literally thin blades or in some severe cases literally roll the metal back on the leading edge of the blades destroying their aerodynamic properties. In some cases I have seen air density cause stalls when the engines was improperly rigged, ran fine in cold air but stalled in hot(thin) air.
In the case of model turbines these are basically smaller versions of the old 50's era centrifugal engines(think F-86 or Mig-15). These engines are essentially 2 impellers mounted back to back and connected together on the same shaft. The front impeller draws in air and spins it outward at a 90 degree angle into a duct called a scroll. It's the centrifugal force that is imparted to the air that results in a pressure rise. Next this compressed air is directed into the combustion chamber where the fuel is injected and ignited, once the burning process begins it is self sustaining and will burn as long as there is fuel to feed the flame, much like a blow torch. The gasses created by the burning process expand and are directed onto the aft impeller which causes it to rotate, think of a child's pinwheel and you will have the idea. The rotation of the aft (power turbine) drives the forward (compressor turbine). An interesting fact that most people are not aware of is that turbine engines use 70% of the power that they make simply to sustain their own operation, the remaining 30% is what we see as thrust or horsepower that turns a shaft in the case of turbo-shaft engines.
Things I witnessed when full scale turbines stalled were broken and burned internal parts in the compressor and power turbine sections, 20-30 foot long flames shooting out of the back(usually) or the front of the engine in some cases. In all cases the engines would make a loud banging noise, imagine the sound a machine gun makes but much louder and deeper in tone. In some cases the stalls were so severed that the engines failed completely and essentially came apart with catastrophic results. Have you ever noticed that in pictures of older full scale jets like the F-4 or F-105(among others) that there is a red stripe painted around the fuse about 2/3rd's of the way back on the airframe? What that line signifies is where the turbine wheels will leave the airframe should a catastrophic failure occur. All personnel are trained to never stand beside the aircraft on either side next to that line while the engines are running for up to a 100 feet(or more) from the aircraft. where turbines are concerned you are safer standing in front of or behind the aircraft than beside it.
I'm not sure this will answer your question but it should be informative to those who have heard the term compressor stall but had no idea what it meant.
I spent my military time as a turbine engine mechanic and have witnessed compressor stalls in full scale aircraft(US ARMY Heli's). A turbine engine is a precarious balancing act of RPM/temperature/pressure that has to be maintained within tight parameters or you can get a compressor stall. In the case of the full scale aircraft this is done electro-mechanically with ECU and/or a mechanical fuel control, depending on the vintage and type of the engine we are talking about.
Essentially a compressor stall occurs when the combustor pressure rises beyond that being created by the compressor (cold)section of the engine. Since the internal pressure is always attempting to find it's way out of the engine and when everything is working properly it exits to the rear. If, for any reason, the pressure in the combustor(hot) section rises beyond that being created by the compressor you get a pressure reversion that flows backward toward the spinning compressor(rear to front of the engine). Since these engines are designed to flow in one direction only, inlet to exhaust nozzle, the reversed flow of air essentially acts like a brake and tries to stop(stall) the spinning compressor. Now when you consider that these compressors are spinning upward of 120,000 rpm you can imagine the catastrophic results that can take place when such things happen.
Stalls usually occur when throttling up quickly or rolling off the throttle quickly. In full scale aircraft the causes can be many but usually are the result of an improper rigging of the inlet or variable stator vane systems, cold section bleed bands that don't open at the right time, a mechanical or electrical fuel control unit that is damaged or out of adjustment or, in many cases, erosion of the compressor blades/vanes that leads to improper(too loose) tolerance of the parts which results in a drastic drop in efficiency(reduced airflow and pressure). Erosion is caused by dirt and debris ingestion(FOD) and can literally thin blades or in some severe cases literally roll the metal back on the leading edge of the blades destroying their aerodynamic properties. In some cases I have seen air density cause stalls when the engines was improperly rigged, ran fine in cold air but stalled in hot(thin) air.
In the case of model turbines these are basically smaller versions of the old 50's era centrifugal engines(think F-86 or Mig-15). These engines are essentially 2 impellers mounted back to back and connected together on the same shaft. The front impeller draws in air and spins it outward at a 90 degree angle into a duct called a scroll. It's the centrifugal force that is imparted to the air that results in a pressure rise. Next this compressed air is directed into the combustion chamber where the fuel is injected and ignited, once the burning process begins it is self sustaining and will burn as long as there is fuel to feed the flame, much like a blow torch. The gasses created by the burning process expand and are directed onto the aft impeller which causes it to rotate, think of a child's pinwheel and you will have the idea. The rotation of the aft (power turbine) drives the forward (compressor turbine). An interesting fact that most people are not aware of is that turbine engines use 70% of the power that they make simply to sustain their own operation, the remaining 30% is what we see as thrust or horsepower that turns a shaft in the case of turbo-shaft engines.
Things I witnessed when full scale turbines stalled were broken and burned internal parts in the compressor and power turbine sections, 20-30 foot long flames shooting out of the back(usually) or the front of the engine in some cases. In all cases the engines would make a loud banging noise, imagine the sound a machine gun makes but much louder and deeper in tone. In some cases the stalls were so severed that the engines failed completely and essentially came apart with catastrophic results. Have you ever noticed that in pictures of older full scale jets like the F-4 or F-105(among others) that there is a red stripe painted around the fuse about 2/3rd's of the way back on the airframe? What that line signifies is where the turbine wheels will leave the airframe should a catastrophic failure occur. All personnel are trained to never stand beside the aircraft on either side next to that line while the engines are running for up to a 100 feet(or more) from the aircraft. where turbines are concerned you are safer standing in front of or behind the aircraft than beside it.
I'm not sure this will answer your question but it should be informative to those who have heard the term compressor stall but had no idea what it meant.
Exelent!
however modelturbines: our flames are mostly 3-10 feet and causes most of the time loss of eyebrows and other facial hair......
as for the build of the turbines . there are still manufacturers using them after the 50,s.
Solar T-61 for instance.
most flame outs and stall i see are cuased by the lack of air , to much heat , or improper connections of fueltanks.
but you are right sometimes it is safer to stand in the back then from the side. however not to close
07-05-2006 | 11:35 AM
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RE: Compressor stall?? Help!
Interestingly the auxiliary power unit(APU) turbines used in the US ARMY helicopters, Chinook/Apache/Blackhawk, are designated T62 and are basically the same as model aircraft turbines, albeit a bit larger. I recall the first time I ever worked on one I instantly wondered if they could be shrunk down a little smaller and used for models.....that was in 1986, imagine my surprise when I got back into modelling in 2001!
Anyone that want's to quote me please feel free to do so. My explanation is overly simplistic but I think most who read it will get the idea.
If anyone has any other questions regarding this subject please feel free to ask.
Anyone that want's to quote me please feel free to do so. My explanation is overly simplistic but I think most who read it will get the idea.
If anyone has any other questions regarding this subject please feel free to ask.
07-05-2006 | 01:50 PM
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RE: Compressor stall?? Help!
Very good Scott. Compressor stall was a common thing when we were handling US Navy aircraft in the 60s. It usually happened just as we were giving the pilot the command to pull out of the parked position and then a very loud whoop! Too fast on throttle at low thrust. It is pretty loud when you are standing out in front of the aircraft even with ear protection. It usually included a big white puff of condensation at the front of the intake as the air was quickly sucked back in. I’ve also been on an airliner and heard compressor stall and the pilot came on the intercom shortly after to explain all was well.
To clarify: The Mig 15 was centrifugal flow, thus the fat fuse http://www.midwaysailor2.com/eltoro/eltoro-008b.jpg and the F-86 and our FJ2s, 3s and 4s were axial flow http://www.centennialofflight.gov/es...E/Aero11G4.htm
Miss those good old FJs, liked the 2s and 3s best! It is too bad that more kids don’t get opportunities to work around fighter aircraft nowadays. Working on the ramp was hard work, but I love jet aircraft so it was a labor of love and we even got paid for it!
To clarify: The Mig 15 was centrifugal flow, thus the fat fuse http://www.midwaysailor2.com/eltoro/eltoro-008b.jpg and the F-86 and our FJ2s, 3s and 4s were axial flow http://www.centennialofflight.gov/es...E/Aero11G4.htm
Miss those good old FJs, liked the 2s and 3s best! It is too bad that more kids don’t get opportunities to work around fighter aircraft nowadays. Working on the ramp was hard work, but I love jet aircraft so it was a labor of love and we even got paid for it!
07-05-2006 | 02:41 PM
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RE: Compressor stall?? Help!
8178,
I always thought the Sabre had a US made copy of the Rolls Royce Nene just like the Mig 15 did, guess I was wrong on that point!
Like I said earlier maintaining the pressures inside a turbine is a precarious balancing act and if something isn't right it all goes south pretty quickly. On all of the older and even some of the current designs, there is a metal strap that is wrapped around the engine at the aft end of the compressor section. This strap covers a series of slots and is held tight to seal those slots off and prevent air from flowing out of them. The bart is referred to as a bleed band and is used to dump excess pressure when the pilot rolls off the throttle quickly. Since the compresor is spooled up to a very high rpm at full throttle it carries a great deal of energy and takes a few seconds to sspool down. Consequently since the compressor drum is still turning it is still making pressure, excess unneeded pressure when your attempting to power down, in this case. The band was controlled by the fuel control or, in the case of the J-79 or F-100 in the Phantom or Eagle, was controlled via the engine ECU. When the FC or ECU senses the lower power demand it opens the band to dump excess pressure from the compressor outboard before it can build and create a stalling condition.
I have always wondered how the small model turbines deal with this issue and I can only guess that it's because of this that they are somewhat slower to spool up(or down) than we would like them to be(turbine lagg as it were). The ECU that controls a models turbine has an incredible amount of number crunching to do and I suspect that by varying the fuel flow to maintain rpm, and thus pressures, it's all done in the programming.
Just remember, if something changes....like an air leak in a fuel line, your turbine is goign to have problems because the system isn't programmed to compensate for such things. For a model turbine to function correctly everything has to be just right.
I always thought the Sabre had a US made copy of the Rolls Royce Nene just like the Mig 15 did, guess I was wrong on that point!
Like I said earlier maintaining the pressures inside a turbine is a precarious balancing act and if something isn't right it all goes south pretty quickly. On all of the older and even some of the current designs, there is a metal strap that is wrapped around the engine at the aft end of the compressor section. This strap covers a series of slots and is held tight to seal those slots off and prevent air from flowing out of them. The bart is referred to as a bleed band and is used to dump excess pressure when the pilot rolls off the throttle quickly. Since the compresor is spooled up to a very high rpm at full throttle it carries a great deal of energy and takes a few seconds to sspool down. Consequently since the compressor drum is still turning it is still making pressure, excess unneeded pressure when your attempting to power down, in this case. The band was controlled by the fuel control or, in the case of the J-79 or F-100 in the Phantom or Eagle, was controlled via the engine ECU. When the FC or ECU senses the lower power demand it opens the band to dump excess pressure from the compressor outboard before it can build and create a stalling condition.
I have always wondered how the small model turbines deal with this issue and I can only guess that it's because of this that they are somewhat slower to spool up(or down) than we would like them to be(turbine lagg as it were). The ECU that controls a models turbine has an incredible amount of number crunching to do and I suspect that by varying the fuel flow to maintain rpm, and thus pressures, it's all done in the programming.
Just remember, if something changes....like an air leak in a fuel line, your turbine is goign to have problems because the system isn't programmed to compensate for such things. For a model turbine to function correctly everything has to be just right.
07-05-2006 | 06:39 PM
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RE: Compressor stall?? Help!
Great info Scott! Thanks for taking some time to educate us a bit! Now I need to find out what " Isn't just right, so I can get back to flying!!
Thanks again Scott.
Thanks again Scott.
07-06-2006 | 03:21 AM
#11
RE: Compressor stall?? Help!
ORIGINAL: Shogun
8178,
I always thought the Sabre had a US made copy of the Rolls Royce Nene just like the Mig 15 did, guess I was wrong on that point!
Like I said earlier maintaining the pressures inside a turbine is a precarious balancing act and if something isn't right it all goes south pretty quickly. On all of the older and even some of the current designs, there is a metal strap that is wrapped around the engine at the aft end of the compressor section. This strap covers a series of slots and is held tight to seal those slots off and prevent air from flowing out of them. The bart is referred to as a bleed band and is used to dump excess pressure when the pilot rolls off the throttle quickly. Since the compresor is spooled up to a very high rpm at full throttle it carries a great deal of energy and takes a few seconds to sspool down. Consequently since the compressor drum is still turning it is still making pressure, excess unneeded pressure when your attempting to power down, in this case. The band was controlled by the fuel control or, in the case of the J-79 or F-100 in the Phantom or Eagle, was controlled via the engine ECU. When the FC or ECU senses the lower power demand it opens the band to dump excess pressure from the compressor outboard before it can build and create a stalling condition.
I have always wondered how the small model turbines deal with this issue and I can only guess that it's because of this that they are somewhat slower to spool up(or down) than we would like them to be(turbine lagg as it were). The ECU that controls a models turbine has an incredible amount of number crunching to do and I suspect that by varying the fuel flow to maintain rpm, and thus pressures, it's all done in the programming.
Just remember, if something changes....like an air leak in a fuel line, your turbine is goign to have problems because the system isn't programmed to compensate for such things. For a model turbine to function correctly everything has to be just right.
8178,
I always thought the Sabre had a US made copy of the Rolls Royce Nene just like the Mig 15 did, guess I was wrong on that point!
Like I said earlier maintaining the pressures inside a turbine is a precarious balancing act and if something isn't right it all goes south pretty quickly. On all of the older and even some of the current designs, there is a metal strap that is wrapped around the engine at the aft end of the compressor section. This strap covers a series of slots and is held tight to seal those slots off and prevent air from flowing out of them. The bart is referred to as a bleed band and is used to dump excess pressure when the pilot rolls off the throttle quickly. Since the compresor is spooled up to a very high rpm at full throttle it carries a great deal of energy and takes a few seconds to sspool down. Consequently since the compressor drum is still turning it is still making pressure, excess unneeded pressure when your attempting to power down, in this case. The band was controlled by the fuel control or, in the case of the J-79 or F-100 in the Phantom or Eagle, was controlled via the engine ECU. When the FC or ECU senses the lower power demand it opens the band to dump excess pressure from the compressor outboard before it can build and create a stalling condition.
I have always wondered how the small model turbines deal with this issue and I can only guess that it's because of this that they are somewhat slower to spool up(or down) than we would like them to be(turbine lagg as it were). The ECU that controls a models turbine has an incredible amount of number crunching to do and I suspect that by varying the fuel flow to maintain rpm, and thus pressures, it's all done in the programming.
Just remember, if something changes....like an air leak in a fuel line, your turbine is goign to have problems because the system isn't programmed to compensate for such things. For a model turbine to function correctly everything has to be just right.
you hit the nail there the ecu is the important bit.
and you can imagine how hard it is to find a good setting for every section of the world.
as for spool up , the newer generation turbines have incredible spool up.
some of us have turbines that spool up from like 60N to 160N in less then 1 second
it is so bizar that we had one guy in norway that had a eurosport with a merlin and if flown at low speeds and he would throttle up.
the torque would move his eurosport to its side.
i also have a few Solars here and i am very tempted to use one as a turboprop
and the otherone as a Thrust turbine (i just remove the powersection)
07-06-2006 | 08:28 AM
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RE: Compressor stall?? Help!
After reviewing all the brilliant and eloquent technical input, it seems Jim's implied question (about what might have changed in his situation to cause the symptoms noted) remains unanswered.
Some related considerations:
1. Most of us here in Colorado on the east side of the Rockies (and that includes Jim) fly our jets at flying sites whose altitude is anywhere between 5,200 and 6,800 MSL. So, there is a significant difference in pressure altitude given that virtually all ECU parameters are typically established and validated at MSL. Jim did not mention if there was an ECU firmware update involved when he got the P-120 back. Being curious, I would want some insight into what ECU parameters were changed (if any) and some discussion with Bob Wilcox might be helpful.
2. The P-120s seem to be happy in any environment but at our altitude, we've found that not all the smaller turbines (i.e., the P-60 variety) are "happy" when fitted with FOD screens and, in fact, do exhibit the symptoms of a classic "compressor stall." Operating the P-60 without the FOD screen might prove insightful.
My 2¢ worth.
Mike
Some related considerations:
1. Most of us here in Colorado on the east side of the Rockies (and that includes Jim) fly our jets at flying sites whose altitude is anywhere between 5,200 and 6,800 MSL. So, there is a significant difference in pressure altitude given that virtually all ECU parameters are typically established and validated at MSL. Jim did not mention if there was an ECU firmware update involved when he got the P-120 back. Being curious, I would want some insight into what ECU parameters were changed (if any) and some discussion with Bob Wilcox might be helpful.
2. The P-120s seem to be happy in any environment but at our altitude, we've found that not all the smaller turbines (i.e., the P-60 variety) are "happy" when fitted with FOD screens and, in fact, do exhibit the symptoms of a classic "compressor stall." Operating the P-60 without the FOD screen might prove insightful.
My 2¢ worth.
Mike
07-06-2006 | 09:42 AM
#14
RE: Compressor stall?? Help!
This comment doesn't help Jim but. The Mig 15 and the F-94C had the same British engine design. The F-94C had an afterburner and the Mig had water/alcohol injection to augment power. I think the F-80, T-33 and F-94A, B, and C were the only Centrifigal compressors in US Air Force planes. Starting with the F-84 on were all axial compressors.
I am building a F-94C from scratch and hope to test fly this fall.
I personally have been plagued by a few problems caused by our high density altitude. The RAM 500 is very difficult to get it to acelerate rapidly at our alititude and my personal engine has yet to be flown here. It works OK in Dallas, TX. The WREN 54's and WREN Super Sport seem to be able to handle the altitude with ECU ajustments.
I am building a F-94C from scratch and hope to test fly this fall.
I personally have been plagued by a few problems caused by our high density altitude. The RAM 500 is very difficult to get it to acelerate rapidly at our alititude and my personal engine has yet to be flown here. It works OK in Dallas, TX. The WREN 54's and WREN Super Sport seem to be able to handle the altitude with ECU ajustments.
07-06-2006 | 09:45 AM
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RE: Compressor stall?? Help!
Mike,
I have seen the term Compressor Stall brought up here many times but never have I read a decent description of what a compressor stall actually is so I thought an explanation was over due. While I couldn't offer specific help to the original posters problem I felt that perhaps added knowledge of how a turbine functions might aid him in diagnosing and fixing it himself.
I had noted that the original post sat here for at least 2 days with no input from anyone so I offered what I could.
I have seen the term Compressor Stall brought up here many times but never have I read a decent description of what a compressor stall actually is so I thought an explanation was over due. While I couldn't offer specific help to the original posters problem I felt that perhaps added knowledge of how a turbine functions might aid him in diagnosing and fixing it himself.
I had noted that the original post sat here for at least 2 days with no input from anyone so I offered what I could.
07-06-2006 | 09:57 AM
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RE: Compressor stall?? Help!
Scott replied:
. . . While I couldn't offer specific help to the original posters problem I felt that perhaps added knowledge of how a turbine functions might aid him in diagnosing and fixing it himself.
. . . While I couldn't offer specific help to the original posters problem I felt that perhaps added knowledge of how a turbine functions might aid him in diagnosing and fixing it himself.
Cheers.
Mike
07-06-2006 | 08:04 PM
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From: Oslo, NORWAY
RE: Compressor stall?? Help!
Hi, I agree with others here, it was a good explanation of how a jet engine works! But I have read that radial compressors like in miniature jet engines for rc use- (i don't like saying "turbines" as most people do, because it is inaccurate, we have all kinds of turbines; water turbines, wind turbines, gas turbines etc 
the turbine (wheel) is only half the engine. a micro gas turbine or jet engine is better to say! [8D] ) -can not stall!(?) to me it seems logical, because an axial compressor is just like a wing of an aircraft, it can of course stall if the incidence is too great, or the speed of the flow is too great, and you get flow separation over the entire upper surface of the blade/wing which is a stall. But I don't know if an axial compressor really can stall, because all it does (as explained by you) is to rotate the air so it is sent radially outwards and compressed by sentrifugal forces. It would have been great if a jet engine company could explained this!
the turbine (wheel) is only half the engine. a micro gas turbine or jet engine is better to say! [8D] ) -can not stall!(?) to me it seems logical, because an axial compressor is just like a wing of an aircraft, it can of course stall if the incidence is too great, or the speed of the flow is too great, and you get flow separation over the entire upper surface of the blade/wing which is a stall. But I don't know if an axial compressor really can stall, because all it does (as explained by you) is to rotate the air so it is sent radially outwards and compressed by sentrifugal forces. It would have been great if a jet engine company could explained this!
07-07-2006 | 12:47 AM
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RE: Compressor stall?? Help!
Actually the compressor design has very little to do with a compressor stall, axial and centrifugal flow designs can both have induced stalls if the operation of the engine gets out of tolerance. In fact most axial flow compressors actually flow their air onto a final stage which is a centrifugal compressor face that directs the compressed air through a scroll into the combustor housing. To visualize this simply imagine an axial flow compressor drum that is attached to the front of the centrifugal compressor impeller, the 2 compliment each other very well. These are known as a compound compressor design.
In general centrifugal compressors are less efficient than their axial flow siblings, but they are much simpler to design, produce and maintain. Axial flow compressors make more sense when the engine is being installed in an aircraft because the designs can retain a much smaller engine diameter, which lend themselves to fitting into aircraft fuselages. Another point to consider is that of maximum CFM flow, axial designs have multiple stages that can ingest and compress huge volumes of air while still retaining a modest inlet diameter. To get the same performance from a centrifugal compressor the design would have to be increased in overall size and diameter which presents the aircraft designers with weight and fit issues when installing them into an airframe.
When discussing stalls in a turbine engine we are not actually talking about a stall that is aerodynamic in nature, like that that of a wing. In the context of this discussion the term stall simply means that the internal air pressure rises beyond limits in the aft section of the engine and when it reaches the aft end of the compressor it attempts to impart a limiting force, essentially it is acting as a brake and acts upon the spinning compressor as such. Since the compressor is turning at a very high speed and carries a huge amount of inertia the effects tend to be non-linear resulting in a violent pulsation that can cause all of the things I described above.
The word stall has many meanings but I think this is the most accurate with regard to this discussion:
v. stalled, stall·ing, stalls
To halt the motion or progress of; bring to a standstill.
It is entirely possible for aerodynamics to induce compressor stalls. In the case of supersonic aircraft the jet inlet design is crucial for proper operation and prevention of compressor stalls.
In the end it's ALL about controlling airflow with a very high degree of precision.
In general centrifugal compressors are less efficient than their axial flow siblings, but they are much simpler to design, produce and maintain. Axial flow compressors make more sense when the engine is being installed in an aircraft because the designs can retain a much smaller engine diameter, which lend themselves to fitting into aircraft fuselages. Another point to consider is that of maximum CFM flow, axial designs have multiple stages that can ingest and compress huge volumes of air while still retaining a modest inlet diameter. To get the same performance from a centrifugal compressor the design would have to be increased in overall size and diameter which presents the aircraft designers with weight and fit issues when installing them into an airframe.
When discussing stalls in a turbine engine we are not actually talking about a stall that is aerodynamic in nature, like that that of a wing. In the context of this discussion the term stall simply means that the internal air pressure rises beyond limits in the aft section of the engine and when it reaches the aft end of the compressor it attempts to impart a limiting force, essentially it is acting as a brake and acts upon the spinning compressor as such. Since the compressor is turning at a very high speed and carries a huge amount of inertia the effects tend to be non-linear resulting in a violent pulsation that can cause all of the things I described above.
The word stall has many meanings but I think this is the most accurate with regard to this discussion:
v. stalled, stall·ing, stalls
To halt the motion or progress of; bring to a standstill.
It is entirely possible for aerodynamics to induce compressor stalls. In the case of supersonic aircraft the jet inlet design is crucial for proper operation and prevention of compressor stalls.
In the end it's ALL about controlling airflow with a very high degree of precision.
07-07-2006 | 07:29 AM
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From: Oslo, NORWAY
RE: Compressor stall?? Help!
Thanks for explaining again, I agree with you again, since this is another kind of stall due to pressure loss the revers (wrong) way. I thought a wing (blade) stall also had something to do with this, but that may be minor or not at all. But if that is the case, I guess it only can happen to axial compressors. (like stalling a ducted fan because of too high rpm caused by a too big engine than what the impeller blade angle is made for.)
07-07-2006 | 01:36 PM
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RE: Compressor stall?? Help!
Some quick questions. On your turbine have you tracked your TGT to see if there has been a significant increase, on its idle temp. If so the compressor blades might be degrading therefore causing the compressor stall to occur when a load is put on the turbine. Remember most of the air going through a turbine is used for cooling if the compressor is not working correctly you will see an increase in TGT, compaired to what it was before.
07-07-2006 | 03:55 PM
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RE: Compressor stall?? Help!
Orest,
I have logged all the stats for just about every flight ( I'd say 98%), so I do have a good track record of the temps, rpm's, voltages etc. I apologize but I am not familiar with the TGT. Would that be the Exhaust Gas Temp....EGT? If so then yes we can go back and look for any trends.
And to everyone else that has taken a minute to chime in:
I did get a chance to talk to the great folks at JetCat and I am going to take both the P-60 and the P-120 out for some more trail runs and try and take good notes of all the stats if and when the problem occurs again. The P-60 may not be a compressor stall issue but may be a tad to much fuel on start up possibly due to the fuel pump breaking in. Either way, if I can gather more info, then it will be easier for them to diagnose what is happening.
Thanks everyone for all the help, hopefully we can figure this out and I will let you know what we find!
I have logged all the stats for just about every flight ( I'd say 98%), so I do have a good track record of the temps, rpm's, voltages etc. I apologize but I am not familiar with the TGT. Would that be the Exhaust Gas Temp....EGT? If so then yes we can go back and look for any trends.
And to everyone else that has taken a minute to chime in:
I did get a chance to talk to the great folks at JetCat and I am going to take both the P-60 and the P-120 out for some more trail runs and try and take good notes of all the stats if and when the problem occurs again. The P-60 may not be a compressor stall issue but may be a tad to much fuel on start up possibly due to the fuel pump breaking in. Either way, if I can gather more info, then it will be easier for them to diagnose what is happening.
Thanks everyone for all the help, hopefully we can figure this out and I will let you know what we find!
07-07-2006 | 04:07 PM
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RE: Compressor stall?? Help!
Sorry I have my APART, and Instrument Check rides on th 13th so the UH-60 lingo is bleeding over ( Trying to fit 100 Gigs of info into a 10 Meg Brain ) . TGT (Turbine Gas Temp). Same as EGT but messured in a different place. Good luck hope you track this bug down and kill it.
) . TGT (Turbine Gas Temp). Same as EGT but messured in a different place. Good luck hope you track this bug down and kill it.
07-07-2006 | 05:54 PM
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RE: Compressor stall?? Help!
EGT, Exhaust Gas Temperature is typically measured in the tail pipe of the engine. TGT, Turbine Gas Temperature is typically measured inside the engine either right in front of or right behind the gas producer turbine rotor wheel(s). This is also sometimes referred to as TOT or Turbine Outlet Temperature, the terms are basically interchangeable.
In the case of model turbines the aft centrifugal turbine takes the place of the individual turbine rotor(s), both perform the same task of driving the compressor or N1 section of the engine.
In the case of model turbines the aft centrifugal turbine takes the place of the individual turbine rotor(s), both perform the same task of driving the compressor or N1 section of the engine.
