Departure Citation jet from tiny uphill Alp strip.

Lance F said:
Greg, I'm not sure I buy that statement. Seems to me there's a lot of mechanical compression going on before that air meets the JetA. What am I missing?
Click to expand...

Your inlet pressure to a turbine engine is always going to be whatever is available from outside. The purpose of turbochargers on piston engines is to provide what would be sea level (or higher) inlet pressure to the engine at higher altitudes. So, just like a naturally aspirated piston engine, your available air (and thus available power) will go down with altitude.

Now, some turbines are "flat rated" to a particular horsepower up to a certain altitude. This would be like flat rating your 200 HP Mooney to about 120 HP by limiting manifold pressure to 20".
 
Ted DuPuis said:
Your inlet pressure to a turbine engine is always going to be whatever is available from outside. The purpose of turbochargers on piston engines is to provide what would be sea level (or higher) inlet pressure to the engine at higher altitudes. So, just like a naturally aspirated piston engine, your available air (and thus available power) will go down with altitude.

Now, some turbines are "flat rated" to a particular horsepower up to a certain altitude. This would be like flat rating your 200 HP Mooney to about 120 HP by limiting manifold pressure to 20".
Click to expand...

The inlet pressure to a turbocharger is also whatever is available from the outside. No difference. Then you have spinning vanes increasing that pressure before it gets to the combustion chamber. No difference. The energy to spin those vanes comes from the exhaust. No difference. Pressurization, if the aircraft has it, comes from the discharge side of those spinning vanes. No difference.

I think we're just playing with semantics here, but I still don't see how you can call a turbo/fan jet "normally aspirated".
 
Lance F said:
I think we're just playing with semantics here, but I still don't see how you can call a turbo/fan jet "normally aspirated".
Click to expand...

Both the turbine engine and the piston engine compress air just before combustion. In a piston engine, this compression is caused by the piston's motion in the cylinder, while in a turbine, the air gets compressed via an axial-flow compressor. In the case of a turbocharged engine, the air is pre-compressed. That is, before it gets into the cylinders. For both a turbine engine and a normally aspirated piston engine, there is no pre-compression, hence Ted calling the turbine engine normally aspirated. Pretty much semantics, I think.
 
Well, isn't the compression ratio of a turbine engine something like 40:1? If what I remember there is correct, that would be equivalent to over 200" of manifold pressure on an 8.5:1 recip engine at sea level. That definitely would require multi stage supercharging.
 
Are turbine engines effected by shock cooling like piston engines are?? If you went all the way to idle at fl300 and did a prolonged descent, no bueno???
 
Lance F said:
The inlet pressure to a turbocharger is also whatever is available from the outside. No difference. Then you have spinning vanes increasing that pressure before it gets to the combustion chamber. No difference. The energy to spin those vanes comes from the exhaust. No difference. Pressurization, if the aircraft has it, comes from the discharge side of those spinning vanes. No difference.

I think we're just playing with semantics here, but I still don't see how you can call a turbo/fan jet "normally aspirated".
Click to expand...

The turbine engine has a fixed OPR (overall pressure ratio), which represents the total compression from inlet of the compressor to the outlet. A turbocharged piston engine equivalent would vary depending on altitude for how hard the turbo has to work.

A turbine engine isn't turbocharged, but you could think of it as a turbocharger, and that would be accurate.

jpower said:
Both the turbine engine and the piston engine compress air just before combustion. In a piston engine, this compression is caused by the piston's motion in the cylinder, while in a turbine, the air gets compressed via an axial-flow compressor. In the case of a turbocharged engine, the air is pre-compressed. That is, before it gets into the cylinders. For both a turbine engine and a normally aspirated piston engine, there is no pre-compression, hence Ted calling the turbine engine normally aspirated. Pretty much semantics, I think.
Click to expand...

Not all turbines are axial-flow, or exclusively axial-flow. The Garrett (formerly AiResearch, now Honeywell) TPE-331 has two centrifugal compressors, one after the other. The multiple axial stages with a single centrifugal stage is also a common theme, reference PT-6, RR250, CT7, TFE731, CFE738.

Henning said:
Well, isn't the compression ratio of a turbine engine something like 40:1? If what I remember there is correct, that would be equivalent to over 200" of manifold pressure on an 8.5:1 recip engine at sea level. That definitely would require multi stage supercharging.
Click to expand...

There are a multitude of different OPRs for turbine engines. 40:1 would be on the high side.

I think the confusion lies in the fact that the two engines work differently, and the super/turbocharger on a piston engine serves a different function than what the compression stages do in a turbine.

The naturally aspirated comment remains valid, though, and makes sense for why most turbine engines can't make the same power at 6500 MSL as they can at sea level.
 
us AAirways said:
Are turbine engines effected by shock cooling like piston engines are?? If you went all the way to idle at fl300 and did a prolonged descent, no bueno???
Click to expand...

The prolonged descent in either case isn't the issue, it's the immediate application or removal of heat, which then causes a bunch of thermal effects on the rest of the engine.

To say that they aren't impacted wouldn't be accurate. There are actually some interesting tests that are done with respect to rapid throttle movements because of what can happen, especially when the throttle is jockeyed back and forth. Mostly it comes down to clearances in the exhaust side of the engine that could cause operational issues.

In most cases, though, you likely won't see much of an impact, because life limits will require the parts that would get some detrimental effects to get scrapped before you'd run into issues. Also, absolute temperatures can be something of a factor. Some airlines get additional engine life by having derated takeoffs. The longest on-wing time I've heard for an airline engine is 50,000 hours, much above the norm. But, they take several steps to be kind to their engines.
 
Ted DuPuis said:
The turbine engine has a fixed OPR (overall pressure ratio), which represents the total compression from inlet of the compressor to the outlet. A turbocharged piston engine equivalent would vary depending on altitude for how hard the turbo has to work.

A turbine engine isn't turbocharged, but you could think of it as a turbocharger, and that would be accurate.



Not all turbines are axial-flow, or exclusively axial-flow. The Garrett (formerly AiResearch, now Honeywell) TPE-331 has two centrifugal compressors, one after the other. The multiple axial stages with a single centrifugal stage is also a common theme, reference PT-6, RR250, CT7, TFE731, CFE738.



There are a multitude of different OPRs for turbine engines. 40:1 would be on the high side.

I think the confusion lies in the fact that the two engines work differently, and the super/turbocharger on a piston engine serves a different function than what the compression stages do in a turbine.

The naturally aspirated comment remains valid, though, and makes sense for why most turbine engines can't make the same power at 6500 MSL as they can at sea level.
Click to expand...

Ok, just seems like an invalid comment to me. What would be valid IMO would be saying a Centrifugal Turbine is like a NA engine while an Axial Turbine is like a supercharged engine since a centrifugal turbine has one stage of compression where as an axial turbine has multiple stages of compression. Only a rocket engine will make the same ultimate power at altitude as it will at sea level.

Now, if you want to build a recip rocket engine, we can do that...:wink2:
 
Last edited:
Henning said:
Ok, just seems like an invalid comment to me. What would be valid IMO would be saying a Centrifugal Turbine is like a NA engine while an Axial Turbine is like a supercharged engine since a centrifugal turbine has one stage of compression where as an axial turbine has multiple stages of compression. Only a rocket engine will make the same ultimate power at altitude as it will at sea level.
Click to expand...

I'm not sure why you separate centrifugal and axial compressors in that way, because it's not accurate. Each one is a single stage and treated as such, and they are typically combined in turbine engines in one form or another to achieve the desired pressure ratio. The real difference is that a single centrifugal stage makes more compression than a single axial stage.

There's a compressor section sitting on my desk from an engine that 5 axial stages and 1 centrifugal stage. As I said, TPE-331s have two centrifugal stages.
 
Mike I said:
Nose gear was flying...definitely not the mains! Here's another angle, from the bottom of the runway.

Click to expand...

^^^Better Video :yikes:

Someone needs to make a composite of both...
 
Ted DuPuis said:
Not all turbines are axial-flow, or exclusively axial-flow. The Garrett (formerly AiResearch, now Honeywell) TPE-331 has two centrifugal compressors, one after the other. The multiple axial stages with a single centrifugal stage is also a common theme, reference PT-6, RR250, CT7, TFE731, CFE738.
Click to expand...

Also the little model ones are all centrifugal that I know of. But most of the decently-sized ones are axial flow, no?
 
Ted DuPuis said:
The prolonged descent in either case isn't the issue, it's the immediate application or removal of heat, which then causes a bunch of thermal effects on the rest of the engine.

To say that they aren't impacted wouldn't be accurate. There are actually some interesting tests that are done with respect to rapid throttle movements because of what can happen, especially when the throttle is jockeyed back and forth. Mostly it comes down to clearances in the exhaust side of the engine that could cause operational issues.

In most cases, though, you likely won't see much of an impact, because life limits will require the parts that would get some detrimental effects to get scrapped before you'd run into issues. Also, absolute temperatures can be something of a factor. Some airlines get additional engine life by having derated takeoffs. The longest on-wing time I've heard for an airline engine is 50,000 hours, much above the norm. But, they take several steps to be kind to their engines.
Click to expand...

How many hrs do airline engines usually last for?
 
Ted DuPuis said:
I'm not sure why you separate centrifugal and axial compressors in that way, because it's not accurate. Each one is a single stage and treated as such, and they are typically combined in turbine engines in one form or another to achieve the desired pressure ratio. The real difference is that a single centrifugal stage makes more compression than a single axial stage.

There's a compressor section sitting on my desk from an engine that 5 axial stages and 1 centrifugal stage. As I said, TPE-331s have two centrifugal stages.
Click to expand...

True with the TPE-331. I'm just trying to relate it to stages of compression since there's no real other way to compare a turbine with either a naturally aspirated or supercharged recip in that type of analogy. A supercharged recip has multiple stages of compression before combustion as does an axial flow turbine. Most centrifugal turbines have a single stage of compression before combustion as does a naturally aspirated recip. Regardless of of NA recip, SC recip, or turbine regardless of type, they will all be subject to the variations in initial atmospheric pressure as to what their terminal power will be. That was my only point. The only way to escape that is to carry along the oxygen in a tank like a rocket.
 
Lance F said:
The inlet pressure to a turbocharger is also whatever is available from the outside. No difference. Then you have spinning vanes increasing that pressure before it gets to the combustion chamber. No difference. The energy to spin those vanes comes from the exhaust. No difference. Pressurization, if the aircraft has it, comes from the discharge side of those spinning vanes. No difference.

I think we're just playing with semantics here, but I still don't see how you can call a turbo/fan jet "normally aspirated".
Click to expand...

Just curious, have you ever operated a jet engine at a high altitude airport on a hot day?
 
I think what Ted and Greg are saying is that the air available to both normally aspirated piston engines and jet engines starts to drop off immediately with DA while a turbocharger will provide a piston engine with the same amount of air up to a certain altitude, so the effects of DA are not as noticeable.
 
jpower said:
Also the little model ones are all centrifugal that I know of. But most of the decently-sized ones are axial flow, no?
Click to expand...

Correct. As with anything, there are compromises that make each type of compressor more optimal for particular configurations.

us AAirways said:
How many hrs do airline engines usually last for?
Click to expand...

There's a difference between "last" and "on-wing." Like with other engines, they can be overhauled or have appropriate parts replaced, typically at a shop (i.e. Off-wing). Common on-wing times I've seen are 25k hours for mature engines.

Henning said:
True with the TPE-331. I'm just trying to relate it to stages of compression since there's no real other way to compare a turbine with either a naturally aspirated or supercharged recip in that type of analogy. A supercharged recip has multiple stages of compression before combustion as does an axial flow turbine. Most centrifugal turbines have a single stage of compression before combustion as does a naturally aspirated recip. Regardless of of NA recip, SC recip, or turbine regardless of type, they will all be subject to the variations in initial atmospheric pressure as to what their terminal power will be. That was my only point. The only way to escape that is to carry along the oxygen in a tank like a rocket.
Click to expand...

The issue I think is that people are trying to compare turbine and piston engines in a somewhat direct manner, which isn't a good way to do it. About all they have in common is they both burn fuel and eventually crate some sort of horsepower that turns into thrust. Otherwise, they work very differently. But, turbines are naturally aspirated.

A few years ago, I didn't understand this very well, either. But being immersed in them helps.

Everskyward said:
I think what Ted and Greg are saying is that the air available to both normally aspirated piston engines and jet engines starts to drop off immediately with DA while a turbocharger will provide a piston engine with the same amount of air up to a certain altitude, so the effects of DA are not as noticeable.
Click to expand...

Yep.
 
Ted DuPuis said:
There's a difference between "last" and "on-wing." Like with other engines, they can be overhauled or have appropriate parts replaced, typically at a shop (i.e. Off-wing). Common on-wing times I've seen are 25k hours for mature engines.
Click to expand...

There is also life after flying. As a pup engineer at a steam turbine generating facility, we also had a ~100 MW gas turbine peaking unit that was built around overhauled airline engines (these days they'd be combined-cycle, but still a GT core). Yes, electric power is important, but not quite the mission-critical 'six nines' operation, and we had a great deal more redundancy, both within the unit and grid-wide. I don't recall what the age of the engines was, either in the air or on the ground.
 
RotorAndWing said:
Just curious, have you ever operated a jet engine at a high altitude airport on a hot day?
Click to expand...
Sure have. Most recently Bogota (8,361') and Medellin (7,027'), Colombia in a Hawker800XP. High enough? I forget actual temps, but they were above standard. OBVIOUSLY there is less thrust available. I am only questioning applying the term "normally aspirated" to a jet engine since the air is compressed mechanically before combustion. Suck, SQUEEZE, bang, blow.
 
Great Video. Looks like FUN!!

I did a Mountain Flying class a few years ago and landed in Los Alamos (KLAM) 7171 Alt. One way in and one way out. A large building is at the end of the runway and you feel like you are going to leave tire tracks on the roof. Oh and the fence on the south side of the airport is Restricted Airspace.

jhausch said:
^^^Better Video :yikes:

Someone needs to make a composite of both...
Click to expand...
 
Lance F said:
Sure have. Most recently Bogota (8,361') and Medellin (7,027'), Colombia in a Hawker800XP. High enough? I forget actual temps, but they were above standard. OBVIOUSLY there is less thrust available. I am only questioning applying the term "normally aspirated" to a jet engine since the air is compressed mechanically before combustion. Suck, SQUEEZE, bang, blow.
Click to expand...

Actually, the suck squeeze bang blow makes the comparison simpler.

In a turbine, squeeze is performed by the multiple compressor stages, in a piston, it's a the piston moving up.

A turbo piston engine would be more like:

Squish, suck, squeeze, bang, blow.

The squish being the turbocharger.
 
Ted DuPuis said:
Actually, the suck squeeze bang blow makes the comparison simpler.

In a turbine, squeeze is performed by the multiple compressor stages, in a piston, it's a the piston moving up.

A turbo piston engine would be more like:

Squish, suck, squeeze, bang, blow.

The squish being the turbocharger.
Click to expand...

jet3b.jpg
 
Bill Jennings said:
Yes, but would not a ramjet be equivalent to an NA piston and a turbojet equivalent to a turbo piston?

:D
Click to expand...

Nope. Ramjets are a whole 'nother kettle of fish.
 
OK, I give up. OTOH I don't think we'll ever hear of Gulfstream calling a G650 "normally aspirated." :D
 
Lance F said:
OK, I give up. OTOH I don't think we'll ever hear of Gulfstream calling a G650 "normally aspirated." :D
Click to expand...

I give up, too.

As for the marketing department, since when have they ever been truthful about the laws of physics? :D
 
Lance F said:
Sure have. Most recently Bogota (8,361') and Medellin (7,027'), Colombia in a Hawker800XP. High enough? I forget actual temps, but they were above standard. OBVIOUSLY there is less thrust available. I am only questioning applying the term "normally aspirated" to a jet engine since the air is compressed mechanically before combustion. Suck, SQUEEZE, bang, blow.
Click to expand...

Same with a piston engine. It's the air pressure at the beginning (intake) that makes the difference.
 
Everskyward said:
I think what Ted and Greg are saying is that the air available to both normally aspirated piston engines and jet engines starts to drop off immediately with DA while a turbocharger will provide a piston engine with the same amount of air up to a certain altitude, so the effects of DA are not as noticeable.
Click to expand...

I let those with the ability to explain better than me do the heavy lifting as far as explanation, but "Yep".
 
Everskyward said:
I think what Ted and Greg are saying is that the air available to both normally aspirated piston engines and jet engines starts to drop off immediately with DA while a turbocharger will provide a piston engine with the same amount of air up to a certain altitude, so the effects of DA are not as noticeable.
Click to expand...

^^^^^This says it well, I think. Makes sense to me...
Are their turbines where they must be "held back" at sea level? Is that where I hear them talking about thermodynamic hp? If so, I suppose one could say they have a turbine with supercharging ability....?

Here's my take:

Normally Aspirated
Normally: under normal or usual conditions; as a rule.
Aspirate: draw (fluid) by suction (from a vessel or cavity)*.

Turbo (or Turbo Supercharging): Using a turbine or turbine like device to "charge" above normal.
 
jhausch said:
Here's my take:

Normally Aspirated
Normally: under normal or usual conditions; as a rule.
Aspirate: draw (fluid) by suction (from a vessel or cavity)*.

Turbo (or Turbo Supercharging): Using a turbine or turbine like device to "charge" above normal.
Click to expand...

Now, with the above in mind, explain turbonormalizing. ;)
 
jhausch said:
^^^^^This says it well, I think. Makes sense to me...
Are their turbines where they must be "held back" at sea level? Is that where I hear them talking about thermodynamic hp? If so, I suppose one could say they have a turbine with supercharging ability....?

Here's my take:

Normally Aspirated
Normally: under normal or usual conditions; as a rule.
Aspirate: draw (fluid) by suction (from a vessel or cavity)*.

Turbo (or Turbo Supercharging): Using a turbine or turbine like device to "charge" above normal.
Click to expand...

There are turbines that have to be "held back". We typically refer to that as a "flat rating", basically saying the engine can produce more, but we're not letting it for various reasons. Think of it like having a manifold pressure restriction.

I think the way you have it is pretty much accurate, and is why turbine engines aren't turbocharged.
 
You must log in or register to reply here.
Back
Top