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ATA1011Tristar
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Turbojet Sizes In Turbofans?

Wed Mar 05, 2008 12:55 am

Hello Everyone!

I have always enjoyed seeing NW's DC-9's, among other aircraft, buzzing around the skies of Minneapolis. Seeing these airplanes in the sky and takeoff at MSP has prompted a question in me. My question has to do with bypass ratios. I noticed the DC-9-50 has an engine thrust rating of approximately 16,000 lbf per engine. I also found that the E190 has an engine thrust rating of 18,500 lbf per engine. I compare these two planes on engine thrust ratings only, not size or economics. With these stats in mind, one can say that these two engines have an approximately equal thrust rating.
The main difference is found in the bypass ratios. I don't know the exact bypass ratios, but I would not be surprised if the E190's was over double the bypass ratio of the DC-9. I know high bypass turbofans develop most of their thrust with their fans bypassing air. Yet, the DC-9 almost makes the same amount of thrust as the E190. Finally, my question:

Is the turbojet engine in the DC-9, turning a smaller fan, larger than the turbojet engine, turning a bigger fan, in the E-190?

It seems like it must be if the DC-9 has a smaller bypass fan, yet similar thrust.

Thanks!
 
AirEMS
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RE: Turbojet Sizes In Turbofans?

Wed Mar 05, 2008 12:59 am

I think that you might be in the wrong forum..... My guess is if you post in the Tech/Ops Forum you could get a zillion answers!!! again just an Idea


-Carl
If Your Dying Were Flying
 
speedracer1407
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RE: Turbojet Sizes In Turbofans?

Wed Mar 05, 2008 2:48 am

I think what you mean by "turbojet engine" in your post is the "core" of the turboFAN engine. Both the JT8D on the DC-9 and the CF34 on the E-190 are turbofan engines, but the difference, as you noticed, is the bypass ratio.

So while I'm no aero-engine engineer, I'm fairly certain that the JT8D's core is significantly larger than the CF34's, which is a big reason why the CF34 is much more fuel efficient: small core turning big fan for the same (or more as installed on the E-190) power as the old JT8D.
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Starlionblue
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RE: Turbojet Sizes In Turbofans?

Wed Mar 05, 2008 4:50 am



Quoting ATA1011Tristar (Thread starter):
Is the turbojet engine in the DC-9, turning a smaller fan, larger than the turbojet engine, turning a bigger fan, in the E-190?

As Speedracer1407 mentions, those aren't turbojets, they are cores of turbofans. But yes, there are wide ranging similarities.  Wink

Anyway as he also mentions the bypass ratio (air bypassing core divided by air traversing core) of the CF34 is significantly higher. So yes, the JT8D has a proportionally larger core and smaller fan. IIRC the bypass ratio is around 1:1. Modern turbofans frequently exceed 10:1.
"There are no stupid questions, but there are a lot of inquisitive idiots." - John Ringo
 
ATA1011Tristar
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RE: Turbojet Sizes In Turbofans?

Fri Mar 07, 2008 8:04 pm

Thanks for the replies everyone!

I am glad to have the turbojet/core issue resolved. I hesitated to call the core of a turbofan a turbojet because a turbojet is not part of an engine, but rather an engine in its entirety. I just couldn't think of a better word  Smile.

Another question I have:

Is there an equation to take an engine's thrust rating and bypass ratio and find out how much thrust comes from the core and bypass fan respectively?
 
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Starlionblue
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RE: Turbojet Sizes In Turbofans?

Fri Mar 07, 2008 11:52 pm



Quoting ATA1011Tristar (Reply 4):

Is there an equation to take an engine's thrust rating and bypass ratio and find out how much thrust comes from the core and bypass fan respectively?

Not sure. The bypass ratio and the proportion of thrust that comes from the fan are of course related but there are other factors like blade shape and size that come into play.
"There are no stupid questions, but there are a lot of inquisitive idiots." - John Ringo
 
thegeek
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RE: Turbojet Sizes In Turbofans?

Sat Mar 08, 2008 12:36 am



Quoting ATA1011Tristar (Reply 4):
Is there an equation to take an engine's thrust rating and bypass ratio and find out how much thrust comes from the core and bypass fan respectively?

I would doubt it. If you didn't have a mixed flow nozzle, it would be likely that the bypass jet would be slower than the core jet, and therefore provide a lower proportion of the thrust at higher speeds as compared to the start of the take off run.

One further comment, excluding the JT8D-2xx series engines, most low bypass turbofans have multi stage fans, while high bypass turbofans only use a single stage fan. (or at least that's my understanding)
 
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Jetlagged
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RE: Turbojet Sizes In Turbofans?

Sat Mar 08, 2008 12:54 am



Quoting Thegeek (Reply 6):
I would doubt it. If you didn't have a mixed flow nozzle, it would be likely that the bypass jet would be slower than the core jet, and therefore provide a lower proportion of the thrust at higher speeds as compared to the start of the take off run.

As an aircraft accelerates, the difference between the core jet velocity and aircraft velocity decreases. Fan velocity will always be aircraft velocity plus a small increment. So at higher speeds, fan air speed and core jet speed will get closer together, so the fan is always providing significant thrust.
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thegeek
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RE: Turbojet Sizes In Turbofans?

Sat Mar 08, 2008 2:03 am



Quoting Jetlagged (Reply 7):

So, you believe that the bypass thrust doesn't lapse as speed increases. Are you saying that the air in the bypass duct is always running at least as fast as the aircraft's air speed?

Still means that you can't have an equation to determine the proportion of thrust from the bypass flow vs core flow, unless you know the core jet velocity, bypass jet velocity and the aircraft's velocity relative to the air.
 
ex52tech
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RE: Turbojet Sizes In Turbofans?

Sat Mar 08, 2008 3:17 am



Quoting Thegeek (Reply 6):
most low bypass turbofans have multi stage fans, while high bypass turbofans only use a single stage fan. (or at least that's my understanding)

Yup, the low bypass fan engines do have multistage fans, or at least the older ones were developed that way. JT3D, and JT8D have two stage fans.
The F404,F110, and F136 for the F-35 all have three stage fans.
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tdscanuck
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RE: Turbojet Sizes In Turbofans?

Sat Mar 08, 2008 3:39 am



Quoting ATA1011Tristar (Reply 4):
Is there an equation to take an engine's thrust rating and bypass ratio and find out how much thrust comes from the core and bypass fan respectively?

Not using those two parameters alone. You'd also need the velocity increment across the fan and core.

Tom.
 
ATA1011Tristar
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RE: Turbojet Sizes In Turbofans?

Sat Mar 08, 2008 4:02 am

Quoting Starlionblue (Reply 5):
Not sure. The bypass ratio and the proportion of thrust that comes from the fan are of course related but there are other factors like blade shape and size that come into play.

So, for example, you could have a very large fan in a turbofan engine leading to a high bypass ratio. The fan could be terrible at moving air though, so it would not create as much thrust as one would think it should for its size. This inefficiency in moving air would then throw off any assumption regarding what percentage of thrust out of the total the fan would create?

Jetlagged, Thegeek, & Tdscanuck:

Let me make sure I got this right. The percentage of the total thrust created by either the fan or the core would be altered by the speed of the plane because it would increase or decrease the difference between the core or fan exhaust speed and the plane's speed? If so, that is some complicated stuff! What you said would seem to make sense, though.

Everyone:

It does not matter that there is no equation. I asked about the equation only because I wanted to use it to compare the core of the JT8D to other plane's cores. Instead, I shall ask an easier question:

Is it possible that the JT8D has a bigger core than does the PW2000 or other engines with high bypass ratios?

Thanks!

Edited: Tdscanuck posted while I was writing my post, and I did not see his before I posted.

[Edited 2008-03-07 20:06:17]
 
DeltaGuy
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RE: Turbojet Sizes In Turbofans?

Sat Mar 08, 2008 4:38 am



Quoting Ex52tech (Reply 9):
Yup, the low bypass fan engines do have multistage fans, or at least the older ones were developed that way. JT3D, and JT8D have two stage fans.
The F404,F110, and F136 for the F-35 all have three stage fans.

Don't forget the F100 on the F-15 and F-16.

Low bypass, more power for fighter-type operations...putting an engine such as a CFM-56 on an Eagle wouldn't help it much...too much bypass!

DeltaGuy
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Starlionblue
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RE: Turbojet Sizes In Turbofans?

Sat Mar 08, 2008 10:07 am



Quoting ATA1011Tristar (Reply 11):
So, for example, you could have a very large fan in a turbofan engine leading to a high bypass ratio. The fan could be terrible at moving air though, so it would not create as much thrust as one would think it should for its size. This inefficiency in moving air would then throw off any assumption regarding what percentage of thrust out of the total the fan would create?

Well, I wouldn't use the word "terrible".  Wink I would say that modern fans are better designed for moving air efficiently given new materials and new shapes.
"There are no stupid questions, but there are a lot of inquisitive idiots." - John Ringo
 
tdscanuck
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RE: Turbojet Sizes In Turbofans?

Sat Mar 08, 2008 11:30 pm



Quoting ATA1011Tristar (Reply 11):
The percentage of the total thrust created by either the fan or the core would be altered by the speed of the plane because it would increase or decrease the difference between the core or fan exhaust speed and the plane's speed?

I think that's correct. The pressure at the inlet is a non-linear function of speed, and the pressure at the fan face and core face is going to be different (since the core is behind the fan).

Tom.
 
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Jetlagged
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RE: Turbojet Sizes In Turbofans?

Sun Mar 09, 2008 12:49 am



Quoting Thegeek (Reply 8):
So, you believe that the bypass thrust doesn't lapse as speed increases. Are you saying that the air in the bypass duct is always running at least as fast as the aircraft's air speed?

I never said it didn't lapse, but I suggest core thrust lapses faster. Of course fan air flow is always faster than aircraft forward speed, otherwise the bypass air would be negative relative to the aircraft.

You're right that there is no single equation to calculate the ratio between core and bypass thrust for all conditions. You need to know all the velocities and mass flows.
The glass isn't half empty, or half full, it's twice as big as it needs to be.
 
thegeek
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RE: Turbojet Sizes In Turbofans?

Mon Mar 10, 2008 2:56 am



Quoting Jetlagged (Reply 15):
otherwise the bypass air would be negative relative to the aircraft.

Unless it was accelerated by the nozzle. I always assumed that the air in the bypass duct was moving forward relative to the outside air, as it had been slowed down by the inlet (but gained in pressure). What you're telling me is that a turbofan works more like a UDF, only difference being the duct.
 
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Jetlagged
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RE: Turbojet Sizes In Turbofans?

Mon Mar 10, 2008 11:46 am



Quoting Thegeek (Reply 16):
Unless it was accelerated by the nozzle. I always assumed that the air in the bypass duct was moving forward relative to the outside air, as it had been slowed down by the inlet (but gained in pressure). What you're telling me is that a turbofan works more like a UDF, only difference being the duct.

You can calculate thrust in terms of momentum change:

Fan Net Thrust = mass flow * (outlet velocity - freestream velocity)

If the air was moving forward relative to the outside air the fan would be producing negative thrust. The fan does a lot of work on the air, increasing pressure which can be converted into velocity by the shape of the duct.
The glass isn't half empty, or half full, it's twice as big as it needs to be.
 
thegeek
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RE: Turbojet Sizes In Turbofans?

Mon Mar 10, 2008 12:38 pm



Quoting Jetlagged (Reply 17):
If the air was moving forward relative to the outside air the fan would be producing negative thrust

Not if it subsequently was accelerated by the nozzle! I understand all this, but I'm still none the wiser. My understanding previously had been that: the inlet decelated the intake air (raising its pressure), the fan compressed it a bit more, then the nozzle expanded it again back to a speed higher than the aircraft's speed.

Quoting Jetlagged (Reply 17):
increasing pressure which can be converted into velocity by the shape of the duct.

I'd call that a nozzle, not a duct.

Quoting ATA1011Tristar (Reply 11):
Is it possible that the JT8D has a bigger core than does the PW2000 or other engines with high bypass ratios?

It's possible in theory, but I don't think it is likely in practice.
 
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Jetlagged
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RE: Turbojet Sizes In Turbofans?

Mon Mar 10, 2008 10:27 pm



Quoting Thegeek (Reply 18):
Not if it subsequently was accelerated by the nozzle! I understand all this, but I'm still none the wiser. My understanding previously had been that: the inlet decelated the intake air (raising its pressure), the fan compressed it a bit more, then the nozzle expanded it again back to a speed higher than the aircraft's speed.

True enough. However, the important velocity for thrust calculation is the air velocity at the exit of the bypass duct (yes it's nozzle shaped but it's still a duct), which we agree is faster than freestream air and always will be if the fan is producing thrust. Internal velocity variations are irrelevant. The overall velocity change is the key.

The fan may not raise the bypass air pressure much but it does an awful lot of work on it, increasing it's energy. That is the source of fan thrust, although it manifests itself in terms of momentum change.
The glass isn't half empty, or half full, it's twice as big as it needs to be.
 
tdscanuck
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RE: Turbojet Sizes In Turbofans?

Tue Mar 11, 2008 3:41 am



Quoting Thegeek (Reply 16):
I always assumed that the air in the bypass duct was moving forward relative to the outside air, as it had been slowed down by the inlet (but gained in pressure). What you're telling me is that a turbofan works more like a UDF, only difference being the duct.

It may be at very high cruise speeds. A turbofan does work like a UDF with a duct, except that the pressure at the fan face is a little higher because of the inlet.

Quoting Thegeek (Reply 18):
the inlet decelated the intake air (raising its pressure), the fan compressed it a bit more, then the nozzle expanded it again back to a speed higher than the aircraft's speed.

The fan compresses it a lot more (more than the inlet, I believe), but your general schema is correct.

Quoting Thegeek (Reply 18):
Quoting Jetlagged (Reply 17):
increasing pressure which can be converted into velocity by the shape of the duct.

I'd call that a nozzle, not a duct.

In aerodynamics, a duct is just the thing around any internal flow. A nozzle is a structure the accelerates the flow (opposite of a diffuser). In normal jet-engine speak, the bypass flow goes through the fan duct. The nozzle is the thing directing the core flow. That's just convention though...they're both ducts, and they're both nozzles.

Tom.
Tom.

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