LASoctoberB6
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Fan Blades

Sun May 27, 2007 9:50 am

can jet engines use the same concept of prop jets in terms of blade deflection on reverse thrust? ie a J31s fan blades can move to different angles to change the direction of air...why cant jets do that? sorry if that made no sense....
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411A
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RE: Fan Blades

Sun May 27, 2007 10:41 am

In short, too many blades and the requirements for a very complicted hub assembly...an engine cold stream thrust reverse assembly at the rear is far more cost effective and slightly lighter in weight.

Of course, it can fail as well, or as in the case recently in FRA, fall off altogether.
 
bio15
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RE: Fan Blades

Sun May 27, 2007 5:27 pm

If the compressor section of a turbofan engine blew forward, no air would get to the combustion chamber. Or at least not in the right direction:

Jet engines are designed to flow in only one direction. That means that the blades, spark plugs, combustion chambers and such, are shaped and positioned for the engine to flow rearward only. Even the 'jet engine' section of a turboprop flows in one direction. The big difference is that the fan section of a turbofan is cased inside the cowling and it represents a vital part of the air supply to the compressor-combustion sections. If you would alter the pitch of these blades then the air supply would be either insufficient or excessive and the engine just doesn't work as it should. The fan generates thrust and sucks the required air at the same time.

On the other hand, the engine section of a turboprop is significantly independent of the propeller itself. The propeller is connected to the engine turbine by a shaft just as the turbofan, but the propeller is not a vital part of air supply to the engine. You can change the propeller pitch and the engine would go on running independently providing the necessary torque that the propeller is demanding.

Most turboprop engines are "free shaft", which means that one part of the turbine section of the engine is shafted to the outside of the engine and this turbine section and shaft rotate freely. You can use that shaft to move anything, and the engine will run independently. These engines have other aplications; attaching a propeller to the shaft is just one of them.

Alfredo

[Edited 2007-05-27 10:57:44]
 
tf39
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RE: Fan Blades

Mon May 28, 2007 11:00 am

Maybe the closest thing to this is the Unducted Fan (UDF). The UDF has variable blades but I'm not sure to what extent (of power) they provide reverse thrust.


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There's also this bad boy Prop Fan as well from the AN-70:


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Just throw a duct around the fan blades  Wink
 
Blackbird
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RE: Fan Blades

Mon May 28, 2007 11:08 am

That AN-70 with the Propfan looks awesome!

Andrea Kent
 
Blackbird
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RE: Fan Blades

Mon May 28, 2007 11:21 am

I asked this question in the chatroom yesterday, I think, and I think it warrants a Tech-Ops Post, so here it goes: Is it possible to have a variable-cycle engine with (in addition to variable guide-vanes) a variable-pitch fan and some variable-pitch blades?

Also... since blades have AoA limits, is it also possible to have the leading-edge of the compressor blade droopable (to increase each blades "lift"), perhaps configured in such a way that as the blade pitch changes, the droop adjusts to allow it to operate at the higher AoA?

I'd love to hear your responses,

A Kent
 
2H4
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RE: Fan Blades

Mon May 28, 2007 11:50 am




Quoting TF39 (Reply 3):
The UDF has variable blades but I'm not sure to what extent (of power) they provide reverse thrust.

That's a good question. Does anyone here know the answer?


2H4

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zenarcade
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RE: Fan Blades

Mon May 28, 2007 12:34 pm

"Wind tunnel testing demonstrates that a counterrotating propfan can deliver reverse thrust levels up to 60% of takeoff thrust (Groeneweg,Bober, 1988). A typical turbofan engine can only provide between 40-50% of takeoff thrust using thrust reverser hardware."
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2H4
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RE: Fan Blades

Mon May 28, 2007 12:48 pm




Quoting Zenarcade (Reply 7):

Bingo. Thank you, Zenarcade.


2H4

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jetmech
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RE: Fan Blades

Mon May 28, 2007 1:45 pm

Quoting Blackbird (Reply 5):
variable-cycle engine...... is it also possible to have the leading-edge of the compressor blade droopable

Anything is possible (almost), but is it worth the tradeoff? Many ground based turbines use intercooling and / or preheating to increase the thermodynamic efficiency of the cycle, but the tradeoff is not worth it with an aircraft gas turbine. The increased weight and drag caused by an aircraft mounted intercooled and preheated gas turbine would more than likely negate any thermal efficiency benefit. The only variable cycle engine that I can think of is the ramjet / turbojet configuration of the J-57's fitted to the SR-71.

Ditto for leading edge droops on fan blades. Gas turbines are optimised for cruise, so the added complexity, weight and expense of drooped fan blades would only be of benefit for the small amount of time when the engine is operating off cruise conditions. I'm sure it is technically feasible, but economically unjustifiable.

Pretty much the entire compressor section of a CF6 or JT9 is fitted with variable stator vanes (VSV's) which is complex, costly and heavy, but in this case, the VSV's are pretty much essential to prevent compressor stall at certain operating points. Older RR's used to have VIGV's only, but IIRC, the newer Trents have VIGV's and two stages of VSV's. Again, it adds expense, complexity and weight, but the tradeoff of stall / surge free compressor operation is more than worth it.

Regards, JetMech
JetMech split the back of his pants. He can feel the wind in his hair :shock: .
 
Blackbird
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RE: Fan Blades

Tue May 29, 2007 2:21 am

Jetmech,

How are intercooler and preheating systems used on ground-based turbines?

BTW: The engine concept I had thought of was a supersonic (Mach 3 or so) variable-cycle engine, which involved a variable fan-bypass at low-speed to provide turbofan performance, and a fan or core-bypass at high-speed into an outer-annular combustion chamber to produce high-velocity airflow, while avoiding loss in pressure ratio during the bypasses...

Some of the variable-geometry components would be used during the cruise portion of the flight.


Andrea Kent
 
Grunf
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RE: Fan Blades

Tue May 29, 2007 4:46 pm

Quoting Blackbird (Reply 10):


How are intercooler and preheating systems used on ground-based turbines?


Here are some links that explaiin the concept more torroughly:
http://www.benwiens.com/energy2.html#energy1.13
http://www.energysolutionscenter.org/DistGen/Tutorial/CombTurbine.htm

This schematics is particulary good:
http://www.energysolutionscenter.org...Images/TurbinePartsandOptions.jpeg

[Edited 2007-05-29 09:52:49]
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F14D4ever
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RE: Fan Blades

Wed May 30, 2007 9:15 pm

Quoting TF39 (Reply 3):
The UDF has variable blades but I'm not sure to what extent (of power) they provide reverse thrust.



Quoting 2H4 (Reply 6):
That's a good question. Does anyone here know the answer?

Once upon a time I developed the code to calculate GE36 (UDF tm) thermodynamic performance, including reverse thrust. I can't give numbers, but I just looked up the component map evaluation subroutine and can verify that it did in fact have thrust reversing capability.

I know whom to ask.
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prebennorholm
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RE: Fan Blades

Thu May 31, 2007 9:12 am

A "reversed" fan on a turbofan would certainly be the most certain way to create a compressor stall in the core. No variable anything could prevent that.

A turboprop is a different sort of animal. The dynamic forces - on an aircraft wing or a propeller - increase with the square of the speed. That means that the thrust or braking force created near the propeller root, where the engine intake is placed, is practically zero. The bulk of the thrust is created by the 20% outmost propeller tips.

In addition many popular turboprop engined are "reversed flow" engines. The air intake is in the back of the engine, and the turbines are up front - makes the gear box assembly easier to produce. The engine gas flow is turned 180 degrees both at the intake and exhaust.

And most often they have at least one centrifugal compressor stage. A centrifugal compressor is, unlike an axial compressor, impossible to stall.

Quoting JetMech (Reply 9):
The only variable cycle engine that I can think of is the ramjet / turbojet configuration of the J-57's fitted to the SR-71.

Dear JetMech, I guess that you mean the J-58 engine, not J-57. You were better than 98% right, not bad at all.  Wink
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Starlionblue
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RE: Fan Blades

Thu May 31, 2007 9:27 am

Quoting Prebennorholm (Reply 13):

In addition many popular turboprop engined are "reversed flow" engines. The air intake is in the back of the engine, and the turbines are up front - makes the gear box assembly easier to produce. The engine gas flow is turned 180 degrees both at the intake and exhaust.

I guess it is more efficient or they wouldn't be doing it. But it seems like a lot of energy is wasted turning the air around at both ends...
"There are no stupid questions, but there are a lot of inquisitive idiots." - John Ringo
 
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jetmech
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RE: Fan Blades

Thu May 31, 2007 10:50 am

Quoting Prebennorholm (Reply 13):
Dear JetMech, I guess that you mean the J-58 engine, not J-57. You were better than 98% right, not bad at all.

I knew I should have looked that one up before posting  blush  !



http://www.wvi.com/~sr71webmaster/j58airflow.jpg

Regards, JetMech
JetMech split the back of his pants. He can feel the wind in his hair :shock: .
 
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Jetlagged
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RE: Fan Blades

Thu May 31, 2007 11:58 am

I think the Olympus 593 (Concorde) is just as much a variable cycle engine as the J58 (the intake ramps being equivalent to the spike). The system of intake ramps and doors was extremely complex and essential to optimum performance.

The complex intake and nozzle arrangements of all supersonic jet engines mean all of them run differently subsonic and supersonic.
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BreninTW
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RE: Fan Blades

Thu May 31, 2007 12:21 pm

Quoting 411A (Reply 1):
Of course, it can fail as well, or as in the case recently in FRA, fall off altogether.

Must have missed that one -- what happened?

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