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Fan Blades  
User currently offlineLASOctoberB6 From Japan, joined Nov 2006, 2380 posts, RR: 1
Posted (7 years 3 months 6 days 2 hours ago) and read 4044 times:

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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17 replies: All unread, jump to last
 
User currently offline411A From United States of America, joined Nov 2001, 1826 posts, RR: 8
Reply 1, posted (7 years 3 months 6 days 1 hour ago) and read 4025 times:

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.


User currently offlineBio15 From Colombia, joined Mar 2001, 1089 posts, RR: 7
Reply 2, posted (7 years 3 months 5 days 18 hours ago) and read 3974 times:

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]

User currently offlineTF39 From United States of America, joined Jul 2006, 110 posts, RR: 0
Reply 3, posted (7 years 3 months 5 days 1 hour ago) and read 3827 times:

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


User currently offlineBlackbird From , joined Dec 1969, posts, RR:
Reply 4, posted (7 years 3 months 5 days ago) and read 3821 times:

That AN-70 with the Propfan looks awesome!

Andrea Kent


User currently offlineBlackbird From , joined Dec 1969, posts, RR:
Reply 5, posted (7 years 3 months 5 days ago) and read 3819 times:

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


User currently offline2H4 From United States of America, joined Oct 2004, 8955 posts, RR: 60
Reply 6, posted (7 years 3 months 5 days ago) and read 3808 times:
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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




Intentionally Left Blank
User currently offlineZenarcade From Canada, joined Nov 2006, 85 posts, RR: 0
Reply 7, posted (7 years 3 months 4 days 23 hours ago) and read 3785 times:

"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."


If a plane falls on the tarmac and no one is there, does it make any sound? - Starlionblue
User currently offline2H4 From United States of America, joined Oct 2004, 8955 posts, RR: 60
Reply 8, posted (7 years 3 months 4 days 23 hours ago) and read 3779 times:
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Quoting Zenarcade (Reply 7):

Bingo. Thank you, Zenarcade.


2H4




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User currently offlineJetMech From Australia, joined Mar 2006, 2687 posts, RR: 53
Reply 9, posted (7 years 3 months 4 days 22 hours ago) and read 3760 times:

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.
User currently offlineBlackbird From , joined Dec 1969, posts, RR:
Reply 10, posted (7 years 3 months 4 days 9 hours ago) and read 3647 times:

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


User currently offlineGrunf From Sweden, joined Jan 2007, 54 posts, RR: 0
Reply 11, posted (7 years 3 months 3 days 19 hours ago) and read 3523 times:

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]


Drink more milk, less kerosene!
User currently offlineF14D4ever From United States of America, joined May 2005, 319 posts, RR: 4
Reply 12, posted (7 years 3 months 2 days 14 hours ago) and read 3393 times:

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.



"He is risen, as He said."
User currently offlinePrebennorholm From Denmark, joined Mar 2000, 6429 posts, RR: 54
Reply 13, posted (7 years 3 months 2 days 2 hours ago) and read 3335 times:

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



Always keep your number of landings equal to your number of take-offs, Preben Norholm
User currently offlineStarlionblue From Greenland, joined Feb 2004, 17027 posts, RR: 67
Reply 14, posted (7 years 3 months 2 days 2 hours ago) and read 3332 times:

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."
User currently offlineJetMech From Australia, joined Mar 2006, 2687 posts, RR: 53
Reply 15, posted (7 years 3 months 2 days 1 hour ago) and read 3315 times:

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.
User currently offlineJetlagged From United Kingdom, joined Jan 2005, 2546 posts, RR: 24
Reply 16, posted (7 years 3 months 2 days ago) and read 3301 times:

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.



The glass isn't half empty, or half full, it's twice as big as it needs to be.
User currently offlineBrenintw From Taiwan, joined Jul 2006, 1633 posts, RR: 1
Reply 17, posted (7 years 3 months 1 day 23 hours ago) and read 3294 times:

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?



I'm tired of the A vs. B sniping. Neither make planes that shed wings randomly!
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