sstsomeday
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What About A Variable-pitch Turbofan Engine?

Fri May 07, 2010 7:56 am

True, a jet engine has more blades than a prop A/C has propellers, and the cowl encases the blades... but beyond that, it seems to me that the principal of thrust is basically the same.

I am wondering if the fixed position for jet engine blades makes for less efficiency. Is the difference essentially the cowl, which allows the air to be forced/constricted through the blades, whereas the propeller does not have that advantage?

Or would the complexity of a variable-pitch jet engine design prohibit any performance advantage?

Further, the blades behind the jet blast (the ones that drive the larger turbofans up front) - what if THEY were variable pitch? Wouldn't that offer an efficiency advantage between take-off mode and cruise mode?

Clearly this would have already been invented by engineers if it would work so I guess I'm asking... why doesn't it?
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Faro
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RE: What About A Variable-pitch Turbofan Engine?

Fri May 07, 2010 8:54 am

Just a guess but I would think that the added weight of the actuation mechanism for the rotation of the (fan) blades would require a significantly larger containment ring around the fan assembly, which would drive up the weight of other engine components in a cascading effect.

Rotating turbine blades is a very interesting concept; simply, given the extreme thermal and mechanical stress that they are already designed to resist *in a fixed position*, I don't know if it's possible to ensure the same safety and reliability criteria in more than one turbine angle geometry. Maybe not today but perhaps in the future?

Faro
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thegeek
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RE: What About A Variable-pitch Turbofan Engine?

Fri May 07, 2010 10:10 am

I think variable geometry stators achieves more or less the same effect.
 
Fly2HMO
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RE: What About A Variable-pitch Turbofan Engine?

Fri May 07, 2010 8:34 pm

Quoting SSTsomeday (Thread starter):
Clearly this would have already been invented by engineers if it would work so I guess I'm asking... why doesn't it?

Oh it's been invented alright:

http://en.wikipedia.org/wiki/Turbomeca_Astafan

What Is A Turbomeca Astafan? (by KELPkid Feb 25 2010 in Tech Ops)
 
sstsomeday
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RE: What About A Variable-pitch Turbofan Engine?

Fri May 07, 2010 8:44 pm

Quoting Fly2HMO (Reply 3):
Oh it's been invented alright:

http://en.wikipedia.org/wiki/Turbomeca_Astafan

What Is A Turbomeca Astafan?

Thanks - very interesting.
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aeroweanie
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RE: What About A Variable-pitch Turbofan Engine?

Fri May 07, 2010 8:47 pm

 
A342
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RE: What About A Variable-pitch Turbofan Engine?

Fri May 07, 2010 10:04 pm

And don't forget about the Russians:

http://motor-s.ru/NK93_en.htm

But the question is: Where do we draw the line between a turbofan and a ducted propfan?
Exceptions confirm the rule.
 
Fly2HMO
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RE: What About A Variable-pitch Turbofan Engine?

Fri May 07, 2010 11:02 pm

Quoting A342 (Reply 6):
And don't forget about the Russians:

http://motor-s.ru/NK93_en.htm
Quote:
The engine NK-93 - double-loop engine of the extra high degree of the double-looping is intended for the perspective passenger medium /long -haul aeroplanes of the large passengers seating capacity and also for the cargo-type, military and transport aeroplanes.

That has to be the best auto-translation I've ever seen!         

At least I hope that was a computer doing the translation  
 
tdscanuck
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RE: What About A Variable-pitch Turbofan Engine?

Sat May 08, 2010 3:18 am

Quoting SSTsomeday (Thread starter):
True, a jet engine has more blades than a prop A/C has propellers, and the cowl encases the blades... but beyond that, it seems to me that the principal of thrust is basically the same.

The thrust principle is exactly the same.

Quoting SSTsomeday (Thread starter):
I am wondering if the fixed position for jet engine blades makes for less efficiency

Strictly in propulsive terms, it's less efficient. In terms of overall system (weight, cost, complexity, reliability, efficiency, etc.) it's the right balance.

Quoting SSTsomeday (Thread starter):
Is the difference essentially the cowl, which allows the air to be forced/constricted through the blades, whereas the propeller does not have that advantage?

That is the major difference between a fan and prop; it basically translates to much higher disc loading.

Quoting SSTsomeday (Thread starter):
Or would the complexity of a variable-pitch jet engine design prohibit any performance advantage?

It might not negate the performance advantage, but it would almost certainly negate cost, weight, and reliability.

Quoting SSTsomeday (Thread starter):
Further, the blades behind the jet blast (the ones that drive the larger turbofans up front) - what if THEY were variable pitch? Wouldn't that offer an efficiency advantage between take-off mode and cruise mode?

The problem is that those blades are also the primary load path between the core, the fan case, and the forward engine mount. To make them also variable would be very complex, and very heavy.

Tom.
 
sstsomeday
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RE: What About A Variable-pitch Turbofan Engine?

Sat May 08, 2010 8:38 pm

Quoting tdscanuck (Reply 8):
The thrust principle is exactly the same.

Your post obviously very informed and informative.

What I'm gleaning form all these posts is that the scenario of variable pitch jet engine blades has been attempted, but due to the challenges you site, only in application to a small number of small A/C.
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packcheer
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RE: What About A Variable-pitch Turbofan Engine?

Sun May 09, 2010 5:00 am

you also have to remember that the main goal of the fan blades on a jet engine is to get air to the other stages of the engine, such as the compressor, while the main goal of a propeller is to create thrust.

The reason I say this is that a variable pitch prop can actually increase/decrease its effect on the aircraft without affecting in the engine driving said prop. That would be a very hard part in that changing the fan blades angles in a jet engine can have significant impact on the flight ops.

One thing that intrigues me however is the abiltiy to get rid of "lag" in jets. In engines like Turbo fans, there is spool up time. Pilot needs thrust, pushes lever forward, engine takes a bit of time to spool up to the desired .

With variable angle fan blades, the engine could "in theory" stay spooled up at a higher level, while reducing the angle on the fan blades. (think high RPM with a flat blade that you would find in landing of GA aircraft.)
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tdscanuck
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RE: What About A Variable-pitch Turbofan Engine?

Sun May 09, 2010 7:14 pm

Quoting SSTsomeday (Reply 9):
What I'm gleaning form all these posts is that the scenario of variable pitch jet engine blades has been attempted, but due to the challenges you site, only in application to a small number of small A/C.

I think this is an accurate statement; it's not technically impossible, just impractical for the majority of missions and applications (with current technology). If the design drivers change to put a much higher premium on efficiency, or we get some new materials or architectures that mitigate some of the negatives, it's entirely possible we'd see the idea gain traction. Engine designs change so slowly that I think we're talking multiple decades though.

Quoting Packcheer (Reply 10):
you also have to remember that the main goal of the fan blades on a jet engine is to get air to the other stages of the engine, such as the compressor, while the main goal of a propeller is to create thrust.

This isn't really true; the fan is technically the first compressor stage but only about 10% of the fan is actually feeding air to the core; the main goal of the fan blades is to generate bypass flow and thrust. On a modern high-bypass jet the core is basically a gas generator that's feeding the low-pressure turbine to power the fan to provide thrust.

Quoting Packcheer (Reply 10):
The reason I say this is that a variable pitch prop can actually increase/decrease its effect on the aircraft without affecting in the engine driving said prop. That would be a very hard part in that changing the fan blades angles in a jet engine can have significant impact on the flight ops.

This is certainly an effect that would have to be dealt with, but I'm not sure it would be "very hard". One of the technical engine guys like Lightsaber could confirm, but my gut says that proper scheduling of the low pressure compressor variable stator vanes would be sufficient to deal with the variable compression from the variable pitch fan. An alternative would be to have a fixed airfoil in the fan where it overlaps the core, and just sweep the outboard bypass portion of the blades. That would fix the operability problem, although it would make for some weird aerodynamics at the transition point.

Quoting Packcheer (Reply 10):
One thing that intrigues me however is the abiltiy to get rid of "lag" in jets. In engines like Turbo fans, there is spool up time. Pilot needs thrust, pushes lever forward, engine takes a bit of time to spool up to the desired .

With variable angle fan blades, the engine could "in theory" stay spooled up at a higher level, while reducing the angle on the fan blades. (think high RPM with a flat blade that you would find in landing of GA aircraft.)

Good point; you could eliminate the lag associated with acceleration of the N1 spool. You'd still have to wait for the N2 (and N3 if applicable) to spin up to provide the extra power as you increase the fan pitch, but those spools respond much more quickly.

Tom.
 
kurtverbose
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RE: What About A Variable-pitch Turbofan Engine?

Sat Oct 18, 2014 10:02 pm

Thread resurrection in view of the RR Ultrafan.



One thing missed from above is the thrust reverser can be ommitted with a VP fan.

Wonder what else has changed to make a VP fan advantageous now?
 
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akiss20
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RE: What About A Variable-pitch Turbofan Engine?

Sun Oct 19, 2014 4:29 am

Quoting kurtverbose (Reply 12):
One thing missed from above is the thrust reverser can be ommitted with a VP fan.

You wouldn't want to go into reverse on the fan seeing as you will completely stall the core.
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737tdi
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RE: What About A Variable-pitch Turbofan Engine?

Sun Oct 19, 2014 4:45 am

To be honest I don't know why you would want a variable fan set. 90% of the time the engine is running at one +/- power setting. Short haul this is less but for most it is true. Once the aircraft is in cruise there are little to no throttle/engine speed settings. There may be slight changes but they are minimal. Engines are optimized for cruise with a built in performance enhancement for takeoff. Variable stator vanes and variable bleed valves to enhance take off performance. By the way that is not a copy/paste, just from my little brain. Engine engineers look at all of this and variable fan blades aren't in the list of things needed. I just can't see them ever being a viable solution for power or performance. To what I said before, these engines are optimized for cruise flight.
 
kurtverbose
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RE: What About A Variable-pitch Turbofan Engine?

Sun Oct 19, 2014 6:56 am

Quoting akiss20 (Reply 13):
You wouldn't want to go into reverse on the fan seeing as you will completely stall the core.

I think it doesn't actually go into reverse, just acts as an aerodynamic brake so the core can still work.
 
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akiss20
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RE: What About A Variable-pitch Turbofan Engine?

Sun Oct 19, 2014 2:40 pm

Quoting kurtverbose (Reply 15):
I think it doesn't actually go into reverse, just acts as an aerodynamic brake so the core can still work.

The pumping capability would be basically completely destroyed. Even at idle I doubt the fan could deliver the corrected flow for the core to keep functioning.
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kurtverbose
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RE: What About A Variable-pitch Turbofan Engine?

Sun Oct 19, 2014 4:52 pm

It's been done before with a bypass duct, as posted earlier in the thread. Anyway, I'm sure RR have given it a thought.
 
Aircellist
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RE: What About A Variable-pitch Turbofan Engine?

Sun Oct 19, 2014 5:24 pm

Pratt&Whitney has also worked on the Advanced Ducted Prop which was to be a ducted full feathering and reversing prop looking like a fan, but the program has been shelved. Information about it is scarce to find. There is a reference to it in the german Wiki page on the PW1000G.

http://de.wikipedia.org/wiki/Pratt_%26_Whitney_PW1000G

Maybe Lightsaber would know something…
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