Posts: 801
Joined: Fri Dec 05, 2003 11:26 pm

Adjustable Pitch In Turbofans?

Thu May 20, 2004 1:01 pm

As a general aviation pilot, I don't get it. In small airplanes, the propeller pitch matters a lot in a plane's performance. Adjustable pitch is preferable and more efficient.

As I understand it, in turbofans, the fan works more or less like a propeller. Yet, it does not look like the pitch is adjustable.

Another thing that seems different is that there is no impact on the sense of rotation of the fan. For instance, in a small plane you have to use the rudder when you start rolling for take off. I've heard that with turbo fans, there were not such effect at all. Is that really so?
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RE: Adjustable Pitch In Turbofans?

Thu May 20, 2004 3:22 pm

Variable inlet guide vanes (VIGV) and variable stator blades have been around for a while to avoid compressor stalls.

As for variable fan blades i don't know of any but i don't know much  Big grin
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RE: Adjustable Pitch In Turbofans?

Thu May 20, 2004 5:42 pm

The greatest advantage afforded by the hi-bypass fan engine is its lo-bypass thrust produced during takeoff. Approximately 75% of total thrust at takeoff on a modern airliner's fan engine is produced by the fan's bypass flow. In cruise the fan produces about 60% of the total combined thrust of the fan and core flows.

As Liamksa pointed out above, the variable stator blades provide most of the necessary adjustments to airflow into the higher pressure compressor(s). Nozzle design maximizes the low bypass flow and mix with the core exhaust.

Variable pitch N1 fans might be a good idea but I believe the engineering necessary to do so would outweigh the benefits.

Most modern jet engines are not used with their maximum thrust in cruise as the airframe is the limiting factor on forward speed.

On your second question, if you are referring to the torque produced by a single engine propellor driven aircraft being the reason to add extra rudder, to a large extent, this torque is negated on multi-engine, non-centreline thrust engines. There is also, from what I remember on tail-draggers, the P-factor to take into consideration, also requiring rudder input to counter. Again, not a factor on modern transport aircraft, or even light jet aircraft.

Hope that answers some of your question!

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RE: Adjustable Pitch In Turbofans?

Thu May 20, 2004 7:23 pm

For instance, in a small plane you have to use the rudder when you start rolling for take off.

I must admit I didn't even see this the first time around.  Big grin There are a few reasons why you need rudder (usually right) with the application of power on the takeoff roll for light aircraft.

Firstly the airflow from the prop corkscrews around the fuselage and strikes the fin at an angle of attack on the port side inducing a yaw to the left - remedy = right rudder.

Secondly the torque reaction to the engine tries to spin the airframe / fuselage in the opposite direction, increasing the friction on the left wheel, again a yaw to the left - remedy = right rudder.

Thirdly theres the P-factor. The downgoing blade strikes the airflow at a greater angle of attack -> more thrust -> thrust displaced to the starboard side -> yaw to the left -> and you guessed it right rudder!

Lastly, in taildraggers, as you lift the tail theres the gyroscopic effect precession (force acting 90 degrees and ahead on the spinning mass ie: prop) which again yaw to the left - remedy = right rudder.

(note this is all for a conventional clockwise rotating as view from the cockpit engine).

And as to how this applies to turbofans  Nuts - i'm a spam can man  Big thumbs up
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RE: Adjustable Pitch In Turbofan?

Thu May 20, 2004 8:46 pm

I read some where in either a RR publication I get at work or a P&W that they are actually working on a variable fan for their engines. I will see if I can find it some where.

Sometimes all you can do is look them in the eye and ask " how much did your mom drink when she was pregnant with you?"
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RE: Adjustable Pitch In Turbofans?

Thu May 20, 2004 9:17 pm

I think there are a couple of reasons that you don't see variable pitch blades in turbofans. The first is simply that the mechanical mechanism for such a thing would be extremely heavy and expensive. The rotating energy in the front hub and first stage blades is tremendous. Building a mechanism that could turn all 40 or 50 plus blades, plus remain fail proof under all that loading would be a huge task.

Second, I don't think a turbofan would benefit from variable pitch blades. The engine works by creating more pressure as you move forward to aft. Each stage is designed to take air at given pressure from the stage before. If that pressure becomes variable, then each stage will have to become variable. You could try to compensate with variable vanes, but then they are doing double and triple duty and I wonder if they could change angle of attack enough to compensate for a large first stage variation in pressure??

Air transport is just a glorified bus operation. -Michael O'Leary, Ryanair's chief executive
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RE: Adjustable Pitch In Turbofans?

Fri May 21, 2004 4:35 am

Yo Shawn:

Very good point, reinforcing my thoughts above. It is important to remember that the fan section of the engine, driven by the N1 turbine(s), serves the dual purpose of providing high-bypass airflow to provide the substantial percentage of thrust metered out by the engine, and equally important, providing the next stage of the compression cycle with a sufficient volume of air under many variable conditions - from takeoff speeds right up to MMo.

No easy task. The variable guide vanes came into being quite a long time ago in an effort to equalize pressures throughout the operating range/speed of the engine. Other means to prevent compressor stalls were such things as interstage bleed valves, those secondary bypass doors on older turbo jets such as the DC8, and more efficient FCU's.

The most recent exercises in extracting more bang-for-buck are ceramics. To date, I believe they have been introduced but only into the non-moving parts of the engine - fixed stator/guide vane assemblies. We're probably another decade away from seeing ceramics introduced into the moving parts.

Why ceramics? The hotter you can make the engine run, the more power can be extracted from it. So far, most internal combustion engines, jet engines included, are limited in their power output by the susceptibility to heat of their component parts. Ceramics can handle temperatures that turn steel into liquids yet still retain incredible strength.

Nice topic folks!

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RE: Adjustable Pitch In Turbofans?

Fri May 21, 2004 12:32 pm

Pratt and Whitney and NASA are testing one as we speak. Read this article: http://articles.findarticles.com/p/articles/mi_pasa/is_/ai_1774285580

[Edited 2004-05-21 05:33:35]
"Failure to prepare is preparing to fail"--Benjamin Franklin
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RE: Adjustable Pitch In Turbofans?

Sat May 22, 2004 9:32 am

What if you take just the bypass portion of the N1 fan and make it variable leaving the compressor part fixed pitch (like it is currently)? (This assuming you could get the mechinism and the blades strong enough to take the loading of the out tips biting more air).
The other idea, if you guys remember the old wind helicopters with safety ring that went around the outside of the rotors? What if you applied the same concept to the n1 fan to increase strength of the blades?
"And remember, Keep your stick on the ice"--->Red Green
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RE: Adjustable Pitch In Turbofans?

Sat May 22, 2004 2:14 pm

It's usually wrong to comment before you "google", but to the best of my recollection, there was the V2500 Superfan that was supposed to have this feature. I think it was destined for the A-340 before it got the hairdryers.

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