Moderators: richierich, ua900, PanAm_DC10, hOMSaR

 
User avatar
767333ER
Topic Author
Posts: 1174
Joined: Wed Jun 15, 2016 5:14 am

Variable Pitch Turbofan Questions

Wed Aug 31, 2016 6:27 am

I have a couple of questions regarding variable pitch turbofans such as RR Ultrafan.

1. RR believes that the Ultrafan will be able to reverse thrust by putting the fan into negative pitch allowing them to remove the tradition cascade vane reverses and all the heavy and complex equipment that is involved with those. How though would a negative fan pitch reverse work without interrupting airflow to the engine?

2. The variable pitch fan seems to be the next step after the geared fan which seems to doing well in the PW1000G (yes I know it's not the first geared fan) and will likely become the norm eventually. What benefit does the variable pitch offer? Does it offer similar benefits to a constant speed prop and would it be operated like one?
Last edited by qf789 on Fri Sep 02, 2016 9:03 am, edited 1 time in total.
Reason: spelling mistake in title
Been on: 732 733 734 73G 738 752 763 A319 A320 A321 CRJ CR7 CRA/CR9 E145 E175 E190 F28 MD-82 MD-83 C172R C172S P2006T PA-28-180

2 ears for spatial hearing, 2 eyes for depth perception, 2 ears for balance... How did Boeing think 1 sensor was good enough?!
 
User avatar
767333ER
Topic Author
Posts: 1174
Joined: Wed Jun 15, 2016 5:14 am

Re: Variable Pitch Turbofn Questions

Wed Aug 31, 2016 6:28 am

Please excuse the typo in the title.
Been on: 732 733 734 73G 738 752 763 A319 A320 A321 CRJ CR7 CRA/CR9 E145 E175 E190 F28 MD-82 MD-83 C172R C172S P2006T PA-28-180

2 ears for spatial hearing, 2 eyes for depth perception, 2 ears for balance... How did Boeing think 1 sensor was good enough?!
 
jonathanpun
Posts: 28
Joined: Sun Mar 23, 2008 3:07 pm

Re: Variable Pitch Turbofn Questions

Wed Aug 31, 2016 9:06 am

Interesting, I want to know too.
But, see p.12 in http://aerosociety.com/Assets/Docs/Events/746/GBD_Propulsion_211014_RR_1.pdf, seems like RR recognizes the reverse as one of the challenges as well.

Btw, just realise there is a similar post: viewtopic.php?t=773449
 
User avatar
BaconButty
Posts: 913
Joined: Tue Aug 06, 2013 3:42 pm

Re: Variable Pitch Turbofn Questions

Thu Sep 01, 2016 9:48 pm

Reverse thrust mode requires flow to turn sharp lips of cold nozzle & core splitter.

I'm just dropping this into the conversation as if I know what it means and didn't just copy and paste from here. :-)
Down with that sort of thing!
 
ninspeed
Posts: 54
Joined: Mon Feb 29, 2016 5:31 am

Re: Variable Pitch Turbofan Questions

Fri May 05, 2017 4:52 pm

Once the fan rotates and reverses the flow of air, there would have to be a moveable flap that changes the intake into the compressor section. This would make the air do a 180 causing a major restriction of air into the compressor that I think is causing the issue....
 
kurtverbose
Posts: 583
Joined: Thu Jun 26, 2014 9:33 pm

Re: Variable Pitch Turbofan Questions

Fri May 05, 2017 6:09 pm

Why don't they just eliminate the fan from the core flow and induct the core air through the hub?
 
User avatar
speedbored
Posts: 2230
Joined: Fri Jul 19, 2013 5:14 am

Re: Variable Pitch Turbofan Questions

Fri May 05, 2017 6:48 pm

I presume that the twist in the fan blades will help - in the initial stages of reverse thrust, it will still provide some positive flow near the centre and then as the blades are turned further, there will effectively be little to no flow through the blades near the hub, helping with the u-turn required from air going to feed the core.

It can't be insurmountable as it has been done on turboprops for years. It is just a different order of magnitude in terms of flow speeds and pressures.
 
User avatar
keesje
Posts: 14435
Joined: Thu Apr 12, 2001 2:08 am

Re: Variable Pitch Turbofan Questions

Tue May 09, 2017 12:13 pm

I've been wondering where the thrust reverse (massive) airflow inflow should come from (from the sides?) without stalling the compressor..

I concluded this likely won't be feasible / practical.

However optimizing fan blade AOA's for various flight stages (take-off / cruise) could be where RR is aiming, in combination with a gear box removing the LPT.

Image
"Never mistake motion for action." Ernest Hemingway
 
benbeny
Posts: 250
Joined: Thu Nov 03, 2016 1:44 pm

Re: Variable Pitch Turbofan Questions

Tue May 09, 2017 4:29 pm

I'm wondering about the second question too. Why don't we create variable pitch fan?
 
User avatar
DocLightning
Posts: 22208
Joined: Wed Nov 16, 2005 8:51 am

Re: Variable Pitch Turbofan Questions

Wed May 10, 2017 12:46 am

If the twist is done right, then when the blades are pitched to "reverse," the very inner portion of each blade could still have a positive AOA while the outer portion would have a negative AOA. That might allow flow to continue into the core while reversing thrust.

As for why the variable pitch at all, not only could you eliminate the heavy and complex TR mechanism, but at present an engine that is being spooled up for a static start has an AOA that is too high and that robs efficiency. In flight, the AOA may be slightly less than optimal. If the AOA can be varied based on the speed of the incoming air, perhaps that could add a few percent efficiency through the entire flight envelope.
-Doc Lightning-

"The sky calls to us. If we do not destroy ourselves, we will one day venture to the stars."
-Carl Sagan
 
User avatar
77west
Posts: 1011
Joined: Sat Jun 13, 2009 11:52 am

Re: Variable Pitch Turbofan Questions

Wed May 10, 2017 7:10 am

I think the first variable pitch design will probably still have conventional reverse thrust, unless they really can prove it won't cause any issues using the fan.

Wonder if we'll ever see variable pitch compressors / turbines... imagine the engineering to do that!
77West - AW109S - BE90 - JS31 - B1900 - Q300 - ATR72 - DC9-30 - MD80 - B733 - A320 - B738 - A300-B4 - B773 - B77W
 
WIederling
Posts: 10043
Joined: Sun Sep 13, 2015 2:15 pm

Re: Variable Pitch Turbofan Questions

Wed May 10, 2017 7:26 am

77west wrote:
Wonder if we'll ever see variable pitch compressors / turbines... imagine the engineering to do that!


Isn't that done on the stator side already (variable pitch?)

Reversing the fan would loose you one pressure stage ( the fan ).
airflow through the core engine is rather low with high BPR >>10.
Murphy is an optimist
 
User avatar
77west
Posts: 1011
Joined: Sat Jun 13, 2009 11:52 am

Re: Variable Pitch Turbofan Questions

Wed May 10, 2017 7:40 am

WIederling wrote:
77west wrote:
Wonder if we'll ever see variable pitch compressors / turbines... imagine the engineering to do that!


Isn't that done on the stator side already (variable pitch?)

Reversing the fan would loose you one pressure stage ( the fan ).
airflow through the core engine is rather low with high BPR >>10.


Yes but much easier to do on a stator compared to a compressor turning at 10,000rpm!! In fact I don't see variable compressors ever, the technical challenges and reliability implications would probably kill the idea. Think of one failed pitch control unit, leads to uneven stress and vibration on that disc, next minute it spews its blades through the rest of the core...
77West - AW109S - BE90 - JS31 - B1900 - Q300 - ATR72 - DC9-30 - MD80 - B733 - A320 - B738 - A300-B4 - B773 - B77W
 
rbretas
Posts: 72
Joined: Mon Aug 22, 2016 10:21 am

Re: Variable Pitch Turbofan Questions

Wed May 10, 2017 11:19 am

Does the stator change pitch during flight? I thought it was only during startup or idle.
 
User avatar
akiss20
Posts: 960
Joined: Tue Sep 04, 2007 9:50 am

Re: Variable Pitch Turbofan Questions

Wed May 10, 2017 10:47 pm

rbretas wrote:
Does the stator change pitch during flight? I thought it was only during startup or idle.


Variable vane schedules are a function of corrected N2 and is never really constant for any significant range of corrected N2. So yes, they will rotate typically during flight with power changes (especially going to F/I for descent).
Change will not come if we wait for some other person or some other time. We are the ones we've been waiting for. We are
 
User avatar
kitplane01
Posts: 1856
Joined: Thu Jun 16, 2016 5:58 am

Re: Variable Pitch Turbofan Questions

Thu May 11, 2017 12:46 am

77west wrote:
WIederling wrote:
77west wrote:
Wonder if we'll ever see variable pitch compressors / turbines... imagine the engineering to do that!


Isn't that done on the stator side already (variable pitch?)

Reversing the fan would loose you one pressure stage ( the fan ).
airflow through the core engine is rather low with high BPR >>10.


Yes but much easier to do on a stator compared to a compressor turning at 10,000rpm!! In fact I don't see variable compressors ever, the technical challenges and reliability implications would probably kill the idea. Think of one failed pitch control unit, leads to uneven stress and vibration on that disc, next minute it spews its blades through the rest of the core...


Propeller driven turbine aircraft have one unit per prop, and yet get the job done reliably. It's not obvious that a variable pitch fan is significantly different that a four or five bladed variable pitch prop.
 
User avatar
PITingres
Posts: 1363
Joined: Fri Dec 21, 2007 1:59 am

Re: Variable Pitch Turbofan Questions

Thu May 11, 2017 5:41 pm

kitplane01 wrote:
Propeller driven turbine aircraft have one unit per prop, and yet get the job done reliably. It's not obvious that a variable pitch fan is significantly different that a four or five bladed variable pitch prop.


A shrouded fan will have a much higher blade count, making the mechanism a lot more complex and it still has to fit into a tight space. It will also have tight blade to shroud clearance requirements, so it has to be something that doesn't allow extra radial play. It wouldn't surprise me to find that those two requirements make the difference "significant".
Fly, you fools! Fly!

Who is online

Users browsing this forum: aca763er, DEN2021, Starlionblue and 17 guests

Popular Searches On Airliners.net

Top Photos of Last:   24 Hours  •  48 Hours  •  7 Days  •  30 Days  •  180 Days  •  365 Days  •  All Time

Military Aircraft Every type from fighters to helicopters from air forces around the globe

Classic Airliners Props and jets from the good old days

Flight Decks Views from inside the cockpit

Aircraft Cabins Passenger cabin shots showing seat arrangements as well as cargo aircraft interior

Cargo Aircraft Pictures of great freighter aircraft

Government Aircraft Aircraft flying government officials

Helicopters Our large helicopter section. Both military and civil versions

Blimps / Airships Everything from the Goodyear blimp to the Zeppelin

Night Photos Beautiful shots taken while the sun is below the horizon

Accidents Accident, incident and crash related photos

Air to Air Photos taken by airborne photographers of airborne aircraft

Special Paint Schemes Aircraft painted in beautiful and original liveries

Airport Overviews Airport overviews from the air or ground

Tails and Winglets Tail and Winglet closeups with beautiful airline logos