Videokarma.org

Go Back   Videokarma.org TV - Video - Vintage Television & Radio Forums > Things with Motors

We appreciate your help

in keeping this site going.
Reply
 
Thread Tools Display Modes
  #1  
Old 05-24-2016, 03:37 PM
rca2000's Avatar
rca2000 rca2000 is offline
VideoKarma Member
 
Join Date: Mar 2003
Location: cincinnati,ohio
Posts: 2,090
Jet engine Question...

On a large turbofan jet engine...I always thought that the front fan was driven by the turbine shaft. BUT...i have seen MANY instances of the front fan just "freewhelling in the wind" on a parked Jet plane that as not even powered at all. If the turbine shaft was connected to the fan..HOW could simple wind make it rotate? It would seem to be WAAYYY too much mass for that. Also...I have also seen pictures of the front fan spinning MUCH slower--it appeared and sounded like--than the jet engine..Maybe 1500-2000 RPM..but jet engine spin at MANY thousands of RPM when running--even at idle--right?

So...is the front fan driven...OR does it just spin by AIR PRESSURE ?
Reply With Quote
  #2  
Old 05-24-2016, 04:20 PM
jr_tech's Avatar
jr_tech jr_tech is offline
VideoKarma Member
 
Join Date: Sep 2009
Posts: 4,510
Good explanation here:

https://en.m.wikipedia.org/wiki/Turbofan

Concentric shafts for low and high pressure components.

jr
Reply With Quote
  #3  
Old 05-24-2016, 07:12 PM
rca2000's Avatar
rca2000 rca2000 is offline
VideoKarma Member
 
Join Date: Mar 2003
Location: cincinnati,ohio
Posts: 2,090
STILL no explanation of why the fan can sit and slowly spin just sitting on the tarmac...with a sleeping engine..

BUT maybe it explains why the fans seem to go a LOT slower than the engine ..(gear reduction)..so it does not spin too fast--..or it would be VERY noisy..
Reply With Quote
  #4  
Old 05-24-2016, 10:14 PM
jr_tech's Avatar
jr_tech jr_tech is offline
VideoKarma Member
 
Join Date: Sep 2009
Posts: 4,510
Why wouldn't a large, efficient fan blade supported by nearly frictionless bearings turn in the wind? My attic fan turns a lot when we we have a bit of east wind.

jr
Reply With Quote
  #5  
Old 05-25-2016, 12:34 AM
rca2000's Avatar
rca2000 rca2000 is offline
VideoKarma Member
 
Join Date: Mar 2003
Location: cincinnati,ohio
Posts: 2,090
I guess I figured all of the OTHER stuff it was connected through--gears, power shaft, power turbine--would be too much drag on the fan for it to just turn in the wind...UNLESS..there is some sort of a clutch between all of it, which does not engage till the power shaft is spinning sufficiently fast..
Reply With Quote
Audiokarma
  #6  
Old 05-25-2016, 12:59 AM
jr_tech's Avatar
jr_tech jr_tech is offline
VideoKarma Member
 
Join Date: Sep 2009
Posts: 4,510
I may be looking a this wrong, but it looks like there is no direct connection between the low pressure components and the high pressure components. If pushed by wind, the large front fan only has to turn a few very small rotors at the rear of the engine through the centermost of the concentric shafts. I bet you could easily spin that puppy by hand!

jr
Reply With Quote
  #7  
Old 05-25-2016, 02:05 AM
Electronic M's Avatar
Electronic M Electronic M is offline
M is for Memory
 
Join Date: Jan 2011
Location: Pewaukee/Delafield Wi
Posts: 14,758
Quote:
Originally Posted by jr_tech View Post
If pushed by wind, the large front fan only has to turn a few very small rotors at the rear of the engine through the centermost of the concentric shafts. I bet you could easily spin that puppy by hand!

jr
I'm picturing some one yelling "contact" to an idiot sitting in front of the turbine preparing to spin it by hand to start it...
__________________
Tom C.

Zenith: The quality stays in EVEN after the name falls off!
What I want. --> http://www.videokarma.org/showpost.p...62&postcount=4
Reply With Quote
  #8  
Old 05-26-2016, 01:37 PM
jr_tech's Avatar
jr_tech jr_tech is offline
VideoKarma Member
 
Join Date: Sep 2009
Posts: 4,510
Found this video of a small turbofan... it is easily turned by hand:

https://m.youtube.com/watch?v=BEghFV2y2rY

jr
Reply With Quote
  #9  
Old 06-01-2016, 09:11 PM
quaddriver quaddriver is offline
VideoKarma Member
 
Join Date: Jan 2007
Location: Moved to Pittsburgh to be closer to Primantis
Posts: 94
the hi bypass fans are driven by the exhaust.

on a dead engine they are oft turned by an electric motor to keep oil flowing on colder days
__________________
QuadDriver

Pioneer Silver-face Collector
Reply With Quote
Reply


Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off

Forum Jump


All times are GMT -5. The time now is 09:40 AM.



Powered by vBulletin® Version 3.8.4
Copyright ©2000 - 2024, Jelsoft Enterprises Ltd.
©Copyright 2012 VideoKarma.org, All rights reserved.