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dhawald3
Topic Author
Posts: 23
Joined: Tue Jun 28, 2011 3:10 pm

Why fuel driven actuators?

Fri Nov 12, 2021 2:23 pm

Why do Jet engine actuators like VSV, IGV are operated by fuel pressure and not pressure from the hydraulic system?
 
GalaxyFlyer
Posts: 8939
Joined: Fri Jan 01, 2016 4:44 am

Re: Why fuel driven actuators?

Fri Nov 12, 2021 2:29 pm

Because there is fuel pressure available internal to the engine. Using hydraulics would introduce new lines from the pumps. Basically, it’s simpler
 
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Starlionblue
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Re: Why fuel driven actuators?

Sat Nov 13, 2021 12:16 am

Isn't this also a bit of a chicken and egg problem?

There is typically no hydraulic pressure before the first engine has started. So you can't run the actuators on hydraulics during engine start because hydraulic pressure is provided by the running engines.

The fuel pumps, however, will be running.
 
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Horstroad
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Re: Why fuel driven actuators?

Sat Nov 13, 2021 1:42 am

The engines can't run without fuel. They can run without hydraulics...
 
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fr8mech
Posts: 8426
Joined: Mon Sep 26, 2005 9:00 am

Re: Why fuel driven actuators?

Sat Nov 13, 2021 4:39 am

Starlionblue wrote:
Isn't this also a bit of a chicken and egg problem?

There is typically no hydraulic pressure before the first engine has started. So you can't run the actuators on hydraulics during engine start because hydraulic pressure is provided by the running engines.

The fuel pumps, however, will be running.


Small point...the aircraft fuel pumps don't provide for engine actuators. That pressure comes right from the engine driven pump through the fuel control. The required pressure is generated almost soon as the engine is turning.

Horstroad wrote:
The engines can't run without fuel. They can run without hydraulics...


Past simplifying the plumbing and potential for contamination, this is probably the most important.
 
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77west
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Joined: Sat Jun 13, 2009 11:52 am

Re: Why fuel driven actuators?

Sat Nov 13, 2021 10:14 am

Simplifying it way down: If the engines don't have fuel pressure, there is no need for VSV/IGV actuators etc. If the engines don't have fuel pressure on the ground - possibly normal, but not a major if not. In the air - VSV/IGV are the least of your problems.
 
Dalmd88
Posts: 3218
Joined: Fri Jul 28, 2000 3:19 am

Re: Why fuel driven actuators?

Sat Nov 13, 2021 11:26 pm

There is also the airframe/engine divide. Hydraulics are considered airframe components. Yes there is a hydraulic pump mounted to the engine, but the engine manufacturer does not provide it with the engine. The fuel pump is an engine component so they can use the excess muscle fuel pressure to run engine required items like the VSV/IGV.

But wait the T/R system is typically run by hydraulic pressure. Correct, and the T/R is also considered to be an airframe part.
 
Redbellyguppy
Posts: 261
Joined: Sat Apr 01, 2017 3:57 am

Re: Why fuel driven actuators?

Sun Nov 14, 2021 1:19 am

I once flew an aircraft that had pneumatically actuated thrust reversers. There’s an infinite supply of air, too, in an engine that is running.
 
StereoTechque
Posts: 209
Joined: Sat Nov 28, 2015 7:24 am

Re: Why fuel driven actuators?

Sun Nov 14, 2021 1:58 am

Redbellyguppy wrote:
I once flew an aircraft that had pneumatically actuated thrust reversers. There’s an infinite supply of air, too, in an engine that is running.


At the cost of engine efficiency of course.
 
GalaxyFlyer
Posts: 8939
Joined: Fri Jan 01, 2016 4:44 am

Re: Why fuel driven actuators?

Sun Nov 14, 2021 3:49 am

You’re not using the pneumatics for T/Rs very often, so not an issue. Pneumatics on the 727 IIRC and on the Challenger/CRJ
 
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fr8mech
Posts: 8426
Joined: Mon Sep 26, 2005 9:00 am

Re: Why fuel driven actuators?

Sun Nov 14, 2021 12:18 pm

GalaxyFlyer wrote:
You’re not using the pneumatics for T/Rs very often, so not an issue. Pneumatics on the 727 IIRC and on the Challenger/CRJ


Yup, the B727. It seemed to me that those actuators froze up more often than the hydraulic ones on other aircraft. And, you never know you have a leaking line either, until it just stops working.
 
milhaus
Posts: 119
Joined: Fri Dec 10, 2010 4:19 pm

Re: Why fuel driven actuators?

Sun Nov 14, 2021 1:06 pm

Engine systems have to be independent from airframe systems as much as possible. And then fuel is much more frendlier fluid for mechanics then Skydrol or Hyjet.
 
Wacker1000
Posts: 306
Joined: Tue Jan 07, 2014 6:36 pm

Re: Why fuel driven actuators?

Mon Nov 15, 2021 6:10 pm

dhawald3 wrote:
Why do Jet engine actuators like VSV, IGV are operated by fuel pressure and not pressure from the hydraulic system?


You double the chances of a system failure causing an ISFD if you do as you're describing. It is less safe.
 
bigb
Posts: 1608
Joined: Fri Nov 07, 2003 4:30 pm

Re: Why fuel driven actuators?

Tue Nov 16, 2021 1:28 am

GalaxyFlyer wrote:
You’re not using the pneumatics for T/Rs very often, so not an issue. Pneumatics on the 727 IIRC and on the Challenger/CRJ


The CRJ-700/900/1000s TRs were hydraulically actuated. The older challengers/CRJ100/200 are pneumatically. 747-400/767 GEs are pneumatically actuated 747-400 PWs and 747-8s are hydraulically actuated.
 
acmx
Posts: 31
Joined: Mon Aug 13, 2018 7:49 pm

Re: Why fuel driven actuators?

Tue Nov 16, 2021 2:18 am

The vbv’s and vsv’s move in response to changes in the engines operation, like acceleration or deceleration (among other things). So using the fuel control and fuel pressure to actuate them makes the most sense. Using airframe hydraulics, even from the engine driven pump, wouldn’t make sense. If the engine is running you have fuel pressure available to use, that’s not necessarily always the case with airframe hydraulics.

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