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Why fuel driven actuators?
Why do Jet engine actuators like VSV, IGV are operated by fuel pressure and not pressure from the hydraulic system?
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- GalaxyFlyer
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Re: Why fuel driven actuators?
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?
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.
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.
Re: Why fuel driven actuators?
The engines can't run without fuel. They can run without hydraulics...
Re: Why fuel driven actuators?
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.
Re: Why fuel driven actuators?
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.
Re: Why fuel driven actuators?
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.
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.
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- Redbellyguppy
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Re: Why fuel driven actuators?
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.
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- StereoTechque
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Re: Why fuel driven actuators?
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.
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- GalaxyFlyer
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Re: Why fuel driven actuators?
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
Re: Why fuel driven actuators?
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.
Re: Why fuel driven actuators?
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.
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- Wacker1000
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Re: Why fuel driven actuators?
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.
Re: Why fuel driven actuators?
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.
Re: Why fuel driven actuators?
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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