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skwabie
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 Posted: Jan 28, 2011 - 04:17 PM
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Enthusiast
Joined: Jun 08, 2007 - 09:33 PM
Posts: 31
Status: Offline
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Hi guys,
I've been reading the F-16's Dash 1, and realized that prolonged AB use (especially at low-med altitudes) could cause the trapped fuel condition when feeding from external tanks, since their fuel transfer rate is limited.
BUT, I've also been reading the fuel system section. The fuel from external tanks go into the wing tanks, the wing tanks go into the fuselage tanks, then into the reservoir. So WHY isn't the internal wing tanks contributing to the insufficient fuel flow, instead of the fuselage tanks???
To put it into pictures, according to what I understand, here's what actually happens:
But why doesn't this happen?:
Is it different mechanisms among the tanks' fuel transfer, or different pressure within them....? Hope someone could enlighten me. Thnx!! |
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Sponsor
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 Posted: May 26, 2013 - 6:04 AM
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F-16.net Sponsor
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exfltsafety
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Posted: Jan 28, 2011 - 04:53 PM
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Senior member
Joined: Aug 05, 2009 - 08:11 PM
Posts: 281
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During the high fuel flow conditions stated in the flight manual, the external tanks can not supply fuel as fast as the engine can use it. All the internal tanks (wings and fuselage) can supply fuel as fast as the engine can use it by virtue of the siphon and powered transfer systems. During prolonged AB use, there could still be fuel in the external tanks when the internal tanks become empty.
Your first block picture diagram is not correct. Your second block picture diagram is correct. |
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gunzo
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Posted: Apr 28, 2011 - 07:35 PM
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Newbie
Joined: Sep 29, 2006 - 10:57 AM
Posts: 15
Status: Offline
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[quote="skwabie"]Hi guys,
I've been reading the F-16's Dash 1, and realized that prolonged AB use (especially at low-med altitudes) could cause the trapped fuel condition when feeding from external tanks, since their fuel transfer rate is limited.
BUT, I've also been reading the fuel system section. The fuel from external tanks go into the wing tanks, the wing tanks go into the fuselage tanks, then into the reservoir. So WHY isn't the internal wing tanks contributing to the insufficient fuel flow, instead of the fuselage tanks???
To put it into pictures, according to what I understand, here's what actually happens:
[img]http://i27.photobucket.com/albums/c181/Me262a-1a/Capture_084.jpg[/img]
But why doesn't this happen?:
[img]http://i27.photobucket.com/albums/c181/Me262a-1a/Capture_085.jpg[/img]
Is it different mechanisms among the tanks' fuel transfer, or different pressure within them....? Hope someone could enlighten me. Thnx!![/quote]
The 'flow' is wrong ..
typical AB flow on the 1/229 engines are roughly between 50-60k lbs at SL -10k
according to engineers from LM, internal fuel transfers have no issues sustaining up to these fuel flows
if I remember correctly .. 30k lb transfer rates only if you have 370gal tanks .. otherwise its lower with the 300gals..
TRAPPED fuel logic kicks in when the internal is 500lb less than full with totaliser showing more than total internal ( ie plus external tanks ) and AB usage not within the last 30s ( I think .. can't remember .. its been a while :p )
technically this should take care of any situation since the tanks refill at 500lbs/s .. but once you start using internal tank fuel .. the internal tank becomes pressurised to continue the siphon effect.. this basically reduces the flow capability of the transfer between external to internal hence giving rise to "trp fuel" |
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skwabie
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Posted: Apr 28, 2011 - 08:20 PM
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Enthusiast
Joined: Jun 08, 2007 - 09:33 PM
Posts: 31
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I actually sorta gave up for it's a bit hard to explain...
Understand your words very well Gunzo. But my question is why doesn't the internal wing tanks help when the fuel rate can't keep up.
What happens in reality is 30K pph goes from the 370s to the internal wing tanks, 30K pph goes from internal wing to fuselage, and 40K pph goes from fuselage to the rsvr. Hence fuselage is "starved". But why couldn't it be that, 40K pph goes from internal wing to the fuselage, thus the starvation happens to the internal wing, not the fuselage?
It probably involves, how internal wing tank transfers fuel to the fuselage when 370s are carried. I know without the 370s the internal wing tanks have no problem sustaining the max AB fuel flow. Perhaps something changed when bags are carried. |
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exfltsafety
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Posted: Apr 28, 2011 - 10:25 PM
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Senior member
Joined: Aug 05, 2009 - 08:11 PM
Posts: 281
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The internal wing tanks do help until the fuel level in them is low enough to interrupt the siphon transfer between the internal wing tanks and the fuselage tanks.
Where do you get the following about what happens in reality? It's not reality in my mind.
Quote:
What happens in reality is 30K pph goes from the 370s to the internal wing tanks, 30K pph goes from internal wing to fuselage, and 40K pph goes from fuselage to the rsvr.
Like I said before, the following is more accurate:
Quote:
But why couldn't it be that, 40K pph goes from internal wing to the fuselage, thus the starvation happens to the internal wing, not the fuselage?
There's no difference in transfer of fuel from the internal wing tanks with or without 370s installed.
And your 40K pph flow rate is fine for use in trying to understand the scenario. A prolonged flow rate above what can be transferred from the external tanks can ultimately result in running out of internal fuel while still having some in the external tanks. |
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skwabie
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Posted: Apr 29, 2011 - 08:45 AM
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Enthusiast
Joined: Jun 08, 2007 - 09:33 PM
Posts: 31
Status: Offline
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exfltsafety wrote:
The internal wing tanks do help until the fuel level in them is low enough to interrupt the siphon transfer between the internal wing tanks and the fuselage tanks.
Where do you get the following about what happens in reality? It's not reality in my mind.
Quote:
What happens in reality is 30K pph goes from the 370s to the internal wing tanks, 30K pph goes from internal wing to fuselage, and 40K pph goes from fuselage to the rsvr.
Like I said before, the following is more accurate:
Quote:
But why couldn't it be that, 40K pph goes from internal wing to the fuselage, thus the starvation happens to the internal wing, not the fuselage?
There's no difference in transfer of fuel from the internal wing tanks with or without 370s installed.
And your 40K pph flow rate is fine for use in trying to understand the scenario. A prolonged flow rate above what can be transferred from the external tanks can ultimately result in running out of internal fuel while still having some in the external tanks.
Thanks vey much exfltsafety, I understand what you're saying now. My mind didn't "connect" during your 1st post...
Will do some checks and reply back... |
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