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mikeperkins
Joined: 22 May 2007
Posts: 123
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 Posted: Thu Jun 14, 2007 6:58 am Post subject: Unequal fuel flow |
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Conducting a fuel-flow test on the ground is always a good idea. The fuel rate should be measured by removing the fuel hose from the carb.....anywhere else and the head pressure is going to be different and therefore yield incorrect results. Also, fuel flow should be measured on the ground with the airplane set in three different attitudes: the cruise-flight attitude, approach attitude, and max climb. The FAA wants about 1.5 times your max fuel usage in any attitude. Feeding from each tank separately and all combinations thereof, including all tanks, is also necessary. This will reveal obstructions or problems with fuel line routing.
When it comes to uneven fuel flow between the tanks, the tank pressures can vary. Many Kitfoxes have ram-pressure caps (with the bent-forward vent pipes). but whether an airplane has ram caps or not, the fuel flow difference mostly reflects the difference in tank pressures and fuel-flow turbulence. A sharp 90-degree bend, for example, will create a lot of turbulence, even at low flow-rates. Essentially, the turbulence creates a pressure drop. Each pressure drop subtracts from the tank pressure. Obviously, this kind of testing can only be done in flight or with very sophisticated ground equipmment. Therefore, fuel flow is related to pressures and pressure drops - in other words, the sums and differences of all pressures.
For whatever reason, my Kitfox has about a 30% difference between flow when tanks are near full, but as the tanks empty towards 1/4 full, the tank quanitiy seems to even out.
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Guy Buchanan
Joined: 16 Jul 2006
Posts: 1204
Location: Ramona, CA
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| Posted: Thu Jun 14, 2007 11:03 am Post subject: Unequal fuel flow |
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At 10:01 PM 6/13/2007, you wrote:
| Quote: | | I will try not to confuse, there is two problems, one the vent line and two the tanks draining differently. |
John, I've been trying to figure this out, since it has come up before.
| Quote: | | The vent line is only to get the gas (air) out of the header tank. If the vent is left open, there is a possibility that the header tanks could empty and the engine will suck air through the vent line. |
This presumes BOTH tank fuel lines are blocked, right? If either fuel line is flowing, there is no way to pull vapor through the vent line as the level in the vent line is the same as the level in the flowing tank. (Fluid seeks it's own level.)
| Quote: | | Now, let me explain that, each of the fuel tanks has a line that has a loose 90 degree turn to get to the tank (just behind your head), this turn is usually left fairly loose or large so the wing can fold with out tearing the line out. This half loop can if left too loose, can get a bubble in it during a turn. This bubble can cause a “vapor lock” and the tank will stop feeding. If it stops feeding the header tank empties and sucks air from the vent line. |
There is no vapor lock, the high hose simply acts as a dam over which fuel cannot flow. The tank will gravity feed, however, down to the level of the highest point of the hose. (If the loop rises above the tank then you can't gravity feed. You must siphon.) Then if there's suction you can siphon the fluid over the dam, even with an air bubble. The only time the siphon will fail is if it pulls air instead of fluid. If the flow is very slow the bubble will reside in the top of the loop and fuel will flow past it. If the flow is fast it may pull the bubble along with it. If your pump isn't self priming this bubble can cause it to stop pulling. (As in a 582 oil pump.)
I tried all of these scenarios with water and clear hose and couldn't generate your "vapor lock" no matter what I did. The fluid in my "tank" always drained down to the level of the top of the hose. Then if I used suction, (via siphon or mechanically,) I could continue to drain the "tank" until I sucked air.
Guy Buchanan
San Diego, CA
K-IV 1200 / 582-C / Warp / 100% done, thanks mostly to Bob Ducar. [quote][b]
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_________________
Guy Buchanan
Deceased K-IV 1200
A glider pilot too. |
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john(at)leptron.com
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| Posted: Thu Jun 14, 2007 3:09 pm Post subject: Unequal fuel flow |
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Good grief Guy, J
Ok, it is not a vapor lock; I was trying to get the point across. The fact remains that; we can loose an aircraft due to this problem. In addition, if the vent line is open you will suck air instead of fuel when the level of fuel equals the top of the hose.
John Oakley
From: owner-kitfox-list-server(at)matronics.com [mailto:owner-kitfox-list-server(at)matronics.com] On Behalf Of Guy Buchanan
Sent: Thursday, June 14, 2007 1:02 PM
To: kitfox-list(at)matronics.com
Subject: RE: Unequal fuel flow
At 10:01 PM 6/13/2007, you wrote:
I will try not to confuse, there is two problems, one the vent line and two the tanks draining differently.
John, I've been trying to figure this out, since it has come up before.
The vent line is only to get the gas (air) out of the header tank. If the vent is left open, there is a possibility that the header tanks could empty and the engine will suck air through the vent line.
This presumes BOTH tank fuel lines are blocked, right? If either fuel line is flowing, there is no way to pull vapor through the vent line as the level in the vent line is the same as the level in the flowing tank. (Fluid seeks it's own level.)
Now, let me explain that, each of the fuel tanks has a line that has a loose 90 degree turn to get to the tank (just behind your head), this turn is usually left fairly loose or large so the wing can fold with out tearing the line out. This half loop can if left too loose, can get a bubble in it during a turn. This bubble can cause a vapor lock and the tank will stop feeding. If it stops feeding the header tank empties and sucks air from the vent line.
There is no vapor lock, the high hose simply acts as a dam over which fuel cannot flow. The tank will gravity feed, however, down to the level of the highest point of the hose. (If the loop rises above the tank then you can't gravity feed. You must siphon.) Then if there's suction you can siphon the fluid over the dam, even with an air bubble. The only time the siphon will fail is if it pulls air instead of fluid. If the flow is very slow the bubble will reside in the top of the loop and fuel will flow past it. If the flow is fast it may pull the bubble along with it. If your pump isn't self priming this bubble can cause it to stop pulling. (As in a 582 oil pump.)
I tried all of these scenarios with water and clear hose and couldn't generate your "vapor lock" no matter what I did. The fluid in my "tank" always drained down to the level of the top of the hose. Then if I used suction, (via siphon or mechanically,) I could continue to drain the "tank" until I sucked air.
Guy Buchanan
San Diego, CA
K-IV 1200 / 582-C / Warp / 100% done, thanks mostly to Bob Ducar. | Quote: | | http://www.matronics.com/Navigator?Kitfox-List |
01234
[quote][b]
| | - The Matronics Kitfox-List Email Forum - | | | Use the List Feature Navigator to browse the many List utilities available such as the Email Subscriptions page, Archive Search & Download, 7-Day Browse, Chat, FAQ, Photoshare, and much more:
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Guy Buchanan
Joined: 16 Jul 2006
Posts: 1204
Location: Ramona, CA
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| Posted: Thu Jun 14, 2007 6:07 pm Post subject: Unequal fuel flow |
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At 04:07 PM 6/14/2007, you wrote:
| Quote: | Good grief Guy, J
Ok, it is not a vapor lock; I was trying to get the point across. |
I though so, but wanted to make sure. The fuel injected spam cans I fly get vapor lock on a hot start so I get confused. I just wanted to clarify what you were seeing. I guess I should have just asked, eh? :-[
Guy Buchanan
San Diego, CA
K-IV 1200 / 582-C / Warp / 100% done, thanks mostly to Bob Ducar. [quote][b]
| | - The Matronics Kitfox-List Email Forum - | | | Use the List Feature Navigator to browse the many List utilities available such as the Email Subscriptions page, Archive Search & Download, 7-Day Browse, Chat, FAQ, Photoshare, and much more:
http://www.matronics.com/Navigator?Kitfox-List |
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_________________
Guy Buchanan
Deceased K-IV 1200
A glider pilot too. |
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