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fuel flow for CH701
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kissell



Joined: 05 Dec 2006
Posts: 18
Location: Beavercreek, Ohio

PostPosted: Wed Sep 17, 2008 11:42 am    Post subject: fuel flow for CH701 Reply with quote

Study of the Zenith CH701 two tank fuel system, Bob Kissell, N701UB
<?xml:namespace prefix = o ns = "urn:schemas-microsoft-com:office:office" />
After completing my Zenith CH701 with two wing tanks I discovered on my first cross country flight that I had unbalanced fuel burn. The fuel system in my aircraft is built per the Zenith drawings, which is comprised of two ten gallon tanks, one in each wing that are connected together at the gascolator located at the low point in the system. This anomaly set about an investigation to explain this event and predict what would happen if one tank ran dry before the other

To answer these questions several tests were devised and conducted. The first test was to determine if fuel would be supplied from a tank with fuel, when the other tank is empty. The test was devised using the following logic. The Rotax 912ULS that my aircraft uses, can under the most demanding throttle setting use fuel at no more than 6.5 gallons/hr.. This information is available from the Rotax engine specifications. The fuel in the Zenith design is pumped (drawn) from the gascolator to the engine. I propose that if the natural gravity flow from the tank to the gascolator can supply fuel at a rate that exceeds the engine demand then the fuel pump never draws fuel from the tank(s), it really just lifts fuel from the gascolator to the carb’s and the fuel flows by gravity from the tank(s) to replace the fuel lifted. This is perhaps a different way to look at the Zenith system but I found it made the problem tractable for me.

The fuel system in my aircraft is built per the Zenith drawings, which is comprised of two ten gallon tanks, one in each wing that are connected together at the gascolator located at the low point in the system. The fuel system uses ¼” ID fuel lines and I have added a fuel filter in each wing root. The goal of the test was to determine how much fuel could flow from a single tank and out the gascolator and compare that rate to the worst case demand of the 100 HP Rotax. Note it really takes two people to do this test safely. Both fuel lines from left and right tanks were pinched off by use of two clamps and the cockpit fuel shutoff was switched off. The drain fitting from the bottom of the gascolator was then removed. A small funnel placed into an empty water bottle that could hold 16.9 ounces and was used to catch the fuel. A stop watch was used to measure how long it took to fill the bottle after one of the clamps was removed. This process was repeated for the other tank as well.

The flow was able to fill the 16.9 ounce bottle in 30 seconds. Since there is 128 ounces of fuel in a gallon this flow rate indicates a flow of 15.84 gallons/hr from a single tank. The other tank resulted in a similar flow rate. This test proved to me that fuel flow to the gascolator from even one tank could easily keep pace with the fuel demand of the engine. In fact if one tank is empty, the flow from the other could supply the engine and the difference flow into the other tank.

This test still did not answer why I had experienced uneven fuel burn. To answer that question another test was conducted with some simple math. The Zenith design uses fuel tank caps that are vented around the edge of the cap. I had wondered if a pressure difference was developing in the tanks that influenced fuel flow. To determine this I drilled holes thru the top of the caps and inserted ¼” tubing which ran into the cockpit and connected to a water manometer. This enabled me to measure the differential tank pressure in flight.

In level cruise flight at 78 mph IAS, the right tank measured -2.875 inch of water and left measured -2.25 inch of water. This converts to pressures of (right) -0.10386 PSI and (left) -0.08129 PSI. The difference pressure is 0.0226 PSI, since fuel weighs 0.02597 lbs/inch, the fuel head required to equal this pressure is 0.869 inch. The fuel level difference required amounts to less than 1”. This result suggest that pressure difference was not the dominate term in my unbalanced fuel burn. It was notable that the pressures on the tanks were negative. A snorkel vent was constructed and tested at 70 mph and found to produce about 0.1 PSI. Each fuel cap was modified with a snorkel vent facing the slip stream to pressurize each tank to a positive pressure of 0.1 PSI.

I will now explain the importance of this pressure if it exists. In a perfect world the pressure above both tanks would be equal and any difference in fuel height (head) would result in fuel flowing from the higher tank, through the gascolator then into the tank with the lower fuel level. This would continue until the level of both tanks was equal. If a slight pressure was applied above the fuel level to one tank then the level fuel would flow from the pressurized tank to the unpressurized tank until a fuel head develops an opposing pressure equal the pressurized tank. Keep in mind that as fuel flows from the source tank the level drops and the level in the other rises, so the head that must develop in the tank is really only half of the total required, since half is a rise in one tank, while half is a drop in the other.

To understand how a slight pressure can create a problem, a little math is needed.

Gasoline weighs 6 pounds per gallon and there is 231 cubic inches in a gallon. Thus an inch of gas weighs 6/231 pounds or 0.02597 pounds per cubic inch and would exert 0.02597 PSI on the bottom of the tank. If a pressure difference between the tanks were to develop of say 0.2 PSI, which doesn’t seem like much, it would take (0.2)/(0.02597) or 7.7 inches of fuel head to equal this pressure. To develop this head requires that the fuel flow from one tank into the other until the head pressure matches the offending tank pressure.

One can easily see that in the case when both tanks start out full, such as at the beginning of a cross county, that fuel pushed into the already full tank must vent overboard. Also to develop the required head the pressured tank must lose all of the fuel head. This is because no additional head can develop in a full tank. It’s not really as bad as this would suggest, because while this is occurring, fuel is being burned which creates room for this head to develop without continued fuel loss.

This simple analysis indicates the importance of minimizing the difference pressure between two tanks. The best way to do this is to add a vent tube that connects the top of both tanks together and use two vented caps. I did not know about this problem until I had completed my aircraft so it was too late for me to do this. For you builders that are still building your wings, I would recommend that you modify your tanks and add a hose fitting to the top of each tank. Then when plumbing your aircraft run a fuel grade hose between the two tanks. Back to my problem, the manometer test I conducted indicated that a slight pressure difference did exist between tanks and contributes to uneven fuel burn. I added snorkel vents to both caps to create a slight positive pressure of near equal value on both tanks to minimize the difference pressure. So long as a bug doesn’t plug one of my vents things should work fine.

The conclusions I have reached are thus, the Zenith fuel system which ties both tanks together will supply needed fuel, even if one tank runs dry before the other. This is because the supply line is able to flow fuel to the gascolator at 15.8 gallons/hour and the engine only requires at most 6.5 gallons/hour. Since there is excess fuel flow capability the two tanks will try to remain equal in level and can do so providing there is not a significant difference in pressure above the tanks. The difference pressure should probably not exceed more than about 0.1 PSI (3.3 inches of fuel head). The best way to minimize the pressure difference is to have a common vent line that connects the two tanks together above the fuel level. Also it would be wise to have vents in both caps so that if one were plugged there is a redundant vent.

If any of you builders out there have arrived at a different conclusion please post or send me you efforts. I would really like to put this to bed so I can fly safe and have peace of mind regarding this problem.


Hope this is not to long for you all, sorry if it is but I wanted to get it all out there for consideration and opinions...
Thanks..Bob Kissell

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n85ae



Joined: 14 Mar 2007
Posts: 403

PostPosted: Wed Sep 17, 2008 12:09 pm    Post subject: Re: fuel flow for CH701 Reply with quote

Great post, and a good experiment!

That's exactly what my experience with my Kitfox has been as well. It's
easy to discount the effects of the vents, and the vent plumbing but it
is a very real phenomenon. Have ground tested my fuel flow from both
tanks and they are very close, but in flight is a different matter. Even in
carefully coordinated flight I get uneven flow.

I think there's one 801 crashed when fuel flow stopped, which makes me
very suspicious of the stock caps as the culprit.

Jeff


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Curt.Thompson(at)verizon.
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PostPosted: Wed Sep 17, 2008 1:10 pm    Post subject: fuel flow for CH701 Reply with quote

Lots of great info Bob. Thanks for describing your test.

If I put a vent line between the tanks, can that line dip and have a low spot? What if fuel sloshed up into that line? Let’s say the dip was 3.3 inches and it filled with fuel. Seems to me that there would have to be a 3.3 inches of fuel head pressure (0.1 psi) differential between the tanks before the air would vent between the tanks. Is there a way to route that line between the tanks without any dips or is there a way to keep fuel out?

Curt
CH701, plans building.


From: owner-zenith701801-list-server(at)matronics.com [mailto:owner-zenith701801-list-server(at)matronics.com] On Behalf Of bob
Sent: Wednesday, September 17, 2008 12:43 PM
To: Zenith701801-List(at)matronics.com
Subject: fuel flow for CH701


Study of the Zenith CH701 two tank fuel system, Bob Kissell, N701UB
<Snip>

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kenryan(at)alaska.net
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PostPosted: Wed Sep 17, 2008 5:08 pm    Post subject: fuel flow for CH701 Reply with quote

Let's say each fuel cap is vented to the outside air. Wouldn't that be the
same as tieing them together with a vent line? What would make tying them
together with a vent line better than just having two vented fuel caps?

On 17 Sep 2008 at 15:42, bob wrote:

Quote:
The best way to minimize the pressure difference is to have a common vent line that connects the two tanks together above the fuel level.


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PostPosted: Wed Sep 17, 2008 5:49 pm    Post subject: fuel flow for CH701 Reply with quote

I think what some have said is that there is a little negative pressure
(vacuum) at the vent in the cap. And then the vacuum is not the same on
both sides. Attaching a tube to each cap and facing them forward causes a
positive pressure. But again, this is not equal on both sides.

Curt
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PostPosted: Wed Sep 17, 2008 5:53 pm    Post subject: fuel flow for CH701 Reply with quote

My 801 does the same thing. I have both tanks tied together with a vent line and still the left tank will drain twice as fast as the right one.. My guess it has to do with the airflow swirling around the fuselage and wings,,,, coming off the prop in a circular motion. The right wing gets hit on the top and the left wing gets hit on the bottom.... Anyone got any other ideas ????
Ben Haas
N801BH
www.haaspowerair.com

-- "n85ae" <n85ae(at)yahoo.com> wrote:
--> Zenith701801-List message posted by: "n85ae" <n85ae(at)yahoo.com>

Great post, and a good experiment!

That's exactly what my experience with my Kitfox has been as well. It's
easy to discount the effects of the vents, and the vent plumbing but it
is a very real phenomenon. Have ground tested my fuel flow from both
tanks and they are very close, but in flight is a different matter. Even in
carefully coordinated flight I get uneven flow.

I think there's one 801 crashed when fuel flow stopped, which makes me
very suspicious of the stock caps as the culprit.

Jeff


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PostPosted: Wed Sep 17, 2008 6:01 pm    Post subject: fuel flow for CH701 Reply with quote

I started with the Zenith supplied vent caps. The 801 has such a massive amount of lift the low pressure area above the wing will suck out fuel through the vented caps if I have anything more then 7/8 full tanks of fuel. One my first flight it stained the top of the right wing, yeah, I used Dupont Imron aircraft paint (at) 300.00 + a gallon too. Boy, was I pissed and I even stopped at the Dupont booth at Osk and explained my problem. Those guys said , " yup, 100LL blue dye will stain the paint". So much for aircraft grade crap !!! I epoxied up my caps and that fixed that issue. I also have aux vents that protrude below the wings with the front of those tubes cut at an angle.

Ben Haas
N801BH
www.haaspowerair.com

-- "Ken Ryan" <kenryan(at)alaska.net> wrote:
--> Zenith701801-List message posted by: "Ken Ryan" <kenryan(at)alaska.net>

Let's say each fuel cap is vented to the outside air. Wouldn't that be the
same as tieing them together with a vent line? What would make tying them
together with a vent line better than just having two vented fuel caps?

On 17 Sep 2008 at 15:42, bob wrote:

Quote:
The best way to minimize the pressure difference is to have a common vent line that connects the two tanks together above the fuel level.
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PostPosted: Thu Sep 18, 2008 2:05 am    Post subject: fuel flow for CH701 Reply with quote

Ben

my 701 has the same swirl but it is the right tank which drains first...

Why may only by known by the Great Gull Smile

Peter
701/912ULS


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PostPosted: Thu Sep 18, 2008 4:43 am    Post subject: fuel flow for CH701 Reply with quote

No, the more vents you have the better. They do on occasion get stopped
up with bugs, trash, etc. If you only have one you are asking for
trouble, not a matter of if, when. Larry, N1345L

Ken Ryan wrote:
Quote:


Let's say each fuel cap is vented to the outside air. Wouldn't that be the
same as tieing them together with a vent line? What would make tying them
together with a vent line better than just having two vented fuel caps?

On 17 Sep 2008 at 15:42, bob wrote:


> The best way to minimize the pressure difference is to have a common vent line that connects the two tanks together above the fuel level.
>

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PostPosted: Thu Sep 18, 2008 4:47 am    Post subject: fuel flow for CH701 Reply with quote

That's correct. I have four tanks, each vents with tubes to the bottom
of the wing. Vent caps can be trouble as you point out, plus water can
get in. Larry, N1345L

n801bh(at)netzero.com wrote:
Quote:
I started with the Zenith supplied vent caps. The 801 has such a massive amount of lift the low pressure area above the wing will suck out fuel through the vented caps if I have anything more then 7/8 full tanks of fuel. One my first flight it stained the top of the right wing, yeah, I used Dupont Imron aircraft paint (at) 300.00 + a gallon too. Boy, was I pissed and I even stopped at the Dupont booth at Osk and explained my problem. Those guys said , " yup, 100LL blue dye will stain the paint". So much for aircraft grade crap !!! I epoxied up my caps and that fixed that issue. I also have aux vents that protrude below the wings with the front of those tubes cut at an angle.

Ben Haas
N801BH
www.haaspowerair.com

-- "Ken Ryan" <kenryan(at)alaska.net> wrote:


Let's say each fuel cap is vented to the outside air. Wouldn't that be the
same as tieing them together with a vent line? What would make tying them
together with a vent line better than just having two vented fuel caps?

On 17 Sep 2008 at 15:42, bob wrote:


> The best way to minimize the pressure difference is to have a common vent line that connects the two tanks together above the fuel level.
>

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jetboy



Joined: 22 Jul 2006
Posts: 233

PostPosted: Thu Sep 18, 2008 4:09 pm    Post subject: Re: fuel flow for CH701 Reply with quote

The CZAW kits came with non vented caps that have a 60mm high gooseneck tube welded onto the cap with the opening facing downwards and forward, end of the tube is also cut at an angle so appears elliptical.

I've had no problems with these, much easier than adding crossfeed venting, although I have an andair selector valve fitted between the seat backrest and usually feed one at a time
Ralph


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NYTerminat(at)aol.com
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PostPosted: Thu Sep 18, 2008 5:09 pm    Post subject: fuel flow for CH701 Reply with quote

Someone want to fabricate these fuel caps for a retrofit? I would buy a set.

Bob Spudis




In a message dated 9/18/2008 8:10:02 P.M. Eastern Daylight Time, sanson.r(at)xtra.co.nz writes:
Quote:

The CZAW kits came with non vented caps that have a 60mm high gooseneck tube welded onto the cap with the opening facing downwards and forward, end of the tube is also cut at an angle so appears elliptical.

I've had no problems with these, much easier than adding crossfeed venting, although I have an andair selector valve fitted between the seat backrest and usually feed one at a time
Ralph

--------
Ralph - CH701 / 2200a




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n752ms(at)softcom.net
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PostPosted: Thu Sep 18, 2008 6:37 pm    Post subject: fuel flow for CH701 Reply with quote

Bob.

Look at a Wag-Aero catalog. They have them.

Mark S.
701/912ULS

[quote] ---


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PostPosted: Thu Sep 18, 2008 6:54 pm    Post subject: fuel flow for CH701 Reply with quote

Thanks Mark, just looked online, couldn't find the cap with the pressure tube built in. Do you have a part number?

Bob




In a message dated 9/18/2008 10:37:39 P.M. Eastern Daylight Time, n752ms(at)softcom.net writes:
[quote] Bob.

Look at a Wag-Aero catalog. They have them.

Mark S.
701/912ULS

[quote] ---


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craig(at)craigandjean.com
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PostPosted: Thu Sep 18, 2008 7:05 pm    Post subject: fuel flow for CH701 Reply with quote

Aircraft Spruce 05-03458 might work if you bent the tube down farther. Kind
of expensive at $100. It's described as "Fuel Cap PA-12 thru PA-18 with
180HP Engine"

-- Craig


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PostPosted: Thu Sep 18, 2008 7:16 pm    Post subject: fuel flow for CH701 Reply with quote

Mark S,
I looked at the Wag-Aero catalog on-line and couldn't find a high gooseneck tube welded onto a cap. I have reproduced their fuel cap catalog below. Can you tell me which item from Wag-Aero to purchase?
Les
Products Manufacturer Model SKU Price Flange Vented Fuel Cap [/url] E-482-000 $5.75 [url=http://store.wagaero.com/product_info.php?products_id=14151]Homebuilder's Fuel Tank Parts, Aluminum Weldable Neck [/url] E-448-000 $6.85 [url=http://store.wagaero.com/product_info.php?products_id=1916]Flange Vented Fuel Cap [/url] E-482-000 $5.75 [url=http://store.wagaero.com/product_info.php?products_id=1918]Pressure Fuel Cap, Aerobatic [/url] 55010-3 E-333-000 $27.25 [url=http://store.wagaero.com/product_info.php?products_id=1923]Piper Fuel Cap PA-24, FAA/PMA'd [/url] 21821 E-333-100 $89.50 [url=http://store.wagaero.com/product_info.php?products_id=1925]Piper Fuel Cap PA-24, 30, 39, FAA/PMA'd Piper 27221 E-333-200 $112.75 Piper Fuel Cap PA-23, FAA/PMA'd [/url] 17672 E-333-300 $124.60 [url=http://store.wagaero.com/product_info.php?products_id=1928]Piper Fuel Cap PA-23, FAA/PMA'd [/url] 16097 E-333-400 $102.50 [url=http://store.wagaero.com/product_info.php?products_id=1956]Homebuilder's Fuel Tank Parts, Aluminum Weldable Neck [/url] E-448-000 $6.85 [url=http://store.wagaero.com/product_info.php?products_id=1958]Homebuilder's Fuel Tank Parts, Aluminum Filler Neck [/url] E-324-000 $13.70 [url=http://store.wagaero.com/product_info.php?products_id=2005]Aeronca Wing Kit Aeronca E-476-000 $768.00 Aerobatic Header Tank [/url] E-475-100 $252.75 [url=http://store.wagaero.com/product_info.php?products_id=3943]Fuel System Kit Wag-Aero W-014-000 $1,460.00 Luscombe Fuel Tank, LH 11.5 Gallon Luscombe E-473-002 $575.00 Luscombe Fuel Tank, RH 11.5 Gallon Luscombe E-473-001 $575.00 GATS Jar Fuel Tester [/url] G-781-000 $22.95 [url=http://store.wagaero.com/product_info.php?products_id=12848]Non-Vented Fuel Cap Aeronca 1-2593-2 125932 $12.86

[quote] From: owner-zenith701801-list-server(at)matronics.com [mailto:owner-zenith701801-list-server(at)matronics.com] On Behalf Of Mark Sherman
Sent: Thursday, September 18, 2008 7:36 PM
To: zenith701801-list(at)matronics.com
Subject: Re: Re: fuel flow for CH701

Bob.

Look at a Wag-Aero catalog. They have them.

Mark S.
701/912ULS

[quote] ---


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b.carl@sympatico.ca



Joined: 20 May 2008
Posts: 76

PostPosted: Sat Sep 20, 2008 5:20 pm    Post subject: fuel flow for CH701 Reply with quote

Listers

This message is in reply to several messages on this topic.
During the last 13 years I have gone from the original Zenair large nose tank with two wing tanks, to removing the nose tank and just keeping the two wing tanks, to the present system with two wing tanks and a collector behind the baggage compartment.
The biggest problem with the nose tank was the nose heavy c of g (especially on floats) and the requirement to have the battery at the extreme rear. The problem of uneven feeding was controlled with valves at the wing roots and a main shut-off valve at the IP sub panel. The problem with just the wing tanks was the inability to safely use all the wing fuel without risking fuel starvation.
The addition of the 8 litter collector with a sight gauge and low level warning light allows me to safely drain all the available fuel from the wing tanks and safely land with as little as 4-6 litters of useable fuel.
The other advantage is that my battery is now in the engine compartment saving me 12 feet of high amperage cable running the length of the a/c and the stator now gets full battery voltage.
Save weight in cable and I don't have to carry a lot of wing fuel that can't safely be used.
This works for me.
Regards
Carl
701/912

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SafeAirOne



Joined: 18 May 2008
Posts: 17
Location: Manchester, NH

PostPosted: Sun Sep 21, 2008 7:30 pm    Post subject: Re: fuel flow for CH701 Reply with quote

Great study of the issue, Bob!

I was curious about this problem and its solution in production aircraft. A web search revealed the same complaint from everything from Yaks to Cessnas. Of course, the Yaks are low-winged, but the discussion in their forum eventually turned to the problem in Cessnas.

See:

http://www.aviatorair.com/Site/Aviator_Forum.html

From what I gather, unless you can get a common vent line completely above the fuel level, (and we can't do this with any degree of elegance in the 701/801) you can only minimize the issue, as Cessna has, and not eliminate it.

Interesting to note that the full tank is the one feeding the engine and siphoning the fuel out of the emptier tank to fill the vacuum created in the feeding tank.
-Mark
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Curt.Thompson(at)verizon.
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PostPosted: Mon Sep 22, 2008 9:39 am    Post subject: fuel flow for CH701 Reply with quote

Great find on that info Mark. Kind of confirms my thought about that
interconnecting vent tube needing to always be above the tank.

I don't quite get how the fuel is coming from the fullest tank though. If
the fuel was coming from the fullest tank then why wouldn't the level in
that tank go down? Is the posting saying that the fuel is siphoning across
the interconnecting vent tube?

I also don't see how putting a vent tube on both tanks would help either.
Wouldn't that be more likely to allow a differential pressure between tanks?

Curt
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Curt.Thompson(at)verizon.
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PostPosted: Mon Sep 22, 2008 11:46 am    Post subject: fuel flow for CH701 Reply with quote

Here is a Cessna Tech Note on "Uneven Fuel Feeding in Single Engine
Cessna's":
http://www.moonflight.org/Documents/Fflow.pdf

There is a lot of good stuff there. It has more detailed information about
most of this and explains how fuel is siphoned from the less full tank to
the fuller tank via the interconnecting vent line. It has the whys and
when's the fuel system was changed. Also some simple tests I had not
considered.

It answered my question about why one fuel cap is vented and the other is
not. BTW, it is not always vented but only vents as a backup after the
vacuum has built up after a primary vent failure.

I always noticed that the vent tube is located behind the strut. I thought
it might be there to reduce the chance of a bug plugging it in flight.
Turns out it is to reduce the chances of freezing. The vent tube location
and the strut fairing location (condition) are critical.

When I started this thread a few weeks ago, my primary concern was not
"unequal fuel flow". I think that can be explained by unequal tank pressure
or level due to various causes. My big concern is how "one tank completely
drains before the other tank starts to flow". The only explanation I've
seen for that so far is that an air bubble gets trapped in the line from one
tank.

Curt


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