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kissell

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

Posted: Wed Sep 17, 2008 11:42 am Post subject: fuel flow for CH701

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  

 

   [quote][b]

n85ae

Joined: 14 Mar 2007
Posts: 403

Posted: Wed Sep 17, 2008 12:09 pm Post subject: Re: fuel flow for CH701

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

Curt.Thompson(at)verizon.
Guest

Posted: Wed Sep 17, 2008 1:10 pm Post subject: fuel flow for CH701

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>  

  

        [quote][b]

kenryan(at)alaska.net
Guest

Posted: Wed Sep 17, 2008 5:08 pm Post subject: fuel flow for CH701

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.

Curt.Thompson(at)verizon.
Guest

Posted: Wed Sep 17, 2008 5:49 pm Post subject: fuel flow for CH701

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

--

n801bh(at)netzero.com
Guest

Posted: Wed Sep 17, 2008 5:53 pm Post subject: fuel flow for CH701

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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n801bh(at)netzero.com
Guest

Posted: Wed Sep 17, 2008 6:01 pm Post subject: fuel flow for CH701

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. 

<========================bsp;      - The Zenith701801-List Email Fo===============================================  
	




____________________________________________________________ 

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peterlthomson(at)gmail.co
Guest

Posted: Thu Sep 18, 2008 2:05 am Post subject: fuel flow for CH701

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 



Peter

701/912ULS

lrm(at)skyhawg.com
Guest

Posted: Thu Sep 18, 2008 4:43 am Post subject: fuel flow for CH701

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. 

>     

 ------------------------------------------------------------------------

 Checked by AVG - http://www.avg.com

lrm(at)skyhawg.com
Guest

Posted: Thu Sep 18, 2008 4:47 am Post subject: fuel flow for CH701

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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 ____________________________________________________________

 Planning for retirement? Click for free information on 401(k) plans.

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jetboy

Joined: 22 Jul 2006
Posts: 233

Posted: Thu Sep 18, 2008 4:09 pm Post subject: Re: fuel flow for CH701

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

NYTerminat(at)aol.com
Guest

Posted: Thu Sep 18, 2008 5:09 pm Post subject: fuel flow for CH701

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

	


 

  



Looking for simple solutions to your real-life financial challenges? Check out WalletPop for the latest news and information, tips and calculators.

  [quote][b]

n752ms(at)softcom.net
Guest

Posted: Thu Sep 18, 2008 6:37 pm Post subject: fuel flow for CH701

Bob.

  

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

  

 Mark S.

 701/912ULS

  

 [quote]   ---

NYTerminat(at)aol.com
Guest

Posted: Thu Sep 18, 2008 6:54 pm Post subject: fuel flow for CH701

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]     ---

craig(at)craigandjean.com
Guest

Posted: Thu Sep 18, 2008 7:05 pm Post subject: fuel flow for CH701

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

lgold(at)quantum-associat
Guest

Posted: Thu Sep 18, 2008 7:16 pm Post subject: fuel flow for CH701

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]     ---

b.carl@sympatico.ca

Joined: 20 May 2008
Posts: 77

Posted: Sat Sep 20, 2008 5:20 pm Post subject: fuel flow for CH701

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

  

   [quote][b]

SafeAirOne

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

Posted: Sun Sep 21, 2008 7:30 pm Post subject: Re: fuel flow for CH701

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

do not archive

Curt.Thompson(at)verizon.
Guest

Posted: Mon Sep 22, 2008 9:39 am Post subject: fuel flow for CH701

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

--

Curt.Thompson(at)verizon.
Guest

Posted: Mon Sep 22, 2008 11:46 am Post subject: fuel flow for CH701

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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