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sonar1(at)cox.net
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 Posted: Thu Jun 29, 2006 4:56 pm Post subject: Fuel system poser |
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Hi guys:
I've got 70 hours on my CH701, and loving it, but I have a real poser
for you thinkers.
On two flights, one fuel tank has failed to feed. I landed once with ten
gallons on one side, and two on the other. Today, it was 8 on one side,
and one gallon on the other. And they were different tanks! Once the
right one, and once the left!
There is a bubble in the line of the tank that is not feeding. I thought
it might be a vacuum from the caps, so they have been changed.
What could be simpler? Two high tanks with lines to the gascolator,
through the fuel filter, and to the engine.
Am I flying sideways? I'm stumped!!
Fred Sanford N9701 Santa Barbara, Ca. 70 hours having fun!
do not archive
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mike.sinclair(at)ATT.NET
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| Posted: Thu Jun 29, 2006 7:05 pm Post subject: Fuel system poser |
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Fred
Suspect what may be happening is that even with the vented caps and the
tanks up high, there is not enough actual pressure from fuel depth to
overcome a bubble in the line up in the wing root area. I've got a header
tank and transfer fuel from the wing tanks to the header. I have clear lines
from the fittings at the wing root, down the cabin frame ahead of the doors,
then they run through standard fuel line to the header. I went with the bing
blue fuel line in this area so that I can tell when the wing tanks are
empty. I can also see after fueling and during preflight if there is air in
the line. If there is air I make sure the header tank valve is off, open the
valve for the wing tank, open the drain on the gascolator, and hit the
switch for the electric fuel pump (mounted slightly higher than the
gascolator, but just barely), and see the bubble pulled right on down and
out. I lose less than a cup of fuel in the process. Part of my preflight is
to make sure there is no visible air in the down line, because the fuel just
don't want to transfer if I see air. First time I ran into this I had about
a 3rd of a tank full in the header and 14 gallons in the wings. I was
rocking the wings and other related manuvers for about 10 minutes before I
could get fuel to flow from one of the tanks. All that gas and I was getting
real serious about a place to stop before I would run out of gas. I had this
problem a couple other times and finally figured out what was happening. Now
with no air showing I still see little bubbles at the start of transfer so I
do know the fuel is flowing. Little air bubbles good, big air bubble bad!
Hope this gives you a place to look.
Mike Sinclair Just over 100 hours now on mine.
fred sanford wrote:
| Quote: |
Hi guys:
I've got 70 hours on my CH701, and loving it, but I have a real poser
for you thinkers.
On two flights, one fuel tank has failed to feed. I landed once with ten
gallons on one side, and two on the other. Today, it was 8 on one side,
and one gallon on the other. And they were different tanks! Once the
right one, and once the left!
There is a bubble in the line of the tank that is not feeding. I thought
it might be a vacuum from the caps, so they have been changed.
What could be simpler? Two high tanks with lines to the gascolator,
through the fuel filter, and to the engine.
Am I flying sideways? I'm stumped!!
Fred Sanford N9701 Santa Barbara, Ca. 70 hours having fun!
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jnjkimbell(at)hotmail.com
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| Posted: Fri Jun 30, 2006 5:26 am Post subject: Fuel system poser |
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Fred, I have 36 hrs on my 701 and have the same situation. After 5000+ hrs
flying small AC I hope I am correct in believing that on a gravity feed 2
tank system, that when one goes dry, the fuel in the tank with remaining
fuel will overcome any air (forcing air into empty tank) and the weight of
the fuel will feed engine. If the bubble was below (or beyond) the fuel
selector valve, it should not have effected which tank used fuel first.
Along this line: Has any one ran one tank dry (until engine quits) and then
changed over to other Tank? Also, on Rotax 912, did engine windmill or did
you have to use elect. starter. Joe
---
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Peter Barthold
Joined: 24 Apr 2006
Posts: 25
Location: Germany
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| Posted: Fri Jun 30, 2006 6:37 am Post subject: Re: Fuel system poser |
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Fred,
I'm far away from being an expert here, just a shot in the dark:
Are your fuel caps the only source of ventilation? If yes, are they located on top of your wing profile (or at least somewhere near that)? Then they are in the low pressure zone of your airfoil and air is possibly sucked out of your tanks by the vents. I would tape some flexible hose temporarily to the fuel opening and fix the open end into the airstream. If I am correct, then your quickie ram air vent will work and you will have enough fuel pressure. Next step would be a Cessna type ram air system.
All this is not my wisdom of course but can be found in tony Bingelis' books
Hope this helps
Greetings from Germany
Peter
HDS TD VW
Tail done Wings in progress
www.petersprojekt42.de
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dredmoody(at)cox.net
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| Posted: Fri Jun 30, 2006 7:00 am Post subject: Fuel system poser |
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Hi Guys,
Even with high mounted external auxillary tanks on my ultra-not-so-light, I have found that a bubble in the line can sit there obstinately defying the pressure of the fuel which seemingly should push it on down the tube.
What is apparently happening is that the bubble is trying to "rise" as it would it a large container so it resists the downward, gravity induced flow in the line. As long as that continues, the bubble acts as a partial obstruction requiring the fuel to flow around it. The effective cross-sectional area of the line is drasticallt reduced and very little fuel flows past the bubble at a ridiculously slow rate.
That phenomenon is why I have chosen to put a fuel pump near each wing tank in my 601XL, hoping that the pressure of the pump will be more effective at moving the bubble through the line if/when it happens. The bubbles I'm most concerned with are the ones caused by warm gasohol vaporizing under low pressure on the suction side of the pump.
Ed Moody II
---- Joe and Joan <jnjkimbell(at)hotmail.com> wrote:
| Quote: |
I hope I am correct in believing that on a gravity feed 2
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| Quote: | tank system, that when one goes dry, the fuel in the tank with remaining
fuel will overcome any air (forcing air into empty tank) and the weight of
the fuel will feed engine.
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| Quote: | >
>
> Hi guys:
> I've got 70 hours on my CH701, and loving it, but I have a real poser for
> you thinkers.
> On two flights, one fuel tank has failed to feed. I landed once with ten
> gallons on one side, and two on the other. Today, it was 8 on one side,
> and one gallon on the other. And they were different tanks! Once the right
> one, and once the left!
> There is a bubble in the line of the tank that is not feeding.> > What could be simpler? Two high tanks with lines to the gascolator,
> through the fuel filter, and to the engine.
> Am I flying sideways? I'm stumped!!
|
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JERICKSON03E(at)aol.com
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| Posted: Fri Jun 30, 2006 3:41 pm Post subject: Fuel system poser |
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In a message dated 6/29/2006 10:06:17 PM Central Daylight Time, mike.sinclair(at)ATT.NET writes:
| Quote: | Suspect what may be happening is that even with the vented caps and the
tanks up high, there is not enough actual pressure from fuel depth to
overcome a bubble in the line up in the wing root ar |
Both comments cite use of vented caps.
Question. Are the caps the P.N. E-482-000 as specified by 7-V-11? If so, do they have the additional vent, a 1/8 in hole drilled through the cap by the builder?
If not, what is the exact configuration of the caps in use during the no fuel feed problem?
Regards, Jerry
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NYTerminat(at)aol.com
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| Posted: Sat Jul 01, 2006 8:11 pm Post subject: Fuel system poser |
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Jerry,
Thanks, I missed the drilling of the 1/8" hole. Does that hole go straight up through the cap? What about water entering the tank? I have had the left tank burn off sooner than the right. I have fuel shut off valves at the door headers so I can shut off the side that has less fuel than the other to balance them out. Lately they have been going down pretty evenly.
Bob Spudis
N701ZX / 55hrs
In a message dated 6/30/2006 7:43:03 P.M. Eastern Daylight Time, JERICKSON03E(at)aol.com writes:
| Quote: | Both comments cite use of vented caps.
Question. Are the caps the P.N. E-482-000 as specified by 7-V-11? If so, do they have the additional vent, a 1/8 in hole drilled through the cap by the builder?
If not, what is the exact configuration of the caps in use during the no fuel feed problem?
Regards, Jerry
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JERICKSON03E(at)aol.com
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| Posted: Sun Jul 02, 2006 9:49 am Post subject: Fuel system poser |
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In a message dated 7/1/2006 11:14:44 PM Central Daylight Time, NYTerminat(at)aol.com writes:
| Quote: | Jerry,
Thanks, I missed the drilling of the 1/8" hole. Does that hole go straight up through the cap? What about water entering the tank? I have had the left tank burn off sooner than the right. I have fuel shut off valves at the door headers so I can shut off the side that has less fuel than the other to balance them out. Lately they have been going down pretty evenly.
Bob Spudis
N701ZX / 55hrs
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Bob,
The reason I asked is that on previous list postings about fuel feed problems no mention was made of the cap configuration, per the ZAC print. What changes from the static gravity fuel flow tests, to the in flight system operation is the air pressure in side the tank.
Change in the fuel level, head, and change in the tank pressure will effect the ability of the fuel to gravity feed.
As to the location of the hole, I plan to drill through the middle hole already located in the spring tensioned that is riveted in place in the cap. But only thru the inside section.
The fuel caps on the 701 demo at SAC, have been modified to include a ram air tube with an L brazed or welded to it and to the cap. That is what I am planning on.
Some time ago a list 701 builder mentioned that in addition to using a ram air tube, like Tony Binges suggests, he also soldered shut the four edge vents inside the cap. That makes since to me because those vents live in the upper wing surface low pressure area, during flight. So, it seems like the pressure would reduce in the tank, due to out flow suction through the four edge vents in the cap. Make since?
So that would result in the ram air tube feeding the tank, with no leakage at the edge vents.
The cap gasket seal might also need work to assure a good seal. Prevent fuel siphoning, Prevent air siphoning too. Perhaps a viton type seal rather than pressed cardboard??
At any rate, if the gravity flow is OK during 14.7 static tests, ,,,, and has less flow during flight, something has changed.
I'm thinking that the inside tank air pressure becomes less than atmospheric, resulting in less fuel flow.
Also as fuel is used from the tank, there is less fuel head to aid in gravity flow.
As to bubbles living in the fuel lines, and blocking fuel flow, it seems so sensitive as to be in need of a serious fix. Would it be more prone to occur during hot weather,, or when using auto fuel, or just after draining the gascolator?? Just wondering out loud here.
Good luck with solving the problem, and please do keep us posted.
Jerry
CH 701 SP ~90%
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NYTerminat(at)aol.com
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| Posted: Sun Jul 02, 2006 7:55 pm Post subject: Fuel system poser |
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In a message dated 7/2/2006 1:50:20 P.M. Eastern Daylight Time, JERICKSON03E(at)aol.com writes:
| Quote: |
The fuel caps on the 701 demo at SAC, have been modified to include a ram air tube with an L brazed or welded to it and to the cap. That is what I am planning on.
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Jerry,
Thanks for your input, the demo may have a header tank which I believe required the ram air tube.
Bob Spudis
do not archive
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