Imbalanced fuel flow: was Re: REFLECTOR:Low fuel light
Brett Ferrell
reflector@tvbf.org
Thu, 24 Jul 2003 11:29:32 -0400
Theoretically? You should not have to, if the wings are approximately level
the head pressure of the strake with fuel "should" keep the sump full as
long as it can feed the flow the engine requires. In fact, it will
back-feed the "empty" stake to the level it contains (again assuming
adequate flow), so that strake will never be truly empty (fluids seek their
own level, everything else being equal). That's the whole point of having a
sump rather than two separate lines to the engine, the tanks should feed
about equally, and should be able to use all of both tanks.
If the engine could outdraw the gravity flow from a single strake, it would
try to suck air through the vent or the other strake drain line, and that
would, well, suck.... Worse, even if it couldn't pull air all of the way to
the engine, as the head pressure on the fuel pump sags, it could cavitate
and become ineffective long before that. Does anybody know if aircraft fuel
pumps can operate without head pressure on the inlet?
A turn towards the full strake could get a little exciting, but the sump
should hold enough buffer to outlast a turn, but this is exactly a situation
where the sump needs to vent air inorder to refill, though assuming the
other tank is dry it could vent through that line.
The only things that should affect the rate of flow between the tanks is if
the wings aren't level (check your ball), a leak (through the cap or
otherwise - which should only matter if the vent lines can't equalize the
pressure), or if there's a big difference in the line losses in the fuel
drain lines between the strakes (shouldn't be the case, but could be if
there were debris on your strake drain screen).
Brett
----- Original Message -----
From: "Chuck Jensen" <cjensen@dts9000.com>
To: <reflector@tvbf.org>
Sent: Thursday, July 24, 2003 11:11 AM
Subject: RE: Imbalanced fuel flow: was Re: REFLECTOR:Low fuel light
> Scott,
>
> Good idea. I'll give that a look-see. In turns, I feel lucky if I keep
the
> ball anywhere near the lubber lines, but in straight and near-level, that
> should be easy to see.
>
> If a person thinks about the problem a little (which always gives me a
> headache), it would only take a wing being a couple inches low to make
quite
> a difference in fuel feed between the tanks.
>
> I am still wondering if the left tank goes to 0, will I have to fly in a
> slight slip to keep my right tank feeding? Or will the usual variations
of
> flight continue to keep the sump filled as the right wing goes up
> momentarily? If it will, then no problem....except, of course, when I
turn
> a right base onto final with a quartering head wind from the right. My
> rough calculations (and bad luck) indicate I would run out of fuel about
1/2
> mile short of the threshold. My headache is getting worse.
>
> Chuck
>
> -----Original Message-----
> From: reflector-admin@tvbf.org [mailto:reflector-admin@tvbf.org]On
> Behalf Of Scott Derrick
> Sent: Thursday, July 24, 2003 9:56 AM
> To: reflector@tvbf.org
> Subject: Imbalanced fuel flow: was Re: REFLECTOR:Low fuel light
>
>
> Chuck,
>
> I'd bet your plane is not trimmed correctly. I had imbalance tanks after
> flying a short while when I first bought my plane. I futzed with the
> caps. no change.
>
> The ball was not quite centered within the lubber lines when I was
> flying straight and level.
>
> So I tried this. I flew a while until the tanks were 5-6 gallons
> different, then I held a rudder in to put the ball on the opposite side
> it was sitting on but all the way on the other side of the lubber line.
> Flew a very short while and the tanks evened out.
>
> If you shim out the correct rudder so the ball is exactly in the middle
> I think your imbalance will be cured. Its a simple fix, just shim out
> the correct rudder to get the ball centered. Its an easy thing to try, I
> used a dab of JB Weld to shim it out.
>
> Scott
>
> Chuck Jensen wrote:
> > Good stuff on the vent system. I've been greasing (petro jelly) the O
> > rings on the fuel caps but didn't even think about the O ring on the
> > locking lever. During a long cruise, I'll get a 9 gallon indicated
> > differential between the tanks, so these efforts might reduce that
> > differential. I wasn't too concerned as I understand some differential
> > is almost expected.
> >
> > Though I've never gone there, what happens when left tank is 0 and the
> > right is 9 gallons? Will the rest of the fuel draw from the right tank
> > or will I have to pull off on the side of the road and hitch a ride for
> > gas? I'd hate to think I was carrying 9-10 gallons of useless ballast in
> > my tanks but I'm not (intentionally) going to find out. Anyone been
> > there, done that?
> >
> > Chuck
> > XL RG 540
> >
> > -----Original Message-----
> > From: reflector-admin@tvbf.org [mailto:reflector-admin@tvbf.org]On
> > Behalf Of Ronnie Brown
> > Sent: Thursday, July 24, 2003 7:46 AM
> > To: reflector@tvbf.org
> > Subject: Re: REFLECTOR:Low fuel light
> >
> > I think the two primary reasons for the fuel tank imbalance are (1)
> > airplane flying out of trim, i.e. one wing low; (2) a fuel tank cap
> > leak.
> >
> > Enlarging the vent system to 3/8" instead of the original 1/4" will
> > greatly improve the impacts from (2) a leaking fuel cap. 3/8
> > tubing has double the area of a 1/4" tube and will pass twice as
> > much air. The reason this matters is that we are only talking about
> > 2 feet of fuel level from the bottom of the sump to the top of the
> > tanks. This is less than 1 psi. It doesn't take much of a cap leak
> > to pull a slight vacuum on that tank which causes that tank to fill
> > up. Keep those O rings vaselined and also make sure the cap is
> > pulling all the way down (adjust the lock nut on the back side of
> > the cap). There is also a small O ring on the shaft of the locking
> > lever that needs to be lubricated. If you want to check how tightly
> > your caps are sealing, connect a tube to the vent line, blow in it
> > (keep the pressure less than 1 psi!) and spray the caps with a
> > water-dish detergent mixture.
> >
> > The original Velocity design had separate vents for each tank -
> > which led to lots of problems. It is impossible to create two vent
> > outlets that have the same static pressure at 200 mph!!! Hence the
> > single vent system now used. It is also important that the vent
> > tubing slope upward to the manifold or you can trap some fuel and
> > create a tank vent pressure imbalance which will lead to different
> > fuel levels in the tank.
> >
> > Ronnie
> >
> > ----- Original Message -----
> > From: KeithHallsten <mailto:KeithHallsten@quiknet.com>
> > To: reflector@tvbf.org <mailto:reflector@tvbf.org>
> > Sent: Wednesday, July 23, 2003 11:20 PM
> > Subject: Re: REFLECTOR:Low fuel light
> >
> > Yes! Everything you say "SHOULD" be true. Unfortunately, we
> > continue to get these reports from the flying population
> > regarding odd behavior of the fuel/vent system!
> >
> > I think that at times the vent system supplies air to the sump
> > tank just a TINY bit more easily than the longer path through
> > the strakes supplies fuel to the sump tank. It's probably a
> > transient event, just persisting long enough to actuate the "low
> > fuel" warning and scare the bejesus out of the pilot. I see
> > little risk in shutting the vent line to the sump tank, thereby
> > assuring that the flow into the sump tank must come from the
> > strake tanks.
> >
> >
> >
> > ----- Original Message -----
> > From: Ronnie Brown <mailto:romott@adelphia.net>
> > To: reflector@tvbf.org <mailto:reflector@tvbf.org>
> > Sent: Wednesday, July 23, 2003 6:55 PM
> > Subject: Re: REFLECTOR:Low fuel light
> >
> > If the vent lines slope upward to the manifold from the
> > three tanks, there shouldn't be any problems trapping air or
> > fuel. The fuel should go down and the vent air goes up.
> > And the fuel in the sump tank vent should seek the same
> > level as in the fuel in the main tanks, thereby keeping the
> > sump tank full and the alarm off.
> >
> > Ronnie
> >
> > ----- Original Message -----
> > From: KeithHallsten <mailto:KeithHallsten@quiknet.com>
> > To: reflector@tvbf.org <mailto:reflector@tvbf.org>
> > Sent: Wednesday, July 23, 2003 8:51 PM
> > Subject: Re: REFLECTOR:Low fuel light
> >
> > Rene',
> >
> > I'm not flying yet, so my comments can be discounted
> > accordingly. I plan to install a manual vent shut-off
> > valve between the sump tank and the vent manifold. The
> > result will be that the vent system can allow the main
> > strake tanks to drain, but can't introduce air into the
> > sump tank. I have a theory that the vent line to the
> > sump tank causes more problems than it solves, once the
> > initial "burp" of air is allowed to escape from the sump
> > tank (before engine start).
> >
> > Keith Hallsten
> >
> >
> > ----- Original Message -----
> > From: Rene Dugas <mailto:rdugas@bayou.com>
> > To: reflector@tvbf.org <mailto:reflector@tvbf.org>
> > Sent: Wednesday, July 23, 2003 4:00 PM
> > Subject: RE: REFLECTOR:Low fuel light
> >
> > Thanks Jack,
> >
> > I felt that maybe the boundary layer was causing a
> > vacuum allowing air to enter the sump but not the
> > tanks. I don not want to over pressure my tanks and
> > rupture them at high pressures. I can splice a gage
> > into the line but wanted to glean info from the
> > informed (flying) few. Since all three tanks are
> > connected to the manifold I remain perplexed.
> > Thanks for the info on the Vision. Two engine
> > builders told me significant fluctuations with the
> > engine pump are common but no pressure numbers were
> > available. I talked to Lycoming. He air might be
> > in the line but I don't see how. Thanks
> >
> > Rene'
> >
> >
> >
> > -----Original Message-----
> > From: reflector-admin@tvbf.org
> > [mailto:reflector-admin@tvbf.org] On Behalf Of Jack
> > Sheehan
> > Sent: Wednesday, July 23, 2003 4:35 PM
> > To: reflector@tvbf.org
> > Subject: Re: REFLECTOR:Low fuel light
> >
> >
> >
> > Rene'
> >
> > I can not say much about the low fuel light except
> > that I have not had that on my XL-RG and I am not
> > extending the vent. Mine is as you described in the
> > plumbing except the vent line under the belly is cut
> > off almost flush with the fuselage. I have tested my
> > sump warning light durin the test flight period by
> > getting pretty low on fuel and then pushing the nose
> > over and maintain a dive for enough time for the
> > sump light to start to flicker The low pressure
> > warning from the VM1000 is a common indication. I
> > have spoken with Vision micro about it and the good
> > news is the VM1000 is working fine and the bad news
> > is the lack of pressure is real. it is the way the
> > lycoming pump works and if you have an analog system
> > with low sensitivity you never see it. They have
> > sent me a snubber to install in the sensor line
> > which will in theory reduce thesensitivity. ther is
> > a similar snubber device in the manifold pressure
> > system. I will let you know when I get some free
> > time to install it.
> >
> > Jack
> >
> > N55XL
> >
>
>
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