Imbalanced fuel flow: was Re: REFLECTOR:Low fuel light
Scott Derrick
reflector@tvbf.org
Thu, 24 Jul 2003 07:56:11 -0600
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
>