REFLECTOR: Uneven Fuel Flow

[Alex Balic]

Yea, the fuel cap would vent enough air to allow an imbalance in the tanks
that the vent would not fully compensate for- remember, that there is very
very little head pressure between the top of the tank and wherever the two
tanks T together, ( especially because fuel is significantly less dense than
water) so almost any discrepancy in pressure would cause one tank to drain
faster than the  other.  Think of it this way- if the tanks are say 10"
deep, you could model that by having a u shaped piece of aquarium air line
with  8 or 10" of fuel in both sides of the U- how  hard do you think you
would have to blow against one side of the tube to cause a big differential
in the height of the fuel - not much, so even a little bit of pressure,
would favor one tank to drain before the other.  Especially if the vent is
getting ram pressure.

 

A flat vent exit would surely preclude getting jammed up by a bug or clod of
dirt- watch out for the mud daubers though- that 3/8 line is just right for
them. 

 

From: reflector-bounces at tvbf.org [mailto:reflector-bounces at tvbf.org] On
Behalf Of Brooke Wolf
Sent: Thursday, September 29, 2011 3:02 PM
To: reflector at tvbf.org
Subject: Re: REFLECTOR: Uneven Fuel Flow

 

AH HA!  So what you are saying Alex, is that a fuel cap leak could be large
enough to overwhelm the plumbed fuel vent.  ie, more air could escape
through the cap than could be replaced by the vent resulting in a vacuum
which would inhibit in fuel transfer from the main tank to the sump.

 

For the record, the bottom of my vent is cut off parallel with the bottom of
the fuselage.  I also have a one way check valve inside the cabin as an
alternate vent.  So far, in my 45 hours of flying, I have had no fuel feed
issues.  Both sides feed evenly.

 

Brooke 

 

 

 

From: "Alex Balic" <velocity_pilot at verizon.net>

Date: September 29, 2011 3:14:09 PM EDTReply-To: Velocity Aircraft Owners
and Builders list <reflector at tvbf.org>

To: "'Velocity Aircraft Owners and Builders list'" <reflector at tvbf.org>

Subject: Re: REFLECTOR: Uneven Fuel Flow



Yes, there is a LOT of suction at the fuel cap area- it is the highest point
in the chord- even though the vent is allowing air back in, there is enough
of a pressure imbalance that with the slight head pressure of the tanks, the
fuel would not feed evenly. Try blowing through 6 feet of 3/8 line by mouth,
and you will see that there is quite a bit of resistance to flow- and that
is without any fittings on there.  The vents are designed to allow pressure
to equalize in the tanks based on fuel being removed by the engine running,
not air being sucked out via the fuel cap.

-----Original Message-----
From: reflector-bounces at tvbf.org [mailto:reflector-bounces at tvbf.org] On
Behalf Of John Abraham
Sent: Thursday, September 29, 2011 9:37 AM
To: 'Velocity Aircraft Owners and Builders list'
Subject: Re: REFLECTOR: Uneven Fuel Flow

Above the fuel cap is a low pressure.  This creates a vacuum on the tank if
the cap is loose.  If it is loose enough you will see fuel coming out of the
tank.  Have you ever flown a piper when a fuel cap came off on take-off?  An
entire tank will be sucked out in a matter of a minute or so.  Your fuel
vent line is pressurizing the tank with ram air.  If you were to cut your
vent flat or scarf it backwards then you can also create a vacuum on the
tank that will not let the tanks feed very well.

-----Original Message-----
From: reflector-bounces at tvbf.org [mailto:reflector-bounces at tvbf.org] On
Behalf Of Jones Nick
Sent: Thursday, September 29, 2011 10:16 AM
To: Velocity Aircraft Owners and Builders list
Subject: Re: REFLECTOR: Uneven Fuel Flow

I'm with you on this one. I hope someone can explain this.

Nick Jones
XL-RG
N10CN


________________________________
From: reflector-bounces at tvbf.org [reflector-bounces at tvbf.org] On Behalf Of
Brooke Wolf [bwolf1 at tds.net]
Sent: Thursday, September 29, 2011 09:41
To: reflector at tvbf.org
Subject: REFLECTOR: Uneven Fuel Flow



Ok, at the risk of showing my ignorance, I need to chime in here.

How could a minute leak in a fuel cap affect fuel flow out of a tank when
that same tank has about a 3/8 inch hole in it vented to the outside (aka
fuel vents)?

Brooke




 

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