REFLECTOR: Fuel Vent System - 3

[Bob Jackson]

John,

 

I don't think your in-cabin (back-up) vent is high enough on the inside of
the firewall -- it need to be up as close to the ceiling as possible (or at
least the vent outlet line coming out of it), otherwise fuel is going to
slosh from the tanks, through the block, and out at least the normal main
vent line (which looks like in your case goes out of the bottom of the
fuselage, just forward of the bulkhead.  And worse yet, that sloshing could
cause even a small amount of fuel to slosh out your red check valve into the
cabin if the sloshing is energetic and cyclic enough to activate the check
valve (which should normally remain closed unless a vacuum develops in the
block).  When the main tanks are full, the full level will be at rest only
an inch or two the level of your block (it will be almost at the of the top
of the spar with full tanks).

 



 

Even if the back up cabin check valve isn't momentarily sloshing open (I
don't see any blue fuel dye stains on the carpet, except a little behind the
right fitting which looks like a one time occurrence, probably resolved by
tightening the fitting)  -- then with the block looped so low to the top of
the spar, you're going to have quite a lot of fuel coming out of the main
vent with full fuel when you get it sloshing (on take-off or in turbulence).
Your main fuselage fuel vent line is also pretty close to the retract
openings, so if fuel sloshes over the vent block that you show in the photo
and out this vent line, air turbulence will also probably cause fumes extend
out to the sides and to come in the retract openings and be smelled by your
passengers.

 

I'm confused by the fact that you only apparently have one 'in' and one
'out' into your block -- that would indicate to me that maybe this section
of the plumbing is only venting the sump tank (and maybe the 'overflow
vents' that you're referring to in each wheel well are the vents for each of
the main tanks).  If so, those are way too low as well.  

 

I would say you need to:

1.	Extend the routing of the two fuel vent/overflow lines you describe
in each wheel well into the cabin,
2.	Buy or make a new aluminum block that has four openings to it,
3.	Route your newly extended vent lines from each wheel well into two
of these aluminum block inlets,
4.	Route the vent line from the sump tank into a third aluminum block
inlet,
5.	Route a new aluminum line from the top 'outlet' hole in the aluminum
block in a loop that goes all the way to the top of the fuselage ceiling,
and then down out of the fuselage as the main vent out,
6.	Add a 'tee' to this main vent out line down a foot or so from the
ceiling, and plumb your red check valve off of this tee.

 

This is more or less the way Richard Gentil's photo shows his as being
plumbed, except his must have all three inlet lines already teed together
before going into the block, since he shows his check valve coming out of
one of the block side outlets, and apparently his main vent line coming out
of the other.  I don't think Richard's routing is quite as good because his
check valve won't benefit from the vent loop I described that could go all
the way to the top of the fuselage before heading to the vent hole on the
bottom of the fuselage (and I think it would be more likely to also vent
some fuel in the cabin occasionally).

 

You should do as Scott recommends and read the online manual section on fuel
plumbing and follow it's recommendations.  Did you build the plane or buy it
from someone else?

 

Just my 2 cents worth...

 

Bob Jackson

N2XF

 

  _____  

From: reflector-bounces at tvbf.org [mailto:reflector-bounces at tvbf.org] On
Behalf Of aminetech at bluefrog.com
Sent: Sunday, January 30, 2011 10:48 AM
To: Velocity Aircraft Owners and Builders list
Subject: REFLECTOR: Fuel Vent System - 3

 

 

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