REFLECTOR: Fuel Vent System

[Al Gietzen]

I think my problem is that, in flight, the pressures at the 2 overflow vents
are never perfectly in balance and that results in an air flow in one vent
and out the other one.  Of course fuel vapor will be in that air and get
into the cabin via the gear opening.  Maybe blocking one vent would fix this
issue, assuming one vent is adequate.  I'll get some more pics today so you
experts can get a better look at this unusual setup.

 

John

If you can blow in one vent and have air come out the other; then clearly I
do not understand what the setup is.  If the lines just come from the top of
the tank inside, and into the wheel well as you suggested; then the only
other connecting path is through the sump tank to the other side.  No air
could flow that way unless the tanks were empty; and the level in the sump
somewhere below the fittings at the top. 

Al



--- ALVentures at cox.net wrote:

From: "Al Gietzen" <ALVentures at cox.net>
To: "'Velocity Aircraft Owners and Builders list'" <reflector at tvbf.org>
Subject: Re: REFLECTOR: Fuel Vent System
Date: Sun, 30 Jan 2011 22:07:00 -0800

John wrote:

The line enters the wheel well at the top inboard side.  I suspect it simply
connects to the fuel tank at that point.  If I overfill the tanks, gas will
come out the overflow. 

If that is so; you are losing fuel out of those lines any time the tanks are
near full; and you should definitely consider some re-plumbing.  There is no
purpose or function for an “overflow line”.  You need a vent line that
allows air in (or out); but does not allow fuel out under any normal flight
conditions.

What Bob has described sounds about right.  One thing I will add to that is
the manifold block needs to be higher than the existing your existing small
block which is only a few inches above the top of the strake level.
Visualize the strake being full of fuel. Then visualize – or put a stand
under the nose wheel – to simulate a high AOA, equivalent to your max climb
rate. The manifold block must be at a higher point (horizontally) than the
highest part of the strake tank; otherwise fuel will flow out of the vent
lines when the tank is full and the plane is at that AOA.

My vent manifold was mounted high on the firewall as shown in the manual.
During my flight test phase I noticed evidence of fuel streaking back from
the vent exit under the fuselage. It didn’t seem to make any sense; and the
only thing I could think was that there somehow was foaming of the fuel
coming back to the sump with the fuel return from the fuel pressure
regulator (fuel injection system); which would then carry some fuel up the
vent line.  I later realized that with my somewhat higher powered rotary, my
initial climb was frequently about 2000 fpm; and at that AOA the high point
at the front of the strake was as high as the vent manifold. This would
result in some fuel going out the vent line.

I understand the factory discovered a similar issue with the
high-performance XLs.  I think their solution was to add a loop at the
manifold exit extended a bit forward at the top of the fuselage before going
back and down out the bottom.  I had solved it by installing a small
‘accumulator’ tank (a few cu. in.) in place of the manifold while still
thinking it was a foaming issue.  Fortunately it served the same purpose as
a loop because the small amount of fuel coming through the ¼” lines would
collect there; and drain back when the AOA was reduced.

Long story to make the point about the manifold placement.  With a standard
200hp engine on an SE, high on the firewall is fine; but anyone with more hp
on as SE RG may want to take extra measures. 

Al

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