REFLECTOR: Fuel Vent System

Mon Jan 31 10:28:01 CST 2011

John,

Al's is a great point about how the height of the vent loop above the top of
the spar at the firewall is so important in preventing fuel venting during
high pitch angle climbs in high performance Velocity's.  I had forgotten
about how with fully topped-off fuel tanks, the vent line fuel will be all
the way up at the elevation of the fuel in the front of the strakes.  

If I did my math right and the front of the strake fuel tank is about 8 ft
forward of the fuel vent line on the front of the firewall, with the ten
degrees pitch required for a 2000 fpm climbout (about 10 degrees total pitch
angle, when you add approx. 2 degrees AOA to the required 8 degree climb
angle for a 2000 fpm climb with a 140 kt forward speed), then the fuel level
at the manifold will be about 17 inches above the top of the spar on the
firewall!

Looking at Andy's great photo of his interior fuel plumbing layout, you can
see that 17 inches above the spar is going to put the static fuel level in a
10 degree climbout very nearly at the top of the fuselage, if not above!
That's why Al's accumulator (probably good enough to prevent at least
sloshing overflow) and the factory's forward vent loop extensions are
required for high performance climb angles.  (I'm repeating Andy's photo
below, since it shows the standard plumbing layout so well).

These high pitch angle climbs will create not just occasional venting, but a
steady state streaming of fuel out of the vent -- and onto the hot exhaust
pipes and spinning prop behind!  That doesn't sound like a good situation!  

Also, even if you can't climb steady state at 2000 fpm, most Velocity's have
an initial climb angle of at least 5 degrees.  A nominal 5 degree climb
angle combined with the fuel sloshing of the take-off rotation, the dynamics
of raising of the gear, and just getting your attitude properly stabilized
in the climb (my 'attitude' is sometimes not properly stabilized, even on
the ground!) means that almost all of us are likely to be venting some fuel,
especially during take-offs and the initial climb.  The tanks don't
necessarily have to be full either, since the sloshing of fuel for partially
filled tanks can be as violent, and more dynamic, than with the full fuel
case. 

These are new good things to be thinking about!

Thanks Al!

Bob

N2XF

PS  I'm glad you're going to post some more pictures of the 'overflow' lines
in your wheel wells and where they go, John -- I can't personally imagine
where they go and what they do, unless it's like Al says they're not open to
the outside and are instead just the normal strake tank vent or fuel drain
lines.

  _____  

From: reflector-bounces at tvbf.org [mailto:reflector-bounces at tvbf.org] On
Behalf Of Al Gietzen
Sent: Monday, January 31, 2011 1:07 AM
To: 'Velocity Aircraft Owners and Builders list'
Subject: Re: REFLECTOR: Fuel Vent System

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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