fuel venting
Posted: Sun Feb 19, 2012 10:39 am
Hi Dick
Cessna's seem to also have compromises. It is amazing how complicated
just venting a tank can be. FWIW my goal was no mechanical/rubber valves
as they will plug/leak sooner or later.
No restrictions on filling the tank which pretty much means using
inverted J tube(s) and venting from the high point or directly from the
filler neck which means vent the cap or run the vent line from the
filler cap area.
A cross vent line to aid in even fuel flow is desireable.
No gross leakage due to parking on a slope or a flat tire which means
vent from the cap or an inverted J tube. Venting from the fuel cap area
out to the opposite wing tip will also work without having to use an
inverted J tube. One degree of dihedral is less than 2" of rise from the
tank to the wingtip. If cross venting from inboard tank area (the top of
sight tube) significant fuel may flow from the top tank to the lower
tank and possibly out the filler cap with a flat tire. Note the MAM
supplied fuel caps will leak and indeed mine have a small vent
incorporated. However if the wee cap vent should plug and seal
completely then expanding fuel (parked in sun) will come out the vent
unless vented from the filler cap area.
Since all lower wing vent locations tend to eject small slugs of fuel
even if plumbed from the far side of the tank or far side of the far
wing tank that was not ideal from an environmental point of view. (I
would never tolerate a car that did that or that even had a poor fitting
gas cap) A surge tank in the vent line was undesireable from a weight
and complication point of view and difficult to implement without more
dihedral.
If using only one J tube in wing root area and cross venting from the
top of the sight gauge, any slugs of fuel in the cross vent could affect
the sight gauge reading a bit. Two inverted J tubes (one in each wing
root) might be desireable or running a separate vent line (independant
of the sight gauge) out to the filler neck area might be preferable.
Conclusion A: Inverted J tubes on gas caps is entirely acceptable but
somewhat delicate and subject to damage.
Conclusion B: If cross venting from the sight gauge port - use inverted
J tubes on the caps or use two inverted J tubes (one in each wing root)
and make sure there is a least a small vent in the gas caps.
Conclusion C: Cross venting is best done from the filler cap area. If
doing that anyway then one inverted J in a wing root is acceptable and
the gas caps can be left stock.
Conclusion D: It is difficult and time consuming to improve on what is
in the manual ;)
I ended up with one inverted J in one wing root and ran the cross vent
lines through the tanks to the filler neck area. That scheme would also
work with flush fuel caps. My sight gauges do not share any lines with
anything else.
The only surprise so far is that my header tank will burp out through
the vent line with surprising force. Bubbles don't percolate up slowly
but are pushed up by the 3' head of fuel coming down from the wing tanks
to the header. That seems to have ceased once the air was out of the
system. I will remake the vertical J tube out of streamline tubing to
incorporate a vertical surge tank into it or drill another port into a
fuel tank root bulkhead if that proves troublesome but it looks fine so
far.
Ken
wampach wrote:
-----------------------------------------------------------------
List archives located at: https://mail.dcsol.com/login
username "rebel" password "builder"
Unsubscribe: rebel-builders-unsubscribe@dcsol.com
List administrator: mike.davis@dcsol.com
-----------------------------------------------------------------
Cessna's seem to also have compromises. It is amazing how complicated
just venting a tank can be. FWIW my goal was no mechanical/rubber valves
as they will plug/leak sooner or later.
No restrictions on filling the tank which pretty much means using
inverted J tube(s) and venting from the high point or directly from the
filler neck which means vent the cap or run the vent line from the
filler cap area.
A cross vent line to aid in even fuel flow is desireable.
No gross leakage due to parking on a slope or a flat tire which means
vent from the cap or an inverted J tube. Venting from the fuel cap area
out to the opposite wing tip will also work without having to use an
inverted J tube. One degree of dihedral is less than 2" of rise from the
tank to the wingtip. If cross venting from inboard tank area (the top of
sight tube) significant fuel may flow from the top tank to the lower
tank and possibly out the filler cap with a flat tire. Note the MAM
supplied fuel caps will leak and indeed mine have a small vent
incorporated. However if the wee cap vent should plug and seal
completely then expanding fuel (parked in sun) will come out the vent
unless vented from the filler cap area.
Since all lower wing vent locations tend to eject small slugs of fuel
even if plumbed from the far side of the tank or far side of the far
wing tank that was not ideal from an environmental point of view. (I
would never tolerate a car that did that or that even had a poor fitting
gas cap) A surge tank in the vent line was undesireable from a weight
and complication point of view and difficult to implement without more
dihedral.
If using only one J tube in wing root area and cross venting from the
top of the sight gauge, any slugs of fuel in the cross vent could affect
the sight gauge reading a bit. Two inverted J tubes (one in each wing
root) might be desireable or running a separate vent line (independant
of the sight gauge) out to the filler neck area might be preferable.
Conclusion A: Inverted J tubes on gas caps is entirely acceptable but
somewhat delicate and subject to damage.
Conclusion B: If cross venting from the sight gauge port - use inverted
J tubes on the caps or use two inverted J tubes (one in each wing root)
and make sure there is a least a small vent in the gas caps.
Conclusion C: Cross venting is best done from the filler cap area. If
doing that anyway then one inverted J in a wing root is acceptable and
the gas caps can be left stock.
Conclusion D: It is difficult and time consuming to improve on what is
in the manual ;)
I ended up with one inverted J in one wing root and ran the cross vent
lines through the tanks to the filler neck area. That scheme would also
work with flush fuel caps. My sight gauges do not share any lines with
anything else.
The only surprise so far is that my header tank will burp out through
the vent line with surprising force. Bubbles don't percolate up slowly
but are pushed up by the 3' head of fuel coming down from the wing tanks
to the header. That seems to have ceased once the air was out of the
system. I will remake the vertical J tube out of streamline tubing to
incorporate a vertical surge tank into it or drill another port into a
fuel tank root bulkhead if that proves troublesome but it looks fine so
far.
Ken
wampach wrote:
Subject: FW: [rebel-builders-d] DIGEST - rebel-builders fuel venting
If I may suggest, take a look at a Cessna 177 it has very little
dihedral as does the Moose. It vents at the wing tips as I recall the
right tank is vented from the outboard high point over to the left wing
tip. The left tank set up is the same and vents at the right tip. This
reduces the 150/152 type of drip problem when parked on the ramp in a
left wing low condition. The 210 also vents at the tips but I don't
think it cross-vents as the 177 does.
Some of you may want to look into the ideas that Cessna used here to
help with a proven idea. A look into the service manual and pilot
operating hand book of these models will give you block diagrams and
schematic diagrams, and the parts manual will show the complete lay out
of all the lines, parts etc.
Also take a look at the inside of the gas caps. In the event of a vent
problem the little silicon rubber "mushroom valve" is designed to allow
air to enter the tank as fuel is used and blocks the out flow if the
tank is trying to over-flow when parked on a slope and fuel is trying to
flow from the high tank to the low one (it's a gravity thing. This was
a service letter item years ago and I think a darn good idea.
I plan to look into the two systems before I close up the wings on mine.
I have used Cessna, Piper and Beech systems ideas in the planning for my
Moose. Mostly I like the idea of using proven engineering in areas
where mistakes can cost you in more ways than just the pocket book, if
you understand my drift here!
Dick Wampach, AP/IA
SR108
-----------------------------------------------------------------
List archives located at: https://mail.dcsol.com/login
username "rebel" password "builder"
Unsubscribe: rebel-builders-unsubscribe@dcsol.com
List administrator: mike.davis@dcsol.com
-----------------------------------------------------------------