Group,
The current discussion regarding wing and header tank design has
included:
1. Gravity flow from the selected wing tank to the header (assuming the
wing tanks are high enough)
2. Independent main tank and header air vents
3. Fuel return to the selected tank
Based on this, I have a concern:
When both fuel pumps are turned on, or perhaps even one pump, a large
volume of fuel would be pumped from the header tank back to the
selected tank. For example, the pump Tracy sells is 40 GPH and 95
psi. So if both pumps were on, the maximum flow could be 80 GPH
(limited of course by the resistance of the system and the actual
system pressure).
The flow FROM the header tank through the pumps and back to the
selected tank seems like it could easily exceed the gravity-fed flow
back INTO the header tank from all but very large fuel lines. I'm
afraid my fluid mechanics skills faded years ago, so I can't calculate
the actual pressure drops and flows. I haven't had much luck finding
standard flow charts for tubing at different pressures, either. I
suppose I could do a gravity test and see if 1.3 GPM would flow through
a -6 tubing, valve and fittings.
Anyway, it would seem that the pumps could potentially empty the header
tank, because its vent air could also flow into the tank and has less
resistance to flow than a liquid.
Am I missing something here? Should the header's vent have a one-way
valve with a set pressure before it opens to help prevent this problem?
Or should the header be strong enough to resist a certain level of
vacuum and not use a vent at all? The latter would have to pull fuel
from the selected tank.
This potential problem would not be possible if the fuel return was to
the header tank, but then there is the heat buildup issue. There also
might be a 'surge' issue, where the engine fuel usage dropped quickly,
and increased the return. This would result in a large return to the
header tank, even if it were full. It seems the resultant pressure
might momentarily exceed the height of the header vent and dump fuel
unless a one-way valve was present.
Now I know why Paul recommends not mucking with a fuel delivery system
unless one is well qualified! Unfortunately there is no FI return
system designed for my plane as of yet.
Comments and suggestions are welcome.
Scott Gettings
Glass Goose/Renesis
I think you are right Scott. Best to use very large hoses or
a large passage from the wing tank to a tall and skinny header tank.
I recommend -8 line for fuel flow anyway although some people
tell me -6 is good to 300 HP. If I were inclined to use
a one way valve or valves I would use the flapper type between
the wing tanks and the header tank.
I would not use a one way valve in the header vent line.
Most one way valves of that size have a breakaway pressure
which just might be strong enough to over come the suction
of the HP pressure pumps.
Gravity should do the job feeding the high pressure pumps
from a tall and skinny header tank. Again the HP pumps intake
needs to be near the bottom of the tall and skinny header tank.
One HP pump should be a back up only so both on at the same
time should not be needed.
I would be nice to preflight them both one at a time.
Paul Lamar
Scott,
I have a Sea Hawk and I bonded in a return line to the tanks with a one way
valve. 15 hours of ground and high speed water taxing and the engine hasn't
missed a beat. Just make sure you are returning to the same tank that you
are feeding from.
Bill Hyde
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