Here's further discussion on the fuel tank cap question. (And a reduction of my previous confidence (always good for me)).
On the question of whether the fuel cap vent holes should face forward or
aft,
I wrote this:
> > The vent holes definitely face forward. I was once told that was to
partially
> > pressurize the tanks to assist the fuel pump. I don't know if that's
the reason
> > or if it's simply so the venting doesn't allow siphoning. Either way,
they
> > definitely face forward.
I got an interesting message saying there may be a reason some caps are on
with
the vents backward. (Since he sent it to me privately, I'll assume he
preferred
anonymity.)
>> When I had a 415-C that was causing me problems with uneven wing tank
feeding
>> I was told how to correct it by the chicken ranch farmer from Ark.
>> (Leonard Page)
>> What he said was to cut the rivets that hold the plate to the bottom of
the
>> cap, turn the fuel cap around and re-rivet so the vent holes are facing
the
>> the back. After I did this I never had uneven fuel feeding. When I last
saw
>> the plane it was in Anchorage, Alaska with the vent holes facing the
back.
Here's some further discussion, please chime in with the best facts you
have.
Can we pin this issue down to a certainty, somehow?
Discussion:
The fuel should feed evenly from both tanks. I think that if there's a big
unevenness between the tank levels there's either a problem with the cross
feed
lines that should be fixed, or perhaps one vent hole is plugged and the
other
open, creating a pressure differential. Reversing both the caps would
remove
the ram-air pressurization.
By my calculations, there may be about 6.2" of differential at 105 mph if
one
tank had no ram air and the other did (at 3k feet). (No guarantees on the
calculation, it's the best I could do.
Calculations for peer review:
Water is supported by pressure at about:
32' water/14.7 psi
384" water/14.7 psi
or 26.1" water/psi
Ratio of weight water/fuel = 8 lb. water /6 lb. fuel) = (.75
water/fuel)
(same pressure will support 1.333 times as much fuel)
giving 26.1" water/psi / (.75 water/fuel) = 34.83" fuel/psi
Dynamic pressure of 105 mph = .179 psi
( Reference for dynamic air pressure:
http://aero.stanford.edu/StdAtm.html )
(34.83" fuel/psi) * .179 psi = 6.2 inches
Summary: If there's overflow, check for a clogged cap vent. If both vents
are
clear but there's level differential, look for clogging in the cross-feed
fuel
lines.
The vented cap design seems to have been intentional. Fred's team probably
had
a reason for it. By the design, the caps are made to fit the tank holes
with
the vents pointing forward. (If the lineman puts them on backward, it can
be
devilish getting them off.)
What do you think? (I apologize for imposing this on you all, this kind of
discussion is my kind of fun and, yes, I do lead a sheltered life.)
--
Ed Burkhead
Peoria, Ill.
Ercoupe N3802H, 415-D
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