On 8/17/2010 8:02 PM, [email protected] wrote:
wouldn't simply venting both tanks connect the air spaces through the
atmosphere?
That works for our normal (small) wing tanks.
First of all, if you have a header tank, none of this applies as the
engine is not drawing fuel from the wing tanks.
If you remove the header tank, then it becomes an issue. On the
surface, it would seem that if both tanks are vented to the atmosphere,
that should be sufficient. However, as others have said, there are
subtle differences that can crop up between the venting of the two
tanks, like leakage of one of the cap gaskets, differences in airflow
around the two cap vents, etc. It only takes a small pressure
differential to have a big affect on flow rates. Anyone who has a lot
of time working on Cessnas that draw from both tanks simultaneously will
tell you that, even if all the requisite plumbing is in place, you can
still have problems. There is a vent tube located behind the lift strut
on one side, and miss-positioning it by as little as 1/4" can make the
difference between normal operation and running out of fuel with a full
tank on one side.
The required connection between the airspaces is intended to overwhelm
any potential differences in tank venting simply by virtue of the fact
that the connection is bigger than the vent(s). This works fine, and
tank pressures are equalized, as long as there is no slug of fuel
trapped in the line (sort of reverse vapor lock...). The easiest way to
insure against this is to have the line level or inclined downward
toward both tanks, and enter the tanks above the normal fuel level.
Unfortunately, on the Ercoupe family of planes, this is not practical as
such a line would enter the cabin an inch or two above the wing. If you
connect the two inboard ends of the tanks together, the lines enter
below the level of the fuel in the tank(s) and therefor do not connect
the airspaces until at least one tank draws down to the level of the
line. Once this happens, things generally start to equalize and
ultimately, both tanks are emptied at about the same time. Fine for
transfer tanks but not good enough for main tanks.
From CAR 4a.606:
/If two or more tanks have their outlets interconnected they shall be
considered as one tank and the air space in the tanks shall also be
interconnected to prevent difference in pressure at the air vents of
each tank of sufficient magnitude to cause fuel flow between tanks.
Mechanical pump systems shall not feed from more than one tank at a time
except by special ruling from the Administrator./
(Note, the last statement describes the 30 gallon fuel system STC...)
CAR 3 and Part 23 contain similar wording.
--
John Cooper
Skyport East
www.skyportservices.net
