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From: "sboese" <sboese@uwyo.edu>
To: "'Rotary motors in aircraft'" <flyrotary@lancaironline.net>
Subject: fuel system issues
Date: Tue, 6 Dec 2005 16:09:43 -0700
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In-Reply-To: <list-860110@logan.com>

Thanks Todd, Rusty, and Al for the responses to my fuel system concerns.

Todd=92s observation that the capacitive fuel gauge indications are =
reliable
when the tanks are near empty is what I needed to hear.

I took Rusty=92s suggestion and investigated the source of the bubbles =
in the
fuel lines further.  When the high pressure fuel pump in the plane is
running, there are no bubbles between the tank and the pump, no bubbles =
in
the fuel rail or lines between the pump and the pressure regulator, and
there are bubbles in the return line between the pressure regulator and =
the
wing tank.  As a separate test, I connected a fuel pressure regulator to =
the
RX7 donor car=92s fuel tank which was filled with fuel and had the =
original
submerged fuel pump in it with a clean inlet sock.  Running the pump in =
this
setup gave the same results:  bubbles in the return line, but nowhere =
else.

Al=92s suggestion is probably correct that the bubbles are air which was
originally dissolved in the fuel rather than fuel vapor as I had =
originally
thought.  Fuel vapor should rapidly recondense, but redissolving the air
into the fuel would probably be a slower process, thus the bubbles =
persist.
Using the submerged fuel pump as he recommended did not change what was
observed, but still is a good idea for the reasons he stated. =20

I don=92t think there is a problem when the fuel is returned to the =
large wing
tank where it would be unlikely for the bubbles to get into the line
supplying the pump.  It could be an issue, however, if the fuel is =
returned
to a smaller header tank depending on how the header tank is constructed =
and
vented.

If the fuel is recirculated long enough, and if the bubbles are really =
air;
the dissolved air should be eventually nearly eliminated from the =
system,
but I didn't run the pump long enough to see if this is true.  I =
haven=92t
tried the tests with avgas, either.

My apologies to the list if this is common knowledge. =20


Steve Boese, WY, RV6A, 13B NA, EC2, RD1A =20
=A0