Hi Frans,
you are absolutely as right
******as usually*********
(ask Ilona about these stars ;)
but still I am brave enough to consider that fuel
will not be significantly "aged" when dropping or
in fact spraying it from top of the tank. That
spray will contact air just a split second during
the drop distance from 0 to max say 40 cm. Notice
you will use this fuel in hours or at least mix it
soon to the fresh fuel when refueling again.
Anyway - if you have a manual tap or an automatic
one-way-valve - there is no reason to instal it (a
return line) to the top of the tank.
BTW have you any idea what=B4s a typical fuel
temperature rise from the tank outlet to the tank
inlet? My guess is it is just couple of degrees
only or even zero - flow rate is so high.
Anyway - thanks you helped me/us to understand
that return line potential danger IN TIME (and to
do something to it). Among us is living at least
one person who would have been very happy about
that extra valve during his awful incident (IMHO).
Funny, we (me & my wife) feel comfortable over the
shoreless sea or in the most thickiest cloud but
so far we cannot even think about flying VFR over
hostile & high terrain like Ilona=B4s parent=B4s Alps.
I understand it=B4s just our lack of information.
And also lack of turbo, oxygen and BRS...
Raimo
HA-MDO
http://www.flyfinland.fi/view/10153/
OH-XRT
http://www.flyfinland.fi/view/9490/
-----Alkuper=E4inen viesti-----
From: Frans Veldman
Sent: Thursday, September 19, 2013 11:55 AM
Subject: Re: Europa-List: Fuel return line
one-way-valve
<frans@privatepilots.nl>
On 09/19/2013 07:07 AM, rparigor@suffolk.lib.ny.us
wrote:
> The 914 returns a lot of fuel to the tank
> compared to the 912S. By
I have a 914 AND I have a one-way-valve in the
fuel return line. No
problems, although with the engine at idle the
fuel pressure tends to
climb to the upper edge of the green zone of my
fuel differential
pressure gauge. But maybe that would even be the
case without the one
way valve. At anything else than idle the fuel
pressure is in about the
middle of the green zone.
> I think I remember reading it may
> not be a great idea to let fuel (especially auto
> fuel) cascade as I
> think it may somehow accelerate spoiling the
> fuel? Did you ever hear of
> this?
I have the same fear. Remember the returning fuel
has an increased
temperature (might even be hot) due to its travel
through the heated
engine bay, pumps, pressure regulator, etc! That's
the reason why we
have a fuel return line after all; by keeping more
flow in the system
than the engine requires we prevent vapor lock.
Fuel consists of many
components, and they all vaporize at a different
rate. It doesn't feel
right to have hot fuel cascading downwards from
the top of the tank,
splattering around, having its components vaporize
partly, then
condensing against the cold tank walls, and
dripping into the fuel pool
again. Also the surrounding oxigen together with
the high temperature of
the return fuel vapor might oxidize some of the
fuel components at an
increased rate. We all know that fresh fuel is
different than "old" fuel
and elevated temperatures and lots of contact with
oxigen might play a
role in this "aging" process. My feeling is that
it is healthier to
arrest the returning fuel immediately on arrival
in a pool of cold fuel
without letting it get into contact with air and
oxigen first.
For what it is worth, I try to use Mogas as much
as possible, even when
flying in the French Alps (around FL100), have
done this dozens of times
(Ilona's parents live there so we tend to visit
them often with our
little aircraft) and I never had any problems with
the fuel. Maybe it
just helps to keep the fuel temps down and not
inviting too much
interaction with oxigen molecules.
> I suppose could put a dip tube to the bottom of
> the tank to
> prevent this?
So then what would be the point to route the fuel
to the top of the tank
first while there is a shorter route to the same
spot?
> On the 914 you don't want much in the way of
> back pressure
> as it can cause problems with proper functioning
> of the fuel pressure
> regulator.
One more reason to feed the fuel back at the
underside of the tank.
Shorter hose, and less elevation of the fuel
required to deliver it
where it ends up anyway.
Of course, a one way valve gives some resistance,
but apparently it is
not too much. A valve can fail, but the most
dangerous fail is when it
remains shut. If this happens, you will discover
it during engine start
up. With the low fuel demand during that stage
there is much return
flow, and if the valve stays shut it will quickly
flood the engine
(apart from your fuel pressure needle pegging
against the stop) Once
open, it seems utterly unlikely that the valve
will spontaneously close
against the fuel pressure during flight. Once
open, it will stay open as
long as fuel pressure is pushing against it and
holding it there.
It gives some piece of mind that there is not an
unclosable fuel hose
going to the engine bay. If the hose breaks, the
fuel flow will stop by
itself.
Frans
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