Yup, I've said my part in the past (can search the archives) on this issue,
and yes, first hand experience, too, with air in the line on take-off. Don't
have to remove the gascolator, just draining it with a normal water check
will do it. And of course, the sequence of opening and closing your tank
valves with low fuel in flight is most important.
Walter
-----Original Message-----
From:
mike.davis@dcsol.com [mailto:
mike.davis@dcsol.com] On Behalf Of Wayne
G. O'Shea
Sent: Sunday, December 04, 2011 8:40 AM
To:
rebel-builders@dcsol.com
Subject: Re: [rebel-builders] fuel guage
I guess the big difference between a double ported tank and a single with
tee'd fuel sight gauge is in most fuel situations, until really low on fuel,
the dual port generally has fuel on both outlets. On the sight gauge
system.. the top fitting is always in air once a gallon or so of fuel is
burned off from full tanks.
Other than that in a static situation it, as you say, the sight gauge should
always have fuel to the same height as the tank level. You seem to have a
good grasp of the air bubble blocking fuel flow theory.. and probably one of
the major issues. The other, like I noted, the line can't act as a syphon to
get fuel flow going again if an air bubble indeed stops it especially when
low on fuel. It will draw the air down the sight gauge first before it tries
to pull fuel. I've witnessed that first hand when I couldn't get FOKM's fuel
system to prime with any less than 10 gallons of fuel in a tank when it had
a tee'd system. Mind you this, in the realm of flight, may not be an issue
since you should never have an empty fuel line to the carb in the first
place.
What I will point out though, there were obviously enough reported issues,
or questions against this system that Murphy changed it to add the seperate
lower sight gauge fitting. It must have truly been a serious issue that
Darryl believe was factual........ as Darryl rarely adds a part.. hell 2
parts to a kit for no good reason. lol
Now another air issue that causes hickups.... when you remove your
gascolator for inspection and put it back on. You turn the fuel back on and
check it for leaks. Then you fire it up, taxi out to the runway and at just
about 50 feet AGL while rotating under full power the engine coughs..
doesn't it Walter!!! So if the air makes it down your line from above
somehow, we're in the same boat if you happen to be at a high power setting.
You would think with the carb bowl being float operated.... any air that
gets there would just sit above the fuel level and not be an issue, but I
know many cases of engines coughing from the "gascolator air bubble" at full
power. Standard procedure when reinstalling a gascolator should be to use
the primer to bleed the air off the unit... at least it is in this hangar.
Would suggest if your engine coughs I'd be pumping hard on that primer to
see if there's air down there.
Another thought while typing this... keep in mind that no matter how hard
most of us try to keep the fuel lines running down hill to the carb, many
lines have a rise at the firewall to get into the gascolator that is
mandated in Canada to be above the lower firewall lip Therefore if there
happens to be air in the lines.... and if it makes it far enough down and
finds the low point in the system, it could return to the tank or it could
go forward into the gascolator/carb.
Wayne
----- Original Message -----
From: <
tjhickey@iowatelecom.net>
To: <
rebel-builders@dcsol.com>
Sent: Saturday, December 03, 2011 10:26 PM
Subject: Re: [rebel-builders] fuel guage
In the spirit of seeking understanding, I just for the life of me can't
see
how teeing into the same outlet port as the one used to route fuel to the
engine can cause air to be induced into the line. UNLESS, the fuel flow is
somehow being restricted from leaving the tank, and then the first fuel to
be drained to the engine will be the fuel in sight gauge. ( I am assuming
gravity feed here, no suction from a fuel pump.)In a static situation the
level of the fuel will be the same in the sight gauge as in the tank,
assuming that the top of the sight gauge is plumbed back into the top of
the
tank and there is a continuous path for the fuel to enter the sight gauge.
Perhaps the bottom of the sight gauge needs a restriction to prevent rapid
removal of the fuel due to any reason.
I followed somewhat the discussion from a year or so ago about Charlie
Eubanks and using two outlet ports, one fore and one aft, which are then
teed into one line to feed the engine. There was strong opinions, perhaps
correct under the given circumstances, that one should not do this. I
think
missing from that conversation was some consideration of the size of the
fuel line. A small diameter line, how small I do not know, can become
blocked by a rising air bubble if the diameter of the line is small. If
the
fuel use (gpm) results in a fuel stream speed that is greater that the
speed
of the bubble trying to rise to the top, then that is a problem. The
bubble
will not be able to clear, and will be transported to the inlet of the
carburetor. Once the bubble gets there, it seems to me that it should
vent,
and fuel catch back up to feed the bowl. That may take a few seconds,
which
can seem like a lifetime if the two seconds are the ones immediately
following pushing in the throttle.
Charlie's Luscombe has fore and aft fuel outlets teeing into a common line
from the wing tanks. But he has a 65 hp. engine that burns a lot less fuel
per hour, (slower fuel stream speed) than a 100hp. or larger engine.
Frankly, I have wondered if perhaps one ought not to run a larger diameter
fuel line just to insure good head pressure (less head loss due to flow
restrictions) to the carb.
Don't get me wrong here, I am not saying that anybody is wrong about
anything. People see what they see. I am trying to look into the physical
system to see if I can develop an understanding of what, how, and why.
Tim(I'm heading for the hot tub) Hickey.
-----Original Message-----
From: Ralph Baker
Sent: Friday, December 02, 2011 1:19 PM
To:
rebel-builders@dcsol.com
Subject: [rebel-builders] fuel guage
Wayne,
Your comment on not using a tee in the fuel outlet for the level guage
because of air makes sense. Any suggestions as to how to add a second
outlet for the fuel guage to already sealed pressure tested wings? I'd
rather not open the wing.
Ralph Baker
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