|
|
Paul, I may have spotted a problem in this
reply to Todd. The Facet pumps I remember had a one way flow (internal check
valve). If your hot fuel being bypassed to the sump tank was hot enough to form
a vapor bubble and the vapor tried to flow uphill and caused the check valve to
stop the back flow, the delivery of cool fuel would be stopped or at least
restricted on an intermittent basis as the vapor condensed and was replaced
with more hot gas or vapor.
Plumbed the way you describe, fuel runs
downhill to the top of the sump, this can be considered one continuous body of
fuel and any bubble would migrate up through the main tank and it is already
vented by the main tank vent, if not interrupted by the check valve in the
Facet tank. Before you do anything drastic, why don’t you take a couple
to two liter coke bottles, drill a hole in the tops, and 5 minute epoxy a clear
plastic tube about 3 ft long in the caps. Fill one with water (or gas) and play
with letting one be the main tank, the other be the sump (lower) and hold
various positions to see if you keep a bubble in the sump when the fluid is
coming downhill from the main into the top of the sump. You can see
that if you do the same, but try to have the sump fill from the bottom, you can
keep a bubble trapped in the bottle.
Then plumb in an old facet pump between
the two bottles and see what happens. My guess is that if you take the Facet
pump out and have a fuel line direct from the main to the top of the sump, your
problem goes away. Putting a supplemental vent line in from the top of the sump
to the top of main tank does the same thing – it just by passes the
Facet.
Herb Sanders
-----Original Message-----
From: Rotary motors in aircraft
[mailto:flyrotary@lancaironline.net] On
Behalf Of Paul
Sent: Saturday,
February 12, 2005 12:55 PM
To: Rotary motors in aircraft
Subject: [FlyRotary] Re: Vapor
lock in sump/header tank.
Hi, Todd....thank you VERY much for going into all that
detail regarding your fuel system and it's problems/fixes. Yes, my main
fuel tanks are the highest point of the system....the sump tank is lower than
the 2 main tanks (both are vented), and the fuel pumps are lower than the sump
tank. The fuel goes out of the bottom of the main tanks and goes downhill
into the fuel selector valve. Then the fuel comes out of the fuel
selector valve , downhill (through a facet low pressure boost pump that I have
yet to use) to the top of the surge tank.The fuel then comes out of the bottom
of the surge tank and runs downhill to the gascolator and then two EFI
pumps. After going uphill through the fuel rail and to the fuel pressure
regulator, it then goes back downhill to the top of the unvented surge
tank. (I hope I didn't confuse everyone with this)
As to how long I waited to restart the
engine....It never really quit. It just wouldn't produce any power.
Immediately, as the plane came to a stop in the mud, I applied power and was
able to turn around and taxi back to the hangar. Unfortunately, I didn't
try a full power run-up (just too darned happy to be on the ground in one
piece), but I would have to guess that I used at least 1/2 throttle to pull me
out of the mud, and I was unable to get that in the air, or else I would have
continued to fly the pattern and land on the runway.
You have really peaked my curiosity...you
stated that you converted to a returnless fuel system....please educate me a
little here....where does the unused fuel (after the fuel pressure regulator)
go? Inquiring minds need to know. Thanks again for helping
out. Paul Conner
----- Original Message -----
Sent: Saturday, February
12, 2005 5:18 AM
Subject: [FlyRotary] Re:
Vapor lock in sump/header tank.
Caution this is a long
story relating my previous vapour lock issues and how they may relate to yours.
Recent comments have differed about the need to vent or not to vent a
sump/header tank. As I understand it you have a sump tank in which it is
gravity fed from your mains, then pumped up to your engine. However IIRC a
previous post in which you said you had copied Ed's system which is a header
tank that has boost pumps feeding it. (I may have that incorrect as I couldn't
find the post in question) Now I went through a couple different versions of a
header tank before scrapping the idea altogether. My first one was always
vented, but this required level control, via the boost pumps as it required
fuel to be pumped up to the tank, then it gravity fed my FI pumps. If not
filled by pumps, it would just drain the tank due to the vent. If pumps were
left on it would over fill and fuel would go out the vent lines which were tied
via a common header to the main tanks so negative draft would draw most
of the fuel back to the feed tank, but not all. I'd planned to use an
electronic level control system to help manage this but supplier problems
caused me to look closely at what was really an unnecessary failure mode.
At this time I decided
to use a non-vented system much like Ed's, however I was certain that I could
improve upon it by increasing the size of the header tank and having it located
on the cool side of the firewall. But I found that it required a vent as there
was no other way to purge out the air from the tank which would accumulate
there when one would run a tank dry before switching tanks (I have 6 tanks so
it is important to completely empty the aux tanks). So I installed a vent with
a pilot accessible valve. This allowed the venting of all air out of the
tank at which time it would be closed. Then the fuel would draw up from the
main tanks to replace the fuel that had been drawn out and not returned through
the return line from the fuel reg, without the need for pumps. I had a clear
sight tube to see the level of the header tank and another short clear section
of hose on the vent line, so that I could see whether I was venting air or
fuel.
This seemed like a fine
system, until it began seeing the heat of flight operations. Ground runs seemed
mostly successful and running a tank dry until the header tank was empty
resulted in approximately a 20 second delay in getting fresh fuel to restart
the engine and completely purging the air within 2-3 minutes. But under flight
conditions it was much different. Heat would cause vapour problems that were
very difficult to deal with. The pressure would build within the header tank
and the sight tube would not always give a true reading as it should. Twice I
ran a tank dry while at +10000' above the airport and had a very difficult time
in getting fuel back resulting in approx 6000' of glider alt loss each time.
Another time I was doing circuits and was on my downwind leg when the pressure
in the tank caused a vapour lock even though I had plenty of fuel in the
tank(s) feeding the header tank. The pressure had just built up to the point
where even the 7 psi boost/transfer pumps couldn't overcome it to keep fuel in
the tank. In this case I was able to make an emergency deadstick landing on an
intersecting runway and fortunately I even kept my speed up enough to coast all
the way to my tie-down spot. That's where the firetrucks met me... kinda
embarrassing!
In each of these cases, I
found that when opening the vent valve to release the pressure, I could see
through the clear vent tube section that I was venting a boiling fuel
(air/fuel) mixture. And it took an uncomfortably long period of time to relieve
that pressure and refill the tank with fresh fuel. I then installed optical fuel
level sensors (hi/low) on the header tank, a vacuum/pressure gauge and a temp
sensor. I found that it would begin by having a vacuum in the tank as the
system would draw in new fuel to replace that which was being consumed, but it
would soon begin to build as a pressure as the fuel heated and would then begin
to push the level down. Opening the vent relived this pressure but it couldn't
be left open, or it would just empty the header tank. Keeping the pump(s)
running would over flow the tank.
Can you see where all
this was going? I surmise that the greater capacity of the header tank (as
compared to Ed's) allowed some heat absorption as it took longer to become
affected, but once heated it was far less manageable. My pilot workload was too
great and became centred around fuel management. I had so much time, effort and
$$$ invested in making that header tank work that I didn't want to let go of
the idea, but one day I just had enough and tore it out of there. I've now
converted to a returnless system with no header tank and I couldn't be happier.
What I'm trying to show
is that there is more going on with heat in the fuel system than expected. I'm
not familiar with your system, but the way I understand it, you have your main
tanks draining by gravity into your sump tank. Your FI return line feeds into
this tank bringing heat from the engine with it. I surmise that it is bringing
enough heat to begin a vapour build-up in your sump tank overcoming the gravity
feed. How long after your emergency landing did you restart your engine? If it
was immediately and it ran fine, then this theory may not be valid, however if
there was sometime elapse (while you kissed the ground & changed your
shorts :-), then it could be that the tank had time enough to cool and/or
relive the pressure allowing more fuel to enter the tank.
One way that I could see
overcoming this would be.....
A vent line from the top of your sump tank
up to the top of both of your main tanks, but not tied into the main tank
vent system. This would allow any fuel vapours to immediately flow out of the
sump tank, eliminating any chance of vapour build-up, while allowing the cool
fuel to continue to be gravity fed to the sump tank. These fuel vapours would
then flow into the main tanks where they should immediately condense,
preventing the loss of any fuel through a direct atmosphere vent system.
This would (may) only work if you have your main tanks located above the sump
tank. Having your FI pumps located at or below the sump pump would surely be a
help as well. But I would not expect the tank to work without a vent or with a
vent to atmosphere.
I hope some of this is
relevant to you and helps.
Todd Bartrim (top posted all the way
to the bottom)
No virus found in this incoming message.
Checked by AVG Anti-Virus.
Version: 7.0.300 / Virus Database: 265.8.7 - Release Date: 2/10/2005
|
|