|
|
On a low wing plane (at least the RV's), it will usually be
impossible to gravity-feed all fuel to a sump tank, since the lowest
point on the a/c will likely be only an inch or so below the low
point in the tank. (I never said 'pump to a pump'; was that a typo?)
While Ed A's vacuum-fed sump is obviously successful, I can't get
comfortable with using that technique unless I exactly duplicate
everything in his system. My personal preference is to keep as much
of the fuel system as possible out of the engine compartment, to
minimize heating of the fuel until it will actually be going through
the injectors.
I think you make my point on life risk vs design decisions. Life
risk should obviously carry more weight, but if it's the only
criteria, we'd find a way to never leave home. Van could have found
a way to pull fuel directly from the bottom skin of his tanks, and
feed it directly to the carb with the lines & shutoff valve
hanging under the fuselage, and there might be a slight reduction in
'life risk'. But the system he chose has had zero problems in
unmodified installations, meaning to me that the choice to keep the
design cleaner was justifiable. Furthermore, difficulty in
maintenance can actually *drive up* life risk. I consider it safer
to make everything except a single mechanical joint (fuel pickup to
bulkhead fitting) the only thing that I can't conveniently inspect.
I tried to keep my primary fuel delivery path to the pump(s) as
close as possible to the 'stock' configuration from the designer. At
least in my feeble mind, this is the best I can do to minimize life
risk, since it's been successful in at least 7,000 flying a/c.
Are we closer to being on the same page?
Charlie
On 09/04/2011 11:34 AM, Al Wick wrote:
Charlie wrote:
<If you must pump to the sump, then the vent can't 'go
anywhere' unless you're willing to risk pumping fuel overboard
Yeah. It seems so. It's a bit strange to think someone MUST
pump to the pump. I'd be quick to question that assumption.
< removing several dozen screws, the vent line and fuel
line, and then removing the tank before I could remove the 6 or
8 screws that <hold on the access plate to the tank
So, once every twenty years you
might have to access the pump. And the number of screws is
your reason to have higher risk fuel design? I try my best not
to let anything get in the way of improving safety. That's why
I changed my fuel design even though I'd never had a problem.
I want that extra safety margin from sump tank with two wet
pumps, self cleaning filter. It's real. It's significant.
<we would choose maintenance ease vs life risk: you have
to be honest; *every* decision makes that choice
Of course not. I'd never agree
with a sweeping assumption like that. Life risk SHOULD always
carry more weight in a decision.
-al wick
----- Original Message -----
Sent: Friday, September
02, 2011 12:21 PM
Subject: [FlyRotary] Re:
vapor lock
I'd have to differ on a couple of points.
If you must pump to the sump, then the vent can't 'go anywhere'
unless you're willing to risk pumping fuel overboard.
On a typical RV, it would require removing several dozen screws,
the vent line and fuel line, and then removing the tank before I
could remove the 6 or 8 screws that hold on the access plate to
the tank. So, there is a bit of a maintenance issue for us. If
not for that, I'd use in-tank pumps. For my application, the
external sump would add more weight/complexity.
To the implication that we would choose maintenance ease vs life
risk: you have to be honest; *every* decision makes that choice.
The safest G/a factory plane in the world uses bolt-on wings
instead of a one-piece wing; a one-piece would be both lighter
and safer.
Charlie
On 09/02/2011 01:04 PM, Al Wick wrote:
What fine work Steve. I love
it when someone converts theory to facts. You measured
that all at sea level. Now just subtract 5.35 from all
your pressure numbers if you fly at 12k ft. You've now
measured all of the variables that affect vapor lock. Only
remaining item is measuring pressure at your pump inlet.
You then can predict exactly how safe your plane is
without ever flying! How cool is that?
I use a submerged fuel pump
for added safety. Two actually. Easy to maintain, just
remove the 6 screws that hold the pumps in place. So there
is zero significance to the argument of maintenance. Can't
imagine how anyone could claim maintenance is more
important than life risk anyway. I return all fuel to my 3
gallon sump. Vent can go anywhere, you don't have to tie
vent into main fuel vents.
Once again, nice work Steve.
-al wick
----- Original Message -----
Sent: Friday,
September 02, 2011 12:04 AM
Subject: [FlyRotary]
vapor lock
For those concerned about the formation
of vapor in a pressurized fuel rail, I've attached a
plot of data collected from local samples of 100 LL
avgas, 87 octane auto fuel with no ethanol, 91 octane
auto fuel with 10% ethanol, and tap water. The water
was measured just as a check on the method.
The data would be considered more of a
true vapor pressure rather than a Reid vapor pressure
due to the method used.
The data indicates to me that if the
fuel pressure in the fuel rail is 35 psi as measured
with a regular gauge referenced to the atmosphere at
sea level, the temperature of 100 LL or 91 octane 10%
ethanol in the rail would have to be in the
neighborhood of 240 deg F for it to form bubbles of
vapor (boil). The sample of 87 octane would require a
temperature of about 215 deg F to form a vapor phase.
My take on this is it may be more
productive to be concerned about the fuel supply to
the high pressure pumps rather than worrying
about "vapor lock" downstream of those pumps. This
seems to be the conclusion reached by the recent
thread on this subject, possibly now supported by
actual data. Of course the data only applies to the
samples I obtained.
The higher temperature tolerance of the
auto fuel with ethanol compared to the auto fuel
without ethanol was surprising to me.
But I only collect data --- it is up to
an engineer to make sense of it ;>)
Steve Boese
RV6A, 1986 13B NA,
RD1A, EC2
--
Homepage: http://www.flyrotary.com/
Archive and UnSub: http://mail.lancaironline.net:81/lists/flyrotary/List.html
|
|