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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
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