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Hello Todd. You describe multiple forced landings
related to fuel. Pretty clear you copied a fuel design that has a very low
vapor safety margin. If you measure fuel pressure at pump inlet and pump temp
you can predicted this failure. I really want to encourage any fact
gathering method. Ed often does an excellent job of calculating these types of
things. Greatly improves understanding. To actually measure how close you are to
failure, you'd have to put those numbers in a spreadsheet. Add atmospheric
pressure info from your instrument panel. Pick up one of those Hodges fuel
testers that measures vapor point of this particular lot of fuel. Next you just
have to apply a hair dryer to your pump and voila, you have measured how safe
your design is. All while sitting on the ground. Use clear tubing and do this on
your workbench and you can see fuel turn to vapor.
I enjoy lurking, witnessing this group improve
their conversions. I noticed this method of returning fuel to pump inlet
was being promoted. No one was offering counterpoints. I'm fully aware this is a
serious life risk. Please question this particular design.
<So as Ed has pointed out, while millions of cars return the fuel to the
tank, millions don't.
This statement defies all logic. You are comparing apples to oranges. All
auto fuel designs that utilize the pumps like ours return fuel to
tank. They do this because it's essential for safety. Only fuel designs that
have variable displacement pumps eliminate return to tank.
Apologize for my use of strong wording. I was
thinking: "omg omg, people are going to die needlessly". My bad.
-al wick
----- Original Message -----
Sent: Tuesday, June 08, 2010 10:31
PM
Subject: [FlyRotary] Re: high/low
pressure pumps question
Hi Al;
Returning the fuel to the fuel
line rather than the tank (thereby to the fuel suction), is I admit not ideal.
Not by a long shot. However in some cases it may be a necessary evil that can
(and so far has) been made to work. In most vehicles there is one tank,
sometimes two so it is relatively simple to return fuel to the source fuel
tank. Most airplanes also have 2 tanks and occasionally one, so returning to
the source tank is again fairly simple and should be the first choice. where
it becomes more complicated is when you have to manage 4 or even 6 tanks. It's
difficult enough to come up with a simple, safe, light and affordable for the
average home builder, way to manage selection of these 6 tanks, but to return
the fuel to the source tank, would become a potential problem. Some builders
simply return to a single tank which they draw down first then switch back to
before it overflows, while others always drawn out of the same tank, and
simply transfer fuel to that tank. Nothing wrong with that and it is ideal for
many. Fortunately were not all the same, which is why were here on this
list.
My reason for 6 tanks is to increase range for
either flights to remote areas with no fuel service or over long stretches of
inhospitable terrain (like passing over my mother-in-law's). The plan of
transferring fuel in my mind allows the distinct possibility of pumping
precious fuel overboard whether it be from directly transferring fuel or via a
fuel return to a tank that is not the source. There is also the possibility of
failure of the transfer equipment, which would result in inaccessible fuel on
board. Sure these are remote possibilities and could be lessened by the
addition of various safety checks and devices all which add complexity and/or
workload to the pilot. Not much but some. So I chose to try to implement a
variation of a return-less system.
Sure as you've
said the failure frequency could be in the hundreds of hours, so in my short
time it has been just fine, but still not proven. However what would be the
incident frequency of lost or inaccessible fuel? More, less? I can't say, but
I do know that if it happened there would be absolutely nothing I could do
about it in flight, so I'd better hope that I didn't really need it. With my
current system design, I have 2 facet pumps and 2 FI pumps that are plumbed in
such a way that is fairly simple for fuel management, but also allows full
redundancy in that any pump failure would not prevent me from being accessing
fuel. The issue of vapour lock remains but the facet pumps 7psi of pressure
has so far proven to be more than adequate for elimination of this threat. So
if this were to happen at least I would have options to try to clear the
vapour. Better to be able to do something than nothing. Now with that
said, I feel the chances of a vapour issue developing halfway through a flight
are even more remote. This would be most likely to rear it's ugly head during
the take-off you described after a long taxi and wait in the south Texas sun.
Or in my case more likely while flying in mountain valleys when working it a
little harder. Exactly the time I wouldn't want it to happen. That's why the
facet boost pumps are always on at these times.
So
as Ed has pointed out, while millions of cars return the fuel to the tank,
millions don't. But, I must add, most or all of them have a true return-less
fuel system which we don't. Some of us are trying to copy that within the
limitation placed upon us. Could we make it a true return-less system? Sure we
could. At the low, low, price of a few more pounds and cost and complexity per
tank. There is no free lunch here. Would I be willing to pay that price, while
dead-sticking it down into a mountain valley. Of course I would, but using
that logic would result in an ultimate safe plane, as it would never get
finished. So here we are discussing ways to make it work without returning
fuel to the tank. And I can say that it has worked well. I just can't say that
it will always work well.
I would never suggest to
anyone that the way I did it is the best, but when asked about it, I'm happy
to report my knowledge based on my experience along with any cautions based on
that same experience. Then in the ensuing discussion I may learn something.
Which seems to be the case here, as Charlie, who started this thread with his
question has shown his idea based on what he's learnt from the discussion and
it has his own subtle differences, with regards to regulator plumbing, that
I'm surprised I overlooked and I think it has real promise. So much so that
I'll consider a possible change to my current system.
Other commitments these days usually restrict me to
lurker status (that always sounds creepy), but the occasional questions
directed at me usually don't go unnoticed and I enjoy the free exchange of
ideas that happen on this list. I appreciate the words of caution, but when
phrased as a command not to go against the grain, it loses it's validity as it
is reminiscent of another place where free thinking is discouraged. We're all
aware of the risks of venturing from the beaten path, which is why we're here
to discuss, dissect and hopefully come up with a better
way.
Al, I do value much of your advice even if it
comes across as harsh, but I figured I'd explain my mindset which I suspect is
shared by a few others here, which is to minimize and take calculated risks,
but not reckless risks.
Todd
C-FSTB
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