Today's Message Index:
----------------------
1. 06:48 AM - Re: Fuel return lines (Gary Casey)
2. 06:56 AM - Re: Who should be allowed (Jerry and Nancy)
3. 08:15 AM - Re: Fuel return lines (Ken)
Message 1
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| Subject: | Re: Fuel return lines |
--> Engines-List message posted by: "Gary Casey" <glcasey@adelphia.net>
Frank said:
<<...Gasoline is a high vapour pressure fluid. If
the net positive head is reduced below the VP of the liquid in question
it will boil...Boiling fuel will vapour lock the pump.>>
Right. Except it's worse than that - gasoline is comprised of a wide
variety of compounds. Some, like propane, naturally exist as gases and they
are merely dissolved in the fluid. So all you have to do is thrash the
stuff about and some of those gases will come out of solution, making a
vapor (sorry Frank, I meant "vapour"), and won't easily re-dissolve. So
boiling is technically the wrong term to use.
<<Vpressure increases rapidly with temperature..So you want to keep the
pumps as cool as possible....This means the firewall is a poor
choice...Forget blast tubes...The Egg powered RV was so equipped..They
don't work well enough.>>
Right again.
<<Hydraulically speaking the best place is INSIDE the tank...Practically
this is difficult to achieve.>>
Right again. Automotive designers came to that conclusion about 30 years
ago. Except I'm not so sure it is all that difficult if you commit to an
in-tank pump before the tanks are built. Then it is just a matter of an
access panel large enough through which to mount the pump.
<<Here is another thought...What if you have a right/left selector before
the pump?...What happens to the fuel during the 1/10th of a second that
fuel does not flow?>>
Here I disagree. As far as I know all normally-used selector valves go to
"both" in between tank selections - one port opens before the other closes.
Otherwise even my fuel-injected Cessna would momentarily quit while changing
tanks.
<<Remember once a pump fills wit vapour it will not pump anymore against a
regulator spring...Must therefore have a small bypass around the
regulator.>>
Yes again. Most automotive pumps, though, have a small internal bypass.
That doesn't help a lot because the fluid that is being bypassed could be
vapor. Also, positive displacement pumps will usually (is "usually"" good
enough for an aircraft application?) re-prime themselves. Many automotive
pumps are hydrodynamic and they have a very difficult time re-priming
themselves.
<<Bottom line at the very least get the pumps off the firewall and into a
nice cool location inside the cabin...Mine are going in the wing roots!>>
Excellent. I think that is a close second to being in the tank and any
other location is a real compromise. I spent some time designing a system
like this and the best compromise I could come up with is a selector valve
that only switched the return flow. On the selector valve was a switch that
changed from one wing root pump to the other. The check valves internal to
the pumps prevented cross-flow. Only problem with that is that if one pump
fails to start it could happen with a dry or nearly dry other tank and then
the fuel in the fuller tank is not available. Not a good thing if you're
over the middle of an ocean, but anywhere in the US it could be considered
to be okay. Another idea that is functionally good, but complex is a vented
header tank with both pumps mounted below. Transfer pumps in the wing tanks
are used to refill the header. The same problem exists - a pump can quit
and some of the fuel is unavailable. High-wing planes have a much easier
time of it as pumps can be mounted well below the tanks and then a selector
valve in the suction side of the pump is probably just fine.
A final thought: I talked to a builder that had what I considered a poor
fuel pump configuration very similar to the one Frank described. He said,
"it works just fine -I've flown it 100 hours without a problem. Before I
would use experience as the sole proof of acceptability I would want maybe
100,000 hours of fault free performance. Sound engineering principles make
for much better proof of acceptability.
Gary Casey
Message 2
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| Subject: | Re: Who should be allowed |
--> Engines-List message posted by: "Jerry and Nancy" <flyguy50@bluemarble.net>
>Original message: dated 3/3/2005 6:36:31 AM Eastern Standard Time,
>engines-list-digest@matronics.com writes:
>It reduces down to this: some people just should not be allowed to
>build Experimental aircraft or engines.
>Monty Barrett
>Barrett Performance Aircraft, Inc.
To Speedy11 and the rest of the list,
Having been around experimental aviation since early Rockford days.. anyone with
eyes open, can attest to the comment above (without intent to flame ANYONE here,
for the recent posts). I can personally tell you of 5 different people I
knew, who might have been better off not building (or flying) their own planes.
They are no longer with us, BTW!! VERY costly mistakes indeed. The FAA is
not a Guardian Angel, nor able to catch every mistake a home-builder might make
on an inspection. But I can certainly relate to the comment above. My dad
also had his own version... when he too was apalled by some peoples actions,
or practices ... "the guy has a 100HP engine, and a 10 HP brain". Sometimes "political
INCORRECTNESS" says it best. Jerry
Do not archive.
Message 3
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| Subject: | Re: Fuel return lines |
--> Engines-List message posted by: Ken <klehman@albedo.net>
Since we are being picky there is another gotcha lurking here that is
sometimes overlooked. Most automotive submersed pump outlets route
through a rubber hose to the fuel tank outlet. If that rubber hose fails
(and they apparently occasionally do) then the pump internal check valve
is useless as the other pump from the other tank will still dump all the
fuel out through the failed hose unless you also use external check
valves. In addition that short piece of rubber hose should be rated for
submersion in fuel (SAE R10 rating I believe). Replacing it with normal
R9 rated efi hose is not a good idea.
Yes high wing fuel gravity fed to a header has advantages. You can put
two submersed pumps in that header if you wish and then there is no loss
of available fuel if one in-wing tank pump fails. There is no fuel
management to contend with. You can put a low fuel warning sensor in the
header tank if you wish. But you will still probably want an emergency
fuel shutoff near the firewall in both the supply and return lines if
the header is behind the firewall. As it happens I did all this but
would probably not do it again as that header tank has significant
construction and mounting issues to make it reasonably crash resistant.
So (space permitting) my preferred installation would be external pumps
on the firewall beside a gravity fed header tank. But with an enclosure
and fresh air to keep it cool ;)
With a low wing machine I agree that in tank pumps start to become
attractive as does the dual port Andair valve with an off position.
Ken
snip
> I spent some time designing a system
>like this and the best compromise I could come up with is a selector valve
>that only switched the return flow. On the selector valve was a switch that
>changed from one wing root pump to the other. The check valves internal to
>the pumps prevented cross-flow. Only problem with that is that if one pump
>fails to start it could happen with a dry or nearly dry other tank and then
>the fuel in the fuller tank is not available. Not a good thing if you're
>over the middle of an ocean, but anywhere in the US it could be considered
>to be okay. Another idea that is functionally good, but complex is a vented
>header tank with both pumps mounted below. Transfer pumps in the wing tanks
>are used to refill the header. The same problem exists - a pump can quit
>and some of the fuel is unavailable. High-wing planes have a much easier
>time of it as pumps can be mounted well below the tanks and then a selector
>valve in the suction side of the pump is probably just fine.
>
>
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