Bob Darrah wrote:

Paul wrote:

Snip.

Your wing tank vent could be plugged and if it feeds into
an unvented header tank there goes your fuel supply.

If it feeds into a vented tank, you have the same problem, except when
 you switch tanks, you can get the air out of the header tank.

Not exactly the same Bob. The engine does not stop right away.
It must deplete the fuel in the vented header tank. That is why
I said; "if you see your header tank going down then is the time
to land".

If you see the level going down in your unvented header tank it is time
to land even if you just took off with full wing tanks.

True, but you have already pulled  a dangerouly high vacuum inside inside
 your tank with the plugged vent for this to happen. (fuel tank collaps? wing?)
 This collapse is the only thing that I can see that a vented header tank would
prevent.

I think the transfer pump would cavitate long before the wing collapsed.
It would be a good thing to check before you flew your airplane.

A gravity feed header tank is best.

?????

To a low mounted carb.

An unvented gravity feed header tank is an excellent back up.

???

To a low mounted carb.

Use a float switch if neccessary to control the wing tank pump
to maintain the fuel  level in the header tank.

If you need an additional fuel pump and float switch to keep the engine
running that is another failure source.

True. But then you have the safety of knowing exactly how much fuel
is in the header tank when you are low on gas. Nothing like
the big surprise you get when the engine stops because your high
pressure
pump(s) is/are in the main wing tank(s) and you ran both wing tanks dry. 

BTW Old VW bugs had what amounted to a header tank and it worked great. 
You ran out of gas on the main tank and switched to the aux tank. You 
then knew you had to find a gas station quick.

. The EFI high pressure fuel injection pump feeding
>from the header tank must run all the time of course.

A back up is called for there.

Yes!!!!!
.

Pump current drain is an issue when your electrical system
goes out. You will fly further if you can dump some electrical
power load. I would have an on and off switch on every pump.
I would have dual pressure pumps and run only one at a time.
A preflight check should be to test both pumps.

Yes.

I would also use a sealed or high vented center sump tank  near
the floor with both wing tanks feeding it. In fact a
transparent vent line mounted vertically on the side of the cockpit
will make an excellent, low cost, very simple  and
very reliable fuel gage.

If the vent line is in a higher pressure area than the wing tank vents,
the fuel will not gravity feed into the header tank.  If it is in a lower
pressure area, you will suck fuel out the vent.  That vent has to be
at the same pressure as is in the fuel tanks!

There is nothing stoping you from running both vent lines out the
same hole in the airplane or using a manometer to check both
pressures.

I would then have a back up mechanical wobble pump
to fill the header tank if necessary. This too will extend
your alternator-out flying range.

Not likely,  your ign quits, too!  The wobble pump would have to be
good (and your arm) to deliever fuel at 40 psi to the injectors 
to be of much help.

Not until your battery goes dead. Just because the alternator quits does
not mean
the engine quits. I believe Tracy's computer will work down to eight
volts or
so. Tracy? Help me out here Tracy.

A 40 amp hour battery means it is supposed to deliver 40 amp for one
hour
more or less with no help from the alternator.

The wobble pump is mainly used to back up the transfer pump and off load
the electrical drain from the battery if the alternator dies. It is a 
low pressure pump that re-supplies the header tank. I agree! If the high
pressure pump fails you are in deep trouble. 

I do however see no real proplem in a manual high pressure pump. 
The design is different but the  HP required out of your arm is not 
unreasonable. A high pressure pump drawing 10 amps at 12 volts is 
120 watts. At 746 watts per HP that is a 0.16 HP pump. The average
person 
can develope 0.25 HP more or less continuously and a burst at .75 HP. 
( I learned all that consulting on the Gossummer Condor :-))

Besides you can manually pump your tires up to 40 psi can you not? 

IMHO!!!!

Bob Darrah

 
Paul Lamar
 
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