Fuel System Musings, comments encouraged

On Oct 20, 6:43*am, flybynightkarmarepair wrote:
RE the Ercoupe example: I don't like header tanks from a crash safety
standpoint, and another vent, and a return line souunds like MORE
plumbing to me. *Plus, this is a VW conversion, and if I used a
mechanical fuel pump it would be on the TOP of the engine, plus they
are not sealed like aircraft mechanical fuel pumps are, so this is a
no-go.

The Ercoupe design has worked safely and well for over fifty years.
The lack of a header tank does not guarantee crash safety - nor does
it's presence necessarily increase the danger.

The one pump in that design can either be mechanical or electric, it
matters not. There are no vents involved in the design. The float
gauge holes in the caps provide adequate venting. If you are forced to
park outside in the rain, you simply put a cap over the cap. There is
one line connecting the two tanks which are located at the wing roots.
If one wing is parked, or flown, a little low it makes no difference.
There is a tee in the line that leads to the inlet of the pump. The
outlet of the pump goes to the header tank. The overflow of the header
tank goes to one wing tank.

IMHO, you are making a mountain out of a molehill. This ain't rocket
science and you aren't building a space shuttle. Gravity flow to a
carburated engine is the safest and most reliable fuel feed possible,
until you are attacked by Feherenghi using anti-grav phasers. Listen
carefully - putting any pump, electric or mechanical, in the fuel line
to the carb will restrict or prevent fuel flow in the event of pump
failure. Be safe - use gravity.

My 2 cents worth.

Rich S.

On Oct 20, 5:45*pm, wrote:
On Oct 20, 6:43*am, flybynightkarmarepair wrote:

RE the Ercoupe example: I don't like header tanks from a crash safety
standpoint, and another vent, and a return line souunds like MORE
plumbing to me. *Plus, this is a VW conversion, and if I used a
mechanical fuel pump it would be on the TOP of the engine, plus they
are not sealed like aircraft mechanical fuel pumps are, so this is a
no-go.

The Ercoupe design has worked safely and well for over fifty years.
The lack of a header tank does not guarantee crash safety - nor does
it's presence necessarily increase the danger.

I've read an accident report where 2 POB died in an otherwise
survivable Ercoupe crash due to the fuel tank splitting and soaking
them in gas. Ignition happened, and they became human wicks.

Others have noted ways of mitigating this via fuel cells, and while
you make a pretty good argument, I'm not convinced. Fred Weick, the
designer of the Ercoupe (although perhaps not this part of it...) also
had significant input to the most numerous low wing production design,
the Piper Cherokee, and it DOES NOT use a header tank. Strictly
circumstantial, to be sure, but draw your own conclusions.


The one pump in that design can either be mechanical or electric, it
matters not. There are no vents involved in the design. The float
gauge holes in the caps provide adequate venting. If you are forced to
park outside in the rain, you simply put a cap over the cap.

I don't like this at all. I don't like relying on the nut between the
headphones to keep me safe from a significant hazard (water in the
gas) in an airplane with as small a fuel capacity as the one I'm
building. I want a screened vent of at least 3/8 in. dia. that exits
the BOTTOM of the wing, but vents the TOP of the tank.

There is
one line connecting the two tanks which are located at the wing roots.
If one wing is parked, or flown, a little low it makes no difference.
There is a tee in the line that leads to the inlet of the pump.

I recall a LONG argument on this forum (or maybe it was on a Zenith
601 forum - LOTS of that design fly with a system VERY much like
you're advocating) about unporting a tank at low fuel levels and
sucking air, that I don't want to rehash, but I'll just note that I'm
firmly in the LEFT/RIGHT/OFF fuel selector camp for low wing planes
with wing tanks.

Gravity flow to a
carburated engine is the safest and most reliable fuel feed possible,

I agree, I agree, to the extent that I'd rather build a high wing
airplane if plans for one that meets my mission requirements were
available when I was first looking. The Aerosport Quail is the only
HIGH wing all metal VW powered single place homebuilt aircraft I'm
aware of, and it's not clear you can actually get plans, although a
source iin Oregon is rumored.

For a low wing plane with fuel in the wings, which I'm well convinced
is safer than fuel in the fuselage, pumps are a necessary evil, and
since they can fail, I'll take two please. William Wynne has done
more engine installations by far than I have, and he's going this way;
so am I.

================================================== ============

Anybody have anything to say about the fittings? That was my initial
interest. Should I ditch the pumps I have and just bite the bullet
and go with AN fittings per the exemplar? Do all those unions make
sense?(heavy little suckers! Think of a ball of solid brass the
diameter of a quarter for the 1/8" NPT size, and the diameter of a
Susan B. Anthony dollar for the 1/4" NPT size, and they lighten the
wallet to the tune of $13-20 per at the same time they increase empty
weight) - given that NPT fittings don't seal anyway metal-to-metal,
and you can "clock" them with SOME degree of freedom as long as
they're at least slightly more than finger tight, relying on Loctite
Pipe Sealant to keep them leak free?

On Oct 20, 5:45*pm, wrote:
This ain't rocket
science and you aren't building a space shuttle. Gravity flow to a
carburated engine is the safest and most reliable fuel feed possible,
until you are attacked by Feherenghi using anti-grav phasers.
-------------------------------------------------------------------

Agree.

As with most engines, the VW fuel pump is driven at cam speed but it
is NOT driven off the cam shaft, a significant difference with regard
to its mechanical durability. It is driven off the crankshaft via its
own gear-train. The pump is a two-chamber type in which the
mechanical input serves only to extend a compression spring. The
spring is then free to retract, the amount of retraction determined by
the quantity of fuel drawing into the lower chamber during the
extension cycle. In effect, the pumping action is performed by the
spring rather than by the engine.

Mechanically, the pump is isolated from the engine by a push-rod that
is actuated by a pinion gear. The flexible diaphragm that makes up
the floor of the lower pumping chamber has a useful service life in
excess of 1000 hours, the push-rod slightly more than that. Output
pressure is typically between 8 and 16 ounces, depending on the
spring. Any flow-control valve, such as the ball-valve in the stock
Solex carburetor, which typically has a pressure between 8 ounces and
two pounds is sufficient to shut off delivery. However, any fracture-
failure in the delivery circuit will cause the pump to spray gasoline
in all directions. Wear in the push-rod is reflected by its length
which may be meausred by removing the pump (two studs). Volkswagen
provided three pump push-rods of different lengths.

Failure of the flexible diaphragm offers a bit of visual warning in
the seepage of fuel at the parting line of the pump. This is only
valid for the early model, rebuildable fuel pumps, which is what I
prefer to use on my VEHICLES. In an airplane I'm more comfortable
with a gravity-fed system, even if that includes a header tank in the
fuselage.

The push-rod actuates a bell-crank in the base of the pump body, which
is a white-metal casting. The bell-crank's pivot bears on the casting
without provision of bushings or bearings. In a high-time pump it is
common to see the bearing-holes worn to an oval. The pivot itself is
held in place by a cotter key (early) or snap-ring (late) which is
also subject to wear. The pallet of the bell crank calls for periodic
lubrication with axle grease, maintenance it seldom sees when the
vehicle is maintained by the typical owner. (I believe the service
interval was every 12,000 miles or annually but I'd have to check the
Factory Service Manual. I do it about once a year and have never had
a mechanical failure due to lack of lubrication. I have had the pivot
and the fastener break.)

In my opinion, each builder is responsible for answering such
questions for themselves. I can offer an opinion and may provide an
example to support it but ultimately, that's the limit of my interest
and responsibility; I can always refuse to ride in the thing. (And
have, on several occasions.) Personally, I don't care for pumps and
would eliminate them if possible, especially if they are engine-
driven. The next best choice would be an electrically driven pump (or
any number of them) located where a failure would not douse the engine
with gasoline. Even so, at best all this would do is reduce the risk
of an IN-FLIGHT fire. Following a crash or bad landing, there is
usually some amount of fuel at the scene regardless of where the fuel
tank is located and no matter how the fuel is delivered to the engine.

-Bob Hoover

flybynightkarmarepair wrote:
My blog has become a way for me to think out loud about my project.
Putting things down in a narrative helps me crystallize my thoughts,
and give me something to refer back to when the time comes to move
from thinking to doing.

Anyway, I've been THINKING about my fuel system, and have put down my
thoughts, I encourage those with more experience to comment either
here or via the blog.

http://users.lmi.net/~ryoung/2008/10...tings-and.html

Lots of good stuff in the links, especially the Sacramento Sky Ranch
ones.


I'm in what I hope are the closing stages of a build of a 601XL with a
William Wynne built Corvair engine. I have the exact fuel system shown
in the photo.

Go with the AN stuff. You can get them from Summit Racing and they don't
cost that much.

For those that suggest a pumping into a header tank. That is exactly
what the older 601HD and HDS did. There just isn't room for it in the
601XL if you have any kind of radios behind the panel.