Hi Chris,
A fuel pump and a TRANSFER pump are functionally quite different even
though the hardware and terminology are identical. The Ercoupe "fuel
pump" is merely a transfer pump and is not critical to flight.
1. Way more fuel than the engine can burn is sucked out of the balance
line between the two wing tanks and pushed under pressure through the
restrictor.
2. The restrictor reduces the huge excess of fuel transferred at full
pump pressure to a lesser excess which is forced under relatively low
pressure into the fuselage, or header tank. The flow restriction
causes the pump to cavitate; thus reducing the "load" of a pump
operating at full capacity to that of the required capacity. A tip of
the hat to Fred Weick for that bit of "magic on the cheap".
3. The lesser excess still causes the fuel level in the header tank to
rise until it reaches the tank's overflow intake, which it enters and
flows by gravity back to one or more wing tank(s).
4. The header tank cap is vented to atmosphere in every ATC 718 and
787 airframe to assure that fuel in the header tank can flow freely by
gravity to the gascolator, carburetor and engine. The header tank
cannot be pressurized in normal operation and still function as
intended.
Without the restrictor the pump would transfer fuel at a rate in excess
of the header overflow system to return and as the fuel level rose to
the level of the fuel cap fuel would be ejected (visualize Old Faithful
geyser) and distributed across the windscreen and likely further. This
happens more than it should when the restrictor is removed or omitted
by a mechanic unfamiliar with the Ercoupe (et al) design.
It is true that once all fuel in the wing tanks the engine could not
burn has been thus ejected, the pump has no measurable load whatsoever
;<)
Regards,
William R. Bayne
.____|-(o)-|____.
(Copyright 2009)
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On Apr 29, 2009, at 13:47, Chris wrote:
On the restriction of the fuel pump outlet. It seems to me that it
would be more efficient to let the fuel pump free run without the
restrictor and let part of the pressure bypass back to the inlet.
Wouldn’t that take some of the load off the pump? Am I missing
something here?
Chris
99674 in restoration
‘--o-O-o--’
