PA28-181 Fuel pressure drop off - Inspired by the Gascolotor thread.

Roy Page wrote:
: Dave,
: Thanks for your input to this discussion.
: Your findings are almost identical to mine in my PA28-181.
: I think that your comments are valid but would indicate that the mechanical
: fuel pump on all PA28's is marginal.
Given the explanation I got from Petersen on the autogas STC, it would seem
the FAA agrees.

: A previous explanation of the pressure drop centered on the pressure
: transducer not venting sufficiently in climb.
Interesting, but with as crappily as a PA28 climbs, it would have to be damn
near closed off. A 1000' change in altitude is only 1/2 PSI change in cabin pressure
and takes between 1 and 3 minutes to do in a Cherokee. I'm not saying it's
impossible...just seems a more unlikely explanation. Of course, that won't stop me
from experimenting on mine and seeing if I can see that.

: A situation is emerging in this discussion which clearly shows that many
: PA28's exhibit this fall in fuel pressure.
: Apparently without causing any fuel starvation to the engine.
As long as there's enough pressure to fill the carb bowl, everything is fine.
Sure there's a *slight* change in float bowl level with a varying input pressure, but
it's probably not enough to measure, let alone care.

: Whilst a number of people have identified causes, no one as yet as come up
: with a fix.
: It seems that I had better take Cory's advice and cover up the gauge :-)
: Thanks again.

Now you're talking.

Just FYI... while my electric fuel pumps are from the autofuel STC and are
different from stock, when they're off the system is the same. In a full-power,
full-rich, power-on-stall attitude, the mechanical pump can only muster about 1 psi on
the gauge IIRC. Not exactly comforting. In a less aggressive attitude, it's more
like 2 psi. In cruise or any attitude with the electric pumps on, it's rock-solid at
5 psi. I *do* seem to recall that with the stock (non-autogas STC) electric pumps,
the pressure would still fluctuate some at high power, aggressive attitude settings.


-Cory


************************************************** ***********************
* Cory Papenfuss *
* Electrical Engineering candidate Ph.D. graduate student *
* Virginia Polytechnic Institute and State University *
************************************************** ***********************

It seems, from the initial comments that a fuel pressure fall off during
climb in PA28 aircraft is seen more often than I thought.
Well where do I go from here .....

You join the Cherokee Pilots Association, where if you had been a member
(and if you had read the organization's "bible", "Cherokee Tips and Hints")
you would have known that this fuel pressure thing is a common, benign trait
of all Cherokees.

My Warrior did it, and my Pathfinder, with the O-540 engine, does it, too.
It's weird, but (if you look closely at the fuel gauge) the "green" goes all
the way down to 0.5. "Normal" fuel pressure stretches across a very wide
range!
--
Jay Honeck
Iowa City, IA
Pathfinder N56993
www.AlexisParkInn.com
"Your Aviation Destination"

Picking up on Kyle's transducer pressure differential explanation, I suspect
the static air pressure between the engine and the firewall may increase as
the nose is raised from level flight. This could be caused by ram air
entering the bottom of the cowling just forward of the firewall. No actual
altitude change would be needed.


"Aaron Coolidge" wrote in message
...
Roy Page wrote:
: I bought our Piper Archer [PA28-181] just over a year ago from a
Californian
: owner and flew it home to it's new base in Ohio.
: On the flight home, we noticed on long climbs that the fuel pressure
fell
: off from an indicated mid scale reading to about one third scale.

This has always happened on my PA-28-180 as well. It is more noticable in
hot weather. As long as there is positive pressure to the carb there
should
be no problems.

--
Aaron C.