DJA727@aol.com wrote:
> Here is a question that comes after thinking quite a bit.
>
> It maintains the fuel pressure at about 3.5 psi above the airbox
> pressure. The normal range is from 2.0 to 5.0 psi delta above airbox
> pressure.
> ...
> I have been trying to figure a way to measure that parameter and have
> not found a cheap way, since a delta pressure sensor to do that job
> is very expensive, I think.
I've seen spec sheets where there's two versions on the same sheet. One
is sealed; the other is called differential and has a second hose barb
where the other one is sealed. One does need to look for a fuel/solvent
resistant type, though.
> It worked fine, except for the fact that the fuel pressure reading
> jumped 2.2 psi with the second fuel pump turned on. This has always
> bothered me that the fuel pressure is supposed to be regulated,
> yet it significantly increases with the second fuel pump activated
> along with the first.
Wonder if that's back pressure in the return line. I would tee into the
return line at a low point level with tank bottom, and with a full tank.
Clear tubing then pointing skyward. Measure increases in level with
0, 1 and 2 pumps. One psi = 27.7" of water at sea level, or 36.9" of
fuel. Don't forget to add say .6 psi for worst case pressure induced
by the tank vent in cruise.
> Should the fuel pressure be measured in the line from the regulator
> to the carb?
I believe that will same pressure, looking at internal design of the
regulator.
> If I institute the policy of taking off with one pump, that would
> solve this puzzle, but I am not sure what is correct.
Upon failure it's maybe 5 seconds 'til silence, and if early in
climbout, I don't like the idea of dealing with even first an aural
alarm by trying 2nd pump at a time where delay in setting up for forced
landing can be bad. The FAA has an AC advising mfrs (other than
airliners) how to deal with the requirement to meet specified failure
probabilities in critical systems. Amazing how mere redundancy
simplifies compliance!
Good luck!
Fred F.
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