Hi Al, thank you for a very informative explanation.

-- Ian
(I think my chemistry teacher tried, and failed to teach me this about
liquids a long time ago...)


-----Original Message-----
From: Rotary motors in aircraft [mailto:flyrotary@lancaironline.net]On
Behalf Of Al Gietzen
Sent: Monday, June 13, 2005 11:48 AM
To: Rotary motors in aircraft
Subject: [FlyRotary] Fuel System


Do you have a number that represents a minimum head pressure to prevent
vapour lock, some suggestions for a filter and the pressure altitude I
should limit my flying to

Ian;

I know you asked this of the “other” Al, but here’s my $.02 worth.

We know that there are fractions in auto fuel that have (partial) vapor
pressures at ambient conditions that are ½ - 2/3 of atmospheric (absolute).
This suggests that we don’t have to draw the pressure down very far below
atmospheric (negative gauge pressure, psig) for incipient bubble formation.
And as temp goes up, or our altitude goes up, the margin decreases.  So
reiterating what Leon pointed out, we should have a system which maintains
some positive gauge pressure at the pump inlet; enough so even at altitude
we have a something around zero guage or better.

Remember, we are talking ‘at the pump inlet’.  If you have some line, and/or
some filtering before the pump, that loss has to be accounted for.  I have
about 18” of level 3/8 line from the bottom of my sump tank to the pump, so
I want a minimum of about 1 ft head of fuel to the pump inlet (that’s only
about 0.5 psig) when I’m down to that last gallon of gas (which should never
happen except when I’m testing on the ground).  I’ll have over 2 ft of head
with full tanks.  Generally something less than could be OK, but we want
margin for those hot days, time on the taxiway; whatever.

BTW; speaking of the other Al, Al Wicks, I would only comment that
statistical failure analysis requires statistically significant numbers of
cases, which for much of what we are doing doesn’t exist.  But there were
good points buried in there about determining root causes; and measuring
and/or calculating whatever unproven aspects we can to verify function PRIOR
to taking to the air.  It was disappointing to see rude and even vulgar
remarks here.  We can look for what can be of value and either ignore, our
simple say we disagree and why, for stuff we don’t agree with.

Al G.