Hans,
You are right that they do not have a requirement, spelled out, but it
is implied. Rotax does have parts for it and information is in the new
Rotax installation manual. Copy attached (check currency), page 69.
I follow their instruction requirements very closely. Part numbers are
in the parts catalogue.
UMA does make a pressure differential gauge to measure airbox vs. fuel
pressure. I can do the math so I just use the raw fuel pressure and
manifold pressure.
Explanation: My normal fuel pressure is 4-5psi. When I go to boost I
glance at it to see if the pressure rose IAW the boost.
We know that 29.92 in HG is 14.7 psi so 2 inches of Hg is about 1 psi.
ISH OK.
Example: On takeoff, at a boost pressure of 40 inches, that is 10
inches over atmosphere (that's 5 psi ish) so the fuel pressure must be
above 2.3 ish min plus 5 psi or a min of 7.3 to keep the fuel going to
the carbs to prevent fuel starvation as the operations and installation
manuals state..
If I cruise at a fuel pressure of 5 psi at 30 inches MP at low altitude,
that is OK, if I climb at 34 inches I'm looking for 6 ish and on
takeoff, 8.
I don't cruise at high altitude normally, but as you climb, the outside
pressure decreases and that lowers the fuel pump inlet head a bit, so as
you climb your MP or outside pressure is around 17 inches HG at 15,000
MSL. (Remember, you loose 1 in HG per 1000 or so.) That means outside
air pressure is 8-9 psi or so in round numbers below sea level pressure
of 29.92 inches or 14.7 PSI. If you push the MP above 30 inches then,
the engine may start to see some fuel starvation, so turn on the other
pump when going above 10,000 and you can run max continuous power up to
15000 or so. Above 15,000 I haven't gone, but I imagine that one has to
pay attention. Let's see. MP gauge reads 34 inches at 15,000 feet,
minus 17 is 17 inches HG and that is about 8 psi, so 2.3 plus 8 is 10.3
at the fuel pressure meter... That needs both pumps to do, and it does
it well.
So install a good MP gauge and a descent fuel pressure gauge or buy one
of the UMA differential gauges and press on. I prefer to know my raw
pressure. It is a preference for troubleshooting. When my math skills
go, I'll buy the other gauge I guess.
Hope this wasn't too confusing. Check my math, I am getting older.
Bud Yerly
Custom Flight Creations, Inc.
----- Original Message -----
From: Hans J. Danielsen<mailto:hansjd@online.no>
To: europa-list@matronics.com<mailto:europa-list@matronics.com>
Sent: Sunday, October 17, 2010 1:10 PM
Subject: Europa-List: Fuel Pressure Indication
During my studying in order to rectify my too high fuel pressure
indication (which for my 914 installation most probably is caused by a
restriction - one way or another - in the fuel return line), I'm
astonished to (re)learn that there is no requirement from Rotax to
install a fuel pressure instrument.
The Installations Manual states: "There is no standard connection
provided to measure the fuel pressure. For general observation and at
engine troubles fuel pressure readings would be helpful". The text
further describes how to go about fitting of an extra banjo bolt etc.
In other words: It's entirely up to the builder - or to follow local
country requirements - to install such an indicator.
Nevertheless: The Rotax Operations Manual issue a warning (page 2-4)
which says: "Exceeding the max admissible fuel pressure will override
the float valve of the carburetor and to engine failure".
I would say that a check of the pressure would be damned "helpful" in
this respect! Or, how would I know at all of any ill funktion before the
donkey elects to stop over the biggest lake in Norway???
Now, can someone explain to me why the Rotax factory has such an
indifferent approach to this issue in the Installations Manual, while
the Operations Manual "paints the devil on the wall" in pointing out the
dangers. And, there is no follow up whatsoever in procedures or check
lists for high fuel pressure, as with other abnormalities. In other
words: Rotax issue a Warning based on an abnormality you wouldn't stand
a chance to know - if you didn't find it "helpful" to install the
indicator system!
This can't be right!
Hans
LN-HJD, #334
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