Today's Message Index:
----------------------
1. 04:48 AM - Re: Disappointment in Rotax 9 series (Gilles Thesee)
2. 05:44 AM - Re: Disappointment in Rotax 9 series (Thom Riddle)
3. 06:07 AM - Rotax 914 regulator mounting location (Kevin Klinefelter)
4. 06:30 AM - Re: Rotax 914 regulator mounting location (Gilles Thesee)
5. 03:26 PM - Re: Disappointment in Rotax 9 series (Mark Burton)
6. 03:32 PM - Re: Disappointment in Rotax 9 series (Thom Riddle)
7. 04:45 PM - Re: Disappointment in Rotax 9 series (Mark Burton)
8. 05:33 PM - Re: Disappointment in Rotax 9 series (rampil)
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| Subject: | Re: Disappointment in Rotax 9 series |
Ivan a crit :
> Supposedly the "altitude compensating carburetor" is one that self
> compensates for changes in atmospheric pressure and adjusts the fuel
> flow accordingly. Thus even without a mixture control as you go up in
> altitude with a constant power setting the carburetor compensates and
> decreases the fuel flow, thus giving the engine less fuel when there
> is less oxygen and thus doesnt let the mixture get rich but keeps it
> about constant. This has been my finding on my Jabiru 3300 since the
> EGT(a measure of mixture) stays about the same as my altitude is
> increasing. There is a little error due to different temperatures
> from day to day which thus a colder day with the same barametric
> pressure can lean out the mixture slightly as evidenced by slightly
> higher EGT's for that day. The carburetor ONLY compensates for air
> pressure and not air temperature(which indirectly may affect pressure.)
Ivan,
Thank you for clarifying your opinion.
The point was, there is no technical difference between the Bing 64
carbs on a Rotax vs a Jabiru.
Both provide *some degree* of altitude compensation.
BTW, just for the fun, the cutaway drawing in the Jabiru documents was
borrowed from...a Rotax manual ;-)
But no problem here, since the Rotax drawing comes direct from Bing, and
the carb internals are the same.
Best regards,
--
Gilles
http://contrails.free.fr
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| Subject: | Re: Disappointment in Rotax 9 series |
Giles,
You are correct in that there are many combinations of mp/rpm that will produce
equivalent power, which is as you stated, limited by atmospheric conditions in
a normally aspirated engine. That is why Rotax provides tables for the operators
that show the most efficient combination of mp/rpm to achieve standard power
outputs of 55%, 65% and 75% of max.
What is interesting to me is that the combinations Rotax recommends are markedly
different for the 912 and 912S (80 vs 100 hp) engines. See attachments for these
tables. Your statement that higher MP vs rpm combinations are more effecient
is evident most of all in the 80 hp engine table.
Without looking at the Katana manual, I don't recall if the particular mp/rpm combinations
shown in their table are the same as recommended by Rotax. If not,
then perhaps there is room for improvement in fuel flow by conforming to the
Rotax recommended settings.
--------
Thom Riddle
N221FA Allegro 2000 912UL
N197BG FS1/447
--------------------
Believe nothing, no matter where you read it or who has said it, not even if I
have said it, unless it agrees with your own reason and your own common sense.
- Buddha
Read this topic online here:
http://forums.matronics.com/viewtopic.php?p=163452#163452
Attachments:
http://forums.matronics.com//files/912_power_151.jpg
http://forums.matronics.com//files/912s_power_175.jpg
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| Subject: | Rotax 914 regulator mounting location |
I am trying to decide where to mount the regulator in my 914
installation. I hear these regulators fail if you draw more than 14 amps
or so, possibly due to heat? Mine will be used as a backup, switched
off, unless the IRalternator I have mounted on the vacpump pad fails.
Has anyone mounted it in the air intake area near the air filter? I am
thinking of bolting it to the passenger foot well front in the air
intake plenum. Seems like it would get cool air,and maybe some heat sink
advantage from contact with the aluminum CD6.
It would be easier to just put it on top of a foot well. Anyone have any
advice?
Thanks, Kevin
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| Subject: | Re: Rotax 914 regulator mounting location |
Kevin Klinefelter a crit :
> I am trying to decide where to mount the regulator in my 914
> installation. I hear these regulators fail if you draw more than 14
> amps or so, possibly due to heat?
Heat, yes.
Have a look at http://contrails.free.fr/elec_ducati_en.php
> Mine will be used as a backup, switched off, unless the IRalternator I
> have mounted on the vacpump pad fails.
>
> Has anyone mounted it in the air intake area near the air filter? I am
> thinking of bolting it to the passenger foot well front in the air
> intake plenum. Seems like it would get cool air,and maybe some heat
> sink advantage from contact with the aluminum CD6.
Cool air, yes. But remember that anything protruding into the airflow,
albeit internal, will generate drag, unless the airflow has really
slowed down.
>
> It would be easier to just put it on top of a foot well. Anyone have
> any advice?
You can put it in any location you see fit, provided cooling air is
sufficient and wires are not overly long.
Best regards,
--
Gilles
http://contrails.free.fr
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| Subject: | Re: Disappointment in Rotax 9 series |
Howdy folks,
I don't have anything to add re altitude compensation (or lack of it) but you may
be interested in the following:
I have in my (912 powered) aircraft an instrument that calculates %power from the
engine RPM and differential manifold pressure. By differential, I mean it measures
the difference between manifold pressure and the ambient pressure at the
engine side of the firewall rather than absolute pressure which is the norm.
You can think of DMAP as being an indication of throttle position.
Now as %power is closely related to fuel flow, you can easily calculate an estimated
fuel flow from RPM+DMAP. And, of course, by integrating the estimated fuel
flow you end up with an estimate of fuel used.
So it gives you a similar readout capability to using a fuel flow meter but without
any fuel flow sensors to leak or clog up (or purchase!)
It's turned out to be amazingly accurate for the low-level flying that I do. I
would say that the error was no more than 5%, probably better.
Cheers,
Mark
Read this topic online here:
http://forums.matronics.com/viewtopic.php?p=163573#163573
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| Subject: | Re: Disappointment in Rotax 9 series |
Mark,
That sounds very interesting. Is this system a commercially available set up or
did you put it together? I'd like to learn more about how you did it and how
you estimate the power and fuel flow from the RPM and DMAP.
The more info, the better. I think others might also be interested.
--------
Thom Riddle
N221FA Allegro 2000 912UL
N197BG FS1/447
--------------------
Believe nothing, no matter where you read it or who has said it, not even if I
have said it, unless it agrees with your own reason and your own common sense.
- Buddha
Read this topic online here:
http://forums.matronics.com/viewtopic.php?p=163575#163575
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| Subject: | Re: Disappointment in Rotax 9 series |
Hi Thom,
The instrument I describe is actually one of my CSC units that I sell. The %power
calculation has been in the commercial unit for some years now but the estimated
fuel flow stuff is only in my own aircraft because I want to give it a really
good evaluation before I consider making it available to others.
As for the %power calculation, it's straightforward. I based my code on the curves
given in the Rotax 912 documentation. I interpolate in 2 dimensions (RPM and
MP) to produce an estimate of %power. It's not rocket science.
The problem with using absolute MP is that for a constant throttle setting, the
measured MP goes down as you climb so the calculated %power goes down as well.
But by using differential MP the calculated %power doesn't change much with
altitude so you end up with a %power figure that relates to throttle position.
i.e. for a given throttle position, the displayed %power doesn't change much
with varying altitude.
Fuel flow is simply calculated based on the assumption that it scales linearly
with %power. If you look at Rotax's curves, they match pretty well so it's not
an unreasonable assumption.
As I said, it gives remarkably accurate results and would make a good backup system
to cross-check other fuel gauges against.
Cheers,
Mark
Read this topic online here:
http://forums.matronics.com/viewtopic.php?p=163601#163601
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| Subject: | Re: Disappointment in Rotax 9 series |
Mark,
Percent power relative to engine rated max decreases with DA.
What ever fraction of max power (75% for example) at SL some
arbitrary throttle setting will decrease with altitude if the throttle
stays put. I don't believe you are saying otherwise.
Just to avoid hand waving, Engine power for an Otto cycle recip run
rich of peak is a function of MAP, and RPM. As a first linear
approximation, Power declines 3.3% per inch Hg MAP decline at fixed
RPM. Power also declines linearly with decrease in RPM. For example,
at fixed MAP, if the RPM declines from 5800 to 5500 (a drop to 94.8
percent of max RPM), the engine power declines to 95%. Therefore,
absolute pressure in the manifold is central, as is RPM relative to max
RPM speed.
What your device does, if I understand it is to display percent power
available as a percent of the max at any particular altitude. My
question is Why? What's it good for?
--------
Ira N224XS
Read this topic online here:
http://forums.matronics.com/viewtopic.php?p=163614#163614
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