Today's Message Index:
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1. 04:44 AM - Re: Rotax 912 Idle Speed Setting (Thom Riddle)
2. 05:23 AM - Re: Rotax 912 Idle Speed Setting (h&jeuropa)
3. 07:38 AM - Re: Rotax 912 Idle Speed Setting (Roger Lee)
Message 1
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| Subject: | Re: Rotax 912 Idle Speed Setting |
Roger,
Nor am I arguing, merely discussing what I know from personal
experience.
1) I mentioned 80 hp engines because I know the difference in
compression ratios between the 100 and 80 hp does make the gearbox
abuse worse if the engine is not idling smoothly. Also because it is my
understanding that Hugh's Allegro is also running the 912UL.
2) The CT has much more effective flaps than the Allegro, is about 100
lbs heavier than the Allegro empty and thus the behavior comparisons on
final are not really valid. This behavior also depends somewhat on the
pitch of the prop. What I said about behavior on final approach in an
ALLEGRO, I said from personal experience in an airplane essentially
identical to Hugh's.
3) I've never seen a Rotax 912 series engine idle REALLY smoothly
without the balance tube connected. This includes the CT with Rotax
912ULS at Lockwood being balanced by the instructor Dean Vogel on April
14th of this year. It includes our Allegro w/ 912UL. It includes the
Diamond Katana w/ 912F. It includes my old Titan Tornado with 912UL.
All of the above with vacuum gages exactly matched and idle at 1800
rpm. Incidentally, when setting the idle in the 1400-1500 rpm range and
the carbs are synchronized properly there is no difference in the
smoothness compared to 1800, at least not on the 80 hp engines.
Roughness is the thing we are trying to avoid. If it idles smoothly at
1450 rpm then there is no damage being done to the gearbox. At
Lockwood, Dean set the idle on the CT at 1800 rpm so I don't know how
it would have been at 1500 rpm or lower. I do know for absolute
certainty that the gages were reading identically, the rpm was 1800 and
without the balance tube the engine was running rougher than when the
balance tube was reconnected. The balance tube is an integral part of
the intake system, not a nice to have option. If it was not needed it
would not be there. Unless one has a dynamic balancer at hand, the
degree of roughness we are talking about is a subjective thing. Your
idea of smooth running may be different than mine. Unless we can
quantify our subjective assessments, then were are not going to get any
further in this part of the discussion.
4) In THEORY, the starting carburetor does not work much above 1800 rpm
because the vacuum at the starting carb fuel exit port is insufficient
to draw fuel into the intake manifold. That is the reason not to set
idle above this rpm. It can cause difficult or impossible starting when
cold. Have you ever activated the starting carb IN FLIGHT with the
engine running at various rpm? If not you may be in for a surprise. Try
it at varying rpm from 3500 - 5500 and see what happens; you might
learn something. Hint: The response is different at different rpm.
One last question for you, Roger:
Why does the Rotax factory say the acceptable range in idle rpm is
between 1400-1800 if anything under 1800 rpm causes damage? Does your
instructor know more about Rotax engines than Rotax?
Thom in Buffalo
FAA Powerplant Mechanic since 1972
Rotax afficionado for only six years and still learning
Retired mechanical engineer and machine designer
Message 2
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| Subject: | Re: Rotax 912 Idle Speed Setting |
Duncan,
We use the Twinmax and it works just fine.
Jim & Heather
Europa XS
Read this topic online here:
http://forums.matronics.com/viewtopic.php?p=111305#111305
Message 3
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| Subject: | Re: Rotax 912 Idle Speed Setting |
Hi Thom,
My instructor was Eric Tucker, he is Rotax's main man for engines. He is the top
dog for Kodiak out of the Bahamas. If you want the straight scoop on anything
he's the guy to ask. I have found a number of Rotax dealers who don't know what
they are talking about and you just have to shake your head at some of the
answers.
I agree that the crossover is necessary. There is a difference because of the way
the 80hp is made v.s. the 100 hp., that's why the difference in idle rpm's
for the 1800rpm recommendation. Try and set a 912uls at 1400-1500rpm and watch
it shake. The 80hp is quite different. It is taught in all 912uls classes and
taught by Rotax/Eric Tucker. It has not just been bored and stroked as many think.
Your engine should be almost as smooth when balanced right without the crossover
as with the crossover. Over the last 5 years I have balanced 912uls's
dozens of times and I'm very anal about making the sync perfect.
I would recommend the 3 day Rotax 912uls class to anyone that owns a 912uls. It
was a very informative class and an eye opener. The class is offered around the
US and it cost $300. What I learned was that some of these long time 912uls
owners and A&P's didn't really know what they were talking about when I went
to some of the Fly-In's or called a dealer for info because they had never had
any formal training. There attitude was if it didn't break right away it must
be right.
You ask why the Rotax factory says 1400-1800rpm? Because of the many different
applications, prop inertia, mountings, ect.. Rotax has changed a number of things
over the last 5 years because they finially get enough engine hours from field
use to understand why it would be better done a different way. They even
teach some differences in class v.s. the manual. When I was in class I had 2 pages
of changes to make in my manual to bring it up to the newer way of thinking
from Rotax. Look at all the service/AD bulletins over the last several years.
Rotax makes changes all the time, some we never know about because we don't
need to make the change in the field they just decided there was a better way.
All the so called "choke" does on the 912uls is a fuel bypass. It introduces more
fuel at start-up.
Bottom line I have learned with any engine be it auto, ATV, motorcycle, ect, people
are going to do what they think is right or was told to by another person.
I have learned that has a pretty big error factor. That's why I went an took
a couple of the Rotax classes so I would know the difference.
Thom,
Please do not take any offence as we are only chatting and having a good open debate.
I respect all your opinons and ideas.
Have a good day.
Roger Lee
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Roger Lee
Tucson, Az.
Read this topic online here:
http://forums.matronics.com/viewtopic.php?p=111338#111338
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