Choke Management

[facthunter]

  What about balancing the carb and the technique to shut them down smoothly. The Bowl breather tube and the float problem and the rubbers they are hooked to the engine by. The gearbox service and the sprag clutch starter problems and throttle linkage adjustment  quirks. the ODD alternator and oil return system . I've been involved with the things so it's not just my imagination.  Nev

[IBob]

Let's see:
1. Balancing the carbs: adjusted the cables as they bedded in to synchronise movement at full throttle. No further adjustment required for last 150+hrs.
2. Bowl breather tube: piped to airbox downstream of air filter, job done.
3. Carb floats. Yep they definitely had issues there, I have replaced one set as directed and supplied FOC.

4. Carb to manifold rubbers. Swap 'em out when the time comes, or maybe you don't believe in periodic maintenance?

5. Gearbox service and sprag clutch. I expect no issues at all as I use unleaded fuel as recommended, and the more recent ignition units give excellent starting.
6. Throttle linkage? Mine works fine.
7. Not sure what your problem is with the alternator. Is there a problem with the alternator?
8. Oil return system: it works fine. Surely that is a the measure of a good machine: not all the theory, but how well does it work???
9. Starting and stopping technique: simple.

 

Okay Nev, you don't LIKE the 912, it offends your sensibilities, I guess.
You're entitled to that.

FWIW I'm ambivalent about the 0-200.

[facthunter]

Same response always .  Deaf to ANY adverse comment and you asked me, and I foolishly responded against my better judgement.  Nev. 

[IBob]

Hardly, Nev: I responded to each of the items you raised, including acknowledging the matter of carb floats.

You want more detail????

[RFguy]

alright you two. stop it.

[facthunter]

I can go into it to a greater depth too but it would be as pointless and unproductive as discussing the Jabiru motor is on this forum.   (OR any other motor probably). Nev

[RFguy]

The rotax is pretty forgiving of misalignment of ... everything... at cruise and WOT settings.

 

It does not however, like idle settings. the inlet manifold is suboptimal, the gearbox doesnt like it.

 

However, that's hardly an operational  disadvantage to all the other fine attributes of the engine. 

 

I'd suggest warm up oil cooler bypass  and water cooler bypass, to limit the warm up idle time where the engine is unhappy.
Or a sheet of cardboard.....

On warm up, I idle the club rotax at the top end of the rotax permitted warmup RPM , and use a sheet of cardboard on both coolers.

 

 

[Yenn]

IBob. How did you synchronise the throttles? I think there is more to it than you say you did.

[IBob]

Hi Yenn, a certain amount of it was in understanding what was in the kit, and working with that for best results, so bear with me if I go there first:

The Savannah has a throttle bar across the width of the firewall, with arms connected to L and R throttle knobs, and with a pair of arms or quadrants side by side at the LH end that operate a pair of bowden cables. The short LH cable goes to the LH carb, the longer RH cable to the RH carb.
I would guess this is a fairly common setup.

Note that this is very positive when closing the throttle and the cables are being pulled, but when opening the throttle the cables are not pushed: they rely on the carb throttle arm springs to draw the cables. So for this to work well there can be no binding in the cables.

Accordingly, I went to some trouble to ensure that all parts of this setup were well aligned and free moving. Also in the case of the RH cable, I was very careful to install it with sweeping curves so as to minimise cable friction. And for the LH cable I ensured there was a small but sufficient amount of curve in the outer to allow the engine to move on the mounts.
I was greatly helped in all this by a series of pics supplied by the then Australia agent, Reg Brost, which proved to be gold when it came to making a tidy engine instal.

 

Here is a pic of the cables coming off the throttle bar, they are the lower cables:

 

And a pic of the engine bay showing the routing of the cables to the carbs:
Note I have since seen a factory build where they do an even nicer job of curving that RH carb cable: they fit a little cable fastening near the top LH side of the airbox front, which makes for an even softer curve.

 

continued below..............
 

[IBob]

continued................

 

The rest has been very straightforward:

Pretty much the only synchronisation tool you have is adjusting the relative length of those cables.

So I did this:
1. First checked idle stop and idle jet settings as per the LM manual. This is quick and easy, and as noted the idle jet settings were only approximate, so worth doing.

2. With everything in place, I cut and fitted a stop block at the firewall: when the throttles are fully closed, the spare central lever on the throttle bar bottoms on this block. This prevents the carb throttle arms, which are light in construction, from being flexed or bent, upsetting adjustment, if the throttles are closed firmly (as you do when stopping the engine).

3. I reasoned that since the engine spends much of it's time at 75% power or above, this was where I wanted best synchronisation. So I push the throttle knob to fully open, this puts some slack in the cables. I then gradually close the throttle, watching the two throttle arms to see them both start to move off their stops simultaneously.
This required several small cable adjustments in the first 40hrs, but in the 200hrs since, while I run this simple check regularly, I have not had to touch it.
Surprisingly also, given the different cable lengths, the fully closed appears to be pretty much perfectly synchronised too.

 

And that's it. I inspect all this carefully at service intervals, and lubricate the cables with light oil (I'm not sure about that, but it seems to be working okay in this environment.)



 

Edited December 11, 2022 by IBob

[IBob]

And........that was met by a deafening silence.......)
 

Here is someone operates both an 0-200 and 912:

https://www.rotax-owner.com/en/912-914-technical-questions/4678-o-200-vs-rotax-912-costs

 

Second post down:
"As for balancing the carbs, most of the the problems I've run into are not with the carbs but with improperly installed or poorly maintained throttle cable systems. I check my carb balance at every annual but haven't had to adjust them in two years."

 

As for the 0-200 vs 912 question, he loves them both, but wonders if the 912 may require overhaul sooner.

[RFguy]

Bob, I read through that thread, 

I would agree a problem for Rotax is the lack of suitably familiar mechanics. and parts are priced to deter rebuilds IMO.

They are a little fussy and complex compared to an O200  but that's what you get for the advantages of - weighing 30% less and not ever having to worry about engine temps, and not have a bulk strip if you have a prop strike....and most will go to TBO with just a gearbox overhaul to 1k hours. (ULP)

I think Mark's rotax throttle linkage solves the multi  throttle cable problem with a nice hammer..

[IBob]

Thanks RFGuy, yes that post is 9 years old, what you say seems to sum it up as we stand now. To which I would add the estimated NZ$27,000 fuels saving that is 5L per hr over 2000hrs.

 

Mark is certainly making a nice job of the alternative throttle linkage. And quite possibly others face challenges that I don't, and so will benefit from that.

[RFguy]

Not sure about the fuel saving calcs. fuel is fuel. I would think because the O200 is a bit happier at lean of peak, the O200 might be lower fuel cost, apples for apples ULP.   weight advantage would depend on the aircraft.

[IBob]

I saw the 0-200 quoted at 22L/hr at 75% but perhaps that figure was wrong?

[RFguy]

specific fuel consumption kg/kW/h is going to be the same, I would think. 

 

[IBob]

Yes. I'm not thinking straight there............(

[RFguy]

now, nit picking : 
While the O-200 can go lean of peak,

the Rotax 912ULS runs at 10.8:1 compression- higher efficiency .

The O200 at 7:1 compression. 

Yes, taking into account the higher compression ratio, MUCH higher, indeed the rotax shoudl do MUCH better than the O-200. But not all things are equal, losses, friction, heat etc all very different engines....

 

 

 

Edited December 12, 2022 by RFguy

[IBob]

I'm confident in what I write about fitting running and maintaining the 912 (though always more to learn there too).

I know nothing about the 0-200 other than what I can dredge up online.
I don't know enough to be running comparisons.

[RFguy]

For the ideal  otto cycle engine , thermal efficiency  = 1-( 1 / (c^(1.4 -1) )).

 for 7:1  = 54.1%
for 10.8:1  = 61.4%

IE 1.13x better !  so in theory, maybe  22 litres>>19.37., Hmmm not far from the rotax numbers !
But in practice, real outcomes are less.  engines have different cooling, oiling etc shaft input costs, temperatures the CC runs at, RPM, friction etc. 
 

 

https://www.primescholars.com/articles/influence-of-compression-ratio-on-the-performance-characteristics-of-a-sparkignition-engine.pdf

Edited December 12, 2022 by RFguy

[Thruster88]

19 minutes ago, RFguy said:

specific fuel consumption kg/kW/h is going to be the same, I would think. 

 

Agree, my beech/lycoming 0-320 burns 29 lph @65% power = 104hp. Rotax 912 @100% power = 100hp burns 27.5lph according to rotax graph. That is not even a bees dick of difference. 

 

The airframe consumes the energy. Rotax is economical because they are in small light aircraft.  

[RFguy]

What is the compression ratio of that particular   O-320  Thruster ?
cant really compare 100%  on rotax  (full rich) to 65% on beech (Not full rich) 

Edited December 12, 2022 by RFguy

[IBob]

I'm still not biting, Glen.

Shortly you'll set off that guy who always claims to be flying round at vast speeds on 12L/hr, while entirely ignoring the fact that his aircraft is light, and his engine is not working very hard.
You can't make any headway with this stuff. It's like the GA vs ulight attitude you run into occasionally.......or all the sh*t people eternally tell themselves about where the 3rd wheel is......(

[RFguy]

ahh Bob I have found the difference in the O200 and the rotax in the thermal efficiency numbers  see above....

 

so it is likely the rotax really is better, even if the gain is only half that just on thermal efficiency gain of 13% due to higher compression ratio.

 

 

 

 

[Thruster88]

1 minute ago, RFguy said:

What is the compression ratio of that particular   O-320  Thruster ?
cant really compare 100%  on rotax  (full rich) to 65% on beech (Not full rich) 

8.5:1    It is the high compression 160hp version. 

 

Why can't we compare rich and lean mixtures, if the engine is built to take it and the cooling was totally sorted by the airframe factory, in 1963?