I make a point of setting the idle speed on my aircraft as low as possible as I need to fly into some extremely short strips sometimes. I find that if the engine will just about run during the initial slow running checks, it usually results in a smooth even idle of perhaps 5-600rpm when warm.. just right. Also I have found that with a metal prop, a slightly lower idle can be used.

I say this as a stick and string PFA type, rather as a licensed engineer however..

Kingy

We find that the limiting factor on how low you can idle is the roughness of the engine at tickover. We are using a Rotax 912 and if you let it get below 1100 RPM or so you are afraid the engine is going to jump out of its mountings. While warming up, we usually increase the revs to 1700 or so and hold it on the brakes. No amount of carb balancing will seem to allow an idle of less than 12-1300 RPM with any chance of keeping the screwdriver in the adjusting screw slot.

We are a bit paranoid about engine stability as we found all 4 mounting brackets broken during an inspection last year. It has taken 7 months to get it all repaired and the engine is much smoother at low revs now.

What stable idle speed do others get down to on this engine?

Sorry Gengis ... it was the way you wrote that made it look like it was the first time you had experienced it. I know you would have known about it of course.

I see your point about throttle wide open, and I guess that probably is the best as your options are increased. However, many a well set up forced landing has been ruined by an engine stuttering back to life at the wrong moment ... hence "switches off" once committed.

Muffin ....

My mate with a Zenair 701 has a Rotax 912, it often stops as he rounds out due to its inability to tick over smoothly at speeds low enough to produce no thrust. He prefers that to a higher tick over, the reason is as demonstrated by Gengis. And yes they do shake about a bit at low tick over don't they? :uhoh:

SS

Rotax 912 startup vibrations
 

As has been said, the startup vibrations from the 912 is pretty horrible, and the 912s even worse. Rotax' solution to this is a "slipper clutch", which seems to make a huge difference to startup and low-RPM vibrations. I've never done one myself, but I believe that it adds about 1½kg and can be retro-fitted, presumably Skydrive can give you chapter and verse.

Incidentally, the analysis I did on this particular aeroplane showed that it was the final approach (float!) that was most extended rather than the ground roll - so if you are really determined turning the switches off at about 100ft would be the way ahead.

Would I have the nerves to do that? Nope, I'd go to an airfield with a longer runway!

G

A 912 with properly balanced carbs purrs like a pussy cat on idle.

Alll are a bit rough on start up, the 912S being rougher on start up and slightly rougher on idle, but still not bad when running right. Its the price you pay for a high compression ratio and 20% more power.

Genghis, what aircraft were you testing?

I had a look at the Rotax web site, which specifies an idle speed of 1400 RPM on the 912. There is also an SB that says you must not idle below 1200 due to the risk of damage. I tried the experiment today in a 15kt headwind. I touched down at normal speed where the numbers would be if we had any. The runway has a slight up slope and I did not touch the brakes at all. I used up most of the 450 metre runway despite the up slope. I then repeated the exercise landing at the same speed but cutting the mags on touch down. I rolled to a halt in less than 200 metres.

I expected a reduction but was surprised how much it was. The stationary idle prior to takeoff was 1800 RPM and with power off prior to landing about 2000 RPM presumably due to windmill effect. It was also interesting how long the prop took to stop when I cut the mags due to the same effect.

This is a useful exercise to try for yourself so you get a feel for how short you can land if you really have to.

Sorry, not really my information to give out - and irrelevant to the context of the thread.

G

Another thing you can do at idle check is to pull back the mixture slightly and watch the revs. If they go up, then the mixture may set a bit rich at idle. Not really critical but just something to mention at next maintenance.

Jemima has a 912S. It is mounted on a bracket on the root tube by 4 bolts into the bottom (only) of the block. The engine wobbles around a good bit on start up and shut down but actually runs quite smoothly the rest of the time although it does not much like full idle during descent.

A number of the non certified 912S engines were supplied without the clutch for a while. The clutch needs regular adjustment and they hoped to do away with that. The reason given for the clutch was to safeguard the crank in the event of a prop strike. Unfortunately a considerable number of engines without the clutch have developed gearbox vibration at about 3k+ revs after about 70 hours and mostly the owners have been obliged to go with Rotax suggested upgrade of installing the clutch. (Ouch $$$$)

When I first set up the engine I used the factory recommended idle at about 1400 revs but as the weather got colder it occasionally got lumpy. Our group (about twenty of us) were discussing it at the time and we reckoned we might go for 1600 to a max of about 1800 for the Winter. Finally I've set mine for 1600 year round.

Last November I had the engine quit on landing when I pulled it to idle. Twenty seconds later she restarted and idled just fine. I did all the checks and then flew another circuit (close in!) and the same thing happened. According to Rotax the 912S is not subject to icing but I have seen a couple of pictures of a thin film of ice formed on the inside wall of the carb, I reckon it is just enough to close off the air when the butterfly closes but does not affect airflow when the throttle is open. As soon as she quits the throttle body warms up and so she runs.

As it happens it was only a few days before her annual and she now has new plugs, also I have only flown since in weather dry enough to turn running bull into jerky in about twenty minutes. so no re-occurrencies. We'll see. . . . .