Sport Pilot Talk

N918KT

I went flying today and the instructor told me that the Rotax 912 engine rpm can't go below 1,900 or 2,000, otherwise it's gonna stall. Is it true? The plane I flew today was the Storm Century.

IIRC, 2 years ago when I was flight training in the Evektor Sporstar I believe that I saw a sign by the tachometer that says "don't go below 1,900 rpm" or something like that.

Is this true for all LSAs with 912s or only true for the Storm Century and the Evektor Sportstar that the engine will stall if gone below 1,900 rpm?

roger lee · Posted: Thu Mar 17, 2011 7:00 pm Post subject: Rotax rpm

Hi Kevin,
Two questions here. Do I absolutely have to idle above this rpm or is it just a little better to idle a little higher?
The 912ULS will run as low as 1500, but vibrates too bad. The normal idle debate is more about vibration and not stalling. Yes too low and it will stall, but should never above 1650.
If you have a 912UL (80hp) you can idle lower at 1500-1800, but if you have a 912ULS (100hp) then 1700-1900 is recommended because of the higher compression. I set all my clients no matter what aircraft with a 912ULS at 1750 for an idle rpm. You should idle at around 2200-2300 rpm for the first warm up of the day when the engine is cold so as to cut back on unwanted vibrations. There is no real need to idle a warm 912ULS at 2000 rpm. I do because it will run a little smoother, but it is not necessary. Being able to pull back the throttle to 1750 may help a few for landing. I always land with power and not at idle so for me it doesn't make any difference.
There is a little play here and each pilot needs to make up their own mind what they like. Most I know are 1750-1800 for the 912ULS.

N918KT · Posted: Tue Mar 29, 2011 1:14 pm Post subject:

So about the engine stalling if all the way idle, does that happen on SOME LSAs with a Rotax 912 like the Storm Century or Evektor Sportstar or does that happen for ALL LSAs with the Rotax 912? If not all LSAs, are there any LSAs that won't stall if the engine is at its lowest power setting?

roger lee · Posted: Tue Mar 29, 2011 5:28 pm Post subject: All Engines will stall

Any engine will stall if you get the rpm low enough. Why would anyone want to set themselves up for failure or a possible mishap by setting an idle that low. That's a fools Russian Roulette. If you set the 912ULS (100HP) idle at 1700-1800 for idle on the ground it won't stall providing you don't throw any other factors into the mix. As you approach at a reduced throttle or at idle the engine will also have a little help from the wind passing over the blades. If you have an engine set to 1750 and that's where it idles on the ground it won't idle there on approach at 50-60 knots of speed, it will be higher. I never set any clients idle rpm below 1700 and most are 1750-1800. Lower than 1700 and you start to cause too much vibration due to the 10.5:1 compression ratio. It's a long term wear issue. The 80 hp doesn't have this problem. It would behoove this instructor to take a Rotax class so he knows the engine as well as the the flight characteristics.

Education never stops or is limited because very few things in life are static. Just people become static.

ibgarrett · Joined: 04 Dec 2008 Posts: 147 Location: Westminster, CO

I'm reading this slightly different - at least from the initial post. You said "instructor told me that the Rotax 912 engine rpm can't go below 1,900 or 2,000, otherwise it's gonna stall"

Did your instructor mean the plane will stall or the engine will stall? I assume he means the engine, but it could've been his way of saying the engine needs to be running above a certain level to remain in flight.

I'm not an A&P, so take what I say with a grain of salt, but you should be able to run a warmed-up Rotax at 1700 or 1800 RPM without stalling. At least that has been in the case of the planes I've been in.

A side note, in the plane I'm flying now we didn't have the throttle set quite right and if you held the throttle all the way back to what idle should be, the engine would just quit. Fortunately the only time I experienced this was as I was crossing the threshold of the runway. What a nice quiet landing that was.

Needless to say - that "problem" has been fixed now.

Jim Stewart · Joined: 12 Oct 2006 Posts: 259

To further muddy the waters, I can authoritatively say that a 912ULS set to idle at 1400 will die if you simultaneously pull the throttle to idle and yank the nose up to do a power-off stall. Been there, done that, set the idle to 1800.

ibgarrett · Joined: 04 Dec 2008 Posts: 147 Location: Westminster, CO

wow - that must've been an interesting experiment Jim... :D

drseti · Joined: 28 Nov 2009 Posts: 1387 Location: Lock Haven PA

N918KT wrote: the instructor told me that the Rotax 912 engine rpm can't go below 1,900 or 2,000, otherwise it's gonna stall.

This is a prime example of why it is so important for instructors to use precise terminology. One should always specify, and differentiate between, "engine stall" and "aerodynamic stall," as the case might be.

That said, I don't believe "stall" (in either sense of the word) was the right word to use in this case. What your instructor should have told you (and as Roger has already explained here) is that, at low RPM, the Rotax 912 ULS vibrates quite a bit. This is primarily caused by backlash in the gearbox, and can be damaging. If you slowly advance the throttle after starting a cold engine, you can actually feel the point at which the engine smooths out. This is what you should consider your minimum idle speed in normal operation -- not because of stall of any kind, but just to keep the gears from clattering, chattering, and slowly grinding each other down.

roger lee wrote: It would behoove this instructor to take a Rotax class so he knows the engine as well as the the flight characteristics.

I would strongly urge anyone owning, operating, maintaining, giving instruction in, or taking instruction in a Rotax-powered aircraft to take the Rotax Service-level course (the first of three levels offered -- the other two are Maintenance and Heavy Maintenance). These are great little engines (really the highest power-to-weight ratio in the industry), and quite reliable (if properly maintained, they will significantly exceed their rated 2000 hour TBO). But, they are completely different animals from the Continental and Lycoming engines with which most of us are familiar.

Jim Stewart · Joined: 12 Oct 2006 Posts: 259

ibgarrett wrote: wow - that must've been an interesting experiment Jim... :D

It wasn't an experiment. It happened during the 5 hours familiarization with a Flight Design pilot. The engine had previously died while taxiing. I asked him if it was going to happen in the air and he said no. When it did, I slugged him in the shoulder.

roger lee · Posted: Wed Mar 30, 2011 9:24 pm Post subject: Engine rpm

Hi Jim,

Out of curiosity was this event at Lockwood?

drseti · Joined: 28 Nov 2009 Posts: 1387 Location: Lock Haven PA

ibgarrett wrote: A side note, in the plane I'm flying now we didn't have the throttle set quite right and if you held the throttle all the way back to what idle should be, the engine would just quit. Fortunately the only time I experienced this was as I was crossing the threshold of the runway. What a nice quiet landing that was.

The exact same thing happened to me, two years ago, in a Jabiru 2200-powered KitFox. Fortunately, I plan all my landings as glider landings (throttle to idle on downwind; maintain constant airspeed; glide to touchdown; manage descent with flaps), and try to teach my students to do the same, so: "Engines? We don't need no steenkin' engines!" :wink:

Jim Stewart · Joined: 12 Oct 2006 Posts: 259

roger lee wrote: Hi Jim,

Out of curiosity was this event at Lockwood?

No. Out here in California.

jeepinbanditrider · Joined: 17 Mar 2011 Posts: 21 Location: T67

Seems like it would be a simple procedure to adjust the engine's throttle stop so after warmup it will idle at 1900-2000 RPM with the throttle full closed.

But all of my mech. experience is with ground based vehicles so I'm probably wrong.

drseti · Joined: 28 Nov 2009 Posts: 1387 Location: Lock Haven PA

jeepinbanditrider wrote: Seems like it would be a simple procedure to adjust the engine's throttle stop so after warmup it will idle at 1900-2000 RPM with the throttle full closed.

You're pretty much on target. I adjust mine for 1800 rpm at the stop (warm engine). That's right in the middle of the manual's recommended range of 1700 to 1900. On startup cold, this gives 1400 to 1500 rpm, so immediately after starting, I check for oil pressure, and then release the choke and advance the throttle -- one minute at 2000, then advance to 2500 until the oil comes up to temperature (+50C, +122F). All this is covered in that Rotax service-level course Roger and I were mentioning.

roger lee · Posted: Thu Mar 31, 2011 11:26 am Post subject: Idle rpm

1750-1800 rpm at idle for a warmed up Rotax 912 is a good place to be. A side note for some: The idle screws can't be just turned. You need to set the idle with your vacuum gauges on, other wise they will be out of sync.