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Minimum T/O RPM

bobg56

Well Known Member
I remember reading that Rotax 912 ULS engines were to have a minimum RPM of 5200 at climb out so as not to over load the engine or cause detonation. I don't see this anymore in the Operators Manuel or SL 912-016 which just mentions octane requirements and manifold pressures. My RV12 gets about 5150 rpm at VY which to me is close enough. I wonder if they got off that requirement?
 
Rotax had it in the Operating Manual about not over loading the engine below 5200 Rpm on the T/O climb but I looked for it and can no longer find that, I'm curious to know if Rotax revised this...I know a few people with Rotax engines in Zeniths and Kit Fox and they climb out 4800-4900. Although they may have in accurate tachs.
 
Rotax had it in the Operating Manual about not over loading the engine below 5200 Rpm on the T/O climb but I looked for it and can no longer find that, I'm curious to know if Rotax revised this...I know a few people with Rotax engines in Zeniths and Kit Fox and they climb out 4800-4900. Although they may have in accurate tachs.

I would take Rotax's advice about the 5200. My RV-12 climbs good, and cruises good, when I see 5200 rpm at Vy of 75 kts. 5200 RPM is 100 rpm over the peak torque at 5100 rpm on the motor. The motor is making good HP and good torque at that rate. You'll know it by how hard you have to step on the right rudder to center the ball on climb out. Takes a good amount of pressure.
 
5200 RPM at Vy is just a smidge to fine from my experience, if someone is looking for the best all around performance.
I have found 5150 to be about the sweet spot unless flying near gross in hot conditions on a regular basis.

This value would be based on a take off in average temp at the airpot altitude commonly flown from. different altitude airports on different temp days, will result in slightly different values.
 
I agree, 5200 would be a bit too much, probably even exceed 5800 in level WOT, I can get about 5650 and I'm happy with my pitch setting. R.O.A.N. has the 5200 RPM as the minimum target, even suggesting 5500-5800 for climb out, they have this info in their Video section regarding causes of Detenation, although it's dated 2010...I'm just wondering if ROTAX is backing off from their stance on this...RPM like this would be good if you wanted climb performance and nothing else.
 
I agree, 5200 would be a bit too much, probably even exceed 5800 in level WOT, I can get about 5650 and I'm happy with my pitch setting. R.O.A.N. has the 5200 RPM as the minimum target, even suggesting 5500-5800 for climb out, they have this info in their Video section regarding causes of Detenation, although it's dated 2010...I'm just wondering if ROTAX is backing off from their stance on this...RPM like this would be good if you wanted climb performance and nothing else.


I see about 5650- 5700 at WOT, level flight at 4500 ft ASL, depends on the day. Cold mornings, around 45 to 50F here in So CA, with altitude pressure of 30.20 are on the high end. August summer mornings with temps near 80 to 85 F are the lower end. Motor seems to run much richer in the summer time, a little bit leaner in the winter time, looking at the color of the spark plugs.

My interest is in cruise performance, around 6000 to 9000 ft at 5200 to 5350 rpm. I like seeing my EGT's no higher than 1400, 1300's, even 1200s is better. If your EGT's are up there, you're lugging the motor and working it too hard.

The Rotax is a short stroke, overbore built motor. Designed for rpm, less for torque and lugging the motor. Spinning fast and lightly loaded over more power strokes per minute is what it does well, less work performed, per powerstroke. It allows you to pull the throttle back about 1 1/4" from the firewall at around 5200 rpm cruise, and keep the needle in the main jet region of the Carb, instead of running full rich, at full throttle, at least that's what I am seeing, around 4500 ft.

Keep in mind, Bombardier / Rotax's engineering experience is in building snowmobile and Personal Watercraft powerplants. They all wind out, pretty high.
 
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This is typical. EGT's closely matched near 1300F while making published cruise speed at just under 5500 RPM.

"How sweet it is" - Jackie Gleason
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Can you provide a reference to info that substantiates this?

Scott,

from July through October here in So CA, on any given day with temps in the high 70's to 90 F on the ground with the airport I take off out of, at 1004 MSL, it's very, very common for days to have a temperature inversion to 5500 to 6000 ft.

It's also common for me to see full throttle exhaust temps at or around 1450 F on initial climb out, to about 2500 ft, where I will then drop the nose from a 75 kt climb to level and about 5200 rpm, trimmed up level. Usually the exhaust temps will drop 100 to 125 F pretty quickly, once level, to 1320 -1350F

The other really important observation I need to make is watching my oil temps on hot days like this. A climb from airport at 1004 to 3000 ASL will usually have my oil temps right around 230F and edging into the yellow range. At that point I have to give up the 85 kt XC climb rate, throttle back to 5100 and fly level for a couple of minutes, before I attempt to climb shallow at 250 -300 fpm under light engine load. The inversion factor and the temp going up as I increase in altitude, magnifies the temperatures oil and exhaust run at.

It takes a long time to get to 6000 ft to get above the inversion and have the air temps start dropping as you go higher so that your exhaust and oil temps start dropping down again too. It was that way down here for probably 4 months with the inversions, during the forest fire season.

It's rarely a problem since late November, so far, through the winter months, the oil or exhaust temps being on the higher side. But July through October, absolutely. Seems to be common, at least on some 2012 RV-12 Legacy models. SN120212
 
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Scott,

from July through October here in So CA, on any given day with temps in the high 70's to 90 F on the ground with the airport I take off out of, at 1004 MSL, it's very, very common for days to have a temperature inversion to 5500 to 6000 ft.

It's also common for me to see full throttle exhaust temps at or around 1450 F on initial climb out, to about 2500 ft, where I will then drop the nose from a 75 kt climb to level and about 5200 rpm, trimmed up level. Usually the exhaust temps will drop 100 to 125 F pretty quickly, once level, to 1320 -1350F

The other really important observation I need to make is watching my oil temps on hot days like this. A climb from airport at 1004 to 3000 ASL will usually have my oil temps right around 230F and edging into the yellow range. At that point I have to give up the 85 kt XC climb rate, throttle back to 5100 and fly level for a couple of minutes, before I attempt to climb shallow at 250 -300 fpm under light engine load. The inversion factor and the temp going up as I increase in altitude, magnifies the temperatures oil and exhaust run at.

It takes a long time to get to 6000 ft to get above the inversion and have the air temps start dropping as you go higher so that your exhaust and oil temps start dropping down again too. It was that way down here for probably 4 months with the inversions, during the forest fire season.

It's rarely a problem since late November, so far, through the winter months, the oil or exhaust temps being on the higher side. But July through October, absolutely. Seems to be common, at least on some 2012 RV-12 Legacy models. SN120212

Ok, but you said "I like seeing my EGT's no higher than 1400, 1300's, even 1200s is better."

What makes them "better" if they are in the 1200's"?
 
d
Ok, but you said "I like seeing my EGT's no higher than 1400, 1300's, even 1200s is better."

What makes them "better" if they are in the 1200's"?

Let me ask you the opposite? What makes them worse? I am all ears.


Keep in mind, I have a muffler sitting an inch away from my oil cooler, by Van's original design... what's an extra 200F worth when I am trying to cool oil, too? Am I better off, backing off out of the throttle, lowering my attitude, increasing my airspeed, and lowering my rpm, oil and exhaust temps or not, when trying to climb out, putting less load on the engine? I am surrounded by mountains here around Los Angeles, everywhere I fly. I live down in a 1000 ft elevation valley.


IMHO, if your carbs are jetted correctly, high exhaust temps can be caused by several things. Too low an octane fuel, too lean on the jetting, too advance ignition timing ( a non issue on a 912ULS) or increased load on the engine. We can control octane, and load on the engine. Once fuel is in the tank, all we can control is load on the engine, for that batch of fuel. Pitch of prop has an effect on that engine load.
 
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How do these numbers look for prop pitch? Two blade Sensenich.
Obviously the GPM is a calculation but it is spot on when I measure remaining gallons in the tank. I fly at gross and typically 120 knots, my average (measured over many flights, in all weather) is 4.8 GPH.

climb WOT, 5070RPM, 750 fpm, 3500MSL, 76TAS, 27.2hg, 67f OAT, EGT 1203 / 1259, 5.0GPM

cruise 5370 RPM, 3600 MSL, 119TAS, 25.1hg, 120TAS, 64f OAT, EGT 1256 / 1309, 4.9GPM

cruise 5170 RPM, 3600 MSL, 115 TAS, 24.3hg, 63f OAT, EGT 1280 / 1327, 4.2GPM
 
d

Let me ask you the opposite? What makes them worse? I am all ears.


Keep in mind, I have a muffler sitting an inch away from my oil cooler, by Van's original design... what's an extra 200F worth when I am trying to cool oil, too? Am I better off, backing off out of the throttle, lowering my attitude, increasing my airspeed, and lowering my rpm, oil and exhaust temps or not, when trying to climb out, putting less load on the engine? I am surrounded by mountains here around Los Angeles, everywhere I fly. I live down in a 1000 ft elevation valley.


IMHO, if your carbs are jetted correctly, high exhaust temps can be caused by several things. Too low an octane fuel, too lean on the jetting, too advance ignition timing ( a non issue on a 912ULS) or increased load on the engine. We can control octane, and load on the engine. Once fuel is in the tank, all we can control is load on the engine, for that batch of fuel. Pitch of prop has an effect on that engine load.

Lower EGT's can be an indicator of being excessively rich.
In reality, the Bing carbs are conservatively rich all of the time but particularly at mid range throttle.
That is why the iS (which typically has higher EGT's than the ULS) is able to be so much more fuel efficient and why EGT's in the 1200's are not particularly desirable or necessary.

BTW, lower EGT's has a very negligible effect on oil temp, and low EGT's caused by a rich mixture could be a negative influence on CHT/coolant temp.
 
I'm curious, Jon, what are the EGT's on your IS motor in the summer time, there in UT, at full throttle and Vy?

1350 to 1400 in full power climb, then 1400 to 1450 in cruise with the power back to ECON (about 5150)
 
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How do these numbers look for prop pitch? Two blade Sensenich.
Obviously the GPM is a calculation but it is spot on when I measure remaining gallons in the tank. I fly at gross and typically 120 knots, my average (measured over many flights, in all weather) is 4.8 GPH.

climb WOT, 5070RPM, 750 fpm, 3500MSL, 76TAS, 27.2hg, 67f OAT, EGT 1203 / 1259, 5.0GPM

cruise 5370 RPM, 3600 MSL, 119TAS, 25.1hg, 120TAS, 64f OAT, EGT 1256 / 1309, 4.9GPM

cruise 5170 RPM, 3600 MSL, 115 TAS, 24.3hg, 63f OAT, EGT 1280 / 1327, 4.2GPM

Suggestions?
Should I start a new thread?
 
Out here in PHX summers I’ll do anything to run cooler. I have had to return to the airport twice for excessive oil and CHT temps in summer. Each time the result of having to sit too long in the run up area watching the endless maerry-go-round of touch and goes.
 
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