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57 minutes ago, Franky said:

Why not follow the manufacturer’s recommendations?

 

 

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Shame on you. You just took the fun out of  the debate😊

 

As most  Rotax 9 operators know & the graph you have provided proves, the recommended sustained operating range is 5200-5500 RPM - unfortunately the end of the story/debate

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3 minutes ago, skippydiesel said:

As most  Rotax 9 operators know & the graph you have provided proves, the recommended sustained operating range is 5200-5500 RPM - unfortunately the end of the story/debate

It doesn't show that at all. It has e.g. settings for 55% power cruising.

 

What it shows is that 5200-5500 are the recommended/required rpm for full throttle at low altitude.

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With the correct fixed pitch prop there's no worries but there will be more knowledge required  re manifold pressure where you have any inflight adjustable pitch fitted or if you have experimented with props that hold revs down. Nev

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2 hours ago, aro said:

It doesn't show that at all. It has e.g. settings for 55% power cruising.

 

What it shows is that 5200-5500 are the recommended/required rpm for full throttle at low altitude.

I suggest that it would be impractical for Rotax to develop a similar graph for every density altitude or even for blocks of altitude so they have come up with a fairly frequently used altitude  ( "pressure altitudes below  3500ft") to illustrate their recommendation for engine management in this area.

 

I quote: "Continuous use of engine speeds below 5200 RPM must follow the manifold pressure graph below" which then has the words (in the shaded area below 5200 RPM) "Limited operation in this envelop area"

 

I dont know what this means to you however I like to be "kind" to my engine, so try to operate it in the envelop where its many parameters are close to optimum - I believe this to be between 5200 & 5500 RPM

 

So you want to get the best (whatever your definition) from the engine; Personally I have longevity/reliability over time at the top of my list; So, for sustained cruise operations, in what RPM range would you say Rotax have recommended you operate???? 

Edited by skippydiesel
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"limited" operation means the power available in that area has limits by the MP you can use.. It doesn't mean LIMIT your operation in this area. Rarely does running a motor near the max rpm permitted extend engine life. Also most cruise power is not below 75% as a general statement but when holding  or best range cruise is done you may be well below 75% power. A lot depends on your planes drag characteristics and whether you have overpowered it.  IF you fly at higher levels you just won't have the power available either. Nev

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30 minutes ago, facthunter said:

"limited" operation means the power available in that area has limits by the MP you can use.. It doesn't mean LIMIT your operation in this area. Rarely does running a motor near the max rpm permitted extend engine life. Also most cruise power is not below 75% as a general statement but when holding  or best range cruise is done you may be well below 75% power. A lot depends on your planes drag characteristics and whether you have overpowered it.  IF you fly at higher levels you just won't have the power available either. Nev

Nev I understand "limited" In this context includes - load & duration/time. 

 

I agree that airframe characteristics come into it.

 

Operating below optimum may also be below or at max torque; The analogy I would use (perhaps not perfect) - its a bit like doing 90-100 kph, in overdrive, with minimal load/passengers, on a level road - all well and good as long as the gradient remains level (constant relatively low load). Great fuel econamy, engine only lightly loaded BUT add a gradient/stiff head wind/towing/acceleration (ie increase load) and it will probably be easier on the engine if you go down a gear and increase RPM. Of course you dont have to but this may negatively impact of engine service life and would certainly produce acute discomfort in myself.

 

So in the Rotax 9 situation, I like to know that the engine is not "lugging";  Without selectable  gears (no CS prop) my options are limited to operating in the RPM range that is unlikely to be causing sub optimal engine conditions. If loitering at low speed (60-80knots) , constant altitude, I am happy to have the engine RPM low eg 4000 RPM (or even lower depending on aircraft weight & configuration) for a fuel burn of about 8 L/hr. When I want to climb or increase speed (load) I increase my engine RPM to a minimum of 5200 RPM. I usually cruise (travel) between 5200 & 5400 RPM, 100-110 knots, for a fuel burn of 13-14L/hr.

 

My new aircraft comes with a CS - I am so  endorsed but its been a very long time since flying with a CS prop, so will have to get up to speed on this significant change in pilot work load/understanding.

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4 hours ago, skippydiesel said:

I suggest that it would be impractical for Rotax to develop a similar graph for every density altitude or even for blocks of altitude so they have come up with a fairly frequently used altitude  ( "pressure altitudes below  3500ft") to illustrate their recommendation for engine management in this area.

 

I quote: "Continuous use of engine speeds below 5200 RPM must follow the manifold pressure graph below" which then has the words (in the shaded area below 5200 RPM) "Limited operation in this envelop area"

 

The shaded area is above 27" manifold pressure. Manifold pressure is reduced when you close the throttle. The reference to 3500 feet is because the air pressure reduces as you climb, so the manifold pressure possible at full throttle reduces.

 

It is absolutely permitted to close the throttle and reduce RPM and cruise at a lower power setting because your manifold pressure is reduced by closing the throttle.

 

The relationship between manifold pressure and rpm is set by the propeller. If you have a constant speed propeller, you have to manage manifold pressure to make sure you don't exceed allowable manifold pressure for the RPM.

 

If the propeller is fixed pitch or ground adjustable, the pitch determines whether the engine is overloaded. If you are at less than 5200 rpm at wide open throttle you are lugging the engine (too high manifold pressure with too low rpm). In that situation Rotax recommend you close the throttle and reduce RPM by at least 100, because that shows that the manifold pressure has been reduced. It's not lugging at a lower rpm because with a fixed pitch propeller the load from the propeller reduces.

 

4 hours ago, skippydiesel said:

So, for sustained cruise operations, in what RPM range would you say Rotax have recommended you operate???? 

They have provided a number of points on the chart:

5500 rpm 27"MP - max continuous power

5000 rpm 26"MP - 75% power

4800 rpm 26"MP - 65% power

4300 rpm 24"MP - 55% power

 

Without a manifold pressure gauge we don't know the exact power setting, but it's pretty safe to assume if you get 5200+ at WOT at climb speed with a fixed pitch prop, manifold pressure will be OK in cruise.

 

If you're only using 13-14 l/h at 5200+ you should be well below 75% power so well below 26" MP. 75% power fuel consumption is about 19l/h. 14 l/h should be around 60% power. With a variable pitch prop you would probably be better at around 4700-4800 rpm. However with a fixed pitch prop and a faster aircraft, 5200 minimum at WOT in climb means higher rpm at cruise speeds.

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It’s a good job Rita’s donks are very robust otherwise where would be smoking holes everywhere if we believe the copious qty’s of wives tales here!😂😂

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If you want to disagree with something, perhaps you can tell us what? Knowledge isn't worth much if you don't put it out there to test.

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I’ll just stick with cruise at 55% power on a fixed pitch prop that at sea level is properly set up to absorb that power at that throttle setting and has WOT that is exactly per the graph.  
 

and anyone who thinks the minimum/proper cruise power setting is well over 75% can have the fun of finding out all the bits of the engine that need work when run at high power.

 

I’ll just keep plodding along on a half hours engine that exceeded its years life nearly twice but issued bugger all oil and has compressions that are unchanged in the last 200hrs and 10 yrs.  

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4 hours ago, aro said:

 

The shaded area is above 27" manifold pressure. Manifold pressure is reduced when you close the throttle. The reference to 3500 feet is because the air pressure reduces as you climb, so the manifold pressure possible at full throttle reduces.

 

It is absolutely permitted to close the throttle and reduce RPM and cruise at a lower power setting because your manifold pressure is reduced by closing the throttle.

 

The relationship between manifold pressure and rpm is set by the propeller. If you have a constant speed propeller, you have to manage manifold pressure to make sure you don't exceed allowable manifold pressure for the RPM.

 

If the propeller is fixed pitch or ground adjustable, the pitch determines whether the engine is overloaded. If you are at less than 5200 rpm at wide open throttle you are lugging the engine (too high manifold pressure with too low rpm). In that situation Rotax recommend you close the throttle and reduce RPM by at least 100, because that shows that the manifold pressure has been reduced. It's not lugging at a lower rpm because with a fixed pitch propeller the load from the propeller reduces.

 

They have provided a number of points on the chart:

5500 rpm 27"MP - max continuous power

5000 rpm 26"MP - 75% power

4800 rpm 26"MP - 65% power

4300 rpm 24"MP - 55% power

 

Without a manifold pressure gauge we don't know the exact power setting, but it's pretty safe to assume if you get 5200+ at WOT at climb speed with a fixed pitch prop, manifold pressure will be OK in cruise.

 

If you're only using 13-14 l/h at 5200+ you should be well below 75% power so well below 26" MP. 75% power fuel consumption is about 19l/h. 14 l/h should be around 60% power. With a variable pitch prop you would probably be better at around 4700-4800 rpm. However with a fixed pitch prop and a faster aircraft, 5200 minimum at WOT in climb means higher rpm at cruise speeds.

Thanks for that Aro, I think I follow - quite the educator BUT this still does not quite explain away all my concerns; Low engine speed results in sub optimal spark timing, less than optimal fuel air flow/mixing/combustion (volumetric efficiency), low oil flow/pressure and coolant flow. Perhaps I am too focused on ground based engines however I have always understood engines to have a optimal RPM range and that low speeds (for a given engine) are not good for longevity.

 

Contrary to the old expression "revs= rebores" (American propaganda ?) engines  that are designed to thrive at higher RPM's (like Rotax 9's) do so. The very fact that LyCons, operating at roughly half the operating RPM's of a Rotax 9, have no greater expected in service life, is pretty indicative of error of this theory.

 

"However with a fixed pitch prop and a faster aircraft, 5200 minimum at WOT in climb means higher rpm at cruise speeds." Makes little sense; With a fixed pitch prop, RPM in climb is controlled by attitude, therefor your engine speed in climb does not have any great relationship to cruise RPM (controlled by throttle position). 

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7 hours ago, pmccarthy said:

Due to the short stroke, Rotax average piston speed is similar to Lyc/Con. Wear in bore etc should be similar.

Possibly, however rotations (cycles) are roughly double, so even if your statement is correct everything else that is experiencing friction, erosion, cyclic impacts is happening twice

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Nikasil  bore treatment is a high life, low wear, no erosion/corrosion deal that is done on aluminium surfaces.  Load reversals (Cycles) affect fatigue life.  Stresses are four times when Revs double. (Obey square Law) Nitrided shafts and cylinders last much longer where wear is a factor.  Not all Lycoming cylinders are nitrided. . Aero engines are not as robust as marine engines for example. Hp for mass is more important than HP for capacity.  Long total hours motors develop more cracks. I wouldn't rebuild a cont/lyc more than twice as a principle. There are also deck height limits that might get you before then anyhow.  Nev

Edited by facthunter
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Just out of curiosity do any of you 912 drivers do a mag check at idle before shutdown & if so what is the a) idle speed and b) rpm drop?

 

I've done this all my flying career with Lycoming, Continentals & now the Jab 3300. It can identify a faulty plug.

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41 minutes ago, kgwilson said:

Just out of curiosity do any of you 912 drivers do a mag check at idle before shutdown & if so what is the a) idle speed and b) rpm drop?

 

I've done this all my flying career with Lycoming, Continentals & now the Jab 3300. It can identify a faulty plug.

kgwilson I picked up that habit from my Savannah instructor. I am not sure how much it proves, as it is my understanding that the ignition modules tend to fail by degrees, the first indication being failure when cold.

But I do it anyway, at 3000RPM, where I see a drop of maybe 200RPM.

----------------------------------------------------------------------------------------------------------------

As an aside to that: I used to see a quite uneven drop when doing this, with distinctly rough running on one ignition module.  However, this cleared up while I was chasing a minor radio issue, as follows:

 

I had started at the plugs and leads, could tell nothing much from the plugs: we have a reasonably long taxi in from the strip, and my plugs have always looked kinda terrible after this, but the engine is new, starts and runs like clockwork etc, so I've come to accept that's how the plugs look after a long taxi. I took them out, looked at them, put them back, checking the leads and caps in the process.

 

What I did notice while doing this was a certain amount of rubbing where the leads for the lower plugs pass down between the cylinders. These leads sit inside a heatproof sleeve, but there is nothing to prevent this sitting against the heads, so I cut a couple of square rubber guides from large grommets, centered the leads through these. I also cut the cable ties off these leads, then fitted new ones with the leads centered in this manner.

 

None of this improved my radio issue, but since then I get equally smooth running on each mag during my pre-shutdown 3000RPM mag check.

Edited by IBob
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1 hour ago, kgwilson said:

Just out of curiosity do any of you 912 drivers do a mag check at idle before shutdown & if so what is the a) idle speed and b) rpm drop?

 

I've done this all my flying career with Lycoming, Continentals & now the Jab 3300. It can identify a faulty plug.

Hmm! When I do a shut down mag check on a LyCon its to determine that there is a good earth (dead short).

 

Rotax 9's have have an inconsistent (rough) low speed idle at the best of times and wouold seem to be sensitive to seasonal temperature variations - not sure that an idle speed "mag" check would give you much information.

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I would always run on ONE to get the idle speed as slow as possible for a better cut, That was supposed to be normal practice. The Importer  Bert Flood, used to hang around Lethbridge when I was there. He was THE MAN for tuning Bultaco's in the 70's. He used to spend time at the factory in Spain. Nev

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