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The most accurate depiction of power output is torquemeter and RPM. A torquemeter is used to trigger autofeather often. With a Jet with all derivatives fuel flow is a pretty good indicator of power when manouvering at low levels. Nev

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The Rotax machine I fly reg (not mine) uses around 23 LPH & 5200 rpm, I plan 24 and that works. Not sure what the fuel usage is at full power as you don't get pull power at t/off and I don't like the sound of the beast WOT level flight!

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Thruster88 - My aircraft can cruise at 100 knots at 4800 rpm for well under 13 L/h - I flight plan at 14 L/hr, to be conservative, accommodate the increased fuel consumption from having the occasional passenger or a higher speed cruise 5000 - 5200 rpm/ 105-110 knots.

 

Note my fuel flow is for the mission (taxi/TO/climb/cruise/decent/taxi) not just cruise.

 

My mate, in his much faster 912 ULS powered aircraft, flight plans at 18 L/hr but sees 17 L/hr/ 5400 rpm at 130 knots .

 

The Rotax chart you refer to must be a worst case (cover the but) scenario for a heavy/draggy aircraft.

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There's not a lot of point in quoting individual in cruise figures. That depends on the clean nature of your plane, your prop and how you are operating it all in combo.. .Comparisons are hard to validate as there are too many variables. IF you quote both at a certain horsepower OUTPUT you do then have a basis for comparison IF you accept that as a valid one.. One engine is at full power where it's not recommended to be operated continuously so we are told. The F/F Graph shows no sign of using a higher rate of fuel flow as it gets there either so it's not over rich as some are in the take off range. Nev.

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Skippydiesel ... heavy draggy planes have nothing to do with fuel consumption other than if you try to fly them faster than they will go at a power setting.

 

Matching a prop to a rotax 912 on an airframe is always a compromise - same for any engine but this tread is on the 912.

 

The simple way to tie your airframe/engine Back to the rotax is If you are getting the fuel flow at the rpm that is in the graph your engine is delivering the power from their graph and your prop is absorbing that power.

 

If you are burning less fuel than the rotax graph at a throttle setting you are producing less power than the graph and that’s due to the prop needing less power to be spun at that airspeed- you could increase pitch or blade area to absorb more power if you wanted to reprop or repitch.

 

If you are burning more fuel than the graph would indicate at that rpm you have a rep problem if being over propped and you you should consider lowering pitch or reducing blade area as your engine can not produce enough power to turn the prop and the engine is being swamped.

 

The pitching and swamping issue is really an issue I two strokes - not enough prop to absorb the power and the engine will overlean as it over speeds the throttle setting and will over heat the egt and you are at real risk of melting an engine - robin440 engine would melt a piston in under a minute.

 

Overprop a two stroke and you may get to a super rich mix that can actually stop your engine.

 

Rotate 912 and all four stokes are really forgiving of props. But if you understand that the published fuel and power graphs of the 912 are for full absorbed power at the power max for that rpm you can interpret what your see on lph burn on set rpm.

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Fair comment Kasper BUT in my real world of fixed or ground adjustable props the pitch is set for the conditions our aircraft must perform safely in - in my case I have a very dodgy home field, that requires short field performance on every TO. This means that my prop is set for advantage TO/fine or at least the best compromise I can come up with. So its not working so hard (below optimum) at cruise (100-110 N indicated) 4800-5200 rpm and I get a very low fuel consumption - I can go to 54/5500 at 500 ft above actual sea level on a cool day and get 120 N indicated but in truth my aircraft feels like everything is struggling a bit - much happier back at 48-5200 ish.

 

Now to your comment on heavy/draggy air frames - for the same indicated speeds (TO, climb & cruise) my fuel consumption rises with increased load (passenger and or goods) as I expect. I have no great experience of 912's in draggy air frames however I read with interest and no little smugness, the fuel consumption of many other forum members who fly dedicated STOL aircraft. You might argue that the fuel consumption per hour at the same engine rpm is much of a muchness (& you would probably be technically correct) but when I see 17-19 l/hr being quoted for sub 100 knot air speeds, I call that a higher fuel consumption ie my aircraft will consume (in still air) 13 l to fly 100 N theirs will consume 17-19 L to go the same distance.

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Well skippy it’s horses for courses. The stols get in and out with having to worry about short strips. Yep they are draggy and they use their power to get under 100nts. You’d be laughing at me - 11lph on my 80hp 912 is cruise and that’s 60nts.

 

You may be happy running your engine in cruise up over 5000 - I run mine at 4300 and it will outlive me.

 

You may also say that converting my speed/fuel to comparative to yours is not great - mine will burn 18.3l per 100nm - just like the stols.

 

But like them I’m am not using my aircraft to travel - I use it to enjoy a type of flight that’s suited to me. And I can operate out of 120m runways easily down at sea level and even up here at 3300ft on my back paddock I’m in and out in under 200m.

 

Horses for courses. For me if I want to get somewhere by air I use mr qantas and get there a hell of a lot faster than 130nts

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Kasper - I was not intending to make negative criticism, only to highlight fuel consumption/ trip differences. As Bluea said horses for courses.

 

The Zephyr can easily TO , full fuel, one pilot, on grass in 100 m - less on a sealed runway. & climb out at 1500 ft/min - BUT you dont get that wind in your face feeling.

 

One point I would highlight - Rotax recommend running their 912 range above 5000 rpm (actually 5200).

 

My engine, currently at 920 hrs, should (barring any unforeseen failures) easily make 2000 hrs and possibly beyond. I always filter fuel on tank filling, preflight "burp", use a good healthy battery to start, warm my engine befor flight, TO at max rpm, reducing rpm on achieving "safe" altitude, climb at 5200rpm or higher, keep a close eye on temperatures so as to remain "in the green" carry power on decent to slow cooling. All servicing as per"the book" - does not require topping up the oil between services, carby balance is a near perfect as I can get it (100 hr checks) leak down/pressure are consistently within specifications, prop statically & dynamical balanced. In short you do not need to "baby" the engine by running at low rpm"s which may actually be bad for its service life expectancy.

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Behind what engine? Was it fitted with a mixture control?

I fitted an Air Fuel Ratio gauge when fiddling with a different system. I left it on when I put carbs back on and found the metering not nearly as precise as I thought. Taking off at about 1500 amsl with afr ratio of about 12.5:1 at full power you can watch the afr drop as you climb and will get down to just over 11:1 Around 7000 amsl, which significantly reduced power, not only because of lower atmospheric pressure, but an over rich mixture. Maximum power occurs between about 12.1:1 and 12.5:1, any richer or leaner and revs start to drop off, although over rich seems to reduce power quicker than leaning.

With the other system fitted, I could adjust the mixture as I climbed which provided optimum power for the altitude.

 

Mine is a 912 ULS (2007 model) I am often above 8500ft. No issues at all. In fact we cruised all the way home from Townsville to country NSW over 2 days above 8500ft for most of the way! :D

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Mine is a 912 ULS (2007 model) I am often above 8500ft. No issues at all. In fact we cruised all the way home from Townsville to country NSW over 2 days above 8500ft for most of the way! :D

You probably won’t feel any roughness until about 10:1, but I’d lay money that at those altitudes if you could lean it out you would be making better power.

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Mine is a 912 ULS (2007 model) I am often above 8500ft. No issues at all. In fact we cruised all the way home from Townsville to country NSW over 2 days above 8500ft for most of the way! :D

I've just done 25 hrs in a week.

Alot of it over 8500.

Yes, it runs perfectly fine, but it's not running at it's most efficient.

You are probably throwing away a couple of litres an hour at least, at that altitude.

To know what it really should be using, the best comparison would be how much fuel an injected engine was using at that altitude.

I also think it's doesn't like burning avgas at those altitudes and egt's. It's a bit cold for a clean burn.

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They've used plenty of "Pistons" at high altitude and polar areas. Oil getting thick is a worry but fuel still burns if it's mixed ok. Your fuel flow rate should still be proportional to shaft horsepower which does get way down in thin air. Leaning will make a lot of difference at height. The Bing carby does not cope with such levels. Nev

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There is another thread running here right now about VH-SGS, the Sonerai that ended up with various world records and a 24,000 ceiling. It is powered by a 912. And he had leaning. Does anyone know how he did that?????

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There is another thread running here right now about VH-SGS, the Sonerai that ended up with various world records and a 24,000 ceiling. It is powered by a 912. And he had leaning. Does anyone know how he did that?????

I don't know of any other commercial system tha the one in post 46, unless he made his own.

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  • 2 weeks later...

I've just done 25 hrs in a week.

Alot of it over 8500.

Yes, it runs perfectly fine, but it's not running at it's most efficient.

You are probably throwing away a couple of litres an hour at least, at that altitude.

To know what it really should be using, the best comparison would be how much fuel an injected engine was using at that altitude.

I also think it's doesn't like burning avgas at those altitudes and egt's. It's a bit cold for a clean burn.

Yep! Agreed. Sometimes though, getting above the crap takes you that high, and I am not that concerned about the couple of litres an hour. I rarely burn Avgas. ONLY if I have to!

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You probably won’t feel any roughness until about 10:1, but I’d lay money that at those altitudes if you could lean it out you would be making better power.

 

Yeah right! I have no way to do it though - so it is what it is! :)

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Hi skippy..........nothing special.....11,400' and climbing. No doubt running very rich but no sign of rough running. Made it to 12,000 then went home.....)

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11,400 feet

 

Thanks Bruce - sorry but I didn't pick that up - that's terrific - I actually already knew that a 912 ULS could keep performing, at least up into the low teens - I would however council you to be a bit circumspect, about such achievements (assuming NZ has the same altitude restrictions Au has).

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Thanks Skippy. We had obtained clearance for 12,000, which is why we stopped there. We also thought ATC would be having something of a laugh as they tracked our progress.................)

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