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Rodr

Rotax 912

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Building my second Savannah this being a S series question what model of the Rotax 912 is everyone buying using comments on carby versus injection would be app ?

 

 

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In my humble opinion - Bother engines are 100 hp so look at the 912 S cost of purchase, the fuel savings and the slightly unknown cost of additional complexity. Compare these cost with the 912 ULS.

 

Without doing the exercise myself, I would guess that the 912 S benefits will only appeal to an aircraft/owner doing very high hours per year.

 

 

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ULS is cheaper and less complex. The fuel saving is not enough to make a monetry difference really. There are heaps of cases about the IS being way to complex electrically...unless you have a carb ice issue where you fly stick with carbs....just get rid of the throttle system on the Sav and put in a McFarlane twin cable throttle

 

 

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Thanks guys it’s just what I wanted to hear having had twoULS and no problems it makes a lot of sense. Yes I bought the twin throttle cable from aircraft spruce back in 2011 on the first Sav XL wouldn’t use anything else cannot understand why ICP continue with that horrible setup.

 

Cheers

 

 

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Building my second Savannah this being a S series question what model of the Rotax 912 is everyone buying using comments on carby versus injection would be app ?

Hi Rod,

 

I have a ULS in my Savvy S, and I have got about 15 litres/hr over the 3 years or so that I have been flying her (about 260 hours). I cruise around 4800-5000 revs. That is mostly solo flying, and I weigh around 70kg, which may have contributed, but I still reckon that's not bad......

 

Cheers,

 

Neil

 

 

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Hi Rod,

I have a ULS in my Savvy S, and I have got about 15 litres/hr over the 3 years or so that I have been flying her (about 260 hours). I cruise around 4800-5000 revs. That is mostly solo flying, and I weigh around 70kg, which may have contributed, but I still reckon that's not bad......

 

Cheers,

 

Neil

Hi Neil,

 

I dont fly a Savannah but the Zephyr I fly is towed by the very same Rotax 912 ULS -

 

My prop is set for TO advantage, I flight plan at 13L/hour at 100 knots indicated. Cruise 4800-5000 rpm. Cruise climb 90 knots at 5200 rpm. I weigh a little less than you but often have a passenger (no change to flight plan).

 

Routinely get sub 10 L/hr around the patch a 60-100 knots.

 

I would be cruising at 115 + knots and about 5200 + rpm to consume 15L/hr in cruise.

 

My point is that your engine is consuming way to much fuel at your nominated rpm.

 

 

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My point is that your engine is consuming way to much fuel at your nominated rpm

But the Sav is way draggier, and usually doesn't have an in flight adjustable prop.

 

 

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But the Sav is way draggier, and usually doesn't have an in flight adjustable prop.

My prop is ground adjustable and litres per hour has little to do with drag, much more to do with power setting hence my focus on your usual cruise rpm which is the same as mine 4800-5000.

 

Your fuel consumption should be similar to mine, irrespective of air speed but apparently not. So the question is why is your engine gobbling so much more fuel than mine???

 

 

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Skippydiesel, I think if you canvas Sav owners, you will find they all report fuel consumption similar to Neil S.

 

So either 4000+ Sav owners have all got it wrong.........or you may need to examine your assumptions about the relationships between drag/pitch/rpm and fuel consumption?

 

 

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Let's cut to the chase, basic physics and thermodynamics.

 

A 912ULS pitched to wide open throttle at 5800rpm is developing its rated 100hp. At 5000rpm, with the same pitch on the propellor its developing around 65% power, 65hp obviously.

 

A very good guideline for brake specific fuel consumption for a carburetted petrol aero engine is around .45lb/hp/hr. Or about 272gm/kw/hr if you prefer.

 

So it follows that to develop 65hp for an hour you'll use .45 x 65 equalling around 29.25lb of fuel, 4.0 imperial gallons or so.

 

If you like it in metric the numbers are 272gm x 49kw = 13.3kg, or 18 litres or so.

 

Even if your engine is 10% more efficient in terms of BSFC then you'll still be burning around 15/16 litres per hour.

 

While a particular 100hp 912 may be burning 12 or 13 litres per hour at a constant cruise setting then rest assured the power output is commensurately low.

 

If I'm just mucking around in my 100hp Savannah, low power settings and lots of idle time yes my fuel burn is down in those sorts of ranges. But if I'm cruising at 5000rpm ( My prop is pitched to WOT is around 5700) then yes I burn in that range of 17/18litres per hour on a trip.

 

There is no magic.

 

Peter

 

 

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Fallowdeer - your science/argument sounds good.

 

Seems to me that there could be a factor or two giving the different real life fuel consumption's:

 

  • My tacho is over reading ie I am actually cruising at a lower RPM.
     
  • My prop pitch is finer so my engine is not working so hard at cruise RPM - this would then buy in to the draggy airframe idea. My airframe does not require the same expenditure of power to stay aloft at a given air speed so even if the engine is turning at the same RPM as another airframe it is using less power/fuel to do so.
     
  • My Hobbs is over reading - my fuel calculations are based on Hobbs time against actual fuel put into the tank.
     
  • My airspeed is inaccurate - highly likely
     
  • I only fly in cool weather - more efficient combustion, prop, wing
     

 

 

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Hi Neil,

I dont fly a Savannah but the Zephyr I fly is towed by the very same Rotax 912 ULS -

 

My prop is set for TO advantage, I flight plan at 13L/hour at 100 knots indicated. Cruise 4800-5000 rpm. Cruise climb 90 knots at 5200 rpm. I weigh a little less than you but often have a passenger (no change to flight plan).

 

Routinely get sub 10 L/hr around the patch a 60-100 knots.

 

I would be cruising at 115 + knots and about 5200 + rpm to consume 15L/hr in cruise.

 

My point is that your engine is consuming way to much fuel at your nominated rpm.

Hi Skippy,

 

Thanks for your reply - very interesting.

 

The book figure for the Savvy S at that rpm (5000) is actually 18.5 litres/hour....., so my figure of 15 litres/hour looked pretty reasonable to me.

 

My experience of other 912ULS powered LSAs (I learnt to fly in a Texan), was similar, so your figures look amazing to me.

 

I'd be interested to hear what other pilots running 912ULS powered planes get....

 

Cheers,

 

Neil

 

 

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Whenever there's a conversation about fuel burn vs RPM in a Rotax the very first thing to clarify is (assuming ground adjustable propellor) what the wide open throttle setting is.

 

Without similar WOT settings then one engine at 5000RPM is likely to be producing very different amounts of power compared to another engine also at 5000. Those who (erroneously, and I have seen a few) have their 912 pitched WOT at 5200RPM will clearly be putting out more power at 5000 than someone who is at 5000 but who has WOT at 5800.

 

As an exaggerated example compare fuel used in any engine under load and then fuel used at same RPM but no load.

 

This is why power charts have RPM and manifold pressure.

 

Peter

 

 

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About 19 lph at 5200-5300 rpm in my case.

 

 

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Whenever there's a conversation about fuel burn vs RPM in a Rotax the very first thing to clarify is (assuming ground adjustable propellor) what the wide open throttle setting is.

Without similar WOT settings then one engine at 5000RPM is likely to be producing very different amounts of power compared to another engine also at 5000. Those who (erroneously, and I have seen a few) have their 912 pitched WOT at 5200RPM will clearly be putting out more power at 5000 than someone who is at 5000 but who has WOT at 5800.

 

As an exaggerated example compare fuel used in any engine under load and then fuel used at same RPM but no load.

 

This is why power charts have RPM and manifold pressure.

 

Peter

Very interesting -

 

  • The ATEC aircraft (Zephyr & Fayeta) have exceptionally wide flight "envelopes" - so low stall & "slippy" in cruise.
     
  • I have set my prop pitch to static (tied down) WOT 5200 rpm. This translates to Short Field 5200-5400 rpm @ 60 knots, 1200-1500 ft/min. Normal 5600-5800 rpm @ 80-90 knots, 1000-500 ft/min.
     
  • Unless very very gradual, all climbs are at a minimum of 5200 rpm as per Rotax recommendation..
     

 

My home field is just that, a paddock on the side of a hill, with lots of "issues" on departure/arrival ( I do not take passengers from this field). My TO technique is always a short filed departure to a safer altitude where I level out, gain speed ( & rpm) and after a short interval, to make even safer altitude, throttle back.

 

Departures from a conventional airfield, grass or "paved", is much less frenetic with normal/even climb profile and more consistent (high) RPM & air speed.

 

If I could afford it I would purchase an in-flight adjustable prop to better manage my engine RPM in the variose stages of flight.

 

 

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I got 15 l/h over 250 hours in a Texan with 912ULS. Check Hobbs time vs tacho time and is there an air switch?

 

 

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I got 15 l/h over 250 hours in a Texan with 912ULS. Check Hobbs time vs tacho time and is there an air switch?

I have some 500+ hrs in my Zephyr - over this time the fuel consumption has remained constant for a given load and power setting. No air switch - hobbs starts/ stops on power on/off (so moments befor/after engine run). Actual fuel burn for a planned trip is slightly lower than calculated ie 13 L/h @ 100 knots indicated, is conservative - go figure.

 

 

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I got 15 l/h over 250 hours in a Texan with 912ULS. Check Hobbs time vs tacho time and is there an air switch?

My average fuel burn on a 2-3 hour flight (912ULS) based on tacho time is about 17 lph but steady cruise (112-115 kts IAS at about 3000 feet and 5200-5300 rpm) is about 19 lph.

 

 

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My average fuel burn on a 2-3 hour flight (912ULS) based on tacho time is about 17 lph but steady cruise (112-115 kts IAS at about 3000 feet and 5200-5300 rpm) is about 19 lph.

Rgmwa, this sounds more like what I would expect, although I am confused by 17 & 19 Lph. What Lph you flight plan at?

 

 

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Talking to a bloke at echuca fly in for roast day last month , he built a really great sling 4 seater powered by a 914 but at 110 cruise it burnt 28 litres two up, I know it is a lot more plane than my Sabre which uses about 18 at around 120, I reckon it was maybe underpowered.

 

 

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