Choke Management

[RFguy]

I dont think we can compare rich and lean mixtures because power output is different for different mixtures - we really need to compare apples for apples- IE 100% or 75% power.

 

theroretical thermal efficiency :

7:1 (O-200) : 54.1%

8.0 (Jabiru most -depends on head recession ) 56.4

8.5 (Beech 23 with Lyco) 57.5

rotax 912ULS 61.4%
real life differences will be about half  to three-fifths of that 
 

so Bob, your original statement of lower fuel costs with the rotax are probably true.

 

[BrendAn]

On 11/12/2022 at 8:28 AM, RFguy said:

The rotax is pretty forgiving of misalignment of ... everything... at cruise and WOT settings.

 

It does not however, like idle settings. the inlet manifold is suboptimal, the gearbox doesnt like it.

 

However, that's hardly an operational  disadvantage to all the other fine attributes of the engine. 

 

I'd suggest warm up oil cooler bypass  and water cooler bypass, to limit the warm up idle time where the engine is unhappy.
Or a sheet of cardboard.....

On warm up, I idle the club rotax at the top end of the rotax permitted warmup RPM , and use a sheet of cardboard on both coolers.

 

 

is that because of the cooler weather in canberra, because the tecnam i have been doing lessons in warms up to operating temps while we taxi out to the runup bay. on first startup he runs it at about 2200 rpm while we do the first checks then back to idle before taxi . 

[IBob]

BrendAn, I believe Rotax recently lifted their initial warmup recommendation to 2200RPM, and certainly mine has always felt 'happier' at that initially. Once warmed, I pull it back to 2000RPM, I don't usually go below that.

As for warming up, it depends what you are referring to: while the head coolant temperatures in mine come up fairly promptly, the oil temp is much slower. This seems to be usual, and some fit a thermostatic bypass to the oil radiator, with varying results as the oil tank still requires a fair bit of warming.

[Thruster88]

I believe the initial rpm limits on 9 series Rotax is due to the oil pump sucking that cool thick oil out of the tank, cavitation?  There is nothing in the cylinder or crank that would cause any limit.  

 

Good thread drift, that's how conversion goes.  

Edited December 12, 2022 by Thruster88

[RFguy]

5 minutes ago, Thruster88 said:

I believe the initial rpm limits on 9 series Rotax is due to the oil pump sucking that cool thick oil out of the tank, cavitation?  There is nothing in the cylinder or crank that would cause any limit.  

 

Good thread drift, that's how conversion goes.  

Indeed. suck limit is -4.5psi IIRC.

[skippydiesel]

I find it quite extraordinary that those comparing Rotax 912ULS fuel consumption with the 0-200 (or others in the same hp class) go on to bag the 912's , because it may not be motivating an aircraft weighing in at many more kgs. That Rotax tend to be used in lighter aircraft, is their good luck (fuel wise) -  hardly a failing.

Those that choose to fly heavy (er) iron must bear the additional running cost, irrespective of the engine used.

My ATEC Zephyr, at a nominal 300 kg, was capable of 100 knots indicated, sipping around the 12 L/hr (single POB) and 120 knots at 18L/hr (sea level). The combination of an efficient (Rotax) engine, light and aerodynamic airframe,  all contribute to excellent fuel efficiency.

[BrendAn]

4 hours ago, IBob said:

BrendAn, I believe Rotax recently lifted their initial warmup recommendation to 2200RPM, and certainly mine has always felt 'happier' at that initially. Once warmed, I pull it back to 2000RPM, I don't usually go below that.

As for warming up, it depends what you are referring to: while the head coolant temperatures in mine come up fairly promptly, the oil temp is much slower. This seems to be usual, and some fit a thermostatic bypass to the oil radiator, with varying results as the oil tank still requires a fair bit of warming.

ts and ps in the green after taxi to runup bay. cfi is an l4 that does a lot of work on rotax engines , he is a good operator.

[BrendAn]

and the tecnam returns consistently around 10lph during training.

[RFguy]

7 hours ago, BrendAn said:

and the tecnam returns consistently around 10lph during training.

so its mostly descending.....

[BrendAn]

36 minutes ago, RFguy said:

so its mostly descending.....

Well circuits are descending and climbing aren't they. Even cruising at 90 knots it is excellent on fuel. Faster means more fuel of course.

[RFguy]

10lph average sounds a bit low for a ol tecnam.

well what do we have,  TO roll, climb to 1000' in favourable condix , total 90 seconds at 27 lph. = 675mL

probably low cruise 65% for 120 second  at 15lph = 0.5L

base, final average 30% for 150 seconds  maybe 8 lph =333mL
total time 360 seconds, total fuel 1.508L. equates to 15.08lph

well that's my guess, anyway .

 

[RossK]

32 minutes ago, RFguy said:

10lph average sounds a bit low for a ol tecnam.

well what do we have,  TO roll, climb to 1000' in favourable condix , total 90 seconds at 27 lph. = 675mL

probably low cruise 65% for 120 second  at 15lph = 0.5L

base, final average 30% for 150 seconds  maybe 8 lph =333mL
total time 360 seconds, total fuel 1.508L. equates to 15.08lph

well that's my guess, anyway .

 

Here is a man who is very good with numbers, I doubt there is much guesswork in your numbers RF 😉

My empirical evidence supports this:

15L/hr for circuits in the Sportstar is what I regularly see.

 

[skippydiesel]

31 minutes ago, RFguy said:

10lph average sounds a bit low for a ol tecnam.

well what do we have,  TO roll, climb to 1000' in favourable condix , total 90 seconds at 27 lph. = 675mL

probably low cruise 65% for 120 second  at 15lph = 0.5L

base, final average 30% for 150 seconds  maybe 8 lph =333mL
total time 360 seconds, total fuel 1.508L. equates to 15.08lph

well that's my guess, anyway .

 

Hmmm! Never flown a Tecnam but my Zephyr/912ULS could loiter at 70-80 knots @ 8L/hr. Short field take off/climb out power delivers 22-24L/hr . Base/ Final, idle power not sure but would guess 3-5L/hr

 

So using your times that would be:

 

Take off 0.4L

Down wind 0.26L

Base/Final (glide approach) 0.21L

Total 3.2L

[RFguy]

you sure about your addition in those calcs  SKippy? 

and Tecnam is like a flying square box compared to the Zephyr.

Ross, yeah sort of depends on the downwind leg a bit- you need time to look at the windsock, do bumfish, figure out the radio chaos .. etc and of course extra fuel used if you find yourself a little low mid final and end up dragging it.
But I think those numbers are reasonable for a student/training

Edited December 12, 2022 by RFguy

[skippydiesel]

20 minutes ago, RFguy said:

you sure about your addition in those calcs  SKippy? 

 

Let me see:

 

TO & Climb: 24L/Hr /60 min x 1 min=0.4L

Down Wind: 8L/Hr/60min x 2min = .26L

Base & Final: 5L/Hr/60min x 2.5 min =0.2L

 

Total .85L - even better than my appalling addition in the first go.

 

My guess: A 300 kg aircraft, 75kg pilot & 47kg fuel all in a low drag aircraft, with long wings, makes for very low fuel consumption:

[Geoff_H]

18 hours ago, RFguy said:

I dont think we can compare rich and lean mixtures because power output is different for different mixtures - we really need to compare apples for apples- IE 100% or 75% power.

 

theroretical thermal efficiency :

7:1 (O-200) : 54.1%

8.0 (Jabiru most -depends on head recession ) 56.4

8.5 (Beech 23 with Lyco) 57.5

rotax 912ULS 61.4%
real life differences will be about half  to three-fifths of that 
 

so Bob, your original statement of lower fuel costs with the rotax are probably true.

 

Is thermal efficiency mechanical energy out divided by chemical energy in (ie fuel energy)?

[facthunter]

Hotter running motors are  More thermally efficient. That's how Turbines have upped their efficiency as well as high bypass ratio fans to improve THRUST.  Higher RPM in Pistons increases friction on a  squared law basis.  Compact combustion chambers. LOW surface area/ volume are more thermally efficient. Less heat loss to cooling system . This is why a Mazda Rotary is so inefficient and thirsty. Nev

Edited December 13, 2022 by facthunter

[BrendAn]

3 hours ago, RFguy said:

10lph average sounds a bit low for a ol tecnam.

well what do we have,  TO roll, climb to 1000' in favourable condix , total 90 seconds at 27 lph. = 675mL

probably low cruise 65% for 120 second  at 15lph = 0.5L

base, final average 30% for 150 seconds  maybe 8 lph =333mL
total time 360 seconds, total fuel 1.508L. equates to 15.08lph

well that's my guess, anyway .

 

Yes . I made it up. I didn't really check the fuel computer and dip the tanks . I was surprised at how good it is in fuel. Better than the j160 I had been in before.

[RFguy]

"Is thermal efficiency =mechanical energy out divided by chemical energy in (ie fuel energy)?"
Geoff, as u know, the thermal efficiency is the fraction of the heat converted to work.  Going back to the classic otto cycle 1,2,3,4  adiabatic  state chart, a higher T diff between the lower line and the higher line,  provides for work able to be done .
I'm not going to draw a picture. people can look at this :

https://www.sciencedirect.com/topics/engineering/otto-cycle

 

[RFguy]

10 minutes ago, BrendAn said:

Yes . I made it up. I didn't really check the fuel computer and dip the tanks . I was surprised at how good it is in fuel. Better than the j160 I had been in before.

"Yes . I made it up."  tut tut ! at least you admit it. 

the J160 should be better  overall .  a more slippery plane.  just the way it is being flown I suspect. J160 also have smaller donk.

 

[facthunter]

It applies to the CARNOT Cycle  equally.  Nev

[BrendAn]

6 minutes ago, RFguy said:

"Yes . I made it up."  tut tut ! at least you admit it. 

the J160 should be better  overall .  a more slippery plane.  just the way it is being flown I suspect. J160 also have smaller donk.

 

That was a joke. I didn't make it up.next lesson I will take a photo of the log.

[skippydiesel]

21 minutes ago, BrendAn said:

Yes . I made it up. I didn't really check the fuel computer and dip the tanks . I was surprised at how good it is in fuel. Better than the j160 I had been in before.

Maaaate! tut tut, etc etc BUT you are not so far out IF you are not trying to race around the circuit AND your climb out to 1000ft is around the 22-24L/Hr AND your low speed cruise is about the 10-12L/hr 

[BrendAn]

2 hours ago, skippydiesel said:

Maaaate! tut tut, etc etc BUT you are not so far out IF you are not trying to race around the circuit AND your climb out to 1000ft is around the 22-24L/Hr AND your low speed cruise is about the 10-12L/hr 

Seriously,did you not see my post where I said I was joking . Like I said. I can take a photo of the aircraft log for f**ks sake.  Why wouldn't it be 10 lph . Climb out to 1200 ft. Cruise the circuit at 4500rpm. Descend on idle.  Stall practice we climb to 3000 ft and back up again after each stall. Cruise at 4500 rpm also. Rf is convinced I am wrong but I know I am not. The plane is not being thrashed around at high rpm so it gets great economy.

[RFguy]

LOL.

 

actually they like being thrashed. rotaxes dont like being babied. you are paying per hour..

though yes, you are training, so different needs apply...

Edited December 13, 2022 by RFguy