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Disappointment in Rotax 9 series
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rampil

Joined: 04 May 2007
Posts: 870

Posted: Sat Feb 09, 2008 8:46 am Post subject: Disappointment in Rotax 9 series

Hi All,



I have been a big fan of my 912uls since I started flying with it three

years ago. Part of the reason was the modern carb design in the Bing 64 

with automatic (at least partial compensation for altitude in the) mixture.

All their advertising states this.



Well, unless someone can prove the following data wrong now, this

Altitude compensation is a complete crock.  I really hope someone

has some data to prove this wrong.



I just happened upon the new Katana DA20-100 POH with official

performance charts of the certificated 912 and 912S engines. The AC

also has a Hoffman CS 2 blade prop.  The charts clearly show

progressive increase in fuel flow with altitude at a set %max power.

This is of course the exact opposite of Lycomings and Continentals

and the Rotax/Bing claims.



Can someone please show me how this interpretation is wrong (no

handwaving please, just documented facts).



Maybe the GreenSky HAC or real fuel injection is the way to go!



I attach some charts which show the data

912S Range KatanaDA20-100.png

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_________________
Ira N224XS

jstewart(at)jkmicro.com
Guest

Posted: Sat Feb 09, 2008 9:22 am Post subject: Disappointment in Rotax 9 series

rampil wrote:



 	  Quote: 	
		  



Hi All,



I have been a big fan of my 912uls since I started flying with it three

years ago. Part of the reason was the modern carb design in the Bing 64 

with automatic (at least partial compensation for altitude in the) mixture.

All their advertising states this.



Well, unless someone can prove the following data wrong now, this

Altitude compensation is a complete crock.  I really hope someone

has some data to prove this wrong.



I just happened upon the new Katana DA20-100 POH with official

performance charts of the certificated 912 and 912S engines. The AC

also has a Hoffman CS 2 blade prop.  The charts clearly show

progressive increase in fuel flow with altitude at a set %max power.

This is of course the exact opposite of Lycomings and Continentals

and the Rotax/Bing claims.



Can someone please show me how this interpretation is wrong (no

handwaving please, just documented facts).



Are you sure you have the right charts?  The second chart
	


shows a range of  1900 - 2380 RPM which seems unlikely

for a 912.

ami(at)MCFADYEAN.FREESERV
Guest

Posted: Sat Feb 09, 2008 11:26 am Post subject: Disappointment in Rotax 9 series

As part of some recent testing of a different propeller, a full power climb 

from 2500' to 6500'+ showed constant RPM and EGTs all the way.

Fuel flow dropped steadily from 4.0 Imp GPH to 3.7 Imp GPH.

This suggests to me that the mixture was altitude compensating between those 

altitudes. The reduction in fuel flow was consistent with a reduction in 

engine power resulting from the lowering air density; not to be confused 

with mixture.



But this was with a constant throttle opening (WOT), whereas the DA20 

figures are for increasing throttle opening, necessary to maintain the 

stated power level at the different altitudes.



Duncan McF.

---

rampil

Joined: 04 May 2007
Posts: 870

Posted: Sat Feb 09, 2008 12:34 pm Post subject: Re: Disappointment in Rotax 9 series

Followup-



JStewart:  Think prop rpm, not engine RPM

NB:  The data I appended was for the DV-20-100, not the DA-20

engine is the rotax 912S3



Just off the phone with Bud Yerly about this.  He recounted a flight test

recently with no measured fuel flow, but with EGT declines with altitude.

_________________
Ira N224XS

imap8ntr(at)cox.net
Guest

Posted: Sat Feb 09, 2008 1:29 pm Post subject: Disappointment in Rotax 9 series

Hi

 The best "factual" data is what I  have as my experience.  I have owned a 912UL for about 4 years and  definitely it does not have an altitude compensating carburetor.  As I go  up in altitude I must allow for increased fuel usage. As I go from 2000 ft to  9500 ft I increase my fuel consumption by 20%.  This makes sense to me  since I am burning richer as I go up in altitude.  The new LSA's have the  Bing compensating carburetor and thus no mixture control.

  

 My Jabiru engine has the Bing  compensating carburetor.  As I go up in altitude I dont see much change in  my EGT because in stead of the engine running richer,  the carburetor  compensates to keep the mixture the same as evidenced by a contant EGT.  So  I know for a fact that the Bing compensating carburetor works well as  stated.  I dont know why Rotax states the fuel cosumption is increasing  with altitude.  My engine does the opposite and I run most efficiently at  high altitude on my cross countries.

  

 Hope this helps. I  have not  been pleased with Rotax support as a whole.  Certainly there have been so  many service bulletins out regarding the 912 that I almost wish I had my old 582  back.

  

 Ivan

  

  

 ---

Gilles.Thesee(at)ac-greno
Guest

Posted: Sat Feb 09, 2008 1:46 pm Post subject: Disappointment in Rotax 9 series

Ivan a écrit :

 	  Quote: 	
		   I have owned a 912UL for about 4 years and definitely it does not have 

 an altitude compensating carburetor.  As I go up in altitude I must 

 allow for increased fuel usage. As I go from 2000 ft to 9500 ft I 

 increase my fuel consumption by 20%.  This makes sense to me since I 

 am burning richer as I go up in altitude.  The new LSA's have the Bing 

 compensating carburetor and thus no mixture control.

  

 My Jabiru engine has the Bing compensating carburetor.  As I go up in 

 altitude I dont see much change in my EGT because in stead of the 

 engine running richer,  the carburetor compensates to keep the mixture 

 the same as evidenced by a contant EGT.  So I know for a fact that the 

 Bing compensating carburetor works well as stated.  I dont know why 

 Rotax states the fuel cosumption is increasing with altitude.  My 

 engine does the opposite and I run most efficiently at high altitude 

 on my cross countries.

  


	


Ivan,



Would you care to elaborate on the "compensating" and "non compensating" 

carbs ?

As far as I could see, there is no particular difference between the 

Bings 64 on the Rotax, and the Bing 64 on the Jabiru. Also, except for 

the particular engine jetting, there is no apparent technical difference 

between the carbs in each engine documentation : the Jabiru manual shows 

the same picture as the Rotax.



In my opinion, the Bing carb provide *some degree* of altitude 

compensation due to the constant vacuum design.



Best regards,

-- 

Gilles

http://contrails.free.fr

rampil

Joined: 04 May 2007
Posts: 870

Posted: Sat Feb 09, 2008 7:35 pm Post subject: Re: Disappointment in Rotax 9 series

Hi Gilles,



I think you are an experienced thoughtful guy,  but what does it mean

to have an opinion on these carburetors <"In my opinion, the Bing carb provide *some degree* of altitude compensation due to the constant 

vacuum design. ">.  On the other hand, what you told Ivan is absolutely

true from my hands on experience with both engines,



We are not talking about health food nonsense or placebo. We must talk

engineering, these are deterministic systems, they either work

or they do not work.



The DATA says they do not work, in fact, they perform WORSE

than just leaving full rich on a Lycoming.  



I may well be wrong due to lack of sufficient data, but the DATA

so far says the 912S3 is CONTRACOMPENSATING.



Please explain to me my error.  Opinions can not count 



Has anyone performed actual, careful engine performance flight

test.  I'd like to, but my new EFIS  has the output of my FloScan

bouncing all over creation to the point where it is unusable.



Does anyone have DATA to share?

_________________
Ira N224XS

Thom Riddle

Joined: 10 Jan 2006
Posts: 1597
Location: Buffalo, NY, USA (9G0)

Posted: Sun Feb 10, 2008 5:38 am Post subject: Re: Disappointment in Rotax 9 series

rampil,



I do not know where you got your data but attached is a snapshot of the cover page from my Katana Manual and the page with the performance and fuel consumption table data. 



See attachments.



As you can see there is nothing indicating a variance in fuel flow AT A CONSTANT POWER SETTING regardless of variations in altitude. 



Per my training, knowledge and experience, fuel flow is a nearly linear function of power being produced, regardless of altitude at which it is being produced. I have never had a fuel flow meter on any of the airplanes I've owned with the 912 series engine but have flown a good bit of cross country time in them at known power settings at widely varying altitudes, and the fuel required to fill the tank after these flights has always been within 1/2 gallon of my projections based on time and power setting during these long continuous power flights at varying altitudes. Also, the EGTs on my 912 read pretty constant at a constant power setting at varying altitudes.



Thom Riddle

Retired Mechanical Engineer

FAA Certificated Powerplant Mechanic

http://riddletr.googlepages.com/a%26pmechanix

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rampil

Joined: 04 May 2007
Posts: 870

Posted: Sun Feb 10, 2008 6:06 am Post subject: Re: Disappointment in Rotax 9 series

Hi Tom,



Again, the data was for the DV-20-100 912S3

not the DA-20 912.



Yes my DA-20 manual looks the same (I have about 150 hours in them)

but the manual does not specify what altitude the measurement was

made, and since my time was in a rental, I did not bother to try.

Re the DA-20 manual, certainly you dont believe the carbs compensated

up to the max altitude of 13000?  Nobody I spoke every claim claimed

they compensated for altitude over 8000, including Eric Tucker who

I spoke to at Oshkosh 2005 at some length when he was hanging out at

the CPS booth.



Unrepentant Electrical Engineer

AME Eastern Region

_________________
Ira N224XS

Thom Riddle

Joined: 10 Jan 2006
Posts: 1597
Location: Buffalo, NY, USA (9G0)

Posted: Sun Feb 10, 2008 7:15 am Post subject: Re: Disappointment in Rotax 9 series

rampil,



I appreciate your bringing this increased fuel flow with increased altitude data to us. It was news to me and a bit of a surprise. The good news is that at higher altitudes you get higher TAS with a given power setting, which helps compensate for the increased fuel flow.



If my current airplane were certificated experimental I would be tempted to try the adjustable mixture control (HACman) that you mentioned. But then again, most of my current flying is at low altitudes so it might not be an economical solution for me anyway. I'm happy that I can run methanol free 87 octane autogas in our 912UL and burn between 4.0-4.2 gph at 75% power.



I still like the 912 series better than any other engine I've flown with.

_________________
Thom Riddle
Buffalo, NY (9G0)

Don't worry about old age... it doesn't last very long.
- Anonymous

hagargs(at)earthlink.net
Guest

Posted: Sun Feb 10, 2008 10:31 am Post subject: Disappointment in Rotax 9 series

The carbs on my Europa are almost indistinguishable from the carbs on my

old BMW pushrod motorcycle.  So there should be no moaning and groaning

that you are not getting "altitude compensating" carbs.  What you have are

"constant velocity" motorcycle carbs.  Those diaphrams on top of the slides

keep you from opening up the bores too much for the mass of air that can

flow through them and get a relatively good mixture.  Supposedly to give

you better or smoother throttle response when cranking it on a motorcycle. 

As noted  before fuel consumption is directly related to hp produced. 

Leanable carbs only reduce fuel consumption by allowing you to keep the

gas/air (oxygen) ratio proper (stoiciometric?).  You can keep the throttle

wide open.  What you probably get with the bings is the  fact at altitude

the  mass of air going through at altitude will not allow the diaphrams to

pull the slides up all the way even though the throttle is opened up fully.



The 914 shouldn't care as it should think it is at sea level to 16K I

believe. 



Steve Hagar

A143

Mesa AZ 

 	  Quote: 	
		   [Original Message]

 From: Thom Riddle <riddletr(at)gmail.com>

 To: <rotaxengines-list(at)matronics.com>

 Date: 2/10/2008 8:21:21 AMa

 Subject: Re: Disappointment in Rotax 9 series



 

<riddletr(at)gmail.com>
	


 	  Quote: 	
		  

 rampil,



 I appreciate your bringing this increased fuel flow with increased

altitude data to us. It was news to me and a bit of a surprise. The good
	


news is that at higher altitudes you get higher TAS with a given power

setting, which helps compensate for the increased fuel flow.

 	  Quote: 	
		  

 If my current airplane were certificated experimental I would be tempted

to try the adjustable mixture control (HACman) that you mentioned. But then
	


again, most of my current flying is at low altitudes so it might not be an

economical solution for me anyway. I'm happy that I can run methanol free

87 octane autogas in our 912UL and burn between 4.0-4.2 gph at 75% power.

 	  Quote: 	
		  

 I still like the 912 series better than any other engine I've flown with.



 --------

 Thom Riddle

 N221FA Allegro 2000 912UL

 N197BG FS1/447

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

 Believe nothing, no matter where you read it or who has said it, not even

if I have said it, unless it agrees with your own reason and your own
	


common sense.

 	  Quote: 	
		   - Buddha





 Read this topic online here:



 http://forums.matronics.com/viewtopic.php?p=163287#163287

hdwysong(at)gmail.com
Guest

Posted: Sun Feb 10, 2008 10:35 am Post subject: Disappointment in Rotax 9 series

 	  Quote: 	
		  Has anyone performed actual, careful engine performance flight

test.  I'd like to, but my new EFIS  has the output of my FloScan

 bouncing all over creation to the point where it is unusable.

	




Hello Ira.  We saw the same erratic behavior out of our FloScan and EFIS (MGL RDAC-X).  The solution was to pull the signal line (white wire) up through a resistor to clean up the signal.  I'll send you a sketch of the schematic if you're interested.

 

D

  [quote][b]

rampil

Joined: 04 May 2007
Posts: 870

Posted: Sun Feb 10, 2008 1:36 pm Post subject: Re: Disappointment in Rotax 9 series

Hi D,



Thanks for the offer. Supposedly the EFIS I have has a 10k ohm

pullup to the 5v rail internally. A pullup makes sense when the 

optical transistor inside the Floscan does not generator enough 

voltage swing on it's collector.  I don't think that is the problem in my

case, but I will check the waveform with my scope.



I think my problem is that the company has a strange idea of how

much digital filtering (smoothing after conversion  from pulse interval

to flowrate) they do to this particular signal.

_________________
Ira N224XS

rampil

Joined: 04 May 2007
Posts: 870

Posted: Sun Feb 10, 2008 1:53 pm Post subject: Re: Disappointment in Rotax 9 series

Steve,



I think we both know what a carb is supposed to do.

You are just off track when you compare aircraft to motorcycle

applications.  I cant think of the last time I gunned my rotax throttle

at the stop line 



The constant "depression" in Euro-speak or constant pressure in USA

English is in fact supposed to provide altitude compensation, the goal

in the case of the Rotax, to run just rich of peak at all altitudes.



This is the stated and advertised goal of using the Bing 64 instead

of a single conventional carb.



What is in fact happening is that the engine IN FACT is running richer

and richer with DA when it should should reduce fuel flow to match the 

reduced air mass charge.

As for increased TAS with DA, well you don't need a Rotax for that!

A rubber band motor will do exactly the same thing!

_________________
Ira N224XS

Gilles.Thesee(at)ac-greno
Guest

Posted: Sun Feb 10, 2008 2:20 pm Post subject: Disappointment in Rotax 9 series

rampil a écrit :

 	  Quote: 	
		   



 Hi Gilles,



 I think you are an experienced thoughtful guy,  but what does it mean

 to have an opinion on these carburetors .  On the other hand, what you told Ivan is absolutely

 true from my hands on experience with both engines,



 We are not talking about health food nonsense or placebo. We must talk

 engineering, these are deterministic systems, they either work

 or they do not work.



 The DATA says they do not work, in fact, they perform WORSE

 than just leaving full rich on a Lycoming.  



 I may well be wrong due to lack of sufficient data, but the DATA

 so far says the 912S3 is CONTRACOMPENSATING.

   


	


Ira,

Not sure what your point is, but what I'do first is double check the 

printed data.

Especially when they seem indicate something contrary to engine physics.



 	  Quote: 	
		   Please explain to me my error.  Opinions can not count 

   


	


I may have missed something, but could you provide a link to this 

particular manual ?



 	  Quote: 	
		   Well, unless someone can prove the following data wrong now, this

 Altitude compensation is a complete crock.

I'd say it the other way "unless you can prove our engines and carbs are 
	


wrong, these data you're referring to are not reliable".



Best regards,

-- 

Gilles

http://contrails.free.fr

jetboy

Joined: 22 Jul 2006
Posts: 233

Posted: Sun Feb 10, 2008 2:22 pm Post subject: Re: Disappointment in Rotax 9 series

I would like to add here that although the carbs used on the 912 are similar to those on the Jabiru, the Jabiru ones now have a much different profile needle which gives the standard Continental / Lycoming aero engine profile of leaner mixture below 75% power and progressively richer above 75%.  This saves 1 or 2 litres/hr of fuel flow over the original needle profile. Above 8,000 feet the needle should not be able to enrich the mixture even at full throttle setting although I have not tested this recently. they wont be able to compensate above this altitude however the Jabiru needle keeps a much leaner mixture due to the different taper.



You might consider purchasing more appropriate needles for the 912.

Downloading the latest Jabiru SB-18 gives good guidance.



Ralph 



 	  Gilles.Thesee(at)ac-greno wrote: 	
		  Ivan a ï¿½crit :

 	  Quote: 	
		   I have owned a 912UL for about 4 years and definitely it does not have 

 an altitude compensating carburetor.  As I go up in altitude I must 

 allow for increased fuel usage. As I go from 2000 ft to 9500 ft I 

 increase my fuel consumption by 20%.  This makes sense to me since I 

 am burning richer as I go up in altitude.  The new LSA's have the Bing 

 compensating carburetor and thus no mixture control.

  

 My Jabiru engine has the Bing compensating carburetor.  As I go up in 

 altitude I dont see much change in my EGT because in stead of the 

 engine running richer,  the carburetor compensates to keep the mixture 

 the same as evidenced by a contant EGT.  So I know for a fact that the 

 Bing compensating carburetor works well as stated.  I dont know why 

 Rotax states the fuel cosumption is increasing with altitude.  My 

 engine does the opposite and I run most efficiently at high altitude 

 on my cross countries.

  



	


Ivan,



Would you care to elaborate on the "compensating" and "non compensating" 

carbs ?

As far as I could see, there is no particular difference between the 

Bings 64 on the Rotax, and the Bing 64 on the Jabiru. Also, except for 

the particular engine jetting, there is no apparent technical difference 

between the carbs in each engine documentation : the Jabiru manual shows 

the same picture as the Rotax.



In my opinion, the Bing carb provide *some degree* of altitude 

compensation due to the constant vacuum design.



Best regards,

-- 

Gilles

http://contrails.free.fr

_________________
Ralph - CH701 / 2200a

rampil

Joined: 04 May 2007
Posts: 870

Posted: Sun Feb 10, 2008 2:45 pm Post subject: Re: Disappointment in Rotax 9 series

Gilles,



I wish it were so simple as to have wrong data.



Here is the URL you seek for your own official copy straight

from Austria.



http://www.diamond-air.at/dv20_afm_bas+M52087573ab0.html

I refer you to page 73 in the PDF corresponding to page 5-04 on paper.

_________________
Ira N224XS

Gilles.Thesee(at)ac-greno
Guest

Posted: Sun Feb 10, 2008 3:39 pm Post subject: Disappointment in Rotax 9 series

rampil a écrit :

 	  Quote: 	
		   I wish it were so simple as to have wrong data.

   


	


Ira,



Had a look at the manual.

Concentrated on the 75 % power numbers. The 8000 ft numbers are in close 

agreement with every other 100 hp Jabiru/Rotax engine : in the 22-23 

L/hr ballpark. At this altitude, 75 % power is obtained with full 

throttle and max allowable RPM. Nothing wrong with those data.

Now at lower altitudes, one can choose between different MP/RPM 

combinations. The lower the RPM and the higher the MP, the lower the 

fuel burn for a given % output. So it is normal to burn *less* at those 

settings. Any atmospheric engine behaves like that.



So I'd rather say : at lower altitudes it is possible to burn slightly 

less fuel for a given % output. At 1000 ft you also can use the 8000 ft 

settings. You'll burn a little more fuel because you'll be at part throttle.

But at 75 % any atmospheric 100 hp engine will burn about 22-24 L/hr at 

8000 ft : Lyc, Conti, Rotax, Jabiru or whatever.

At 75 %, our 914 (same carbs) burns around 22 L/hr.



Best regards,

-- 

Gilles

http://contrails.free.fr

imap8ntr(at)cox.net
Guest

Posted: Sun Feb 10, 2008 4:46 pm Post subject: Disappointment in Rotax 9 series

Gilles

Supposedly the "altitude compensating carburetor" is one that self 

compensates for changes in atmospheric pressure and adjusts the fuel flow 

accordingly.  Thus even without a mixture control as you go up in altitude 

with a constant power setting the carburetor compensates and decreases the 

fuel flow,  thus giving the engine less fuel when there is less oxygen and 

thus doesnt let the mixture get rich but keeps it about constant.  This has 

been my finding on my Jabiru 3300 since the EGT(a measure of mixture) stays 

about the same as my altitude is increasing.  There is a little error due to 

different temperatures from day to day which thus a colder day with the same 

barametric pressure can lean out the mixture slightly as evidenced by 

slightly higher EGT's for that day.  The carburetor ONLY compensates for air 

pressure and not air temperature(which indirectly may affect pressure.)



And that is the way I see it as evidenced by by EGT probes.  My fuel usage 

is basically constant at about 5.1 gal/h on cruise at 75% power (2850 RPM) 

on my 120hp engine.



Hope this lightens things rather than confues them,

Ivan

Phoenix,  AZ

---

Gilles.Thesee(at)ac-greno
Guest

Posted: Mon Feb 11, 2008 4:48 am Post subject: Disappointment in Rotax 9 series

Ivan a écrit :

 	  Quote: 	
		   Supposedly the "altitude compensating carburetor" is one that self 

 compensates for changes in atmospheric pressure and adjusts the fuel 

 flow accordingly.  Thus even without a mixture control as you go up in 

 altitude with a constant power setting the carburetor compensates and 

 decreases the fuel flow,  thus giving the engine less fuel when there 

 is less oxygen and thus doesnt let the mixture get rich but keeps it 

 about constant.  This has been my finding on my Jabiru 3300 since the 

 EGT(a measure of mixture) stays about the same as my altitude is 

 increasing.  There is a little error due to different temperatures 

 from day to day which thus a colder day with the same barametric 

 pressure can lean out the mixture slightly as evidenced by slightly 

 higher EGT's for that day.  The carburetor ONLY compensates for air 

 pressure and not air temperature(which indirectly may affect pressure.)


	


Ivan,



Thank you for clarifying your opinion.

The point was, there is no technical difference between the Bing 64 

carbs on a Rotax vs a Jabiru.

Both provide *some degree* of altitude compensation.



BTW, just for the fun, the cutaway drawing in the Jabiru documents was 

borrowed from...a Rotax manual 

But no problem here, since the Rotax drawing comes direct from Bing, and 

the carb internals are the same.



Best regards,

-- 

Gilles

http://contrails.free.fr

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