Carby heater on a Rotax 912

[Blueadventures]

Hi  A friend is doing some maintenance to a Rotax UL (80 hp) installed on a Lightwing.

 

He is asking for any details on the attached carby heater unit; wiring details etc.

 

He said it was wired into the ac wires (Yellow) that go from engine into the rectifier regulator.

 

He would like to know if it can be wired direct via a switch and fuse to 12 Volt DC or if it must connect to the yellow wires (ac).

 

Any advice is appreciated by him.

 

 

 

 

 

 

 

[Downunder]

I've not seen one using the ac power before.

 

The ac output is about 25 to 40v, depending on rpm. The good thing I guess is that it is pre regulator so not loading that system up?

 

My (non electrical) opinion is you probably wouldn't get the same heat with 12V......

 

 

[scsirob]

From the pictures it looks like it is on the input side of the carby. That makes no sense. No amount of available electric power on that side will significantly raise the input air temperature enough to avoid carb ice.

 

There's other electric carb heaters producing about 50W that attach to the butterfly side where it may help a bit, and they are hardly effective. I'd opt to remove this pre-heater contraption and find a way to install proper hot air carb heat from near the exhaust pipes

 

 

[Mewp]

I wonder if it is trying to heat the carb body close to the venturi and not the air passing through.

 

 

[facthunter]

IF you keep the carb body above freezing it won't have any ice forming on it. The advantage of electric it's not dependent on the engine being hot, and it doesn't cause significant loss of power. You can just have it on anytime the plane is in use AC or DC doesn't matter with a heating element. Nev

 

 

[skippydiesel]

Problem with posting pictures is that people like me see them and make uncalled for comment - Apologies in advance no offence intended-

 

What looks like the left carbi throttle cable is badly damaged & rubbing against the fuel pipe. You may have different throttle opening/closing characteristics between carbi's (out of balance)

 

The float bowls look stained with petrol residue - are the gaskets  leaking?

 

Most of the tubes, cables, wires in the photo's are not adequately secured against vibration/movement/abrasion.

 

Ordinary hose clamps have been used to secure the fuel lines to carbi spigots - use fuel injector clamps or similar even pressure/non crimping clamps.

 

Carbi air vent lines look distinctly non standard

 

Sorrrry!

 

 

[Blueadventures]

Problem with posting pictures is that people like me see them and make uncalled for comment - Apologies in advance no offence intended-

What looks like the left carbi throttle cable is badly damaged & rubbing against the fuel pipe. You may have different throttle opening/closing characteristics between carbi's (out of balance)

 

The float bowls look stained with petrol residue - are the gaskets  leaking?

 

Most of the tubes, cables, wires in the photo's are not adequately secured against vibration/movement/abrasion.

 

Ordinary hose clamps have been used to secure the fuel lines to carbi spigots - use fuel injector clamps or similar even pressure/non crimping clamps.

 

Carbi air vent lines look distinctly non standard

 

Sorrrry!

Hi Skippy absolutly agree.  I was calling past his airfield on my way to Mackay after doing some work out of town.

 

Regarding the carby heater it has somehow shorted out the rectifier / regulator, item US and a replacement one fitted. 

 

I do not know about these units and there is no branding on it.  Hence this thread putting the inquiry out there. 

 

If no details can be found it may be worth his while replacing it with a current day carb heater unit; I have the Irish made electric unit and I am happy with it.   He does want carby heat, fitted to and working on the engine.

 

Yes, the fuel system and wiring need correction and appropriate securing; and this is being done; including under the instrument panel.

 

Regarding the carby vent line; it is of the required type and lengthby rotax; however he is advised not to join into a lenghtened hose arrangement.  This is to be rectified by way of fitting a larger i.d. hose / drain tube that the carb vent goes into, so that no low pressure air passing suction occurs and prevent fuel discharge onto the exhaust system.

 

Will be nice to discover the unit details and wiring instructions; as it was wired to the charge side of things and was only energised at about 300 rpm (engine).

 

Regards

 

Mike

 

 

[skippydiesel]

Hi Mike,

 

I gather the aircraft is not yours, so decisions are up to someone else , however you might like to inform him of the following:

 

I fly an 19 old ATEC Zephyr/912ULS with over 800 hrs on the engine - never had carbi ice.

 

Recently, I temporarily fitted one of those cheap, off the shelf stand alone (battery powered) indoor /outdoor (or fridge) thermometers (from EBay about $5/ JayCar about $25). Nice little LCD read out with remote thermocouple on the end of a long lead. Stuck read out  head on panel, using Velcro - passed thermocouple out cabin air vent  forward in fuselage (suck a bit of tape over it for security) & into engine bay - securing very close to left  carbi air filter - went flying.  

 

Temps inside engine bay consistently 20 C above ambient!!

 

No need for carbi heat on my installation - could be a similar story with your owners set up. The few dollars spent on the indoor/outdoor thermometer may be well spent.

 

 

[Blueadventures]

It is a friend's aircraft.  He does want carby heating.

 

I will always have carby heat available. 

 

I have experienced the need twice; one example is in my earlier skyfox Ca21 with a rotax 912 UL 80hp. For info; If you have never flown a skyfox tail dragger they need plenty of rpm (above 4,600 in my experience) to maintain or increase height (Due to the wing design). So when on decent and you get icing, rpm well below above and the pilot don't have or use carby heat you will continue on the current rate of decent and if short of runway you be on the ground off airfield and hopefully no damage if luck is with you.  The other before that in a motor glider.

 

In my option pilots should not rely on past history of never having icing as I believe it can occur when the conditions exist. Comment made with sincere intentions. 

 

Cheers

 

Mike

 

 

[skippydiesel]

It is a friend's aircraft.  He does want carby heating.

I will always have carby heat available. 

 

I have experienced the need twice; one example is in my earlier skyfox Ca21 with a rotax 912 UL 80hp. For info; If you have never flown a skyfox tail dragger they need plenty of rpm (above 4,600 in my experience) to maintain or increase height (Due to the wing design). So when on decent and you get icing, rpm well below above and the pilot don't have or use carby heat you will continue on the current rate of decent and if short of runway you be on the ground off airfield and hopefully no damage if luck is with you.  The other before that in a motor glider.

 

In my option pilots should not rely on past history of never having icing as I believe it can occur when the conditions exist. Comment made with sincere intentions. 

 

Cheers

 

Mike

I have experienced carbi ice  - in a C172 so I agree with all you have said however you haven't addressed my consistent 20C above ambient - its a bit like having carbi heat on all the time.

 

Oh! and one other thing  - with reference to your 6400 rpm, I have never flown a 912UL (80 hp) but in the 912 ULS (100 hp) climb rpm is 5200 or better and  normal cruise (fixed pitch prop) 4800- 5400 rpm.

 

And another - it is good practice to maintain some power on decent as it keeps everything warm (20C) so reducing chance of icing - if you have carbi heat (on befor power reduction) same applies. 

 

 

[Blueadventures]

I have experienced carbi ice  - in a C172 so I agree with all you have said however you haven't addressed my consistent 20C above ambient - its a bit like having carbi heat on all the time.

Oh! and one other thing  - with reference to your 6400 rpm, I have never flown a 912UL (80 hp) but in the 912 ULS (100 hp) climb rpm is 5200 or better and  normal cruise (fixed pitch prop) 4800- 5400 rpm.

 

And another - it is good practice to maintain some power on decent as it keeps everything warm (20C) so reducing chance of icing - if you have carbi heat (on befor power reduction) same applies. 

My post was 4,600 rpm not 6400. To mean that anytime flying S&L at 4,600 my aircraft was desending.  With any icing and engine not developing greater than 4600 I could not climb.  At the occasion I mentioned the end of runway gate was open and flew through the opening. If closed  could not climb.  Out come would have  been different.  Iceing occurred during the flight, close to airfield and diverted direct to runway.

 

 

[skippydiesel]

Sorry misunderstood and quoted mixed numerals

 

 

[scsirob]

I wonder if it is trying to heat the carb body close to the venturi and not the air passing through.

It's attached to the input side of the carb. The largest risk of ice forming is at the opposite side, at the butterfly. Especially with low power settings and the butterfly almost closed.

 

The engine is a huge air pump. At 4600 rpm and 1200-1350cc displacement, you are looking at about 40 liter of air per second passing through that venturi. That's a lot of cooling capacity. Having 50W or 100W of electric heater on the outside of the carb body won't make a dent in the temperature of the mixture passing the butterfly.

 

 

[pmccarthy]

I suspect 20 degrees above ambient will not make much difference to icing.

 

 

[skippydiesel]

I suspect 20 degrees above ambient will not make much difference to icing.

Hummm! - put it like this: Everything inside my cowling is pretty much 20C or higher . On decent I carry power to maintain that heat soaked environment. If I had carbi heat I would switch over to an air supply heated by the exhaust system (dont go much for the electrical concept). I have no idea what temperature this incoming air is likely to be but it pretty much just has to maintain the carbi body above freezing (no chance of ice adhering) as it is now.

 

Being conveyed by a RAA aircraft means no cloud (high moisture) flying, although I have passed through the odd light shower. By accident of design (simplicity) I suspect my carburettors, (tucked up warmly inside my cowling) are outside the temperature/moisture environment conducive to icing. Soooo no further ahead.

 

 

[M61A1]

Hummm! - put it like this: Everything inside my cowling is pretty much 20C or higher . On decent I carry power to maintain that heat soaked environment. If I had carbi heat I would switch over to an air supply heated by the exhaust system (dont go much for the electrical concept). I have no idea what temperature this incoming air is likely to be but it pretty much just has to maintain the carbi body above freezing (no chance of ice adhering) as it is now.

Being conveyed by a RAA aircraft means no cloud (high moisture) flying, although I have passed through the odd light shower. By accident of design (simplicity) I suspect my carburettors, (tucked up warmly inside my cowling) are outside the temperature/moisture environment conducive to icing. Soooo no further ahead.

While probably adequate for our temperate climate and conservative flying, I'm pretty sure that the actual requirement (can't recall which standard) for carb heat is to produce at least a 30°C rise when applied.

 

 

[facthunter]

IF the body of the carburetter is hot enough the ice will not  form on it or stick to it. I have flown with such a set up and it's left on all the time. You don't have to get the intake air above the  temp where ice is likely to form . Perhaps you might get some ice forming downstream of the carb but that is unusual and only likely with long intakes which are the most troublesome. Hot intake air will reduce power available and IF you are descending for a while your source of hot air will cool off. That's why you should apply FULL carb heat in cruise for about a minute before descending if icing condition may exist, AND descend with some power on, if the condition warrant it.

 

   You don't need to be in cloud to get carb icing. Your carburetter resembles a refrigerator  as it has gas expanding  near the throttle and the venture.(Adiabatic) cooling and also evaporation of the fuel in the same area. Just pour avgas on your hand and wave it in the breeze. You are less likely to get carb icing in very cold air as it holds less moisture than warm air. Some of the driest air on earth exist at the south pole. Near the coast of after showers in the tropics with an OAT of say 25 C you can get carb ice. Warm with a high Relative humidity means a lot of actual moisture is in the air.  You carb can cool it to a below zero C. Injection near the inlet port has much less likelihood of icing. It's also probable that a slide type carburetter mounted directly on the motor and getting some heat from it to the carb body, would not be susceptible to icing. Individual set ups will vary  a lot.. Certain engines will REQUIRE a stipulated temp rise occurrs  That would be for things like a Continental 0-200 which generally uses the same carb set up on all of that type of motor and would have been tested extensively. Nev

 

 

[M61A1]

All I was trying to say Nev, is that there is a clearly defined design standard  for carb heat temp rises. 

 

The figure I quoted came from an engineering order by Bill Whitney, I couldn’t find the standard he was quoting though.  

 

 

[facthunter]

 I agree with that and Bill knows his stuff. It's up to the designer to prove the engine as installed isn't dangerous.  I've heard a figure quoted also for a specified temp rise and it might be  an IF it does "this" all is covered sort of thing. Not only pistons have icing problems with engines. I think the Bristol Brittania went back to the drawing board after icing tests near Darwin. .