Carb icing on a 912

bencmaster · Post by **bencmaster** » Thu Feb 14, 2008 9:38 am

I always had carb heat, like most folk, until this plane.

Now I note a roughness in this odd, rather warm foggy weather. The engine sometimes stutters and I have noted sticking choke leaver and similar throttle (on landing)

It seems obvious to me that it is icing, but I am amazed when the 'in cowl' temperatures are as high as they are.

I'm not sure I can get the PFA to agree any extra weight up front on my plane, but if they did what 912 system would seem to owners to work the best?

Ben

Rod1 · Post by **Rod1** » Thu Feb 14, 2008 4:02 pm

Contact skydrive. There are a number of systems, but the SD one is very light and LAA approved.

Rod1

John Price · Post by **John Price** » Fri Feb 15, 2008 3:22 pm

I have the Skydrive unit fitted in my Europa. It only took about 2hrs to fit, plus a bit of glueing time. Since then we have had no instances of carb icing. Fitted probably only adds about 1 lb to the weight of the aircraft. No paperwork needed just get your inspector to sign it off.

John.

NJ673 · Post by **NJ673** » Sat Feb 23, 2008 5:06 pm

Ben, are you sure its carb ice?

The 912 is quite resilient to carb ice depending on how the installation is configured. For example in the RANS S6 the coolant radiator is above the rear of the engine and the carbs sit in the warm downdraft. Never say never ! but i never had carb ice in the S6. If your installation is configured this way, your problem may not be carb ice?? If this is how your rad is situated do you have the carb drip trays fitted??

In installations where the coolant radiator is below the engine, for example the Europa, carb ice is definately possible even given the engines natural resilience. So if thats how the coolant rad is situated in your storch then fitting the water jacket system from Skydrive is one way to go. I had that fitted to a 912S engine I had and it worked fine.

There is an electric heater system for the Bing carb on the Jabiru 2200 by ST Aviation, and one in process of developement for the 912 carbs by Conair Sports. Those are a much easier method (and cockpit selectable if you feel the need) but I am not sure on LAA approval status for either. Someone else will no doubt know, you can always ask engineering!

Fun looking plane by the way!! I am a little envious

Gordon

Pete · Post by **Pete** » Sat Feb 23, 2008 8:24 pm

excuse my ignorance, but how can a battery powered heater give enough heat to warm an inlet manifold.

I assume that the electric heater takes no more than 20 amp, = 250 watts

If a 2l engine is turning at say 1800 rpm at part throttle, it is sucking in something like 2000 litres of air per min which has a mass of about 2.2 kg, which at high RH will contain something like 30 grams of water.

As I recall one gram of water = 1cc and 1 cc of water needs 1 calorie to raise it's temp by 1 deg C, one calorie is 4.2 watts*sec so to raise the temp of the water by 1 deg C we need 30 calories which is approx 130 watts.

Now according to Boyles Law if the manifold pressure is 20 inches, then the inlet temperature will be 20/29 of the absolute temp in degrees K ie minus quite a bit, lets say -30C if the OAT is 20C, so to avoid carb icing we need to raise the inlet temp by perhaps 30C - that will need 3900 watts

Possibly my maths is wrong I left school a long time ago !!

Steve Brown · Post by **Steve Brown** » Sat Feb 23, 2008 10:16 pm

With an elec carb heat you are not heating the air going through the carb at all.
You only need to heat the carb body and the throttleplate /venturi. And the heat transfer between the inlet air/fuel is not that high so it doesnt cool the carb that much.
My home made electric carb heat featured in PFA mag a while back maintains my O-200 carb at 9-13 deg C in virtually all conditions conducive to carb ice ie -5 deg C & above.

And its got full (one-off) LAA Mod approval.

Works well and uses about 6 amps / 85 W.

Regards
Steve

Pete · Post by **Pete** » Sun Feb 24, 2008 6:48 pm

I would dispute the "does not cool the carb that much bit", just touch the inlet manifold upstream from your carb with the engine running, not so easy on an O-200, but on my old Luton Minor, which had exposed pipes, I can tell you they were very cold, and I have observed ice forming on the outside of the pipes whilst flying

Of course an electric carb heater cannot heat the air, however the crucial bits of the carb ie the throttle butterfly and spindle will be hit by a significant proportion of the water in the air, which can be super cooled - ie at up to minus 30 or so. Even if the heat exchange effect is only 5% efficient, that's still absorbing 200 watts from the butterfly and spindle.

Your electric heater may appear to work fine at -5C under a cloudless sky, but I am unsure I would rely on it in humid conditions at +10C.

Steve Brown · Post by **Steve Brown** » Sun Feb 24, 2008 9:37 pm

I realise there is heat transfer between the air/fuel mix & the carb/spider - being a function of the contact area (not the air volume) , characteristics of the fluid & specific heat of the metals but there is not much heat transfer in the 1.75" dia & 1.5" length of the venturi & throttle plate which is all we are interested in heating. The lower carb body isnt heated as it holds the fuel and is insulated by the carb joining gasket, while the inlet spider is insulated from the heated carb bit by a phenolic spacer. The mass of the upper carb body that needs keeping above freezing is only about 0.5kg.

Anyway it certainly works and the proof is in the pudding is in the eating.

Bumble bees aren't supposed to be able to fly and some carb aircraft seem to survive without being fitted with any carb heat - so sometimes theory isnt always bourne out in practice.

I have carried out controlled fully instrumented testing as part of the LAA mod approval and part of that was testing in just those conditions you correctly cite - ie at 10 deg C , severe icing conditions, just below dark grey cumulus cloudbase at 2000' at all throttle settings for about 20 -30 mins - no icing symptoms and the carb temp kept between 7-13 deg C. I then switched it off and after about 5 mins I was getting rev drops and when the conventional carb heat was added we got the familiar stumble as the ice melts. That single event really convinced me it was providing good additional protection.

Won't ice up at idle either which is much more convenient too.

I've still got conventional heat and use it occasionally at cruise for keeping my routines going and as a periodic check for symptoms & of course for pre landing / approach checks as normal too

I think it adds safety at very little cost or downside

NJ673 · Post by **NJ673** » Sun Feb 24, 2008 10:44 pm

Pete

Steve has summed it up so I have not much to add, but bear in mind that on the 912 and Jabiru the inlet manifold is very short and on the 912 is an aluminium casting, so a good conductor of the cylinder head heat. On the Bing the elect heaters heat the throttle shaft/butterfly, keeping it ice free, it doesnt actualy heat the carb, nor the air. The upside of this is that in theory there isnt a power reduction or danger of detonation at high power settings, ergo it can be permanantly on.

Incedentaly the water jacket system works in a similar fashion but it heats the carb this time and again is on all the time.

You dont say what engine you had on the Luton but if it was a VW with single carb then that is a very long steel manifold to keep warm which is why VW heated it with exhaust gasses via a aluminium jacket. Most aero conversions seemed to dispose of that system. I put it back on and made a copper manifold for my VP, then lagged it, to help overcome just the conditions you describe.

I found the VW to be very suseptable to carb ice and it needed a working hot air system, whether a carb heater would work on the VW i dont know, but I do believe the aircooled VW fraternity have used an electric carb heater system to overcome carb ice on buggies and bugs with aftermarket exhaust and inlet systems which dont incorporate the original VW manifold heating jacket.

I need to track one down for fitting to my Beach Buggy

.

Gordon

Simon Clifton · Post by **Simon Clifton** » Mon Feb 25, 2008 12:53 pm

Steve

Is your electric heater wired through a separate switch, or does it just come on with your master switch? Or maybe something else?

Simon C
~~~~~~

Rod1 · Post by **Rod1** » Mon Feb 25, 2008 1:33 pm

I thought there was a requirement under cs-vla for the battery to be able to sustain normal flight loads for 30min? If so then the electric heater, if it was not possible to switch it of, would flatten my 8ah bat in less than 30min on its own. My system is an option which comes with the airframe kit and works like a conventional lyk or continental carb heat. This is very light and simple. I have never had carb ice in my 70 hours of operation, but at least I have a means of dealing with it if it ever does happen.

Rod1

Steve Brown · Post by **Steve Brown** » Mon Feb 25, 2008 2:33 pm

It is designed & wired in accordance with CS-VLA and CS-E through a separate relay/switch so the load can be connected / disconnected by the pilot. It is also thermostatically controlled so average current load is variable depending on the ambient temperature.

Pete · Post by **Pete** » Mon Feb 25, 2008 6:40 pm

Hi Steve,

I take your word for it, certainly you appear to have the evidence.

I notice that Con Air are somewhat more circumspect in the claims for their water heated gizmo - perhaps to fend of future legal issues!!

Do Jabiru have any plans to provide a similar device ?

Brian Hope · Post by **Brian Hope** » Mon Feb 25, 2008 8:16 pm

Jabiru, or at least ST Aviation, has been supplying an electric heater that fits onto the side of the carburettor for two or three years now. Mike Moulai has one fitted to his Jab 2200 powered Kolb and reckons it works very well.

Steve Brown · Post by **Steve Brown** » Mon Feb 25, 2008 10:00 pm

Ref the product liability issues that plague our society and prevent many products coming to market as a result :

Rutan got so fed up with it all that in 1991 he called out a warning placard to be fitted to his homebuilts that said something like

MANDATORY/GROUND Check for proper placards in cockpit. Install "You may die if you fly this airplane" placard.

He does have a good sense of humour!!