Carb Ice and 912 Rotax's

[facthunter]

In Flying "assume" is not a word I'm comfortable with.  If you are not sure check it. Nev

[RFguy]

So the Italians DO have a form of carb heat...  and not cheap. Nice post Danny, very interesting.

[danny_galaga]

1 minute ago, facthunter said:

In Flying "assume" is not a word I'm comfortable with.  If you are not sure check it. Nev

Zactly.

[IBob]

6 minutes ago, danny_galaga said:

Oh, KISS applies here. See attached.

 

Ah, gotcha. The Savannah 'carb heat' just takes air from over the muffler into the airbox, so is divorced from other systems and not at risk of complicating them.

[BrendAn]

1 hour ago, RFguy said:

Like everything Italian. cheap.

Aren't they double the price of a jabiru

[facthunter]

Ferrari's, Lamborghini's, Alfa Romeos, leather shoes. Fine clothes, The Opera.    Nev

Edited December 18, 2022 by facthunter

[Blueadventures]

9 hours ago, BrendAn said:

The tecnam I have been training  in has no carb heat. He has had the plane for years and is on his second engine now. He has never had icing except cold idle 1st start on a cold morning. He just bumps the throttle up a bit for a few seconds and then its fine. The club tecnam is the same. The horsepower loss is negligible. I know it's not my plane but it's real world experience. I would not bother with carb heat if you don't need it. Kiss principal.

Thanks for the model and I read the carb and also cabin heat are listed as an option; therefore a preference choice of the buyer.  Cheers.

[Kiwi]

I know of a Tecnam 2008 that if you pull the knob that is labelled Carb heat, your feet get hot.

[Ian]

1 hour ago, RFguy said:

somewhere around there is a nice FAA chart of likelihood of icing versus temperature and humidity

P1xT1/V1 = P2xT2/V2, Bernoulli and heat of evaporation, explains why icing occurs. But the reason why carbs are significantly more prone to icing than throttle bodies or fuel injection is due to the design of carburettors.

Engines with carburettor have a butterfly valve which reduces airflow and thus reduces power and  creating lower pressure behind the valve.

In addition carburettors utilise the venturi effect to suck fuel into the airstream, further lowering the pressure and associated temperature. https://en.wikipedia.org/wiki/Bernoulli's_principle

Then on top of this is the cooling effect of fuel being vapourised.

This all adds up to a significant cooling of the air below freezing, causing moisture in the air to become supercooled which freezes directly on contact with the internal carb structure.

 

While some carburettor planes are less likely to suffer from carb ice than others, the fundamental design of carburettors makes them susceptible to icing and because fuel delivery can be impacted by small changes in the airflow cause by icing significant power loss can occur without much ice accumulating.

 

This is a flaw of carburettors and it mitigated effectively by carb heat. Going without carb heat or equivalent poses a risk in some weather conditions. 

 

There's a good story on induction heating being required on Ellison throttle bodies in ideal icing conditions over New Guinea which is probably about as bad as it gets.   https://josteve.typepad.com/blog/2010/04/the-puffin-has-landedin-oz.html 

 

[Blueadventures]

1 hour ago, Kiwi said:

I know of a Tecnam 2008 that if you pull the knob that is labelled Carb heat, your feet get hot.

Well get your feet out of the carby throats:)

[Blueadventures]

1 hour ago, Kiwi said:

I know of a Tecnam 2008 that if you pull the knob that is labelled Carb heat, your feet get hot.

Scat hose hookup issue???  Usually two outlets on muffler shroud to two places Cabin heat and Carb heat.  Just a guess,  Cheers.

[Kiwi]

28 minutes ago, Blueadventures said:

Scat hose hookup issue???  Usually two outlets on muffler shroud to two places Cabin heat and Carb heat.  Just a guess,  Cheers.

 

No, the aircraft doesn't have carb heat, or airbox....

Incorrect cable fitted from factory 

[IBob]

AUTO CARB HEAT:
The Savannah XL and S take air in to the airbox via a NACA scoop on the top of the cowling. The airbox has a temperature probe, and I noticed that in a hard climb, the airbox temperature would rise.
The reason was that the increased angle of attack altered the airflow through and round the cowling with warm undercowl  air being taken at the 20-25mm gap between the NACA scoop fibreglass and the airbox inlet, instead of cold air from the NACA scoop.

This was easily fixed by extending the airbox inlet with an aluminium tube, held in place with a hose clamp.
I left a gap of approx 3mm to allow drainage in the event of rain while parked.
The extension is marked with a black felt pen line at the RH end of the airbox.
(The scat tube is warm air from the muffler for those times when you do actually want carb heat).