carb heat //was//Re: Which engine?

[Aerodude45]

Why isn't carb heat needed with dual mikunis? Is there not enough of a pressure-drop in that set up to cool the air to the point where it's an issue during "flying season" OAT? What about for flying in colder temps (30-50 Deg F)?

[Dan_]

Why isn't carb heat needed with dual mikunis? Is there not enough of a pressure-drop in that set up to cool the air to the point where it's an issue during "flying season" OAT? What about for flying in colder temps (30-50 Deg F)?

The "conventional wisdom" has been that the carb is close enough and behind enough of the hot head.  

I have my doubts, but there have been no efforts to document the actual temperatures at the venturis that I know of.  

Spring time is the worst around here when cool temps and moist air are prevalent most all day.  

I would urge extra caution running the engine at low RPM in cool moist air.  Especially after an extended cruise.  

A high compression engine with a light wood prop is very easy to stop...

Either the conventional wisdom in the case of 1/2 VWs with 2 carbs is factual, or everyone has been lucky apparently.

We had a very high time Cessna 180 pilot here who had the carb heat disconnected, and regularly flew jumpers that way.

He always took off on the pavement and landed on the grass between the runway turn-offs.  

He never had a problem because he never had to add power during a landing until the beautiful spring day he overshot the grassy spot.

His disbelief in carb heat was fallacy, but he had been "lucky" up till this point.   It cost him a plane worth at least 50,000...

Steve once posted a low cost carb temp device.  I guess no one ever built one.

I will look for the post.

[Aerodude45]

Thanks a lot! Yeah, I never had trouble with my last 1/2 vw with dual carbs, but I always wondered about that. I'd be VERY interested in rigging up a carb temp probe, though! I live in Wisconsin, so I have no shortage of cold, wet weather for testing.

[Dan_]

So far, haven't been able to find Steve's post on the "neo yahoo" site.  I sent him a PM about it.  

I did find something interesting however...

Looks like a one wire hook-up to a digital voltmeter will give a reading for $9.95.   There is one with a better coating on the wire on down the page for $14.95

http://www.adafruit.com/category/56   (third one down the page)

[Aerodude45]

I could certainly rig that up to my carb, probably even before starting engine testing. I'm actually trying to use a microcontroller (probably arduino) to datalog temps. I'll prioritize carb air temp and get the stuff I need this month.

[Steve]

So far, haven't been able to find Steve's post on the "neo yahoo" site.  I sent him a PM about it.  

I did find something interesting however...

Looks like a one wire hook-up to a digital voltmeter will give a reading for $9.95.   There is one with a better coating on the wire on down the page for $14.95

http://www.adafruit.com/category/56   (third one down the page)

The Temp read on this Harbor Freight instrument might do: http://www.harborfreight.com/lcd-automotive-multimeter-with-tachometer-kit-95670.html

[Dan_]

I could certainly rig that up to my carb, probably even before starting engine testing. I'm actually trying to use a microcontroller (probably arduino) to datalog temps. I'll prioritize carb air temp and get the stuff I need this month.

Cool..!  

That adafruit.com site seemed to have a lot of arduino stuff.  

Maybe keep poking around there, and keep us posted on what you decide to go with.

BTW, while searching posts I ran across one from Steve pointing out that he had made his head cooling shroud direct heat specifically on the carb body...

[Dan_]

The arduino has a temp/humidity sensor for $2.50.  Project link here...  There is an Oled screen available which is supposed to be many times more viewable in daylight than an LCD display

I just checked the Metars for the local airport here.  Temp is 94 and dew point is 65.   The chart below shows I am at the border line for icing both at cruise and glide power, but just into serious icing at glide power.  

Point being, it can happen anytime --any where if you don't mitigate the hazard.

Following the relative humidity line down gives about 50%.   Some of these charts have a rain, fog and clouds graphic on the 100% relative humidity line. 

William Wynne shared his carb ice incident so that all can learn from it...  http://www.flycorvair.com/carbice.html

[Dan_]

More Arduino...  specifically how to do up the sensor suitable to read carb body temp.  You can connect 4 to the same wire and read them all on a 4 line LCD...

https://arduino-info.wikispaces.com/Brick-Temperature-DS18B20#TS




Also this bit of info quoted from the FAA "Pilots handbook of Aeronautical knowledge".

Carburetor Air Temperature Gauge
Some aircraft are equipped with carburetor air temperature gauge, which is useful in detecting potential icing conditions.   Usually, the face of the gauge is calibrated in degrees Celsius, with yellow arc indicating the carburetor air temperatures where icing may occur.   This yellow arc typically ranges between -15 °C and +5 °C (5 °F and 41 °F).   If the air temperature and moisture content of the air are such that carburetor icing is improbable, the engine can be operated with the indicator in the yellow range with no adverse effects.    If the atmospheric conditions are conducive to carburetor icing, the indicator must be kept outside the yellow arc by application of carburetor heat.    Certain carburetor air temperature gauges have red radial, which indicates the maximum permissible carburetor inlet air temperature recommended by the engine manufacturer.    If present, green arc indicates the normal operating range.   

Outside Air Temperature Gauge 
Most aircraft are also equipped with an outside air temperature (OAT) gauge calibrated in both degrees Celsius and Fahrenheit.   It provides the outside or ambient air temperature for calculating true airspeed, and also is useful in detecting potential icing conditions.

Perhaps I can let it rest now...

[Aerodude45]

There is a lot of engine data I'd love to be able to record using stuff like this (EGT, CHT, Oil Press, CAT, OAT, and % humidity). I'm undecided as to whether I want all of that data staring me in the face on digital displays when I'm flying, but trying to set something up to do it would be fun and a great way to gather test data cheaply. I'll have to continue research to determine if I can find sensors designed for these applications, but for now I think I'll buy an arduino, OLED display, and temp sensor. If I can make a minimum viable product out of that, I'll start adding features to it later.

William Wynne's straightforward approach to aviation safety is awesome. That article was very informative.

Yes, Carb ice can and does happen even when you don't suspect it. I'll try my best to test whether the conventional wisdom on a 1/2vw is true, or if we're all just lucky. If I don't get the arduino set-up working, I'll try the other possibilities listed here. I'd be interested in exploring whether I can do better than carb body temp, and drill/ tap a hole in the top of the carb just past the throttle slide so I could install a temp probe flush with the inner wall. Would have to be done very carefully, and the probe itself would have to be in a threaded housing. Also, it may call for safety-wire, as a temp probe falling into the carb and blocking airflow would be a bad deal.

My intention is to be able to provide some sort of carb temp alongside OAT and % humidity. I would also like to try running the engine on the ground under conditions most likely to cause icing to determine if and when it happens. 

[Aerodude45]

Just a quick update. My engine runs and I will be recording CAT through some means soon in the Wisconsin winter. With El Nino this year the temperature is higher than usual, but it may actually be in the sweet spot for carb icing more of the time.

[Dan_]

 My engine runs and I will be recording CAT through some means soon in the Wisconsin winter. 

Congrats!  Send some pics or video.  Go-pro had to go to 4k to stay ahead of the cell phones...

[stevejahr]

Note that carb body temperature and air temperature are really only secondary indicators here.  Temperature drops as pressure drops, thus air temperature will drop across the venturi of the carb body/slide proportional to the pressure drop.  Temperature also drops due to evaporation of the fuel emitted into the airstream through the carb.  So you have two different affects contributing to the carb icing phenomenon.

Personally rather than spend a bunch of time rigging up sensors and trying to predict carb icing I would rather just make the big jump to multi-point EFI and eliminate the entire issue.  A Megasquirt to control fuel and spark could be a beautiful thing.

[Dan_]

So you have two different affects contributing to the carb icing phenomenon.  

Yes and they both combine to read on a carb body temp gauge...  

This information and current humidity will allow you a margin of safety you may need, especially using car gas.

William Wynne had carb ice in Florida, fortunately he lived to tell about.