Carb heat

[kookaburra]

Hi everyone,

New RV7a owner here and still a beginner with building and maintenance. I am to add the carb heat attachment in this photo with the duct flange and Exhaust Heat Muff attached to the FAB. The RV I bought doesn't have this and it's from 2010, but I couldn't find any references to the installation in the builder manual or the revisions (https://www.vansaircraft.com/service-information-and-revisions/filtered-air-box-fab-instructions/)

Could anyone guide me on where the instructions are / how to put this in?

Thanks!

 

[PaulvS]

Hi everyone,

New RV7a owner here and still a beginner with building and maintenance. I am to add the carb heat attachment in this photo with the duct flange and Exhaust Heat Muff attached to the FAB. The RV I bought doesn't have this and it's from 2010, but I couldn't find any references to the installation in the builder manual or the revisions (https://www.vansaircraft.com/service-information-and-revisions/filtered-air-box-fab-instructions/)View attachment 70268

Could anyone guide me on where the instructions are / how to put this in?

Thanks!

The instructions for the carb heat door are on page 4-5 of the above linked document, though there isn't a lot of detail or a diagram, but you can work it out as you build it.
IMHO the design is kinda ineffective because of the gap between the duct flange and the top of the air box. It is just going to allow unheated air to be sucked in.
Some builders, myself included, make a different arrangement for the connection to the FAB and I can post a pic if there is interest.

 

[blaplante]

The effectiveness is certainly influenced by how tightly the door's baffle fabric seals into the normal intake. Most that I've seen don't seal well at all. Then cold ram air just blows in.

 

[Bonhomme]

The instructions for the carb heat door are on page 4-5 of the above linked document, though there isn't a lot of detail or a diagram, but you can work it out as you build it.
IMHO the design is kinda ineffective because of the gap between the duct flange and the top of the air box. It is just going to allow unheated air to be sucked in.
Some builders, myself included, make a different arrangement for the connection to the FAB and I can post a pic if there is interest.

I am interested in your solution.

 

[PaulvS]

I am interested in your solution.

The first pic is of the Van's DL07 duct flange with standoffs for the hot air connection to the FAB, which I did not use . The standoffs are to allow the flapper door actuating arm to move and the opening is 2.5" x 5/8", both sides, so area 3.13 sq inches and that allows unheated air to enter. The SCAT tube is 2" dia, area 3.14 sq inches for heated air to enter. So 50% of the opening is going to let in cold air and not hot air.



I fabricated a new actuating arm that flits through a narrow slot in the top of the FAB, so that the duct flange for warm fits directly onto the box without any significant gap for cold air to enter.



The other part that differs from the plans is a removable nose piece and use of flexible SCAT for ram air induction. This change is to allow for better relative movement between the engine and the cowl so as to put less stress on the carburetor from the FAB.

 

[Tdeman]

The instructions for the carb heat door are on page 4-5 of the above linked document, though there isn't a lot of detail or a diagram, but you can work it out as you build it.
IMHO the design is kinda ineffective because of the gap between the duct flange and the top of the air box. It is just going to allow unheated air to be sucked in.
Some builders, myself included, make a different arrangement for the connection to the FAB and I can post a pic if there is interest.

Mine is built to plans, and I would not agree with it being “ineffective”. I remember thinking it wouldn’t work while building, but the reality is that it works well. I see about a 20 deg F increase in carb temp when operating around freezing level.

When flying home in November, racing the “atmospheric river” back over the Cascades, I started to notice occasional and slight hiccups, and carb temp hovering around 30. Carb heat on full, cleared it up and had the carb back up at 50 deg F for the remainder of the flight home.

 

[PaulvS]

Mine is built to plans, and I would not agree with it being “ineffective”. I remember thinking it wouldn’t work while building, but the reality is that it works well. I see about a 20 deg F increase in carb temp when operating around freezing level.

When flying home in November, racing the “atmospheric river” back over the Cascades, I started to notice occasional and slight hiccups, and carb temp hovering around 30. Carb heat on full, cleared it up and had the carb back up at 50 deg F for the remainder of the flight home.

Nice pic and you wouldn't want a forced landing in that territory...

Local builders who have used a carb heat system that delivers 100% of the air from the heated duct have experienced at least 50 degree F increase in carb temp when used. Carb ice can form in many situations even if it is not near freezing temp outside. I went with this solution because I had the choice and I'm sure that other RV pilots are probably happy with their solutions.

 

[Tdeman]

Nice pic and you wouldn't want a forced landing in that territory...

Local builders who have used a carb heat system that delivers 100% of the air from the heated duct have experienced at least 50 degree F increase in carb temp when used. Carb ice can form in many situations even if it is not near freezing temp outside. I went with this solution because I had the choice and I'm sure that other RV pilots are probably happy with their solutions.

Thanks. I sure didn’t mean to put down any other solutions, I was just chiming in that the standard configuration works better than I expected to.

From my relatively narrow experience, my carb temp never seems to drop below about 29 deg F (granted, the coldest OAT I’ve flown in is 10 degrees F). So the 20 deg increase always seems like plenty.

I have two questions about your setup. Did you increase the size of scat tubing, and what type of manifold pressure change do you see when applying carb heat? It is my understanding that the open sides per the plans are designed to allow airflow above what the 2” scat can provide.

 

[Dan 57]

My carbed O-360 engine, standard FAB/carb heat, has now close to 2.2Kh. During the 1'900 (bought the aircraft around 300 hours old) I've flown her, some were in dire conditions... all T ranges, many different kind of clouds, light rain, torrential rain, light freezing rain, fog, many times in moderate icing. By which I mean windshield iced over, and up to 2cm ice on the leading edges. Never had carb ice during those encounters...
I've had the engine starting to stumble, then pulled the carb "heat", engine ran normally instantly. No variation in MP before or after the action. I strongly suspect ice building up on the ram air inlet to the point of blocking it, selecting carb "heat" effectively acting as alternate air. This has happened 3 times in a vast amount of hours flown in carb ice prone weather.

Beware that this has only been my own experience during this time, with a pretty standard configuration, YMMV!
Used to have a carb T probe/indication, this has been ditched in the meantime as there's no purpose really. Sporting the standard Mickey Mouse carb heat system, testing shows a minute 10-20 RPM drop.

 

[PaulvS]

Thanks. I sure didn’t mean to put down any other solutions, I was just chiming in that the standard configuration works better than I expected to.

From my relatively narrow experience, my carb temp never seems to drop below about 29 deg F (granted, the coldest OAT I’ve flown in is 10 degrees F). So the 20 deg increase always seems like plenty.

I have two questions about your setup. Did you increase the size of scat tubing, and what type of manifold pressure change do you see when applying carb heat? It is my understanding that the open sides per the plans are designed to allow airflow above what the 2” scat can provide.

I retained the 2 inch SCAT tubing between the heat muff and the FAB. The original "stove pipe" type heat muff had a slot cut in it to reduce the constriction caused by the close fit around the exhaust pipe, per my understanding.

Crossover Type Exhaust Heat Muff

Straps on Crossover Type Exhaust

store.vansaircraft.com

I don't yet have any performance data for my own installation (not flying yet) but other builders who I copied reported that manifold pressure drops slightly when carb heat is selected. Some of the drop is probably related to ram air.

It is interesting that thers is a range of carb heat solutions that seem to work well enough. The original plans did not depict carb heat and the parts were optional. The flapper door in the FAB would have functioned as an alternate air door that allowed warmer air from inside the lower half of the cowl to enter the carb. I understand that RVs with Lycomings are generally not prone to carb icing.