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Airbox Carb Heat Plumbing Question

647jc

647jc

Well Known Member
I'm installing the standard Van FAB on an O320 RV9A. Have read in the forums that just drawing in warm air from inside the cowl didn?t work very well so I purchased an EC100-020 heat muff from Wicks per forum recommendations. The muff fits over the exhaust crossover and the design is such that air must be ?sucked? through it.

I?ve noticed several pictures on the forum showing similar muff installations, some using Vans Carb Heat muff of similar ?sucking? design but then connecting the muff to the air box using Vans Carb Heat connector which has about a ?? standoff ?to allow hot air to escape when carb heat is not selected and allows full operation of lever opening alternate air door? according to Van.

For a pressurized muff installation the stand-off would definitely be desired but for a 'sucking' muff design the ? inch standoff would really impede the air box from sucking through the muff so I wonder if this combination of components is a very good idea. I?m thinking of using a regular tight fitting aluminum duct fitting on top of the air box but am concerned with interfering with the carb heat flap / door. I'm also concerned with restricting the engine breathing through a 2" scat tube when carb heat is fully applied.

Would be interested in comments and pictures of alternative installations.
 
647jc said:
I'm also concerned with restricting the engine breathing through a 2" scat tube when carb heat is fully applied.
Click to expand...

According to some, such as the NTSB, the 2" scat tube isn't enough. That 1/2" standoff, will also let additional cowl air in. I'd leave it.

L.Adamson
 
I have the same carbheat muff you speak of on my 0320 E2D in my 6A. The proof is in the pudding, in that on run up, I see a 60 RPM drop when carbheat is applied. I would leave it as is IMHO. Works great.

Regards,
 
Fin,

Yes, those were some of the posts that concerned me. The NTSB report that the 2" scat tube was not large enough to remove the carb ice (not enough heat, air flow, didn't really say). I'm assuming the NTSB concern was the 2" hose tightly sealed to the airbox did not allow enough air flow to the engine but I really don't know. Correction the NTSB report stated the 2" hose was NOT tightly sealed but installed per Vans recomendation with a stand-off at gap at the air box. I see others are still installing the heat tube tightly sealed to the airbox so there is definitely some confusion or at least varied interpretation of the subject. Anyway, I have decided to use the Van airbox connection with the 1/2" standoff. This configuration with a heat muff will be better than just drawing warm air from inside the cowl but will not provide air as warm as a tightly sealed connection but at least should not starve the engine of air either.
 
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Carb Heat

During my last oil change I noticed 3 of the 4 spot welds were broken on the flange that the SCAT tube connects to on top of the FAB. I have the same setup utilizing the heat muff from Wicks. When the flange was being repaired I also noticed the extruded hinge was getting worn. The hinge was worn to the point it was difficult to close the door when moving by hand. It didn't take very long to make the repairs. The picture was taken a long time ago; it identifies the welds I'm referring to.

The plane has been flying for a little less than 3 years, 400 hours or so.

flangevi1.jpg


Gary Kremers
N715AB
 
Gary,

Your picture of the tube flange that developed the cracked welds is the same flange I got with my baffle kit (old style). The pictures at Vans web store for the airbox flange and the baffle flange both show a different design for the flange, it shows a plate with a hole & sort of rim with a 2" tube pressed through it, don't see any welds on it, wonder if this is a new or very old style device they are showing. Looks to be a better design to me than the welded version, hopefully the picture is not from a very old design they no longer sell.

cat-med_carbheat-connector.jpg
 
Same design

Joe,
The replacement part recieved from Vans (1/16/2008) has the same welds as the original.

Gary
 
Give Rick a call at Robbins Wings. His setup is very simple and is very much like vans, however it does suck all the air through the two inch scat, but it is way hotter. In my opinion Vans setup is on the cool side, so I ordered one for my 7. I don't think that the 2" scat would pose a problem, as we only use carb heat very sparingly.
 
jrvssgl said:
Give Rick a call at Robbins Wings. His setup is very simple and is very much like vans, however it does suck all the air through the two inch scat, but it is way hotter. In my opinion Vans setup is on the cool side, so I ordered one for my 7. I don't think that the 2" scat would pose a problem, as we only use carb heat very sparingly.
Click to expand...

A few questions come to mind, when using the 2" scat tube only; with no additional air from the mounting flange.

First; the scat tube with an area of approx. 3.14 sq. inches, is about 1/2 the area of the ram inlet.
The scat tube is also ribbed, which causes an additional loss of airflow due to friction. It's also not getting the effect of ram air, and forces a 90 degree change in airflow direction, which farther deceases the amount of air available to the engine.

The air would indeed be "hotter" because of a possible improvement in muff design; the loss of mixing somewhat cooler cowl air through the now closed flange sides; and the fact that the "restricted" airflow will absorb heat faster.

What I don't know, is the engines exact needs of air flow under various operating conditions, including a partially blocked carb due to icing. I don't know the point, at which the engine is starting to starve for air, based on the inlet sizing.

What I do know, is that the 2" scat tube only, is at least 50% restrictive, compared to the ram inlet. I also know that an inspector of a 1999 RV6A accident, felt that the 2" scat tube wasn't enough air to melt carb ice; but I don't know the full particulars.

L.Adamson
 
Advised to change to closed flange.

I just want to point out that a couple of you in the forum are making a decision based on false facts.

The NTSB inspector clearly says that the problem was because of the inability to clear ice as it was a stock vans settup, thus being an open flange.

Here is what he said.

According to the Van's Aircraft construction and operating manual, one method of building the
internal carburetor heat system is to "run a 2-inch air hose from a heat muff and position it to
feed into the alternative air inlet of the carb[uretor] air box without being attached and closed."
According to the FAA inspector who examined the aircraft following the accident, this is the
method by which the pilot constructed the airplane. According to the inspector, the 2-inch hose is
not large enough to adequately supply enough heat to the carburetor to sufficiently melt the ice.
The aircraft was issued an airworthiness certificate by the FAA on August 24, 1995.


In my opinion I believe that installing a closed conector on the airbox is a better option. Even though the two inch scat can appear restrictive, it will probably run the engine just fine, and deliver more heat to where it is important. We don't fly on carb heat all the time, just when we need it, so I do not think that being restrictive is the issue. It will not harm the engine being restrictive, it will just run a little richer.

Another point is that if you have developed carb ice, your carburator is already restricted. You want to hit that ice with as much heat as you can to clear the ice. Having an open FAB adapter, with a partially blocked carb might not allow enuf suction from the heat muff to melt the ice. This to me sounds like exactly what the inspector was trying to say above.
 
Jarvis,

You are correct; I misinterpreted the NTSB report and have corrected my earlier post. So, in my opinion I guess the issue is still up in the air.

1. Should I disregard Vans recommended installation instructions that use a stand-off gap connecting the scat tube to the airbox based on the NTSB report?

2. If I establish a tight seal of the scat tube, will the 2" tube provide enough air flow to an engine experiencing carb icing to keep it running?
 
Airflow from carb heat ... Could be tested

647jc said:
2. If I establish a tight seal of the scat tube, will the 2" tube provide enough air flow to an engine experiencing carb icing to keep it running?
Click to expand...

Joe, would that not be easy to test? I believe Vern Little has blocked off the side openings of his carb heat attachment. A quick flight, and then test carb heat on, 75% power, see if how much EGT changes. Maybe even try 100% for short while, and monitor EGT.

Maybe Vern could chime in, or someone else that has blocked the side openings on the carb heat scat tube support of the air box.
 
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Is it false?

jrvssgl said:
I just want to point out that a couple of you in the forum are making a decision based on false facts.

The NTSB inspector clearly says that the problem was because of the inability to clear ice as it was a stock vans settup, thus being an open flange.

Here is what he said.

According to the Van's Aircraft construction and operating manual, one method of building the
internal carburetor heat system is to "run a 2-inch air hose from a heat muff and position it to
feed into the alternative air inlet of the carb[uretor] air box without being attached and closed."
According to the FAA inspector who examined the aircraft following the accident, this is the
method by which the pilot constructed the airplane. According to the inspector, the 2-inch hose is
not large enough to adequately supply enough heat to the carburetor to sufficiently melt the ice.
The aircraft was issued an airworthiness certificate by the FAA on August 24, 1995.


In my opinion I believe that installing a closed conector on the airbox is a better option. Even though the two inch scat can appear restrictive, it will probably run the engine just fine, and deliver more heat to where it is important. We don't fly on carb heat all the time, just when we need it, so I do not think that being restrictive is the issue. It will not harm the engine being restrictive, it will just run a little richer.
Click to expand...

I'm going to farther confuse the problem. I know that plane in this NTSB report and have flown in it. But I haven't seen the owner for quite a few years since a fly-in, where the plane had been re-built.

For some reason in the back of my mind, I thought the flange with air gap was added, after this accident. But I'm no longer sure, as it's too long ago. However, I did revue my early ninties George Orndorff construction tapes for his RV6A. His plane had a horizontal carb heat muff, and a flange with "no" gap.

I've also went through years of the Matronics archives, but havn't come to a conclusion; although the RV6A accident was mentioned several times. Within the archives, a few had closed up the sides, and felt it worked okay. Others felt that the open flange was working fine. The date of these posts were around 2001, and that icing accident occured in 1999.

Therefor, was the flange added "after" the icing accident, where the inspector felt the 2" scat tube was insufficiant..............or not?

L.Adamson
 
OK, I wasn?t going to mention this and risk starting some conspiracy theory but might as well add a little mystery to the discussion and perhaps confuse the matter even more. The published NTSB current synopsis for this incident reads as follows:

The Van's Aircraft construction and operating manual states that one method of building the carburetor heat system is to 'run a 2-inch air hose from a heat muff and position it to feed into the alternative air inlet of the carb[uretor] air box,' which is the method by which the pilot constructed the airplane. According to the inspector, the 2-inch hose is not large enough to adequately supply enough heat to the carburetor to sufficiently melt the ice.

The NTSB quote posted in the forum is from the Factual report and reads:

According to the Van's Aircraft construction and operating manual, one method of building the internal carburetor heat system is to "run a 2-inch air hose from a heat muff and position it to feed into the alternative air inlet of the carb[uretor] air box without being attached and closed." According to the FAA inspector who examined the aircraft following the accident, this is the method by which the pilot constructed the airplane. According to the inspector, the 2-inch hose is not large enough to adequately supply enough heat to the carburetor to sufficiently melt the ice.

Notice the words ?without being attached and closed? are either missing from the NTSB synopsis or have been added to the NTSB factual report. Other than those additional 5 words the descriptions are most identical.

See NTSB ID: DEN99LA062

I sent an email to Vans this afternoon asking for advice and will pass that on when I get it. Thanks to all who have contributed to the discussion.
 
647jc said:
I sent an email to Vans this afternoon asking for advice and will pass that on when I get it. Thanks to all who have contributed to the discussion.
Click to expand...

I've been reading some more Matronics archives from the mid ninties era, and builders were wondering how to attach a scat tube, since the carb heat door lever was in the way. The majority of my plane was purchased in 1996, but my air box & carb heat attachment including the open flange was purchased a few years later.

L.Adamson
 
Lycosaurus said:
Joe, would that not be easy to test? I believe Vern Little has blocked off the side openings of his carb heat attachment. A quick flight, and then test carb heat on, 75% power, see if how much EGT changes. Maybe even try 100% for short while, and monitor EGT.

Maybe Vern could chime in, or someone else that has blocked the side openings on the carb heat scat tube support of the air box.
Click to expand...


I'll let you know on Wednesday. It's still the middle of winter up hear (which means rain and low stratus), but we are projected to have some clear weather later this week.

Vern
 
Please keep it coming guys. This is a confusing matter indeed. I am at that point in my build and was planning a closed flange. I guess I am on to something else until I can get more info and make the decision of what to do. I am looking forward to Vern's flight test result. Thank you to everyone who takes the time to research and post good information.
 
Vans recommended configuration

In response to a couple inquiries regarding the airbox carb heat configuration, I received the below two email responses from Bruce Reynolds at Vans;

"The alternate air door on the top of the air box was originally there without
any heat muff of any kind. The theory was that warm air in the lower half of
the cowl was adequate to prevent carb ice. It was never thought to be warm
enough to melt ice in the carb. Since then we have offered a small muff and
connection to enhance the heat supplied to the carb. It is still our contention
that this is to be used to prevent carb ice, not melt ice that has already
formed. Since the heat door does not bypass the filter, you are always
getting filtered air unlike the carb heat system on some factory planes. It is
OK to fly with the carb heat on any time that you think that there may be a
chance of carb ice."

"Yes, the Vent DL-07 is to be used to connect the EA CARB HEAT MUFF with a short piece of 2" SCAT tube. That is the only combination of parts that we have for that purpose. This arrangement was never intended to provide hot air to melt ice and should not be used for IFR operations."

So, the configuration I will be using is with the standoff connection rather than an air tight seal of the scat tube to the airbox. My RV9A will not be IFR so this configuration should be in compliance with Vans recommendations. We should acknowledge Bruce making the point that this configuration is not intended to melt ice but rather to prevent ice. Good information to know, don?t wait until you get icing before you turn the heat on.
 
jrvssgl said:
Please keep it coming guys. This is a confusing matter indeed. I am at that point in my build and was planning a closed flange. I guess I am on to something else until I can get more info and make the decision of what to do. I am looking forward to Vern's flight test result. Thank you to everyone who takes the time to research and post good information.
Click to expand...

Here's the data from my test flight today:

O-320 A1AC2, dual mags. Wicks heat muff, sealed FAB flange.
Sea level temp/dew point (degrees C) 5/2
Altimeter 30.22 inHg
75% power 1800'. Carb temperature:65F, with carb heat 77F (12F rise)
MAP 24.4, RPM 2390. 50 rpm drop when carb heat applied. Peak power mixture.

45% power 1800'. Carb temperature 74F, with carb heat 87F (13F rise)
MAP 21.4, RPM 2090. 40 rpm drop when carb heat applied. Peak power mixture.

I don't have definitive data from before modification to the flange, but I recall about 7F rise.

Key point to note: rpm drop is only about 50 rpm, and does not appear to be due to the restriction of the 2" SCAT duct. Evidence is that at lower power settings the drop is similar, so my conclusion is that it's the mixture change that reduces the rpm. I didn't try re-leaning.

One more note: it takes about 1 minute for the temperature to stabilize after applying carb heat. In addition, the carb heat knob must be held out, because it has a tendency to creep open. My next mod will be to put a friction lock of some kind on the control.

Vern
 
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carb temps

I fly my RV6a in the clouds routinely and the carb heat issue has been a concern. I usually just pull full carb heat anytime the carb temp gets close to freezing to allow for some error. The standard Vans setup will raise the temp 10-15 degrees in cruise flight which is often not enough to raise the carb temp above freezing. I have never experienced carb ice in this aircraft but sure am interested in what Joe and others are finding.
 
and for what it is worth

On the helicopters we fly, carb heat is an issue. A tightly sealed system with three inch scat tube directly into the airbox will raise the carb temp 60 degrees or more.
 
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