Airbox Carb Heat Plumbing Question

647jc · #1 02-10-2008, 12:46 PM

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.

L.Adamson · #2 02-10-2008, 12:54 PM

Quote:

Originally Posted by 647jc

I'm also concerned with restricting the engine breathing through a 2" scat tube when carb heat is fully applied.

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

painless · #3 02-10-2008, 12:59 PM

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,

Finley Atherton · #4 02-10-2008, 02:20 PM

Joe,
See http://www.vansairforce.com/communit...ad.php?t=25856 post #16 and a few others after for more information.

Fin
9A Flying

647jc · #5 02-10-2008, 03:00 PM

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.

GaryK · #6 02-10-2008, 04:13 PM

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.

Gary Kremers
N715AB

647jc · #7 02-10-2008, 04:54 PM

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.

GaryK · #8 02-10-2008, 05:25 PM

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

Gary

jrvssgl · #9 02-10-2008, 06:30 PM

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.

L.Adamson · #10 02-10-2008, 09:37 PM

Quote:

Originally Posted by jrvssgl

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.

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