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Carb Heat "Flapper" wear

B

Bob Axsom

Well Known Member
After approximately 500 hours I discovered during the annual condition inspection that the the back face of the flapper valve and the underside of the metal top of the filter air box are worn about half way through. I do not know what the failure mode will be except reduction in power when that interface wears through but I intend to coat one surface with Dow Corning 736 (red RTV) at this point to reduce the wear. The hinge is also badly worn and I am replacing it with a stainless steel hinge. There is also a small crack in the filter air box metal top at the fwd L/H edge of the heated air inlet opening that I am working on my repair for (some form of doubler probably involving extruded angle stock). Strong temptation to do away with the whole carb heat system but conscience prevails - so far. Time to sit back and think elegant fix thoughts.

Bob Axsom
 
Interesting, Bob

I wonder where all that metal went.:(
I assume that it is the black (smoker) powder and would stop in the air filter. What is the effect to powdered AL in the engine?
Maybe when it is that fine it just ignites and gives you more power.:cool:

Kent
 
Notes From the Field

Bob Axsom said:
..... .There is also a small crack in the filter air box metal top at the fwd L/H edge of the heated air inlet opening that I am working on my repair for (some form of doubler probably involving extruded angle stock)...
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I just completed the first significant repair since "Darla!" first flew in 2005. At 250 hours, what can be loosely considered the "carb heat" function failed. A 2" long crack was noted at the bend on the FWD R/H side of the VA-130 airbox top plate. Especially noteworthy was the hole in the VA-122 arm was enlargened and the shaft on the Bowden cable bug nut was worn down from its original diameter too! I repaired the local area on the FAB using a combination of .063 angle stock and an .063 strap doubler sandwiching the fiberglass FAB in between the two pieces of repair stock.

I was not too keen to replace the steel VA-122 lever arm because to do so I would have had to virtually disassemble the entire FAB to get at it.

I decided to ream the badly enlarged hole to .280 which was close to what it was anyway, then press fit a .282 O.D. bushing into the hole. I installed a new Bowden bug nut using washers on either side of the "bug nut" and operation is once again normal.

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While I was at it: I have always strongly disliked the stock carb heat muff. It is next to useless. Upon application of carb heat, I never could observe any noticable RPM drop so I removed and replaced it and I am glad I did. Upon removal, I noted that the "muff" left obvious wear marks on the exhaust cross over pipe and conceivably could have eventually worn through the cross over pipe necessating a truly expensive repair, not to mention potential carbon monoxide issues. Piece of junk! I replaced it with an EC-100 SS muff from Wicks. With shipping, it cost $68.43. The subsequent test flight revealed a definite 30-40 RPM drop now but I am still not quite happy. I reviewed the archives on VAF then called and ordered the well regarded 6" heat muff made by Robbins Wings. It's cost with shipping is $45. When it gets here, I'll quickly replace the Wicks unit and note any meaningful improvement in performance.

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Rick6a said:
...the hole in the VA-122 arm was enlargened and the shaft on the Bowden cable bug nut was worn down from its original diameter too! ...I decided to ream the badly enlarged hole to .280 which was close to what it was anyway, then press fit a .282 O.D. bushing into the hole. I installed a new Bowden bug nut using washers on either side of the "bug nut" and operation is once again normal.
Click to expand...

Rick,

I had the same thing, and my fix was to drill another hole farther down on the arm. Now that hole is enlarging, so some more permanent solution is clearly needed.

Any other solutions to this problem?
 
I just inspected this area yesterday, at about 250 hrs tt and there is no problem on my airplane.

One thing I did was fabricate a stiffener for the FAB that attaches it to the cranckase and limits vibration. It runs from the most forward attach bolt up to the crankcase.

Sorry, no photos. I installed it just before my first flight because I had heard that this was a problem area.

Vern
 
ROBBINS WINGS carb heat

Rick:

My build partner insisted on the ROBBINS WINGS carb heat. He was right, it works great.

Barry
Tucson RV9A
 
Thank you all gentlmen

for good info.
Vern can you explain in general what kind of stiffener you installed. Material, appr lenght, what point on the case to attach? I am about to start working on my FAB, already have Vans carb heat kit.
 
Preload helps.....

....if you'll pull the cable housing forward in the Adel clamp to slightly bend the cable. It seems to stop any vibrating motion.

Regards,
 
I too had the hole wearing larger, so I put a piece of reinforced silicone tubing over the lever. The bolt now rides only on the silicone, so it can't wear the lever any more.

At around 1050 hours, the hinge is getting fairly worn though. I'll probably replace it in the next year or so. The flap itself is also wearing a little as Bob described, not sure what I'll do there. It might make sense to make the flapper from some non-metallic material, but I need to think about that a bit. Perhaps reinforced rubber sheeting backed by aluminum? Use the rubber as a hinge? Hmmm.

And yes, I will fall directly out of the sky because the stop nut is not engaging enough threads...

 
When I had the hole problem

It seems like a couple of years ago that I had the hole wear problem. To fix that I had an extra rod end bearing with a threaded stud in the bearing. I made a small fitting out of 1/16" aluminum angle as a link between the control wire and the rod end bearing and hard attached the stud to the lever using the worn out hole, washers and nuts so there is no longer any relative motion in the hole. That has held up well. Taking all of the front end of the FAB to replace the hinge was a bit messy but with stop drilling, doublers, sandwiching and a lot of patience it can be done (was done). Now I will wait a couple of hundred hours and see if the stress has been transfered to the next weakest point. Oh yes, during the rework, the control wire broke off at the attach point on the aluminum link. I fixed that by buying a new ratcheting cable from Aircraft spruce and pulling the wire out of the new housing and using it to replace the old one in the old installed housing. I like Rick's action to get a better carb heat input source.

Bob Axsom
 
Vladyspassky said:
for good info.
Vern can you explain in general what kind of stiffener you installed. Material, appr lenght, what point on the case to attach? I am about to start working on my FAB, already have Vans carb heat kit.
Click to expand...

Vlad, it should be pretty obvious when you look at it. The stiffener can be 0.040 or 0.063 angle. Once your Fab is mounted, you'll see where it goes-- straight up from the front bolt on the Fab attachement straight up to the first crankcase bolt it reaches.

I think that vibration of the FAB is causing the carb heat bushing wear.
 
Here's what I did to modify the flange:


IMG_0728_1.JPG


IMG_0730_2.JPG


IMG_0732_1.JPG


I've also been using the Wicks Muff for about 3 years.

Vern
 
Rick6a said:
2d6s4yt.jpg


2vlmhc5.jpg
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That flange opening is for some extra "cowl air". The two inch scat tube isn't enough. This design change was put into effect, after an early model RV6A had to perform an emergency landing due to potential carb icing. The "road" landing did some wing damage thanks to fence posts. I've flown this particular aircraft, and it has since been repaired.

L.Adamson --- RV6A
 
L.Adamson said:
That flange opening is for some extra "cowl air". The two inch scat tube isn't enough......L.Adamson --- RV6A
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Is that to say you disagree with Robbins Wings advice to RV builders to fabricate a 2" "air tight" flange over the FAB?
 
That 2 inch scatt tube has about 6 square inches for air flow area while the regular input has about 12 square inches of air flow area. A pretty big difference and the extra air sucked in the sides of the flange is also warm air.
 
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RV7AV8R said:
That 2 inch scatt tube has about 6 square inches for air flow area while the regular input has about 12 square inches of air flow area. A pretty big difference and the extra air sucked in the sides of teh flange is also warm air.
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Ooops, the scat tube are is onlt 3.14sq inches of area so the differenece is even bigger 1:4. (I used the circumference instead of area calculation originally)
 
RV7AV8R said:
Ooops, the scat tube are is onlt 3.14sq inches of area so the differenece is even bigger 1:4. (I used the circumference instead of area calculation originally)
Click to expand...
Let me try to put this in a way my few brain cells can process: First, we really do not know how efficient the carb heat muff was or how well it was installed on the RV that was forced down due to "potential" icing and apparently prompted the design change. If the heat muff installed on the subject RV-6A was the marginal design that Van's includes in its FWF kits....well, really no surprise there, is it? As to the use of an air tight 2" scat tube connection as Robbins Wings suggests along with its vastly superior carb heat muff....where icing may occur, is it not better to push (strictly arbitrary numbers used for illustration) 200? air through 3.14 sq inches of area rather than 100? air through an area that is sized 1:4 larger?
 
Rick6a said:
Let me try to put this in a way my few brain cells can process: First, we really do not know how efficient the carb heat muff was or how well it was installed on the RV that was forced down due to "potential" icing and apparently prompted the design change. If the heat muff installed on the subject RV-6A was the marginal design that Van's includes in its FWF kits....well, really no surprise there, is it? As to the use of an air tight 2" scat tube connection as Robbins Wings suggests along with its vastly superior carb heat muff....where icing may occur, is it not better to push (strictly arbitrary numbers used for illustration) 200? air through 3.14 sq inches of area rather than 100? air through an area that is sized 1:4 larger?
Click to expand...

Rick, I don't know about the thermal differences, I am just saying the 2 inch scatt tube doesn't by itself provide as much area to suck in air to the engine than the regular input area of the FAB. In fact about 1/4 the area so some extra input area around the scatt tube sounds reasonable to me.
 
Rick6a said:
Let me try to put this in a way my few brain cells can process: First, we really do not know how efficient the carb heat muff was or how well it was installed on the RV that was forced down due to "potential" icing and apparently prompted the design change. If the heat muff installed on the subject RV-6A was the marginal design that Van's includes in its FWF kits....well, really no surprise there, is it? As to the use of an air tight 2" scat tube connection as Robbins Wings suggests along with its vastly superior carb heat muff....where icing may occur, is it not better to push (strictly arbitrary numbers used for illustration) 200? air through 3.14 sq inches of area rather than 100? air through an area that is sized 1:4 larger?
Click to expand...

I suppose you'd have to "lab" test it...........or have they? As I'm in the HVAC business, I see what happens to furnaces and air conditioners when they starve for air. The furnace limits out, the blower can pulsate, and A/C coils freeze up. I just don't know at what point the benefit of higher heat will outweigh the potentially restricted airflow. As to myself, I'd at least keep the open side flange & see what difference the muffs make.

L.Adamson --- RV6A
 
Does anyone know the diameter of the throat in our MA-4-5? If the area of the scatt tube at 3.14 sq inches is much bigger than the carb intake area than this disccussion is much to do about nothing. My guess only is the carb intake area is about the same size as the scatt tube and I would want the intake area before the carb to be much bigger that the carb throat itself. I notice on Vans muff design the exhaust pipe blocks off part of the scatt tube suction area, and in this version of their muff they have an extra side hole to allow more air in the scatt tube (i am assuming that is why).
 
2" scat with standoff gives a good temp rise.

It works, as you can see in this blog entry I have the Robbins Wings parts, both the carb muff and the scat hose, stood away from the FAB.

In the attached graph you will see good temp rises when I pull carb heat.

I am not sure what rise is considered necessary. If I wanted more I could close off the gap a little more since I have it open on 3 sides. I really want to be sure the engine gets the air its looking for though.
 
Steve, it is interesting to note even with using the stand off and 'three sides open' that there was a slight drop in manifold pressure when you pulled carb heat. Not unexpected since the flow path of the heated air is different when it is pulled. It does suggest to me that an even more restricted flow path would further lower manifold pressure. That may be an insignificant drop but at some point of restricted air intake it may also be significant.
 
I think the MP drop is more the result of the ram effect when the flapper shots the air coming in and the engine has to draw from the lower cowl, than the restriction of small scat.
 
I have the Wicks carb heat muff. It works OK but I have been thinking if it can be improved. This muff covers about 6 to 7"? of the exhaust pipe (see Rick's photo post #4) but has a wide, full length opening to draw in air from the engine compartment. The outlet flange is about half way along the muff on the opposite side to the inlet. I speculate that with carb heat on, most of the air is being drawn over the exhaust pipe adjacent to the muff outlet and that relatively little is being drawn from around the ends of the muff. I have thought about installing a cover (say 2" wide) over the middle part of the inlet (adjacent to the outlet) so as to force the air to enter the muff nearer the ends and then travel over more length of the exhaust pipe before leaving the muff. The only problem I see is that the extra cover may restrict air movement slightly when the carb heat is off possibly causing the pipe to heat excessively. I doubt if this would be an issue as there is still plenty of open area and the gap at the FAB flange would also allow circulation.
Any thoughts would be appreciated.

Fin
9A

Edit. Apologies. I just read the thread again and realized I have drifted off topic.
 
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RV7AV8R said:
Does anyone know the diameter of the throat in our MA-4-5? If the area of the scatt tube at 3.14 sq inches is much bigger than the carb intake area than this disccussion is much to do about nothing.
Click to expand...

There will be static pressure losses along the airflow path, so bascially the throat size isn't the best measure to go by. As an example; going to my HVAC work; it's like comparing the area of a blower cage versus the required duct size. The duct is much larger than the blower opening into a furnace or air handler.

L.Adamson --- RV6A
 
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