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whifof100ll

Well Known Member
I had the same problem Chris Pratt and Paul Dye had others have with FAB and cowl interference described here
http://www.vansairforce.com/community/showthread.php?t=26129

I decided to remake the FAB top plate by starting with raw material. When removing the top plate, I found cracks in it just outside of all 4 bolt heads. If left unattended, the air box may have eventually fallen off the engine.

I am an engineer and I have also had several colleague with metallurgical experience inspect the cracks. We agree that the cracks are fatigue type cracks and we do not believe that they were caused by the interference. I have just over 200 hours on my RV-6. I made the plate from 6061 instead of 2024, which is what Van's provides.

Yesterday I visited a friend that has an RV-7 with about the same amount of time on it. Strangely enough he was making a new top plate because his original was cracked. He had also doubled his up near the induction opening. I think I like his approach better.

Generally, when cleaning the air filter I may not have looked closely at the top plate to check for cracks. I will now and I encourage others to do the same.
 
Do you have any pictures Dale? Please post some. I am finishing my FAB now and it would be great to see useful modifications. I am also making extra support attached to engine sump as Vern did.
 
100 hours

I built my original top plate from scratch to avoid interference with the cowl. 100 hour inspection this week revealed major cracks at all 4 bolts and no sign of contact with the cowl.

Andy
 
I wonder if the contact area might be at the FAB-to-lower-cowl carb opening as opposed to someplace further aft. I may also check for diminishing FAB clearances as the engine mounts age.

Steve
 
Pics

Vladyspassky said:
Do you have any pictures Dale? Please post some. I am finishing my FAB now and it would be great to see useful modifications. I am also making extra support attached to engine sump as Vern did.
Click to expand...


Unfortunately I did not think to take pics. Next time I pull the airbox I will take some and post them.

Just a reminder to make sure you get drilled head bolts for the airbox top plate and safety them. This was something I overlooked initially.

Regards,
Dale
 
Steve said:
I wonder if the contact area might be at the FAB-to-lower-cowl carb opening as opposed to someplace further aft. I may also check for diminishing FAB clearances as the engine mounts age.

Steve
Click to expand...

In my opinion Steve's thought is at least part of the issue.
Think about this...
The engine moves around but the lower cowling does not. Since the airbox is attached to the engine it has motion relative to the cowl inlet.

I have seen many, many, different airbox to induction inlet seal installations on RV's (every one is of course trying to get the best induction efficiency that they can). Many of them are tight enough for teh cowl to be resisting teh motion of teh engine though the airbox. Guess what part of the airbox is going to loose this battle.
I am not saying that this is the cause of all airbox mounting plate cracks (their are numerous other factors that can come into play), But I think it is the reason for a lot of them.
Something to consider...
 
Here is a picture of the FAB plate on my Airflow Performance setup at about 400 hours:



The arrows point to the important locations. There is a relatively sharp edge right there on the AFP mount, which clearly causes a stress riser. I replaced this anodized one (which is also problematic for fatigue cracking...) with another 2024 plate. That one also went about 400 hours and cracked similarly. I then made some custom little brackets which could be rounded off nicely. They also distribute the load a lot better. I'm at around 300 hours on that latest mod, we'll see how that goes.

Most of the RV's I know of have this problem at several hundred hours. Just keep an eye on them.
 
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I've cracked 3 so far...

I check the FAB mounting plate everytime the cowl comes off. Cracks look exactly like those in the posted photo. I, too, attribute the cracking to fatigue due to vibration. I hypothesized that the airbox seal transmitted high loads to the plate due to the relative motion (vibration) between the engine and the cowl. After the most recent cracking incident, I shortened the FAB by about 1/2" and replaced the rubber seal material with a section of SCAT tubing in an effort to "soften" the joint. This set-up has resulted in the longest plate life (no cracks so far), but it is too soon to tell for sure.

Dean Pichon
Bolton, MA
RV-4 - flying since 2001
 
I havent seen cracks yet on my mounting plate, but what IS happening is my FAB is being pulled off of the bottom of the AFP (the intake). The first time this happened I thought that I just had not snugged down the clamping bolt on the donut-shaped retaining ring that keeps the FAB attached to the AFP throttle body. But it has happened again - it becomes apparent on pre-flight because I can see the misalignment between the cowl inlet hole and the FAB inlet. Not sure what to do about this issue - maybe do what Dean did and use some scat. Dean, do you have any pictures? How is the transition done betwen the scat and the FAB, and also between the scat and the cowl inlet?

I recall that Vans instructions advised the builder to take the time to make a nice tight fitting interface between the FAB and cowl so as to maximize air intake ( a bit of ram air effect), but I think my tight fit has come back to haunt me.

erich
 
My FAB

At 526 Hours:

Note that black lines are also cracks

airbox.jpg


Solution:

Doubler plate and brace
No more problems since. Now at 1380 hours


exhaust_brace.jpg
 
FAB slippage

erich weaver said:
I havent seen cracks yet on my mounting plate, but what IS happening is my FAB is being pulled off of the bottom of the AFP (the intake). The first time this happened I thought that I just had not snugged down the clamping bolt on the donut-shaped retaining ring that keeps the FAB attached to the AFP throttle body. But it has happened again - it becomes apparent on pre-flight because I can see the misalignment between the cowl inlet hole and the FAB inlet. Not sure what to do about this issue - maybe do what Dean did and use some scat. Dean, do you have any pictures? How is the transition done betwen the scat and the FAB, and also between the scat and the cowl inlet?

I recall that Vans instructions advised the builder to take the time to make a nice tight fitting interface between the FAB and cowl so as to maximize air intake ( a bit of ram air effect), but I think my tight fit has come back to haunt me.

erich
Click to expand...

Hi Erich,

If your FAB is slipping on the fuel servo air inlet, I would look to the tightening sequence for the installation hardware. The donut should be installed and tightened onto the servo prior to the 4 bolts that hold the mounting plate to the donut. Once the donut is installed, these 4 bolts may be tightened.

I will look for a couple of photos of my modified FAB. Basically, I bought several diameters of SCAT tubing and selected the size that best fit over the end of the FAB. I pulled a turn or two of the wire reinforcement from between the plies of the tubing to allow the tube to conform to the profile of the FAB opening. I then used RTV and large head pulled rivets to attach the SCAT tube to the FAB. I then trimmed the length of the tube until I had what I deemed a reasonable preload on the cowl inlet flange. I shaped the opening of the SCAT tube (by bending the wire) to fit the opening in the cowl. Sombody with more time or skill could construct a fiberglass transition piece to scarf onto the FAB to provide a circular opening that would easily accept the SCAT tube. Imagine an all fiberglass FAB...

Good luck,

Dean
 
Huh. Not quite sure how the tightening sequence would affect this, but I am not discounting what you are saying either - things have a way of looking a little different when you actually have the objects in front of you. I'll try your suggestion when I get back to the airport and think it over more at that time - certainly couldnt hurt, eh? Thanks for the description of the scat transitions and I look forward to any photos you may have.

regards
erich
 
erich weaver said:
Huh. Not quite sure how the tightening sequence would affect this, but I am not discounting what you are saying either - things have a way of looking a little different when you actually have the objects in front of you. I'll try your suggestion when I get back to the airport and think it over more at that time - certainly couldnt hurt, eh? Thanks for the description of the scat transitions and I look forward to any photos you may have.

regards
erich
Click to expand...

Erich, I thought I had posted something in these forums, but couldn't find it with a quick search. I had calculated that, when the 1/4-20 bolt that clamps the ring onto the servo was tightened to nominal torque, the clamping force was something like a ton or so. Hard to imagine it moving. The sequence of tightening the ring's bolt first is so that the four bolts holding the plate to the ring don't prevent the ring from contracting around the servo as the ring's bolt is tightened. Best method is to loosely tighten the ring to the servo, then snug the bolts to the plate. Now you can align the snoot to the lower cowl. Then remove the airbox and torque the 1/4-20 bolt (IIRC) on the ring.
 
I keep thinking that this would be a good part for someone to offer an improved version of. I have certainly replaced several of them.

For a couple of reasons, I cut back the seal material on the air box so that it doesn't touch the cowl anymore. I thought that would help but noticed yesterday that the plate is cracked once again.
 
AlexPeterson said:
Best method is to loosely tighten the ring to the servo, then snug the bolts to the plate. Now you can align the snoot to the lower cowl. Then remove the airbox and torque the 1/4-20 bolt (IIRC) on the ring.
Click to expand...

Ahhh, that makes sense. I'll bet I did screw this up.

Wow, I am mystified by Larry's plate cracking even with no contact with the cowl. The FAB isnt very heavy and would seem to be adequatly supported. I was sure that the cracking folks are experienceing had to be due to differential movment between the engine and the cowl.

erich
 
Use thicker aluminum

I made a new plate out of thicker aluminum, about twice as thick as stock. I also moved the FAB over about an inch and ended up having to notch half of the top seal in the filter, not a big deal, the filter can only go in one way now and you have to get it aligned right. This was a pain in the #### but not really tough and I ended up with a solution that won't rub on the cowl (probably why many of them crack anyway).

Randy
8A Finishing, FWF
 
Double plate (mostly)

Part of the casting on the rear of my carb extended below the mounting surface. In order to address this problem and strengthen the mounting plate, I put a large doubler on the mounting plate.

Basically, I have 2 mounting plates riveted and prosealed together. I cut the top plate even with the rear of the carb mounting surface (I think I used .063). The full size bottom plate then clears the casting on the carb. This way all 4 carb mounting bolts are through double thick material and most bolts for the FAB are also through the doubler.
 
Run a brace from a front sump bolt down to the carb plate. Mount the base of the brace close to the back of the carb heat inlet.
 
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