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  #1  
Old 03-07-2009, 07:35 AM
whifof100ll whifof100ll is offline
 
Join Date: May 2005
Location: Mandeville, Louisiana
Posts: 179
Default Airbox Pirep, check for cracks

I had the same problem Chris Pratt and Paul Dye had others have with FAB and cowl interference described here
http://www.vansairforce.com/communit...ad.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.
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RV-6 Flying, XPIO360 Catto 3bld AFS3500EE
'68' C177
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  #2  
Old 03-07-2009, 08:03 AM
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Vlad Vlad is offline
 
Join Date: Oct 2008
Location: Utah
Posts: 8,151
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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.
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  #3  
Old 03-07-2009, 08:14 AM
andyrv andyrv is offline
 
Join Date: Feb 2005
Location: Leawood, KS
Posts: 275
Default 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
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  #4  
Old 03-07-2009, 08:57 AM
Steve Steve is offline
 
Join Date: Jan 2005
Location: Roy, Utah
Posts: 1,144
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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
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  #5  
Old 03-07-2009, 09:48 AM
whifof100ll whifof100ll is offline
 
Join Date: May 2005
Location: Mandeville, Louisiana
Posts: 179
Default Pics

Quote:
Originally Posted by Vladyspassky View Post
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.

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
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'68' C177
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  #6  
Old 03-07-2009, 10:29 AM
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Vlad Vlad is offline
 
Join Date: Oct 2008
Location: Utah
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Quote:
Originally Posted by whifof100ll View Post


Just a reminder to make sure you get drilled head bolts for the airbox top plate and safety them.


Regards,
Dale
I have newer Firewall Forward Kit with drilled head bolts. Thanks for reminding
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  #7  
Old 03-07-2009, 02:12 PM
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rvbuilder2002 rvbuilder2002 is offline
 
Join Date: Jul 2005
Location: Hubbard Oregon
Posts: 9,035
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Quote:
Originally Posted by Steve View Post
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
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...
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  #8  
Old 03-07-2009, 03:46 PM
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AlexPeterson AlexPeterson is offline
 
Join Date: Sep 2005
Location: Maple Grove, MN
Posts: 2,333
Default

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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RV6A N66AP 1700+ hours
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Last edited by AlexPeterson : 03-07-2009 at 03:50 PM. Reason: hit the wrong button
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  #9  
Old 03-08-2009, 05:25 AM
Dean Pichon Dean Pichon is offline
 
Join Date: May 2006
Posts: 532
Default 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
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  #10  
Old 03-09-2009, 10:54 AM
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erich weaver erich weaver is offline
 
Join Date: Aug 2006
Location: santa barbara, CA
Posts: 1,682
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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
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