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-   -   Airbox Pirep, check for cracks (https://vansairforce.net/community/showthread.php?t=40668)

chuckwn 03-09-2009 12:09 PM

My FAB
 
At 526 Hours:

Note that black lines are also cracks



Solution:

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



Dean Pichon 03-09-2009 02:57 PM

FAB slippage
 
Quote:

Originally Posted by erich weaver (Post 306654)
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

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

erich weaver 03-09-2009 03:37 PM

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

AlexPeterson 03-09-2009 05:47 PM

Quote:

Originally Posted by erich weaver (Post 306725)
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, 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.

n5lp 03-09-2009 06:01 PM

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.

asav8tor 03-09-2009 08:13 PM

Problem: cracks

Solution: steel

No more cracks

erich weaver 03-09-2009 08:24 PM

Quote:

Originally Posted by AlexPeterson (Post 306771)
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.

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

az_gila 03-09-2009 10:40 PM

Except that...
 
Quote:

Originally Posted by asav8tor (Post 306827)
Problem: cracks

Solution: steel

No more cracks

...you might just move the stress points to the carb mounting lugs, which are a lot more expensive to replace if they crack.

N208ET 03-10-2009 07:34 AM

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

Rick_A 03-10-2009 08:14 AM

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.


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