Cracking FAB to servo mounting plate

[walkman]

Seems like on my RV-8 stop drilling cracks in the FAB to servo mounting plate, or making a replacement, is an annual event.

Is there anything that folks are doing to mitigate this issue? Some sort of isolation mount perhaps, or a thicker and/or software plate?

 

[Walt]

Likely cause by the airbox hitting the cowl.

 

[walkman]

Likely cause by the airbox hitting the cowl.

seems to be pretty good separation, but I'll double check

 

[blaplante]

I like the software idea. Then you can just reboot it.

Seriously ---
I assume this is a vertical induction with the chin inlet?
I agree w/Walt, most likely engine shake on start / stop is causing the airbox to strike the cowl. You'll probably need a borescope (go in from the bottom aft of the cowling) to see how close the box (nose) is to the cowl.

Photos are always helpful....

 

[JonJay]

If it’s vertical induction, it’s a common problem. I just fabricated my out of 4130 per some early adopters and moved on.
My airbox has plenty of clearance .

 

[Dean Pichon]

Likely cause by the airbox hitting the cowl.

What Walt said. I went thru a bunch of plates before I was able to resolve the issue. I "softened" the connection between the cowl and airbox to reduce the loads seen by the plate. I used a piece of appropriately sized SCAT tubing, re-shaped to match the air inlet in the cowl. The other end is riveted and pro-sealed to the airbox. I haven't replaced a plate in 15 years.

 

[Ralph Inkster]

My solution, I rivet a .063 doubler to the top of the new replacement top plate.
Some folks replace the .063 plate with a .125 plate.
Some other folks fashion a metal strut to support & stabilize the front of the FAB housing to the front oil pan bolts.
As others have said, insure that the housing is not interfering with the cowling & that the inlet snorkel rubber seal is not too stiff.

 

[rvbuilder2002]

A lack of flexibility between the nose of the airbox, and the inlet of the cowl is the primary cause fatigue cracking in the plate.
Switching to a more flexible, interface will help a lot.
When mine cracked, I chose to remake the plate from .125 thick material using the original as a pattern.
No more cracking after doing that.

 

[Carl Froehlich]

The airbox is out of balance (fore and aft). It is only a matter of time before you get cracks as it bounces around.

On the first build (RV-8A) I fixed this by replacing the aluminum FAB plate with 0.032” stainless steel. On the second build (RV-10) I took preemptive action by adding a scrap piece of J channel angle between the top forward bolt on the FAB and the engine case.

Carl