Suitable alloy for new FAB mount (VA-131-C) ?

Geez, I'm really at a stage where nothing seems to fit quite right. Like many other before me, I need to make an FAB mount plate with more offset. With stock setup there's only maybe 1/8" between FAB fiberglass and side of the lower cowling scoop...and I figure I need at least 1/2" to account for engine waggle. I've already notched the cowling to clear the mixture arm, and will glass up a "speed blister" to cover it.

The good news is that I won't need to rivet the aluminum recess into the FAB itself for mixture arm.

What's a good alum. alloy to make a new VA-131-C out of? I'll probably go with 0.090 thickness, as some have experienced cracks with the 0.063 stocker. I'd like to get something that's available through ACS. Stock piece has the number 3073A5, but I can't find anything that matches up.

Thanks!!

The one I am presently using is made from Titanium that I purchased on eBay. Have replaced two of the stock aluminum ones before I upgraded to the Titanium. Ti is difficult to work with but it should last forever. If you stick with aluminum, I would recommend doubling the thickness. I feel my earlier cracks are from having the carb mounted as far back on the B_ _ sump as Lycoming could locate it. The Fiberglass FAB is full length as supplied by Van to just fit properly.

Thanks, Gary. I think I'll stick with thicker aluminum, and definitely run a support strut forward to the snorkel.

Would 2024-T3 be suitable?

Lots of us have replaced these. I used .050 4130. I was fresh out of titanium. Really Gary?
I also had two crack and chose to go that route.
I, like you, had to modify the FAB to offset it. I believe this contributes to the cracking issue. Supporting the FAB with a bracket has proven a good solution if you stick with aluminum.

I also had to make a new offset FAB plate, I went with the .063 2024-T3. I made some large area washers to help support the plate better, I had to shape them a tad to fit. You can see the ring of my "shrinking" airfilter, there wasn't room for a washer on the one bolt.


I also installed a small brace to the front of the FAB plate. Only 95 hours so far, hopefully my luck continues.

Great, guys. Thank you for the photos and suggestions.

I wonder if fitting a cork gasket between this piece and the throttle body would help with the cracking issue. It would prevent the plate from flexing against the sharp machined edges of the throttle body.

Update: I just ordered some 0.125 2024-T3. ACS had it in a convenient 1' x 1' square for $18.

rightrudder said:

Great, guys. Thank you for the photos and suggestions.

I wonder if fitting a cork gasket between this piece and the throttle body would help with the cracking issue. It would prevent the plate from flexing against the sharp machined edges of the throttle body.

Update: I just ordered some 0.125 2024-T3. ACS had it in a convenient 1' x 1' square for $18.

Click to expand...

Sorry, I forgot to mention that I did make and am currently running a cork gasket for the carb to FAB mounting plate.

I have not installed the FAB on my 9A yet, is there a consensus about how far to shift it?

The metalurgists on VAF will more accurately describe the problem and solution, but aluminum will crack after a certain amount of flex cycles. The cycles are likely from the engine intake pulsations. Go to stainless and problem solved.

crabandy said:

I also had to make a new offset FAB plate, I went with the .063 2024-T3. I made some large area washers to help support the plate better, I had to shape them a tad to fit. You can see the ring of my "shrinking" airfilter, there wasn't room for a washer on the one bolt.

I also installed a small brace to the front of the FAB plate. Only 95 hours so far, hopefully my luck continues.

Click to expand...

I oversized the holes slightly and took the oversized washer route and got about 500 hours out of my second plate with no brace.
The crack, both times, emanated from the aft right bolt hole that secures the plate to the injector body, not from the front, and not from the edge of the injector body.
I believe if you do not have an offset, and the FAB is done correctly, you would never see a crack even in aluminum for a long time. Many are flying out there with their original plates and no cracks.

Following is a copy and paste of a 1-5-2010 post by DanH I found by searching "FAB Cracking" which explains the issue much better than I did above







Originally Posted by AlexPeterson View Post

"The most likely cause is not engine movement, but the tremendous acoustic vibration going on in an induction system."

Ya'll should listen to Alex. The issue is number of stress cycles and the rate at which they accumulate per flight hour. Aluminum has no knee in its S-N curve; given enough cycles all aluminum parts fail. At low load per cycle it merely requires more cycles.

Aluminum fatigue strength is usually quoted for 10 million or 100 million cycles, an arbitrary figure on a slope which steadily (although exponentially) declines to zero strength. Ten million cycles may seem like a lot (and it is if we're talking wing spars), but it's nothing in the context of the vibe Alex describes. Assume a 2400 RPM cruise, or 4800 intake cycles per minute, or 288,000 per hour. It only takes 35 hours to reach 10 million cycles, and that doesn't count the harmonics.

Two choices...either greatly reduce material stress (to push failure way out into the far exponential reaches of aluminum's S-N curve), or change material. I'd suggest ya'll should quit fooling around and make steel mount plates. Steel does have a knee in its S-N curve, so stress below the knee doesn't fail the part no matter how many cycles you apply.
__________________
Dan Horton
RV-8 SS
Barrett IO-390

mdoyle said:

I have not installed the FAB on my 9A yet, is there a consensus about how far to shift it?

Click to expand...

It's gonna depend on how much clearance you have between the FAB and the cowl...for your particular application. The only way to know is mount the cowling and carb/fuel servo and see what the fit is. I'm shooting for about 3/4" of clearance, which means I'll need to offset the plate a little more than 1/2". For reference, I've got the I0-320 D1A with the AvStar fuel servo.

The 2024-T3 came today from ACS, and after the aluminum chips settled, this is how it turned out. Offset is 5/8" from original, and I cut the outer perimeter bigger by about 1/8". Lots of material between the servo studs and the hole, as it's much smaller than the original "square hole." Mixture arm clears nicely, and I've got the 3/4" I wanted between FAB and cowling.

On to the endless trimming of the cowling...

I was developing crack with the 0.063 so I increased the thickness to 0.090. Problem solved for at least another 150 hours then I sold the plane. Built a RV 9 next with IO320 and again had a rack at 75 hours. My tech advisor Tom Berge suggested making a plate out of 5 layers of carbon Fiber. 125 hours larger it still looks good.

2x VA-131C available

Hi guys,

I have two VA-131C's available. Clearing out my garage.
Make me an offer..