Plenum/Baffle Q

[glenn654]

For those of you who run a plenum, have you ever had any problems of cracking baffles?

Was thinking the plenum cover may be too restrictive of movement of the baffles and cause cracking.

Glenn Wilkinson

[BillL]

Originally Posted by glenn654

For those of you who run a plenum, have you ever had any problems of cracking baffles?

Was thinking the plenum cover may be too restrictive of movement of the baffles and cause cracking.

Glenn Wilkinson

Interesting thought, I was wondering if the plenum would stiffen the back baffle by #4 on the 360's and prevent cracks in the oil cooler region.

[hydroguy2]

A plenum will not necessarily prevent cracking of the baffle at #4 with the cooler hanging off it. At least it didn't for me

Mine cracked during phase 1 I think. I added a stiffener and all is well since. I have a IO-360 180hpwith a SW8406cooler hanging at angle off #4 and a plenum.

[DanH]

Originally Posted by glenn654

For those of you who run a plenum, have you ever had any problems of cracking baffles?

Yes.

The -8 with an angle valve is sort of a special case. There is very little clearance between the port cylinders and the upper cowl, in particular above the 1/4-20 threaded mounting ears on the tops of the heads. In order to maximize clearance (and create a flange for plenum lid nutplates), I made very short baffle sidewalls that tipped inward at the ears. Because the I expected a significant load across the span of the intake openings, the front cylinder baffle walls were made from 0.060" 2024T3.

Early photos:





Both sides developed stress cracks at the most forward 1/4-20 mounting ear, as well as cracking the ends of the formed flange over the crankcase, behind the prop. The cracking above the cylinder heads was partly due to the plenum bumping the inside of the cowl during start up, but that would not explain cracking at the ends of the formed center flange. Remember, pressure recovery tries to blow up the plenum lid like a balloon, and the numbers are quite high; 136 lbs sq ft at sea level and 200 knots, times about 75% with the cowl exit open. The lid is about 4 sq feet, so there is about 400 lbs of force trying to lift the lid off the engine. In this case, the rubber inlet ducts add no strength or stiffness across the span of the intake openings, unlike a more conventional fiberglass snout.

The cure was to (1) attach the plenum lid directly to the mounting ears on the front cylinders, and (2) eliminate the formed center flange, placing the fasteners there in shear. The lid was reworked to add the necessary mounting ears and a wrap down over the forward face of the center case baffle:





The attach points at the cylinder heads got clamp plates over the glass, which seem to do a good job:



BTW, a fiberglass plenum lid really isn't all that stiff. Glass is low modulus (stretchy, in lay terms), and at engine compartment temperatures the epoxies we use are almost like hard rubber.

[N941WR]

Dan, your work puts everything I try to do to shame! Is the inside of your exhaust polished as well?

Originally Posted by hydroguy2

A plenum will not necessarily prevent cracking of the baffle at #4 with the cooler hanging off it. At least it didn't for me

Mine cracked during phase 1 I think. I added a stiffener and all is well since. I have a IO-360 180hpwith a SW8406cooler hanging at angle off #4 and a plenum.

Same here!

After replacing the cracked baffles, here is how I kept the crack from coming back:

[rzbill]

Originally Posted by BillL

Interesting thought, I was wondering if the plenum would stiffen the back baffle by #4 on the 360's and prevent cracks in the oil cooler region.

Probably,
The corner structure that cracks is not the problem. It is the flexible baffle section at the inboard end of the cooler that is the problem.

This J-Channel Reinforcement has been completely successful (350 hrs) in preventing the corner cracks.

I imagine similar stiffening can come from a proper plenum top.

[N941WR]

Originally Posted by rzbill

Probably,
The corner structure that cracks is not the problem. It is the flexible baffle section at the inboard end of the cooler that is the problem.

This J-Channel Reinforcement has been completely successful (350 hrs) in preventing the corner cracks.

I imagine similar stiffening can come from a proper plenum top.

Bill, that might have saved us a lot of work by keeping the rear baffle from flexing at the rivet line. Here are two pictures of a crack that developed in the exact same place on twp different airplanes. . The lower picture is from my plane.

Lionclaw's plane does not have a plenum and mine does, which is why his has the stiffener above the cylinder and my does not.

Originally Posted by Lionclaw

Same problem/fix on my O-320 as well. Van's really needs to update their baffle kits with this fix.

Originally Posted by N941WR

My crack was exactly like Andy's, it went from yellow arrow to yellow arrow:

[Weasel]

Originally Posted by DanH

Yes.


BTW, a fiberglass plenum lid really isn't all that stiff. Glass is low modulus (stretchy, in lay terms), and at engine compartment temperatures the epoxies we use are almost like hard rubber.

Dan, Would you envision a glass plenum to be soft enough to attach to the baffle on say cyl's 1 and 5 and 2 and 6 and then do the clamp attachment on cyl's 3 and 4 to keep the stress on the aluminum baffle down?

[Rupester]

At 200 hrs, I have no evidence of baffle cracking ... fingers crossed. During the build I did add a .063" doubler all the way around the rear baffle opening at the oil cooler. That was in addition to the .063" doubler that Vans now provides for the outboard bolts at that same location.
The good news is, this area is visible and accessible for close inspection any time the top cowl is off!

[BABALU]

What is the objective of the baffles and Plenom ?