Check Those Heater Shrouds!

[FlyingMonkey]

Hey all...

In the process of performing my annual condition inspection, I found some issues with my muffler and heater shroud. 

First, some engineering notes:

The heater shroud is a piece of 0.060" aluminum that is rolled into a cylinder.  A tab is cut into it (before rolling presumably), and folded out to provide a "shelf" for the air inlet (also 0.060" aluminum) to sit on.  Inside the shroud the inlet has a tab on each side folded out and riveted to the inside of the shroud housing.

Some failings I noted of the build:

*  The slots cut in the shroud to form the sides of the inlet shelf are square cut with no clean up or rounding.  The square corners of these slots are stress risers and highly prone to crack formation.  One of mine has a 1" long crack growing out from it.

*  The tabs that attach the inlet to the shroud are flat parts attempting to rivet to the rounded surface of the shroud.  The tabs are not fully formed to the inside radius of the shroud.  As a result, the rivets form a poor joint, leading to loosening of the rivets over time and increased vibration of the inlet, exacerbating the tendency of the inlet slots or other areas to crack.

*  All the attachment points for the inlet are on the lower side (rivets to the shelf and the two tabs).  There is nothing attaching the upper part of the inlet to the upper area of the shroud.  This allows the inlet to flex the lower part of the shroud like a lever, and further increases the chances of crack formation. 

Here is the crack I found, stop drilled:

Here are the ruined tabs I found inside when I removed the shroud:



To fix these problems, I did the following (all metal is 6061-T6 aluminum and all rivets stainless steel blind rivets):

* Stop drilled any cracks.

* Put a large 0.060" doubler under the shelf supporting the inlet, to provide support to the cracked area and increase stiffness and vibration resistance of the shelf.

* Cut the tabs off and sealed the area with 0.060" patches.

* Riveted a bracket to the top of the inlet, with long "wings" that then are secured against the upper shroud half by the factory clamp.  This creates a very stiff, vibration-resistant inlet area, while still allowing adjustability and easy removablility of the shroud.

Here are pictures of the doubler I made, and the upper winged bracket and how it attaches.  This to me beats the $790.15 that FD-USA quoted me for a new shroud, and will probably last longer.  If this one has issues I'll probably make a new one from scratch and do some more re-engineering to improve it further.  There is one minor engineering mistake I made here, but if nobody points it out I won't either. 

Yes, I will be replacing the rubber on the inlet, but since it's warming, I won't be using heat anymore for at least six months,  and I want to finish all this work and go fly... I will probably leave that until next fall.

There was also one small hairline crack discovered in the muffler itself under the shroud.  I had a buddy put a bead of weld on it so that is fixed now as well.

 

[GrassStripFlyBoy]

If only someone would finish up that dandy aftermarket exhaust system!  So ripe for improvement on exhaust.  I'd give up heat to have a simple / clean / light weight pipe.

Nice work on the repairs Andy.

[FlyingMonkey]

9 minutes ago, GrassStripFlyBoy said:

If only someone would finish up that dandy aftermarket exhaust system!

 

There are options...

 

http://www.rotax912exhaust.com/en/

 

http://toucanexhaust.com/

 

https://aircraftexhaust.net/rotax/

[Roger Lee]

Good job.

I just had to fix a broken one. The aluminum that attaches that air intake vibrates too much in the wind and many over time are cracking and or breaking. I just fixed one last week on a 2007 CTSW. You can stop this vibration damage by putting a short 1/2" wide brace from the air intake and then bend it flat and outward and then put it under the clamps. You pop rivet it onto the air intake part. Makes it solid as a rock and the cost is just a couple of dollars. The aluminum can be purchased at Ace Aviation (Ace Hardware).

[Towner]

Roger, can you show a picture of this work?

[Roger Lee]

Hi Tom,

I don't have any pictures of this and the plane isn't here.It's more or less just an extra beefer support bracket I made. 

[FlyingMonkey]

Roger's repair description sounds exactly like the the piece I riveted to the top of my inlet in the last pic I posted above.

[Roger Lee]

Nice job Andy

 

[Madhatter]

When you have the shroud off check the muffler for cracks,  I had six cracked welds on the shroud standoff rings due to them being incorrectly welded with 700 hrs total time. You have to remove the muffler, tape up the stack ports ,attach the exhaust of a shopvac to the tailpipe and spray soap solution on the muffler. If you have cracks they will show up. This is a HUGE issue on certified aircraft and the liability is through the roof for mechanics, there is no difference with LSA. When I worked as a mechanic for a piper distributor I had to weld cracks in house many years ago. Today no one would even consider it due to liability. With many closing up access holes in the CT cabin to reduce airflow in winter, CO is a real possibility.  The cheap CO visual detectors are worthless. 

[Runtoeat]

Good info Andy.  Roger and Mad too.  Looks like I better take a good look at this area my 2006 CTSW.

[Tom Baker]

I just repaired one for a customer. I made a new bottom half to the scoop.

[Anticept]

Good job Andy! The top plate is a brilliant idea and I'm going start making them for aircraft that I help out on.

Gusset plates are probably fine for the bottom. Something like this:

Plus, before making such a plate, stop drilling the corners, even if there's no cracking, would probably go a long way too.

Note that my rough pattern above doesn't put any rivets in a line. It makes it harder for crack propagation.

[FlyingMonkey]

Thanks Corey.  Yeah, gussets or a thin crosspiece would work fine.  Mine was cracked pretty badly and I tend to over-engineer things. 

Also, I should have radiused inside the corners better, hopefully *they* won't crack.

[Anticept]

I wouldn't use a crosspiece. Loads would concentrate in it.

Nonetheless the real genius thing was your clever design for the top. Every time I thought about reinforcement, I've always thought of riveting it. I didn't think of making tabs at the clamps could latch on to. Very very clever.

[FlyingMonkey]

22 hours ago, Anticept said:

I wouldn't use a crosspiece. Loads would concentrate in it.

Nonetheless the real genius thing was your clever design for the top. Every time I thought about reinforcement, I've always thought of riveting it. I didn't think of making tabs at the clamps could latch on to. Very very clever.

Thanks!  It's the only way I could think of to secure the inlet to the shroud to reduce vibration and still retain the ability to remove/adjust the shroud without drilling out rivets.