Home brew exhaust heat shields

[Desert Rat]

I was a little concerned about how close the exhaust runs under the throttle cable, so in addition to firesleeving it, I spent some time today busting out a exhaust heat shield.

I made it out of .025 scrap with the tab that the band clamp slides through .032.

I made the first one too short, so I ended up with two. The shorter one I used up by where the heater scat runs over the data plate. There is about a 1" air gap between the pipe and the scat, and the shield stands off from the exhaust pipe about 1/4"

Down underneath, there isn't much clearance. I built it so that it's only standing off the exhaust by about 3/16"

On the upper one I don't know if it's even necessary, but I had it so I put it in. I'm not sure if the aluminum will hold up to the heat that close to the exhaust valve. What do you guys think? Try it out and see, scrounge up some stainless and remake it? File it in the round bin as unnecessary?

For the lower one, is it better to have a 3/16" standoff from the pipe and a bigger air gap above? Or would it be better to have it a bigger air gap below and less space between the shield and the cable?

As always, I'm open to all input.

 

[wirejock]

Heat shield

I was a little concerned about how close the exhaust runs under the throttle cable, so in addition to firesleeving it, I spent some time today busting out a exhaust heat shield.

I made it out of .025 scrap with the tab that the band clamp slides through .032.

I made the first one too short, so I ended up with two. The shorter one I used up by where the heater scat runs over the data plate. There is about a 1" air gap between the pipe and the scat, and the shield stands off from the exhaust pipe about 1/4"

Down underneath, there isn't much clearance. I built it so that it's only standing off the exhaust by about 3/16"

On the upper one I don't know if it's even necessary, but I had it so I put it in. I'm not sure if the aluminum will hold up to the heat that close to the exhaust valve. What do you guys think? Try it out and see, scrounge up some stainless and remake it? File it in the round bin as unnecessary?

For the lower one, is it better to have a 3/16" standoff from the pipe and a bigger air gap above? Or would it be better to have it a bigger air gap below and less space between the shield and the cable?

As always, I'm open to all input.

You do nice work.
I have Antisplat heat shields very similar to yours.
Two port and two starboard to protect the cowl and one to protect the cable.

 

[avrojockey]

I would tent to think the larger the air gap between the heat source and shield the better, but you're probably good either way as long as theirs some gap on each side. I think the purpose of the shield is to block/absorb radiant energy so as long as it not touching the cable you're good.

Not that I have this issue but why shield the cable? Is the interior lubrication needing protection from heat?

 

[Thierry A]

Others might chime in but I think Stainless would be better. Aluminum radiates heat well, so you're potentially moving the heat closer to what your trying to shield.

 

[Desert Rat]

I would tent to think the larger the air gap between the heat source and shield the better, but you're probably good either way as long as theirs some gap on each side. I think the purpose of the shield is to block/absorb radiant energy so as long as it not touching the cable you're good.

Not that I have this issue but why shield the cable? Is the interior lubrication needing protection from heat?

There are quite a few old posts on here where guys have reported throttle cables getting gritty/hard to move. The general consensus is that heat breaks down the liner over time. It seems to be really hit or miss who it happens to, but it seems reasonable.

Based on how close the cable is to the exhaust in that area I decided to go with the nuclear option and install both a heat shield and a piece of firesleeve.

Since it was already rigged, instead of taking that all back apart I just split a piece of firesleeve longways, wrapped it around the cable and secured it with several wraps of safety wire. Then I sealed it up with RTV just to keep oil and mystery crud out of there.

 

[Desert Rat]

Thanks for the kind words Larry. I'm not too proud to admit that I shamelessly ripped this off. Vans sell a very similar one for 15 bucks a pop.

Thierry- You might be right. My reasoning was that Vans and Anti-splat both sell a similar product that's aluminum. Plus I had some down time this afternoon to play with this and thats what I had in my scrap bin

 

[Jetmart]

Not MY Idea

Not my idea. Saw it here on VAF. I insulated my heat shields.

.

 

[NTex]

I was a little concerned about how close the exhaust runs under the throttle cable, so in addition to firesleeving it, I spent some time today busting out a exhaust heat shield.

I made it out of .025 scrap with the tab that the band clamp slides through .032.

I made the first one too short, so I ended up with two. The shorter one I used up by where the heater scat runs over the data plate. There is about a 1" air gap between the pipe and the scat, and the shield stands off from the exhaust pipe about 1/4"

Down underneath, there isn't much clearance. I built it so that it's only standing off the exhaust by about 3/16"

On the upper one I don't know if it's even necessary, but I had it so I put it in. I'm not sure if the aluminum will hold up to the heat that close to the exhaust valve. What do you guys think? Try it out and see, scrounge up some stainless and remake it? File it in the round bin as unnecessary?

For the lower one, is it better to have a 3/16" standoff from the pipe and a bigger air gap above? Or would it be better to have it a bigger air gap below and less space between the shield and the cable?

As always, I'm open to all input.

Nice work.. out of curiosity, how did you fabricate the joggles in the clip that's riveted to the heat shield? Very clean install, though I might agree with others about standoff/material choice. The insulated version might help.

And agree with your assessment about thermal protection, on many certified aircraft, the engine control cables (particularly vernier cables) can get stiff over time. I've seen some guys also firesleeve the OD of the cables to protect them.

 

[rv8ch]

Almost three years on these home made heat shields, and they are still holding up.

http://www.rv8.ch/heat-shield-home-made/

 

[PhatRV]

In the FF kit that Vans shipped out, it included a couple of similar heat shields. But I needed to make a few more using the stainless steel sheets bought from McMaster-Carr.

 

[Desert Rat]

Nice work.. out of curiosity, how did you fabricate the joggles in the clip that's riveted to the heat shield

It’s nothing special, I just measured a bit and then bent them with a pair of flat jawed knipex pliers.

 

[DanH]

I'm not sure if the aluminum will hold up to the heat that close to the exhaust valve. What do you guys think?

Depends on how hot; aluminum weakens quickly at elevated temperature, chart below. And this is a high cycle fatigue application.

I make mine from stainless steel. I also give them two legs, not one, to better resist vibration.

The goal is to reduce heat flux, the quantity of energy reaching the victim component. Let's ignore convective transfer, and consider only energy leaving the hot pipe as radiation, which strikes the inner surface of the shield. Some of the energy is reflected, and some is absorbed. The absorbed energy is conducted through the shield, and re-radiated from the other side, toward the victim.

Obviously for a really good shield we want to reduce absorption, conductivity, and re-radiation.

The measure of energy absorbed and energy radiated is called emissivity, technically a percentage when compared to a perfect black body =1. In our simplified world, it's mostly a matter of surface finish, i.e. shiny = low emissivity and dull = high. Emissivity is low for almost any polished metal (most are below 0.1) but there is a lot of variation in how well those metals stay polished. We want to pick a material which will remain as shiny as possible for the life of the shield. Here aluminum has an advantage over stainless. It's better when polished (ballpark 0.05 vs 0.2), and much better over time. The reference text in front of me gives an emissivity (at 300K, or 80F) of 0.2~0.33 for a heavily oxidized aluminum surface vs 0.3~0.4 for lightly oxidized stainless and 0.7~0.8 for heavily oxidized stainless.

Decreasing conductivity is a function of insulation. Insert a material with low thermal conductivity into the energy path, and less quantity comes out the other side as the rate of transfer is slowed.

So, want a great shield? Structurally, make it from stainless, with two legs for support. Add an insulation layer; 970 fiberfrax felt is cheap and available. Wrap it in aluminum tape, to get low emissivity and to protect the 970. The TDS for most of the tapes lists the materials as "dead soft aluminum", so it's probably 1100 alloy, relatively resistant to oxidation.
.

 

[Desert Rat]

holy cow thats a lot of big words. Kidding, thanks for the input .

I've already got the fiberfrax and speed tape. The next time I happen on some stainless stock that I can glom onto on the cheap I'll revisit this.

 

[Ralph Inkster]

I have an old chunk of SS sheet that was the bottom kick panel on a door. Has been great for whipping up heat shields & brackets over the years.
Some see me as a pack rat, never throwing anything away, much has come in handy, but I dread the coming day when I will have to clear out the hangar…

 

[avrojockey]

There are quite a few old posts on here where guys have reported throttle cables getting gritty/hard to move. The general consensus is that heat breaks down the liner over time. It seems to be really hit or miss who it happens to, but it seems reasonable.

Great to know!

 

[Freemasm]

Great to know!

Macfarlane sells cable with no liner for such applications IIRC. Of course, you’d expect a bit more friction but that would be the trade-off. Ain’t nothing free.

 

[avrojockey]

From what I understand a material conductive ability is not the same as it ability to absorb/reflect radiant energy so AL my be a better material than stainless. I saw a video from a YouTuber (AVE) who broke down a Yeti mug to see why it was so great. They actually take the effort to copper plate the outside of the inside sleeve...because copper does a great job of reflecting radiant energy.

 

[ronschreck]

Waht he said...

Dan Horton's highly scientific post is right on except that he didn't take into account the airflow around the heat shield that will draw considerable heat away from the source and the shield. I used several homemade shields just like yours to protect control cables, fuel and oil lines with great results. I never saw the aluminum degrade but I did have a few break off due to vibration so having two supports on larger shields is a good idea. Nothing wrong with stainless steel, insulated shields but probably overkill.

Just my opinion and worth just what you paid for.

 

[DanH]

Dan Horton's highly scientific post is right on except that he didn't take into account the airflow around the heat shield that will draw considerable heat away from the source and the shield.

Convective transfer would indeed be useful to consider if we were trying to cool the source and the shield, i.e. remove energy from them.

Here our interest is reducing energy transfer to the victim.

Transfer is always high to low. Although the hot pipe and hot shield can heat the air, the warmed air is unlikely to be warmer than the control cable. Without a temperature difference there is no transfer from air to cable.

Other than the previously described heat path through the shield material, conductive transfer isn't relevant, as there is (hopefully!) no physical contact between the really hot parts and the cable.

So, radiant transfer is the dominant transfer mechanism in this example.

BTW, although we tend to hang heat shields on the pipes, there is no reason it must be done that way. I've been running these two cables directly above a 4-into-1 cluster for 1200 hours. The shield is a reflector over an insulator, attached to the cable sheath bracket.

 

[RV8iator]

I love this kind of thread!!

I get smarter just by thinking I understand any of this.

 

[Desert Rat]

So, I was challenged by some of these responses to see if I could do better. It was also pointed out me on another thread that I wasn't done yet and needed two more heat shields on the pilot side where the fuel line runs.

So, yesterday I swing by my local Metal Supermarket for some stainless. The only stainless they had in stock in nice thin 24 gauge was 301 in a soft annealed condition, so thats what I ended up with.

I fabricated a couple of shields for the pilot side pipes with two attach points, with the inner surface covered with aluminum speed tape, and with the outer surface encapsulating a layer of 1/16" fiberfrax under more speed tape.

While I was at it, I removed the aluminum one below the throttle cable and added a 1" wide strip of fiberfrax down the spine, which is the part closest to the cable. I only went with one inch here because that was the scrap I still had on the bench and I had already taped up the rest of the roll before I though of it.

The new ones ended up at about an oz. heavier than the basic aluminum ones I made the other day. Which isn't much, but yeah, it's something.

So as it stands, I've got a bare aluminum one on the r/h side by the heater scat, an aluminum one with a partial fiberfrax treatment underneath, and two soft stainless ones with a full aluminum tape and fiberfrax treatment on the l/h side.

It will be interesting to see how this all holds up.

On a side note, They will tell you that you should wear gloves when working with stainless because of the sharp edges, but that's for rookies, not for trained professionals such as myself. Surely I can grab this piece and hit the edges on a debur wheel. I've done stuff like this forever, no problem....right?

 

[71459]

On a side note, They will tell you that you should wear gloves when working with stainless because of the sharp edges, but that's for rookies, not for trained professionals such as myself. Surely I can grab this piece and hit the edges on a debur wheel. I've done stuff like this forever, no problem....right?

That's right. The rule is if you don't have some dried blood in a few places inside your airframe, you're not trying hard enough.