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  #21  
Old 01-19-2022, 05:28 PM
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AlexPeterson AlexPeterson is offline
 
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Quote:
Originally Posted by Darin Watson View Post
Somewhat challenged to quantify, but I would say it was chilly at -10C with “normal chothing” prior to shirtsleeves now and at -20C you want to start to zip up your jacket. My wife and I were up at 17,500 on a cold day over Montana once and the OAT was reading -53C on the way to Mexico, and it did get uncomfortably cold…I think it was -30ish on the ground that day.
Is that a typo, -53C? That is -63F. Count me as impressed if you survived a flight in that OAT in an RV!
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  #22  
Old 01-19-2022, 07:40 PM
rmarshall234 rmarshall234 is offline
 
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I once made a skydive from 25,000 msl when the OAT was -40F with a windchill factor of over -100F. That was cold! See why I want a heat muff in my airplane.
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  #23  
Old 01-20-2022, 06:45 AM
elev666 elev666 is offline
 
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Originally Posted by rmarshall234 View Post
I once made a skydive from 25,000 msl when the OAT was -40F with a windchill factor of over -100F. That was cold! See why I want a heat muff in my airplane.
It looks as though we may have found The infamous D B Cooper!!
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  #24  
Old 01-20-2022, 07:21 AM
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Default Horton to the rescue

I am waiting for Dan Horton to come out of retirement and help with this thread.

IIRC - He torched the bronze already or doing it now.

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  #25  
Old 01-20-2022, 07:23 AM
WiscoMick WiscoMick is offline
 
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Are there any metallurgists out there that could give an opinion on making Darin’s fin assemblies out of Aluminum vs. his SS? Obviously, AL is much easier to source and work with. Thanks in advance.
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  #26  
Old 01-20-2022, 07:58 AM
Gusmax Gusmax is offline
 
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Early Mooney's used a very long spring tightly wrapped around the 2 cross-over pipes and then the heat muff fit over the spring. The heat seemed very good with that set up.
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  #27  
Old 01-20-2022, 08:33 AM
KayS KayS is offline
 
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Quote:
Originally Posted by WiscoMick View Post
Are there any metallurgists out there that could give an opinion on making Darin’s fin assemblies out of Aluminum vs. his SS? Obviously, AL is much easier to source and work with. Thanks in advance.
i also wonder to make these things from aluminum sheet. melting temp of 2024 is around 900 - 1200 F. we know that exhaust gas temps can get higher but i have no idea how hot the exhaust pipe itself gets. stainless steel has around 2500 F melting point. so there's a significant difference. but my gut feeling tells me that we should get away with aluminum.

anyone could shed some light?

Last edited by KayS : 01-20-2022 at 08:36 AM.
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  #28  
Old 01-20-2022, 08:49 AM
Freemasm Freemasm is offline
 
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Quote:
Originally Posted by KayS View Post
i also wonder to make these things from aluminum sheet. melting temp of 2024 is around 900 - 1200 F. we know that exhaust gas temps can get higher but i have no idea how hot the exhaust pipe itself gets. stainless steel has around 2500 F melting point. so there's a significant difference. but my gut feeling tells me that we should get away with aluminum.

anyone could shed some light?
You nailed on the first strike. Significantly different alphas, expansion rates as related to temperature, would be strike two (but overcome-able). Solid contact between the tube and fin material is important. Tensioning the fin wrap is the typical answer.

Strike three would be stainless and aluminum don't play well from a galvanic standpoint. The heat they would be exposed to would greatly accelerate this electrolysis/corrosion. Once this corrosive layer separated/insulated the "feet" of the fin from the hot exhaust tube surface, the local heat conduction (tube to fin) would rapidly shift towards zero making the fins next to worthless.

Designed a lot of HEx's a while back; one of the many things I've gotten to do as a professional. Most finned tubes utilized "wrapped" fins along the tube. For some critical applications, the fins and tubes were a single element. We had a vendor in Tulsa OK that would mechanically extrude fins from very thick walled pipe. I was an impressive feat of machining, if you're a geek.

Last edited by Freemasm : 01-20-2022 at 08:53 AM.
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  #29  
Old 01-20-2022, 09:07 AM
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AlexPeterson AlexPeterson is offline
 
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Quote:
Originally Posted by Freemasm View Post
You nailed on the first strike. Significantly different alphas, expansion rates as related to temperature, would be strike two (but overcome-able). Solid contact between the tube and fin material is important. Tensioning the fin wrap is the typical answer.

Strike three would be stainless and aluminum don't play well from a galvanic standpoint. The heat they would be exposed to would greatly accelerate this electrolysis/corrosion. Once this corrosive layer separated/insulated the "feet" of the fin from the hot exhaust tube surface, the local heat conduction (tube to fin) would rapidly shift towards zero making the fins next to worthless.

Designed a lot of HEx's a while back; one of the many things I've gotten to do as a professional. Most finned tubes utilized "wrapped" fins along the tube. For some critical applications, the fins and tubes were a single element. We had a vendor in Tulsa OK that would mechanically extrude fins from very thick walled pipe. I was an impressive feat of machining, if you're a geek.
Copper might be an ideal material for this. The melting temp is much higher than egt and the thermal conductivity is excellent, about 20x that of SS and almost double that of aluminum.

Re galvanic corrosion - it requires that liquid be present, so it won't occur in this application.
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  #30  
Old 01-20-2022, 09:29 AM
Freemasm Freemasm is offline
 
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Originally Posted by AlexPeterson View Post
Re galvanic corrosion - it requires that liquid be present, so it won't occur in this application.
I won't get into a debate but I'm going to respectfully disagree, Sir.

Catalyst like water or even more so, the contaminants that water tends to deposit, affects the reaction rate (definition of a catalyst). It does not have to be present for electrolysis to occur. It would usually be limited to the first few mills of the surface before the new layer provided some isolation for lack of a better word. The preservative oils that both wick well and have good dielectric properties are a good approach (and the fact that they repel water). At colder temps, the rate could be measured in decades. In exhaust conditions, weeks, days or even hours to start seeing the consequences. Relatedly, there are plenty of nasties in hydrocarbon exhaust that are both corrosive on their own and catalytic to the related chemistry. Water is also a byproduct of combustion. It's a rough environment for most any alloy.

I mentioned before in response to the original post, the improvement of bronze wool over stainless (everything else being equal) would probably be quite measurable; at first. I wouldn't expect that to last.

Try your approach. I've been wrong before and that won't change in the future.

Last edited by Freemasm : 01-20-2022 at 10:48 AM. Reason: Added clarity, hopefully
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