Hazards in higher temp heat muff

[wjohnson]

Fellow carnadians,
I see all the interest and upset people when a EZ type plane is posted for sale on eBay or some other site that has some questionable information. It is a good idea as far as I am concerned to question such listings from a safety point of view.
My prior posting about increasing heat muff temperatures via insulating the heat muff, etc. posed safety issues also. I have been surprised by the lack of relevant comments.
What if I insulated the heat muff and managed to increase the temps enough to overheat the Scat type hose? Wouldn't this be really great to happen particularly during the winter when carb heat is a must? The hose could melt with no carb heat available.
If you think one should proceed with caution, do a little insulating at a time, forget the whole thing, etc. I would appreciate your comments. I do know that there are intelligent/experience people out there!  
One suggestion I rec'd to check for leaking cold leaking ram air was a good one and I will be checking on that, but I expect that I will still want to explore insulating the carb heat muff system.
My VariEze is just about 25 years old now and I have made several changes from standard over the years, but don't plan on doing anything too radical or dangerous.
In the interest of all concerned-- just me, but any other EZ types out there with similar problems, how about laying out your thoughts?
WJohnson

[maprather]

Why are you surprised?  

This may twist peoples' tails, but I don't think any of us have a responsibility to reply to list queries...   You haven't hired us or anything.  This is experimental aviation.  So.. do some tests.  Or, if you don't feel up to it, and if you can't find free/volunteer engineers to help you out, consider hiring a professional.

That said, your query about progressively adding more insulation to the system seems like a good idea.  If I were doing that, I would wire up a thermocouple probe to my multimeter (which is something like this:

http://www.tequipment.net/ExtechEX330.asp

)

and attach the probe to the scat tube where it attaches to the heat muff.
Set it up so that you can fly with it installed, and still observe the temp readings.  You'll likely have to wire up an extension to the thermocouple lead (which will hurt accuracy, but probably by less than 50degF, which you don't care about).  

Hottest temps will probably happen at high power settings, low airspeed, with the carb heat enabled (maybe only partially).  Record what you see.  Add insulation, record the readings, etc.  Raise the temperature (by adding insulation) no higher than the temp limit minus suspected instrumentation error (50degF).  

I would place the probe on the outside of the SCAT hose, underneath the clamp which holds the SCAT to the muff.

If I were adding insulation, I think I would add it to the airbox end of the hose first, which would allow the SCAT tube the most temperature margin.


Best of luck,

Matt-

[Drew]

The only thing I would ask you to consider is how you think about carb icing.  It is not just a winter problem.  There are serious problems at all times of the year.  When I was flying with a carb, I kept one of these in my checklist (http://www.ez.org/carb_ice.htm).  Just to pick off a couple of points:
at 30F, it isn't until I get to about 55% rel humidity where you have a possibility of getting into carb ice at cruise and glide power.  As you get more and more rel humid at this temp, you get into the "serious carb ice at cruise power"

on the other hand, at 80F, when you get to a little over 20% rel humid, you get into the same condition.  As you get more rel humid at this temp, you get into the serious carb ice at glide power (vice cruise power for 30F)

[wjohnson]

Thanks for the responses so far about insulating the carb heat intake system.
I have assumed that many, but not all, EZ types are interested in helping others of a like kind.
I went to school when manual typewriters and key punch cards were in use. So I was not aware of this new fangled temperature gauge that has been mentioned.
However, I surely agree that carb icing can be experienced when it gets to be warm weather. I experienced same just west of Spokane, WA during a nice, warm and clear summer day.
And I know that I am not totally alone, since I expect that carb icing is close to the top of FAA reasons for loss of power when flying.
Also, rattling cages does work!
Thanks, Wayne
Varieze N725EZ

[Drew]

I don't think that insulating the muff is the problem.  There are a couple of things you could check.  1st, are you getting flow from the scat to the cockpit.  If you are not, then it does not matter how hot you are generating the air---you have to have flow.  For flow to generate, you have to have higher pressure inside the cowl than inside the cockpit.  I think most people bring in fresh air to the muff and then send it into the cockpit.  I have a simpler approach and just take cowl air instead.

If you have good flow but it is cold, then you have a heat generation problem.  The ridges on the scat, uninsulated scat, and uninsulated muff are not making a difference for my setup----I am still generating plenty of heat.  If the air is cold, your problem really can only reside in a couple of areas.  Either your inflow of air is moving to fast through the muff to be heated or you don't have enough surface area on your muff to collect heat.

You may want to take a look on some Cozy sites to see pictures of the setup.

Here is what Nat Puffer says from one of his newsletters (and mine works exactly as he describes).

"To get a decent heat muff, we had to design a new 4-pipe exhaust system and find a supplier who could do a good job in building them (see pictures in newsletter #37). The idea was to collect heat from the two longest pipes, #2 and #4 on the right side. The pipes have to be free to move with respect to each other, so the pipes are supplied with only a partial muff welded to pipe #4. The builder completes the muff by wrapping a .016" aluminum shroud around both pipes. The one heat muff can supply heat to either the carburetor or the cabin, if a tee is installed in the line.

We didn't have a chance to test the system until just recently. I hooked up the cabin heat temporarily without a shut-off valve or blower and went for a ride. During the take off roll I could feel the heat pouring out, and after getting airborne. it was darned uncomfortable, even with full fresh air though the vents. I was satisfied I wouldn't need a blower, and made the installation permanent by installing a on/off valve ahead of the firewall, between the back seats. It is operated by a auto choke cable mounted just above the fuel selector valve. I ordered a carbon monoxide detector, to monitor and make sure no carbon monoxide finds its way into the cockpit. The hot air system. when it is in use, gives off some heat to the air going to the oil cooler, with a resulting increase in oil temperature. Since cabin heat is only needed in cold weather, this might turn out to be an advantage in keeping the oil temperature from getting too low. Needless to say, we are much relieved to know that the cabin heat system works well if installed the way described above."