Replies to This Discussion

Permalink Reply by Jerold Ebke on May 1, 2014 at 12:51pm

David,

Did you fabricate that box yourself or what is its origin?

Yes , I believe the pressure difference is the force moving the air.

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Permalink Reply by Don Morrisey on May 1, 2014 at 1:06pm

David, I believe you have it right, pressure in the lower cowl will make the air flow.  Personally I would not be a fan of this, over time the air will eventually smell like an old engine as you are bound to get oil and fuel residues on the engine block etc.  You may want to consider installing a naca duct and supply air to the box that way...ACS has many choices: http://www.aircraftspruce.com/categories/aircraft_parts/ap/menus/ap...

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Permalink Reply by John Austin on May 1, 2014 at 6:26pm

Your heater muff is very similar to the one used on the Jabiru 3300 FWF kit. It is fed by a NACA duct on the side of the cowl. NACA ducts passively take in the airflow presented to it. I had better results by installing a "periscope" scoop in one of the cowl inlets immediately next to the prop hub where it gets more of a ram-air effect. This pressurizes the muff to ensure positive pressure in the muff keeps CO out of the heater air. With the passive NACA duct intake, it's possible to have negative pressure in the muff and suck CO into the heater air!

John

N750A

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Permalink Reply by David J. Beaulieu on May 1, 2014 at 8:22pm

"Periscope scoop" or NACA duct it is...I think I just so happen to have one or the other?  Take a look at object on seat and tell me what you think it is.  The heater muff looks very similar to the one shown in the UL parts catalog, though I suspect it may be a Zenith fabrication for their UL350 FWF package.  Thanks for the quick replies and insight.

Dave

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Permalink Reply by John Austin on May 2, 2014 at 8:57am

That's a NACA duct (don't know what the metal bracket is for?). Here's a picture of the scoop I made that I called a "periscope scoop" :

As you can see, it's sitting in the cowl inlet in a high pressure area (actually, just a hole in the bottom of the inlet works pretty well due to the high pressure), but the "periscope scoop" gains a little more from the ram-air effect. If you think it looks suspiciously like a PVC pipe fitting, you're right! 

There is very little flow (pressure) generated by the NACA duct on the side of the cowl. You can enhance the flow by creating negative pressure in the cabin with the vents, etc., but you run the risk of sucking CO into the cabin heat muff. The much higher pressure of the scoop would tend to force leaking air out of the muff, pushing CO away.

In any event, have a sensitive and reliable electronic CO monitor in the cockpit!

John

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Permalink Reply by Don Morrisey on May 2, 2014 at 12:09pm

I like John's periscope.  On a previous plane, I had fresh air vents fed by a naca duct on the side of the plane and a heater box fed from a ram air vent at the same location of John's periscope.  The volume of air thru the heater box was far superior to that coming through the panel fresh air vents.

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Permalink Reply by John Austin on May 2, 2014 at 2:18pm

It's all part of my "total heat management" system! In the winter, I run SCAT to the heater muff for cabin heat and in the summer, I run the SCAT to my finned oil sump cooler duct for oil temp reduction. I cap-off the original NACA duct on the side of the cowl (originally intended for the cabin heat) in the winter, but in the summer, I connect it to the heater muff to provide a continuous stream of cooling air to carry the heat off - since my muffler and muff are right under the engine, it helps to keep heat from being radiated to the engine. The cabin heat valve dumps the heated air overboard at the cowl outlet if the cabin heat is "off."

John

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Permalink Reply by Bob Pustell on May 2, 2014 at 2:51pm

That last point is very important for a muff type installation. There must ALWAYS be air flowing through the muff to avoid overheating the pipes inside the muff. A cabin heat valve (usually located on the firewall, Van's sells a nice one for a good price) is set up so that there is always flow. When in the cabin heat "on" position the heated airflow from the muff is directed through the firewall and into the cabin. When in the cabin heat "off" position the valve dumps the heated air out the bottom of the valve assembly and it exits the lower cowl along with the rest of the air exiting the bottom of the cowl. The muff is always producing heated air -- it has to be to avoid damage to your pipes.

Carb heat valves are set up the same way -- when the carb heat is "on" the heated air from the carb heat muff enters the carb. When the carb heat is "off" the air continues to flow through the muff but is dumped overboard in some manner and the carb is sucking in unheated ambient air.

For flow, you need higher pressure at the entrance to the ductwork system than you have at the exit point. This is usually accomplished by using ram air or inlet scoops of some kind to take advantage of the "free" higher pressure air available from the motion of the aircraft through the air. As already pointed out the NACA inlet will generate less flow than a scoop type inlet. The advantage of a NACA inlet is that it does not generate any drag, unlike a scoop. Life is full of tradeoffs.

You could generate a pressure difference to produce airflow by installing electric blower(s) in the ducts, but that adds weight and complication. You are already generating higher pressure air by moving the plane through the air so you may as well take advantage of that and use the "free" pressurized air with little or no weight and complication penalty.

The proper installation orientation of a NACA inlet is with the hose connection inside the airframe and the flush surface on the outside. In your picture of your inlet on the seat cushion, the outside surface is face down against the seat cushion and the left side (the side with the round hose connection) would be facing aft.

Most planes use two muffs, one for cabin heat and one for carb heat. Some have creative arrangements that let one muff do both jobs. In those cases you usually loose cabin heat when you turn on the carb heat. I live in cold northern New England. I plan to put two muffs on my plane. One will be a dedicated cabin heat system. The second will be a system that does both carb heat or cabin heat. Unless I am using carb heat I will have dual cabin heaters to improve the climate inside the plane during the coldest part of the winter. If I need to use the carb heat I will temporarily loose half my cabin heat but the dedicated system will still be on the job keeping the cabin habitable until the second system can join back in when carb heat is no longer needed.

In warm climate areas, many builders only have a carb heat system. They do not even bother putting cabin heat into the plane. Each builder needs to make their own choices based on their needs and priorities.

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Permalink Reply by John Austin on May 2, 2014 at 3:12pm

Great points, Bob!

I originally had anemic airflow/cabin heat with my NACA duct. I put an electric blower on the cabin heat duct outlet to create negative pressure and boost the heat. It worked great, however, since I was creating increased negative pressure in the outlet duct, it sucked CO into the leaky heat muff!!! I then went to the scoop to stay in the KISS (keep it simple, stupid!) mode! The blower would have been OK if it was on the inlet side of the muff, pressurizing the heat muff with fresh air, but I decided I didn't want more electrics under the cowl if a passive system would work just as well, which it did!

John

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Permalink Reply by David J. Beaulieu on May 2, 2014 at 10:24pm

Great discussion on a system that can easily be taken for granted..appreciate.

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