My cabin heat is fed outside air through a NACA duct on the side of the cowl (Jabiru 3300).  NACA ducts are great to smoothly and passively flow air, but since it's not an air scoop, the flow is rather anemic!  I recently relocated my flap switch to just under the throttle, leaving a rectangular rocker switch hole beside my cabin heat control, which I had blanked-off.  Long ago, I had made a simple heater fan by securing a 12v fan (new old stock computer server fan) inside a piece of SCAT duct:

I had a 12v accessory plug to power it, and it worked great to boost cabin heat air flow.  I didn't like the dangling power cord, however, and I also later switched my accessory outlet to a USB, so the fan was laid aside and not used again.

With the flap switch relocation, I now had an available switch opening in an excellent location relative to the cabin heat control and I installed a rocker switch and wired it so it illuminates when turned on:

The fan has a diverter vane on the end to force some of the air flow over to the pilot's side.:

This works great and the heat is markedly better in the cabin of my STOL 750!  Just an idea if you have a similar heat valve and need some boosted airflow.



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Nice work John!  Great idea.

I have been thinking about how to increase the cabin heat on my plane.  Same engine and same heat duct so this should work great  Thanks for the idea and the great pictures.  Now I just need to find a fan to fit. 

John B,

This uses a server cooling fan like this:  AVC DB04048b12U

It is 12V and only draws 1.14A.  Has 2 coaxial fans and is very powerful despite its tiny size!  I cut some foam that is used to insulate hot water pipes and rounded-out the flat sides of the square fan case, wrapped with vinyl tape to secure the foam, and then slid it down inside the SCAT (which had the wire removed).  Snugged-down the fan with a zip tie and then used worm drive clamps to secure to the heat valve outlet and the diverter vane I fabricated.


Too bad you're not near me. I've got a small box full of them.

The fan is working great, but it occurred to me that it could work even better if it had a stronger air flow to boost.  I had always thought about putting some sort of air scoop over the NACA duct to boost the flow, but hated to disrupt the clean lines of my cowl.  As I was installing the fan, I thought about the air scoop idea again and came up with this easy mod - I simply cut a piece of piano hinge and clipped the corners on one leaf so it would clear the NACA duct when folded-in and bonded the other leaf to the inside of the NACA duct with 3M VHB tape:

The hinge was positioned so the outer leaf can only open to about 45 degrees to act as a deflector or scoop to force more air into the duct:

The beauty of this is that you can fold it out when it's really cold and need the extra air for the heater and then when it's not needed, it can be folded-in to reduce drag and keep it out of the way:

I purposely slightly bent the hinge pin to make the hinge a bit stiff so it doesn't flop around easily, but the in-flight airflow pressure should keep it open or closed depending on the position it is in prior to flight.

Flew a test flight this afternoon and it worked great - the deflector stayed in position and there was positive air flow through the heater valve with the fan off, and even stronger air flow than before with fan on.  In effect, this will give me 2 settings - "low" heat with fan off and "high" heat with fan on.


This looks even easier and faster than the fan.  I think I will try this one first.  I flew yesterday and with the sun and 22° weather it was not too bad except for my legs getting a bit cold toward the end of the hour.  I think this might make it fairly decent.  Of course the climb rate was unbelievable yesterday in that cold weather.  I was at pattern altitude before I got to the other end of the 6500 ft runway.

Just remember to practice and check the position several times before bonding it with the VHB tape - once it's stuck down, it's there forever! Ha!  An epoxy like West's "G-flex" would be more forgiving - giving you time to position and clamp it in place, but I felt pretty confident and went the VHB tape route.


This looks even easier and faster than the fan.  I think I will try this one first.

One thing to point out - I know you have a Jab 3300/Jab cowl same as me.  The fan will work with the heater set-up as provided with the FWF kit, and for that matter, would work with any cabin heat system that uses a similar cabin heat valve (the valve bypasses the heated air when it is not open to the cabin).

However, I no longer use the standard Bing carb and use an AeroInjector TBI carb instead. (Nothing wrong with the Bing, just wanted to experiment and also gain a manual mixture control).  The AeroInjector pulls filtered air from under the cowl and doesn't use the NACA duct like the Bing did.  So, I use the larger NACA duct (that was used for the Bing's air) for cabin heat now and that's the one I installed the deflector/scoop on.  I use the smaller NACA duct to ventilate the immediate area around the air filter directly attached to the AeroInjector (if you have the standard Bing, the smaller NACA duct is used for cabin heat).

All of this to say that obviously, the deflector/scoop would go on the smaller duct with the standard carb and cabin heat plumbing.  Since that NACA duct is smaller and routed through an air box under the cowl, I suspect the results of putting the deflector on it would be less dramatic, but that's just speculation - who knows? - there might be higher pressure air there to start with and it might work just as well!

Also, if you use VHB tape to secure it in the smaller NACA duct, I haven't looked closely, but the area where you bond it will have to be perfectly flat for the tape to bond the flat aluminum hinge leaf.  If it is slightly curved (and the smaller duct is below the larger one and in an area that starts to curve), you might have to build-up the inner surface to flat with some epoxy resin and fiberglass or use a different method to secure it.


Join us next week when John adds a cowl flap control to operate the hinge while in flight. Same bat time, same bat channel.

Hmmmm ....      ; )



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