Real "Cool"Avionics

Making a ducting teefor cooling air

TESTING HAS VERIF IED THAT FOR most

electronic components or sys-tems the equipment ' s l i f e spancan usually be correlated to oper-a t ing tempera ture . That's whym a n u f a c t u r e r s of ten include astatement in their user's guidesthat suggest or require that an airsource for cooling the electronicequipment be part of the hard-ware installation.

From the baking heat of a sum-mer tarmac and the close proximityof other heat-generating compo-nents to the biting chill at high al-titudes, avionics are often subjectedto more severe temperature varia-t ions t h a n t h e i r ground-basedcounterparts, which live in morecontrolled environments.

With that in mind, when tech-nicians install avionics they oftenprovide a source of cooling air. Acommon i n s t a l l a t i o n t ha t pro-vides this cooling air is a small airscoop riveted to the airframe jus tforward of the aircraft's door. Theair scoop channe l s outside ai rt h r o u g h the fuselage w a l l to aplenum where scat tubing is con-nected. This t ub ing t r ans fe r s thecooler air to the innards of an avion-ics box, like a transceiver.

The installation of the scoop re-quires a 1/4-inch starter hole drilledinto the fuselage skin and enlargedto 1/2 or 5/8 inches with a Green-

lee punch. Additional small holeswill also need to be drilled to acceptthe rivets that will hold the scoopto the fuselage skin. Although thescoop creates l i t t le drag, the realpenalty is the cost and time neededto instal l the scoop. A low-cost al-ternative, which does not required r i l l i ng holes in the fuselage, is aducting tee.

Making a Ducting TeeMost a i r c r a f t have an air duct

system that supplies outside coolair to the cabin. This air is carriedfrom an air scoop or other port toa plenum, where it may be mixedwi th warm air for win te r opera-tions. The ducting tee is simply in-

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serted into the scat tubing before itreaches the heat-mixing plenumand siphons some of the cool airfor the avionics.

The tee is constructed f rom

acrylic tubing, which you can pur-chase from various sources, likeU.S. Plas t ic Corporat ion (vis i twww.usplastic.com). The outside di-ameter (OD) is matched to the in-

side diameter (ID) of the vent sys-tem scat tubing (e.g., 2.25 inches).

A common air i n p u t to theavionics bay consists of a 1-inchlong nipple by 5/8-inch OD. The

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nipple is welded or press-fit to therear of the avionics tray. Thus, thescat t u b i n g between the tee andthe avionics must have a 5/8-inchinside diameter.

For test purposes, our initial in-stallation included tubing withridges on the inside wall. Air flow-ing through the tubing created aw h i s t l i n g sound whose p i tchchanged with aircraft airspeed. 1suspect that this may have beenthe first instance of a flying pipeorgan. Changing the tubing typeto CEET (scat with an inside, aswell as outside, coat ing) e l i m i -nated the resonant sound sinceCEET (available from Aircraf tSpruce & Specialty) has a smoothinner surface.

Cut the large diameter acrylictube to a length of 5 inches. Usea fine-tooth saw because acrylichas a tendency to shatter. 1 useda hacksaw to make most of thecut and f i n i s h e d the las t fewstrokes with a fine-tooth hack-saw blade. Sand the cut surfacewith 100-grit sandpaper. Drill astar ter hole wi th a 1/8-inch bitand gradual ly widen the hole toa 5/8-inch diameter.

Elbow BendingThe elbow that is mounted in-

side of the tee body is simply a 5-inch piece of 1/2- inch ID by5/8-inch OD acrylic tubing bent toa 90-degree angle. Because the tub-ing is rigid, you must heat it be-fore you bend it. Use a largediameter shallow pot filled with 3

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inches of water. A 10-inch diame-ter pot 5 inches high works well.Before heating the water, locatean expans ion sp r i ng at least 6inches long with an outside diam-eter of 1/2 inch. Slide the springinside the cut acrylic tubing. Thiswill prevent the acrylic tube fromcollapsing when it is heated.

Bring the water to a light boil.Dip the ac ry l i c t u b i n g wi th thespring into the hot water so thatapproximately 3 inches of it is im-mersed. Hold the spring on oneend with a pair of pl iers and themiddle of the tubing with anotherp a i r of p l i e r s . A f t e r abou t oneminute of immersion, remove thetubing and attempt to bend it. Ifit does not bend readily, the tub-ing is not warm enough. Give itanother bath in the boiling wateruntil it feels ready. After bendingthe tubing, hold it in the bent po-s i t ion u n t i l i t cools—approxi-mately 30 seconds.

Now you're ready for the f inalassembly. I n s e r t the bent tubeinto the barrel through the drilledhole. Mix a small amount of autobody resin with hardener and dipthe tip of a wooden penci l in tothe solution. Apply a drop to eachside of the outside seam. Wait forseveral hour s for the resin toharden and repeat the process;this time apply the resin to the in-side seam. That should do thetrick! If additional strength is de-sired, a 1/2-inch wide strip of autobody fiberglass cloth dipped inthe mixed resin can be "laid-up"against the tubing junction.

F ina l ly , inser t the assembledducting tee into the scat tubing,and your avionics will keep theircool. E&

I am Indebted to Dick Wilkinsonwho took the photographs for thisand other articles I've written andwho, in this instance, flew with meto gain a better understanding of thetechnical details.

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