Real "Cool" Avionics Making a ducting tee for cooling air

TESTINGHAS V E R I F I E D THAT FOR most

electronic components or systems the e q u i p m e n t ' s life span can usually be correlated to opera t i n g t e m p e r a t u r e . That's why m a n u f a c t u r e r s often include a statement in their user's guides that suggest or require that an air source for cooling the electronic e q u i p m e n t be part of the hardware installation. From the baking heat of a summer tarmac and the close proximity of other heat-generating components to the biting chill at high altitudes, avionics are often subjected to more severe temperature variations t h a n t h e i r ground-based counterparts, which live in more controlled environments. With that in mind, when technicians install avionics they often provide a source of cooling air. A common i n s t a l l a t i o n t h a t provides this cooling air is a small air scoop riveted to the airframe just forward of the aircraft's door. The air scoop c h a n n e l s outside air t h r o u g h the fuselage w a l l to a plenum where scat tubing is connected. This t u b i n g transfers the cooler air to the innards of an avionics box, like a transceiver. The installation of the scoop requires a 1/4-inch starter hole drilled into the fuselage skin and enlarged to 1/2 or 5/8 inches with a Green-

lee punch. Additional small holes will also need to be drilled to accept the rivets that will hold the scoop to the fuselage skin. Although the scoop creates little drag, the real penalty is the cost and time needed to install the scoop. A low-cost alternative, which does not require

d r i l l i n g holes in the fuselage, is a ducting tee.

Making a Ducting Tee Most a i r c r a f t have an air duct system that supplies outside cool air to the cabin. This air is carried from an air scoop or other port to a plenum, where it may be mixed w i t h warm air for w i n t e r operations. The ducting tee is simply in91

serted into the scat tubing before it reaches the heat-mixing plenum

and siphons some of the cool air for the avionics. The tee is constructed from

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acrylic tubing, which you can purchase from various sources, like U.S. Plastic Corporation (visit

www.usplastic.com). The outside diameter (OD) is matched to the in-

side diameter (ID) of the vent system scat tubing (e.g., 2.25 inches). A common air i n p u t to the avionics bay consists of a 1-inch

long nipple by 5/8-inch OD. The

nipple is welded or press-fit to the rear of the avionics tray. Thus, the scat t u b i n g between the tee and the avionics must have a 5/8-inch inside diameter. For test purposes, our initial installation included tubing with ridges on the inside wall. Air flowing through the tubing created a w h i s t l i n g sound whose p i t c h changed with aircraft airspeed. 1 suspect that this may have been the first instance of a flying pipe organ. Changing the tubing type to CEET (scat with an inside, as well as outside, coating) e l i m i nated the resonant sound since CEET (available from Aircraft Spruce & Specialty) has a smooth inner surface. Cut the large diameter acrylic tube to a length of 5 inches. Use a fine-tooth saw because acrylic has a tendency to shatter. 1 used a hacksaw to make most of the c u t a n d f i n i s h e d t h e l a s t few strokes with a fine-tooth hacksaw blade. Sand the cut surface with 100-grit sandpaper. Drill a starter hole w i t h a 1/8-inch bit and gradually widen the hole to a 5/8-inch diameter.

Elbow Bending

The elbow that is mounted inside of the tee body is simply a 5inch piece of 1/2-inch ID by 5/8-inch OD acrylic tubing bent to a 90-degree angle. Because the tubing is rigid, you must heat it before you bend it. Use a large diameter shallow pot filled with 3 93

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inches of water. A 10-inch diameter pot 5 inches high works well. Before heating the water, locate an e x p a n s i o n s p r i n g at least 6 inches long with an outside diameter of 1/2 inch. Slide the spring inside the cut acrylic tubing. This will prevent the acrylic tube from collapsing when it is heated. Bring the water to a light boil. Dip the a c r y l i c t u b i n g w i t h the spring into the hot water so that approximately 3 inches of it is immersed. Hold the spring on one end with a pair of pliers and the middle of the tubing with another p a i r of p l i e r s . A f t e r a b o u t one minute of immersion, remove the tubing and attempt to bend it. If it does not bend readily, the tubing is not warm enough. Give it another bath in the boiling water until it feels ready. After bending the tubing, hold it in the bent position u n t i l it cools—approximately 30 seconds. Now you're ready for the final assembly. I n s e r t the bent tube into the barrel through the drilled hole. Mix a small amount of auto body resin with hardener and dip the tip of a wooden pencil i n t o the solution. Apply a drop to each side of the outside seam. Wait for several h o u r s for the resin to harden and repeat the process; this time apply the resin to the inside seam. That s h o u l d do the trick! If additional strength is desired, a 1/2-inch wide strip of auto body fiberglass cloth dipped in the mixed resin can be "laid-up" against the tubing junction.

Finally, insert the assembled ducting tee into the scat tubing, and your avionics will keep their cool. E& I am Indebted to Dick Wilkinson who took the photographs for this and other articles I've written and who, in this instance, flew with me to gain a better understanding of the technical details. 95