nuts & bolts

craft & technique A Breath of Fresh Air Choosing a cockpit ventilation system for your homebuilt Greg Laslo Photography by Ken Ibold

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new pilot doesn’t get very far into his logbook before he realizes just how roasty-toasty an airplane can get when the sun’s out. Indeed, a poorly ventilated cockpit is better suited for incubating whooping crane eggs than fledging new pilots. Naturally, when you’re building an airplane of your own, you’re hoping to avoid the “inconvenience” of launching a sauna with wings. To do so, you must pull the fresh air in to push the hot air out. There are two general strategies to collect air from the outside—scoops in the fuselage and holes in the canopy or window. Given the different materials involved, each strategy requires a different installation process. Understanding the difference can help you blow through the process like a breeze through the trees.

Open Doors The fuselage scoop takes a couple of forms—the hinged clamshell door, such as on the Bellanca/American Champion aircraft; a generic air scoop that projects outward from the aircraft; and the NACA scoop, the ubiquitous divot chipped out of the side of the fuselages of RVs, Zenith CH 601s, Lancairs, and other solidbodied kits. A clamshell door usually operates by pulling a knob to open and close it. When you’re installing one of the vertical mail-slot-shaped vents, be sure there’s a fair amount of friction in the connection linkage to permit you to control how much air flows into the vent. Too little friction will cause the door to blow completely open, which means ventilation will have two settings: all or nothing.

The most elegant ventilation system features an eyeball air vent connected by SCAT tubing to a NACA inlet. 102

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If you’re building your own, the door must be big enough that it projects out of the aircraft’s boundary layer into the slipstream to pull air into it. Similarly, a generic air scoop must also project into the slipstream; only in this case, it’s there all the time, which is a high-drag solution to your ventilation problem. NACA scoops allow you to use that laminar airflow to your environmental advantage by efficiently sucking the air that blows by the fuselage into the cabin. They’re typically molded into composite airframes, and you’ll only need to cut an outlet hole in the back of the shape to allow air to flow across the shell. A die grinder is perfect for that job. In a metal kit, such as a Vans Aircraft, the shape has probably already been cut in the skin. All you

A screen on the NACA inlet will help keep out bugs and debris.

have to do is rivet and ProSeal the scoop onto the back of it. You can also use washers drilled to the size of your rivet for extra support. If a hole is not there, locating the ducts will probably be a function of where they fit easiest, so long as they’re out of the way of the engine exhaust. Remember, the vent is shaped like an arrowhead, pointing in the direction of travel, and the typical location is slightly below and in front of the instrument panel. In a composite aircraft, use a template to cut into the fuselage and bond the vent to the backside. In aluminum aircraft, use a thin sheet of aluminum to make a template and cleco it to the inside of the fuselage. Scribe the outline of the vent with a knife, then drill a hole into the side of the airplane so you can snip out the general shape of the vent. Finally, fold the skin along the scribed lines so it breaks to form the vent shape. The piping for the air can either be a molded piece of composite material, aluminum, or plastic for a short run—these usually come with the NACA kit—or, if it’s a longer distance, use 2-inch wire-reinforced impregnated-cloth Aeroduct air hoses attached to flanges with hose clamps. An eyeball swivel vent is the customary service end of the vent system, and it rotates to adjust the flow of air entering the cabin. The Aeroduct hose attaches to a flange on the back of the vent through the instrument panel. Other fuselage ventilation options are also available. A fairing with a fresh-air hole could be installed in the engine air inlet of the cowling, although this option requires a long vent tube to run through the inhospitable environment of the engine compartment, around the engine, and into the cockpit. It also requires a screen over the intake hole to keep bugs and other airport critters out. In fact, screens are a pretty good idea for every fuselage vent for that reason. The Cessna-style leading-edge-ofthe-wing intake channeled into a EAA Sport Aviation

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craft & technique Snap vents are sized from 2 to 3-1/4 inches, and some are tapered to collect more airflow. The vents are typically installed low in the window in such a location as to allow the pilot to push open the vent or twist the “nozzle” to adjust the airflow by decreasing the intake hole size. Obviously, it should be located in front of the passenger who’s to benefit from the breeze. RANS uses these on their S-7 kits, with the suggested placement for the pilot’s hole in the lower corner of the windscreen, and the passenger’s in the lower corner of the pilot’s window. Snap vents are also one solution— though an inelegant one—to the problem of air exhausts, particularly in composite aircraft, which are generally of tight enough construction that there are few routes for air to escape. A separate vent allows air to exit the aircraft, creating positivepressure ventilation. A snap-in circular vent is probably the easiest ventilation device to install. All that’s required is drilling a hole in your sheet of acrylic. Easy, right? Actually, you’ll do fine as long as you don’t use a reciprocating saw or a handheld drill. Instead, use a hole saw or circle cutter attached to a drill press. Support and clamp your window to the drill press table, and back the window with a piece of wood. If possible, replace the drill bit in the cutters with a 60-degree, zero-rake drill bit designed for use with acrylic to keep the plastic from riding up the bit and cracking. Or, you could first drill a pilot hole with the same-sized acrylic bit. It goes without saying that you should practice on scrap before you make the Big Cut on your window. Another option is a sliding ventilation window, such as that found on Extra aerobatic aircraft, or a fold-down window, such as is commonly found on aluminum Piper aircraft. In each case, you’ll use a template to spot your opening, and cut the window with an angle grinder with a cut-off wheel, a

Clamshell vent latching mechanism should have enough friction to hold it in place against the wind.

Simple clamshell vent opens into slipstream, so placement is important. vent tube is another option, depending on how your high-wing aircraft is constructed. A modern take on this includes installing a NACA vent on the underside of the wing near the strut, channelling that airflow through flexible hose to eyeball swivel vents on the cockpit’s leadingedge wing roots, as on the Bearhawk bush-plane kit. Other builders have cut small—say, 4 inches wide— intakes into the leading edge of their wings outside the propeller wash and run the plumbing to the instrument panel.

Window Treatments While fuselage-mounted air scoops require a collection of parts to get fresh air from point A to point B, window-mounted air vents are basically plug-and-play. But since you’re going through an acrylic window instead of an aircraft fuselage, your techniques will be a little different. 104

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Coolin’ With a Cooler router, a Dremel tool, or a coping saw with a bone blade. If possible, set up a jig to keep yourself from making an expensive mistake, and take your time. Cut your vent hole smaller than finished size, and sand-and-test until you get a good fit. Use a Scotchbrite drum on a drill to smooth the edges, then round every one over and sand smooth with fine wet-dry abrasive paper. Be sure to gently round the corners of the cut so you don’t cause a stress crack. With your vent opening complete, you’ll have to attach the vent cover. Use a 60-degree, zero-rake bit to drill the rivet or bolt holds for the window, but don’t use a lot of pressure on the drill to make the cut; let the drill do the work. If you need some lubrication, use a squirt of soap-and-water solution. Oversize your holes slightly, about a third larger than your hardware.

Aircraft A/C on the Cheap Dry adiabatic rate aside, recreational cruising altitudes can be pretty rough on a pilot and her passengers during the summertime, especially in an aircraft with a bubble canopy. Wouldn’t it be great to have the same climate-control system in your airplane that you have in your car? Short of installing a full air-conditioning system, the Kooleraire 12-volt air conditioner offers a homebuilt solution to the problem of keeping you cool, assuming you’re not already running on the ragged edge of weightand-balance. The device fits into a 25-quart Igloo or 28-quart Coleman chest cooler and pumps out the ice-chilled air from the cooler. According to the company, its 100-cfm fan will keep up to a 7-by-7-by5-foot space chilled. It’s powered by a 12-volt cigarette lighter or a rechargeable battery pack, and the unit doesn’t reduce the capacity of your cooler. For more information about the $39.95 unit, visit www.kooleraire.com. Acrylic shrinks and expands at different rates than the screw or rivet you’ll use to attach the piece, and if it has to expand against that metal it might crack your canopy. A tight fit is all you need to keep the attachment from leaking in inclement weather. The steps you’ll take to install cockpit ventilation into your air-

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plane depend on how you decide to wrangle the cooler air outside— whether through a fuselage scoop or with a window-mounted vent. Either way, by properly installing the appropriate ventilation system for your airplane, you’ll take a huge leap toward keeping yourself and your passengers comfortable in all but the hottest desert scorchers.

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