Low-altitude aircraft with unpressurized cabins often make use of a ventilation system in the cockpit -- some complicated, others as simple to operate as the CC3251 snap vent. These vents open and close with the rotation of a vent tab on the inside of the cockpit windshield, which is typically made of acrylic plastic or polycarbonate material called Lexan. Just cut the right-sized hole in an out-of-the-way spot, then snap your vent on and go.
Remove the side panel of your airplane's windshield, according to your plane's specifications. Some windshields will wrap around the cockpit, requiring you to remove the entire assembly. In any case, this typically involves removing interior screws or bolts attaching the panel to the cockpit.
Lay down your sheet and place the windshield on top of it. Determine the placement of your snap vent. Mark the location of the center with a push pin. Pick a spot out of the pilot's forward field of vision.
Center your piece of plywood on your drill press table and clamp it down. (Using a rotary saw or hand drill could damage your windshield.) Lay the section of windshield across the wood, centering on where you made your snap vent mark. Clamp the windshield down with vises.
Drill a 3 1/4-inch hole for your snap vent. When halfway through the hole, flip the windshield and cut the rest from the other side. Sand the edges of the hole when finished.
Snap the female portion of your snap vent into the hole you've drilled from the outside of the windshield. Snap the inside half of the vent onto the outside half. Test out the vent by rotating the inside handle. With the handle upward, the vent is closed; in any other position, the vent will allow various amounts of air flow.
Return your windshield to your aircraft with your new CC3251 snap vent installed.