Experimenter

MAR 2015

Experimenter is a magazine created by EAA for people who build airplanes. We will report on amateur-built aircraft as well as ultralights and other light aircraft.

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28 Vol.4 No.3 / March 2015 HINTS FOR HOMEBUILDERS Removing & Replacing Avionics Dick Koehler shows how to remove and then replace avionics from the instrument panel. Dick is a Technical Counselor for EAA Chapter 186, A&P aircraft mechanic with Inspection Authorization (IA), and SportAir Workshop instructor. Cutting Holes With a Fly Cutter Sebastian Heintz and Roger Dubbert from Zenith Aircraft demonstrate how to use a fl y cutter to cut a large hole in an aluminum fuel tank used to mount the fuel sending unit. Cutting Dacron Sailcloth Brian Carpenter from Rainbow Aviation Services shows how to cut Dacron sailcloth by modifying a soldering iron into a hot knife. Joining Foam Cores EAA Technical Counselor Mike Busch shows how to join foam core pieces together to make larger foam sections. HINTS FOR HOMEBUILDERS VIDEOS HERE ARE SOME OF THE LATEST HINTS FOR HOMEBUILDERS ADDED TO THE MORE THAN 450 HINTS CURRENTLY AVAILABLE HERE: I WAS DOING AN ANNUAL last month when the checklist called for testing the stall warning system. Most certificated aircraft have a stall warning horn, and most homebuilts do not. On certificated aircraft, there are two basic types for general avi- ation aircraft. The Safe Flight version has a mechanical tab that extends from the lower front edge of the leading edge. It is located at the airfl ow separation spot on the leading edge where the airfl ow reverses direction just prior to stall. The reversing airfl ow pushes the tab upward, and the tab actuates a microswitch, setting of a warning horn and/or light in the cockpit. The other common type of warning system is on many Cessnas. It is fully pneumatic in operation. Again, the "sensor" is on the lower leading edge, but in this case it is just a slot in the leading edge skin. When the airfl ow reverses direction, the air pressure reverses from pressure to suction. The "sen- sor" port is connected to a reed horn in the cockpit with a sec- tion of hose. The horn will buzz when air is sucked through it, providing the pilot with an aural indication that a stall is about to happen. On the fi rst electromechanical type of stall warning system, testing involves turning on the master switch so that the system is energized, and then gently fl ipping the sensor tab upward with your fi ngers and listening for the cockpit buzzer, or seeing the light. This test checks the functionality of the system, not the ac- curacy, but it is the best one can do during the annual. On the second Cessna system, the best way to test the system is to stand on a stool, put your mouth over the port, and suck gently. If all is well, you will hear the buzzer. This can be a rather disgusting exercise if there are dead bugs and whatever on the leading edge. As an alternative to using your mouth, Aircraft Spruce & Specialty sells a suction bottle for only $10 that is de- signed to provide suf cient suction to set of the horn. Anyway, back to the annual. It was a Cessna, and I did not have the $10 suction bottle; I did not particularly want to suck on the leading edge, so I used a blower nozzle on a hose connected to the compressor. With the compressor set to about 100 psi, the blower nozzle put out a mighty blast of air. The trick then was to angle the blast of air up and over the leading edge, like the airfl ow would be following, as the aircraft approached a stall. I was doing this, but due to the noise of the rushing air, I could not hear if the horn was activating. I asked the owner to operate the air nozzle while I went into the cockpit to listen for the horn. Don't Do This! Unintentional damage BY RICHARD KOEHLER Photography by Richard Koehler

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