Build Your Own Stall-Warning Device Certified aircraft have had them for decades– but not Experimentals. By Michel Suire
If you’ve flown in a certified aircraft used to attach the device to the leading During cruise and other maneu- that was built within the last 50 years, edge and fine-tune the point where the vers where there’s plenty of airspeed to it probably was equipped with a stall- warning horn sounds. prevent a stall, the tab on the leading warning device. However, there’s no edge remains in the airflow that goes requirement for stall warning systems in How It Works under the wing, and the microswitch homebuilts. That doesn’t mean it’s not a As a wing moves forward, the air splits remains off. However, as angle of attack good idea to have one. And fortunately, into two flows. One flow passes above increases and airspeed decreases, the tab it’s easy to build your own. the wing, while the other goes under. moves into the airflow moving above The device shown in this article uses a The point on the leading edge where the the wing. When the air pushes the tab microswitch and small tab to activate a flow splits is called the cutoff point. This up (typically about 5 knots above stall warning horn about 5 knots before the point isn’t fixed—it moves up or down speed), it activates the microswitch and stall. An adjustable mounting plate is according to the wing’s angle of attack. sounds the horn.
36 KITPLANES June 2016 www.kitplanes.com & www.facebook.com/kitplanes During normal cruise, the cutoff point is As angle of attack increases and the wing located at Point C. stalls, the cutoff point moves to Point D. Positioning the Tab and Fine-Tuning the System Mounting Plate You can move the mounting plate up You will probably have to do some test- or down by adjusting the screws that ing to correctly position the tab and attach the mounting plate to the leading mounting plate. As shown in the draw- edge. If the tab is too close to Point C, ing below, the tab needs to be somewhere the horn will sound too early. And if the Schematic for the electrical circuit that between Point C and D in level flight. If tab is too close to Point D, the horn will activates the warning horn. the tab is always above Point C, the horn sound too late. will be constantly on. If the tab is always As you make your adjustments, the outboard portion of the wing where below Point D, the horn will never come don’t forget: the ailerons are located, providing lateral on. To locate Point C: • If the warning is too early, move the control at the beginning of the stall. • Place the fuselage reference in a plate down. Stall strips can be made from wood Build Your Own horizontal position. • If the warning is too late, move the and are typically triangular in shape • Use a bubble level or place a plumb plate up. measuring 8 inches long by .25 inches bob tangentially to the leading edge. tall. The strips are positioned the same • The contact point will be Point C, Stall Strips as the tab, i.e., between Points C and D. which will be the upper limit posi- In order to improve the stall character- The exact position will be determined by tion of the tab. istics of some airplanes, it may be neces- successive tests. Stall-Warning Device sary to add stall strips. These strips are typically placed on the inboard portion Conclusion of the leading edge, close to the fuse- A stall warning device isn’t something lage. The goal is for the strips to create you’re likely to use on every flight. But turbulence, which causes the inboard should you ever get distracted, it can be portion of the wing to stall first. Mean- a very nice thing to have. If it only saves while, air is still flowing smoothly over you once, it’s worth it. J
The microswitch is activated by a small metal tab that extends through a slot in The tab and mounting plate should be Like the tab and mounting plate, stall the mounting plate. The plate attaches to located between Points C and D. strips are also located between Points C the wing’s leading edge with screws. and D.
Photos and Illustrations: Michel Suire KITPLANES June 2016 37