According to my SZD-55 manual (issue IV, March 1993), such an AOA sensing device is mandatory in Canada (page 12a of the Flight Manual/POH). The SP3 system works on the pitot and pressure from another port just below the nose pot (about 1.5" down). It works reliably giving audio warning 3-5 kts above the stall. Once properly adjusted (one-time deal) it is reliable, and has a very low draw (higher when it is alerting - which usually only happens in the flare - or if you're about to stall).

"Electronic STALL WARNING SYSTEM designed to meet the requirements of JAR 22.207 and FAR 23.207 as well. Awarded the O.S.T.I.V. prize, being installed on gliders since 1991 and certified in Poland. Easy to install. No aerodynamic burden. Only one additional pressure sensing hole in the nose of the glider, or in the lower surface of the wing of the aeroplane. Ideal for sailplanes, motogliders and ultralight aeroplanes. The device signals close to stall condition in the range between 5 and 10 percent over the stall speed in straight and circling flight over the whole loading and ballast range. You set the alarm threshold only once, individually for your glider. How does it work ? The difference between pressures Pt and Pc is the function of the two variables : speed and angle of attack. The device measures, calculates and detects the moment when the combination of these two parameters informs you about an approaching stall."
see - http://www.olk.com.pl/indexen.php?bo...roducts&lpx=26

There is a discussion on this topic on the DG website at:http://www.dg-flugzeugbau.de/ueberziehwarnung-e.html.

Tested and available since 1991 - why re-invent the wheel?

2D