Posted for "Colyn Case on earthlink" <colyncase@earthlink.net>:

Marv,

do you understand how the pss aoa figures all this out from the very short
calibration procedure?
the latter afaik only tells the box where zero lift is for flaps off and
flaps on.
even if it's a linear relationship to pressure, it would need at least one
other point on the curve to do the interpolation.

Colyn
 
[The algorithm that Jim used to calculate AOA accurately is what made his product patentable.  Up until he started his research the data that was being used from similar probes (what they use on the Dynon, and a couple other AOA offerings) was simply the input from an alpha probe.  The alpha probe is a lot like a pitot tube but instead of being pointed directly into the relative wind to collect ram air in a single hole, it has angled upper and lower surfaces (imagine a right angle shaped wedge tilted 45 degrees so the upper surface looks up, the lower surface down) with a port in each surface and depending on the relative angles between those ports and how the relative wind struck them you'd get a differential pressure.  Unfortunately, that sort of probe only provides 2 of the data points required to accurately calculate AOA... Jim brought on the next step by incorporating dynamic pressure (the difference between pitot and static) and discovered that if he divided the wing differential pressure by the overall airframe dynamic pressure the results correlated exactly to the AOA.  During the calibration process the data points that are being stored are actually points on the L/D curve.... the zero-g maneuever identifies the zero degree angle of attack, and by flying and then recording the port indications at 15% above the stall the instrument learns that (15%) data point on the curve.  Then, using those 2 data points the rest of the curve is calculated and the results displayed on the panel indicator.  That's about it.   <Marv>            ]