Old 2nd Jan 2019, 23:44
  #76 (permalink)  
Join Date: Feb 2009
Location: Seattle
Posts: 379
Originally Posted by gums View Post

according to FCeng

Well, FCeng says that the vane deflection requires a "0.5 gain factor" someplace in the sftwe or A2D chip/device, wherever that is.. Is the "gain" two degrees of wing chord to free stream flow? Is it sensed AoA times two or divided by two? Hard to believe the AoA difference between the sensor and the wing chord remains constant when airflow about the nose doesn't seem to be at a constant angle when the plane changes the actual AoA, so a linear "correction" seems too easy. Who does that signal processing? And most important....and according to latest diagram we have of the "system"....

Our diagram shows analog AoA is sent from the SMYD and thence to the stick shaker. But it has been stated that AoA is also used to "correct" airspeed and maybe that value is used for the STS versus raw pitot dynamic pressure. Where do the ADIRU's , cockpit displays and FCC boxes and the MCAS get AoA? From the SMYD? . There must be plenty of room for mischief here. Ya think?

Gums sends...
Gums - 737 Max conversion from AOA vane to AOA body is an approximation that uses a scale factor of slightly more than 0.5 plus a small second order term and a constant offset. This is clearly a simplified fit to a non-linear relationship that was designed to be sufficiently accurate over the range of interest. A vane movement of 2 degrees equates to a free stream AOA change of about 1 degree. The sensor base plate has eight holes and a pair of alignment pins that look from pictures I have seen to be located roughly fore / aft along the horizontal line of symmetry of the circular base plate. I do not know the details about whether or not installation is critical with regard to which pin is forward and which pin is aft. I suspect that the same sensor is designed to work on either side of the plane so there must be something in the wiring or signal processing down stream that differentiates between left and right and sorts out whether clockwise motion is positive or negative AOA change.
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