After some prodding from Curt, I finally spent a few hours yesterday tracking down the "pitch down" discontinuity in the Citation.
Well, I didn't find a discontinuity. I can now graph the lift curve from a Surface (a real one, part of the real aircraft, not an isolated test instance) and verify that it's valid and correct looking through the entire AoA regime. But I think I *did* find the problem: it seems that I, er, "misdocumented" the incidence and twist parameters in the YASim configuration. The README.yasim file states that these numbers are positive for positive AoA (i.e. a positive incidence on a wing generates extra lift, and a negative twist causes the wing tips to stall after the root). But the code was interpreting the number as a rotation about the YASim Y axis, which points out the left wing and therefore is positive *down*. Oops. The reason the citation exhibited this especially is just luck: the file lists an incidence of 3.0 (which is relatively high, and the inversion bug therefore puts the wing 3 degrees closer to a negative stall) the solver happens to generate a nose-down cruise configuration of about 1.5 degrees, and the elevator authority is actually quite high (which causes higher pitch rates under autopilot control). So the bottom line is that Curt was right: it *was* the negative AoA stall (probably the tail's, not the wing's) happening too soon. :) I'm a little leery of changing this in code this close to a release -- the risk of breaking working aircraft is too high. For the short term, this can be fixed in the Citation-II.xml file by simply negating the incidence and twist values on the wing. I did this and tried the autopilot in a maximum speed cruise at low level (which should produce the highest nose-down AoA) without any odd behavior. Curt, can you try that and see if it appears to fix the handling issues? Likewise, anyone with a YASim aircraft that makes use of incidence or twist values is encouraged to try the same modification and report any problems. We can go back after the release and fix the code and all the aircraft files. Andy _______________________________________________ Flightgear-devel mailing list [email protected] http://mail.flightgear.org/mailman/listinfo/flightgear-devel 2f585eeea02e2c79d7b1d8c4963bae2d
