Lee Elliott wrote: > On Thursday 10 Nov 2005 20:20, Andy Ross wrote: > >>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 > > > I'll try to check the ones I've done over the weekend. The one > that concerns me most is the B-52F. The wing incidence is set > to 6 and the twist to -4 and I'm starting to wonder how it > manages to fly at all.
Nose down. The fuselage is about 5 deg down when in level flight. > > I got some good info on the B-52F from someone who flew around > 3000 hrs in that model and around 6000 hrs total in all models, > apart from the A/B, and it was flying to within around 10 kts or > so of what it should have been doing and was climbing at about > the right rate. > > LeeE > > > _______________________________________________ > Flightgear-devel mailing list > [email protected] > http://mail.flightgear.org/mailman/listinfo/flightgear-devel > 2f585eeea02e2c79d7b1d8c4963bae2d > _______________________________________________ Flightgear-devel mailing list [email protected] http://mail.flightgear.org/mailman/listinfo/flightgear-devel 2f585eeea02e2c79d7b1d8c4963bae2d
