Josh Babcock > > 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. > >
The negative incidence issue might also explain some odd values I was forced to put into the B29 config to make it fly well. I'll try an updated version later. Vivian _______________________________________________ Flightgear-devel mailing list Flightgear-devel@flightgear.org http://mail.flightgear.org/mailman/listinfo/flightgear-devel 2f585eeea02e2c79d7b1d8c4963bae2d
