On Friday 11 Nov 2005 02:47, Josh Babcock wrote: > 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
Depending on weight, alt and speed, 5 deg nose-down could be correct. The incidence of +6 degrees is correct but I had to estimate the twist. I should be able to have a look at it sometime this weekend. Ta for having a look. LeeE _______________________________________________ Flightgear-devel mailing list [email protected] http://mail.flightgear.org/mailman/listinfo/flightgear-devel 2f585eeea02e2c79d7b1d8c4963bae2d
