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. 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
