[Sorry for the delay. This one was hard, and had to wait for the weekend for an investigation.]
David Megginson wrote: > Andy: unfortunately, none of your suggestions helped (details > below). How are you modelling washout in YASim? From the violent > roll that comes with every stall, it looks like all of the wing is > stalling simultaneously, so the plane loses roll control from the > ailerons at the same time as it loses lift from the wings. Wow, good call. The lack of washout modelling is indeed the problem. And the details turn out to be great fun, to boot: What happens in the current (no washout) implementation is that the aircraft nears a stall some (just a tiny bit) non-zero roll rate. The wing going down therefore experiences a higher AoA than the upward wing. In normal flight modes, this has a pro-stability effect. The asymmetric lift distribution opposes the roll rate. Once the wings are beyond the stall point, however, increasing AoA's decrease lift and this causes a pro *roll* moment. That's a divergence, and the aircraft very rapidly rolls off into a wild departure. I've always understood washout the way you explained it. It's there to keep the ailerons unstalled and the plane controllable at high AoAs. I figured this was something that could be "modelled away" by simply increasing the stall width to reflect the fact that the whole wing stalls gradually at different AoA's. But that is a demonstrably minor effect; you can pump the aileron effectiveness up to ridiculous magnitudes in YASim and you will still be doing snap rolls. The real reason for washout (or at least a better physical explanation) is this: the washout that maintains the tips below stall AoA keeps as much of the "stable" derivative as possible out on the wing tips where the moment arm is long. If the early stall happens near the fuselage on a short moment arm, then the overall behavior will still be stable, not divergent, even past the peak of the "whole aircraft" lift curve. It also explains why aircraft which are "normally" stable in the stall can sometimes do divergent snap rolls (c.f. Luke and Ryan's anecdotes). If you pull the AoA high enough to put the tips in the stall too, then the divergent mode reasserts itself. You can do this with an accelerated maneuver, for instance. Also, flying at aft c.g. configurations results in more relative elevator authority which could plausibly overwhelm the washout at the tips. Cool. I learned something this weekend. A "snap roll" is a physically well-defined thing: it is a roll executed in a post-stall environment where the roll-moment-due-to-roll-rate coefficient is divergent. So anyway, YASim needs to model washout. In principle, this should be pretty easy. Each wing segment (Surface object, as currently implemented) gets its own orientation already. We just need to decide on a way to specify it to the solver. Would a linear interpolation between "base" and "tip" incidences work? I don't know much about washout design as implemented on typical aircraft. A fancier mechanism would allow you to specify washout as an interpolated curve per-station curve along the span, but that sounds like it might be overkill to me. Does anyone have a preference? Andy -- Andrew J. Ross NextBus Information Systems Senior Software Engineer Emeryville, CA [EMAIL PROTECTED] http://www.nextbus.com "Men go crazy in conflagrations. They only get better one by one." - Sting (misquoted) _______________________________________________ Flightgear-devel mailing list [EMAIL PROTECTED] http://mail.flightgear.org/mailman/listinfo/flightgear-devel
