On Mon, 2003-01-06 at 11:31, Andy Ross wrote: > [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?
I'm sure any twist is hard enough to manufacture, much less that which is non-linear, so I'm sure linear will be just fine. > > Andy -- Tony Peden <[EMAIL PROTECTED]> _______________________________________________ Flightgear-devel mailing list [EMAIL PROTECTED] http://mail.flightgear.org/mailman/listinfo/flightgear-devel
