On Thursday 07 August 2003 03:53, Jim Wilson wrote: > Lee Elliott <[EMAIL PROTECTED]> said: > > > > The B-52 is proving quite a tricky one to get right. Some of the > > characteristics almost seem mutaully exclusive and it can be hard to > > reconcile them. For example, I found max speeds of 554 kts @ 21000 ft and > > 495 kts at 46500 ft but I can't get a working solution that'll climb that > > high. Another characteristic is the take-roll & clearence for a fifty ft > > obstacle these are 7000ft & 9100ft respectively, which is difficult to > > reconcile with the speed and alt ratings. Yet another 'unusual' > > characteristic are the flaps. These are pretty big but only have two > > positions - fully deployed (35deg) or fully retracted, with no in-between > > settings and can tend to produce huge pitch changes, which is the last thing > > you want in a relatively under-powered a/c like the B-52 at low speed. > > > > In other words, I dunno why it's not stalling properly. > > > > Try this (diffs to current CVS version): > > http://www.spiderbark.com/fgfs/b52.diff > > For some reason YASim isn't going to work with an approach AoA of 0.5...even
Oops! That wasn't right - I've got a few versions I've been trying and I think I may have sent the wrong one. I'd been experimenting to try to establish trends. I'd guess in real life it should be less than 2 deg. From what I've been told and from what I can make out from pics and video, the approach is very shallow and it should be a little bit nose down. > if that is the correct figure. I'm not sure if the angle is supposed to > include the wing incidence (which is considerable toward the tips). The wing incidence is 6 deg so if that should be included in the approach AoA I guess it should be 7 or 8 deg. I dunno about this. > > Set cruise close to where you want the ceiling and use full throttle. The way > the solver works these aren't exactly real cruise figures but more like max > performance figures. Also IIRC the approach help approximate stalls. 554 @ 21000ft & 495 @ 46500ft were the two maxes I've tried. I can get 554 @ 21000ft but I couldn't get it over 38000ft, and it took ages to crawl there. This was still below the rated speeds. I haven't been able to get it to solve using 495kts @ 46500ft. Yet;) > > My quick calcs show 26.0m for wing length. The aircraft has a 56m wingspan. Strange - I'll have to check my measurments again. > > I messed with a few other wing values...to get it closer to what it should be. > Experiment with individual changes to see the results. IIRC you should target > about 10 (5-20) for drag coefficient. 200 or less for LR and near -0.9 or so > for approach elevator. > > I decreased stall AoA and width for the hstab. And finally I increased the > thrust of the engines to 17,000lb max (B-52H). I figured the hstab would have a pretty high stall AoA as it's basically a delta. Increasing the engine thrust is really making it a different a/c. The problem with doing a G or an H is that they have no ailerons and I don't want to have to figure that out untill I've got the F working better. The real a/c was able to get to that height with lower rated engines so there should be some way of getting close with YASim. Another characteristic that's out is the range - that's on the low side too, and I think the two probably go together. I've got an idea I haven't calculated the weight correctly and it's too high. > > This configuration gets closer...about 34000ft. You might want to just > overstate the thrust a little (think I read somewhere that they've actually > used up to 8x22,000lb engines), as it doesn't seem anything else is really > incorrect. > > Also, be careful to decrease vertical speed as you get higher. It is very > easy to get behind the power curve at high altitude. > > Best, > > Jim Ta for the diff - it'll be interesting to have a look at it. LeeE _______________________________________________ Flightgear-devel mailing list [EMAIL PROTECTED] http://mail.flightgear.org/mailman/listinfo/flightgear-devel
