On Wed, 2002-06-05 at 17:32, Andy Ross wrote: > Jim Wilson wrote: > > Not sure if this relates to the air pressure issue, seems to be a bit more > > than that would account for. A few days ago someone reported unusual flap > > effects, prior to the corrections to the YASim flight model. > > It's a known bug. Curt found it a few months back. The fix is > nontrivial, and I've been lazy. I'll see if I can get some time this > week to work on it. > > > It appears that not only do the flaps not increase drag in the 747, but they > > decrease it. > > Not exactly. Flaps do increase drag at the same AoA. But they also > increase lift, which means that you need to hold a lower AoA to avoid > climbing. The lower AoA produces less induced drag.
Induced drag is a function of the vortices surrounding the wing. Those vortices vary in strength with lift, not angle of attack. Since you will make the same lift after the flap extension as before (to maintain the same trimmed condition), the lift will be the same and so will the induced drag. Profile drag will increase significantly and skin friction some (more wetted area exposed) so, all in all, an increase in drag should be observed. Note that one reason transports have such huge flap deflections for landing is not only to make CL and be able to fly at lower speeds, but also to make drag so that steeper glideslopes are possible. With this configuration, quite a bit of thrust is required to hold the standard 3 degree glide slope (at max flap) so spool up times are shorter. That is goodness because it increases the chances of escaping a windshear encounter. Flaps need to > increase the drag that *would* have been produced had the aircraft > been flying without flaps at an AoA high enough to produce the same > lift. It's that level of reverse-intuition that makes the solution > hard to implement. There is nothing non-intuitive about it. Don't think in terms of angle of attack. > > > Under the right conditions, not observed in a > > reproducable/reportable way (but generally lower airspeeds and > > altitudes) the aircraft can speed up drastically and skyrocket into > > the air at a rate over 10kfps. > > This seemed to be implied in a bug report last week, too (someone > posted a screenshot at 140k MSL). I've never seen this effect. It's > more worrisom, as it's clearly non-physical. Whatever forces are > generated by the YASim flap handling, they should be some reasonable > multiple (i.e. near 1.0) of the force generated without flaps. They > should clearly *not* be this big. > > This isn't part of the bug I detail above, and points to a blowup > condition somewhere else. Like I said, I've never seen it happen. If > you could play around and see if you can get something > pseudo-reproducible (even something like "do this, and it'll blow up > one time in ten"), I'd be grateful. > > 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 > -- Tony Peden [EMAIL PROTECTED] We all know Linux is great ... it does infinite loops in 5 seconds. -- attributed to Linus Torvalds _______________________________________________ Flightgear-devel mailing list [EMAIL PROTECTED] http://mail.flightgear.org/mailman/listinfo/flightgear-devel
