Re: [Flightgear-devel] YASim flaps flap
On Wed, 2002-06-05 at 20:54, Andy Ross wrote: Tony Peden wrote: Induced drag is a function of the vortices surrounding the wing. Those vortices vary in strength with lift, not angle of attack. Not so. The induced drag of an aircraft in high-speed cruise is much lower than an aircraft in level flight at stall speed. I probably used lift where I should have used CL. So, at cruise, the dynamic pressure is high and the CL required much lower than at approach speeds where the dynamic pressure is relatively low. Induced drag goes with CL^2 and CL doesn't change much across a flap transition. The lift in these situations is the same (and equal to weight, of course). Think about it: if induced drag depended solely on lift, there would be no such thing as the back side of the power curve. Drag would go asymptotically to some constant as airspeed dropped to zero -- that clearly isn't right. For real aircraft, drag always increases with AoA. In reality, induced drag looks much more like a function of AoA than it does a function of lift. Well, in real reality induced drag is just a metaphor; so it isn't really a function of anything. Maybe you mean to say that induced drag is a function of lift *coefficient* -- as on a drag polar plot? This is true, but unhelpful to me. I don't know what the lift coefficient is until I calculate the AoA. And we're back to where we started. Hmmm... Andy -- Andrew J. RossNextBus 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
[Flightgear-devel] YASim flaps flap
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 appears that not only do the flaps not increase drag in the 747, but they decrease it. Holding an altitude at 3000ft, with idle throttle, flaps position one clearly decreases the rate at which airspeed is falling. Going to flaps position 3 actually _increases_ airspeed (with throttle idle) and at the same time starts pushing the aircraft up in elevation. This is without the David's latest air density patch (using CVS from about 3:30pm Today 2002-06-05). 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. Best, Jim ___ Flightgear-devel mailing list [EMAIL PROTECTED] http://mail.flightgear.org/mailman/listinfo/flightgear-devel
Re: [Flightgear-devel] YASim flaps flap
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. 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. 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. RossNextBus 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
Re: [Flightgear-devel] YASim flaps flap
Jim Wilson writes: 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 appears that not only do the flaps not increase drag in the 747, but they decrease it. Holding an altitude at 3000ft, with idle throttle, flaps position one clearly decreases the rate at which airspeed is falling. Going to flaps position 3 actually _increases_ airspeed (with throttle idle) and at the same time starts pushing the aircraft up in elevation. This is without the David's latest air density patch (using CVS from about 3:30pm Today 2002-06-05). 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. I think that even from the beginning, yasim flaps have increased drag, but have always reduced drag so the end result of lowering flaps is that your speed increases and/or you climb faster (or descend less rapidly). Andy is aware of this issue, but says that it is hard to fix. Curt. -- Curtis Olson IVLab / HumanFIRST Program FlightGear Project Twin Cities[EMAIL PROTECTED] [EMAIL PROTECTED] Minnesota http://www.menet.umn.edu/~curt http://www.flightgear.org ___ Flightgear-devel mailing list [EMAIL PROTECTED] http://mail.flightgear.org/mailman/listinfo/flightgear-devel
Re: [Flightgear-devel] YASim flaps flap
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. RossNextBus 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
Re: [Flightgear-devel] YASim flaps flap
Tony Peden wrote: Induced drag is a function of the vortices surrounding the wing. Those vortices vary in strength with lift, not angle of attack. Not so. The induced drag of an aircraft in high-speed cruise is much lower than an aircraft in level flight at stall speed. The lift in these situations is the same (and equal to weight, of course). Think about it: if induced drag depended solely on lift, there would be no such thing as the back side of the power curve. Drag would go asymptotically to some constant as airspeed dropped to zero -- that clearly isn't right. For real aircraft, drag always increases with AoA. In reality, induced drag looks much more like a function of AoA than it does a function of lift. Well, in real reality induced drag is just a metaphor; so it isn't really a function of anything. Maybe you mean to say that induced drag is a function of lift *coefficient* -- as on a drag polar plot? This is true, but unhelpful to me. I don't know what the lift coefficient is until I calculate the AoA. And we're back to where we started. Andy -- Andrew J. RossNextBus 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
Re: [Flightgear-devel] YASim flaps flap
Tony Peden wrote: Induced drag is a function of the vortices surrounding the wing. Those vortices vary in strength with lift, not angle of attack. Not so. The induced drag of an aircraft in high-speed cruise is much lower than an aircraft in level flight at stall speed. The lift in Anderson defines induced drag as the drag created by the presence of downwash - which arguably is very much related to wingtip vortices. You can *visualize* the physical generation of induced drag by picturing the lift vector tilting backwards - again, according to Anderson. Probably both of you are right in a sense. ___ Flightgear-devel mailing list [EMAIL PROTECTED] http://mail.flightgear.org/mailman/listinfo/flightgear-devel
