Re: [Flightgear-devel] Citation "pitch down" divergence. Fixed?
Lee Elliott wrote: > On Friday 11 Nov 2005 02:47, Josh Babcock wrote: > >>Lee Elliott wrote: >> >>>On Thursday 10 Nov 2005 20:20, Andy Ross wrote: >>> After some prodding from Curt, I finally spent a few hours yesterday tracking down the "pitch down" discontinuity in the Citation. Well, I didn't find a discontinuity. I can now graph the lift curve from a Surface (a real one, part of the real aircraft, not an isolated test instance) and verify that it's valid and correct looking through the entire AoA regime. But I think I *did* find the problem: it seems that I, er, "misdocumented" the incidence and twist parameters in the YASim configuration. The README.yasim file states that these numbers are positive for positive AoA (i.e. a positive incidence on a wing generates extra lift, and a negative twist causes the wing tips to stall after the root). But the code was interpreting the number as a rotation about the YASim Y axis, which points out the left wing and therefore is positive *down*. Oops. The reason the citation exhibited this especially is just luck: the file lists an incidence of 3.0 (which is relatively high, and the inversion bug therefore puts the wing 3 degrees closer to a negative stall) the solver happens to generate a nose-down cruise configuration of about 1.5 degrees, and the elevator authority is actually quite high (which causes higher pitch rates under autopilot control). So the bottom line is that Curt was right: it *was* the negative AoA stall (probably the tail's, not the wing's) happening too soon. :) I'm a little leery of changing this in code this close to a release -- the risk of breaking working aircraft is too high. For the short term, this can be fixed in the Citation-II.xml file by simply negating the incidence and twist values on the wing. I did this and tried the autopilot in a maximum speed cruise at low level (which should produce the highest nose-down AoA) without any odd behavior. Curt, can you try that and see if it appears to fix the handling issues? Likewise, anyone with a YASim aircraft that makes use of incidence or twist values is encouraged to try the same modification and report any problems. We can go back after the release and fix the code and all the aircraft files. Andy >>> >>>I'll try to check the ones I've done over the weekend. The >>>one that concerns me most is the B-52F. The wing incidence >>>is set to 6 and the twist to -4 and I'm starting to wonder >>>how it manages to fly at all. >> >>Nose down. The fuselage is about 5 deg down when in level >>flight. >> >> >>>I got some good info on the B-52F from someone who flew >>>around 3000 hrs in that model and around 6000 hrs total in >>>all models, apart from the A/B, and it was flying to within >>>around 10 kts or so of what it should have been doing and >>>was climbing at about the right rate. >>> >>>LeeE > > > Depending on weight, alt and speed, 5 deg nose-down could be > correct. The incidence of +6 degrees is correct but I had to > estimate the twist. > > I should be able to have a look at it sometime this weekend. > > Ta for having a look. > > LeeE > > > ___ > Flightgear-devel mailing list > Flightgear-devel@flightgear.org > http://mail.flightgear.org/mailman/listinfo/flightgear-devel > 2f585eeea02e2c79d7b1d8c4963bae2d > Yeah, look at a picture of one in flight. The wings are mounted at a high AOA so it can make four point landings at low airspeeds and low descent rates. The b47 had a similar setup, but only the gear was level, the entire fuselage pointed up in the air on that one. Several soviet bombers with bicycle gear also had that look. Josh ___ Flightgear-devel mailing list Flightgear-devel@flightgear.org http://mail.flightgear.org/mailman/listinfo/flightgear-devel 2f585eeea02e2c79d7b1d8c4963bae2d
Re: [Flightgear-devel] Citation "pitch down" divergence. Fixed?
On Friday 11 Nov 2005 02:47, Josh Babcock wrote: > Lee Elliott wrote: > > On Thursday 10 Nov 2005 20:20, Andy Ross wrote: > >>After some prodding from Curt, I finally spent a few hours > >>yesterday tracking down the "pitch down" discontinuity in > >> the Citation. > >> > >>Well, I didn't find a discontinuity. I can now graph the > >> lift curve from a Surface (a real one, part of the real > >> aircraft, not an isolated test instance) and verify that > >> it's valid and correct looking through the entire AoA > >> regime. > >> > >>But I think I *did* find the problem: it seems that I, er, > >>"misdocumented" the incidence and twist parameters in the > >>YASim configuration. The README.yasim file states that > >> these numbers are positive for positive AoA (i.e. a > >> positive incidence on a wing generates extra lift, and a > >> negative twist causes the wing tips to stall after the > >> root). But the code was interpreting the number as a > >> rotation about the YASim Y axis, which points out the left > >> wing and therefore is positive *down*. Oops. > >> > >>The reason the citation exhibited this especially is just > >>luck: the file lists an incidence of 3.0 (which is > >> relatively high, and the inversion bug therefore puts the > >> wing 3 degrees closer to a negative stall) the solver > >> happens to generate a nose-down cruise configuration of > >> about 1.5 degrees, and the elevator authority is actually > >> quite high (which causes higher pitch rates under autopilot > >> control). > >> > >>So the bottom line is that Curt was right: it *was* the > >>negative AoA stall (probably the tail's, not the wing's) > >>happening too soon. :) > >> > >>I'm a little leery of changing this in code this close to a > >>release -- the risk of breaking working aircraft is too > >> high. For the short term, this can be fixed in the > >> Citation-II.xml file by simply negating the incidence and > >> twist values on the wing. I did this and tried the > >> autopilot in a maximum speed cruise at low level (which > >> should produce the highest nose-down AoA) without any odd > >> behavior. > >> > >>Curt, can you try that and see if it appears to fix the > >>handling issues? Likewise, anyone with a YASim aircraft > >> that makes use of incidence or twist values is encouraged > >> to try the same modification and report any problems. We > >> can go back after the release and fix the code and all the > >> aircraft files. > >> > >>Andy > > > > I'll try to check the ones I've done over the weekend. The > > one that concerns me most is the B-52F. The wing incidence > > is set to 6 and the twist to -4 and I'm starting to wonder > > how it manages to fly at all. > > Nose down. The fuselage is about 5 deg down when in level > flight. > > > I got some good info on the B-52F from someone who flew > > around 3000 hrs in that model and around 6000 hrs total in > > all models, apart from the A/B, and it was flying to within > > around 10 kts or so of what it should have been doing and > > was climbing at about the right rate. > > > > LeeE Depending on weight, alt and speed, 5 deg nose-down could be correct. The incidence of +6 degrees is correct but I had to estimate the twist. I should be able to have a look at it sometime this weekend. Ta for having a look. LeeE ___ Flightgear-devel mailing list Flightgear-devel@flightgear.org http://mail.flightgear.org/mailman/listinfo/flightgear-devel 2f585eeea02e2c79d7b1d8c4963bae2d
RE: [Flightgear-devel] Citation "pitch down" divergence. Fixed?
Josh Babcock > > Lee Elliott wrote: > > On Thursday 10 Nov 2005 20:20, Andy Ross wrote: > > > >>After some prodding from Curt, I finally spent a few hours > >>yesterday tracking down the "pitch down" discontinuity in the > >>Citation. > >> > >>Well, I didn't find a discontinuity. I can now graph the lift > >>curve from a Surface (a real one, part of the real aircraft, > >>not an isolated test instance) and verify that it's valid and > >>correct looking through the entire AoA regime. > >> > >>But I think I *did* find the problem: it seems that I, er, > >>"misdocumented" the incidence and twist parameters in the > >>YASim configuration. The README.yasim file states that these > >>numbers are positive for positive AoA (i.e. a positive > >>incidence on a wing generates extra lift, and a negative twist > >>causes the wing tips to stall after the root). But the code > >>was interpreting the number as a rotation about the YASim Y > >>axis, which points out the left wing and therefore is positive > >>*down*. Oops. > >> > >>The reason the citation exhibited this especially is just > >>luck: the file lists an incidence of 3.0 (which is relatively > >>high, and the inversion bug therefore puts the wing 3 degrees > >>closer to a negative stall) the solver happens to generate a > >>nose-down cruise configuration of about 1.5 degrees, and the > >>elevator authority is actually quite high (which causes higher > >>pitch rates under autopilot control). > >> > >>So the bottom line is that Curt was right: it *was* the > >>negative AoA stall (probably the tail's, not the wing's) > >>happening too soon. :) > >> > >>I'm a little leery of changing this in code this close to a > >>release -- the risk of breaking working aircraft is too high. > >>For the short term, this can be fixed in the Citation-II.xml > >>file by simply negating the incidence and twist values on the > >>wing. I did this and tried the autopilot in a maximum speed > >>cruise at low level (which should produce the highest > >>nose-down AoA) without any odd behavior. > >> > >>Curt, can you try that and see if it appears to fix the > >>handling issues? Likewise, anyone with a YASim aircraft that > >>makes use of incidence or twist values is encouraged to try > >>the same modification and report any problems. We can go back > >>after the release and fix the code and all the aircraft files. > >> > >>Andy > > > > > > I'll try to check the ones I've done over the weekend. The one > > that concerns me most is the B-52F. The wing incidence is set > > to 6 and the twist to -4 and I'm starting to wonder how it > > manages to fly at all. > > Nose down. The fuselage is about 5 deg down when in level flight. > > > > > I got some good info on the B-52F from someone who flew around > > 3000 hrs in that model and around 6000 hrs total in all models, > > apart from the A/B, and it was flying to within around 10 kts or > > so of what it should have been doing and was climbing at about > > the right rate. > > The negative incidence issue might also explain some odd values I was forced to put into the B29 config to make it fly well. I'll try an updated version later. Vivian ___ Flightgear-devel mailing list Flightgear-devel@flightgear.org http://mail.flightgear.org/mailman/listinfo/flightgear-devel 2f585eeea02e2c79d7b1d8c4963bae2d
Re: [Flightgear-devel] Citation "pitch down" divergence. Fixed?
Lee Elliott wrote: > On Thursday 10 Nov 2005 20:20, Andy Ross wrote: > >>After some prodding from Curt, I finally spent a few hours >>yesterday tracking down the "pitch down" discontinuity in the >>Citation. >> >>Well, I didn't find a discontinuity. I can now graph the lift >>curve from a Surface (a real one, part of the real aircraft, >>not an isolated test instance) and verify that it's valid and >>correct looking through the entire AoA regime. >> >>But I think I *did* find the problem: it seems that I, er, >>"misdocumented" the incidence and twist parameters in the >>YASim configuration. The README.yasim file states that these >>numbers are positive for positive AoA (i.e. a positive >>incidence on a wing generates extra lift, and a negative twist >>causes the wing tips to stall after the root). But the code >>was interpreting the number as a rotation about the YASim Y >>axis, which points out the left wing and therefore is positive >>*down*. Oops. >> >>The reason the citation exhibited this especially is just >>luck: the file lists an incidence of 3.0 (which is relatively >>high, and the inversion bug therefore puts the wing 3 degrees >>closer to a negative stall) the solver happens to generate a >>nose-down cruise configuration of about 1.5 degrees, and the >>elevator authority is actually quite high (which causes higher >>pitch rates under autopilot control). >> >>So the bottom line is that Curt was right: it *was* the >>negative AoA stall (probably the tail's, not the wing's) >>happening too soon. :) >> >>I'm a little leery of changing this in code this close to a >>release -- the risk of breaking working aircraft is too high. >>For the short term, this can be fixed in the Citation-II.xml >>file by simply negating the incidence and twist values on the >>wing. I did this and tried the autopilot in a maximum speed >>cruise at low level (which should produce the highest >>nose-down AoA) without any odd behavior. >> >>Curt, can you try that and see if it appears to fix the >>handling issues? Likewise, anyone with a YASim aircraft that >>makes use of incidence or twist values is encouraged to try >>the same modification and report any problems. We can go back >>after the release and fix the code and all the aircraft files. >> >>Andy > > > I'll try to check the ones I've done over the weekend. The one > that concerns me most is the B-52F. The wing incidence is set > to 6 and the twist to -4 and I'm starting to wonder how it > manages to fly at all. Nose down. The fuselage is about 5 deg down when in level flight. > > I got some good info on the B-52F from someone who flew around > 3000 hrs in that model and around 6000 hrs total in all models, > apart from the A/B, and it was flying to within around 10 kts or > so of what it should have been doing and was climbing at about > the right rate. > > LeeE > > > ___ > Flightgear-devel mailing list > Flightgear-devel@flightgear.org > http://mail.flightgear.org/mailman/listinfo/flightgear-devel > 2f585eeea02e2c79d7b1d8c4963bae2d > ___ Flightgear-devel mailing list Flightgear-devel@flightgear.org http://mail.flightgear.org/mailman/listinfo/flightgear-devel 2f585eeea02e2c79d7b1d8c4963bae2d
Re: [Flightgear-devel] Citation "pitch down" divergence. Fixed?
On Thursday 10 Nov 2005 20:20, Andy Ross wrote: > After some prodding from Curt, I finally spent a few hours > yesterday tracking down the "pitch down" discontinuity in the > Citation. > > Well, I didn't find a discontinuity. I can now graph the lift > curve from a Surface (a real one, part of the real aircraft, > not an isolated test instance) and verify that it's valid and > correct looking through the entire AoA regime. > > But I think I *did* find the problem: it seems that I, er, > "misdocumented" the incidence and twist parameters in the > YASim configuration. The README.yasim file states that these > numbers are positive for positive AoA (i.e. a positive > incidence on a wing generates extra lift, and a negative twist > causes the wing tips to stall after the root). But the code > was interpreting the number as a rotation about the YASim Y > axis, which points out the left wing and therefore is positive > *down*. Oops. > > The reason the citation exhibited this especially is just > luck: the file lists an incidence of 3.0 (which is relatively > high, and the inversion bug therefore puts the wing 3 degrees > closer to a negative stall) the solver happens to generate a > nose-down cruise configuration of about 1.5 degrees, and the > elevator authority is actually quite high (which causes higher > pitch rates under autopilot control). > > So the bottom line is that Curt was right: it *was* the > negative AoA stall (probably the tail's, not the wing's) > happening too soon. :) > > I'm a little leery of changing this in code this close to a > release -- the risk of breaking working aircraft is too high. > For the short term, this can be fixed in the Citation-II.xml > file by simply negating the incidence and twist values on the > wing. I did this and tried the autopilot in a maximum speed > cruise at low level (which should produce the highest > nose-down AoA) without any odd behavior. > > Curt, can you try that and see if it appears to fix the > handling issues? Likewise, anyone with a YASim aircraft that > makes use of incidence or twist values is encouraged to try > the same modification and report any problems. We can go back > after the release and fix the code and all the aircraft files. > > Andy I'll try to check the ones I've done over the weekend. The one that concerns me most is the B-52F. The wing incidence is set to 6 and the twist to -4 and I'm starting to wonder how it manages to fly at all. I got some good info on the B-52F from someone who flew around 3000 hrs in that model and around 6000 hrs total in all models, apart from the A/B, and it was flying to within around 10 kts or so of what it should have been doing and was climbing at about the right rate. LeeE ___ Flightgear-devel mailing list Flightgear-devel@flightgear.org http://mail.flightgear.org/mailman/listinfo/flightgear-devel 2f585eeea02e2c79d7b1d8c4963bae2d
[Flightgear-devel] Citation "pitch down" divergence. Fixed?
After some prodding from Curt, I finally spent a few hours yesterday tracking down the "pitch down" discontinuity in the Citation. Well, I didn't find a discontinuity. I can now graph the lift curve from a Surface (a real one, part of the real aircraft, not an isolated test instance) and verify that it's valid and correct looking through the entire AoA regime. But I think I *did* find the problem: it seems that I, er, "misdocumented" the incidence and twist parameters in the YASim configuration. The README.yasim file states that these numbers are positive for positive AoA (i.e. a positive incidence on a wing generates extra lift, and a negative twist causes the wing tips to stall after the root). But the code was interpreting the number as a rotation about the YASim Y axis, which points out the left wing and therefore is positive *down*. Oops. The reason the citation exhibited this especially is just luck: the file lists an incidence of 3.0 (which is relatively high, and the inversion bug therefore puts the wing 3 degrees closer to a negative stall) the solver happens to generate a nose-down cruise configuration of about 1.5 degrees, and the elevator authority is actually quite high (which causes higher pitch rates under autopilot control). So the bottom line is that Curt was right: it *was* the negative AoA stall (probably the tail's, not the wing's) happening too soon. :) I'm a little leery of changing this in code this close to a release -- the risk of breaking working aircraft is too high. For the short term, this can be fixed in the Citation-II.xml file by simply negating the incidence and twist values on the wing. I did this and tried the autopilot in a maximum speed cruise at low level (which should produce the highest nose-down AoA) without any odd behavior. Curt, can you try that and see if it appears to fix the handling issues? Likewise, anyone with a YASim aircraft that makes use of incidence or twist values is encouraged to try the same modification and report any problems. We can go back after the release and fix the code and all the aircraft files. Andy ___ Flightgear-devel mailing list Flightgear-devel@flightgear.org http://mail.flightgear.org/mailman/listinfo/flightgear-devel 2f585eeea02e2c79d7b1d8c4963bae2d
