Re: [Flightgear-users] CitatiionII
On Thursday 03 November 2005 05:28 am, Shelton D'Cruz wrote: Anyone else notice a strange problem with the Citation II - after a while flying her, she will all of a sudden go up and down in a yo-yo fashion - only happens to the Citation. Are you using real-weather-fetch? Try disabling any changes in wind speed/direction and see if that helps. Instantaneous changes in wind make the FDMs do funny things. Dave ___ Flightgear-users mailing list Flightgear-users@flightgear.org http://mail.flightgear.org/mailman/listinfo/flightgear-users 2f585eeea02e2c79d7b1d8c4963bae2d
Re: [Flightgear-users] CitatiionII
On Thursday 03 November 2005 11:28, Shelton D'Cruz wrote: Anyone else notice a strange problem with the Citation II - after a while flying her, she will all of a sudden go up and down in a yo-yo fashion - only happens to the Citation. Yes, I noticed it before. It's a known problem, I think - a pity too because the 3d model and cockpit are so nicely done... AJ ___ Flightgear-users mailing list Flightgear-users@flightgear.org http://mail.flightgear.org/mailman/listinfo/flightgear-users 2f585eeea02e2c79d7b1d8c4963bae2d
Re: [Flightgear-users] CitatiionII
AJ MacLeod wrote: On Thursday 03 November 2005 11:28, Shelton D'Cruz wrote: Anyone else notice a strange problem with the Citation II - after a while flying her, she will all of a sudden go up and down in a yo-yo fashion - only happens to the Citation. Yes, I noticed it before. It's a known problem, I think - a pity too because the 3d model and cockpit are so nicely done... Is this with the autopilot on? YAsim has a 'feature' such that when a wing gets in a negative aoa regime, it stalls *far* more quickly than in the positive aoa regime. This is my analysis based on my observation (lacking any formal aeronautical engineering training.) So what happens is as you speed goes up, the aoa required to maintain level flight gets lower. At really high speeds the aoa gets really tiny, and then small disturbances or small control input can put you in a negative aoa situation and WHAM you get a negative incidence stall (or whatever that might be called.) If the autopilot is engaged, the wing leveler can often hold the wings level during a hard negative stall, but your pitch obviously goes a bit crazy. If you have the altitude hold on when this happens, it really has to chase to catch up and level things back out. So far no one has been able to spot the reason why negative aoa stalls happen so much earlier than positive aoa stalls ... maybe a sign someplace that should be flipped? This is my biggest beef with the yasim flight model. It works great in most regimes, but if you ever get in a situation where you are over speed (very low aoa) or if you are trying to push the nose down to drop altitude on an approach (for instance) you can get these wild negative aoa stalls when you least expect them. Adding flaps seems to change the nature of the wing and make this problem happen at slower speeds. Curt. -- Curtis Olsonhttp://www.flightgear.org/~curt HumanFIRST Program http://www.humanfirst.umn.edu/ FlightGear Project http://www.flightgear.org Unique text:2f585eeea02e2c79d7b1d8c4963bae2d ___ Flightgear-users mailing list Flightgear-users@flightgear.org http://mail.flightgear.org/mailman/listinfo/flightgear-users 2f585eeea02e2c79d7b1d8c4963bae2d
RE: [Flightgear-users] CitatiionII
Curtis L. Olson .. snip ... This is my biggest beef with the yasim flight model. It works great in most regimes, but if you ever get in a situation where you are over speed (very low aoa) or if you are trying to push the nose down to drop altitude on an approach (for instance) you can get these wild negative aoa stalls when you least expect them. It's often better to reduce throttle to lose altitude on approach. Avoids such problems. Adding flaps seems to change the nature of the wing and make this problem happen at slower speeds. They're meant to aren't they? Regards, Vivian ___ Flightgear-users mailing list Flightgear-users@flightgear.org http://mail.flightgear.org/mailman/listinfo/flightgear-users 2f585eeea02e2c79d7b1d8c4963bae2d
Re: [Flightgear-users] CitatiionII
Is this with the autopilot on? YAsim has a 'feature' such that when a wing gets in a negative aoa regime, it stalls *far* more quickly than in the positive aoa regime. Amazing how questions get answered before they're asked sometimes :) I was seeing similar behaviour with the Cessna 182 this morning when using the KPM40 autopilot I'd attached to it. I'd get very bad oscillations with the Altitude hold set. Having read Curt's guide to autopilots I was thinkin the problem was the parameters I had set which caused the autopilot to continually over-shoot the target rate of ascent, but now I'm wondering whether I was seeing this instead... -Stuart ___ How much free photo storage do you get? Store your holiday snaps for FREE with Yahoo! Photos http://uk.photos.yahoo.com ___ Flightgear-users mailing list Flightgear-users@flightgear.org http://mail.flightgear.org/mailman/listinfo/flightgear-users 2f585eeea02e2c79d7b1d8c4963bae2d
Re: [Flightgear-users] CitatiionII
Vivian Meazza wrote: Curtis L. Olson .. snip ... This is my biggest beef with the yasim flight model. It works great in most regimes, but if you ever get in a situation where you are over speed (very low aoa) or if you are trying to push the nose down to drop altitude on an approach (for instance) you can get these wild negative aoa stalls when you least expect them. It's often better to reduce throttle to lose altitude on approach. Avoids such problems. Sure, if you fly the airplane like a commercial transport or cargo plane you should pretty much stay away from the regime where YAsim's model breaks down. In a simulation, that's a valid suggestion and we tell people all the time to avoid stalls and spins because they just aren't modeled as in real life (and they are really hard to do correctly.) However, I know Andy's intension was to produce plausible behavior across all flight regimes as best as can be guessed at, and there is clearly a bug where stalls come *way* to early in the negative aoa regime. So I'm not disagreeing with your point, just trying to make one of my own. :-) Curt. -- Curtis Olsonhttp://www.flightgear.org/~curt HumanFIRST Program http://www.humanfirst.umn.edu/ FlightGear Project http://www.flightgear.org Unique text:2f585eeea02e2c79d7b1d8c4963bae2d ___ Flightgear-users mailing list Flightgear-users@flightgear.org http://mail.flightgear.org/mailman/listinfo/flightgear-users 2f585eeea02e2c79d7b1d8c4963bae2d
Re: [Flightgear-users] CitatiionII
Curtis L. Olson wrote: However, I know Andy's intension was to produce plausible behavior across all flight regimes as best as can be guessed at, and there is clearly a bug where stalls come *way* to early in the negative aoa regime. Yes, this is a real bug. It's not the stall per se, I think, but a discontinuity somewhere in the lift curve. Every time this comes up I end up re-reading the (admittedly hairy) Surface.cpp code looking for it, and get lost. The stall handling itself, though, is fairly transparent and looks clean. Something else is going on. I should probably take some time and write up a test rig that graphs the lift curve that emerges from the model, but that requires generating a Surface object with real world coefficients, which requires running it through the solver on a real model, which has interactions that kinda obscure the pure behavior of the Surface. Ick. :( Andy ___ Flightgear-users mailing list Flightgear-users@flightgear.org http://mail.flightgear.org/mailman/listinfo/flightgear-users 2f585eeea02e2c79d7b1d8c4963bae2d
Re: [Flightgear-users] CitatiionII
Shelton wrote: I therefore think that it should not be released in the default base package, because it kinda makes the plane pretty much unflyable - real pity as the cockpit is so lovely done. I thought the workaround for this particular aircraft was just don't fly so fast. :) Andy ___ Flightgear-users mailing list Flightgear-users@flightgear.org http://mail.flightgear.org/mailman/listinfo/flightgear-users 2f585eeea02e2c79d7b1d8c4963bae2d
Re: [Flightgear-users] CitatiionII
OH? is that the solution - OK ill give that a try when I get home from work tonight. Regards Shelton. Quoting Andy Ross [EMAIL PROTECTED]: Shelton wrote: I therefore think that it should not be released in the default base package, because it kinda makes the plane pretty much unflyable - real pity as the cockpit is so lovely done. I thought the workaround for this particular aircraft was just don't fly so fast. :) Andy ___ Flightgear-users mailing list Flightgear-users@flightgear.org http://mail.flightgear.org/mailman/listinfo/flightgear-users 2f585eeea02e2c79d7b1d8c4963bae2d This email was sent from Netspace Webmail: http://www.netspace.net.au ___ Flightgear-users mailing list Flightgear-users@flightgear.org http://mail.flightgear.org/mailman/listinfo/flightgear-users 2f585eeea02e2c79d7b1d8c4963bae2d
Re: [Flightgear-users] CitatiionII
On Thursday 03 Nov 2005 17:09, Andy Ross wrote: Curtis L. Olson wrote: However, I know Andy's intension was to produce plausible behavior across all flight regimes as best as can be guessed at, and there is clearly a bug where stalls come *way* to early in the negative aoa regime. Yes, this is a real bug. It's not the stall per se, I think, but a discontinuity somewhere in the lift curve. Every time this comes up I end up re-reading the (admittedly hairy) Surface.cpp code looking for it, and get lost. The stall handling itself, though, is fairly transparent and looks clean. Something else is going on. I should probably take some time and write up a test rig that graphs the lift curve that emerges from the model, but that requires generating a Surface object with real world coefficients, which requires running it through the solver on a real model, which has interactions that kinda obscure the pure behavior of the Surface. Ick. :( Andy This is an interesting topic to me as I've seen it many times while tuning YASim configs but it seemed sort of reasonable behaviour to me. If the AoA of a wing decreases from a positive value (below it's stall angle), through zero, into negative it seems to me that you are not creating a situation where turbulent air passing over the wing un-sticks from the aerofoil surface. Instead you still have good flow but the direction of lift changes. If you imagine a situation where there's no gravity and you have a symmetrical aerofoil you will get equal lift from equal amounts of +ve or -ve AoA but in opposite directions, which is how rudders and sails work. When you throw in real wing aerofoils and gravity I would expect to see some discontinuous behaviour at -ve AoAs. Dunno what exactly :) LeeE ___ Flightgear-users mailing list Flightgear-users@flightgear.org http://mail.flightgear.org/mailman/listinfo/flightgear-users 2f585eeea02e2c79d7b1d8c4963bae2d
Re: [Flightgear-users] CitatiionII
Lee Elliott wrote: On Thursday 03 Nov 2005 17:09, Andy Ross wrote: Curtis L. Olson wrote: However, I know Andy's intension was to produce plausible behavior across all flight regimes as best as can be guessed at, and there is clearly a bug where stalls come *way* to early in the negative aoa regime. Yes, this is a real bug. It's not the stall per se, I think, but a discontinuity somewhere in the lift curve. Every time this comes up I end up re-reading the (admittedly hairy) Surface.cpp code looking for it, and get lost. The stall handling itself, though, is fairly transparent and looks clean. Something else is going on. I should probably take some time and write up a test rig that graphs the lift curve that emerges from the model, but that requires generating a Surface object with real world coefficients, which requires running it through the solver on a real model, which has interactions that kinda obscure the pure behavior of the Surface. Ick. :( Andy This is an interesting topic to me as I've seen it many times while tuning YASim configs but it seemed sort of reasonable behaviour to me. If the AoA of a wing decreases from a positive value (below it's stall angle), through zero, into negative it seems to me that you are not creating a situation where turbulent air passing over the wing un-sticks from the aerofoil surface. Instead you still have good flow but the direction of lift changes. If you imagine a situation where there's no gravity and you have a symmetrical aerofoil you will get equal lift from equal amounts of +ve or -ve AoA but in opposite directions, which is how rudders and sails work. When you throw in real wing aerofoils and gravity I would expect to see some discontinuous behaviour at -ve AoAs. Dunno what exactly :) LeeE ___ Flightgear-users mailing list Flightgear-users@flightgear.org http://mail.flightgear.org/mailman/listinfo/flightgear-users 2f585eeea02e2c79d7b1d8c4963bae2d I would look at some polar graphs. Stuff happens at those AOAs, but it is usually continuous. Even the stall regimes are continuous, though they do have a much greater slope. I haven't ever seen graphs of deep stalls though, so I have no idea what happens there. Josh ___ Flightgear-users mailing list Flightgear-users@flightgear.org http://mail.flightgear.org/mailman/listinfo/flightgear-users 2f585eeea02e2c79d7b1d8c4963bae2d
