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#2410 Cessna 550 Citation II landing bug
Hi there,
First of all, I apologize if Sascha should not be the "Owner". I saw most of the files in Github are last modified by him, hence I put him as the owner.
There is an issue with the latest version (FGaddon) Cessna 550 Citation II . Whenever I try to land it during strong crosswind , it starts sliding /drifting uncontrollably into the wind as soon as the main gears touch the ground. It feels like something pulls it away from the runway. Straightening or steering with the nose gear does not help at all. I don't have such problem with other aircraft. I tried adjusting the gears' sfric & dfric value, set stiction to "1", but nothing fixed it. I was told that this is a rudder issue.
Any ideas?
Thank you.
Is this an issue between the latest model and previous versions ?
What are the models that you don't have this issue with ( i.e comparable types ? )
( What's a "strong" crosswind ? ! )
What you are seeing makes sense in a proper approach in line with the runway, where the plane will be pointing some 10-20 deg into the wind ( 20kt - 40kt side-on xwind ) and, if the nosewheel is kept off the ground then the rudder is all you have to counteract that initial lurch and subsequent weathervaning.
Any chance the plane is floating at touchdown ? If I remember right, it benefits from a stable sink on approach, with a confident ground contact so you can get nosewheel steering asap.
Also, you may wish to alter the rudder power factor, assuming it's a joystick. I use power-3 for the control surfaces, the deflection matches my grip close to the extremes of joystick movement.
I've downloaded the latest model, it's really too dark for me but I agree there's a strong weather-vane effect, it's also seen at rotate and on takeoff roll if the elevator trim is not set nose-down before roll. I can't tell if it's realistic or not, the model is certainly different feeling than the one (2004 Curtis' ) I previously was using.
Google "Citation II Crosswinds" suggests you are not alone !
Edit: The specs I find are dirty stall: 82Kt (!!) Certified XWind 24kt CG Max Aft 30% MAC
According to Vapp = 1.3 * Vs I've set the approach speed to 107; moved the ballast fwd a bit to get CG at 25% MAC, those maybe made a difference but increasing the stall AOA from 6 deg to 16deg ( 'book' value ) made a big improvement , maybe try that to address the VSATB stalling in crosswind.
That's VSTAB stall AOA
Last edit: Huntley Palmer 2020-11-06
Reviewing the history of teh fdm file, the VSTAB aoa changed from 16 deg to 6 deg under the tag: "FDM changes for improved pushback" so, presumably a typo crept in.
Hi Huntley,
Thank you so much for your findings. I have not seen other versions, probably you can share them with me.
I understand about weathervaning, but the aircraft seems to act abnormally when landed on the runway in crosswind. Even with the nose gear down after flaring, there is no way I can control/steer it. I have reached out to one of the members in FG forum (someone whom I loves all the aircraft he built) and he told me this is caused by the rudders (FDM), not gears. Do you mean this behaviour is normal if you land the real aircraft?
Hi VT The posts evolved ! A lot of the models here are, mature. After I loaded the latest version it was clear you are right, there's odd behaviour at rotate as well as during landing.
I restored VSTAB Stall aoa to 16 degrees, from 6deg as it had been in all earlier FDM files and
the behaviour is much improved. I assume the VSTAB aoa change was unintentional.
Hi Huntley,
Great to hear that you were able to see the problem. This is my first time reporting a bug here. So will someone (or you) be fixing the FDM files in FGaddon if the problem has been identified? How does it work?
Thank you.
Yyeass , I sort of try to confirm issues and try to get to the cause. It would be great if the author of the last change to the yasim config ( you got the owner right, too ! ) would confirm the change to VSTAB Stall aoa was unintentional.
If you care to try the fix I suggest, I attach a yasim FDM with only that element changed, without the 'canonical' VApp and elevator changes. Particularly, I'd be interested in your opinion of elevator authority: get yourself into a reely low energy approach with full flaps and
see if the elevator does what you would want; also pull hard back on the elevator at altitude and cruise: I can get a full stall at ~200kts. I know, but it seems a little savage for a bizjet
Hi Huntley,
I'm really bad at this but I will give it a try. May I know if the attached YASIM configuration has been corrected to what I'm expecting or I will need to alter it?
Thank you .
I can see the stall aoa value is different from the original configuration. I will give it a shot.
Yes, it's in the section: VSTAB where I commented the original value with the date: 2020Nv07 so you could search on that. That's the only change from fgaddon latest.
Last edit: Huntley Palmer 2020-11-07
Hi Huntley,
I just test flew it after making the change to aoa value. Unfortunately, the problem still exists. Set right crosswind @ 10 kts. On the second attempt, I touched down at a lower speed around 80 kts, pulled the yoke back further than first attempt. It still swerved right but this time I was able to quickly steer to keep the aircraft on the centerline.
Last edit: scarymovie 2020-11-07
OK
I honestly cannot spend a long time with this version of the model, even the splash screen is too dark for me to see any detail, there's simply too much eyestrain.
If you're absolutely sure the new VSTAB value is employed then the next best hope is for the author to respond with their intent on this setting. I've had zero success in the past in suggesting changes to peoples' FDMs.
Making that change showed a distinct improvement at rotate in a 20kt crosswind, I'd hope you can see at least some difference.
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Hi Huntley,
No worries. That sounds good to me. I really appreciate all your help and effort.
np I just fixed a broken build and hope to try again today, Best !
Hi Huntley, I hope you had a restful weekend. You might want to join the discussion at FG forum, if you are keen.
https://forum.flightgear.org/viewtopic.php?f=4&t=38227