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Vertical stability problems with F-16CJ Block C

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Vertical stability problems with F-16CJ Block C

Postby Streak » Thu Jul 09, 2020 12:48 am

Has anyone in the forum experienced vertical stability problems with the F-16CJ Block C? I have been flying it and I can't seem to stabilize it vertically and there is a lot of pitch oscillation during flight. I have tried assigning a Saitek X52 H.O.T.A.S. joystick hat button to effect nose up and nose down trim to no avail. The version of the jet is v3.33. Please advise. 8)
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Re: Vertical stability problems with F-16CJ Block C

Postby dilbert » Thu Jul 09, 2020 6:29 pm

Hi streak.
I just flew the add-on hangar version, which is 3.22, using both 2018.3.5 and 2019.1.2 in Win10, without encountering the problem you described.
Suggest you try version 3.22. If that doesn't work, you need to describe your FlightGear version and operating system, in order to get further help.

Kind Regards
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Re: Vertical stability problems with F-16CJ Block C

Postby Streak » Fri Jul 10, 2020 3:45 pm

I am presently using FG 2020.1.2. I believe I'm using the same add-on hangar that you have used. The F-16CJ Block 52 has definite vertical stability issues especially during low speed final approaches. I have tried the alternate F-16C Block 40. It has similar but less severe vertical stability problems. I think the root cause is that nose-up and nose-down trim are not working with these two models. I have always ensured that my Saitek X-52 H.O.T.A.S. joystick is calibrated and custom configured for my knob/button preferences on the thrust and joystick controls. As I mentioned in the initial post, I had one joystick hat switch up and down inputs configured for nose-down and nose-up trim respectively. During flight the aircraft have severe pitch down tendencies even when the gear and flaps are retracted. I 've tried to counteract the nose-down tendency with repeated inputs to the joystick hat switch configured for nose-up and and repeat the nose-up input ad nauseum to no evident effect. I am reluctant to try the 2018 and 2019 FG versions as my 2020.1.2 version works well with other aircraft. Is there any other recommendation you may have in addition to the recommendations that you already posed? Thanks in advance for your future reply!
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Re: Vertical stability problems with F-16CJ Block C

Postby Johan G » Fri Jul 10, 2020 4:59 pm

Just a quick thought. I am not familiar with the F-16 and its flight control system, so this might be meaningless, but could you in FlightGear open the property browser (press "/") and check if your control inputs actually affect any trim properties (probably located under /controls/trim/)?
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Re: Vertical stability problems with F-16CJ Block C

Postby Streak » Fri Jul 10, 2020 6:36 pm

I did press '/' in FlightGear and confirmed my inputs do have an effect on the elevator trim value of the F-16C Block 42. I expect the nose-up and nose-down trim also works for the F-16CJ Block 52 and other F-16 variants. Is there a quick way in FG to zero the elevator trim before flight? When I first inspected the elevator trim value, it was way off of 0.0. I believe this is what is causing the pitch problems as the elevator trim value is type double and seems to get set to a unexpected value (not 0.0) before a flight begins . Please advise.
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Re: Vertical stability problems with F-16CJ Block C

Postby PH-JAKE » Sat Jul 11, 2020 4:59 pm

Hi Streak,

You've stated the jet is version v3.33, yet also that you're using the one from the add-on with FG 2020.1.2. I do too (that last one), and if I look at the version it's 3.22 (as found in f16-base.xml). Can you verify which version it is?

Besides, have you ever looked at this one: https://github.com/NikolaiVChr/f16 It's a nicely developing version. Maybe it already addressed the issue you've seen.
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Re: Vertical stability problems with F-16CJ Block C

Postby Streak » Sat Jul 11, 2020 9:27 pm

I must have gotten confused as you are right, I am using version 3.22. However, I have discovered something that is significant to my issue. When I was prompted to enter "/" in FG to verify that my Saitek X-52 H.O.T.A.S. joystick hat button, that I assigned with nose-up and nose-down elevator trim, was effecting a change in the elevator trim (and it was), I input hat button presses to get the elevator trim almost to 0.0. Then the flights of F-16s thereafter didn't have the pitch problem I originally described. So I think the problem is that the elevator trim is not getting set to 0.0 before each flight and remembers an old, stale value from previous flights. What do you think?
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Re: Vertical stability problems with F-16CJ Block C

Postby PH-JAKE » Sun Jul 12, 2020 10:41 am

Streak wrote in Sat Jul 11, 2020 9:27 pm:I must have gotten confused as you are right, I am using version 3.22.


Okay, that's clear then :)


Streak wrote in Sat Jul 11, 2020 9:27 pm:However, I have discovered something that is significant to my issue. When I was prompted to enter "/" in FG to verify that my Saitek X-52 H.O.T.A.S. joystick hat button, that I assigned with nose-up and nose-down elevator trim, was effecting a change in the elevator trim (and it was), I input hat button presses to get the elevator trim almost to 0.0. Then the flights of F-16s thereafter didn't have the pitch problem I originally described. So I think the problem is that the elevator trim is not getting set to 0.0 before each flight and remembers an old, stale value from previous flights. What do you think?


Good question. I've looked in aircraft-data/f16-block-52.xml but couldn't find anything related to trim being stored persistently. Also the Aircraft/f16 definitions show no storage path for this property. A trial run, in which I set an very large trim and then restarted, also showed no persistence. So I'm not sure where you got that impression from.

My experience, from the keyboard, is just to leave the trim as is. It never helped tweaking it. If I need consistent flight I just flick on the AP controls.


PS, shameless plug: If you like to fly the F16 maybe you may like this post too :wink:
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Re: Vertical stability problems with F-16CJ Block C

Postby Streak » Sun Jul 12, 2020 3:03 pm

I'm a really poor keyboard flyer but thanks for the post link anyway! My past descriptions of the issue has been from experience only except for the one recommendation to verify my elevator trim control. The elevator trim value is type signed double if I recall correctly and increments/decrements minutely with joystick hat input making it difficult to get coarse adjustments. I can only rebut that I actually did experience better vertical control when flying after adjusting the elevator trim to near 0.0. I suspect that the elevator trim value is not initialized and is read only by the F-16 flight model. Having said that, the F-16 flight model may read the value as is and use whatever value it defaults to which, if not reset to an initial value, will be whatever value at the associated memory of the variable is. If this is true, the starting elevator trim value may very well be random instead of a stale, previous value as I had suspected earlier in this post thread. Of course all of my views are speculative and I don't have any concrete data to support my suspicions. I there a better way for me to track this issue down? Thanks in advance for your or other's reply.
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Re: Vertical stability problems with F-16CJ Block C

Postby Octal450 » Sun Jul 12, 2020 3:18 pm

The f-16 uses flybywire custom control.

Please film whats happening so we can better assist you.

Kind Regards,
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Re: Vertical stability problems with F-16CJ Block C

Postby Streak » Sun Jul 12, 2020 11:47 pm

Since nobody else sees this issue (given that others on this post thread have tried to reproduce my issue and have not) I am going to conclude that I may be doing something wrong during my flights of the General Dynamics F-16CJ Block 52 version 3.22. I am going to be offline for a while to see if I can determine the cause of the pitch oscillation. Perhaps I have not configured the aircraft properly during flight which would explain the issue. I am going to investigate by reviewing flight recordings that I will create when flying the aircraft.

I would like to say that this aircraft is one of the best that I have flown and I want the authors of the model to know that I congratulate them with a job extremely well done.

Thanks also for all of the replies to this post … they have been very helpful to me.

Best Regards,

Streak
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Re: Vertical stability problems with F-16CJ Block C

Postby Streak » Mon Jul 13, 2020 11:49 pm

Issue resolved! For the first time I viewed the F-16 externally during engine start and observed that when engine was starting the flaps were extended automatically for takeoff. I was not aware of this in previous flights and never retracted the flaps after gaining altitude after takeoff. So, the problem was the flaps had always been extended during my problem flights and I was trying to vertically stabilize by inputting nose-down elevator trim. In this case, I guess, the flaps will always win no matter how much elevator trim is used. Sorry for all the unnecessary concerns! I believe this issue is resolved. I have verified this with a flight where the flaps were retracted after takeoff and achieving a safe altitude. Elevator trim was used and it worked just fine. My apologies for my novice flying of the F-16CJ Block 52 and not being aware that engine start included auto flaps.

Best Regards,

Streak
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Re: Vertical stability problems with F-16CJ Block C

Postby wkitty42 » Tue Jul 14, 2020 12:15 am

glad you figured it out!

i gotta ask, though... wasn't there an indicator on your dash panel about the flaps being extended? or maybe a warning when trying to go too fast with them extended?
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Re: Vertical stability problems with F-16CJ Block C

Postby Streak » Tue Jul 14, 2020 3:17 am

No indicator on the cockpit panel as far as I can tell. But interestingly, the external view doesn't update when the flaps are retracted showing that the flaps are extended. However the flight model behaves as if the flaps are retracted. I think this is a bug and I'm going to open a new post on that.
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Re: Vertical stability problems with F-16CJ Block C

Postby Octal450 » Tue Jul 14, 2020 4:31 am

No worries - and avoid using elevator trim, its FlyByWire, so you should always be around 1G with no inputs.

Kind Regards,
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