Re: [Jsbsim-devel] F-16 Stability problems

[David C. <dp...@gm...>]

>From your description of the problem it sounds like integrator wind-up
could be the issue.  The fix was added to the PID component a few years
ago, but I haven't used it yet.  IIRC,  it involves specifying an error
property that when very near zero causes the PID controller to reset the
integrator.

Dave

On Oct 20, 2016 11:25 AM, "Bruce Clay" <bcl...@gm...> wrote:

> Erik:
>
> I realized that my reply had not popped up on the forum yet.  It seems I
> replied from my work email and that email does not get posted for some odd
> reason.
>
> ---- previous reply ----
>
>   In thinking through your response I guess I have I misunderstanding of
> the way JSBsim works.  I would have expected the PIDs to be in the base
> code not in the aircraft dependent scripts.  If we take a simplistic view
> of a PID as a filtering mechanism how does altitude affect the roll?  The
> problem tends to show up more in the low altitudes say under 10,000 feet
> and around 600 knots.  Neither of these values are near the limits for the
> aircraft.  I do not see the oscillations during extreme maneuvers rather
> just the opposite.  Doing extreme maneuvers seems to stabilize the aircraft
> (i.e stop the oscillations).
>
> I am no expert in the math behind the flight dynamics so I may be
> overlooking something.
>
> Thanks for your help
>
> Bruce
>
> On Thu, Oct 13, 2016 at 2:23 AM, Erik Hofman <er...@eh...> wrote:
>
>> On 10/12/2016 03:46 PM, Bruce Clay wrote:
>> > Erik:
>> >   Thanks for your reply.  Generally when I see the pitch oscillations I
>> > am going less than 600 knots and the actual aircraft is rated at 1300
>> > knots.  Any thoughts where / how the PID controller can be tweaked to
>> > get rif of the problem and make it stable ?
>>
>> Well the JSBSim F-16 can do mach 2.2 at 40,000ft without any problems
>> last time I checked. So it's not just airspeed but speed at a certain
>> altitude.
>>
>> That said, there definitely seems something wrong here.
>>
>> Unfortunately tuning the PID controllers is more of an art form rather
>> than science. So the tuning process can be tedious and time consuming.
>>
>> There are some hints though: Overcompensation for pitch and error build
>> up for roll.
>>
>> The PID controllers are defined in the configuration file and are called
>> fcs/roll-rate-pid and fcs/g-load-pid
>>
>>
>> Erik
>>
>> --
>> http://www.adalin.com - High performance virtual reality audio software.
>>
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>
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