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Re: [Jsbsim-devel] IC vcas
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From: dpculp <dp...@gm...> - 2011-05-08 02:51:03
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Thanks for looking at this Bertrand. The P-80 is a jet, and should
have no problem with trimming.
This could just be a problem with the way FlightGear's interface is
trimming the model, but the vAeroUVW result looks to me like it
probably comes from elsewhere. In fact, it looks like the wind
direction might be getting convoluted with the relative wind.
It's also possible that I'm the only one getting bad trim results.
I'm using JSBSim of 5 May built into FlightGear of about 20 April.
I run more tests when I get back from vacation.
Dave
Sent from my iPhone
On May 6, 2011, at 10:04 AM, Bertrand Coconnier <bco...@gm...>
wrote:
> 2011/5/6 David Culp <dav...@co...>:
>> More data. This is starting the P-80 model in flight using:
>>
>> --altitude=3000 --vc=300 --prop:/controls/gear/gear-down=false
>>
>>
>> The results:
>>
>> FGIC: vcas: 300 , Mach: 0.477795 , vt_fps: 527.818
>> alt_ASL: 3000 , SAT: 507.972 , Sos: 1104.69
>
> FGInitialCondition uses the total Mach (i.e. the total true airspeed
> divided by the sound speed) to compute the CAS. Calculations are
> correct.
>
>> FGAux: vcas: 236.417 , MachU: 0.223568 , vt_fps: 527.818
>> alt_ASL: 3000 , SAT: 507.972 , Sos: 1104.69
>> VAeroUVW: 246.975, 466.467, 1.8277
>> Mach: 0.477795 , mUW: 60999.9 , pt: 1963.95
>
> FGAuxiliary is using the Mach calculated along the x axis of the body
> frame (called MachU) to compute the CAS. As you can see the total TAS
> is the same in FGInitialCondition and FGAuxiliary, however since the
> velocity u is significantly different from the total velocity ('v' is
> almost twice 'u') the CAS calculations are different in FGAuxiliary.
>
>> Sorry, udot doesn't appear to be trimmable
>
> Currently there is a bug in JSBSim where piston engines cannot be
> started while trimming. Therefore there is no thrust and since JSBSim
> trims udot with the throttle, whichever value it picks for the
> throttle, the thrust is the same : zero. Hence the failure to trim
> udot.
>
> Ron has suggested that piston engines are not starting because
> FGAuxiliary returns a total pressure of zero while trimming. However
> forcing the pressure to a reasonable pressure of 2000 psi did not make
> any difference on my side. May be I did not apply the modification in
> the correct place ?
>
>> Trim Results:
>> Angle of Attack: -3.6 wdot: 1.36e-04 Tolerance: 1e-03
>> Passed
>> Throttle: 0.5 udot: 1.18e+02 Tolerance: 1e-03
>> Failed
>> Pitch Trim: -0.19 qdot: -4.19e-11 Tolerance: 1e-04
>> Passed
>>
>> Trim Statistics:
>> Total Iterations: 4
>> Sub-iterations:
>> wdot: 16 average: 4 successful: 1 stability: 15.348
>> udot: 0 average: 0 successful: 0 stability: 12.625
>> qdot: 7 average: 1.75 successful: 4 stability: 9.9286
>> Run Count: 694
>>
>>
>> In this case the airplane flight path is bad, the udot won't trim,
>> the AOA trims to a bad solution. It's been a while since I started
>> in-air, so this may have been around a long time. I hope I'm not
>> the only one getting this result for in-air trim in FlightGear.
>
> I would say that the issue is more related to the trim of the aircraft
> rather than to the CAS calculation. I guess it does not help you much
> but at least I hope it will focus attention to a more probable cause
> for your issue.
>
> Bertrand.
>
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