Re: [Jsbsim-devel] IC vcas

[Tony P. <ton...@co...>]

What is alpha and beta?  The u component should be pretty close to Tas unless those angles are large.

--
Tony
Follow your heart, don't give up, and try like hell to not be stupid.

On May 6, 2011, at 1:04 PM, 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.
> 
> ------------------------------------------------------------------------------
> WhatsUp Gold - Download Free Network Management Software
> The most intuitive, comprehensive, and cost-effective network 
> management toolset available today.  Delivers lowest initial 
> acquisition cost and overall TCO of any competing solution.
> http://p.sf.net/sfu/whatsupgold-sd
> _______________________________________________
> Jsbsim-devel mailing list
> Jsb...@li...
> https://lists.sourceforge.net/lists/listinfo/jsbsim-devel
> _______________________________________________
> The JSBSim Flight Dynamics Model project
> http://www.JSBSim.org
> _______________________________________________
>