Re: [Jsbsim-devel] IC vcas

[Bertrand C. <bco...@gm...>]

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.