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   Compute a linearized model of an airship at a given flight condition.
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   Form:
   g = AirshipLinMod( d, h, theta, alpha, V );
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   Inputs
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 4 INPUTS:
   d           (.)       Airship model data structure
   h           (1)       Altitude                             [m]
   theta       (1)       Flight path angle                    [rad]
   alpha       (1)       Angle of attack                      [rad]
   V           (1)       Total velocity                       [m/s]
 OR
 2 INPUTS:
   d           (.)       Airship model data structure
   x           (.)       Aircraft state object                [m]

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   Outputs
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   g           (.)       State-space data structure

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   Copyright 2003 Princeton Satellite Systems, Inc.
   All rights reserved.
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