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   Generates a state space F-18 model from a range of models. 
   The mach #s range from .35 to 0.7 and altitudes from 15 kFt to 32 kFt.
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   Form:
   [g, machNo, altitude, def] = F18Model( machNo, altitude )
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   Inputs
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   machNo         (1,1)  Mach number
   altitude       (1,1)  Altitude (kFt)

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   Outputs
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   g              (1,1)  State space model
   machNo         (1,1)  Nearest Mach number
   altitude       (1,1)  Nearest altitude (kFt)
   def            {:,3}  State, input and output definitions

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