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From: barrett@hpcnof.UUCP
Newsgroups: net.aviation
Subject: Re: Orphaned Response
Message-ID: <42000006@hpcnof.UUCP>
Date: Mon, 30-Dec-85 13:52:00 EST
Article-I.D.: hpcnof.42000006
Posted: Mon Dec 30 13:52:00 1985
Date-Received: Wed, 8-Jan-86 07:31:37 EST
References: <3089@sun.UUCP>
Organization: 30 Dec 85 11:52:00 MST
Lines: 101

/*
 * alt.c - Written by Dave Barrett 	Dec 29, 1985
 *
 *   Program to play with density altitude computations
 *
 * NOTE: Formulas for density altitude here won't work above 35000 ft or
 *       so because the standard lapse rate is not linear.  
 *
 * cc -o alt alt.c -lm
 */

#include <stdio.h>
#include <math.h>

#define AbsZero      (-273.16) /* or is it -273.15? definition of C is funny */
#define cent(Kelvin) ((Kelvin) + AbsZero)	     /* kelvin to Centigrade */
#define kelvin(Cent) ((Cent) - AbsZero)		     /* Centigrade to kelvin */
#define meters(feet) ((feet) * 12.0*2.54/100.0) /* meters to feet conversion */
#define feet(meters) ((meters) * 100.0/(2.54*12.0))
#define FtoC(F)      (5.0/9.0 * ((F) - 32.0))	  /* Farinheit to Centigrode */
#define CtoF(C)      ((C) * 9.0/5.0 + 32.0)

#define LapseRate (-0.0065)  /* C/m */       /* Standard free air lapse rate */
#define T0        (288.16)   /* K */    /* Standard Temperature at sea level */
#define g         (-9.80655) /* m / s^2 */    /* Earth's gravitational const */
#define R         8.31432    /* kg m^2 / s^2*mol*K */ /* Universal Gas Const */
#define M         0.028966   /* kg / mol */           /* Molar weight of air */
#define P0        1013.25    /* mb=kg / 100*m*s^2 *//* pressure at sea level */
#define y0        0          /* m */           /* true altitude at sea lavel */

#define StdTemp(y) (T0 + LapseRate*(y))  /* K */     /* Standard temperature */
#define StdAlt(T)  (((T)-T0)/LapseRate)  /* m */         /* Inverse of above */

#define Q         (g*M/(LapseRate*R))			    /* dimensionless */
#define k         (P0/pow(StdTemp(y0), Q))          /* mb / K */ /* d P / dy */
#define mu	  (100*k*M/R)	    	  /* kg / m^3 */       /* d rho / dy */


main()
{
    double alt, temp, press;
    double DensityAltitude(), stddensity(), stdpressure(), PressureAltitude();

    /*
    printf("y0   = %.6f\n", y0);
    printf("b    = %.6f\n", LapseRate);
    printf("a    = %.6f\n", T0);
    printf("g    = %.6f\n", g);
    printf("M    = %.6f\n", M);
    printf("R    = %.6f\n", R);
    printf("P0   = %.6f\n", P0);
    printf("y0   = %.6f\n", y0);
    printf("Q    = %.6f\n", Q);
    printf("k    = %.6e\n", k);
    printf("mu   = %.6e\n", mu);
    */

    printf(" Input alt(ft), Temp(C)?");
    scanf("%lf %lf", &alt, &temp);
    alt = meters(alt);
    printf(" alt          = %.6f m\n", alt);
    printf(" temp         = %.6f C\n", temp);
    printf(" Std Temp     = %.2f C\n", cent(StdTemp(alt)));
    printf(" Density      = %.4f kg / m^3\n", stddensity(alt));
    printf(" Pressure     = %.2f mb\n", stdpressure(alt, temp));
    printf(" Density alt  = %.2f ft\n", feet(DensityAltitude(alt, temp)));
}

double stddensity(altitude) 
   double altitude;
{
   return(mu*pow(StdTemp(altitude), Q-1));
}
   
double stddensityalt(density)
   double density;
{
   return(StdAlt(pow(density/mu, 1/(Q-1))));
}

double stdpressure(altitude) 
   double altitude;
{
   return(k*pow(StdTemp(altitude), Q));
}
   
double stdpressalt(pressure)
   double pressure;
{
   return(StdAlt(pow(pressure/k, 1/Q)));
}

double DensityAltitude(altitude, temperature)
   double altitude, temperature;
{
   return(
      stddensityalt(
	 M/R * 100 * stdpressure(altitude) / kelvin(temperature)
      )
   );
}