121 lines
3.5 KiB
C++
121 lines
3.5 KiB
C++
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#include "Math.hpp"
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#include "Atmosphere.hpp"
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namespace yasim {
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// Copied from McCormick, who got it from "The ARDC Model Atmosphere"
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// Note that there's an error in the text in the first entry,
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// McCormick lists 299.16/101325/1.22500, but those don't agree with
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// R=287. I chose to correct the temperature to 288.20, since 79F is
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// pretty hot for a "standard" atmosphere.
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// meters kelvin kPa kg/m^3
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float Atmosphere::data[][4] = {{ 0, 288.20, 101325, 1.22500 },
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{ 900, 282.31, 90971, 1.12260 },
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{ 1800, 276.46, 81494, 1.02690 },
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{ 2700, 270.62, 72835, 0.93765 },
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{ 3600, 264.77, 64939, 0.85445 },
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{ 4500, 258.93, 57752, 0.77704 },
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{ 5400, 253.09, 51226, 0.70513 },
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{ 6300, 247.25, 45311, 0.63845 },
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{ 7200, 241.41, 39963, 0.57671 },
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{ 8100, 235.58, 35140, 0.51967 },
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{ 9000, 229.74, 30800, 0.46706 },
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{ 9900, 223.91, 26906, 0.41864 },
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{ 10800, 218.08, 23422, 0.37417 },
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{ 11700, 216.66, 20335, 0.32699 },
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{ 12600, 216.66, 17654, 0.28388 },
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{ 13500, 216.66, 15327, 0.24646 },
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{ 14400, 216.66, 13308, 0.21399 },
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{ 15300, 216.66, 11555, 0.18580 },
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{ 16200, 216.66, 10033, 0.16133 },
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{ 17100, 216.66, 8712, 0.14009 },
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{ 18000, 216.66, 7565, 0.12165 },
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{ 18900, 216.66, 6570, 0.10564 }};
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float Atmosphere::getStdTemperature(float alt)
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{
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return getRecord(alt, 1);
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}
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float Atmosphere::getStdPressure(float alt)
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{
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return getRecord(alt, 2);
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}
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float Atmosphere::getStdDensity(float alt)
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{
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return getRecord(alt, 3);
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}
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float Atmosphere::calcVEAS(float spd, float pressure, float temp)
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{
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return 0; //FIXME
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}
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float Atmosphere::calcVCAS(float spd, float pressure, float temp)
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{
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// Stolen shamelessly from JSBSim. Constants that appear:
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// 2/5 == gamma-1
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// 5/12 == 1/(gamma+1)
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// 4/5 == 2*(gamma-1)
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// 14/5 == 2*gamma
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// 28/5 == 4*gamma
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// 144/25 == (gamma+1)^2
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float m2 = calcMach(spd, temp);
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m2 = m2*m2; // mach^2
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float cp; // pressure coefficient
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if(m2 < 1) {
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// (1+(mach^2)/5)^(gamma/(gamma-1))
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cp = Math::pow(1+0.2*m2, 3.5);
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} else {
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float tmp0 = ((144/25.) * m2) / (28/5.*m2 - 4/5.);
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float tmp1 = ((14/5.) * m2 - (2/5.)) * (5/12.);
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cp = Math::pow(tmp0, 3.5) * tmp1;
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}
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// Conditions at sea level
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float p0 = getStdPressure(0);
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float rho0 = getStdDensity(0);
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float tmp = Math::pow((pressure/p0)*(cp-1) + 1, (2/7.));
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return Math::sqrt((7*p0/rho0)*(tmp-1));
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}
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float Atmosphere::calcDensity(float pressure, float temp)
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{
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// P = rho*R*T, R == 287 kPa*m^3 per kg*kelvin for air
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return pressure / (287 * temp);
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}
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float Atmosphere::calcMach(float spd, float temp)
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{
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return spd / Math::sqrt(1.4 * 287 * temp);
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}
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float Atmosphere::getRecord(float alt, int recNum)
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{
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int hi = (sizeof(data) / (4*sizeof(float))) - 1;
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int lo = 0;
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// safety valve, clamp to the edges of the table
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if(alt < data[0][0]) hi=1;
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else if(alt > data[hi][0]) lo = hi-1;
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// binary search
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while(1) {
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if(hi-lo == 1) break;
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int mid = (hi+lo)>>1;
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if(alt < data[mid][0]) hi = mid;
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else lo = mid;
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}
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// interpolate
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float frac = (alt - data[lo][0])/(data[hi][0] - data[lo][0]);
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float a = data[lo][recNum];
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float b = data[hi][recNum];
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return a + frac * (b-a);
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}
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}; // namespace yasim
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