private void CalculateFromPressures(float pitotPress, float staticPress)
{
var speedMps = Math.Sqrt(dp_Coef * Math.Max(0, pitotPress - AIS_Baseline));
var speedKts = speedMps * mps2kts;
// Compute the airspeed
airspeed = ((int)(speedKts < 30d ? 0 : speedKts)).ToString();
var baro2X = Math.Pow((double)((staticPress - mbOffset) / seaLevelMp), coefOfPressChange);
var now = DateTime.Now;
var tDelta = now - lastVsiUpdate;
if (tDelta.Ticks == 0)
{
tDelta = new TimeSpan(200);
}
var dT = Math.Min(0.3d, Math.Max(-0.3d, tDelta.TotalMilliseconds / 1000.0d));
lastVsiUpdate = now;
// initial state
if (baro2XLast == 0)
{
baro2XLast = baro2X;
}
if (speedLast == 0)
{
speedLast = speedMps;
}
var kinetticEnergy = speedMps * speedMps / 19.8d; // must be signed to work... lossing velocity needs to drop total energy
var potentialEnergy = 44330d * (1.0d - baro2X);
var totalEnergy = potentialEnergy + kinetticEnergy;
if (totalEnergyLast == 0)
{
totalEnergyLast = totalEnergy;
}
// Altituded change from pressure difference derivation
// --- Given
// k = 44330
// x = 1 / 5.25
// p0 = 1013.25
// --- stubtracting two pressures equations yields;
// [k * (1 - (p2 / p0) ^ x)] - [k * (1 - (p1 / p0) ^ x)]
// -- Then
// k * [(1 - (p2 / p0) ^ x) - (1 - (p1 / p0) ^ x)]
// k * (1 - (p2 / p0) ^ x - 1 + (p1 / p0) ^ x)
// k * ((p1 / p0) ^ x - (p2 / p0) ^ x) => p1^x / p0^x
// k * (p1 ^ x - p2 ^ x) / p0 ^ x
// --- QED
// k / p0 ^ x * (p1 ^ x - p2 ^ x), or
// 11862.610784520926279471081940874 * (p1 ^ x - p2 ^ x)
const double k_over_p02x = 11862.610784520926279471081940874;
// Convert the baro pressure then add the kinnetic energy factor to generate a total energy
totalEnergyVV = vsiFilter.Update(2d * (totalEnergy - totalEnergyLast) / dT);
var vv30 = (int)(3d * k_over_p02x * tdiVsi30.Push((float)baro2X, now) * mps2fpm);
var vsiT = (int)(totalEnergyVV * mps2fpm);
if (displayUpdateCounter-- == 0)
{
displayUpdateCounter = 10;
vsi = (20 * (int)Math.Round(vsiT / 20d)).ToString();
vsi30 = "30s " + (10 * (int)Math.Round(vv30 / 10d)).ToString();
altitude = (int)(/* meters=> 44330*/ 145439.6d * (1.0 - baro2X));
}
baro2XLast = baro2X;
speedLast = speedMps;
totalEnergyLast = totalEnergy;
}
