The attached patch isn't ready to go in, but I was hoping to get some
comments on it. It changes the power calculation to use the Gagg-
Farrar Power Drop Off Rule from "Performance of Light Aircraft" by John
Lowry.
The Gagg-Farrar equation uses a constant to express the power drop off
of a piston engine as air density decreases.
sigma= air_density / standard_density
phi = ( sigma - C ) / ( 1 - C )
power *= phi
In early testing it seems to work well. I used rho_air_manifold for
air_density and replaced suction_loss, which was based in part on
throttle position, with phi. This may resolve one of Torsten's issues
with the FGPiston model.
I've made the constant "C" a configurable item, but I don't have a good
name for it. The patch calls it <gagg-c> but there must be a better
name... I just can't think of one.
Thanks
Ron
Index: FGPiston.cpp
===================================================================
RCS file: /var/cvs/FlightGear-0.9/source/src/FDM/JSBSim/models/propulsion/FGPiston.cpp,v
retrieving revision 1.12
diff -u -r1.12 FGPiston.cpp
--- FGPiston.cpp 30 Nov 2008 10:44:30 -0000 1.12
+++ FGPiston.cpp 17 Dec 2008 06:33:58 -0000
@@ -81,6 +81,7 @@
MinManifoldPressure_inHg = 6.5;
MaxManifoldPressure_inHg = 28.5;
BSFC = -1;
+ Gagg_C = 0.12;
// Initialisation
volumetric_efficiency = 0.8; // Actually f(speed, load) but this will get us running
@@ -206,6 +207,8 @@
BSFC = el->FindElementValueAsNumberConvertTo("bsfc", "LBS/HP*HR");
if (el->FindElement("volumetric-efficiency"))
volumetric_efficiency = el->FindElementValueAsNumber("volumetric-efficiency");
+ if (el->FindElement("gagg-c"))
+ Gagg_C = el->FindElementValueAsNumber("gagg-c");
if (el->FindElement("numboostspeeds")) { // Turbo- and super-charging parameters
BoostSpeeds = (int)el->FindElementValueAsNumber("numboostspeeds");
if (el->FindElement("boostoverride"))
@@ -521,9 +524,10 @@
void FGPiston::doMAP(void)
{
+ double p_inlet=p_amb, boost_factor=1;
+
suction_loss = RPM > 0.0 ? ThrottlePos * MaxRPM / RPM : 1.0;
if (suction_loss > 1.0) suction_loss = 1.0;
- MAP = p_amb * suction_loss;
if(Boosted) {
// If takeoff boost is fitted, we currently assume the following throttle map:
@@ -549,21 +553,22 @@
}
}
// Boost the manifold pressure.
- double boost_factor = BoostMul[BoostSpeed] * suction_loss * RPM/RatedRPM[BoostSpeed];
+ boost_factor = BoostMul[BoostSpeed] * suction_loss * RPM/RatedRPM[BoostSpeed];
if (boost_factor < 1.0) boost_factor = 1.0; // boost will never reduce the MAP
- MAP *= boost_factor;
+ p_inlet *= boost_factor;
// Now clip the manifold pressure to BCV or Wastegate setting.
if(bTakeoffPos) {
- if(MAP > TakeoffMAP[BoostSpeed]) {
- MAP = TakeoffMAP[BoostSpeed];
+ if(p_inlet > TakeoffMAP[BoostSpeed]) {
+ p_inlet = TakeoffMAP[BoostSpeed];
}
} else {
- if(MAP > RatedMAP[BoostSpeed]) {
- MAP = RatedMAP[BoostSpeed];
+ if(p_inlet > RatedMAP[BoostSpeed]) {
+ p_inlet = RatedMAP[BoostSpeed];
}
}
}
+ MAP = p_inlet * suction_loss;
// And set the value in American units as well
ManifoldPressure_inHg = MAP / inhgtopa;
}
@@ -579,7 +584,7 @@
*
* TODO: Model inlet manifold air temperature.
*
- * Outputs: rho_air, m_dot_air
+ * Outputs: rho_air, m_dot_air, Gagg_sigma
*/
void FGPiston::doAirFlow(void)
@@ -588,8 +593,8 @@
double displacement_SI = Displacement * in3tom3;
double swept_volume = (displacement_SI * (RPM/60)) / 2;
double v_dot_air = swept_volume * volumetric_efficiency * suction_loss;
-
double rho_air_manifold = MAP / (R_air * T_amb);
+ Gagg_sigma = rho_air_manifold / 1.225;
m_dot_air = v_dot_air * rho_air_manifold;
}
@@ -636,6 +641,7 @@
if (Running) {
double T_amb_degF = KelvinToFahrenheit(T_amb);
double T_amb_sea_lev_degF = KelvinToFahrenheit(288);
+ double phi=(Gagg_sigma-Gagg_C)/(1-Gagg_C); // Gagg-Farrar Power Drop Off Factor
// FIXME: this needs to be generalized
double ME, friction, percent_RPM, power; // Convienience term for use in the calculations
@@ -649,7 +655,7 @@
if ( Magnetos != 3 ) power *= SparkFailDrop;
- HP = (FuelFlow_gph * 6.0 / BSFC )* ME * suction_loss * power;
+ HP = (FuelFlow_gph * 6.0 / BSFC )* ME * phi * power;
} else {
Index: FGPiston.cpp
===================================================================
RCS file: /var/cvs/FlightGear-0.9/source/src/FDM/JSBSim/models/propulsion/FGPiston.cpp,v
retrieving revision 1.12
diff -u -r1.12 FGPiston.cpp
--- FGPiston.cpp 30 Nov 2008 10:44:30 -0000 1.12
+++ FGPiston.cpp 17 Dec 2008 06:34:04 -0000
@@ -81,6 +81,7 @@
MinManifoldPressure_inHg = 6.5;
MaxManifoldPressure_inHg = 28.5;
BSFC = -1;
+ Gagg_C = 0.12;
// Initialisation
volumetric_efficiency = 0.8; // Actually f(speed, load) but this will get us running
@@ -206,6 +207,8 @@
BSFC = el->FindElementValueAsNumberConvertTo("bsfc", "LBS/HP*HR");
if (el->FindElement("volumetric-efficiency"))
volumetric_efficiency = el->FindElementValueAsNumber("volumetric-efficiency");
+ if (el->FindElement("gagg-c"))
+ Gagg_C = el->FindElementValueAsNumber("gagg-c");
if (el->FindElement("numboostspeeds")) { // Turbo- and super-charging parameters
BoostSpeeds = (int)el->FindElementValueAsNumber("numboostspeeds");
if (el->FindElement("boostoverride"))
@@ -521,9 +524,10 @@
void FGPiston::doMAP(void)
{
+ double p_inlet=p_amb, boost_factor=1;
+
suction_loss = RPM > 0.0 ? ThrottlePos * MaxRPM / RPM : 1.0;
if (suction_loss > 1.0) suction_loss = 1.0;
- MAP = p_amb * suction_loss;
if(Boosted) {
// If takeoff boost is fitted, we currently assume the following throttle map:
@@ -549,21 +553,22 @@
}
}
// Boost the manifold pressure.
- double boost_factor = BoostMul[BoostSpeed] * suction_loss * RPM/RatedRPM[BoostSpeed];
+ boost_factor = BoostMul[BoostSpeed] * suction_loss * RPM/RatedRPM[BoostSpeed];
if (boost_factor < 1.0) boost_factor = 1.0; // boost will never reduce the MAP
- MAP *= boost_factor;
+ p_inlet *= boost_factor;
// Now clip the manifold pressure to BCV or Wastegate setting.
if(bTakeoffPos) {
- if(MAP > TakeoffMAP[BoostSpeed]) {
- MAP = TakeoffMAP[BoostSpeed];
+ if(p_inlet > TakeoffMAP[BoostSpeed]) {
+ p_inlet = TakeoffMAP[BoostSpeed];
}
} else {
- if(MAP > RatedMAP[BoostSpeed]) {
- MAP = RatedMAP[BoostSpeed];
+ if(p_inlet > RatedMAP[BoostSpeed]) {
+ p_inlet = RatedMAP[BoostSpeed];
}
}
}
+ MAP = p_inlet * suction_loss;
// And set the value in American units as well
ManifoldPressure_inHg = MAP / inhgtopa;
}
@@ -579,7 +584,7 @@
*
* TODO: Model inlet manifold air temperature.
*
- * Outputs: rho_air, m_dot_air
+ * Outputs: rho_air, m_dot_air, Gagg_sigma
*/
void FGPiston::doAirFlow(void)
@@ -588,8 +593,8 @@
double displacement_SI = Displacement * in3tom3;
double swept_volume = (displacement_SI * (RPM/60)) / 2;
double v_dot_air = swept_volume * volumetric_efficiency * suction_loss;
-
double rho_air_manifold = MAP / (R_air * T_amb);
+ Gagg_sigma = rho_air_manifold / 1.225;
m_dot_air = v_dot_air * rho_air_manifold;
}
@@ -636,6 +641,7 @@
if (Running) {
double T_amb_degF = KelvinToFahrenheit(T_amb);
double T_amb_sea_lev_degF = KelvinToFahrenheit(288);
+ double phi=(Gagg_sigma-Gagg_C)/(1-Gagg_C); // Gagg-Farrar Power Drop Off Factor
// FIXME: this needs to be generalized
double ME, friction, percent_RPM, power; // Convienience term for use in the calculations
@@ -649,7 +655,7 @@
if ( Magnetos != 3 ) power *= SparkFailDrop;
- HP = (FuelFlow_gph * 6.0 / BSFC )* ME * suction_loss * power;
+ HP = (FuelFlow_gph * 6.0 / BSFC )* ME * phi * power;
} else {
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