fix(mathutils): remove machToKCas temperature dependency (#9771)

flybywiresim · Jan 17, 2025 · 2c9109b · 2c9109b
1 parent 0984434
commit 2c9109b
Show file tree

Hide file tree

Showing 3 changed files with 19 additions and 21 deletions.
diff --git a/fbw-a380x/src/systems/shared/src/OperatingSpeeds.tsx b/fbw-a380x/src/systems/shared/src/OperatingSpeeds.tsx
@@ -402,29 +402,14 @@ function getVfeNIdx(fi: number): number {
   }
 }
 
-/**
- * Convert degrees Celsius into Kelvin
- * @param T degrees Celsius
- * @returns degrees Kelvin
- */
-function convertCtoK(T: number): number {
-  return T + 273.15;
-}
-
 /**
  * Get correct Vmax for Vmo and Mmo in knots
  * @returns Min(Vmo, Mmo)
  * @private
  */
 function getVmo() {
-  return Math.min(
-    Vmo,
-    MathUtils.convertMachToKCas(
-      Mmo,
-      convertCtoK(Simplane.getAmbientTemperature()),
-      SimVar.GetSimVarValue('AMBIENT PRESSURE', 'millibar'),
-    ),
-  );
+  // FIXME use ADR corrected average static pressure
+  return Math.min(Vmo, MathUtils.convertMachToKCas(Mmo, SimVar.GetSimVarValue('AMBIENT PRESSURE', 'millibar')));
 }
 
 export class A380OperatingSpeeds {

diff --git a/fbw-common/src/systems/shared/src/MathUtils.spec.ts b/fbw-common/src/systems/shared/src/MathUtils.spec.ts
@@ -143,4 +143,14 @@ describe('MathUtils.correctMsfsLocaliserError', () => {
     expect(MathUtils.correctMsfsLocaliserError(177.5)).toBeCloseTo(2.5);
     expect(MathUtils.correctMsfsLocaliserError(90.1)).toBeCloseTo(89.9);
   });
+
+  describe('MathUtils.convertMachToKCas', () => {
+    it('correctly converts mach to CAS', () => {
+      expect(MathUtils.convertMachToKCas(0, 1013.25)).toBeCloseTo(0);
+      expect(MathUtils.convertMachToKCas(0.84, 1013.25)).toBeCloseTo(555.634);
+      // FL350 = 238.423 hPa
+      expect(MathUtils.convertMachToKCas(0, 238.423)).toBeCloseTo(0);
+      expect(MathUtils.convertMachToKCas(0.84, 238.423)).toBeCloseTo(287.097);
+    });
+  });
 });
diff --git a/fbw-common/src/systems/shared/src/MathUtils.ts b/fbw-common/src/systems/shared/src/MathUtils.ts
@@ -289,7 +289,7 @@ export class MathUtils {
     return (
       1479.1 *
       Math.sqrt(
-        ((pressure / 1013) * ((1 + (1 / (oat / 288.15)) * (tas / 1479.1) ** 2) ** 3.5 - 1) + 1) ** (1 / 3.5) - 1,
+        ((pressure / 1013.25) * ((1 + (1 / (oat / 288.15)) * (tas / 1479.1) ** 2) ** 3.5 - 1) + 1) ** (1 / 3.5) - 1,
       )
     );
   }
@@ -306,7 +306,7 @@ export class MathUtils {
       1479.1 *
       Math.sqrt(
         (oat / 288.15) *
-          (((1 / (pressure / 1013)) * ((1 + 0.2 * (kcas / 661.4786) ** 2) ** 3.5 - 1) + 1) ** (1 / 3.5) - 1),
+          (((1 / (pressure / 1013.25)) * ((1 + 0.2 * (kcas / 661.4786) ** 2) ** 3.5 - 1) + 1) ** (1 / 3.5) - 1),
       )
     );
   }
@@ -318,8 +318,11 @@ export class MathUtils {
    * @param pressure current pressure hpa
    * @returns Calibrated Air Speed
    */
-  public static convertMachToKCas(mach: number, oat: number, pressure: number): number {
-    return MathUtils.convertTasToKCas(MathUtils.convertMachToKTas(mach, oat), oat, pressure);
+  public static convertMachToKCas(mach: number, pressure: number): number {
+    // Formula from Jet Transport Performance Methods 2009.
+    return (
+      1479.1 * Math.sqrt(Math.pow((pressure / 1013.25) * (Math.pow(0.2 * mach * mach + 1, 3.5) - 1) + 1, 1 / 3.5) - 1)
+    );
   }
 
   /**