Fix inverted tpa_pitch_compensation documentation and comment

docs/Settings.md

@@ -6454,7 +6454,7 @@ Throttle PID attenuation also reduces influence on YAW for multi-rotor, Should b
 
 ### tpa_pitch_compensation
 
-Pitch angle based throttle compensation for fixed wing. Positive values will increase throttle when pitching up, and decrease throttle when pitching down.
+Pitch angle based throttle compensation for fixed wing, used as a proxy for airspeed when no airspeed sensor is fitted. Positive values will increase this virtual throttle (attenuating PID gains) when pitching down, and decrease it when pitching up, since diving increases airspeed and climbing reduces it.
 
 | Default | Min | Max |
 | --- | --- | --- |

src/main/fc/settings.yaml

@@ -1399,7 +1399,7 @@ groups:
         min: PWM_RANGE_MIN
         max: PWM_RANGE_MAX
       - name: tpa_pitch_compensation
-        description: "Pitch angle based throttle compensation for fixed wing. Positive values will increase throttle when pitching up, and decrease throttle when pitching down."
+        description: "Pitch angle based throttle compensation for fixed wing, used as a proxy for airspeed when no airspeed sensor is fitted. Positive values will increase this virtual throttle (attenuating PID gains) when pitching down, and decrease it when pitching up, since diving increases airspeed and climbing reduces it."
         default_value: 8
         field: throttle.tpa_pitch_compensation
         min: 0

src/main/flight/pid.c

@@ -517,7 +517,7 @@ static float calculateTPAThtrottle(void)
     if (usedPidControllerType == PID_TYPE_PIFF && (currentControlProfile->throttle.fixedWingTauMs > 0)) { //fixed wing TPA with filtering
         fpVector3_t vForward = { .v = { HeadVecEFFiltered.x, -HeadVecEFFiltered.y, -HeadVecEFFiltered.z } };
         float groundCos = vectorDotProduct(&vForward, &vDown);
-        int16_t throttleAdjustment =  currentControlProfile->throttle.tpa_pitch_compensation * groundCos * 90.0f / 1.57079632679f; //when 1deg pitch up, increase throttle by pitch(deg)_to_throttle. cos(89 deg)*90/(pi/2)=0.99995,cos(80 deg)*90/(pi/2)=9.9493,
+        int16_t throttleAdjustment =  currentControlProfile->throttle.tpa_pitch_compensation * groundCos * 90.0f / 1.57079632679f; //groundCos is positive while diving; this raises the virtual throttle (and so attenuates PID gains) when pitching down, since diving increases airspeed. cos(89 deg)*90/(pi/2)=0.99995,cos(80 deg)*90/(pi/2)=9.9493,
         uint16_t throttleAdjusted = rcCommand[THROTTLE] + constrain(throttleAdjustment, -1000, 1000);
         tpaThrottle = pt1FilterApply(&fixedWingTpaFilter, constrain(throttleAdjusted, 1000, 2000));
     }