Airspeed Sensor. ARSPD_RATIO. Air density corrections

postelzhuk · July 26, 2024, 2:49pm

User part

I am trying to understand what ARSPD_RATIO is really doing and how to use it.
Calibration process is described here: Calibrating an Airspeed Sensor — Plane documentation

My main questions:

Should ARSPD_RATIO be recalibrated if the weather conditions changed?
What does ARSPD_RATIO mean from physical point of view? Is this parameter related to air density? Or is this parameter just compensate pitot tube imperfections during measurements?

Developer part

Pitot tube equation

As described in classic document: (Pitot Tube) pitot tube should use this equation:

velocity = sqrt(2*(totalPressure - staticPressure) / airDensity)

In Ardupilot, airspeed sensor pressure values converted to speed using such code:

github.com

ArduPilot/ardupilot/blob/605a9d34e3c6b906203d4f67f98c1c72345d4bd1/libraries/AP_Airspeed/AP_Airspeed.cpp#L669


      
                  state[i].airspeed       = sqrtf(MAX(-state[i].filtered_pressure, 0) * param[i].ratio);
                  break;
              case PITOT_TUBE_ORDER_POSITIVE:
                  state[i].last_pressure  = airspeed_pressure;
                  state[i].raw_airspeed   = sqrtf(MAX(airspeed_pressure, 0) * param[i].ratio);
                  state[i].airspeed       = sqrtf(MAX(state[i].filtered_pressure, 0) * param[i].ratio);
                  break;
              case PITOT_TUBE_ORDER_AUTO:
              default:
                  state[i].last_pressure  = fabsf(airspeed_pressure);
                  state[i].raw_airspeed   = sqrtf(fabsf(airspeed_pressure) * param[i].ratio);
                  state[i].airspeed       = sqrtf(fabsf(state[i].filtered_pressure) * param[i].ratio);
                  break;
              }
          #endif // HAL_BUILD_AP_PERIPH
          }
          
          // read all airspeed sensors
          void AP_Airspeed::update()
          {
              if (!lib_enabled()) {

airspeed_pressure is in fact (totalPressure - staticPressure)
param[i].ratio is a ARSPD_RATIO parameter.
solving this gives us a result that:
ARSPD_RATIO = 2 / airDensity
This makes sense because ARSPD_RATIO default value is 2 and airDensity is in most cases close to 1.

TAS (True air speed)

There is a logic in AP_Baro class from converting EAS (airspeed sensor value) to TAS (true air speed). Function _get_EAS2TAS() calculates scale factor EAS2TAS.
This EAS2TAS scale factor is used like this:

tasDataNew.tas = airspeed->get_airspeed(selected_airspeed) * EAS2TAS;

It’s all make sense. EAS2TAS should scale the airspeed for different air density. But I still have some questions.

Questions for developers

I can’t find where TAS (true airspeed) is logged. Only raw airspeed value without EAS2TAS value is logged (ARSP.Airspeed). Can I use ARSP.Airspeed to analyse pitot tube work? Because since there is no air density correction it should always be a slight error in it.
There a code in libraries\AP_Airspeed\Airspeed_Calibration.cpp that makes me totally confused:
ardupilot/libraries/AP_Airspeed/Airspeed_Calibration.cpp at 605a9d34e3c6b906203d4f67f98c1c72345d4bd1 · ArduPilot/ardupilot · GitHub

// calculate true airspeed, assuming a airspeed ratio of 1.0
    float dpress = MAX(get_differential_pressure(i), 0);
    float true_airspeed = sqrtf(dpress) * AP::ahrs().get_EAS2TAS();

2.1 Why we assume that airspeed ratio is 1.0?
2.2 Calibration code use EAS2TAS() for ratio calculation. So resulting ARSPD_RATIO should already be scaled by EAS2TAS. Isn’t it a double scale factor apply for TAS?
2.3 If there is already air density compensation in ARSPD_RATIO should we recalibrate this value when the whether changes?

Thanks