mavlink_messages for HUD: send EAS instead of IAS

[sfuhrer]

This PR makes the MavlinkStreamVFRHUD send true_airspeed_m_s instead of indicated_airspeed_m_s for HUD on groundstation. This makes especially sense when your airspeed setup needs a scale factor (between indicated to equivalent), as otherwise the HUD displays another airspeed than the one that's used in the control modules.

Alternatively we could also display equivalent and not true airspeed. I would prefer true airspeed, such that it is comparable to the groundspeed.

EDIT: it would now send EAS and not TAS.

[dagar]

I don't actually feel that strongly about this however...

• VFR_HUD contains indicated airspeed according to the Mavlink spec. https://mavlink.io/en/messages/common.html#VFR_HUD
  

• The attitude controller gain scheduling is wrt indicated airspeed.
  https://github.com/PX4/Firmware/blob/6d846143dc7461d9a8ca4fd377763a9f1c04d55c/src/modules/fw_att_control/FixedwingAttitudeControl.cpp#L235-L270

• Wouldn't any pilot expect indicated airspeed? (@RomanBapst)

[hamishwillee]

Yes, this would be a break to the spec. If you're going to do this create a PR on MAVLink with a justification that explains why this is not a problem for all the HUD systems out there. You can perhaps add a new extension field for MAVLink 2 messages if you think a HUD might need that too.

[sfuhrer]

@dagar I wasn't aware actually that FW att C still used IAS, made a PR to change that #14868.
I reverted to using EAS (which is equal to calibrated airspeed, CAS, on PX4).
@hamishwillee I've created a PR on mavlink mavlink/mavlink#1375. As we only have one unspecified "airspeed" field in VFR_HUD, editing the comment would be enough, but let me know what you think of that. It doesn't make sense for me to send IAS to the HUD if you have an CAS available, like we have in PX4. After a bit of internet research I think that also real airplanes usually display CAS and not IAS.

[sfuhrer]

@dagar mavlink/mavlink#1375 is merged, I guess we can then do the same here.