The FPV System family has grown about one more sensor, the airspeed sensor. It is based on very expensive but best in class Swiss made digital chip MS4525D5AI001D. On board has power supply allowing to work with voltages bigger and smaller then 5V (3,5-10V) because it is powered from servo bus and everything can happen there.
I put it in shop as a airspeed kit together with Pitot probe. Also probe can be ordered separately.
By default sensor has 25cm long wire with yellow socket (I2C bus) to OSD (not to AP!). If you will need another wire length to your plane please let me know in comment to order. Pitot probe has 75cm silicon hose. It should be cute in half to connect "static" and "total" pressure ports. Ports are described on sensor's board, so it should be easy to connect. Wrong connection will give you negative speed. I don't know how Zbig did it, but he can calculate square root from negative number!
Good idea is to cover the pitot probe tip (cover both total and static holes)) against wind during power on calibration.
Unfortunately sensor measure speed only in one direction. It doesn't work on flight in reverse or side direction.