PX4Flow, Testing Sonar

I have a PX4Flow mounted on a Trad Heli, which I am playing around with. Running the 3.3 beta firmware, I am getting results with the Optflow, but I cannot get any readings at all on the sonar side. I have tried setting it up in mission planner. can anybody point me in the right direction. I have searched in internet.I am using a Pixhawk flight controller.