While working on my Bix3, I discovered some unexpected behaviour. It might be a bug in Arduplane 3.3, it might be me missing something.
I’m fairly new to building UAV’s but I have quite some flying experience. I couldn’t imagine an aircraft not having an airspeed sensor, so I bought one (digital) for my Pixhawk. After configuring everything, with all gear still motionless on my desk and connected to MP, I noticed something strange: The artificial horizon is constantly moving in a range of about 3-4 degrees pitch. I disconnected all sensors to find out which one was causing it, and when unplugging the airspeed sensor the horizon was steady again.
I reconnected it to do some testing, these are the results:
- The airspeed is fluctuating between 0 and 2-3 m/s, which is normal. However the pitch as indicated by the horizon in MP fluctates in the same pace.
- When I press my fingertip on the pitot tube to simulate about 15 m/s speed, it momentarily indicates a pitch up to almost 10 degrees pitch.
- When it reaches 15 m/s (I kept constant pressure so the 15 m/s didn’t change) the pitch returns to 0 degrees and it does not fluctuate anymore.
- When I take my fingertip off the pitot tube, the horizon shows a pitch of about -10 degrees. After a second or so, the situation as described under point 1 is experienced again.
- If I keep the box “Enable Airspeed” ticked in MP, but untick the box “Use Airspeed”, it is also steady. The airspeed still fluctuates as normal but the sofware doesn’t use it anymore. Of course this is not what I want.
My conclusion: When the measured airspeed increases, the pitch as indicated by the horizon in MP increases, and vice versa. This seems very strange behaviour to me, since the Pixhawk was motionless on my desk, so the gyro’s shouldn’t detect anything other than a constant 0 degrees pitch.