Jordi figured out how to make the ArduPilot shield $5 cheaper--now it's just $32.50 with the differential pressure sensor.
Features:
--Airspeed sensor
--Battery voltage sensor
--3.3v power regulator to support 5Hz GPS modules
--Allows programming of ArduPilot without removing GPS
--Easy attachment of other sensors and wireless modems
If you've got an ArduPilot, either 168 or 328, you totally need this. Flying with an airspeed sensor is 100% better!
EagleTree
sells a nice little Pitot tube for $10.00 that looks ideal for this settup. They also provide instructions on placement away from propwash, etc.
The pressure sensor is basically a MEMS version of a Pitot/Static system used on airliners, etc. The airspeed indicator on these has a diaphragm that receives Pitot pressure on one side of it, and Static pressure on the other, and converts the difference into a gauge reading.
My interpretation of the datasheet is that port P1 is the "pitot" port, and P2 is the "static" port, since during operation, P1>P2. You could use some thin tubing that opens to the front of the aircraft to feed P1, and another tube to the side of the aircraft to measure static pressure.
I haven't got one of these yet, but I work on the real deal on the commercial airliners, and a few issues come to mind that may or may not matter at this scale:
1. For accurate pitot readings of ram air, you need good clean air. Commercial jets have their pitot tubes pretty much outside the windows of both captain and 1st officer, to get ram air before the airframe and wings etc can disrupt that airflow. The pitot tubes also stick out far enough from the skin to get air from the "free stream flow" instead of the boundary layer air next to the skin. Propellers also have an effect, since they obviously disrupt the airflow, which is why the Spitfire had its pitot tube halfway along the wing, just below and forward of the leading edge. Perhaps that approach would be best for a front prop-driven UAV.
2. The tubing you use to transfer the air pressure from the environment to the sensor could play a part too. Avoid sharp turns in the tubing and definitely avoid kinks or squashing the tube, particularly as it meets the sensor, or you'll get all sorts of venturi effects which will stuff up the accuracy of your readings.
3. Consider the effect of sideslip or skidding, particularly if you're aiming for aerobatic manoeuvres, where the nose of the aircraft and therefore any pitot tubes and static ports aren't necessarily facing the oncoming flow. Crossfeeds from dual, oppositely placed pitot tubes and static ports should counter this effect, but may be overkill depending on your application. A simple T-junction in your tubing could work though.
4. Finally, the static port could be fed via a tube to the outside of the airframe, with the opening perpendicular to the airflow, or it may be sufficient to leave the tubing off the P2 port on the sensor if sensor is going to be at the static pressure inside the UAV anyway. It depends on how well you seal up your $32.50 Ardupilot Shield!
Hope this explanation and the links provided help peoples understanding of pitot/static systems, airspeed indicators, etc.
Comments
Just to be clear, it's the same shield--just $5 cheaper.
sells a nice little Pitot tube for $10.00 that looks ideal for this settup. They also provide instructions on placement away from propwash, etc.
My interpretation of the datasheet is that port P1 is the "pitot" port, and P2 is the "static" port, since during operation, P1>P2. You could use some thin tubing that opens to the front of the aircraft to feed P1, and another tube to the side of the aircraft to measure static pressure.
I haven't got one of these yet, but I work on the real deal on the commercial airliners, and a few issues come to mind that may or may not matter at this scale:
1. For accurate pitot readings of ram air, you need good clean air. Commercial jets have their pitot tubes pretty much outside the windows of both captain and 1st officer, to get ram air before the airframe and wings etc can disrupt that airflow. The pitot tubes also stick out far enough from the skin to get air from the "free stream flow" instead of the boundary layer air next to the skin. Propellers also have an effect, since they obviously disrupt the airflow, which is why the Spitfire had its pitot tube halfway along the wing, just below and forward of the leading edge. Perhaps that approach would be best for a front prop-driven UAV.
2. The tubing you use to transfer the air pressure from the environment to the sensor could play a part too. Avoid sharp turns in the tubing and definitely avoid kinks or squashing the tube, particularly as it meets the sensor, or you'll get all sorts of venturi effects which will stuff up the accuracy of your readings.
3. Consider the effect of sideslip or skidding, particularly if you're aiming for aerobatic manoeuvres, where the nose of the aircraft and therefore any pitot tubes and static ports aren't necessarily facing the oncoming flow. Crossfeeds from dual, oppositely placed pitot tubes and static ports should counter this effect, but may be overkill depending on your application. A simple T-junction in your tubing could work though.
4. Finally, the static port could be fed via a tube to the outside of the airframe, with the opening perpendicular to the airflow, or it may be sufficient to leave the tubing off the P2 port on the sensor if sensor is going to be at the static pressure inside the UAV anyway. It depends on how well you seal up your $32.50 Ardupilot Shield!
Hope this explanation and the links provided help peoples understanding of pitot/static systems, airspeed indicators, etc.
Simon.
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