Calibration: An airspeed sensors requires calibration before every flight. This is needed to account for the difference in readings the 2 pressure sensors will give.
No 2 sensors will read exactly the same. The calibration is a snapshot that can be different from one moment to the next. Drift: As the sensors are in operation they tend to drift due to temperature changes, how they drift is not always the same so it is hard to compensate for this. Angle of attack: As you might imagine the system only works correctly if the sensor is pointing directly into the wind, but the pitch angle of any aircraft, especially unmanned, can change based on speed, air pressure, temperature or drag.
The pitot tube is hollow and fragile and can easily break during transport or landing. It needs special calibration: Not everyone is a UAV specialist, and properly calibrating an airspeed sensor takes a specialist. The board computer cannot tell if the calibration was done correctly, it just needs to assume it was. Temperature drift: A UAV heats up as it flies. This means there is a drift in temperature that does not correlate to the air temperature.
The board computer will try to compensate for a temperature that is not the actual air temperature. Cross wind: Where manned aircraft usually have multiple airspeed sensors, UAVs normally have only one.
If it flies in a cross wind there will be extra pressure measured on the static port on the side. The effect of incorrect airspeed readings If the airspeed data is missing most vehicles UAVs will not takeoff or enter some failsafe mode.
This page is currently mainly about how to do just that. The altitude and airspeed loops are separated as shown in the diagram below. Basically the throttle and pitch are controlled independently and are not coupled in the control loops. Of course one affects the other but the control loops are independent. Please see the control loops for a more detailed block diagram. The first loop is used to regulate the ground speed and the second the airspeed.
This is done just to ensure that if the ground speed drops below a certain value the airspeed will be increased to compensate in order to maintain a valid GPS heading. If you happen to have a 3axis magnetometer build in your airframe for getting the heading values, maintaining a certain GPS speed for getting a heading is not needed.
The following plot is from an actual test flight after spending some time setting the loop gains Here the possibility to perform real-time tuning through the GCS is a real time saver.
In the test, the an airplane was flying circles at a constant altitude, except in the end of the flight. In the middle there is an example of what happens when the ground speed falls below the setpoint.
Finally the altitude setpoint was changed to verify that the airspeed will be maintained while climbing. The benefits of the airspeed hold are obvious in this example. The throttle adjusts to keep the airspeed close to the setpoint. Sometimes it is very helpful for tuning your aircraft that you only measure the airspeed without controlling you aircraft behavior. This can be accomplished in the following way:. The EagleTree Airspeed Sensor is a low cost module and comes with a very good pitot tube Prandtl style, pitot-static tube that includes static and dynamic ports, it's resolution is 0.
First connect the two tubes coming out the back to the airspeed sensor. The tube coming straight out the back should go into the top port and the tube exiting at an angle should connect to the bottom port on the airspeed sensor.
If you are using Plane in an aircraft with the propeller in the nose, the pitot tube must be mounted out on one wing, at least a foot from the fuselage to be outside the prop flow. You can check the airspeed reading with Mission Planner or another ground station. Just blow on the pitot tube or press your finger over it and observe the response. In still air oscillation between zero and small values is normal. The airspeed varies with the square root of the pressure, so for differential pressures near zero it varies quite a bit with very small pressure changes, while at flying speeds it takes much greater pressure changes to produce a similar change in speed.
If you see mostly 0, 1, 2, with an occasional bounce to 3 or 4, consider it normal. You will not see that sort of variability at flying speeds. The airspeed sensor reading is automatically zeroed by the autopilot during initialization, so it is good practice during windy conditions to place a loose fitting cover over the pitot tube that shields the front hole and the four small side holes from the wind. This cover should be fitted prior to power on and removed before flight.
A normal model flying field circuit pattern or loiter will achieve the required direction changes, cross-country flying will not. See Calibrating an Airspeed Sensor for more details.
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