Airspeed sensors have been a major concern for both manned, and unmanned aviation alike. Yet almost every fixed wing or VTOL vehicle is dependent on it. So what is wrong with these sensors and why are they used?
A standard airspeed sensors consists of a pitot tube, a temperature sensor and 2 pressure sensors. The pitot tube has 2 ports (holes), one at the tip (like an injection needle) which is called the dynamic port, and one at the side which is called the static port. Each of these ports are connected to a pressure sensor. When the tube is pointing forward on a moving vehicle, it can measure the difference between the pressure from dynamic and static port. When factoring in the temperature it can estimate the current speed through air.
Over the years many problems have been identified with the use of airspeed sensors in both manned and unmanned vehicles. There is not one major problem so here is a list in random order;
If the airspeed data is missing most vehicles UAVs will not takeoff or enter some failsafe mode. But usually the data is not missing, it is simply incorrect. With incorrect readings it will either fly too fast and therefor very inefficient, or it will fly too slow and in many cases crash.
Another serious side effect of bad airspeed readings is landing. Fixed wing UAVs are generally loaded to their maximum capacity. That means they need to fly their absolute minimum speed to land without damage. If they land too fast they start tumbling and usually break their wings or worse. VTOL fixed wing UAVs do not have this problem, but still suffer from inefficient or dangerous flight behavior when the readings are incorrect.
During the research & development of the DeltaQuad VTOL UAV it quickly became clear that the airspeed sensor was a major concern. We therefor embarked on achieving the impossible: completely eliminate the danger.
Having members on the PX4 core development team can be a big advantage when pioneering in unmanned aviation. Using this advantage we managed to completely eliminate the need for an airspeed sensor by using a variety of other sensors to estimate our airspeed accurately and consistently.
Achieving accurate and reliable estimation was not easy, and required very specific tuning and testing, but the end result turned out better then we hoped. Using our estimated readings we not only eliminated all the issues in relation to airspeed sensors, we had also increased the efficiency of our vehicle by 20%
So if you are considering the purchase of a fixed wing or VTOL UAV, and you see a probe sticking out in front that look suspiciously like an airspeed sensor, make sure you think twice.