There are a couple of manufacturers of pitot-static probes, temperature sensors, smart probes and air data computers on the market. How can you be sure to make a good choice? What is important? How about the do-it-yourself approach?
Following some statements and recommendations based on our experience:
- In German we have the proverb “Wer misst, misst Mist” which translates to something like “The one who measures measures crap”. Even so a pitot-static probe is easy to understand in practice the underlying aerodynamics is very complicate. Very small changes can affect the measurement result considerably. Static pressure holes alone is a science. What is the best diameter? What is the influence of a small entry burr? Can you drill static holes in carbon fiber? Without testing it is nearly impossible to do it right. But to perform tests you need a wind-tunnel. Without a wind-tunnel you are out of luck. An air data manufacturer usually operates its own wind-tunnels. These wind-tunnels are optimized for air data and not for force measurements.
- Accuracy is very important if you measure air data. A lot of effects like Reynolds number, Mach number, flow angle, temperature, vibration and others influence the measurement result. As there are so many effects, it is very important to minimize every single source of error to keep the sum of the errors at an acceptable level. So it is for example very important to minimize the error of the pressure measurement. Without special measures you usually get a sensor accuracy that is much worse than 1% (read the data-sheet carefully and sum up all errors).
- If you fly at an altitude of 6’000 m (approx. 20’000 ft) the standard temperature is at about −24°C. What does your pressure sensor measure at that temperature? Without special calibration in a temperature oven it will be most likely off quite a bit. What does this mean? Assume at 6’000 m or −24°C the static sensor is off by 2% (this is a realistic number) then you get an altitude error of more than 300m from the sensor alone. But improving the accuracy is not easy. A general rule-of-thump says: to increasing the accuracy by a factor of 10 you need an effort of factor 100.
- Many UAVs fly at low airspeed. At low airspeed the impact pressure gets very low. Half the airspeed gives only one fourth of the pressure. At 250 kn (approx. 460 km/h) the impact pressure is 105 hPa (or 105 mBar), at 125 kn. the pressure is already down to 26 hPa. At 40 kn (approx. 74 km/h) there is a pressure of only 2.6 hPa. The selection of the correct pressure sensor at such low pressure is crucial. Vibration and g-loads are a considerable source of errors at such low airspeed.
UAVs, RPAs and Drones:
The term RPA usually refers to remotely piloted aircraft, but these aircraft are also known as unmanned aerial vehicles (UAVs). The term unmanned aircraft systems (UAS) includes not only the aircraft, but all the ground support equipment and personnel. The term Drones is usually used in the context of military operations. The aircraft flying such missions are precision weapons systems.
Air data is the measurement of the physical characteristics of the air mass that surrounds an aircraft. The two main physical characteristics measured are temperature and pressure. Using these basic measurements allows to compute many other flight parameters like calibrated airspeed, true airspeed, Mach number, pressure altitude, etc