Monday, June 9, 2014

Alfa beta probe



Sensors of angle of attack and sideslip are not often seen on DIY setups. I make here some consideration on the existing measurement methods to evaluate the more suitable for further development.   The sensor should be installed on small and middle sized RC planes; the typical flight envelope does include high angle of attacks and low speed; high frequency measurements are required.
 
Wind direction along with the other air data measurements enable us to estimate aerodynamic parameters and implement real time control. The real time control includes, but not only, stability augmentation, fault detection and reconfiguration. Classical  stall warning devices are based on angle of attack. Angle of attack is fundamental for obtaining maximum range and most economical cruises.

Types and performances of angle of attack devices is wide, the most common principles of measurement are
-Aerodynamic deflection, the device is pretty similar to a mechanical weather vane. For instance see BasicAirData Boom.
Figure 47.1 AOA, AOS winds vanes mounted on BasicAirData probe
-Multi hole probes or MHP, the wind angles are derived from the pressure measured at different pressure taps


Figure 47.2 Multi hole probe from Aeroprobe

-Null seeking, the device is mechanically deflected until zero reading is reached.
Video 47.1 a null seeking device video
 -Hotwire Anemometers, the wind direction and speed is derived by temperature measurements

Mechanical wind vane was already considered and built. Main issue is the dynamic behavior and connected measurement limitations. If the operating airspeed range is wide than problems at extreme speed values can happen; problems arise at booth upper speed limit and lowers speed limit. This sensor can have serious dynamic tracking problems so it will better to use another sensor type.
Hot wire anemometer design seems too fragile, because of low section cables exposed to the air.
Null seeking device cons is the considerable hardware part count, the complexity for a combined AOA and AOS probe seems rather high for a DIY project.

Multi hole probe technology is suggestive because don’t need for moving parts to operate. Sensor probe is self-contained and robust.

Beside the classical standalone probe there is also the possibility to use multiple pressure ports on the fuselage as the NCAR plane
Figure 47.3 NCAR plane AOA and AOS sensor 

Returning to classical MHP, use of this last kind of sensors should be investigated.  The major advantage seems to be the robustness, absence of moving parts, extremely fast response and a capacity to operate at high incidence angles. Typical MHP has five holes facing the airstream; four holes are in a cruciform pattern and the last at the middle point.
Figure 47.4 Flow angle sensor, flow angle arrangement

You can note the static pressure ports on this probe. In the general case the pressure at frontal the ports is measured respect to static pressure. With this layout five differential pressure transducers and one absolute pressure transducer are needed for best performance. When correctly operated this kind of MHP probes can provide static pressure and total pressure value for altitude and airspeed calculations.
It's evident that this probe needs a good amount of electronic to be operated. With the current piezoelectric sensors dimensions that is not a problem anymore.

On a future post I'll numerically evaluate some probe parameter, to be sure that measurements can be carried out with DIY level equipment.
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