Hi
I have been thinking of making these for a while and by reading the discussion forums there seem to be a couple of options outlined here
http://diydrones.com/forum/topics/angle-of-attack-sensor?id=705844%3ATopic%3A78347&page=2
There seem to be 4 options
a)A rotary vane connected to a Hall effect sensor/low friction pot
b)A diode laser(not a clue about this)
c)A 5 hole probe(Seems interesting and doable)
d)Mark Drela's idea of having pressure taps on the top and bottom of the wing to figure out AOA
Has anyone tried these options? I am starting a new topic because the above link is about a year old.
a)How would a rotary vanes position be affected by aircraft attitude or rotation. I would think that these effects need to be removed before measuring AOA/ sideslip
b)Seems most elegant,
c)How would this be calibrated?
d)If i don't know the exact airfoil shape, this is a bit complicated.
Have any of the RC folks had success with these methods?
Srinath
Replies
AOA is not affected by the aircraft attitude... (I may get flamed for that).... Remember that the aerofoil performance is usually defined in terms of C_L and C_D versus AOA.. not airspeed or attitude
AOA (alpha) is the angle of the wind with reference to either; 1. the aircraft axes (longitudinal axis, xz plane), or 2. the Chord line of the wing (this is the definition of AOA), it is often easier to reference AOA to the primary aircraft axes, as the wing is at a known angle of incidence to the aircraft's longitudinal axis in the xz plane..... and usually an Aircraft body fixed coordinate system (Centred on the nominal CG) is used as a reference.
Sideslip (Wind) angle(Beta), similarly is not related to attitude..... but it is the side angle of the wind frame to the aircraft lateral plane (xy plane) ...
Alpha and beta angles can be measured directly, using a wind vane, or indirectly using pressure measurements.
I am currently looking at the best way to measure it on a small UAV... using a wind vane connected to Either; 1. a voltage divider, make sure you use a linear pot, not a log pot, or 2. an optical rotary disk sensor..... (Using and old (computer) mouse optical trackball sensor, .. though their angle resolution is not very good, it can be made better by using a belt or gear drive to increase the angular rotation relative to the wind vanes...... The angle output needs to be integrated to allow tracking of the current wind frame data....
The only problem with aircraft rotation, is that the physical angle sensors may be damped and have a slower response than the actual aircraft.... (if you are performing sedate (regular UAV style) manoeuvres, this won't be a problem...) meaning that the measured angle of attack etc, may need to catch up with the aircraft.... this is why low friction instruments must be used.
Airfoil shape isn't too important in this instance (as long as the plane flies) as you will have to log the data, to work out the AOA which suits your conditions of operation. The good thing about using a physical sensor is that it can easily be calibrated, a pressure sensor must be modelled and tested under known conditions... (wind tunnel, aircraft on the roof of the car at fixed AOA, etc..)
Happy to be corrected.
I found this article on how a laser doppler system could be used to determine everything we need.
If anyone knows more about laser velocimetry, please feel free to respond. Could we do this with a diode laser?
pogosov.pdf