Andy Blackburn wrote:
At 07:00 01 January 2005, Eric Greenwell wrote:

I think the angle of attack range for an unflapped
airfoil is about 10
degrees, which would suggest errors of 0 (at high speed,
for example)
increasing to 1.5% at low speed (or vice versa - depends
on where you
aim the sensor). This could be easily corrected using
using the
airfoil's Cl vs AOA chart. For a flapped airfoil, the
fuselage AOA range
is even smaller, and the errors could likely just be
ignored.


50:1 is an angle of a degree and a bit so if you have
your 'straight ahead' and 'straight down' sensors canted
down/aft by just a degree from true horizontal/vertical,
you'll get a pretty accurate airspeed, but the 'vertical'
speed will likely show zero, I think, since it will
be reading off dust particles that have zero velocity
towards/away from the glider.

Think of the glider flying straight and steady in still air: it is
descending (vertical motion) through the air at whatever it's sink rate
is. So, at least in concept, a laser airspeed sensor pointed straight
down will be able to measure this. Even if the sensor is aimed a few
degrees one way or the other from perpendicular, the error would be very
small, equal to sine of the angle off of perpendicular.

If the airmass is moving, the measurement would be the same, of course,
since the speed measured is the air motion relative to the glider - it's
just easier to visualize what's happening with still air.


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Eric Greenwell
Washington State
USA