Polar Analysis from flight logs?

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.

9B

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.


--
Change "netto" to "net" to email me directly

Eric Greenwell
Washington State
USA

If the sensor is aiming straight down, at a glide angle of 40/1 the air
is going 40" aft for every 1" that it rises relative to the instrument.
If the sensor is aimed slightly aft (1/40, whatever that is in
degrees), the air won't be rising at all relative to the sensor. Right?

So isn't angle crucial?

Also, can the sensors measure speed when the air mostly is going
crossways in front of the sensor?

Greg Arnold wrote:
If the sensor is aiming straight down, at a glide angle of 40/1 the air
is going 40" aft for every 1" that it rises relative to the instrument.
If the sensor is aimed slightly aft (1/40, whatever that is in
degrees), the air won't be rising at all relative to the sensor. Right?

So isn't angle crucial?

OK, I'm persuaded! It now appears the sensor would need to be aimed up
or down rather accurately, or the at least the angle off vertical
measured accurately. Dang - that's harder. Perhaps the inertial system
would be a reasonable way to achieve this, or maybe differential GPS
system with antennas on the nose and tail.


Also, can the sensors measure speed when the air mostly is going
crossways in front of the sensor?

I think it depends on the sensor: some are optimized for speed in line
with the beam, some for speed perpendicular to the beam (cross wind
measurement, like for bullets). Perhaps there are ones that can read the
vector wind? I have no idea how much crossways speed the various sensors
can tolerate.

--
Change "netto" to "net" to email me directly

Eric Greenwell
Washington State
USA

I think it depends on the sensor: some are optimized for speed in line
with the beam, some for speed perpendicular to the beam (cross wind
measurement, like for bullets). Perhaps there are ones that can read the
vector wind?


If so, you would just need a single sensor facing forward. You still
would have the problem of getting it perfectly horizontal, though.


I have no idea how much crossways speed the various sensors
can tolerate.


I am betting that these sensors can only measure speed directly toward
or away from the sensor. Sort of like a radar gun. So I am not
understanding how they could measure a glider's sink rate.

It seems to me that having the sensor hanging freely like a pendulum
(pointing down) would make it measure the vertical component.

"Eric Greenwell" wrote in message
...

OK, I'm persuaded! It now appears the sensor would need to be aimed up
or down rather accurately, or the at least the angle off vertical
measured accurately. Dang - that's harder. Perhaps the inertial system
would be a reasonable way to achieve this, or maybe differential GPS
system with antennas on the nose and tail.

Bob Salvo wrote:
It seems to me that having the sensor hanging freely like a pendulum
(pointing down) would make it measure the vertical component.

"Eric Greenwell" wrote in message
...

OK, I'm persuaded! It now appears the sensor would need to be aimed up
or down rather accurately, or the at least the angle off vertical
measured accurately. Dang - that's harder. Perhaps the inertial system
would be a reasonable way to achieve this, or maybe differential GPS
system with antennas on the nose and tail.

Maybe that would be good enough - especially for flying in smooth air,
like wave flying or early morning test flights. Or, maybe these units
would measure quickly enough, all you'd need would be occasional 5-10
seconds of smooth air.


--
Change "netto" to "net" to email me directly

Eric Greenwell
Washington State
USA