Polar Analysis from flight logs?

At 22:30 30 December 2004, Eric Greenwell wrote:

A Google search turned up laser airspeed sensors that,
in concept, could
be used to measure L/D directly from the glider. Some
of them were good
for the low speeds we need to measure sink rates. So,
have one pointing
forward, one pointing down, divide the forward speed
by the sink rate,
and ta-da! L/D.

Would you need an inertial platform to resolve horizontal
and vertical accurately enough or would eyeballing
it be good enough?

Also, the forward looking and downward looking lasers
would be observing different air, so it's possible
that would cause problems, depending on how far out
they look.

I think it might work - though at a hefty price I bet.

9B

Andy Blackburn wrote:
At 22:30 30 December 2004, Eric Greenwell wrote:


A Google search turned up laser airspeed sensors that,
in concept, could
be used to measure L/D directly from the glider. Some
of them were good
for the low speeds we need to measure sink rates. So,
have one pointing
forward, one pointing down, divide the forward speed
by the sink rate,
and ta-da! L/D.


Would you need an inertial platform to resolve horizontal
and vertical accurately enough or would eyeballing
it be good enough?

Since the concept is to measure speed through the airmass, I don't think
inertial systems would help any, as they are referenced to the aircraft,
not the air.

I don't know if any of the systems would actually give us the vertical
speed accuracy we'd like. A major application seemed to be for
helicopters, which can move very slowly (and even back up), so low speed
sensing is certainly feasible. Speeds over 20-30 knots seemed to be easy
to get with the units I glanced at.


Also, the forward looking and downward looking lasers
would be observing different air, so it's possible
that would cause problems, depending on how far out
they look.

The distances available varied from a "a few feet" to hundreds of feet,
I think. Very application dependent: helicopters couldn't use anything
that sensed closer that the outer edge of the rotor downwash, for
example. I'm guessing one for gliders could be set for, say, 100 feet to
be far enough from the glider's influence on the airmass, and this still
have both the forward and downward airmasses close enough to be the
"same" airmass.

Some of the sensors actually sensed at right angles to the sensor beam
to measure crosswinds, and maybe one of these could be also be pointed
straight down into the same airmass the vertical speed unit was looking at.


I think it might work - though at a hefty price I bet.

Yes, all the units I came across seemed to be high end or developmental,
and rather large to stick into a glider. A careful search might have
better luck.

A mitigating factor for cost is gliders already have a pretty good
forward airmass speed sensor - the ASI - so perhaps only a downward
aimed laser sensor good for measuring 100 to 500 feet per minute at 2%
accuracy would be satisfactory.


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Eric Greenwell
Washington State
USA

"Eric Greenwell" wrote in message
...
snip

Some of the sensors actually sensed at right angles to the sensor beam
to measure crosswinds, and maybe one of these could be also be pointed
straight down into the same airmass the vertical speed unit was looking
at.


I think it might work - though at a hefty price I bet.

Yes, all the units I came across seemed to be high end or developmental,
and rather large to stick into a glider. A careful search might have
better luck.

The sensor I posted a link to was small enough to be mounted on a rifle as a
sight.
http://www.navysbir.brtrc.com/succes...navsea_p3.html

That would seem to be a feasible size, weight and ruggedness for a glider
I've sent an email with some questions I had, but haven't yet received an
answer.

Some of the applications they mention on that website -- Automotive,
Collision Avoidance,
Cruise Control, Parking Aid, would require the cost to be fairly reasonable
(though the volume would be a lot more than sailplanes would ever give
them.)

A mitigating factor for cost is gliders already have a pretty good
forward airmass speed sensor - the ASI - so perhaps only a downward
aimed laser sensor good for measuring 100 to 500 feet per minute at 2%
accuracy would be satisfactory.

If the laser and optics can be made to "see" through the canopy, pointing it
upward might make for an easier install. I've seen compasses mounted on top
of the panel.
OTOH, with it pointing downwards, it might give a good input to a gear
warning device. If it can detect small particles, I'll bet it can detect a
solid surface -- and that the signals are way different.



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Eric Greenwell
Washington State
USA

Tim Ward wrote:

Yes, all the units I came across seemed to be high end or developmental,
and rather large to stick into a glider. A careful search might have
better luck.


The sensor I posted a link to was small enough to be mounted on a rifle as a
sight.
http://www.navysbir.brtrc.com/succes...navsea_p3.html

I thought it was something telescope sized that sat next to the
rifleman.I can see now it's about the size of a flashlite - much smaller
than the airspeed units I saw.

That would seem to be a feasible size, weight and ruggedness for a glider
I've sent an email with some questions I had, but haven't yet received an
answer.

Let us know what they write.


Some of the applications they mention on that website -- Automotive,
Collision Avoidance,
Cruise Control, Parking Aid, would require the cost to be fairly reasonable
(though the volume would be a lot more than sailplanes would ever give
them.)

A mitigating factor for cost is gliders already have a pretty good
forward airmass speed sensor - the ASI - so perhaps only a downward
aimed laser sensor good for measuring 100 to 500 feet per minute at 2%
accuracy would be satisfactory.


If the laser and optics can be made to "see" through the canopy, pointing it
upward might make for an easier install. I've seen compasses mounted on top
of the panel.

Maybe a mirror adapter to peek out the side window. If the beam is small
enough, mounting it in the baggage compartment and pointing up through a
1/2" hole in the turtledeck wouldn't bother me.

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Eric Greenwell
Washington State
USA

At 23:00 31 December 2004, Eric Greenwell wrote:
Since the concept is to measure speed through the airmass,
I don't think
inertial systems would help any, as they are referenced
to the aircraft,
not the air.

I menat an inertial platform for attitude, not velocity.
If you the sensor isn't pointing horizontally/vertically
or parallel/perpendicular to the flight path, you will
likely get confounded results.

9B

Andy Blackburn wrote:

At 23:00 31 December 2004, Eric Greenwell wrote:

Since the concept is to measure speed through the airmass,
I don't think
inertial systems would help any, as they are referenced
to the aircraft,
not the air.


I menat an inertial platform for attitude, not velocity.
If you the sensor isn't pointing horizontally/vertically
or parallel/perpendicular to the flight path, you will
likely get confounded results.

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.

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Eric Greenwell
Washington State
USA

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.


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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.

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Eric Greenwell
Washington State
USA