Polar Analysis from flight logs?

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?

On Thu, 30 Dec 2004 16:51:25 -0800, Eric Greenwell
wrote:


GPS speed and pressure altitude are referenced to the earth, not the air
mass, so they would determine a different L/D than airmass referenced
instruments. In concept, a glider flying at constant speed through
rising and falling air would have a constant L/D according to the laser
airspeed sensors, but a widely varying one based on GPS speed and
pressure altitude.

You are correct - I should have read your posting more carefully!
Happy New Year, btw!


Bye
Andreas

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

"Lars P. Hansen" wrote:

OK, here we go: Could devices like this not also be used to detect thermals?
The description in the link below about how the laser "sees" minute dust
particles in the air seems to be well suited to thermals.


I don't buy the explanation they give in the cited url (http://www.navysbir.brtrc.com/succes...navsea_p3.html)
They pretend the device measures the speed and direction of dust particles
from the shift in the frequency of reflected light, this is well known
as Doppler effect and can only give the radial component (toward or away
from the sensor) of the speed, not its value and direction. For thermals
we are interested in the speed component which is nearly perpendicular
to the measured component, so this would be of little interest. Of course
whith several such devices on the ground, all the 3 compenents of airmass
speed could be measured, maybe this in the intended use of the device as it
is advertised, but in a glider you don't have sufficient vertical distance
for putting 2 devices which could provide an accurate value for the vertical
component of the speed.

After this discussion we are far from the original question, i.e.
can we infer anything about the polar of a glider just from GPS
fligth logs of this glider?

The obvious answer several people gave was: no because the airmass
movement is unknown.

Well, as a former mathematician, I would say this is just what the
name says: an unknown. Would we be able to determine it?

The problem is this is not a single unknown, it is an infinity of them.
And despite the fact that each track log point gives 3 equations, this
would not be sufficient for determining an infinite number of unknown.
Even when considering the finite number of unknowns consisting of the
3 components of the airmass speed at each point of the log, we have
more unknowns than equation since we have also the unknown polar we
want to determine.

So is there a solution?

We can do for other unknowns just what we do with the polar data,
which are also an infinity of unknowns: reduce their number by
assuming a simple model depending of a small number of unknowns, this
is usually done for the polar by assuming a quadratic approximation
depending only on 3 parameters.

In the same way we can assume that the horizontal components of
the wind are constant on the flight area at a given altitude, and
that the evolution with altitude could be carcterized with a few
parameter, e.g. wind speed at 3 given altitudes and polynomial
interpolation between them.

For the vertical component of airmass movement, we can assume that
the pilot is following some speed-to-fly rule caracterized by
a MC setting and the 3 parameters of the glider polar that was
used for making the MC ring or programming the flight computer.
Even these values may be considered as unknowns.

So we have now a small number of unknowns and a comparatively
large number of equations from the flight log, plus the few
ones from the physics relating position to speed, airspeed and
wind to ground speed, sink to height, sink to airspeed according
to the speed to fly rule and so on. Our system is no more undetermined
but overdetermined, probably there is no exact solution, but there
are methods for determining a most likely solution, i.e. values
for the unknowns that minimize the way the equations are not
satisfied (least square method or others).

As I am only a *former* mathematician, I will not go further
in this way, maybe somebody who is a real mathematician will
complete the job, or somebody who is teaching maths will
propose that as a project for students, this is the lazy method
I would have used when I was myself teaching :-)

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. It wouldn't matter what the airmass was doing, since the
measurements are relative to the airmass.
...

The real problem then is to determine where should the down pointing
device exactly point. An error just equal to the gliding angle in
the backward direction will give an infinite L/D, and this just
about 1 degree. The direction relatively to the airframe is variable
with speed and is identical with the direction of apparent weight
only during unaccelarated flight, in this case this is also the direction
of the real weight, but we have no mean to detect that the flight has
no acceleration. Although the method is very simple in theory, I doubt
there is some practical realisation which would produce accurate enough
results for being interesting, even with a lot of money.

Robert Ehrlich wrote:
"Lars P. Hansen" wrote:

OK, here we go: Could devices like this not also be used to detect
thermals?
The description in the link below about how the laser "sees" minute
dust
particles in the air seems to be well suited to thermals.


I don't buy the explanation they give in the cited url
(http://www.navysbir.brtrc.com/succes...navsea_p3.html)
They pretend the device measures the speed and direction of dust
particles
from the shift in the frequency of reflected light, this is well
known
as Doppler effect and can only give the radial component (toward or
away
from the sensor) of the speed, not its value and direction. For
thermals
we are interested in the speed component which is nearly
perpendicular
to the measured component, so this would be of little interest. Of
course
whith several such devices on the ground, all the 3 compenents of
airmass
speed could be measured, maybe this in the intended use of the device
as it
is advertised, but in a glider you don't have sufficient vertical
distance
for putting 2 devices which could provide an accurate value for the
vertical
component of the speed.

It does work, but they use a little different technique.
The Doppler is only measured on particles at the focal length of the
optics.
The assumption is that the airmass (at least locally) is all the same,
and that the Doppler measurement is taken far enough away so the
effects of the airplane on the airmass are negligible.
So you send out two beams -- say, one forward at 45 degrees, one aft at
45 degrees.
It turns out that if you sum the signals from the two beams, you get
the vertical component of velocity, and if you difference the two
signals, you get the horizontal component.
Since we're measuring frequency, we can get sum and difference
frequencies from a mixer, though I have no doubt it runs through a DSP
somewhere.

So you only need one sensor head (though it puts out multiple beams).

By sending out two more beams, to each side, you can also pick up
sideslip information.
The clever thing is that they're using components developed for the
communications field, which helps to keep costs down.

Tim Ward