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