Re: detection of wind speed aloft

From:         dtmedin@cca.rockwell.com (David T. Medin)
Organization: Rockwell Avionics - Collins, Cedar Rapids, IA
Date:         15 Jun 95 14:25:29 
References:   1
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In article <airliners.1995.775@ohare.Chicago.COM>, Andy Tompson <andy@lll-winken.llnl.gov> writes:
> I was recently on a cross country flight and was listening to the cockpit/ATC
> communications, as I like to do (on UAL). There was some discussion about
> finding good altitudes where the headwinds were low. I got to thinking how
> headwinds can be measured from the plane. Now, I guess there are pitot tubes on
> the plane that can be used to the wind speed relative to the plane. Do they
> just subtract groundspeed to get the headwinds? If so, how do they measure the
> groundspeed? Is there some type of correlation as a function of density,
> temperature, and throttle setting (specific to the plane)? Is there some type
> of differential measurement made with data from different parts of the airframe
> to measure headwinds? Maybe this is all relatively simple, so maybe someone can
> clear the air, as it were. Thanks in advance.

Since the time when DME became operational, deriving windspeeds at various
altitudes has been easy. Of course, the weather service (U.S. anyway)
uses balloons (rare anymore) and ground-based wind profiling radars.

Anyhow, using the navigational facility of an aircraft (GPS, LORAN,
DME, RNAV, or even a chart and a clock, etc.), it is easy to get the
ground track and ground speed of the aircraft. Comparing against
heading flown and true airspeed, the direction and magnitude of the
wind aloft can be computed. While most airliners have this function
built-in to the flight management or navigation sybsystems, many
private pilots still carry an old-fashioned slide-rule calculator
called an E6B which renders solutions to the above problem in several
steps (calibrated airspeed to true airspeed, then the trig reduction
for heading).

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       David Medin            Phone: (319) 395-4498
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