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Subject: Re: Gyroscope vs. Fluxgate compass
From: Brian Whatcott (inet@XXX.XXX)
Date: Fri Feb 01 2002 - 18:25:29 EST
At 05:45 AM 2/1/02, Chuck Taylor recalled several interesting
gyro applications to mind in this note:
>Brian Whatcott mentioned gyroscopes. When I served in US Navy ships a few
>decades ago, we used gyroscopes to steer by, with a compass as backup. One
>advantage was that the gyro always pointed to true north. Several gyro
>repeaters
>were slaved off of one master located in the bowels of the ship. That way the
>gyro repeaters on the bridge wings always agreed with gyro repeater at the
>helm.
>The gyro itself was massive in size and I'm sure it was quite expensive.
Notice that Chuck reaffirmed the gyrocompass as pointing True North
with no outside help.
That was quite a trick that Sperry pulled in 1911!
On a ship of reasonable size - above all, a ship that does not travel West as
fast as the Sun (which would not require unusual speed in polar regions,
or in fast vehicles like modern airplanes) then a gyroscope on a
horizontal axis
sees a precessing force due to Earth rotation. A pendulous weight pointing
downwards can oppose the tilt of the gyroscope so that it precesses towards
the true north/south direction.
This mechanism shares some of the qualities of the Sperry artificial horizon
used in airplanes; in those, a little pendulous vane arangement uncovers an
air port to provide a little jet which keeps the gyro horizon 'erect'.
But as far as I know the Sperry gyrocompass was not adopted to airplane use.
The bumpiness has something to do with it - and the excessive
Westerly speeds another.
Instead, bigger airplanes used a flux gate sensor to drive a 'slaved' gyro.
(This may be implemented as the compass card of a Radio Magnetic
Indicator ("RMI") or lately as a Horizontal Situation Indicator ("HSI") or
currently as a compass card surrogate depicted on the
CRT or LCD or Plasma display of a "glass cockpit".)
Small airplanes dispense with the Flux gate sensor, and the gyro stabilized
compass card (now called a "direction indicator") is set to the whiskey
compass
by hand, initially, and rechecked from time to time. (The instrument is
cunningly
contrived to drift at the appropriate rate for a given latitude, so
North/South flight
incurs increasing error)
>When I learned to fly small airplanes, they all had gyros. If I recall
>correctly, power came from a simple air turbine, which would spin up when you
>started the engine. Part of the check list for takeoff was to align the
>heading
>of the gyro with the direction of the runway. Once aligned, it stayed aligned
>for the duration of the flight.
>
>The cost of many things technological has come down quite drastically over the
>years. Has anyone heard of an attempt to design/build a gyroscope
>suitable for
>steering a relatively small boat for a reasonable price (maybe a few
>hundred US
>dollars)? Is such a thing feasible?
>
>Chuck Taylor
> 47d 55.161' N
>122d 11.176' W
Mechanical gyros are being superceded in some applications by Attitude
Heading References, as I mentioned earlier. The solid state devices which
incorporate
little tuning fork oscillators on multiple axes are a step on the path to
replacing
gyros. Not in quite the same league yet, as far as I know....
Brian Whatcott
Altus OK Eureka!
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