NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Gyroscope vs. Fluxgate compass
From: Brian Whatcott
Date: 2002 Feb 1, 17:25 -0600
From: Brian Whatcott
Date: 2002 Feb 1, 17:25 -0600
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!