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Oct 22, 2012

The Gatty Navigation Instrument (Gatty Drift Sight) 1930's

The Gatty Navigation Instrument was a ground speed and wind drift angle indicator developed by Harold Gatty, an Australian-born navigator who lived in California. There is an US patent number US1977762 about it. This instrument was also called the Gatty Drift Sight (Popular Mechanics Magazine, Sep 1931, had a story about it). This sight was widely used by airmen during the late 1930s and eventually Gatty sold it to the US Army Air Corps. Gatty also perfected a 'double drift' method of taking drift reading headings by flying on courses 90 degrees apart to get accurate ground speeds. /1/ /2/ /3/

Harold Gatty and Wiley Post standing by the Winnie May aircraft. The red arrow points the instrument position in the aircraft.

Harold Gatty invented a special combination wind drift and ground-speed  indicator to assist with his navigation duties during the 1931 world flight with Wiley Post.

The operation principe of the Gatty Navigation Instrument

The principle utilized for determining ground speed was based upon comparing the  rate of passage of terrain, as viewed through an eyepiece and two prisms, with  the rate of passage of a series of red marks placed on a film strip, moved by a  clock mechanism at a fixed speed.

The usage of the instrument

The eyepiece was raised or lowered until the two rates were optically the same.  Gatty then read off the distance above the film to which the eyepiece had been moved and consulted a table he had previously made in test flights with the  Winnie Mae at known altitudes and ground speeds.

All Gatty needed to know to use the instrument for ground speed was the altitude  of the airplane above the ground (which could be determined by flying down and  climbing to the cruise altitude or by using the altimeter that was set in  accordance with the previous takeoff point). The instrument was useful over land  or water.

The periscope case of the instrument

For wind drift, grids of the eyepiece were turned until objects on the earth's  surface flowed parallel through the lines of the grids. The angle of "crab"  across the surface could then be directly read by Gatty. The patented version of the instrument used to turn the whole periscope case.

The instrument was mounted near Gatty's seat in the Winnie Mae and the ground  viewing portion extended through the right side of the fuselage where the  external portion was encased in a teardrop fairing.

 The (none turning version) instrument's 3 View

The wind drift and ground-speed indicator was developed by Gatty especially for the around the world flight with Post. This instrument consisted of an adjustable optical periscope through which the ground was viewed. A small motor-driven constant speed device produced moving lines that was superimposed upon the ground image.

The patented version of the instrument had a turning periscope case instead of just turning the eye piece grid of the other version. This version was supposed to be used through a window.

Gatty adjusted the eye piece height until no motion was apparent on the superimposed images. Proportional to the adjustment height Gatty had a reading of the actual ground speed by virtue of previously made calibrations of the equipment. Simultaneously, a rotating grid on the eye piece (or the whole periscope) was turned until the ground image flowed parallel to the grid lines, giving the degree of ground drift occurring.

1) Adjust height until no apprent motion - 2) Rotate eye piece until ground flow is parallel to the grid lines - 3) Read the height and the "crab" angle and consult calibrated tables

The external portion of this optical device protruded beneath the second and third windows on the Winnie Mae's starboard side and was surrounded by a teardrop streamlined fairing, and its small window was enclosed within the fairing.


/1/ Aero Digest - p. 57-58. - January 1932

/2/ Stanley R. Mohler and Bobby H. Johnson -
Wiley Post, His Winnie Mae, and the World's First Pressure Suit -

/3/ US Patent 1977762 - 1931 , 1934

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