"Mechanical Mike" (The Evolution of the Modern Airplane Autopilot)

Wiley Post's Lockheed Vega aircraft, which was called the Winnie Mae, made 1931 a world record for flying around the world in 8 days with Harold Gatty as the navigator. ( Most references /1/ )

Wiley Post and Harold Gatty.

A key item of equipment in the Winnie Mae airplane was at the time new Sperry automatic pilot that Wiley Post referred to as "Mechanical Mike" and which he had tested for about 85 flight hours.

Mechanical Mike was located in the cockpit of Winnie Mae.

Developed by Elmer and Lawrence Sperry in the early 1920s, the "robot pilot" used two gyroscopes and was only 9 by 10 by 15 inches in size. One of these was an azimuth (directional) gyroscope, which provided a datum for the heading control of the airplane. The other was a horizontal gyro, providing a datum for longitudinal and lateral control of the airplane.

Lawrence and Elmer Sperry.

The two gyros were air driven and ran at a speed of 15,000 revolutions per minute. The human pilot could shift from automatic pilot to manual control at will, but he could also change course, climb, or dive by turning remote controls to the autopilot, thus effecting flight changes without disengaging the autopilot. The apparatus was provided with a hand-operated clutch for disengaging the autopilot for takeoffs and landings.

Tthe "robot pilot" used two gyroscopes.

A mechanism was incorporated so that when the airplane banked about its longitudinal axis, the stabilized gyroscope resisted the bank, and an airjet, activated valve opened, which, through an oil-operated servo mechanism, moved the control cables to remove the resistance by adjusting the ailerons to roll the airplane wings level.

A mechanism  rolled the airplane wings level. Attitude changes of the aircraft, produced pressure differentials in airflow in the ducts ending near the discs. The result was a proportional change in air valves that was transmitted to the appropriate hydraulic system which, in turn, moved the proper-control surface (elevator for pitch, aileron for bank, and rudder for yaw). The autopilot, which Post called "Mechanical Mike," required no electrical power and functioned very well.

A similar mechanism on the same gyroscope controlled airplane pitch about the lateral axis (nose up, nose level, or nose down) by moving the control cables forward and backward as necessary to adjust the elevators. The other gyroscope controlled the directional gyro (which was set periodically in accordance with the magnetic compass) and through its oil-operated servomechanism made small adjustments on the rudder to yaw the aircraft back to maintain a predetermined heading.

"Mechanical Mike", the Sperry Autopilot. It was a "three-axis" autopilot, that is, one gyroscope sensed pitch changes and roll (bank) changes by the aircraft, and the other gyroscope sensed heading changes (yaw). The air compressor and the hydraulic pump were engine powered. The gyroscopes were driven by compressed air. Compressed air was also used to correct changes of aircraft attitudes in relation to pitch, bank, or yaw, through a system of small air ducts and semilunar plates.

The entire apparatus weighed 70 pounds and it deserves appreciation that such devices had not received extensive trials at the time Post was planning his world flight!

The Evolution of the Sperry autopilot or gyropilot (Mechanical Mike): It got smaller during 1930's.

In 1933, Post repeated his round-the-world flight, but this time did it solo, with the aid of the auto-pilot and radio compass.

The Sperry automatic pilot used by Post for the 1933 flight differed from previous autopilots in an important manner. Other automatic pilots of the time used electrical "pick-offs" to determine the relative motions of the aircraft and the "fixed" axes of the spinning gyroscopes, and utilized a slip-stream, "wind-powered" spinning gear device as the motive power to enable the autopilot to move the control surfaces. The Sperry autopilot was pneumatic. For electrical autopilots an externally placed small propeller blade was necessary to achieve this motive power. This type of device created drag and was relatively inefficient as a source of power.

Post's Sperry autopilot used airjet pick-offs closely associated with the gyroscopic platforms, a system less liable to mechanical troubles that were common in other autopilots which used varying electrical contacts. The airjet pick-off mechanism used in the Winnie Mae was the first autopilot with this new mechanism. The autopilot was completely mechanical (pneumatic) and did not use electrical power. The diagram of the autopilot (above) illustrates the relationships of the various mechanisms constituting the autopilot.

Hydraulic cylinders were located directly under the unit and connected to the control cables.

The horizontal gyroscopic air pick-offs consisted of two hemispherical discs that were fixed to the gyroscopic platform, and, with the airplane flying straight and level, the edges of the discs received equal pressure from the air jets. If the airplane nose up, and/or banked, as by a gust, one or more of the jets of air would not encounter a portion of the disc, allowing more air to flow out of the respective jet. The difference in airflow activated the associated air valve which then opened a hydraulic valve and caused pistons to be moved which were attached to the respective aileron or elevator surfaces. The proper surfaces moved to return the aircraft to the position where the airflow pickoffs were equal.

The heading gyroscope, which controlled the rudder, was arranged in such a way that the mounting of the hemispherical disc caused die airjets to move with the airplane rather than with the gyroscope. This enabled the pilot to fix the airplane on various headings in the 360° azimuth from North. Otherwise, the mechanism operated identically to that of the horizontal gyroscope.

Sperry autopilot (Mechanical Mike) was located in front of the pilot.

An "elevator" knob at the right of the gyro horizon on the control panel enabled Post to establish a given "pitch attitude" of the airplane to be held by the autopilot. An aileron knob, just above the gyro horizon, permitted a similar setting for bank and a rudder knob, immediately above the direction indicator, was used fo azimuth. A "directional gyro compass" display was mounted immediately to the left of the "artificial horizon," both being centered on the panel in front of the pilot's seat. The lever that shifted the autopilot from manual to automatic was located immediately under the servo-unit hydraulic cylinders, which were mounted just beneath the autopilot gyroscopes in the instrument panel. The hydraulic cylinders were in plain view.

A "caging" knob for the artificial horizon was placed just to its left, and a similar caging knob was just below the directional gyro indicator. The gyros were "caged" during periods of manual, visual flight to save wear and tear on their bearings.

Oil sump and pump and air compressor parts.

Lawrence Sperry was the son of Elmer Ambrose Sperry and his wife, Zula Augusta Goodman. Both father and son were noted inventors. His father was best known for inventing the gyroscopic compass. Lawrence invented a three-way gyrostabilizer, effectively inventing the first autopilot.

REFERENCES

/1/ Smithsonian Annuals of Flight, Number 8, Stanley R. Mohler, Bobby H. Johnson,
     "Wiley Post, His Winnie Mae, and the World's First Pressure Suit",
      Smithsonian Institution Press, City of Washington, 1971

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