In the 1920s, the public was enamored with flying, culminating with the trans-Atlantic flight of Charles Lindbergh in May of 1927. Perhaps second only to the first non-stop flight across the Atlantic, the goal of being the first to fly to the North Pole captured the public imagination. A year before Lindbergh’s flight, attention was focused on the island of Spitzbergen, where the famous polar explorer Roald Amundsen was poised to cross over the Pole on a flight to Alaska in the dirigible Norge. American hopes to be first to fly to the Pole were pinned on Commander Richard Byrd, flying in a Fokker trimotor plane piloted by Floyd Bennett. Byrd’s plan was to fly due north from Spitzbergen to the Pole and return, and success of the flight depended critically on good navigation. In the uncharted arctic, a magnetic compass would not help, partly because it points to the magnetic pole, not the geographic pole.
Survival of Byrd and Bennett depended primarily on knowing the direction to true north. In the land of the midnight sun, in the course of 24 hours the sun makes a complete circle above the horizon. Byrd used the changing azimuth of the sun during the course of the flight to keep on a line of constant longitude on both the northward and southward legs. An ingenious instrument known as a sun compass did the job. Mounted in the cockpit was a clock mechanism that rotated a sighting device once in 24 hours; set correctly before takeoff, the pilot simply had to keep the shadow cast by a pin on marks on a screen. Photos of a sun compass at the Byrd Polar Research Center at Ohio State University are at https://byrdpolarmedia.osu.edu/nGFMC8XkC.
At least, that’s the simple theory. If there’s any cross wind, of course, the direction the plane points (heading) differs from the direction the plane is moving (bearing). To compensate for this difference, Byrd used a “drift indicator” mounted over a hole cut in the bottom of the airplane. A sight wire was rotated until an object on the ground (a feature on the ice in Byrd’s case) was observed to move parallel to the sight wire. The angle between the sight wire and the axis of the airplane told the pilot how much the plane’s heading should differ from the direction indicated by the sun compass.
The fact that Byrd could fly far north and return to make landfall at Spitzbergen nearly 16 hours later demonstrates that he knew the direction he was traveling quite well. The more difficult, and controversial, part of his navigation was knowing just how far he had traveled. The airspeed gauge on the plane would not tell the speed over the ground if a headwind or tailwind were present. Here a second function of the drift indicator was vital. Two wires, one fixed and the other movable, were mounted at right angles to the direction the plane was going. Viewing from above the wires, Byrd used a stop watch to time how long it took a feature on the ice to appear to go between the two wires. The faster the ground speed, the shorter time it took (shown schematically in Figure 1).
Of course, Byrd had to know how high the plane was; the greater the plane’s altitude, the slower the ice appeared to move. This is where Byrd’s small barograph came in (of a type shown in Figure 2). A pen traced the barometric pressure on a piece of paper mounted on a slowly rotating drum. Every few minutes, Byrd would place a calibration strip of paper next to the barograph drum; the calibration strip was marked with altitudes for every fifth of an inch of barometric pressure. There were bound to be some errors in altitudes obtained this way. The barometric pressure at sea level, 30.5 inches of mercury at the start, would change during the
flight, as would the variation of pressure with altitude.
Altitude scales marked on both sides of the drift indicator showed where the movable wire should be placed. When the plane was twice as high, the movable wire was half as far from the fixed wire. In this way, the time it took a feature on the ice to appear to move between the fixed and movable wire was independent of the plane’s altitude and depended only on ground speed. A drift indicator similar to the one Byrd used is shown at http://airandspace.si.edu/collections/artifact.cfm?id=A19570247000.
The barometer thus played a crucial role in Byrd’s dead reckoning of how far he had traveled. Belfort Instruments has samples of the type paper used in Byrd’s barograph; the paper recorded pressures from 31 inches to 19 inches of mercury in a strip only 3 inches wide. A random reading error of 0.1 inches on the barograph corresponded typically to an error of about 18% in altitude, and hence an 18% error in ground speed. Systematic errors would be much worse, however, since they could all be in the same direction. For instance, if the altitudes Byrd obtained were systematically 5% less than the true altitude, then Byrd would overestimate his speed by 5%, and when he thought he had reached the Pole he would actually be about 38 miles (60 km) short of his goal. Some critics have claimed the Fokker trimotor was not fast enough to have reached the Pole and return in the 16-hour duration of the flight, concluding that Byrd overestimated his distance traveled by at least this much. Others, however, claim a faster speed for the plane, and Byrd himself credited a strong tailwind on the return leg as a major factor for his arrival back at Spitzbergen sooner than expected.
Whether Byrd reached the North Pole on 9 May 1926 continues to be argued. Amundsen and his airship left Spitzbergen on 11 May and, since he arrived in Alaska two days later, there is no argument that he must have crossed over or close to the Pole. Partisans of Byrd claim that credit for the first person to reach the North Pole by air rightly belongs to the American naval aviator. Byrd was an instant American hero, greeted by a ticker tape parade on his return to New York followed by a meeting with President Coolidge. But the claim rests entirely on how well Byrd, in a very fatigued condition and in freezing temperatures, could accurately measure his barometric pressure and altitude, set the movable wire on the drift indicator, time how long a feature on the ice moved between the wires, and finally tabulate his ground speed and accumulated distance traveled. If he did all this with minimal error, it was a most remarkable achievement. The flight continues to provide grist for the mill of those arguing over whether Byrd or Amundsen first flew over the North Pole.
Figure 1 – Measuring ground speed with the drift indicator. At higher altitudes, h, the angle ? is set to be smaller, so that the distance traveled over the ground, d, is the same no matter what the altitude of the plane.
Figure 2 – A small barograph similar to the one used by aerial navigators to find their altitude and
dead-reckoning ground speed.
Department of Astronomy
The Ohio State University
Columbus, OH 43210