During the year 1928, a number of interesting altitude flights were made, both in military and commercial ships. These flights included seaplanes with various payloads, landplanes with and without observer, and commercial planes piloted in most cases by women.

The X-C05A powered with a Liberty water-cooled engine.

On October 10th, at Wright Field, Dayton Ohio, Capts. St. Clair Street and A.W. Stevens made a most interesting and valuable flight. It may well be considered the feature performance of the year.

The Army landplane, the X-C05A, powered with a Liberty, supercharged, 12 cylinder, water-cooled engine of 400 h.p. was used in this epoch making trial, the purpose of which was to make satisfactory photographs of the ground while at high altitudes. Incidentally, a value on the "tapeline" altitude above the ground was obtained by the use of the camera.

Capts Street and Stevens, Camera and Oxygen Bottles

Captains St. Clair and Street successfully piloted the ship to a new ceiling while Captain Stevens recorded important temperature data and operated the camera. Six quarts of liquid oxygen were used. Liquid oxygen was chosen in preference to compressed oxygen gas because the apparatus used to control the pressure of the compressed gas sometimes freezes up. The flying suits, gloves, and one pair of goggles were electrically heated. The goggles used by Capt. Stevens had two small peep holes drilled through the glasses to prevent frost from obscuring his vision. The camera used was a Fairchild aerial surveying instrument fitted with a between-the-lens shutter which also was electrically heated. It was found necessary to supply heat to the shutter of the camera in order to prevent the thin metalic leaves from shattering when operating at low temperature.

Barograph

Captain St. Clair Street managed to climb until the altimeter in the cockpit (uncorrected for scale and temperature errors) read 40,220 feet. The climb to the ceiling of the plane was made in 1 hour, 20 minutes. After the last camera exposures were made, it was found that the throttle had become jammed due to the low temperature which proved to be minus 60° Centigrade outside and minus 44° in the cockpit. It was, therefore, necessary to incline the plane downward with the throttle open. To cut the engine off for the glide down, would mean a rapid freezing of the water in the cylinder jackets, resulting in a ruined engine.

After considerable maneuvering, the plane was brought down to the 34,000 ft level where the free air was relatively warmer (minus 48° centigrade) and the throttle became movable after the application of considerable force. At this point, the ship was almost out of fuel but the engine would sputter now and then, enough to keep the water from freezing during a rapid decent.

Cockpit of X-C05A

By masterful handling of the ship, a safe landing was made in a field near Rushville, Indiana about 50 miles northwest of the point of departure. In as much as the ship did not return to the starting point, the F.A.I. could not have recognized the flight as an official record, even had a new altitude mark been established. This flight, however, was of such great interest that Capt. Stevens brought the two barographs to the Bureau of Standards together with photographic negatives and some unusually complete meteorological data obtained during the flight.

The minimum temperature recorded within the barographs was -43° (plus and minus 3°) for each instrument. The official barograph, when checked against a standard mercurial barometer gave a minimum pressure of 152 millimeters of mercury, which is equivalent to 11,538 meters, or to 37,854 ft. This established an unofficial altitude record for two men in a plane.

Of special interest in this remarkable flight was the comparison of altitude determinations made by three different methods.

• The F.A.I. standard atmosphere method, which gives an arbitrary altitude-pressure relation that is used only for purposes of international comparison.



• The photographic method, which, from the knowledge of the focal length of the camera and measurements of the object on the ground and the image obtained on the plate, gives by computation the tape-line altitude above the ground.



• The barometric formula method, which takes into consideration the pressue and temperature of the air column traversed during the flight. The latter method is outlined in N.A.C.A. Technical Report No. 246, pages 6 to 8.

The results of the various altitude determinations for the flight of October 10 are given in the following table:

The above values are all above sea level. The elevation of Wright Field is 800 feet. The elevation of the ground photographed at the ceiling of the flight near Rushville, Indiana is 970 feet. A comparison of the above values indicates that with reference to the photographic method, the barometric formula method gives a value which is 0.9 percent to high, and that the F.A.I. standard atmosphere method gives a value which is 3.6 per cent to low.

Since service altimeters are calibrated in accordance with the U.S. standard atmosphere given in N.A.C.A. Report No. 246, the official records are determined from the barograph calibrated in accordance with the F.A.I. formula of 1920, it is easy to see why there is usually such a discrepancy between the cockpit altimeter and barograph readings obtained at the ceiling of high altitude flights.