....aerodynamic efficiency. The engine on this aircraft, as on the Vega, employed a single-speed, geared blower to provide improved engine power output at the cruise altitudes of the aircraft. The data in table II indicate that the Orion had a maximum speed of 226 miles per hour at sea level and a cruising speed of 200 miles per hour. The corresponding value of the zero-lift drag coefficient CD,O is 0.0210. The value of this coefficient is seen to be remarkably low, even when compared with values for present-day aircraft; and a comparison with corresponding values for the Lockheed Vega gives a good indication of the magnitude of the improvement in aerodynamic efficiency realized by retracting the landing gear. The retractable landing gear had been thought for many years to be too heavy for practical use in aircraft design; however, the spectacular reductions in drag associated with its use on an aerodynamically clean aircraft were found to far outweigh the relatively small increases in weight. The Orion first flew in mid-1931 and was produced in only limited quantities, perhaps because it was not really large enough for an airline transport; then too, there was a growing feeling that airline aircraft should be equipped with multiengines. Later in the 1930's Government regulations disallowed the use of single-engine aircraft for scheduled passenger-carrying operations.
 ...attached to the skin. The Alpha employed a low wing of cantilever construction and a full NACA-type cowling around the radial engine, but incorporated an anachronistic fixed landing gear together with an open cockpit for the pilot. The zero-lift drag coefficient for the aircraft is seen from table II to be about the same as that for the Lockheed Vega discussed earlier. The aircraft was used in limited numbers for mail and passenger operation, and the particular version shown here was employed for transportation of high-ranking military officers. Various forms of stressed-skin metal construction were destined to become the norm for propeller-driven aircraft in the years ahead.
...of this parameter for the previously discussed aircraft. About 75 Boeing 247's were built but the type was not developed further, perhaps because of Boeing's preoccupation with bomber aircraft development during that period of time.
 rows of 7, one behind the other. [note for the Web version: the Wright engine actually only has a single row of nine cylinders, while the Pratt and Whitney engine does indeed have 2 rows of 7 cylinders each. Thanks to Tom Emmert for pointing out this error that appears in the printed version of this book.] The double-row radial engine was extensively used throughout the subsequent development of large high-performance piston-engine aircraft. A comparison of the aerodynamic parameters for the Douglas DC-3 and the Boeing 247D, given in table II, indicates that the zero-lift drag coefficient of the DC-3 is about 17 percent higher than that of the Boeing aircraft; the larger zero-lift drag coefficient of the DC-3 results from the larger ratio of wetted area to wing area caused by the larger fuselage of the DC-3, which was designed to accommodate three-abreast seating as compared with two abreast for the Boeing aircraft. The value of the maximum lift-drag ratio for the Douglas DC-3, however, is 14.7 as compared with 13.5 for the Boeing machine; the higher aspect ratio of the DC-3 is responsible for the larger value of maximum lift-drag ratio. The wing loading of 25 pounds per square foot for the DC-3, as compared with the 16 pounds per square foot for the Boeing 247, reflects the use of split trailing-edge flaps on the DC-3 aircraft.