DOUGLAS, D., "THE AIRPLANE AS A COMMERCIAL POSSIBILITY," SAE Technical Paper 190064, 1919, doi:10.4271/190064.
The factors included in the commercial airplane problem are the practical use that can be made of airplanes, the volume of business that can be expected, the necessary changes from present military types to make an efficient commercial airplane and what the future holds for this new means of transportation. The requirements for passenger transportation, airmail and general express service, are first discussed in detail, consideration then being given to other possibilities such as aerial photography and map-making, the aerial transportation of mineral ores, sport and miscellaneous usage. Changes in the present equipment of engines and airplanes to make them suitable for commercial use are outlined, and special features of aerial navigation, landing fields and legal questions are mentioned.Assuming that the uses of aircraft will expand rather than contract, future development of airplanes is then considered in detail, including service cost data, conditions of flight, service, time schedules, capital invested and expenses. With larger and faster land airplanes working to the limit of their capacity, it can be shown that passengers can be carried at a good profit at as low as 10 cents per passenger-mile.