READING A MACHINE: THE DC-3--A SOLUTION TO MANY PROBLEMS
The purpose of this exercise is to analyze the unique design of the DC-3 which as the first profit-making passenger aircraft revolutionized commercial aviation. So successful was the DC-3 that it served as the prototype for the design of all commercial propeller-driven aircraft, only to be superseded by the jets. It is still being used on airlines in the United States and abroad.
The DC-3 provides an excellent example of the machine as a solution to a series of problems that were confronting the budding aircraft industry in America during the 1930s. For this reason, the DC-3 should not be presented as the artifact to be studied at the outset of the lesson. It would be more instructive to introduce students to the design features of the DC-3's predecessors: the Fokker and Ford Trimotors and the Boeing 247. Each, though successful in its own way, presented problems to be overcome to make a passenger-carrying aircraft financially successful.
There are at least two ways of studying the DC-3's origins.
First, it may be viewed as a logical solution to a series of design problems. It may also be considered as a case study of the inter-relationship between the aircraft industry, the airline industry and the federal government. In fact, both the engineering and entrepreneurial issues were related.
The DC-3 can be viewed as a solution to the problems presented by its predecessors. The Ford Trimotor will serve as an excellent artifact for this study. It is important that the student be aware of the design features of this aircraft. While a scale model would be preferable, photographs should more than suffice.
First, the students should be presented with the five essential elements in the design of a successful commercial aircraft. It would be important for the student to be apprised of the market for air travel in the early 1930s as well as the nature of the competition. The five essential elements are:
Needless to say, rather than being presented in class, these five elements could be easily derived by students in classroom discussion.
The Ford Trimotor, one of the most popular commercial aircraft of the late 20s and early 30s was never profitable as a commercial venture in carrying passengers. Moreover, it failed to meet the requirements of the potential airline passenger; it was slow, noisy, had a limited range, and vibrated mercilessly.
The following are among the most notable of the Trimotor's design features:
- a corrugated aluminum fuselage and wing
- large, non-retractable landing gear
- 3 uncovered engines--one mounted at the front of the fuselage, the other two suspended from the wings.
Each of these features should not be presented as "givens," but as solutions to prior problems confronted in aircraft design. The corrugated aluminum wing was a solution to the structural failure of wings built of lacquered fabric stretched over wood. This faulty design was found to be the probable cause of a tragic crash of a Fokker Trimotor in 1931 which took the life of Notre Dame coach Knute Rockne and did much to increase the public's fear of flying.
The cumbersome fixed landing gear was eminently suitable for a plane of short range which often landed on rocky fields and pastures. Furthermore, at the Trimotor's slow speeds the fixed gear presented negligible aerodynamic resistance.
The three-engine design was a response to a concern for reliability, safety and lack of engine power. (The issue of two or three engines came to prominence recently when the FAA granted permission to allow the Boeing 767, a two-engine aircraft, to fly the transatlantic route. The issue was whether the plane could safely make it to an airfield if it lost an engine over water mid-route. The deciding factor was the record of reliability earned by the latest jet engines.) Fallible as the Trimotor's engines were, they were a solution to a problem which had only been solved a decade earlier: how to get maximum engine power without making the aircraft too heavy to fly to profitably carry freight or passengers. Charles Lindbergh's plane was one of the first to incorporate a "solution"--the development of an air-cooled airplane engine rather than a much bulkier one cooled by water.
While the corrugated aluminum fuselage was an improvement over the fabric wing, it too was subject to structural failure. In general, it tended to be very strong in one direction, and weak in another. The solution as developed by the German engineer Adolph Rohrbach was to design a wing of stretched aluminum sheeting over a rib-and-spar framework incorporating a "honeycomb" design. This new design in the DC-3 was subjected to sophisticated stress-testing by driving a steamroller back-and-forth over the wing.
Much of the concern in designing the DC-3 involved an effort to increase the range and speed over its predecessors. The demand was for an aircraft to profitably carry passengers from New York to Chicago. This was accomplished not only by designing more efficient engines, but by streamlining the plane. In fact, the original plan for the DC-l, the prototype of the DC-3, called for three engines. It was decided that it was the engines themselves which were responsible for much of the aerodynamic drag on the Trimotor. The solution incorporated into the DC-3 was one that had been devised nearly fifteen years earlier by a man named Northrup in designing the Lockheed Vega--a single-engine plane. For streamlining, Northrup covered the engine with a metal cowling. In addition, the Douglas engineers decided to incorporate the engines into the wing mounted on nacelles rather than suspend them beneath the wing. Further incorporated into the design was the principle of quickly demountable engines; that is, engines could be quickly removed and replaced, which was seen to be much more efficient in keeping an aircraft serviceable than having to service an engine permanently mounted on the wing, thereby delaying the entire plane. (While virtually all modern-day commercial jet aircraft do not incorporate the engine into the wing, the prototype of the modern passenger jet, the Comet, did.)
Finally, it was realized that it would be essential to eliminate the drag produced by the Trimotor's landing gear. This was accomplished in the DC-3 by devising retractable landing gear. In its initial form the gear was manually retracted, and then only partially--the wheels were left partly protruding from the engine nacelles. This turned out to be fortunate when in an early test flight of the DC-2 the pilot neglected to lower the gear prior to landing. The partially protruding gear protected the belly of the plane and only the props suffered irreparable damage.
Many of the main features of the DC-3 can be seen as solutions to problems encountered in the Ford Trimotor. It can be demonstrated that virtually all of its features were responses to some problem. For example, the unique and characteristic swept wing of the DC-3 was an attempt to keep the wing in its original location as the center of gravity moved toward the rear. The trailing flaps first employed on the wing were an effort to slow down the plane from 200 mph to under 65 mph in order to land on existing airstrips.
An alternative or additional way of studying the DC-3 is to examine it as a product of the interaction between the budding aviation and aircraft industries as well as government intervention. Commercial aviation in America pursued a course of development distinct from that in Europe. In post World War I Europe, commercial aviation was seen as an essential means of compensating for the massive destruction of the railroads. In addition, distances between commercial centers were closer than in the United States. The London-Paris route was one of the most traveled in commercial aviation's infancy--a distance easily manageable by post-war aircraft. In America it was the New York to Chicago route which was deemed to be the largest market worth exploiting.
Lindbergh's fame for his historic 1927 transatlantic flight has overshadowed the significant role he played in the development of the DC-3. When Jack Frye organized TWA (then Transcontinental and Western Airlines) he hired Lindbergh as a consultant. As a result, the motto "the Lindbergh line" was emblazoned on the fuselages of Frye's aircraft. The DC-l was designed in response to Frye's demand for an aircraft that could comfortably carry l2 passengers at l50 miles an hour from New York to Chicago and was capable of landing on existing airstrips. In fact, Frye's initial preference was for a trimotor. But it was Lindbergh who set what the Douglas engineers found to be the most stringent requirement:
"This ship should be able to take off with a full load from any airport on the TWA route--on one engine!"
The prototype of the DC-3, the DC-1 was designed with this specification in mind, although the engineers were uncertain about the plane's ability to perform such a feat until a successful flight test.
The DC-3 was a solution to a problem presented to Douglas by C.R. Smith, the president of American Airlines. He wanted Douglas to modify the DC-2 to carry passengers overnight in berths. In order to do this profitably the fuselage would have to be lengthened to accommodate more passengers. This new plane, originally designated the DST (Douglas Sleeper Transport) saw little service as a sleeper when its economic potential as a traditional transport was recognized. Its ability to carry 21 passengers in comfortably configured seating lowered the cost per seat mile to assure the economic success of the DC-3.
In lieu of a visit to the Air and Space Museum in Washington, viewing or handling scale models of aircraft would no doubt prove useful in acquainting students with the structural characteristics of various aircraft. There does appear to be one shortcoming involved in this methodology: the way in which plastic models are divided in half for building by the hobbyist bears little or no relation to the was the aircraft is actually assembled and can be very misleading. The innocent student could justifiably assume that the DC-3 was constructed by connecting fuselage halves and inserting the wings to the main fuselage. In fact, the structural integrity of the DC-3 was assured by constructing the fuselage and wing stubs as one unit. The outer sections of the wing were bolted on later and designed to be easily replaced if damaged. In fact, the fuselage of the aircraft may be viewed as a series of segments connected one in front of or behind the other. Douglas engineers profited from this experience in "stretching" the DC-2 to the DC-3 when in later years they kept modifying the DC-8 jet by "stretching" it. Boeing essentially forfeited this option by designing the 707 with such a low profile that stretching it would have proved prohibitive--the tail scraping the ground on takeoff.
It is the thesis of this abstract that while it is perfectly valid to "read" a machine, it might be exciting for students to take this premise one step further--to "read" machines, define their problems, and be encouraged to "solve" them to produce new machines. Indeed, there appears to be one pitfall inherent in this approach. It would be too easy with the advantage of hindsight to lead students to the DC-3 as the only possible solution to the problems encountered in the Fokker and Ford Trimotors. We are perhaps fortunate in that most of today's students are ignorant of the DC-3. Alternative solutions should be encouraged. Certainly, such an approach would be no less valid than the laboratory course which encourages students to "discover" the solutions to problems already resolved by the scientist or the engineer.
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Allen, Oliver E. The Airline Builders. Alexandria, Va.: Time-Life Books, 1981.
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Smith, Henry Ladd. Airways, The History of Commercial Aviation in the United States. New York: Alfred A. Knopf, 1942.