04 October 2015

Birth of a Breed: The Development of the Douglas DC-1

The Douglas DC-1 at its 1933 handover to TWA
(San Diego Air and Space Museum Archives)
The crash of a TWA (Transcontinental & Western Air) Flight 599 on 31 March 1931 served as a major catalyst in the airline industry for adoption of all-metal aircraft. The Fokker F.10 was on a scheduled flight from Kansas City to Los Angeles when it encountered turbulence on its first leg between Kansas City and Wichita. As the Fokker F.10 airline had wings of wood laminate, accumulated moisture had caused a weakening of the glue used which led to delamination and structural failure. Aboard Flight 599 was the famed Notre Dame head football coach Knute Rockne, who along with five other passengers and two crew, were lost in the crash when the aircraft went down between the rural Kansas towns of Bazaar and Matfield Green. As a result of the crash, the Bureau of Air Commerce required operators of aircraft with wood wing structures to undergo frequent inspections- as this was economically unfeasible for most operators, all-metal designs were procured as soon as possible. TWA nearly shut down for good while its Fokker fleet was grounded for  inspections. Most of the types that were procured were interim types- but in 8 February 1933, a new airliner made its first flight that was a quantum leap over anything that had proceeded it, and that was the Boeing 247 which entered service with United Air Lines just fifty days later. All metal with a retractable undercarriage, the Boeing 247 was sleek and much faster than anything that had flown passenger services up to that point. Even the most current designs were instantly obsolete- for example, the Curtiss Condor biplane airliner had entered service with Eastern Air Lines and American Airlines just five weeks before United put its Boeing 247s into service. 

United's main rival on the transcontinental market was TWA- the Boeing 247 could now make the east to west transcontinental run in only 21 hours 30 minutes including technical stops. By comparison, TWA's Fokkers took 28 hours 43 minutes to fly from New York to Los Angeles. When TWA learned of United's order for the Boeing 247, he contacted Boeing about ordering the aircraft for TWA, but at the time, United and Boeing were part of the same holding company, United Aircraft and Transport Corporation and Boeing was contractually bound to deliver 60 Boeing 247s to United before it could supply airframes to other customers. TWA's vice president for operations, Jack Frye, wasted little time in sending a letter to other aircraft manufacturers soliciting interest in building at least 60 three-engined airliners for the airline. That letter went to Consolidated Aircraft, Curtiss-Wright, Douglas Aircraft, General Aviation Manufacturing, Stout (Ford) Aircraft, and Glenn Martin Aircraft. The letter included general performance specifications that any design had to meet: 

Jack Frye of TWA

1. All metal, trimotor preferred, combination structures and biplane would be considered, but the internal structure had to be metal. 
2. Thee engines of 500-550 hp. 
3. Maximum gross weight of 14,200 lbs. 
4. Weight allowance for radio and mail carriage of 350 lbs. 
5. Weight allowance for full instrumentation, including night flying, fuel to fly 1,080 miles at 150 mph, crew of two, at least 12 passengers in comfort. Payload had to be at least 2,500 lbs with full equipment and fuel for maximum range. 
6. Minimum top speed of 185 mph, cruising speed at least 146 mph. Landing speed not to exceed 65 mph, rate of climb at least 1,200 feet/min, minimum service ceiling of 21,000 feet and a minimum service ceiling with one engine out of 10,000 feet. 

The specifications page also emphasized that any design, fully loaded, "must make satisfactory take-offs under good control at any TWA airport on any combination of two engines." This landmark letter from Jack Frye is considered the "birth certificate" of the DC-1. When Donald Douglas received the letter in Santa Monica, he immediately convened a meeting with his top heads- James "Dutch" Kindelberger (of P-51 Mustang fame) who was his chief engineer, Arthur Raymond who was the deputy to Kindelberger, and Harry Wetzel, the Douglas Santa Monica plant director. With the United States in the midst of the Great Depression, it didn't take long for them to decide this was a tremendous business opportunity for the company. Just ten days after Jack Frye sent his letter of proposal, Douglas dispatched Arthur Raymond and Harry Wetzel with a ten-person team by transcontinental train to New York to meet with TWA. The Douglas team by this point had moved quickly and concluded that even though TWA was leaning towards a three engined design, they would offer a twin engined design that would be equal if not superior to TWA's specifications. The design would be an all-metal monoplane with a retractable undercarriage that would have passenger comfort as a priority. With Kindelberger supervising the design work in California and telephoning design details to Raymond and Wetzel as they were enroute to New York, the design was refined as the team prepared for its presentation to the TWA evaluation team which consisted of Jack Frye, Richard Robbins, president of TWA, and Charles Lindbergh, the airline's technical consultant. Also present were the teams from four other aircraft manufacturers, all of whom tendered three-engined designs. 

Having prior flying experience with other Douglas designs, Frye was favorable to the Douglas proposal, but Lindbergh had his doubts that a twin engined design could meet the airline's stringent specifications. Lindbergh in particular was concerned about single-engined performance at TWA's hot and high airports in the southwestern United States. The Douglas team in consultation with the engineerings staff in California rechecked their calculations repeatedly to be sure that their design could take off with a single engine from any of TWA's airports. With considerable trepidation, Douglas instructed his team meeting with TWA that he would agree to a contract provision guaranteeing this particular specification to satisfy Lindbergh's concerns. On 20 September 1932, TWA signed a contract for the first Douglas DC-1 for $125,000 (about $2 million of today's dollars) with options for 60 more aircraft based on the flight tests and performance of the DC-1. As an insurance policy against a possible failure of the DC-1, General Motors, which owned TWA, had its General Aviation Manufacturing subsidiary work on the GA-38X which was a three-engined larger derivative of the smaller single engined GA-43 airliner. Construction had started on the prototype, but work ceased early on when it became clear the DC-1 would be successful. 

Arthur E. Raymond, chief designer for the Douglas DC-1
(General Aviation News)
With a signed contract in hand, Donald Douglas assigned Arthur Raymond as the DC-1 project manager. Assisting him would be the legendary Jack Northrop, who was at the time managing the Douglas El Segundo division and would be managing the structural design of the DC-1. Other able Douglas engineers were put in charge of various systems and components for the DC-1. In addition and a first for a transport aircraft, extensive wind tunnel testing at Cal Tech's facilities in Pasadena, California, would be an integral part of the design and development process. Cal Tech's aeronautical faculty would head this effort to uncover as many issues as possible before any metal got cut for the prototype- it was wind tunnel testing that found the planned wing design was unstable and further wind tunnel testing at Cal Tech showed that sweeping the wing leading edge back addressed the stability issue. This was how the DC-1 and later DC-2 and DC-3 got their unique wing shape. 

With passenger comfort a priority, it was decided that the benchmark would be the interior noise of a Pullman rail car. The DC-1 cabin had to have the same level of noise or less. As the design effort proceeded, the early specifications for the Boeing 247 were published in Popular Mechanics and Arthur Raymond had copies of the article posted everywhere with the admonition "Do Better than Boeing!" It was vital at every step of the design process that the DC-1 be more comfortable than the Boeing 247 and an important design step was Northrop's wing design that would fit at the bottom of the fuselage without intruding into the cabin as was the case with the Boeing 247. Northrop's wing center section was relatively flat with the engine nacelles at the ends, outboard of which the outer wing panels were attached with the needed dihedral for stability. In a unique feature of the day, the passenger seats could also be reclined. Heating, ventilation, and the aforementioned soundproofing efforts were as considerable as any aircraft system to meet Douglas's desire for the DC-1 to be comfortable. The aisle width was a then-generous 16 inches (Americans in those days were nowhere near as obese as they are now) with a cabin height of 6 feet 4 inches. This would lay down the reputation of Douglas aircraft for years to come to be considered passenger-friendly.

Note the faired struts ahead of the wings on the DC-1
(San Diego Air and Space Museum Archives)
Both the Pratt and Whitney 9-cylinder R-1690 Hornet radial engine and the Wright 9-cylinder R-1820 Cyclone radial engine were evaluated with the Douglas team selecting the Hornet as the DC-1's power plant. Fixed pitch metal propellers were to be used for the prototype. Developments of both engines would figure prominently in American aircraft of the Second World War- the Cyclone would be progressively developed into larger versions that would power the B-17 Flying Fortress, the TBF Avenger, the B-25 Mitchell and in its ultimate development, the B-29 Superfortress and Lockheed Constellation. The Pratt and Whitney Hornet radial engine was a modest success but was developed into a twin row radial as the Twin Wasp which would be used on the DC-3 which in turn was developed into the outstanding Double Wasp and Wasp Major engines. The engine development is of course a future topic here at Tails Through Time! The engines of the DC-1 were one of the first transport aircraft to use the NACA cowling which streamlined radial engines by as much as 60%. When the prototype was rolled out in the summer of 1933, there were also faired struts that connected the forward fuselage with the engine nacelles ahead of the wings- in the prototype these carried sensor cables from the engines for test instrumentation but were later removed. 

On 9 April 1934, Dutch Kingleberger and Arthur Raymond filed for Patent No. 94,427 "Design for an Airplane" which described the layout and configuration of the DC-1 and later DC-2 development. Despite the patent's rather sparse documentation, it was issued by the US Patent Office in the following year. 

On 1 July 1933, the DC-1 would make its first flight as the birth of the Douglas breed of airliners. The story of the flight test program and service history of the DC-1 will be the subject of a subsequent article here, but it should be noted that in 1918, a very young Donald Douglas was working for Glenn L. Martin where he designed bombers for Martin but it was a transport derivative of his bomber designs that fascinated Douglas- the GMT or Glenn Martin Transport was a 15 seat aircraft that had a fully enclosed cockpit. Only one was built and it was destroyed in an accident in March 1920. The GMT was the first Douglas-designed transport and it was where Donald Douglas resolved that his dream was to design and build an aircraft that whose sole purpose was to carry passengers in comfort. That was in 1919-1920, the DC-1 when it made its maiden flight in 1933 was just the start of Donald Douglas's dreams coming true. 

Source: Douglas Propliners: Skyleaders, DC-1 to DC-7 by Rene J. Francillon. Haynes Publishing, 2011, pp 9-11, 46-53. 

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