Tri-Turbo Three

By Henry M. Holden

During World War II, the British went a long way in developing jet power. Their first two pure jet aircraft were the de Havilland “Vampire” and the Gloster  “Meteor.”  While these aircraft did not go head-to-head with the German Luftwaffe, they did manage to put a dent in the tidal wave of V-1 buzz bombs. British pilots are credited with destroying many of these bombs using the fledgling jet-powered aircraft. The British realized that these early engines were useful only in a defensive role as they gobbled fuel in huge quantities.


With the United States winning the upper hand in transport aircraft like the Douglas C-54 and the Lockheed  C-69 Constellation the British reasoned that they could compete with these aircraft if they married the existing airframes with the jet engine and geared the engine to turn a propeller. This would give them a fuel efficient engine that was not as fast as a jet, but highly competitive with the Douglas and the Lockheed propeller driven aircraft. A positive factor in the turboprop consideration was the fact that the engines were lighter than the radials and would allow greater payload. In 1949, two competing British firms each obtained Dakota airframes for the purpose of converting them to turbo power. As a result, the British were years ahead of the United States with the introduction of turboprop technology. When the Armstrong-Siddeley and Rolls Royce turboprop aircraft went into in service in 1951, they became the first revenue-earning turbine-powered aircraft in the world.


The British made the Armstrong-Siddeley “Mamba” and Rolls Royce “Dart” conversions with a minimum of modifications to the airframe. While the “Mamba” engines produced fewer vibrations than a reciprocating engine, the trade-off was in higher fuel consumption. To compensate they carried less payload than originally planned.

N4700C Conroy Turbo
            The turbo-powered Dakotas cruised at 200 miles per hour, at 25,000 feet, used 120 gallons of gas an hour, and had a gross take-off weight of 28,000 pounds. These re-engined Dakotas were tested primarily to give British European Airways (BEA, today British Airways) experience prior to the introduction of the Vickers “Viscounts.”  All the conversions were later refitted with standard engines and sold.
            In conclusion, the conversions were not practical. A letter to L.S. Casey, director of the National Air and Space Museum, from A.J. Hergeworth, assistant general manager at Rolls Royce said, “Due to the lack of cabin pressure on the Dakotas, it was not possible to fly as a passenger carrying aircraft at altitudes where the full advantage of the “Dart” engine could be obtained.”
             This conclusion notwithstanding, people still tried to put new feathers on the old Gooney Bird. In 1969, the Turbo-Three Corporation, a division of Specialized Aircraft, proposed an airliner with 1,535 hp Dart turboprop engines with Dowty Rotol propellers, on a modified DC-3 airframe. The aircraft had a retractable tail wheel, fully enclosed main gear, enlarged tail, and a longer, pressurized cabin to hold 30 passengers. The prototype “Super Turbo Three” (STT) had 40 percent more power than the original DC-3, and retained less than 25 percent of the original transport’s design.It had a cruising speed of 250 miles per hour, and a break-even load factor including indirect operating costs of 33 percent, or ten passengers. Certified for commuter and cargo operations, the basic cargo version cost $525,000. The passenger/cargo version ran about $800,000 or eight times what the original DC-3 cost. It had three PT-6 engines turning five-bladed propellers, and developed 3,600 hp, cruised at 230 miles per hour, and had a range of 3,000 miles. The cost of the remodeling was about $575,000, five times the cost of an original  DC-3.
            The Tri Turbo-Three was to be the final enhancement to the DC-3. This conversion increased performance and payload, and provided greater safety, reliability, and reduced operating costs. The cost savings and increased payload would pay for the conversion over a short period of operation, or so the designers thought.
            The Tri Turbo-Three was to be the answer to the universal requirements for a rugged STOL aircraft, with low operating cost, and a convertible passenger/cargo plane capability. Operators were discovering the skyrocketing costs of power plant maintenance and engine overhauls were serious cost considerations. With the economy of operation built into a turbine engine, the conclusion drawn was there was no other aircraft in the same category comparable with a DC-3 with turbine power.
            Some of the features that made the Tri-Turbo Three attractive were, a greater safety factor when an engine had to be shut down, more horsepower available on take-off (47% more than the standard DC-3), new engines with new factory warranties, approximately 3,000 pounds less aircraft weight and, finally, reduced pilot  fatigue from less noise and vibration levels. The Tri-Turbo Three made its first flight May 13, 1969, and then after a number of hours of testing went over to Europe to the Paris Air Show. The results of the show were disappointing. The aircraft impressed everyone with its performance. It could cruise at 215 mph at a gross weight of 32,000 pounds, for 2,250 miles. Unfortunately, no one bought the machine. What also impressed most people was it was an old airframe using what was now becoming old turboprop technology.
            The Tri Turbo-Three superseded the DC-3, the Super DC-3 and “Dart” conversions for sound and compelling economic reasons. The cost of fuel for commercial operators had tripled during the oil embargo in the early-1970s. To obtain the same horsepower from two “Dart” engines required 40 percent more fuel than three PT-6 engines with the same output. Inflation in England and delays in obtaining parts for the Rolls-Royce “Dart” engines had also sharply increased overhaul costs.
             The three PT-6 engines weighted almost 3,000 pounds less than the “Dart” engines, and allowed for a 10,000 pound payload. It also had a higher cruising speed, and required no major airframe modifications. As in the early days of aviation, single engine performance was a significant measurement. In 1975, there were still several advantages to losing 1/3 power rather than 1/2 power on a two-engine aircraft, which appeared to be a step back to the Ford Tri-Motor philosophy.
            If the Tri Turbo-Three was such an improvement, why wasn’t it flying?  There are several reasons. First, although the Tri-Turbo Three was extensively promoted, the company could not find the customers to place the orders. Also, in December 1979, the FAA turned down the proposed modifications on the grounds the aircraft was unsafe as modified. They said, “Installing a third turboprop engine in the nose raises serious safety questions that require a complete recertification of the aircraft. ”The argument had come full circle, back to the early Douglas-Raymond philosophy of the tri motor being inappropriate and was finally ended. The same problems that drove the DC-1 designers away from a tri motor design were still problems 40 years later.
            An FAA spokesman added that, “. . . extensive redesign of the DC-3 would be necessary to meet current certification requirements for an FAA Type Certificate.” Case closed on another attempt to find a replacement for the DC-3. Turboprops, although a significant advance in power plants, had only a fleeting popularity. The pure jet science was making rapid advances, and the Boeing 707, DC-8,   DC-9, and smaller Boeing jets would soon make turboprops obsolete for all but limited uses.

©Copyright Henry M. Holden 1996, 2013

For the complete story on the Douglas DC-3 see “Legacy of the DC-3″

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