The video above is the only video evidence I've seen of the existence of three of the United States' first turboprop aircraft. They were all testbed variants of three of the most successful piston-engined cargo aircraft of the time: The Boeing C-97 Stratofregihter, the Lockheed C-121 Constellation, and the Douglas C-124 Globemaster.
Turboprops are fairly commonplace these days, particularly in the civil market. But in the 1950s, they were a newfangled technology, and to the USAF at the time, it must have seemed like they'd never work.
The idea of a turbine-driven propeller engine was first formulated in the 1920s, almost at the same time Frank Whittle devised the turbojet. No one actually got around to building one until the 1940s, and it was not until after World War II that the U.S., Great Britain and the Soviet Union began seriously investigating the idea.
The first American turboprop engine was the General Electric T31 of 1945, which was little more than an experimental design. This was at a time when plenty of designers still felt warm and cozy with propellers powering anything, and the lack of reliability of pure jet power provided them ample reason for feeling that way.
The Trouble with Turboprops
Initially, turboprops were considered a logical transition stage from piston engines to jets in the late 1940s. Pure turbojets were not powerful enough to reach the performance levels that designers intended, and piston engines had reached the absolute limits of their potential. The U.S. Navy in particular spent a lot of time, money and resources investigating turboprops, since they could both provide the power of a pure jet with the take-off and landing performance of tried and trusted piston engines - quite helpful in the days before steam catapults, angled decks and mirror landing sites.
So various combat aircraft proposals used turboprops, as it was theorized that they provided the best of both worlds. Among the combat aircraft that used turboprops were several notable and spectacular failures:
As jet engines improved significantly through the 1950s, the idea of using turboprops as a bridge to power fighter and bomber aircraft was abandoned. But the initial teething troubles with complex engines like Allison's cantankerous XT40 were eventually overcome with the introduction of simpler designs, like the Pratt & Whitney T34 and Allison T56.
Meanwhile, as pure jet power was pushing fighter and bomber types to the performance levels initially promised, it did not quite provide the level of efficiency needed for large transport aircraft. This provided a golden opportunity for turboprop designers.
Multi-engine aircraft also naturally have a margin of safety much greater than single or twin-engine types. An in-flight failure in one troublesome engine would not prove fatal to an aircraft with four of them. So turboprops could lift greater loads than piston engines at a time before high-efficiency turbofan engines.
The USAF's crop of piston-engined transports at the time, led by the Douglas C-124 Globemaster II, were at their performance limits. The Globemaster's four Pratt & Whitney R-4360 radial powerplants maxed out at 3,800 horsepower each. However the T34, from the same stable, could top out at more than 5,000 horsepower (and in the case of the T34-P-9W, at a phenomenal 7,500 hp). So the technology was there to enable massive gains in payload, range, speed and altitude.
But of course, in an era before computer simulation, the only way to test these ideas was to put then engines on a plane and go fly it. So both the Air Force and the Navy put three of their most venerable piston engine-powered transports, installed T34s on them, and used them as all-turboprop testbeds in the early 1950s.
Boeing YC-97J Stratofreighter
Boeing had prodded the Air Force for some time in the early 1950s to develop a turboprop-powered variant of their successful C-97, itself a transport development of the B-29, and powered by the same Wright R-4360 radial engines as the B-50. With the advent of aerial refueling, it became clear that piston-engined tankers, like the successful KC-97, were shown to be inadequate for refueling pure jet-powered combat aircraft, being barely able to reach a maximum speed greater than the stalling speed of the jets.
In 1955, Boeing finally got the go-ahead to convert two KC-97G airframes (52-2693 and 52-2672) to YC-97J standard, powered by four Pratt & Whitney YT34 turboprops (JP Santiago notes that the Air Force considered re-designating these machines as YC-137s, but relented)
From the day of the first flight on April 19th, 1955, the performance differences were immediately obvious. The YT34s made the YC-97J 5,000 pounds lighter and much more powerful - 5,700 hp per engine, compared to the R-4360's 3,800. As a result, top speed jumped from 375 mph to 417 mph, and the time the plane took to reach 20,000 feet altitude was cut from 50 to just 14 minutes. Boeing's official proposal to the Air Force called for the C-97J to carry up to 53,600 pounds of cargo - a dramatic improvement of the standard C-97's 37,500 pounds.
While the YC-97Js showed much promise, they were already obsolete. Almost exactly one year before the first YC-97J flew, the famous 367-80, the prototype of the 707 airliner, was rolled out in Seattle, and flew for the first time on July 15 of that same year.
It is reported that, upon seeing the 'Dash-80' during its flight trials in 1954, Gen. Curtis LeMay, the head of SAC at the time, convinced the government to order its first fleet of jet tankers from Boeing, rather than wait for Lockheed to produce its as-yet unbuilt Constellation II - the winner of a government competition - as the USAF's first all-jet tanker aircraft. The KC-135 was ordered on September 1, 1954, and the Dash-80 conducted its first refueling test with a B-52 on October 5. The first KC-135As went into service in 1956.
The immediate success of the all-jet KC-135 spelled the end for the C-97J before it started. The two examples built soldiered on as transports and occasional engine testbeds until 1964, when AeroSpacelines purchased both of them. One was converted into a Turbo Super Guppy and used to transport parts of rockets cross-country for the Saturn rocket program, and the other was cannibalized for parts to support the first airframe. The NASA Super Guppy (formerly 52-2693) rests on display at the Pima Air and Space Museum in Tucson, Arizona.
Douglas YC-124B Globemaster II
At about the same time, in 1950, the Air Force announced it would proceed with development of a turboprop-powered variant of its larges strategic transport at the time, Douglas' massive C-124 Globemaster II. The new C-124B, it was said, would have double the horsepower, giving it greater speed, range and payload. Like the YC-97J, it would also be powered by four T34 turboprops, and would be available in a tanker/transport configuration.
Both Douglas and the Air Force harbored great enthusiasm for the improved Globemaster, given how the production version, powered by four R-4360 piston engines, had proven highly effective. The prospect of 'Old Shaky' getting an upgrade must have been a welcome prospect to the troops in Korea who ferried back and forth aboard the big transports. However, more than three years would pass before the new plane actually took to the air.
The lone YC-124B produced (serial no. 51-072) made its first flight on February 2, 1954, flying from Douglas' Long Beach plant up to Edwards AFB. In addition to the new engines, the vertical stabilizer was enlarge, and the cabin was pressurized, permitting operation at higher altitudes, and thus higher efficiency from the engines. Speed also increased, from 298 mph to 375 mph, and in fact, the YC-124's cruising speed was higher than the top speed of the production version.
By the time the YC-124B flew for the first time, the C-130 Hercules was already taking shape. The Lockheed plane would fly in August, and with its STOL potential and a rear-loading ramp enabling the kinds of in-flight drop-offs that the front-loading C-124 could not, the Air Force decided to go with the new plane over an improved old one.
The YC-124B would provide much helpful data to both Douglas and the Air Force un until its retirement in October 1956. Much of that data would inform Douglas' follow-on XC-132 program, a swept-wing heavy lifter which was to be powered by four massive Pratt & Whitney T57s, a turboprop development of the successful J57 engine which was supposed to crank out an incredible 15,000 horsepower. A C-124C was tested with a prototype XT57 installed in an unusual arrangement in the aircraft's nose. It was this machine that revealed problems with the big engine, and when the T57 failed to materialize, the XC-132 (ostensibly to be nicknames Globemaster III) went with it, not getting past the mock-up stage.
Had it been built, the C-132 would have been the largest turboprop-powered aircraft of all time, being 50 feet longer than a standard C-124. However, this failure would inform Douglas' concurrent and successful C-133 Cargomaster program.
Lockheed YC-121F/R7V-2 Super Constellation
Of the three four-engined turboprop testbeds, of the 1950s, Lockheed's adaptation of the Constellation probably came the closest to entering production.
It was the Navy who first ordered two R7V-1 transports (Bu. Nos. 131660 and 131661) to be pulled from the assembly line and converted to R7V-2 standards in 1954, powered by four Pratt & Whitney T34 turboprops. These were given the company model number L-1249A, and the first one made its maiden flight on September 1, 1954. They were delivered to the Navy ten days later, and eventually were handed over to the Air Force.
At virtually the same time, the Air Force became intrigued, and ordered another pair of R7V-1s to be pulled from the line and upgraded as their own L-1249As. The first YC-121F flew on April 5, 1955, and the Air Force took delivery of the pair that July. All four machines were tested successfully, with all four spending time alongside the YC-97Js and YC-124Bs with the Service Test Squadron (Turboprop) of the 1700th Air Transport Group of the Military Air Transport Service at Kelly AFB near San Antonio, Texas.
Again, the performance upgrades were noticeable, particularly in the speed department. The YC-121F's cruising speed was 421 mph, almost double that of the production RC-121D aircraft. Top speed in testing peaked at 479 mph, making it the world's fastest transport aircraft in service at the time (The C-135 had not come on line yet).
Despite this, no YC-121Fs were ordered into production. Lockheed did propose a turboprop-powered Constellation in the USAF competition for its first AWACS aircraft, losing out to what would become the Boeing E-3 Sentry. Interestingly, the EC-121 Warning Star development of the Constellation, used in the preceding "Big Eye" early radar warning role in Vietnam, would be powered by the standard model's piston engines.
Lockheed also proposed a civilian version, dubbed L-1249B. It was to be powered by four Pratt & Whitney PT-2F1 turboprops (the civilian version of the T34), and with a top speed of 415 mph, it was supposed to fly London-New York in nine hours. However, the airlines were not convinced that turboprops were reliable enough at the time, and preferred to stick with piston-engined transports until the new jets arrived.
One Turbo Connie would serve its manufacturer in testing the Allison 501 (military designation T56, also used in the C-130) engines that would eventually be used in the new Lockheed Electra airliner (as can be seen in this video). This testbed was briefly given the unofficial portmanteau 'Elation.' But in the end, all four turboprop-powered Connies would be scrapped.
While none of the above types would go into production, in no way should they be considered failures. The data that each of these aircraft provided contributed greatly to the development of the USAF's later successful turboprop transports - the Douglas C-133 Cargomaster, and the ubiquitous Lockheed C-130 Hercules. While Russia enjoyed early successes in the development of large turboprop engines, culminating in the Tupolev Tu-95 'Bear' family, American turboprops would gain a reputation for performance, low noise, and most of all, efficiency. The advent of engines like Pratt & Whitney's T34 and Allison's highly successful T56 helped ease the growing pains from those early failed attempts. Today's American turboprops owe their existence to these old, long-forgotten test machines.