Flying High This Past Week: 30 March-5 April

A day late on posting the latest edition of Flying High This Past Week, but no worries, here's what's been getting a lot of hits lately here at TAILS THROUGH TIME:

• The Early History of the Air Line Pilots Association, ALPA: Quite obviously the latest article on TAILS THROUGH TIME is going to be getting the most hits in the past week! The early history of Northwest Airlines is weaved into the early history of ALPA as the founder of the union, Dave Behncke, was Northwest's first pilot and flew its first passengers in 1927. The early history of ALPA gives us a good look at the state of the airline industry in the 1920s which was just on the cusp of making the leap into greater technologies led off by the Boeing 247 and Douglas DC-3. Despite the landmark in aviation history those aircraft were, flying for many professional pilots was still a hazardous profession in the years prior and many airline heads of the day tried to do what they could to stamp out ALPA in its early days. Fortunately a strike at a small airline that ran between St. Louis and Chicago thrust ALPA into the national spotlight and won it friends in high places. 
• Francis Gary Powers: After the Return: Best known as the Lockheed U-2 pilot that was shot down over the Soviet Union in May 1960 that ended US overflights of the USSR, his return to the United States was less than hospitable as the Director of Central Intelligence sought to blame Powers for any number of error that resulted in his shoot down despite being cleared by a CIA damage assessment team, the USAF, and a formal board of inquiry. Recognition of Powers' integrity and bravery were finally acknowledged posthumously in 2000 on the 40th anniversary of his shoot down. Note the comment at the bottom of my article by Powers' son, Francis Gary Powers Jr, who is the founder of the Cold War Museum! 
• The First Steps to a Turboprop Transport, Part Two: The Boeing YC-97J was a Stratofreighter that was modified with Pratt & Whitney T34 turboprops so the USAF could gain operating experience with the new class of engines before the Lockheed C-130 Hercules and Douglas C-133 Cargomaster become operational. Two KC-97Gs were converted to use the same engines and propellers as what would be used on the upcoming C-133. 
• Soviet Wild Weasels, Part One: Doctrine/Tactics: This was the first part of a three article series I did back in 2010 on the differences between American and Soviet SEAD (suppression of enemy air defenses) doctrines. The second part looked at the aircraft that functioned as the Soviet equivalent of the Wild Weasels and the third part looked at the missiles used by those aircraft. 
• Frontier Airlines and the Boeing 737-200: In the 1970s, Denver-based Frontier Airlines (the first incarnation, not the current one flying) became one of the most significant operators of the Boeing 737-200. Originally investing in the Boeing 727-100/200, the switch to the 737-200 and its better operating economics for Frontier's route system undoubtedly helped the airline weather the economic roller coaster that buffeted the US economy in the 1970s.

The next article will be posted tomorrow night and it will cover the development of the Boeing flying boom used in air refueling. Remember, every five days a new article is posted here at TAILS THROUGH TIME and you'll never be quite sure until then where in aviation history we'll be flying!

The PT1: Pratt & Whitney's First Turbine Engine

Animation showing the operation of a free piston turbine engine

Most airborne turbine development during the Second World War wasn't focused at first on jet engines but on superchargers that were driven mechanically by the engine to compress the thinner air of higher operating altitudes so that aircraft engines operated as if they were in the richer air of lower altitudes. Turbochargers operated on the same principle and tended to be more complex do the exhaust ducting used to drive the turbines that compressed the air for the engines. During the war, General Electric was one of the top firms in supercharger and turbocharger development. However, as early as the late 1930s, Pratt & Whitney had been sponsoring small research programs at the Massachusetts Institute of Technology (MIT) to forward its own efforts in supercharger development. In early 1941, one of the MIT engineers, Andrew Kalitinsky, along with his Pratt & Whitney liason, John Marchant, began discussing a new sort of propulsion system based on a free piston engine- basically two opposing pistons would compress air in a combustion chamber between them with the exhaust being ducted to drive a turbine which in turn drove a propeller. Turbochargers already were in use that collected exhaust gases and drove the turbine for the supercharger- aircraft like the P-47 Thunderbolt used this method. The difference in a free piston engine is that the pistons aren't connected to a crankshaft, hence being "free" but are used to compress air for combustion to drive a turbine which is what provides the rotational power to a propeller. 

Free piston engines were already in use as air compressors. On German U-boats, for example, a four pairs of free pistons in series were used to generate compressed air for launching torpedoes. Kalitinsky and Marchant's idea for an aircraft free piston turbine engine went up their respective chains of command at both MIT and Pratt & Whitney with all involved interested in the concept. On 6 September 1941 a formal report based on MIT's studies was submitted to Pratt & Whitney titled "Free Piston Gas Turbine Power Plant for Aircraft". The proposed engine had eight pairs of free pistons (eight stages) as a gas generator that drove a turbine that in turn drove a propeller through reduction gear. The exhaust gases after spinning the turbine were discharged through a variable area nozzle for additional propulsive thrust. In addition, the duct work incorporated a burner for extra power like an internal afterburner. The report suggested that such an engine could drive a fighter aircraft to Mach 0.75 at 40,000 feet or power a four-engine bomber at Mach 0.6 at 40,000 feet. 

The proposed fighter engine at low altitudes had nearly all the propulsive power coming from the propeller, but the proportion from the turbine exhaust increased as altitude increased. At the operating altitude of 40,000 feet. 2/3 of the propulsion would come from the prop and 1/3 of the propulsion could come from the turbine exhaust. In the bomber engine at operating altitudes 3/4 of the propulsion came from the prop and 1/4 of the propulsion came from the turbine exhaust. For comparison, consider the widely-used PT6 turboprop engine- about 85% of its propulsion comes from the prop and 15% comes from the exhaust gases. The specific fuel consumption of the proposed free piston turbine was 0.36 lbs/hr/HP for the bomber engine and 0.41 lbs/hr/HP for the fighter engine. This represented about a 30% reduction in fuel consumption over the piston engines of the day. Again, for comparison to a modern turboprop, the PT6 engine has an SFC that ranges between 0.64 to 0.59 depending upon the variant. 

The report summarized the potential advantages of the free piston turbine over the piston engines of the day: 

• 1. Improved fuel economy.
• 2. Reduced weight.
• 3. Reduced cooling requirements. 
• 4. Flexibility in installation due to the smaller size. 
• 5. Since the turbine exhaust contributed to propulsion, the propeller could be smaller.
• 6. Less fatigue stress since the engine torque would be minimal.
• 7. Use of alternate fuels than avgas. 

The PT1 test article at the Pratt & Whitney Museum

A new engine designation system would be needed since the free piston turbine was a departure from Pratt & Whitney's established business. "P" would stand for propeller and "T" would be for turbine. The free piston turbine was launched as a company-funded program on 27 October 1941 as the PT1. The PT1's pistons were the same as that used on the R-1830 radial engine. The initial test engine was built out of cast iron since it wasn't going to be a flight-worthy engine. The main challenge for the small PT1 team was getting the two pistons to oscillate symmetrically at high frequency. First run was in August 1942, but again, the technical challenge was getting just two opposing pistons to synchronize. Imagine getting eight for the proposed engine! By March 1943 the PT1 test article was running as intended. In parallel to the free piston work were two other efforts- work on the turbine and work on the burner. Two types of burners were tested, one before the turbine that could boost power to the prop and a second one that was in the turbine exhaust as a rudimentary afterburner. 

Cross section model of the PT1, showing the opposing cylinders

It's important to realize at this time, the PT1 was a low priority program despite the potential advantages. The US military didn't want resources to be diverted from established radial engine programs that were crucial to the war effort. In 1943 there were only 74 personnel assigned to the PT1 program but the engine had run for 400 hours and 280 hours were run with the turbine. Considering the modest budget, small staff and that the PT1 pushed Pratt & Whitney's metallurgical techniques and limited turbine experience, that run time is quite an accomplishment for the technology of the day. Based on the testing so far, the PT1 team concluded the bomber application was most promising and should be the focus of the team's efforts. A comparison was done with a B-29 compared with a B-29 powered by the PT1. For the same bomb load, the range was better with PT1 engines going from 5200 miles to 8900 miles. On a long distance 2900-mile mission, the bomb load was also heavier, going from 7600 lbs to 25,600 lbs! In March 1945 Pratt & Whitney proposed developing the PT1 further to a 4500 hp demonstrator engine with the military designation T32. By this point, though, jet engines were a better-known quantity and offered even more power for relative simplicity compared to a production PT1 engine. 

At war's end the PT1 program was canceled but it wasn't for nothing. Many of the engineers who worked on the turbine section of the PT1 would go on to have great influence in the design of Pratt & Whitney's jet engines like the J57/JT3 turbojet and much of what was learned in the PT1 went to the the company's first true turboprop, the PT2 program which was started at the end of the war. The PT2 became the T34 turboprop engine that powered the Douglas C-133 Cargomaster and was flown prior to the Cargomaster on a Boeing C-97 Stratofreighter, a Lockheed Constellation, and a Douglas C-124 Globemaster. The PT2/T34 was Pratt & Whitney's first axial flow gas turbine engine that set the stage for later engine designs. 

Source: The Engines of Pratt & Whitney: A Technical History by Jack Connors. American Institute of Aeronautics and Astronautics, Lockheed Martin Library of Flight, 2010, pp161-172. Photos: Wikipedia, www.enginehistory.org (Kimble D. McCutcheon)

The First Steps to a Turboprop Transport, Part Two

A week and a half ago I had blogged about how the USAF was getting turboprop transport experience by setting up a test squadron at Kelly AFB to operated transport aircraft that had been converted to turbine power: 

52-2693 and 52-2672 in flight together.

On 15 June 1954, the headquarters of the Military Air Transport Service (MATS) activated the 1700th Test Squadron (Turboprop) at Kelly AFB, Texas, with the task of developing maintenance procedures and techniques for the employment of turboprop transport aircraft pending the arrival of the C-130 and C-133 into the USAF service. The squadron had three flights with each flight dedicated to a single type for the testing of standard transport aircraft that had been converted to turboprop power. The first of the three flights to be activated would operate the Convair YC-131C. Two aircraft were converted from standard C-131 Samaritan transports (the USAF version of the CV-340 airliner) to use early test versions of the venerable Allison T56 turboprop.

Back in January 2010 I had written a short posting about the second of the demonstrator aircraft that were operated by the 1700th Test Squadron and operated in the second flight of the unit- the Boeing YC-97J, a Pratt & Whitney T34-powered version of the C-97 Stratofreighter. I had recently picked up Cal Taylor's voluminous tome on the Douglas C-133 Cargomaster and he devotes considerable space to the YC-97J and its operational use by the 1700th TS. The YC-97J made its first flight at Edwards AFB on 19 April 1955 and given that it used the same T34 engines as the upcoming C-133, the USAF was keenly interested in flight testing the engine in an operational environment with the YC-97J. From my previous posting about the YC-97J: 

Boeing converted two aircraft (52-2693 and 52-2672, both KC-97Gs) to turboprop power. Pratt & Whitney YT34 turoprop engines (which would later be used on the Douglas C-133 Cargomaster) delivering 5,700 horsepower were substituted for the four R-4360 radial engines. For a brief time the USAF considered redesignating these two Stratofreighters as C-137, but ended up assigning them the designation YC-97J (ironically the C-137 got used for the Boeing 707s used by the military, itself a development of the Model 367-80 prototype).

The conversion to turboprop power shaved nearly 5,0000 lbs off the aircraft's weight as the YT34s were much lighter but more powerful. The first flight was made on 19 April 1955 and the YC-97J demonstrated significant improvements in overall performance. The top speed was 417 mph compared to 375 mph for a regular Stratofreighter and the YC-97J took only 14 minutes to reach 20,000 feet whereas the regular Stratofreighter took 50 minutes!

Inflight study of the YC-97J during its Edwards flight test program.

In addition to using the same T34 engines as the C-133, the YC-97Js also used an early version of the same Curtiss turboelectric three-bladed propellers planned for the C-133. The first YC-97J completed its flight testing at Edwards and was delivered to Kelly AFB on 14 September 1955, nine months after the YC-131Cs had arrived. The second YC-97J arrived at the end of the month. After a short series of flights operating within the continental United States, the USAF authorized the aircraft to begin overwater missions with the first overwater flight being to Kindley Field in Bermuda- the aircraft covered the 1,700 mile route from San Antonio to Bermuda in 4 hours 42 minutes, the fastest time at that point by a prop-driven aircraft. On 26 January 1956, the YC-97J departed for Rhein-Main AB in West Germany staging through Dover AFB in Delaware, then Newfoundland and Scotland. Despite record breaking cold weather on the trip, the YC-97J performed flawlessly without any of the usual maintenance headaches that were commonplace for the piston-driven C-97s. On the leg between Newfoundland and Scotland, four hours were shaved off the usual flight time when using C-124s or C-118s, the run being made in only 6 hours 30 minutes. It was clear that the time savings was tremendous on long distance missions. The international aviation press covered the flight with interest. On an outbound stop in London, the YC-97J was climbing out of Heathrow at 2,500 feet per minute and London ATC asked the pilots to slow the rate of climb as the radar dish was too slow to keep up! The return flight from Frankfurt stopped in Paris, London, the Scotland (Prestwick), Newfoundland (Goose Bay) then Selfridge AFB in Michigan before returning to Kelly AFB. It was the first round-trip trans-Atlantic crossing by an American turboprop aircraft. During the mission to West Germany and back, no engine or prop maintenance was needed and the aircraft's four engines used a mere four quarts of oil for the entire trip. Needless to say, the USAF was very enthusiastic about the aircraft!

In March 1956 the two YC-97Js were put on a scheduled cargo run between Kelly AFB to Ramey AFB in Puerto Rico via Charleston AFB in South Carolina and the return routing stopped over at Brookley AFB in Alabama (now Mobile Downtown Airport). Average flying time between San Antonio and Puerto Rico was 16 hours and despite the stopovers, it was still nine hours faster than what piston-driven USAF transports took to cover the distance. But it didn't stop there- that same month the first YC-97J made the first trans-Pacific crossing by a turboprop aircraft, averaging 360 mph over the 18,000 mile round trip. The longest leg of the route to Tokyo was between Midway Island and Yokota AB outside of Tokyo- on this leg the YC-97J flew at 30,000 feet and averaged 400 mph. 

In preparation for the arrival of the Douglas C-133 Cargomaster, the first group of air crew and mechanics arrived at Kelly AFB from Dover AFB for familiarization training with the T34 engine and its Curtiss propellers. The three-week course had pilots flying an average of 38 hours on the YC-97Js to build turbine experience while the Dover mechanics worked side by side with the Kelly AFB maintenance team to keep the YC-97Js flying. The reliability of the turboprop over the piston engine was now unquestionable and in the summer of 1956, both YC-97Js would fly a total of 46 hours 35 minutes together in a single calendar day as proof of the reliability of the turboprop. The engine overhaul time (TBO) over the course of the test program with the 1700th started out at 150 hours and ended up at 1,000 hours. 

The YC-97J departs San Diego Lindbergh Field.

In addition to its scheduled cargo flights, the YC-97Js were also flown on demonstration flights for interested groups ranging form the US Navy to other defense contractors like Pratt & Whitney and North American Aviation. On a three day demonstration in Connecticut for Pratt & Whitney, the YC-97J made 78 engine starts, 19 takeoffs and landings, 7 air starts and 15 flights without any malfunctions of the engine or propellers. By October, one of the T34 engines became the first American turboprop engine to reach 1,000 flight hours since its last overhaul. It was removed from the YC-97J with 1,001 hours and 20 minutes flight time and in that time, it only needed 44 hours of unscheduled maintenance and used a miserly 392 quarts of oil in that time frame, a fraction of what the regular C-97's piston engines would have used in 1,000 flight hours. The propellers also proved to be extremely reliable and when the first C-133 Cargomasters were delivered to Dover AFB, the engines and propellers were already rated at 1,000 hours TBO, a significant feat in that day. 

The 1700th TS's flight test program with the YC-97Js concluded on 15 November 1956, six weeks ahead of schedule. However, the aircraft were kept operational until 17 January 1957 as they were used in Operation Safe Haven to fly refugees from the 1956 Hungarian Revolution from Europe to new homes in the United States. The first YC-97J, would go on to create more aviation history, though- it was modified to become a Super Guppy transport. Aero Spacelines president Jack Conroy had already flown a piston driven Super Guppy, and aware of the pending retirement of the YC-97Js, acquired one as the turboprop engines made his conversion not only faster, but more efficient. The new turbine Super Guppy used a swing nose instead of a tail break as was the case with the original design and it was put into service with NASA in 1966, its first job transporting the second stage of the Saturn IB rocket from Huntsville, Alabama, where it was built to the Kennedy Space Center in Florida. It was subsequently retired to the Pima Air and Space Museum in Tucson, Arizona. 

Stay tuned for the final installment in this series which will look at the turboprop-powered YC-121F Super Constellation!

Source: Remembering an Unsung Giant: The Douglas C-133 Cargomaster and Its People by Cal Taylor. Firstfleet Publishers, 2005, p29-43. Photos: Smithsonian Institution, SDASM.

During the late 1940s and early 1950s many aircraft manufacturers were conducting studies on the feasibility of converting existing piston-powered transports to turboprop power. The use of turboprops was seen as a low-risk advance that combined proven airframes with higher performance engines without sacrificing fuel economy, one of the weaknesses of jet engines of the day. With the Boeing C-97/KC-97 Stratofreighter in service with the USAF at the time, Boeing had pitched to the USAF several times a turboprop-powered Stratofreighter. All were under the same Model 367 number and at one point in 1953 Boeing went as far to built a partial mockup of the proposed Model 367-41.

The USAF, however, showed little interest in Boeing's proposals but in 1955, decided to investigate further the concept of a turboprop-powered C-97/KC-97 by commissioning Boeing to convert two aircraft (52-2693 and 52-2672, both KC-97Gs) to turboprop power. Pratt & Whitney YT34 turoprop engines (which would later be used on the Douglas C-133 Cargomaster) delivering 5,700 horsepower were substituted for the four R-4360 radial engines. For a brief time the USAF considered redesignating these two Stratofreighters as C-137, but ended up assigning them the designation YC-97J (ironically the C-137 got used for the Boeing 707s used by the military, itself a development of the Model 367-80 prototype).

The conversion to turboprop power shaved nearly 5,0000 lbs off the aircraft's weight as the YT34s were much lighter but more powerful. The first flight was made on 19 April 1955 and the YC-97J demonstrated significant improvements in overall performance. The top speed was 417 mph compared to 375 mph for a regular Stratofreighter and the YC-97J took only 14 minutes to reach 20,000 feet whereas the regular Stratofreighter took 50 minutes!

Both aircraft were flown in regular transport duties as well as trials work by the USAF until 1964, but by the time both aircraft had flown, Boeing and the USAF were shifting their efforts to developing the KC-135 Stratotanker and its even greater potential than the YC-97Js.

The first YC-97J, 52-2693, upon retirement in 1964 was used to provide parts and sections for the prototype Aero Spacelines B-377SG Super Guppy.

Source: International Air Power Review, Volume 20. AIRtime Publishing, 2006. "Warplane Classic: Boeing C/KC-97 Stratofreighter" by Bill Yenne, p128-129.