Y1B-17A (B-17A)

The Strains of High Altitude Bombing

At the high altitudes required for effective strategic bombing, the B-17's engines lost considerable power due to the reduced air density. The solution was turbochargers, which used the exhaust flow to drive a compressor that forced more dense air into the engine cylinders, thus increasing engine power at high altitudes.

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The Y1B-17A received an exhaust-driven turbosupercharger installation connected to its Wright Cyclone radials. This allowed Boeing and Army Air Corps engineers to evaluate turbocharging's impacts on the big bomber's performance at heights where the engine power dropped off.

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Integrating this supercharging ability was vital to unleashing the B-17's full high-altitude potential. The Y1B-17A played a pivotal part in fine-tuning and implementing the system that would enable Flying Fortresses to cruise at higher operational ceilings with better speed and range.

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Turbosupercharging

In May 1937, the U.S. Army awarded Boeing a contract to proceed with construction of an aircraft designated the Y1B-17A and allocated the company Model 299F.

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The core innovation on the Y1B-17A was an exhaust-driven turbosupercharger system to enable high-altitude operations. At the reduced air densities found at stratospheric heights, a typical internal combustion engine cannot develop full rated power without forced induction. A turbosupercharger utilizes the engine's exhaust flow to drive a turbine that compresses and pumps denser air directly into the engine cylinders.

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For the Y1B-17A, the turbocharger units supplied by General Electric were initially installed in shrouded housings above the wing, at the rear of each engine nacelle near the exhaust outlets. With no way to better integrate the bulky hardware into the wing roots, these above-nacelle turbo installations disrupted the aircraft's clean aerodynamic lines.

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But the exposed positioning proved more problematic than just visual - it caused severe airflow turbulence and vibrations whenever the turbos were engaged in flight. The disruptive air turbulence transmitted buffeting forces through the airframe, making this initial turbocharger layout unworkable for production bombers.

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Turbocharger Troubleshooting

Despite efforts by Boeing engineers to correct this turbulence problem, a solution proved elusive when mounted in the top nacelle positions. The company eventually decided the only effective fix was to completely reposition and reinstall the entire turbocharger system underneath the engine nacelles.

 

This major retrofit amounted to stripping out and rebuilding the forced induction configuration from scratch in a new layout. This proved to be an expensive and labor-intensive undertaking for Boeing. Repositioning and redesigning the turbo setup required extensive rework estimated at nearly $100,000 in costs. However, the U.S. Army declined to fund these modifications, leaving Boeing to shoulder the financial burden.

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Undeterred, Boeing's engineers persisted in perfecting a viable turbocharger configuration vital to the B-17's performance objectives. But getting the complex forced induction system working seamlessly required countless man-hours of troubleshooting, testing, and refinements. It wasn't until early spring of 1939 that the major technical hurdles had finally been overcome. The repositioned under-nacelle design finally resolved the airflow and vibration issues that had plagued the original installation.

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The Y1B-17A's turbocharger developmental journey highlighted the iterative challenges in adapting cutting-edge technology to a large, advanced aircraft design like the B-17. But this persistence paid off with a refined, production-viable forced induction system for the bomber. Indeed this location for the turbosuperchargers would remain unchanged for all later models of B-17.

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Performance Increase

After the arduous process of repositioning the turbocharger system beneath the nacelles, the forced induction setup finally proved its value on the Y1B-17A testbed. When combined with the upgraded Wright R-1820-51 engines, the turbochargers enabled dramatic performance improvements for the big bomber.

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With the turbochargers compensating for the thinner air at altitude, the Y1B-17A's service ceiling soared up to an impressive 38,000 feet. This was the high-altitude operating realm required for effective strategic bombing missions while avoiding most anti-aircraft defenses of the era.

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But the turbochargers didn't just allow the B-17 to fly higher - they also provided a boost in maximum speeds at intermediate altitudes. At 25,000 feet, the turbo-charged Y1B-17A could now reach 271 mph. This was a significant increase over the 239 mph top speed of the earlier non-turbocharged models at the much lower altitude of 5,000 feet.

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The Y1B-17A in flight

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The true capabilities of the turbocharged Y1B-17A were put on full display during an ambitious transcontinental flight in July 1939. Major Stanley Umstead, Chief of the Materiel Division Flying Branch, piloted the aircraft on a non-stop journey from Los Angeles to New York.

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At a cruising altitude of 26,000 feet, made possible by the aircraft's turbosuperchargers, Umstead was able to average an impressive 265 mph over the 2,400-mile route. The flight lasted just 9 hours, 14 minutes and 30 seconds from takeoff to landing - a stunningly fast transcontinental time.

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This record-setting mission dramatically illustrated the advantages the turbocharging system provided to the B-17's performance envelope. By allowing the big bombers to cruise at higher altitudes in thinner air, they could leverage the higher true airspeeds available at those heights while having a longer practical range.