19 June 2010

The Turboprop B-17 Flying Fortress

In the late-1950s, many of the surplus Boeing B-17 Flying Fortresses that were used by the Navy, Air Force, and Coast Guard in the air-sea rescue role found their way into the hands of several civilian fire bomber operations that for the first time along with surplus Consolidated PB4Y-2 Privateers, offered a quantum leap in performance with the ability to carry significant loads of fire retardant. In fact, prior to the arrival of the B-17s and PB4Y-2s, no other civilian fire bomber then in use even remotely approached the fire retardant capacity of the converted four-engined bombers. In 1960, the first of about two dozen B-17s were converted with bomb bay tanks for aerial delivery of fire retardants.

The bomb bays were fitted with tanks that could carry 2,000 lbs of retardant. The tank was then subdivided into four compartments, each compartment having its own quick-opening door to empty that tank on a forest fire target. The converted B-17s were based on the -F and -G variants primarily (the USAF's air-sea rescue SB-17G, for example). Stripped of all non-essential equipment, the B-17 offered a significant increase in power in the typical-high altitude areas that most forest fires were found. Most of the conversions flew on contracts with the US Forest Service.

However, by the late-1960s many of the fire bomber B-17s were retired from service as aircraft like the Douglas DC-6 and ex-military Douglas C-54s were converted for the role. In addition, the Wright R-1820 Cyclone radial engines of the B-17s were becoming increasingly difficult to support with spare parts. One enterprising outfit got around this issue by re-engining their B-17 fire bomber with four Rolls-Royce Dart turboprops that once belonged to Vickers Viscount. As the Dart engines were much lighter than the Wright Cyclone radials, the nacelles had to be extended far forward to maintain the center of gravity with the propeller spinners being nearly in line with the nose of the B-17. Only one B-17, N1304N, was converted in 1970. Some sources indicate that the aircraft had the nickname "Batmobile" and she was so overpowered, that with both outboard engines shut down and feathered, she was still faster than a stock B-17 and this was while carrying a full load of fire retardant. When the pilots made their drop, they had to shut down and feather the outboard engines to keep from overspeeding the airframe.

This unique and one-of-a-kind B-17 Flying Fortress was unfortunately lost in the same year it was converted. While fighting a forest fire near Dubois, Wyoming, the engines lost power due to excessive ingestion of heated air and smoke from the fire and the aircraft failed to pull out of a retardant drop.

Source: Boeing B-17 Flying Fortress (Warbird Tech No. 7) by Frederick A. Johnsen. Specialty Press, 2002, p97-99.

1 comment:

  1. The engine mounts were 48 inches long. I designed the engine mounts. They saw the DC-3 turboprop for Jack Conroy that we did at Aircraft Technical Service at Van Nuys. I designed the 63 inch mounts for the same conversion. All was done for reliability. Each Rolls Royce Dart engine was 50% more horse power and 500 pounds lighter. Great project and sad for the outcome. Stan Burak