Manufacturer’s Dilemma: Turboprop vs. Turbojet
Despite the success of Boeing’s prototype jet transport, not everyone in the airline industry was convinced that turbojets were the ultimate answer to airliner development…. Jet engines explode! Jet engines use too much fuel, and they might even catch surrounding airport structures on fire. They are unreliable and uneconomical, and will cause a fortune to be spent on lengthening runways and expanding terminal facilities. The answer is no, we’re not going to fly airliners powered by jet engines!
So went the thinking in the U. S. airline industry immediately after World War II. And why not? The Jumo and BMW axial-flow turbine engines that came out of Germany after the occupation had been years ahead of their time as far as known metallurgy and materials were concerned. These engines lasted, if the fighter squadron was even lucky enough, perhaps 100 flying hours at best before they totally disintegrated.
In England and via license in the United States, centrifugal-flow jet engines seemed to be the answer to those reservations, with the possible exception of catching on fire. Their ruggedness and simplicity made them more viable powerplants, and the military was anxious, if not simultaneously full of trepidation, at the thought of putting them into routine operational use.
As mentioned previously, the notion of using new turbojet engines for commercial airline applications were pipe dreams more than reality prior to 1950, although the wonderful Avro Jetliner would probably have changed all that (see Chapter 1 sidebar “Avro Jetliner: The Other First Jet,” page 17). The de Havilland Comet 1 almost did by 1952. And in 1954, at the introduction of the Dash 80 from Boeing, the tide was finally starting a slow turn in favor of pure jet airliners.
By the late-1940s, however, it was becoming apparent to airplane industry observers that postwar England had warmly embraced jet power, in all its forms. Eventually, from 1948 to 1958 no fewer than 10 different airliner designs came off the drawing boards either incorporating or anticipating the use of turboprop engines. Existing airplanes were also modified with turbine-propeller engines either experimentally or as operational upgrades. Why did they utilize turboprop technology to this extent? England was the leader in pure-jet technology, so why were they dithering with propellers (or airscrews, as they were called in that country)?
Well, the answers to these questions are many. Foremost was that jet engines at the time were just too underpowered to carry enough weight, and hence provide enough payload and range for an airliner. Being underpowered also meant using long runways. (Most piston propliners of the era required as little as 2,500 feet of runway for takeoff while jets would easily require twice that.) Jet engines also took an appreciable amount of time to come up to full-power RPM from flight idle settings (known as “spool-up” time for the axial-flow designs). Pilots had to really anticipate the need for power far in advance. Unlike the piston engine, there was no instantaneous surge of thrust from the jets when one moved the throttle forward, which is never a safe situation in an airplane.
Enter the turbine-propeller combination. In a turboprop package, the jet engine is linked to a propeller via a reduction gearbox. The jet engine is typically spinning at a constant 100-percent rpm, while throttle controls are actually changing only the pitch of the blades, therefore allowing for the all important instantaneous thrust applications by the pilot. The added thrust of turbine efflux combined with the strong and immediate pull of the propeller makes up for the lack of thrust coming from an early pure-jet engine alone. Additionally, the air being moved over the wings from the propeller also adds an appreciable amount of selfgenerated lift. These obvious advantages were not lost on engineers and airline bosses alike, and it became a natural course of action for the world’s airlines, and hence their suppliers, to seriously consider turboprop powerplants.