The Airplane - Jay Spenser [104]
Putting its supercharger expertise to a new use in the meantime, General Electric improved on the Whittle-type engines it produced under license during World War II. It leveraged that success to join Pratt & Whitney and the world’s other traditional aero engine manufacturers in postwar jet engine production. Today, GE is a world leader in this field.
Two factors dictated a transition to turbine propulsion. The first is that jet engines are fundamentally more reliable than piston engines. Instead of back-and-forth stresses, turbine power plants feature smooth, continuous rotation. The second factor was that greater speed was not possible with propellers. It was a matter of fundamental physics. World War II fighters had flown so fast that, although they remained subsonic, the tips of their propellers encountered transonic or even supersonic flow. When this happened, shock waves formed. The airflow separated from the blades and the resultant turbulence turned high thrust into high drag.
Postwar propeller airliners were somewhat slower than World War II fighters, but they too ran into trouble. The reason was that their designers were after payload and range as well as speed. This dictated high takeoff weights, which in turn called for powerful engines and propellers capable of accepting that raw power.
One way to get a propeller to handle more power was to increase its diameter. However, that drove up the tip speeds, triggering efficiency losses at lower cruising speeds. Going to larger-diameter props also complicated airframe design by requiring greater engine spacing on the wing and longer, heavier landing gears for adequate ground clearance.
To avoid those complications, designers could instead give a propeller more blades to handle the added power. This kept the propeller diameter the same, but the blades ended up too close to each other. The result was lost efficiency because, denied sufficient time and forward motion, each blade encountered the affected airflow of the previous one.
In desperation, designers resorted to mounting two propellers, one immediately behind the other, on concentric and counterrotating shafts. Very late-model Spitfires equipped with Rolls-Royce Griffon engines show this practice. It worked aerodynamically, but the excessive complexity, high maintenance, and poor overall reliability of these installations made them more trouble than they were worth.
As the 1950s drew to a close, it was obvious that propellers had carried first-line military and commercial aviation about as far as it could go. Higher speeds and gross weights required jet propulsion.
The Boeing 707 debuted in transatlantic service on October 26, 1958. An instant sensation, it revolutionized air travel and launched a rapid conversion to jets. In its first years of operation, however, the 707 assaulted eardrums, rattled windows, and smeared the sky with dark sooty trails. This was true of the Douglas DC-8 and all other first-generation jetliners as well. The reason was their turbojet engines, which were earsplittingly loud even fitted with noise suppressors. Those “straight turbojets” accelerated a small amount of air to very high velocity, which is inefficient at low speeds (it was like trying to start a car in high gear). Worse still, their hot exhaust produced a flaring blowtorch noise on contact with the surrounding cold air.
In 1960, a better idea took flight: the turbofan. Turbojets and turbofans have the same three internal sections: compressor, combustion chamber, and turbine or “hot section.” To this, however, the turbofan engine—also called a fanjet—adds a fan unit at front that is larger in diameter than the rest of the engine.