WHEN A MODIFIED MD-80 airliner flew at the 1988 Farnborough air show in Britain, it was supposed to represent the future of air travel. One of its rear-mounted jet engines had been replaced with an unusual form of propulsion. This consisted of two rings of short propeller-like blades mounted in the open air, immediately behind the jet housing and spinning in opposite directions. Some called it a propfan, others an unducted fan or open rotor. Whatever the name, many said it marked the return of the propeller for large passenger aircraft.
It was not to be. Although the propfan promised a big reduction in fuel consumption the engine failed to enter commercial service, mostly because oil prices fell, but also because conventional jet engines became more efficient. But the idea never went away completely. Engineers in America and Europe continued to tinker with propfans. In the 1990s Antonov, a Ukrainian firm, launched a military transport plane called the AN-70 with four forward-facing contra-rotating propfans under its wing, although only two prototypes were made. Now, in a more environmentally conscious world where cutting aviation emissions is of increasing importance, propfans may be staging a comeback.
The idea behind a propfan is to combine the fuel economy of a propeller with the speed of a jet engine. Because it is more efficient to move a large volume of air slowly than a small amount rapidly, the fans at the front of modern jet engines have been made progressively bigger. Removing the housing allows the fan to be bigger still.
A fan rather than a propeller (as on a turboprop aircraft) is used because aerodynamic effects cause propellers to lose efficiency at high speed. This happens when the moving tips of a large propeller blade reach supersonic speeds, setting off shock waves which disrupt the airflow. The fans on jet engines, however, have more numerous blades and they are specially shaped, a bit like scimitars, to avoid creating shock waves. This should allow an aircraft using propfans to match the speed of a conventional jet airliner.
The propfan fitted to the experimental MD-80 was made by CFM International, a joint venture between GE, an American engineering group, and Safran, a French company then called Snecma. The pair are now developing a new version, which they reckon could reduce fuel consumption and CO2 emissions by more than 20% compared with the best jet engines currently in use. And the new propfan will be capable of running on unconventional sources of energy, such as synthetic aviation fuel, hydrogen or a hybrid-electric system. Flight tests are scheduled for 2025.
The new CFM propfan differs from older designs in various ways. Like the engines on the AN-70, it will sit under the wings, with an open fan at the front. Unlike the AN-70’s engines, though, it will use a single rotating open-fan. It does have a second ring of blades immediately behind the front fan, but these do not spin. Instead, they act as vanes that guide the flow of air backwards. The new arrangement is less complicated than the older one, and should be easier to maintain.
The new propfan is also lighter, which itself saves fuel and hence emissions. This is not just because the fan housing has been eliminated. The thrust reverser—which on a conventional jet engine uses a series of deflectors to slow an aircraft once it has touched down on the runway—has also gone. Instead of deflectors, the pitches of both the rotating blades and the static vanes can be adjusted on the new propfan to reverse its thrust.
One problem with propfans, though, is noise. This was particularly high inside the cabin of the modified MD-80. The combination of a single ring of blades, and advances in acoustic design should let the engine meet current and anticipated noise regulations, says CFM.
There has been concern about what would happen if a propfan blade broke off. A conventional jet engine is designed to contain a breakaway blade within the fan housing. With no such housing, there is nothing to stop such a breakaway striking the aircraft’s fuselage or wing, with potentially catastrophic results. That could mean parts of the aircraft near the open blades will need to be reinforced.
The best thing, though, is to stop the blades breaking in the first place. For that, new materials will help. One legacy of the MD-80 propfan was the pioneering use of extremely strong blades made from carbon-fibre composites. This technology has progressed, and the fan blades in jet engines and the propellers of modern turboprops are now routinely made from composites. These blades are proving their durability by safely notching up millions of flight hours, says a spokeswoman for CFM.
Mounting the propfans under the wings of an aircraft is more in keeping with the way modern airliners are built. It also makes maintenance and safety checks easier. However, the consortium says it can producer a “pusher” version if an aircraft-maker wants to design a plane with its engines at the rear, like the old MD-80.
New aircraft designs are in the works, but the two main builders of airliners, Boeing and Airbus, have yet to commit themselves to any big, new projects. CFM hopes the propfan will be a contender for a new generation of short- and medium-haul aircraft, although it could be used on large wide-body aircraft, too. If flight tests go well, in a decade or so travellers might find the aircraft they are boarding is once again powered by propellers.■
This article appeared in the Science & technology section of the print edition under the headline “Back to prop swinging”
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