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Modell-Triebwerk mit gegenläufigen Propellern - © DLR Deutsches Zentrum für Luft- und Raumfahrt

Researchers in Hamburg working on quiet airlines

The Hamburg-based COCLEA project is devising solutions for deflecting propeller noise, particularly low-frequency noise

Aircraft engineers from the Hamburg University of Applied Sciences (HAW), Airbus and other research institutions are collaborating in the COCLEA (Comfortable Cabin for Low Emission Aircraft) project to make aero engines with counter-rotating propellers viable for civil aviation.

Quieter, more cost efficient, more comfortable – these are a few of the drivers behind the development of future airliners. To achieve those demands, aircraft designers are using not only new and lighter materials such as carbon-fibre compounds or ceramics. They are also investigating propulsion systems that have rarely been used so far and are examining their impact on aircraft passengers.

One of these systems is an engine driving counter-rotating propellers. This system has been used mainly in military aviation so far with a few exceptions. A turboprop engine powers two propellers installed behind each other and that rotate in opposite directions. This arrangement is more powerful and more economic than a conventional system, where the engine drives a single propeller. Fuel consumption is 30 per cent lower than that of a jet engine. Thus, in an age of ambitious goals for climate protection, counter-rotating propellers would be an almost ideal solution. Thus, the EU is funding their development as part of Counter-Rotative Open Rotor. Yet, the high noise levels have made civilian aviation reluctant to use them.

COCLEA research project

But fuel prices and climate protection policies have prompted a rethink among decision-makers. Today, aviation designers are looking at equipping future short and medium-haul airliners with counter-rotating propellers rather than jet engines. The Hamburg-based COCLEA project is devising solutions for deflecting propeller noise, particularly low-frequency noise. Launched in January 2012, COCLEA will end in September 2016.

Soviet airliner Tupolev Tu-114 as forerunner

Unlike jet engines, propeller noise is particularly loud in the frequencies between 100 and 200 Hertz. These sounds travel into the passengers’ cabin with relative ease. As a result, counter-rotating propellers have rarely been used in civil aviation the only exception being the Soviet airliner Tupolev Tu-114 flown by Aeroflot between 1961 and 1977. The aircraft flew almost as fast as contemporary jets at a higher range and with lower fuel consumption. Although known for its reliability, passengers and crew had to endure a fairly high level of noise and vibrations.

The Tu-114 was developed in the late 1950s from the Tupolev Tu-95 long-range bomber. Tupolev designers took the Tu-95’s wings, engines, stabilizers and landing gear and pared them with a newly designed fuselage and a pressurized cabin for 170 to 220 passengers. The Tu-114 was an economically viable solution for Soviet planners at the time and was replaced by more capable jet airliners in the 1970s.

In the 21st century, lightweight construction and composite materials are changing the equation for sound proofing in the cabin. Contemporary airliners have a lighter structure, but which is prone to vibrations and transmitting noise. Older aircraft made of aluminium are heavier and thus less prone to vibrations. Today’s focus on lighter materials, however, is producing aircraft that tend to vibrate more and produce noise in the cabin.

However, one completed new concept foresees a lightweight sound shield on the exterior of the fuselage. The engines are mounted in pods on the left and right of the aircraft’s rear and form a sound shield that deflect most of the propeller noise. Another concept will use new design approaches to construct the connections between individual fuselage sections reducing the amount of noise transmitted through the entire fuselage.

Making flying economically and ecologically viable

An increase in acoustic comfort for passengers is only possible through a combination of various concepts. Hardly any passenger would fly with an airliner that is cheaper, but noisier. “If they are to generate demand on a global scale, new aircraft must have many attractive features, especially a comfortably low level of cabin noise. New propulsion technologies for economically and ecologically viable will only gain a footing on the market, if they offer an acceptable acoustic environment in the cabin”, says Professor Dr. Ing. Wolfgang Gleine, who is heading COCLEA. Thus, counter-rotating propellers will only become a reality, if aviation engineers can solve the system’s noise problem.

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