Barry Controls Aerospace is demonstrating a new noise-suppression device which is designed for McDonnell Douglas (MDC) DC-9s, MD-80s and other aircraft with tail-mounted engines which are prone to vibration-induced noise.

The Active Tuned Mass Absorber (ATMA) was developed in conjunction with Oregon-based acoustic-vibration specialist Hood Technology, and is expected to receive US Federal Aviation Administration supplemental type certification this month. The system has already been selected by a corporate DC-9 user and by Midwest Express airlines, which plans to equip its fleet of 23 DC-9-31s with it. Flight testing was completed on two DC-9s in August and September 1996, and the system was demonstrated at 1996's National Business Aircraft Association meeting in Florida.

The ATMA system consists of eight absorbers, replacing the original MDC-made tuned vibration absorbers which mount to the engine yoke. The aft cabins of the DC-9/MD-80 series are acknowledged to be prone to high levels of noise induced by out-of-balance. The original N1 absorbers at the front of the engine are able to cut engine tone noise by up to 15dB at the noisiest seats, but are only effective over a narrow 2-3% range of engine RPM. Additionally, the absorbers are "invariably tuned out of specification by up to 10%", says Surrey, UK-based Barry Controls, which adds that the original absorbers are prone to mechanical failure.

"We've basically taken out the Douglas-built system and put in our units, which consist of two N1s and two N2s [at the back mounts], in each engine. We hook them up to the electronics and pick up the frequency of the engines. The information goes through the processor logic and back to the ATMA , and continues to do so in a closed circuit," says adaptive and active systems product line manager, Kevork Kayayan.


Engine sensors

Three sensors mounted on each engine-mounting yoke pick up frequency data and pass them to the processor, which is packaged in a management control unit in the tailcone. The processed response is fed to the four spring-loaded absorbers which are expanded or contracted, depending on the requirement. Sensors mounted on the absorbers compare the frequency response and pass the results back to the processor for comparison with the signals from the yoke-mounted sensors and so-on, in a constant, closed loop.

Test results indicate that both N1 (low-pressure spool) and N2 (high-pressure spool) tones are reduced by up to 25dB at the noisiest seats - normally those in the back four rows. The noise reduction occurs over an rpm range of 65-100% and is most apparent in the DC-9/MD-80 in the back four window and aisle seats along the port side. Ambient noise levels are reduced from around 95dB(A) to 75dB(A) in the most extreme cases. "We get a measured tonal reduction of 90% at the noisiest seat and an overall reduction of around 16.8dB, or a 70% reduction throughout the whole aft cabin," claims Kayayan.

"We think it is a simpler system than any competing system, and we think it is easier to install. It is designed to be fitted in less than one shift, or around 8h, or less. In addition, we believe it is highly reliable and we do not induce any artificial vibration into the yoke which we think is detrimental to the structure," he adds. The system adds around 13.6kg to aircraft weight and consumes about 100W in electrical power.

Andy von Flotow, president of Hood Technology, says that the production system, "...will be basically worth about 15 extra seats to the airlines per flight. At the moment, many of them do not want to use these seats. If that works out it will pay for itself within a few weeks".

Hood Technology, with support from Barry Controls, is also expected to demonstrate the vibration-absorption system to the US Navy at Patuxent River, Maryland, where it will be fitted to a Northrop Grumman E-2C. The USN is experiencing vibration-related avionics failures in its 120-plus fleet of E-2Cs and Grumman C-2s, and plans to evaluate the ATMA in 1997.

Barry Controls, meanwhile, plans to have the system installed on the entire Midwest Express fleet by 1998 and is in negotiations with several other large DC-9/MD-80 operators about similar requirements.

Source: Flight International