NASA and Boeing have been conducting tests and have confirmed that using slots in acoustic liners reduces both drag and noise. IKONICS Precision Abrasive Machining technology can support a variety of precise features, shapes, and sizes – including slot perforations – in composite materials.
NASA and Boeing Tests Confirm Using Slots In Acoustic Liners Reduces Drag and Noise
Tests are focused on industry demand for better fuel economy and less aircraft noise
NASA examined several thru-hole configurations and determined that slots instead of traditional round holes in the acoustic liner (with air passing perpendicular to the slots) had the greatest effect on reducing drag by as much as 50%.
Boeing completed a full scale test on a 737 CFM Leap 1B engine prototype using a slot pattern and results showed that “acoustic and drag benefits prove better than expected.”
IKONICS Precision Abrasive Machining
IKONICS Precision Abrasive Machining can perforate slots in acoustic liners without the challenges associated with traditional machining. IKONICS Technology can support a variety of precise features, shapes, and sizes in composite materials such as carbon fiber and fiberglass. Advantages of the IKONICS Precision Abrasive Machining process include no delamination, stress or pulled fibers.
Acoustic Liner Drag
Excerpt from the NASA report: “Fuel efficiency has been a significant concern for airframers and engine companies developing commercial aircraft products over the last 40 years. Regulatory pressures in other areas besides performance have led to design compromises that trade fuel efficiency for compliance with noise constraints. Acoustic liners have been successfully applied to reduce the radiated noise created by turbofan engines, but with a weight and drag penalty that increases fuel burn. Over the past several decades, research efforts in acoustic liners have mainly focused on understanding liner physics and how to optimize liner impedance (whether uniform or variable) to achieve maximum noise reduction. This work could be applied to reduce the amount of required treatment that, when coupled with advancements in liner materials and manufacturing, would mitigate some of the weight penalty.
Recently, research into lowering liner drag by modification of the facesheet has shown some promise. The majority of acoustic liners developed for production aircraft engines use a facesheet perforated with round holes to provide the necessary porosity. Reducing the hole diameter while keeping the total percent open area (POA) constant has been shown to reduce liner drag. Changing the perforate geometry and orientation, relative to the flow, may create a configuration with improved drag performance.”
Testing Low-Drag Configurations
Excerpt from the Boeing article: “At the same time, analysis of the relative drag of the various configurations of face sheets in combination with the MDOF core was performed at different airflow speeds and across a full frequency range of 400-3,000 Hz. Testing of the low-drag configuration, the face sheet of which incorporates elongated slot-shaped perforations arranged perpendicular to the flow instead of the round holes in current designs, was then conducted in various fan rigs at NASA Glenn Research Center.”