Technical Approach in the FQUROH Project

The FQUROH project employs advanced CFD based on Large Eddy Simulation (LES), which has seen rapid development in recent years, combined with noise measurements from conventional wind tunnel testing to achieve low-noise aircraft design. Current CFD analysis of aircraft noise does not yet provide sufficiently reliable results, such as being unable to adequately predict high-frequency sounds. However, it does provide sufficient knowledge to guide design decisions, such as understanding the mechanisms of noise generation and how to reduce the turbulence that causes it. On the other hand, wind tunnel testing enables highly accurate noise measurement. However, due to equipment limitations, testing cannot be performed on full-scale aircraft; instead, small-scale models are used. As a result, the airflow around the noise source changes due to differences in Reynolds number, again making noise prediction less reliable. In the FQUROH project, we will utilize both approaches complementarily to achieve “low-noise designs” that effectively apply low-noise concepts derived from past research to actual aircraft.

Based on this, we modify the actual aircraft and conduct flight tests to precisely evaluate the noise reduction effects, thereby identifying discrepancies between predicted design results and actual aircraft performance. By thoroughly investigating the underlying physical phenomena causing these discrepancies, we uncover knowledge of low-noise design that can only be gained through real-aircraft testing, thereby advancing the maturity of the techniques. This approach enables us to reliably acquire the design expertise necessary for reducing passenger aircraft noise.

To achieve this objective, highly accurate noise measurements capable of analyzing noise variations during flight tests are also required. In the project, we employed sound source measurement techniques using a phased microphone array with numerous microphones. A total of 195 microphones were installed within a 30-meter diameter area at the testing airfield, and measurements were taken synchronized with the flyover timing of the modified experimental aircraft. The measurement results reveal how the noise levels from each source on the aircraft change across different frequencies. Furthermore, by combining this with high-precision flight path measurement and JAXA's advanced guidance system, Tunnel-In-the-Sky, we achieved highly accurate noise measurements despite the challenges of flight testing, which is susceptible to meteorological influences such as natural wind.

Low-Noise Modifications to the Flaps and Main Landing Gear of the “Hisho”

Based on the low-noise concepts developed thus far, we proceeded with the designs using advanced CFD and wind tunnel testing. Three types of low-noise concepts were applied to the flaps, and four types to the main landing gear.

Flight Demonstration Test at Noto Satoyama Airport

In August 2017, after confirming safety during ground taxiing, test flights were conducted to check flight performance, and in September, the actual noise source measurement tests commenced. A phased microphone array, consisting of 195 microphones with a diameter of 30m as mentioned earlier, was installed alongside the runway to measure the noise. With flight speed 140 kt (72 m/s) and altitude 200 ft (61 m) as baseline conditions, noise source measurements were conducted 222 times over three weeks. These measurements compared configurations with and without low-noise components installed, flight configurations with both flaps and main landing gear extended versus only one extended, and various flight conditions including speed and altitude.

The flight demonstration results confirmed that both the flaps and main landing gear achieved noise reduction of 3dB(A) or more, significantly reducing the stagnant noise during the landing approach. The noise spectrum from the flight test closely matched the pre-flight design predictions, confirming the high effectiveness of the advanced low-noise design utilizing CFD and wind tunnel testing.

Both the flaps and main landing gear have achieved significant noise reduction.

Both the flaps and main landing gear achieved noise reduction of 3 dB(A) or more.

Research and Development Aimed at Regional Jets

Based on the results from the “Hisho” tests, we conducted research and development targeting larger regional jets using CFD and wind tunnel testing. A major difference from the “Hisho” used in flight demonstrations is the presence of high-lift devices called slats on the leading edge of the main wing. To address this, we performed noise source measurements using an actual regional jet, CFD analysis, and wind tunnel tests using a semi-span model.

We designed a device for the slats that aims to reduce noise by suppressing vortex development. This is achieved by adding a “bump” to the cove or extending the trailing edge of the slats. For the main landing gear, which has a different configuration from the “Hisho,” we set the goal of creating a low-noise device that could be stowed and deployed together with the main landing gear during takeoff and landing—a feature not considered for the “Hisho.” We aimed to reduce noise levels suitable for regional jets by refining the low-noise concepts developed for the “Hisho” and devising new low-noise concepts.

The results of this research confirmed sufficient noise reduction effects: for the slats, a total noise reduction of up to 7.3 dB across all frequency bands (Figures 5 and 6), and for the main landing gear, a reduction of 5.5 dB (Figures 7 and 8).

Future Outlook

JAXA partnered with Boeing and Safran Landing Systems to showcase their airframe noise reduction technology on a mid-size airplane that seats 200 and 400 passengers. This collaboration leverages JAXA’s expertise in technology demonstrations using a research aircraft, “Hisho,” based on a business jet and research and development for regional jets, combined with the extensive experience of Boeing and Safran Landing Systems in developing passenger aircraft. Together, the three parties aim to apply the technology practically to reduce airframe noise.