Central Aerohydrodynamic Institute
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Without any noise and delay

We have already written several times about a new generation airliner — a supersonic business jet (SBJ), which is being developed by TsAGI’s scientists. The specialists of the Institute work on its design, study its aerodynamic characteristics, and work on providing high supersonic flight speeds. But the question of compliance with current and prospective environmental requirements on the noise level is still open. This problem is actively studied in the Russian Center of Aeronautical Science; and the seminar of the Academic Council of TsAGI is also dedicated to this problem. We met with Viktor Kopyev, the Head of the Acoustic Department, an expert of the Central AeroHydrodynamic Institute in the field of aeroacoustics, and talked with him about the future research on the prospective airplane’s development.

“Viktor Feliksovich, what problems are being faced in the development of supersonic passenger aviation regarding aeroacoustics?”

“The issue of the noise level of a supersonic aircraft is very relevant. Similar studies are being conducted all over the world by many companies, including such major players in the market, as Boeing and Airbus. The idea of high-speed flight is much in demand, not only in the area of business aviation, but also in the field of passenger transport.

“However, the companies face great difficulties developing a new airliner. This to a greater extent relates to the environmental parameters: sonic boom and environmental noise characteristics, which are controlled by International Civil Aviation Organization (ICAO) standards. The development of these standards is the main task of the ICAO committee on reducing the impact of aviation on the environment (CAEP). Within the committee there is a permanent subdivision on supersonic aircraft, which is currently developing a new standard for sonic boom level. However, there are still no standards for environmental noise level for such advanced aircraft, which creates certain difficulties at the design stage. In fact, the aircraft, that is being developed by our organization today should meet all possible environmental requirements in the future, so it is important to have correct reference values now, and reliably assess where we are, not on the base of good intentions and false assessments, but clearly understanding the real complexity of the task that has been set and regulated by ICAO standards.”

“You have mentioned ICAO standards. Please, tell us more about them.”

"ICAO is a specialized agency of UNO that sets international standards of civil aviation and coordinates its development. Its main objective is to improve the safety and operational efficiency of civil aircraft.

“ICAO subsonic noise standards regulate the maximum noise levels at three control points (takeoff, flyover and landing), taking into account the takeoff weight of the aircraft and the number of engines. As a result, acoustic parameters must not exceed each of these values, and the standards are generally tightened through setting limits on cumulative values (determined by current or prospective requirements).

“In its recommendations ICAO specifies aircraft environmental noise limits for all countries, which are members of this organization. On the base of these recommendations, all countries set their own standards. Thus, if a particular aircraft doesn’t comply with ICAO standards, it automatically violates the national requirements of most countries. That, then, prohibits the aircraft operation in their airspace.”

“You have mentioned before that the standards for a supersonic aircraft are still not developed. What are your reference points for SBJ design and development?”

“As an absolute guideline, ICAO documents tell us to follow noise restrictions for a subsonic aircraft with appropriate takeoff weight. And to ensure that the results of TsAGI’s studies are correctly assessed in terms of the prospective standards, it is necessary to properly use ICAO recommendations: for example, for our project with takeoff weight of 135 tonnes the requirements for 400-tonnes subsonic aircraft will not fit, because they are much looser. Unfortunately, such errors still occur, even among experts.”

“Viktor Feliksovich, what is a source of noise for SBJ and how close are the scientists of the Institute to solving the aeroacoustic problems?”

“As for any aircraft, a supersonic business jet noise level in general will be determined by the noise of the engines (in particular, its fan and jet), the interaction noise, and at landing also by the airflow noise of airframe components such as wings, fuselage, mechanization components, etc. It should be mentioned that the integrated layout, considered by TsAGI, has doubtless advantages in terms of reduced fan noise. In this case the power plant has very long air intakes and an outer duct. This allows locating sound-absorbing constructions on the large area. Additional fan noise reduction will be achieved due to the fact that the air intake is located above the streamline surfaces of the airframe and the sound will be shielded by the fuselage and wing.”

“How do you estimate the acoustic parameters of SBJ in terms of their compliance with ICAO standards?”

"First of all, let’s consider two control points — take-off and flyover, where the jet noise is the dominant one. This noise level can be estimated by special programs that are simple and reliable enough, when talking about such source as the high-speed single jet.

“But at the third control point — landing — the most important components for consideration are an airframe and partially an engine fan. In the world there are still no sufficiently reliable calculation methods for airflow noise of airframe components, so it’s very difficult to estimate the aircraft noise through these methods. In order to perform such assessments, the expensive acoustic chambers were constructed, allowing not only to assess the noise level, but also to consider various measures aimed at airframe noise reduction. The results of flight experiments can be also considered, which allows, but only post factum, to determine what noise components are dominant in the noise value at aircraft landing: a gear, flaps or the whole wing, wing slats, the interaction of the landing gear track with an airframe, etc. However, to assess the noise level of SBJ at landing, it is possible to select the appropriate value using a subsonic aircraft analogue with similar takeoff weight from the special ICAO database, which contains the certified values for all aircrafts. It is unlikely that SBA noise level at landing will be less. Thus, at three control points it is possible to evaluate aircraft noise and compare the values with ICAO standards quite reliably and very easy.”

“The international standards are constantly being toughened. SBJ is believed to be able to easily meet the requirements that ICAO will impose in the future. Is that true?”

"Estimated parameters show that we have a lot of work to do to find the solutions for problems, identified by the world community.

“Some scientists lay their hopes on exotic forms of a sectioned nozzle with means of shielding. But in fact, this will unlikely lead to a significant reduction of noise and even, quite the contrary, may increase it.

“In the shielded chamber of TsAGI we have obtained numerous experimental data on the noise level of various jets. On their base it was determined that the high-speed free jets flowing out from rectangular or sectioned nozzles do not usually offer any advantages in assessing the perceived noise level than compared to thrust-equivalent round jets. These studies comply with NASA works, which were reported at the AIAA Aeroacoustic Conference held in June. In the presence of a shield, when the jet approaches its edge there is a significant increase in broadband noise at low frequencies with its significant reduction at high frequencies. This effect is of the same character as the mechanism of noise level amplification at jet’s interaction with deflected flaps. However, in the case of a rectangular cross-section of the jet, this effect can become stronger, since the major part of the shear layer interacts with the edge. Thus, the question of jet noise shielding when the shield is located in close proximity, remains quite controversial, as the opposing tendencies occur, and, I suppose, only good experiment can help to assess results for a particular aircraft.”

“So, shielding will not protect against excessive noise, will it? In this case, how can it be reduced?”

“It is necessary to continue our studies to find appropriate solutions. The scientists know very well that the acoustic power of the jet is proportional to the relative speed of its discharge raised to the power of eight, so even its small decrease can help to solve the noise level problem. Only by significantly reducing this rate, we can try to apply new solutions. For example, the chevrons or corrugations will be very efficient in the close proximity to the shield. When exceeding a certain discharge rate, no noise-absorbing means, including shielding, sectioning, etc., will help in this regard.”

“The problem of SBJ noise reduction was actively discussed during the seminar of the Academic Council of the Institute. What are the results of this discussion?”

“It’s clear that we need to ensure the compliance with prospective ICAO standards regarding aircraft environmental noise, as otherwise we will not be allowed to operate our aircraft in other countries. Our studies and estimations have shown that in this configuration SBJ will meet only outdated standards, and even then with some limitations. To ensure the compliance with prospective requirements, which will come into force for subsonic aircrafts in 2017, it is in any case necessary to significantly reduce the engine jet discharge rate in takeoff and climbing conditions, and further reduce the fan noise. These conditions should give a definite reference regarding the direction of our further work, which, in my opinion, should be conducted based both on classical results and recent achievements in aeroacoustic studies.”

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