Tactical Jet Research Articles

Development of a far-field prediction model for tactical jet noise spectra

Semi-empirical models for predicting far-field jet noise radiation are computationally efficient. One historical example is NASA SP-8072 (1971), which couples source sound power spectra with frequency-dependent directivity indices to obtain far-field radiated spectra for rockets. This paper applies the SP-8072 framework to develop spectra for the T-7A trainer aircraft. Data at 38 and 76 m arcs are used to obtain sound power spectra and frequency-dependent directivity indices for four different engine conditions ranging from 36% thrust through maximum afterburner. Application of a ground reflection correction model is discussed. The model is applied to obtain results at distances between 19 and 229 m and the performance of the model is evaluated using measured data. [Work supported by ONR Grant No. N00014-21-1-2069.]

Acoustic source characterization of shock-associated noise in an installed, full-scale F404 engine

Shock-associated noise in supersonic jets is an important factor in acoustic radiation, particularly in the upstream direction. A previous paper [L. T. Mathews et al., J. Acoust. Soc. Am. 154, A325 (2023)] discussed a preliminary source characterization of shock-associated noise in a full-scale, installed tactical jet engine. Acoustical holography was used to reconstruct the acoustic behavior near the jet and high-amplitude features were produced that were consistent with shock-associated noise sources. This paper presents a more detailed characterization of shock-related noise in the jet using higher-fidelity methods, such as subarray processing and bandwidth extension. The spatial and spectral characteristics of the shock cell noise sources are discussed. A virtual reference decomposition method is used to examine the source and radiative characteristics of each detected shock noise source. The results are compared with shock noise characteristics in the literature. [Work supported by ONR Grant No. N00014-21-1-2069.]

Recent findings regarding noise from tactical jet engines

This paper reviews the outcomes of several recent investigations into full-scale supersonic jet noise. First, concordance has been shown between a T-7A-installed GE F404 engine’s sound power and classical jet noise theory in both the subsonic and supersonic regimes [M. A. Christian et al., JASA Express Lett. 3, 073601 (2023)]. The results indicate approximate agreement between the “eighth-power” law for subsonic engine conditions and the “third-power” law for supersonic engine conditions, with a radiation efficiency of ∼0.5–0.6% at afterburner. However, a sudden jump in radiation efficiency in the jet’s transonic region is not presently explained. Second, normalized far field-derived sound power spectra for the F404 do not agree well with a curve found in NASA SP-8072 (Eldred, 1971), likely indicating differences between the importance of different Strouhal number regimes in military jet and rocket noise and, possibly, different radiation phenomena. Third, the maximum directivity angle for overall level appears to be primarily driven by the convective Mach number associated with Kelvin–Helmholtz instability waves (M. A. Christian et al., AIAA Paper 2023-3351). Together, these recent findings represent improved understanding of full-scale tactical jet noise phenomena. [Work supported by ONR.]

An investigation of shock cell-related noise in an installed, full-scale tactical jet engine using acoustical holography

Shock cells in imperfectly expanded supersonic jet flows have been found to contribute to the noise radiated by these jets. Acoustic phenomena such as screech and broadband shock-associated noise have been linked to the shock cell structure in jets. Acoustical holography has been used to study various jet noise source and radiation characteristics. Holography-based reconstructions near the acoustic source in an installed, full-scale tactical jet engine have yielded features indicative of the known shock cell structure in the jet. This paper presents these results and investigates the characteristics of these features, including their distribution and relative amplitudes.

Perspectives on Cervical Arthroplasty in Navy and Marine Corps Tactical Jet Aircrew.

Symptomatic cervical spondylosis is a progressive degenerative condition of the cervical spine commonly resulting in functionally-limiting pain, weakness, and/or limited dexterity. Symptomatic cervical spondylosis is believed to occur at higher rates in military aviators than civilian counterparts and is a disqualifying condition for all Navy and Marine Corps aircrew. This condition is non-waiverable for tactical jet (ejection-seat-based) aviators. Medical attrition of experienced tactical jet aircrew from the military aviation community results in substantial cost to the U.S. Government, reduces fleet combat capability, and adversely impacts career progression and retention. The clinical maturation of cervical total disc replacement (TDR) technology over the last 2 decades has revolutionized the treatment of symptomatic cervical spondylosis and enabled a return to duty for hundreds of military service members in non-aviation fields. TDR studies demonstrate equal or superior functional outcomes, rates of symptom resolution, reduced complication and reoperation rates, and lower long-term cost compared to traditional Anterior Cervical Discectomy and Fusion (ACDF). Although initial computational modeling studies have evaluated cervical arthroplasty performance during rotary-wing crash impacts, safety within the dynamic tactical jet environment has not yet been established. The purpose of this article is to review factors relevant to TDR safety and outcomes and to propose a framework to evaluate the safety of TDR in Navy and Marine Corps tactical jet aircrew, to ultimately inform aeromedical algorithms regarding return to flight after TDR.

Transcutaneous and End-Tidal CO₂ Measurements in Hypoxia and Hyperoxia.

BACKGROUND: Transcutaneous measurement of carbon dioxide (CO₂) has been proposed for physiological monitoring of tactical jet aircrew because in some clinical settings it mirrors arterial CO₂ partial pressure (Paco₂). End-tidal monitoring in laboratory settings is known to give high-fidelity estimates of Paco₂.METHODS: The correspondence between end-tidal (PETco₂) and transcutaneous Pco₂ (tcPco₂) was examined in healthy volunteers under laboratory conditions of hyperoxia and hypoxia. Rest and exercise, skin heating and cooling, hyperventilation, and induced CO₂ retention were employed.RESULTS: Neither measure followed all known changes in Paco₂ and tcPco₂ changed when the skin temperature near the probe changed. Bland-Altman analysis showed significant nonzero slopes under most conditions. Regression analysis indicated that oxygen partial pressure (Po₂) in tissue measured as transcutaneous Po₂ (tcPo₂) is an important explanatory variable for tcPco₂ in addition to PETco₂, and that local skin temperature also has an effect. Additionally, absorption atelectasis from breathing 100% O₂ may cause PETco₂ to deviate from Paco₂.DISCUSSION: Even as a trend indicator for Paco₂, tcPco₂ is not useful under conditions that resemble those in the highly dynamic tactical jet aircraft environment. PETco₂ is also not a good indicator of CO₂ status in pilots who breathe nearly 100% O₂.Shykoff BE, Lee LR, Gallo M, Griswold CA. Transcutaneous and end-tidal CO₂ measurements in hypoxia and hyperoxia. Aerosp Med Hum Perform. 2021; 92(11):864-872.

Nonlinear modeling of tactical jet aircraft flyover noise

Acoustic modeling of high-performance military aircraft is used to determine noise impacts and plan training operations. However, modeling accuracy to date has been limited because of nonlinear noise propagation, resulting in shock formation and greater high-frequency spectral levels. While prior work has investigated nonlinearity in noise propagation from static aircraft, this paper investigates the use of a nonlinear propagation algorithm based on the generalized Burgers equation to model noise propagation from F-35 flyovers. Waveforms recorded below the aircraft during a low-altitude flyover are used as the algorithm input and are propagated hundreds of meters. The sound pressure levels and spectra from the resulting waveforms are then compared with measured levels at the same distances. Using nonlinear modeling to predict the sound pressure levels significantly increases broadband spectral accuracy.

Far-field noise radiation characteristics of an afterburning military jet aircraft

Because of the potential noise impacts of tactical jet aircraft, detailed far-field measurements are valuable in developing and validating models. This paper describes far-field measurements taken of the T-7A Red Hawk trainer aircraft. Data were taken at five microphone arcs, over six engine runups from idle to afterburner. The arcs were of varying resolution, but generally spanned 30 to 160 deg (relative to the jet inlet) at distances from 19 to 230 m. The spectra, sound pressure level, and skewness of the time derivative of the pressure waveform are analyzed according to microphone position and engine condition. Measurement consistency over the different runups and across all conditions validates the dataset for future analysis. [Work supported by AFRL.]

Connecting high-power jet noise characteristics with human annoyance

DoD has funded the development of advanced modeling tools for community noise exposures and for exploring noise reduction techniques. However, inadequate understanding of high-power jet noise sources and the effects of radiated noise on people are both limiting factors in mitigating community noise exposure. In the meantime, both ground personnel and community noise exposure lead to annoyance and disturbance, costly noise mitigation, and burdensome restrictions on operations. Therefore, DoD is exploring acoustical metrics that may better correlate jet noise exposure to human response. More broadly, DoD and the jet noise research community desire an increased understanding of jet noise source characterization to best focus on promising strategies for reducing jet noise impacts. This research project seeks to address these two needs by increasing the understanding of tactical jet noise sources and by correlating their physical noise features, such as noise source distribution, directivity, and shock content, with human response related to annoyance, disturbance, and the perception of crackle. This presentation will provide an overview of the project’s objectives. [Work Supported by AF SBIR Program.]DoD has funded the development of advanced modeling tools for community noise exposures and for exploring noise reduction techniques. However, inadequate understanding of high-power jet noise sources and the effects of radiated noise on people are both limiting factors in mitigating community noise exposure. In the meantime, both ground personnel and community noise exposure lead to annoyance and disturbance, costly noise mitigation, and burdensome restrictions on operations. Therefore, DoD is exploring acoustical metrics that may better correlate jet noise exposure to human response. More broadly, DoD and the jet noise research community desire an increased understanding of jet noise source characterization to best focus on promising strategies for reducing jet noise impacts. This research project seeks to address these two needs by increasing the understanding of tactical jet noise sources and by correlating their physical noise features, such as noise source distribution, directivity, and shock content...

Source characterization of full-scale tactical jet noise from phased-array measurements

Application of phased-array algorithms to acoustic measurements in the vicinity of a high-performance military aircraft yields equivalent source reconstructions over a range of engine conditions. Beamforming techniques for aeroacoustics applications have undergone significant advances over the past decade to account for difficulties that arise when traditional methods are applied to distributed sources such as those found in jet noise. The hybrid method, an inverse method approached via beamforming, is applied to jet noise measured along a 50 element, 30 m linear array to obtain equivalent source distributions. The source distribution extent decreases with increasing frequency or with a decrease in engine condition. A source coherence analysis along the axial dimension of the jet plume reveals that the source coherence lengths scale inversely with increasing engine condition. In addition, a method for extending the array bandwidth to frequencies beyond the spatial Nyquist frequency limit is also implemented. A directivity analysis of the beamforming results reveals that sources near the nozzle radiate to the sideline from a relatively stationary point irrespective of frequency, while the noise source origin of downstream radiating noise varies significantly with frequency.

Elevated Environmental Carbon Dioxide Exposure Confounding Physiologic Events in Aviators?

Physiological events (PEs) are a growing problem for US military aviation with detrimental risks to safety and mission readiness. Seeking causative factors is, therefore, of high importance. There is no evidence to date associating carbon dioxide (CO2) pre-flight exposure and decompression sickness (DCS) in aviators. This study is a case series of six aviators with PE after being exposed to a rapid decompression event (RDE) with symptoms consistent with type II DCS. The analysis includes retrospective review of flight and environmental data to further assess a possible link between CO2 levels and altitude physiologic events (PEs). IRB approval was obtained for this study. This case series presents six aviators with PE after being exposed to a rapid decompression event (RDE) with symptoms consistent with type II DCS. Another three aviators were also exposed to a RDE, but remained asymptomatic. All events involved tactical jet aircraft flying at an average of 35,600' Mean Sea Level (MSL) when a RDE occurred, Retrospective reviews led to the discovery that the affected individuals were exposed, pre-flight, to poor indoor air quality demonstrated by elevated levels of measured CO2. PEs are a growing safety concern for the aviation community in the military. As such, increasing measures are taken to ensure safety of flight and completion of the mission. To date, there is no correlation of CO2 exposure and altitude DCS. While elevated CO2 levels cannot be conclusively implicated as causative, this case series suggests a potential role of CO2 in altitude DCS through CO2 direct involvement with emboli gas composition, as well as pro-inflammatory cascade. Aviators exposed to elevated CO2 in poorly ventilated rooms developed PE symptoms consistent with DCS, while at the same command, aviators that were exposed to a well ventilated room did not. This report is far from an answer, but does demonstrate an interesting case series that draws some questions about CO2's role in these aviator's DCS experience. Other explanations are plausible, including the accurate diagnosis of DCS, health variables amongst the aviators, and differences in aircraft and On-Board Oxygen Generation Systems (OBOGS). For a better understanding, the role of environmental CO2 and pre-flight exposure as a risk of DCS should be reviewed.

Comprehensive Aviaton System of Flying Crew Improving Safety in the Aircraft Operation

Abstract The article describes 10 known programmes of practical flight training in military aviation – specialization: Multi-Mission Tactical Jet Pilot, which is the highest level of military pilot training in all types of military and civil aviation – comparable only to the level of training and experience of the pilot-instructor of the Military Aviation School. The presented comparison was developed on the basis of literature research from the point of view of, among others, an aircraft operation engineer and a pilot-instructor.

Numerical validation of using multisource statistically optimized near-field acoustical holography in the vicinity of a high performance military aircraft

Multisource statistically optimized nearfield acoustical holography (M-SONAH) is an advanced holography technique [Wall et al., J. Acoust. Soc. Am. 137, 963–975 (2015)] that has been used to reconstruct the acoustic field from measurements taken in the vicinity of a high-performance military aircraft [Wall et al. 139, 1938 (2016)]. The implementation of M-SONAH for tactical jet noise relies on creating an equivalent wave model using two cylindrical sources, one along the jet centerline and one below the ground as an image source, to represent the field surrounding an aircraft tethered to a reflecting ground run up pad. In this study, the spatial and frequency limitations of using the M-SONAH method to describe the field of a tethered F-35 is explored by using the same measurement geometry as at a recent test, but substituting the sound field obtained from a numerical source for the measurement data. The M-SONAH reconstructions are then compared to numerical benchmarks. A spatial region and frequency bandwidth where bias errors are low are identified and provide validation for the use of this method in tactical jet noise source and field reconstructions. [Work supported by USAFRL through ORISE and the F-35 JPO. Distribution A: Approved for public release; distribution unlimited. Cleared 07/10/2017; JSF17-714.]

Military jet noise source imaging using multisource statistically optimized near-field acoustical holography

The identification of acoustic sources is critical to targeted noise reduction efforts for jets on high-performance tactical aircraft. This paper describes the imaging of acoustic sources from a tactical jet using near-field acoustical holography techniques. The measurement consists of a series of scans over the hologram with a dense microphone array. Partial field decomposition methods are performed to generate coherent holograms. Numerical extrapolation of data beyond the measurement aperture mitigates artifacts near the aperture edges. A multisource equivalent wave model is used that includes the effects of the ground reflection on the measurement. Multisource statistically optimized near-field acoustical holography (M-SONAH) is used to reconstruct apparent source distributions between 20 and 1250 Hz at four engine powers. It is shown that M-SONAH produces accurate field reconstructions for both inward and outward propagation in the region spanned by the physical hologram measurement. Reconstructions across the set of engine powers and frequencies suggests that directivity depends mainly on estimated source location; sources farther downstream radiate at a higher angle relative to the inlet axis. At some frequencies and engine powers, reconstructed fields exhibit multiple radiation lobes originating from overlapped source regions, which is a phenomenon relatively recently reported for full-scale jets.

In-Flight Hypoxia Events in Tactical Jet Aviation: Characteristics Compared to Normobaric Training

Hypoxia continues to be a significant threat in military aviation. In an attempt to counter the hypoxia threat, military jet aviators receive periodic training using a reduced oxygen breathing device (ROBD). This study explored the characteristics of in-flight hypoxia events among tactical jet aviators and compared reported symptoms to those experienced during ROBD training. An anonymous survey was administered to naval aviators prior to aviation physiology training. The survey queried them about previous in-flight hypoxia encounters and the symptoms they experienced. These data were then compared to symptom data from a previous ROBD training survey using Chi-square analyses. Of the 566 aviators who completed the survey, 112 (20%) reported experiencing hypoxia symptoms in a tactical jet aircraft and 64 aviators (57%) indicated they were not wearing the required oxygen mask when the incident first occurred. The results also revealed only 21% of hypoxia events were reported in aviation hazard reports and the three most commonly recorded in-flight hypoxia symptoms were tingling (54%), difficulty concentrating (32%), and dizziness (30%). Chi-square analyses revealed statistically significant differences in frequency of reporting between 5 of 16 symptoms encountered in flight compared to ROBD training. The present investigation is the first survey-based study of hypoxia events in U.S. naval aviation. The study reveals in-flight, mask-on hypoxia has a similar overall reported symptom profile to ROBD training. Further, results suggest increased oxygen-mask compliance among these aviators may be necessary to effectively combat in-flight hypoxia.

The Crew Resource Management Attitudes of U.S. Naval Aviators

The purpose of this paper is to describe the development of, and findings from, a survey designed to measure the Crew Resource Management (CRM) attitudes of U.S. Naval aviators. An attitude questionnaire based upon the Flight Management Attitude Questionnaire (FMAQ) was used. A total of 364 responses were received from U.S. Naval aviators. To obtain acceptable Cronbach's Alpha levels it was necessary to drop nine items from the questionnaire. A comparison of the responses to the questionnaire suggested that tactical jet squadrons may benefit from training on recognizing, and addressing, the effects of stress in other aviators. A more important finding, and counter to expectations, was that senior aviators were significantly more supportive of an open cockpit climate than junior aviators.

The Crew Resource Management Attitudes of U.S. Naval Aviators

The purpose of this paper is to describe the development of, and findings from, a survey designed to measure the Crew Resource Management (CRM) attitudes of U.S. Naval aviators. An attitude questionnaire based upon the Flight Management Attitude Questionnaire (FMAQ) was used. A total of 364 responses were received from U.S. Naval aviators. To obtain acceptable Cronbach's Alpha levels it was necessary to drop nine items from the questionnaire. A comparison of the responses to the questionnaire suggested that tactical jet squadrons may benefit from training on recognizing, and addressing, the effects of stress in other aviators. A more important finding, and counter to expectations, was that senior aviators were significantly more supportive of an open cockpit climate than junior aviators.