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77 CHAPTER 6. INM TAXI NOISE IMPLEMENTATION INM 7 contains the backbone necessary to define detailed taxi aircraft movements and compute noise from taxi operations; however several modifications are necessary to affect a proper solution. These modifications fall into two areas: Operational and Acoustic. One important consideration for taxi noise which has not been a focus in INM (other than to a limited degree for run up operations) is the element of time. Taxi noise contours are directly impacted by the amount of time spent performing taxi operations (taxiing and holding). The treatment of the time element in INM is a common thread in many of the proposed features. Within INM, analysis of taxi operations should be treated much as any other analysis and the user given the flexibility to organize studies, scenarios and cases. There are some extensions to the GUI that will need to be made to accommodate taxi analyses as proposed in this report. These are also described in the following sections. For each of the specific modifications, an importance assessment of low, medium or high has been made. An appraisal of the difficulty of implementation in INM has also been provided with the ranking of easy, moderate and difficult. A ranking of easy is reserved for those features which likely only require a simple flag status check and enabling of a specific algorithm for taxi situations. The difficult implementation ranking is expected to require major changes both to the database, current internal storage arrays, and will likely impact data screens in the GUI. With each taxi noise feature discussion, an indication of a rough order of magnitude level of effort and costs (based on an average $150/hr burdened labor rate) as well as any other required modifications has been provided. 6.1. Taxi Operations In INM, operational trajectories are created in the compute module, by combining ground tracks with flight profiles. In keeping with this structure, the most suitable method of input for taxi motion is via ground tracks and point-to-point profiles. Procedure step algorithms for determining aircraft altitude, speed and thrust based on aircraft performance data as defined in SAE-AIR-1845 are not applicable to taxi operations. 6.1.1. Taxi Tracks User creation of taxi tracks should be consistent with the existing P-tracks feature in INM. Helicopter taxi tracks are not extended using a 100 nmi straight segment on the last defined track, and taxi tracks should be treated similarly. Track dispersion for taxi tracks should not be enabled as it is not a realistic scenario. The Add Track menu (Figure 64) already displays the Taxi track type; however taxi tracks need not be associated with a runway or helipad as they can be used to model tracks to and from run up or maintenance facilities.
78 Figure 64. Track display control screen. Taxi Tracks Importance Difficulty ROM Effort (Hrs) ROM Cost ($) Other Required Features High Easy 80 12k None 6.1.2. Taxi Profiles Taxi profiles should be structured in such a way as to permit the user to string together segments modeling changes in speed and thrust. For taxi operations, the aircraft altitude does not change; therefore, it is important to add the height of engines above the ground to the aircraft-specific data used for taxi profiles. For analyses requiring only low fidelity computations or backwards compatibility with previous studies, a simple two point profile with constant speed and thrust as is currently recommended in the INM manual can be used. As was demonstrated in the sensitivity study, the existing INM track and profile modeling techniques combined with static run up operations can be used to model detailed taxi motions. To reduce the input burden on users performing higher fidelity modeling, it would be beneficial to include the ability to define specific taxi profiles with multiple segments including stationary holds and stationary regions where breakaway thrust is applied. The operations can then be split internally into stationary and moving segments (transparent to the user), and the noise computed in the same manner as currently exists in INM. The benefit to the user of defining and linking holding queues to specific taxi profiles is that it would significantly reduce the amount of external record keeping.
79 Figure 65. Fixed point profile form. Taxi and Hold Profiles Implementation Importance Difficulty ROM Effort (Hrs) ROM Cost ($) Other Required Features Medium Moderate 160 24k Aircraft Engine Height Dataset Aircraft Engine Height Dataset - Assimilation from Public Data Sources Importance Difficulty ROM Effort (Hrs) ROM Cost ($) Other Required Features Medium Moderate 60 9k Taxi and Hold Profiles GUI 6.1.3. Thrust Settings for Taxi Operations The thrust in the NPD data is a label that points to the appropriate source SEL or Lmax data. There is scatter in existing empirical taxi noise data. Current taxi modeling guidelines for emissions analyses suggest a nominal value of 7% thrust, however this is a subject of debate. INM currently requires the user to specify a suitable thrust setting, but this feature is only required if the current NPD structure (see Section 6.1.1) is retained. If the recommendation to create a ND database (with no acoustic thrust sensitivity) is implemented in INM, then this feature is not needed. Breakaway thrust is used by some aircraft types more often than others and reflects a higher taxi engine and noise state. Limited measurements conducted as part of the sensitivity study suggest that noise levels at the breakaway thrust condition for larger commercial aircraft are on the order of 3-7dB higher than idle thrust taxi settings, and last for approximately 10 seconds. Nominal taxi thrusts, speeds, breakaway thrust settings and duration can be implemented in INM as standard taxi profile steps with the flexibility to build more complex user defined taxi profiles.
80 Default INM Assignment of Taxi Thrust Settings Importance Difficulty ROM Effort (Hrs) ROM Cost ($) Other Required Features Low Easy 40 6k None 6.2. Acoustic Algorithms Within INM the distinction between source modeling and propagation modeling is not completely obvious. Expansion of the taxi modeling capability within INM, and in the future AEDT, will need to ensure consistency in propagation and source modeling effects between static and moving operations. 6.2.1. Source Spectra Differences in spectral content between flight and taxi noise suggests that a new taxi- NPD class be created in addition to the existing approach-NPD and departure-NPD datasets. Taxi noise spectra often contain higher frequency content noise due to engine ingestion of a ground vortex. These differences in spectral content can cause an under prediction of taxi noise in communities located at shorter propagation distances from the taxi operations. INM Spectral Class data is located in the system subdirectory SYS_DATA and contained in an encrypted compressed binary format file SPECTRA.BIN. At present INM contains 34 Approach and 34 Departure spectral classes. The most logical extension would be to create 34 taxi spectral classes. Each of these new taxi spectral class data should be based on nominal empirical taxi spectra for the same aircraft types representing each class. The process used for determining the nominal spectra (i.e. fleet mix weighted average) should mirror the process that was used to develop the approach and departure spectra already in INM. With the addition of a new Taxi Spectral Class the Civil and Military Noise Identifiers interface will need to be expanded to point to the appropriate Taxi Spectral Class (Figure 66). Figure 66. Noise identifiers screen.
81 Taxi Spectral Class â GUI & Database Updates for INM 7 Importance Difficulty ROM Effort (Hrs) ROM Cost ($) Other Required Features Medium Difficult 40 6k Taxi Spectral Class Dataset Taxi Spectral Class Acoustics Taxi Spectral Class â Dataset Development Importance Difficulty ROM Effort (Hrs) ROM Cost ($) Other Required Features Medium Difficult 80 12k Taxi Spectral Class Dataset Taxi Spectral Class Acoustics Taxi Spectral Class â Acoustic Implementation in INM 7 Computational Module Importance Difficulty ROM Effort (Hrs) ROM Cost ($) Other Required Features Medium Difficult 80 12k Taxi Spectral Class GUI & Database Taxi Spectral Class Dataset 6.2.2. Taxi-NPD Data Implementation of a taxi-NPD database for INM will require preparation of measurement noise data across a range of aircraft and engine types, and will require either adoption of an existing comprehensive taxi noise database or additional acoustic measurements. In the short term we suggest adoption of the taxi noise data contained in reference (26). This data will need to be processed for each aircraft type from the current form (spectral directivity sound power) to a taxi spectral class for use in INM. This activity is referred to as Taxi NPD Database Development. Additional aircraft types found in the INM database, but not contained in reference (26) (primarily regional jet aircraft, turbo props and general aviation) will also need to be measured under nominal taxi conditions. The new NPD data will need to be displayed in the NPD Data screen (Figure 67). It is proposed that T be used to indicate the Taxi-NPD type. The existing capability in INM to copy/edit/paste records should be supported for taxi NPDs as well. Figure 67. NPD data screen.
82 Taxi NPD - Database Development Importance Difficulty ROM Effort (Hrs) ROM Cost ($) Other Required Features High Difficult 160 24k Taxi NPD â Acoustics Taxi NPD Data â GUI Taxi NPD - Database Development, Augmentation with Measurements Importance Difficulty ROM Effort (Hrs) ROM Cost ($) Other Required Features High Difficult 300 60k* Taxi NPD â Acoustics Taxi NPD Data â GUI * Costs for Measurements include labor and other elements. Taxi NPD - Acoustic Implementation in INM Importance Difficulty ROM Effort (Hrs) ROM Cost ($) Other Required Features High Difficult 40 6k Taxi NPD - Database Taxi NPD â GUI Taxi NPD â GUI Implementation in INM Importance Difficulty ROM Effort (Hrs) ROM Cost ($) Other Required Features High Difficult 40 6k Taxi NPD - Database Taxi NPD - Acoustics 6.2.3. Taxi Longitudinal Ground-Based Directivity INM presently includes the capability for applying a Ground-Based Directivity Adjustment resulting from the normalized noise pattern defined by a 360-degree area in the horizontal plane around a noise source. This directivity adjustment does not include any changes in front of the vehicle, only behind the aircraft. In INM, measurement-based directivity is accounted for in run up operations and flight operations (such as behind the takeoff roll) when the altitude is exactly 0 ft AGL. If the height of the segment is above 0 ft AGL then this directivity adjustment is not applied. INM should be modified that Ground-Based Directivity is applied for all taxi operations. The current Ground-Based Directivity Adjustment should be extended for taxi operations such that it represents a nominal taxi directivity shape, including the increase in noise ahead of the aircraft as was shown in the sensitivity study report. Three possible concepts for taxi directivity are: a) A single taxi directivity pattern applied to all aircraft. b) Suggested: A simplified taxi directivity class pattern which considers different groups of aircraft. i.e. jet versus propeller aircraft and wing versus tail mounted jet types. c) A directivity pattern that is different for each specific aircraft type. It is our suggestion that option b) be implemented in INM; however, this will also require the incorporation of a database which assigns an aircraft directivity class to each aircraft directivity class. This option is consistent with recent research (35) on directivity behind the start of takeoff roll. It is suggested that directivity patterns for taxi and behind the start of takeoff
83 roll be based on classes of aircraft. In the event that this is not feasible in the short-term, a description of option a) which is consistent with the current INM7 single directivity adjustment is also provided. One existing measurement dataset has been identified (25, 26) which contains full spectral directivity for a wide variety of aircraft types and could be adopted as option c) however, considerable scatter between this and other empirical data has been observed. Database Development â Required for Option a) Single Importance Difficulty ROM Effort (Hrs) ROM Cost ($) Other Required Features Low Moderate 80 12k GUI (a) & Physics (a) Implementation in INM Database Development â Required for Options b) Class or c) by aircraft type Importance Difficulty ROM Effort (Hrs) ROM Cost ($) Other Required Features Low Moderate 160 24k GUI (b or c) & Physics (b or c) Implementation in INM Taxi Longitudinal Directivity Physics in INM â Option a) Single taxi directivity pattern Importance Difficulty ROM Effort (Hrs) ROM Cost ($) Other Required Features Low Moderate 40 6k Single Directivity Curve Taxi Longitudinal Directivity Physics in INM â b) Directivity patterns for aircraft classes Importance Difficulty ROM Effort (Hrs) ROM Cost ($) Other Required Features High Moderate 120 18k Directivity Database & GUI Taxi Longitudinal Directivity GUI in INM â Option b) Directivity patterns for aircraft classes Importance Difficulty ROM Effort (Hrs) ROM Cost ($) Other Required Features High Moderate 20 3k Directivity Database & Physics Taxi Longitudinal Directivity Physics in INM â c) Aircraft specific directivity patterns Importance Difficulty ROM Effort (Hrs) ROM Cost ($) Other Required Features Medium Difficult 120 18k Directivity Database & GUI Taxi Longitudinal Directivity GUI in INM â Option c) Aircraft specific directivity patterns Importance Difficulty ROM Effort (Hrs) ROM Cost ($) Other Required Features Medium Moderate 40 6k Directivity Database & Physics
84 6.2.4. Ground-Based Directivity Smoothing at Larger Distances For all moving flight operations in INM the directivity adjustment is modified by a smoothing equation (1), computed as a function of slant range from the observer location to start of takeoff, and is activated for distances greater than 2500 feet. This smoothing accounts for the effects of variations in the heading of the aircraft. The smoothing serves to make the noise source look more circular at very large distances, while at closer distances the details of the directivity pattern are preserved. This directivity adjustment should be used for all operations, where taxi directivity is applied, for both stationary and moving segments. Ground Based Directivity Smoothing at Larger Distances Importance Difficulty ROM Effort (Hrs) ROM Cost ($) Other Required Features High Easy 20 3k None 6.2.5. Engine Installation Effects (Lateral Directivity) Within INM 7 there is a component of the lateral attenuation adjustment of SAE-AIR- 5662 (34) that takes into account the directivity of the sound from an aircraft as a function of engine/aircraft type (jet, prop, helicopter), engine mounting location (fuselage or wing), and depression angle. A graphical illustration of this is reproduced from reference (1) in Figure 68. This is structured such that the difference in directivity for engine fuselage and wing mounted locations has a 0 dB difference directly under the aircraft, and a 3 dB difference in the plane of the wing of the aircraft. When computing taxi noise this engine installation effect should be disabled and the values in the taxi-NPD database should be representative of measurement sites roughly in the plane of the wing, and not below the aircraft. Figure 68. Lateral source directivity from SAE-AIR-5662. Lateral Source Directivity Importance Difficulty ROM Effort (Hrs) ROM Cost ($) Other Required Features High Easy 20 3k Taxi NPD Dataset
85 6.2.6. Noise Fraction Adjustment for Taxi Segments The INM taxi-NPD data represents the noise exposure level associated with a taxi path of infinite length. However, the aircraft taxi path is described by a set of finite-length segments, each contributing varying amounts of exposure to the overall noise metric computed at an observer. The noise fraction algorithm, which is used exclusively for computation of the exposure-based metrics and indirectly for computation of the time-above metrics, computes the fraction of noise exposure associated with a finite-length flight path segment. This fraction of noise exposure is computed relative to the noise associated with a flight path of infinite length. It is based upon a fourth-power, 90-degree dipole model of sound radiation. Because taxi noise sources are not very well represented with 90-degree dipoles, this noise fraction adjustment is not recommended for use with the taxi noise module. The Noise Fraction Adjustment for Behind the Start of Takeoff Roll ensures consistency between the 90-degree adjustment (to the side of the aircraft) and that at 180-degrees (behind the vehicle). As illustrated with INM contours in the sensitivity study, it serves to âfill inâ what would otherwise yield a significant reduction in noise directly behind the vehicle. For taxi noise computed from flight segments this Noise Fraction Adjustment should be reflected towards the front of the vehicle as well as behind and applied for taxi segments. Noise Fraction Adjustments for Taxi Importance Difficulty ROM Effort (Hrs) ROM Cost ($) Other Required Features High Low 0 0k Included with Ground-Based Directivity Smoothing at Larger Distances in 6.2.4
