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7-6 A single directivity adjustment for taxiing propeller aircraft, DIRTXP (in decibels), was developed as a function of azimuth angle from 0o to 180o (nose to tail) and is provided in Equation 7-3. )20680000000015.0()540000003871.0( )000013441.0()00187281.0()0042452.0(20209.8 54 32 ï±ï± ï±ï±ï± ïïï ï«ïïïïïï«ï½TXPDIR Eq. 7-3 The coefficient of determination and standard error of the estimate are 0.99 and 0.25 dBA, respectively. It is important to note that the curve fit and associated statistics refer to how the fit matches the average of the weighted directivities, and not how well the curve fit matches the data from which the average directivity was derived. The resulting directivity pattern represents the U.S. fleet-mix average for taxiing propeller aircraft. 7.3. Directivity Uncertainty Assessment A comparison of the Jet Directivity function with the empirical directivity patterns is provided in Figure 7-3. An uncertainty assessment was performed by comparing the analytical directivity (dBA) with the empiricial directivity for each aircraft type. The mean and standard deviation of the predicted â measured directivity was computed for each angle from 20 to 160 degrees. Table 7-4 itemizes the results of this analysis for the available jet aircraft from which the directivity assessment was derived. Table 7-5 is the average of the Mean, Median and Standard Deviations towards the front (20-90o) Overall (20 to 160o) and aft (90-160o) for all Turbofan (Jet) aircraft types for which directivity data (or surrogate data) was available (Table 7-2). The empirical directivity pattern took into account the US fleet mix distribution, however for the uncertainty assessment no weighting factors were applied. These directivity patterns are applied to all metric computations, SEL, Lmax, EPNL and PNLTmax per established INM/AEDT methodology for application of aircraft directivity behind the start of takeoff roll. An uncertainty assessment was not performed for the turboprop aircraft due to the very limited amount of commercial directivity data and the extensive use of military surrogates as described in Section 7.2. TABLE 7-4 Taxi Jet Aircraft Directivity Uncertainty Angle Analytical Fit Average Pred- Meas Median Pred - Meas Stdev Pred- Meas 20 7.4 0.1 7.6 2.7 25 7.3 0.6 6.8 2.7 30 7.0 0.8 6.3 2.7 35 6.5 0.8 5.8 2.6 40 5.8 0.6 5.1 2.6 45 5.0 0.4 4.5 2.6 50 4.2 0.1 3.7 2.7 55 3.2 0.1 2.8 2.4 60 2.3 0.0 2.0 2.2 65 1.5 -0.1 1.2 1.9 70 0.7 -0.2 0.5 1.6
7-7 TABLE 7-4 Taxi Jet Aircraft Directivity Uncertainty (Continued) Angle Analytical Fit Average Pred- Meas Median Pred- Meas St.Dev Pred- Meas 75 0.1 -0.2 0.0 1.2 80 -0.2 -0.2 -0.2 0.8 85 -0.3 -0.2 -0.2 0.4 90 0.0 0.0 0.0 0.0 95 0.5 0.2 0.4 0.6 100 1.1 0.5 0.8 1.0 105 1.8 0.6 1.3 1.4 110 2.5 0.6 1.9 1.7 115 3.0 0.8 2.4 1.9 120 3.4 0.8 2.7 2.0 125 3.7 0.9 3.1 2.0 130 3.7 0.9 3.2 2.0 135 3.5 1.3 2.6 2.1 140 3.0 1.6 2.0 2.6 145 2.2 1.4 1.3 2.7 150 1.1 0.9 0.4 3.3 155 -0.3 0.8 -0.8 3.4 160 -2.0 0.0 -1.7 2.9 TABLE 7-5 Angle Averaged Taxi Jet Aircraft Directivity Uncertainty Uncertainty Average over 20-90 deg Mean Median St.Dev 0.2 3.0 1.9 Uncertainty Average over 20-160 deg Mean Median St.Dev 0.5 2.2 2.0 Uncertainty Average over 90-160 deg Mean Median St.Dev 0.7 1.3 2.0
