1. Configuration 6 on the NASA HWB Best aircraft was used to determine the approach noise in EPNL dB as a function of approach speed. The three-degree flight path angle is kept fixed for all approach speeds. The approach speed is varied from the minimum speed for the HWB, 97 knots, used in the 2009 preliminary study50to a speed of 140 knots that is more typical of the 777-like aircraft. Figure 9 shows that the airframe noise decreases continuously with reduced approach speed as expected, while engine noise first decreases as approach speed is reduced from 140 knots, has a minimum near 130 knots, and then increases again toward the minimum approach speed of 97 knots. The result is that total aircraft noise does not minimize until 100 knots, a velocity too low by the criteria already discussed. Therefore, the approach speed is fixed at 115 knots for all cases to be discussed subsequently.
2. Figure 10 shows the approach velocity as well as takeoff velocity for the four aircraft models discussed. The HWB 2009 model approached at the speed of 97 knots and also had a slower takeoff velocity of about 140 knots. The SOA aircraft has the highest approach speed of 125 knots and the highest takeoff speed approaching 180 knots. The HWB Best and Heavy aircraft have virtually identical speeds, lower compared to the SOA aircraft. Throttle settings are shown in Figure 11 showing similar profiles. Figure 12 plots the flight path as a function of distance for both approach and takeoff maneuvers. Again, all four aircraft follow the three-degree approach angle. Ground roll for the HWB 2009 aircraft was not constrained and is shorter given high lift characteristics of the HWB and the lighter weight. For the current study the ground roll for the HWB Best and the HWB Heavy was constrained to equal that of the SOA and is therefore much longer. Even so, the resulting flight paths for takeoff show the faster climb of the HWB compared with the SOA, a factor that will influence noise. The angle of attack profiles in Figure 13 show that the HWB Best and Heavy have angles of attack three to four degrees greater than the SOA on both approach and takeoff while the HWB 2009 had angles that were even greater and less practical as was the approach speed of the HWB 2009.
3. The overall aircraft system noise is calculated for the seven configurations listed above and is presented in Figure 14 as cumulative noise relative to the Stage 4 level. Configuration 1 with the standard pylon in the keel position is assessed at a level of 22.0 EPNL dB below Stage 4. As described in the previous assessments of the baseline configuration this level results from many effects but primarily from lower airframe noise on approach, shielding of fan inlet noise, and the faster climb on takeoff. Configuration 2 reduces the aircraft noise to 31.6 dB below Stage 4 due to simple shielding effect from the two diameters of shielding surface that primarily impacts fan exit and core noise but has little impact on jet noise.
4. Starting with Configuration 3, additional shielding surfaces are included and technologies are introduced that can reduce source levels and impact the source distribution so as to enhance the effectiveness of the same shielding surface length. Configuration 3 adds both the inboard verticals and the advanced chevrons that have a significant effect on jet noise shielding effectiveness by relocating peak noise sources upstream for a wide range of frequencies10. The cumulative system noise is 35.1 dB below Stage 4 with the addition of the verticals and these best available chevrons. The verticals potentially have an effect on jet noise shielding but also on the shielding of fan exit and core engine sources. Examining the intermediate calculation steps can determine the net effect of the verticals. Spectral data for the effect of the verticals on jet noise is shown in Reference 10 for the sideline condition and the effect is small. However, for the certification point calculation, the effect on jet noise is to slightly increase noise. Of greater impact is the effect of the verticals on aft radiated fan and core noise. For those components the verticals increase fan exit and core noise about 1 dB at each of the three certification points. The effect is most probably due to engine sources reflected from the verticals and constructively summing in the direction of the ground observers, but further data and diagnostic efforts would be useful. The effect is considered real and emphasizes the importance of considered the engine and airframe as a system and not as isolated components. As a result of this finding, for the final recommendation based on this analysis, the inboard verticals should be repositioned to winglets as in the original BWB concepts. However, the verticals were included in the experimental data and their effect is included for the remaining configurations to be shown.
5. Configuration 5, again shown in Figure 8, adds two more effects simultaneously. First, the elevon deflection above the surface does effectively add more shielding. Second, the active pylon injection from the shelf of the keel pylon also adds additional movement of jet noise sources upstream, increasing jet noise shielding effectiveness. This combination results in an additional 1.5 dB cumulative noise reduction compared to Configuration 3 for a total of 36.6 dB below Stage 4.