05 g properly accounted for and added to figure 4 the

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deceleration at 0.05 g, properly accounted for and added to figure 4, the result is shown in figure 23. This is a more appropriate basis to realistically evaluate and to gain some understanding of the results of the acoustic data described above.
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10 With the deceleration effect considered, the descent rate of the Quiet profile is now around 800 fpm, instead of the scheduled 500 fpm, at and near the airspeed flying over the mics. The result is to increase significantly the noise impact, as can be seen clearly in figure 23. Also shown in the figure, indicated with the asterisks, are the effective descent rates of the HAI-Medium and HAI-Light profiles at the airspeeds when flying over the microphones. The acoustic data set was not dense enough in the region near the asterisks to permit a reliable estimate; however, when interpolating among the available data, the HAI-Medium and HAI-Light profiles appeared to produce noise impact no worse than the Quiet profile. Similarly, the other three conventional approach profiles tested, i.e., 6 deg at con- stant speed of 80 knots, 6 deg deceleration, and 9 deg deceleration, can be interpreted with the help of figure 23. Thus, after taking deceleration effects into account, all three decelerating noise-abatement profiles tested pass through nearly the same region of moderately noisy char- acteristics of the test helicopter, as the microphone array is crossed. This implies that the test helicopter is a poor candidate for assessing the merit of noise reduction through the use of special approach flight procedures, because it has no clearly defined fried egg plot within the operational approach envelope as is the case for a HAI- Light helicopter, for example. Another important effect of deceleration is the change to the directivity of the BVI noise. Deceleration increases the TPP angle-of attack, which tends to flatten the rotor wake, thereby shifting BVI positions further back in the first quadrant. The result is a change in directionality of the BVI noise, moving the BVI lobe more to the starboard side. This and other effects discussed earlier are among the main reasons why it is important to provide accurate measurements of the main rotor TPP angle-of-attack in an acoustic flight test. Although there is a potential substitute for the direct measurement of the TPP with the use of the effective flightpath angle described previously, a thorough evaluation must be made on the quality associated with the estimation of the effective flightpath angle. Also, because of the strong BVI directivity, test methods specif- ically with respect to suitable number of microphones and their locations probably need further developing and improving. The effect of deceleration in a descending approach flight on the effective rate of descent, has important ramifica- tions on the design of an approach profile. The increase in the rate of descent, commensurate with the level of deceleration, should be included in the design process. A potential application of this requirement is given in fig- ure24, which shows a HAI recommendation (ref. 1) on
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