Two options for conversion to approach flight in

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two options for conversion to approach flight in order to avoid the BVI intensive region. Based on the above dis- cussions, the preferred path will have to be the lower one as indicated in the figure. Also, a helicopter that has BVI characteristics as shown in figure 24, which exhibits a more focused BVI intensive region and located more within the operational approach envelope than was the case for the test helicopter, figure 4, would offer much more promise for showing significant benefits in using these noise-abatement approach procedures. Concluding Remarks An acoustic flight experiment was conducted using the NASA/Army RASCAL research helicopter with its recently developed LDGPS-based cockpit display capa- bilities. The flight tests were conducted at Crows Landing Airfield in northern California with an array of four microphones similar to that used in the standard ICAO/ FAA noise certification test. A newly developed, laser- based rotor state measurement system on board the aircraft was used to measure the main rotor tip-path-plane angle-of-attack. An assessment of the noise reduction effectiveness and associated tracking performance of a RASCAL-specific BVI noise-abatement approach profile (called Quiet), two HAI-recommended generic noise abatement profiles—one for medium weight and one for light weight helicopters—and three conventional descend- ing approaches, namely, 9 deg decelerating, 6 deg decel- erating, and 6 deg constant speed at 80 knots, was made. The results showed that 1. Using LDGPS guidance for positioning over the micro- phones, the flight technical errors were small, generally within the flight performance window required for the noise certification test. 2. The Quiet approach profile, which was designed using acoustic flight test data appropriate to the test helicopter but without considering the effect of deceleration on the effective descent rate, showed no reduction in BVI noise from the standard 6 deg or 9 deg decelerating approaches. 3. The effect of deceleration on descending approach flight was found, with the help of the newly developed, laser-based rotor-state measurement system installed on the RASCAL helicopter, to increase the tip-path plane angle-of-attack of the main rotor with an accompanying increase in the effective rate of descent. The effect can influence significantly the BVI noise characteristics. The design of a deceleration approach profile for BVI noise abatement should therefore consider the effect of deceleration. 4. At the centerline microphone location, the mean value of dBAmax increased from HAI-Light to HAI-Medium,
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11 9deg decelerating, Quiet, 6 deg decelerating, to 6 deg at constant speed of 80 knots, with the range being less than 5 dB. Similar ranking was found with the use of SEL noise metric; however, the range was somewhat smaller, being less than 3 dB.
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