Acoustic Flight-Test Preparation

A flight test of a military aircraft will be conducted at NAS Patuxent River. The purpose is to obtain narrowband acoustic data to identify jet-noise sources. This same data can be analyzed in Va-octave bands and used to evaluate predicted ground-based contours. 

The most difficult part of planning the flight test is associated with determining a flight-test profile that is representative of the FCLP mission profile. In order to obtain improved statistical accuracy through ensemble averaging [2], ground-based acoustic measurements are performed with a linear array of microphones, like that shown in Fig. 4.1. 

As can be observed, 12 microphone sensors are utilized that permit acquisition of 12 ensembles for averaging. For the aircraft flight speeds and altitudes associated with the FCLP, the 200-foot microphone spacing is selected to minimize the smear angle. 

The above constraints require that significant deviation occurs between the FCLP mission profile and that adopted for an acoustic study. To understand this, consider the actual flight profile for the FCLP mission shown in Fig. 4.2. At the touchdown point, the engine power is increased to military power (96% N2) and airspeed is 130 knots. Rotation occurs after a 400-foot roll with engine and airspeed maintained. After climbing to approximately 300 ft, engine power is reduced to 90% N2. 

At this point, the aircraft is 1000 ft downrange with an indicated airspeed of 145 knots. The aircraft continues to climb to approximately 1000 ft, where the engine power is reduced to 86% N2 with an indicated airspeed of 130 knots.  

---