Comparative Acoustical Characterization of Sailplane Interiors
Total Page：16
File Type：pdf, Size：1020Kb
COMPARATIVE ACOUSTICAL CHARACTERIZATION OF SAILPLANE INTERIORS Jurica Ivošević1, Dubravko Miljković2, Tino Bucak1 1University of Zagreb, Faculty of Transport and Traffic Sciences, Department of Aeronautics, Vukelićeva 4, Zagreb, Croatia; 2HEP Zagreb, Croatia [email protected], [email protected], [email protected] Abstract: Aircraft interior noise of small engine powered aircraft and their comparison have been investigated by many researchers. This paper deals with a glider aircraft that, after being launched by towing aircraft, continues an autonomous, non-powered soaring flight. The interior noise is predominantly generated aerodynamically by wind slipstream during progressive flight. A number of interior noise measurements have been undertaken on two different types of glider aircraft, Schleicher K7 Rhönadler and Blanik L-13, at different phases of flight with various speeds and wing control surface configurations and some of the results will be presented and discussed. Key words: sailplane, glider, interior, acoustics, noise 1. INTRODUCTION 1.1. Sources of sailplane noise This paper focuses on the comparative acoustical The noise of small engine powered aircraft usually characterization of the interiors of two different types of consists of two major components: powerplant noise glider aircraft, Schleicher K7 Rhönadler and Blanik (engine, exhaust and propeller noise) and aerodynamic L-13, both being used for flight training and operated by (airframe) noise, [1]. In addition to the airframe noise and Aeroklub Zagreb. This contribution to the overall noise structure borne noise present during autonomous flight, image of sailplanes interior should be known, as those noise in a sailplane during a towing phase originates also findings can be used to control and minimize the noise from the towing aircraft (engine, propeller and exhaust effects on crew. Even with no engines, the interior noise system) and towing aircraft slipstream, as illustrated in inside a sailplane can be quite noticeable. The instructor Figure 1, [2]. and student pilot often don’t use headphones so their communication can be impaired, leading to potential decrease of flight safety. According to the noise spectra, the best and the most practical way to reduce high frequency noise inside the aircraft, and in that way to protect pilot from noise harmful effects, is to use the appropriate absorptive materials in the acoustic insulation of aircraft interior parts such as seats, doors, ceiling and floor. However, lower frequencies cannot be efficiently diminished with such techniques. Use of headphones does not contribute to the reduction of the overall level of cabin noise, but significantly contribute to the crew protection from their harmful noise emission. Modern ANC headphones can be used both for communication and the reduction of the low frequency noise. Fig. 1. Aircraft and sailplane noise components 2. SAILPLANE 2.3. Towing aircraft Sailplane is a light glider used especially for soaring. It Towing aircraft was a two seater Piper Super Cub PA-18 takes-off using a towing aircraft or may be launched by with 150 HP engine, commonly used for this purpose, towing winch. Figure 4. Propeller wake from the towing aircraft contributes to sailplane interior noise during a towing 2.1. Schleicher K7 phase, Figure 5. The Schleicher K7 Rhönadler, also known as Ka-7 or K- 7, Figure 2, is a German high-wing, two-seat glider designed by Rudolf Kaiser and produced by Alexander Schleicher GmbH & Co. [3]. It is of conventional wood and fabric construction, with a steel tube fuselage which had fabric covering over wooden formers. Landing gear consists of a non-retractable and unsprung Dunlop monowheel. The pilots are accommodated inline under a Plexiglas canopy, the front portion of which hinges to starboard and the rear portion hinges rearwards. Fig. 4. Towing aircraft Piper Super Cub PA-18 Fig. 2. K7 sailplane 2.2. Blanik L-13 The L-13 Blaník, Figure 3, was designed by in 1956, and was the first Czech glider employing laminar flow wing profiles. It is suitable for basic flight instruction, aerobatic Fig. 5. Towing the sailplane instruction and cross-country training, [4]. 3. ACOUSTIC MEASUREMENTS, RESULTS AND DISCUSSION Measurements undertaken included impulse responses of the cabin and noise levels during various phases of flight. 3.1. Impulse response, resonances and antiresonances The cabin interior of a glider aircraft has dimensions that favor formation of standing waves (resonant modes) in the range of interior induced noise. Impulse responses of cabin interiors are determined using the MLS and TSP method. Cabin impulse response has been measured in static, on-ground conditions. Measurement by TSP method is obviously less noisy and cabin reflections diminish quickly. Frequency response is determined with Fig. 3. Blanik L-13 sailplane active part of impulse response selected. 3.2. Impulse response of Schleicher K7 3.3. Impulse response of Blanik L-13 Fig. 6. MLS impulse response of the K7 cabin Fig. 9. MLS impulse response of the Blanik L-13 cabin Fig. 7. TSP impulse response of the K7 cabin Fig. 10. TSP impulse response of the Blanik L-13 cabin Measured impulse responses of the K7 cabin are shown in Measured impulse responses of the Blanik L13 cabin are Figures 6 and 7. Measurement by TSP method is shown in Figures 9 and 10. Fading oscillations can be obviously less noisy then with MLS method. Despite noted in impulse responses due to the formation of noise, in both cases cabin reflections diminish quickly. standing wave within a cabin. Fig. 8. Frequency response of the K7 cabin Fig. 11. Frequency response of the Blanik L-13 cabin Resonances and antiresonances of the K7 cabin are Resonances and antiresonances of the Blanik L-13 cabin, determined from frequency response, Figure 8, derived Figure 11, are determined and listed in Table 2 from measures impulse response and listed in Table 1. Table 2. Resonances and antiresonances of the Blanik Table 1. Resonances and antiresonances of the K7 cabin L-13 cabin Resonance (Hz) Antiresonance (Hz) Resonance (Hz) Antiresonance (Hz) 1 26,9 1 10,8 2 37,7 48,4 2 75,4 86,1 3 75,4 91,5 3 99,6 110,3 4 113 134,5 4 123,8 139,9 5 177,6 199,1 5 166,8 172,2 6 242,2 296 6 204,5 209,9 3.2. Noise levels Table 4. Noise levels during various phases of flight for K7 and Blanik L-13 sailplane Sailplane interior noise was measured during various phases of flight, listed in Table 3, using Class 1 SLM, Noise [dBA] Figure 12. Measurement position was at the head level. Flight phase Blanik Δ K7 Data from the analyzer were downloaded later to a PC for L-13 post-processing. Measured noise levels corresponding to T/O run* 76,7 82,6 -5,9 various phases of flight and both sailplanes are shown in T/O* 81,1 79,4 1,7 Table 4 and Figure13. Climb 83,2 82,5 0,7 Climbing turn 80,5 83 -1,5 Soar (40 kn) 77 75,4 1,6 Soar (45 kn) 79 77,1 1,9 Descend (40 kn) 76 75,7 0,3 Descend (45 kn) 77,3 76,3 1,0 Descend (50 kn) 78,1 77,9 0,2 Descend w/A (35 kn) 84,9 77,8 7,1 *T/O - Take off 86 84 82 80 78 Leq,dBA 76 74 72 Ka-7 70 Blanik Fig. 12. Measurements of noise levels in L-13 T/O Climb T/O run Soar (40 Soarkn) (45 kn) Climbing turn Descend (40Descend kn) (45Descend kn) (50 kn) Table 3. Flight phases with corresponding values (towed Descend w/A (35 kn) in red, autonomous in green) Flight phase Fig. 13. Noise levels during various phases of flight for Airspeed Height Flight phase K7 and Blanik L-13 sailplane [kn] [ft] * 1 T/O run 30 0 propeller wake from the towing aircraft with engine * 2 T/O 35 10 operating at high power settings. Take off runs are noisy 3 Climb 45 100 because of ground movement of sailplanes rolling on a 4 Climbing turn 50 1000 rough grass surface. Climb and climbing turn are again 5 Soar (40 kn) 40 2300 quite noisy in both sailplanes that are now flying with 6 Soar (45 kn) 45 2100 considerable airspeed of approx. 45-50 knots to 7 Descend (40 kn) 40 2000 accommodate necessary speed of towing aircraft. At these 8 Descend (45 kn) 45 1650 speeds aerodynamic noise is becoming more pronounced. 9 Descend (50 kn) 50 1300 1 Noise levels in spectral domain for various phases of Descend w/A (35 kn) 35 1300 0 flight and for both sailplanes are shown in Figures 14-23. *T/O - Take off Highest spectral components are present at relatively low frequencies below 250 Hz, particularly during Take-off The distinction is made between towed and autonomous run and Take-off phases. Higher frequency components phases of flight in red and green shading within tables. catch up once the sailplanes acquire speed (due to aerodynamic noise caused by airstream around the As can be noted from Table 4 and Figure 13., noise levels fuselage). Roughly said, noise at higher frequency for most flight phases are pretty much the same with the components decrease at the rate of about 6 dB per octave. exception of T/O run where Blanik L-13 is considerably noisier (probably due to its metal construction that doesn’t One can note considerably higher noise at almost all dump vibrations sufficiently) as well as during a descent frequency components for Blanik L-13 during Take off with airbrakes extended where K7 was much noisier. run. During towing flight phases sailplanes are exposed to 120 100 90 100 80 70 80 60 60 50 Leq, dBA Leq, dBA Ka-7 Ka-7 40 Blanik 40 Blanik 30 20 20 10 0 0 8 16 31,5 63 125 250 500 1k 2k 4k 8k 16k 8 16 31,5 63 125 250 500 1k 2k 4k 8k Octave band center frequency, Hz Octave band center frequency, Hz Fig.