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 jurica.ivosevic@fpz.hr, dmiljkovic@hep.hr, tino.bucak@fpz.hr

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 This paper focuses on the comparative acoustical characterization of the interiors of two different types of glider aircraft, Schleicher K7 Rhönadler and Blanik L-13, both being used for flight training and operated by Aeroklub Zagreb. This contribution to the overall noise image of sailplanes interior should be known, as those findings can be used to control and minimize the noise effects on crew. Even with no engines, the interior noise inside a sailplane can be quite noticeable. The instructor 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.

1.1. Sources of sailplane noise The noise of small engine powered aircraft usually consists of two major components: powerplant noise (engine, exhaust and propeller noise) and aerodynamic (airframe) noise, [1]. In addition to the airframe noise and structure borne noise present during autonomous flight, noise in a sailplane during a towing phase originates also from the towing aircraft (engine, propeller and exhaust system) and towing aircraft slipstream, as illustrated in Figure 1, [2].

Fig. 1. Aircraft and sailplane noise components

2. SAILPLANE

Sailplane is a light glider used especially for soaring. It takes-off using a towing aircraft or may be launched by towing winch. 2.1. Schleicher K7 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. 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 instruction and cross-country training, [4].

Fig. 3. Blanik L-13 sailplane

2.3. Towing aircraft

Towing aircraft was a two seater Piper Super Cub PA-18 with 150 HP engine, commonly used for this purpose, Figure 4. Propeller wake from the towing aircraft contributes to sailplane interior noise during a towing phase, Figure 5.

Fig. 4. Towing aircraft Piper Super Cub PA-18

Fig. 5. Towing the sailplane

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 active part of impulse response selected.

3.2. Impulse response of Schleicher K7

Fig. 6. MLS impulse response of the K7 cabin

Fig. 7. TSP impulse response of the K7 cabin

Measured impulse responses of the K7 cabin are shown in Figures 6 and 7. Measurement by TSP method is obviously less noisy then with MLS method. Despite noise, in both cases cabin reflections diminish quickly.

Fig. 8. Frequency response of the K7 cabin Resonances and antiresonances of the K7 cabin are determined from frequency response, Figure 8, derived from measures impulse response and listed in Table 1. Table 1. Resonances and antiresonances of the K7 cabin

Resonance (Hz) Antiresonance (Hz) 1 26,9 2 37,7 48,4 3 75,4 91,5 4 113 134,5 5 177,6 199,1 6 242,2 296

3.3. Impulse response of Blanik L-13

Fig. 9. MLS impulse response of the Blanik L-13 cabin

Fig. 10. TSP impulse response of the Blanik L-13 cabin

Measured impulse responses of the Blanik L13 cabin are shown in Figures 9 and 10. Fading oscillations can be noted in impulse responses due to the formation of standing wave within a cabin.

Fig. 11. Frequency response of the Blanik L-13 cabin Resonances and antiresonances of the Blanik L-13 cabin, Figure 11, are determined and listed in Table 2

Table 2. Resonances and antiresonances of the Blanik L-13 cabin

Resonance (Hz) Antiresonance (Hz) 1 10,8 2 75,4 86,1 3 99,6 110,3 4 123,8 139,9 5 166,8 172,2 6 204,5 209,9

3.2. Noise levels Sailplane interior noise was measured during various phases of flight, listed in Table 3, using Class 1 SLM, Figure 12. Measurement position was at the head level. Data from the analyzer were downloaded later to a PC for post-processing. Measured noise levels corresponding to various phases of flight and both sailplanes are shown in Table 4 and Figure13.

Fig. 12. Measurements of noise levels in L-13 Table 3. Flight phases with corresponding values (towed

in red, autonomous in green)

Flight phase Airspeed [kn]

Height [ft]

1 T/O run* 30 0 2 T/O* 35 10 3 Climb 45 100 4 Climbing turn 50 1000 5 Soar (40 kn) 40 2300 6 Soar (45 kn) 45 2100 7 Descend (40 kn) 40 2000 8 Descend (45 kn) 45 1650 9 Descend (50 kn) 50 1300 10 Descend w/A (35 kn) 35 1300

*T/O - Take off

The distinction is made between towed and autonomous phases of flight in red and green shading within tables. As can be noted from Table 4 and Figure 13., noise levels for most flight phases are pretty much the same with the exception of T/O run where Blanik L-13 is considerably noisier (probably due to its metal construction that doesn’t dump vibrations sufficiently) as well as during a descent with airbrakes extended where K7 was much noisier. During towing flight phases sailplanes are exposed to

Table 4. Noise levels during various phases of flight for K7 and Blanik L-13 sailplane

Flight phase Noise [dBA]

Δ K7 Blanik L-13

T/O run* 76,7 82,6 -5,9 T/O* 81,1 79,4 1,7 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

707274767880828486

T/O ru

n T/OClim

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Climbing t

urn

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Fig. 13. Noise levels during various phases of flight for

K7 and Blanik L-13 sailplane propeller wake from the towing aircraft with engine operating at high power settings. Take off runs are noisy because of ground movement of sailplanes rolling on a rough grass surface. Climb and climbing turn are again quite noisy in both sailplanes that are now flying with considerable airspeed of approx. 45-50 knots to accommodate necessary speed of towing aircraft. At these speeds aerodynamic noise is becoming more pronounced. Noise levels in spectral domain for various phases of flight and for both sailplanes are shown in Figures 14-23. Highest spectral components are present at relatively low frequencies below 250 Hz, particularly during Take-off run and Take-off phases. Higher frequency components catch up once the sailplanes acquire speed (due to aerodynamic noise caused by airstream around the fuselage). Roughly said, noise at higher frequency components decrease at the rate of about 6 dB per octave. One can note considerably higher noise at almost all frequency components for Blanik L-13 during Take off run.

0

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8 16 31,5 63 125 250 500 1k 2k 4k 8k 16k

Octave band center frequency, Hz

Leq,

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Ka-7

Blanik

Fig. 14. T/O Run

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Blanik

Fig. 15. T/O

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Fig. 16. Climb

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8 16 31,5 63 125 250 500 1k 2k 4k 8k 16k

Octave band center frequency, Hz

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Ka-7

Blanik

Fig. 17. Climbing turn

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8 16 31,5 63 125 250 500 1k 2k 4k 8k 16k

Octave band center frequency, Hz

Leq,

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Ka-7

Blanik

Fig. 18. Soar (40 kn)

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8 16 31,5 63 125 250 500 1k 2k 4k 8k

Octave band center frequency, Hz

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Blanik

Fig. 19. Soar (45 kn)

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Fig. 20. Descend (40 kn)

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8 16 31,5 63 125 250 500 1k 2k 4k 8k 16k

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Fig. 21. Descend (45 kn)

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Fig. 22. Descend (50 kn)

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Fig. 23. Descend with Airbrakes

4. SPEECH INTELIGIBILITY

High levels of interior noise in an aircraft (incl. sailplane) considerably downgrade the quality of speech communication. Common in-situ methods of measuring speech intelligibility are Speech Interference Level (SIL), articulation Index (AI) and Speech Intelligibility Index (SII), [5]. The latter provides a means for machine estimation of speech intelligibility under conditions of

additive stationary noise or bandwidth reduction. The SII value can range from 0 (complete lack of intelligibility) through >0.45, considered as minimum acceptable and >0.75 to maximum 1, considered as excellent intelligibility. It is computed from acoustical measurements of speech and noise using SII CALCULATION 1.0 software package [6]. Values for SII during various phases of flight are summarized in Table 5.

Table 5. Speech intelligibility index in various phases of flight (towed in red, autonomous in green)

Flight phase

SII K7 sailplane Blanik -13 sailplane

Normal Raised Loud Shouted Normal Raised Loud Shouted 1 T/O run 0,12 0,35 0,58 0,78 0,03 0,16 0,37 0,59 2 T/O 0,05 0,19 0,42 0,64 0 0,12 0,35 0,57 3 Climb 0 0,03 0,20 0,43 0 0,1 0,31 0,53 4 Climbing turn 0 0,07 0,30 0,53 0 0,09 0,26 0,49 5 Soar (40 kn) 0,02 0,13 033 0,56 0,04 0,23 0,46 0,68 6 Soar (45 kn) 0 0,08 0,29 0,52 0 0,17 0,4 0,62 7 Descend (40 kn) 0,03 0,15 0,36 0,59 0,04 0,25 0,48 0,7 8 Descend (45 kn) 0,02 0,12 0,33 0,55 0,03 0,21 0,44 0,66 9 Descend (50 kn) 0 0 0,13 0,36 0 0,15 0,38 0,6

10 Descend w/A (35 kn) 0,02 0,12 0,31 0,54 0 0,19 0,41 0,63

5. CONCLUSION

From the results acquired by in-situ measurements on two different sailplane constructions, following conclusions can be derived: compared to engine powered aircraft, while taking into consideration sailplane principles of flight (non-powered gliding/soaring, i.e.), sailplane interior is unexpectedly noisy environment. Noise levels are evidently high enough to substantially downgrade both acoustical comfort and speech intelligibility, whereas spectra being strongly corellated with the sailplane type and respective flight phase. Additional engineering efforts should be made (by retrofitting or even from the blueprint stage) in order to assure comfortable ergonomics and flight safety-related communication quality within sailplane interiors.

REFERENCES [1] Miljković D., Maletić M. and Obad M.:

Comparative Investigation Of Aircraft Interior Noise Properties, Proc. AAAA 2007, Graz, 2007

[2] Bucak T., Miljković D. and Ivošević J., Interior

Noise Characterization of a Sailplane Aircraft, Proc. 7th Forum Acusticum 2014, Krakow, 2014.

[3] Schleicher Ka7, Development and Description, http://www.diego-g.com.ar/aeronautica/descargas/Schelaicher%20Ka7.pdf Accessed: 2014-07-13

[4] Blanik L-13 Flight Characteristics, http://planeadores.aeroclub.co/documents/LET L13

Blanik Flight Characteristics.pdf Accessed: 2014-09-23

[5] Bucak, T., Bazijanac, E. and Juričić, B.: Correlation Between SIL and SII in a Light Aircraft Cabin During Flight, Proc. ICSV14, Cairns, Australia, 2007

[6] Speech Intelligibility Index, Available from: http://www.sii.to/html/programs html Accessed: 2014-07-13