5th Congress of Alps-Adria Acoustics Association COCKPIT NOISE ANALYSIS IN A SMALL TRAINING AIRCRAFT

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5th Congress of Alps-Adria Acoustics Association 12-14 September 12, Petrčane, Croatia COCKPIT NOISE ANALYSIS IN A SMALL TRAINING AIRCRAFT Mirela Birt, Jurica Ivošević, Ivana Varešak University of

hr Abstract: Cabin noise in a small training aircraft is an important factor that affects not only the quality of cockpit speech communication, but the human health as well.

Since the noise is much higher during such operation regimes than during the cruising, the instructor and the student can become fatigued thus endangering the safety of flight.

1 5th Congress of Alps-Adria Acoustics Association September 12, Petrčane, Croatia COCKPIT NOISE ANALYSIS IN A SMALL TRAINING AIRCRAFT Mirela Birt, Jurica Ivošević, Ivana Varešak University of Zagreb, Faculty of Transport and Traffic Sciences, Department of Aeronautics Vukelićeva 4, 1 Zagreb, Croatia mirela_birt@yahoo.com, jurica.ivosevic@fpz.hr, ivana.varesak@fpz.hr Abstract: Cabin noise in a small training aircraft is an important factor that affects not only the quality of cockpit speech communication, but the human health as well. Training aircraft make a large number of operations in one day and are distinguished from transport aircraft by a large number of takeoffs and landings. Since the noise is much higher during such operation regimes than during the cruising, the instructor and the student can become fatigued thus endangering the safety of flight. Unlike the external noise that is precisely defined and standardized through international regulations, aircraft internal noise has not yet been particularly standardized. The results of the cabin noise measurements within piston engine training aircraft Diamond DV, both in various operating conditions during the flight, as well as in typical power settings on the ground, is presented and discussed in this paper. Furthermore, a few possible methods of noise reduction will be shown. Key words: cabin noise, small aircraft, noise reduction 1. INTRODUCTION Noise is an unwanted sound which has big enough intensity, is distinguished from other sounds and which is loud enough to be heard. High levels of noise negatively influence concentration, communication and ability to effectively perform cognitive tasks [1]. Before presenting and discussing the cabin noise measurements results in various operating conditions during the flight and in typical power settings on the ground, noise sources in small training aircraft, main methods for reduction of the cabin noise and basic data of the aircraft we used for the measurements will be shortly explained. cylinder and the interaction of cold air and hot exhaust gases. Propeller noise is generated by spinning the propeller and the airflow that appears around the blades. The amount of noise is connected with spinning frequency, number of blades on the propeller and physical characteristics of the blades. Aerodynamic noise is caused by the airflow around the fuselage. The most important cabin noise sources in small training aircraft are engine and propeller. The resultant vibration energy of these two elements transmits to the aircraft structure and then reradiate dominantly in the cabin. Propeller wake resulting in interior noise in an small aircraft cabin is shown at the Fig SMALL TRAINING AIRCRAFT CABIN NOISE SOURCES Small training aircraft cabin noise is mainly generated by the propulsion group composed of engine and propeller and insignificantly by aerodynamic airflow that affects turbulent boundary layer and external structure. Engine noise is generated by internal combustion process. It occurs due to sudden changes in pressure within the Fig. 1. Propeller wake resulting in interior noise in a small aircraft cabin AIR-1 Page 1 of 5

2 3. NOISE REDUCTION METHODS When aiming to reduce the effects of noise on the instructor and the student pilot, it must be consider following aspects: noise sources, transmission path and personal noise protection devices. There are three basic methods used by aircraft manufacturers to reduce noise. These methods are isolation, dumping and absorption and are shown in a Fig. 2. noise signature received by the microphones. Then an inverted sine wave (anti-noise) is created that cancels the most dominant noise picked up by the microphones. The second method measures the vibration signal with an accelerometer at the engine mount. This vibration signal is sent to an electronic device that processes and inverts the vibration sine wave and is then sent back to the engine mount to cancel the vibration. Active noise control method is very sensitive on input parameters changes so it must be very carefully performed and monitored. 4. DIAMOND DV DATA Fig. 2. Soundwave spreading through the aircraft fuselage Except the ones mentioned above popular are usage of tuned dumpers and active noise control. Talking about propeller driven aircraft there is not much to do to reduce noise, so this is the reason why manufacturers concentrate on reducing the noise entering the cabin. First mentioned method is isolation. It is made by placing the non-conductive device between the source and the receiver of the vibration. Dumping is method where dumping material is applied to the skin bulkheads, and floor of an aircraft. Dumping changes the vibration frequency of the part to which it is attached and is very good at lowering the noise level in the cabin by reducing the vibration. Sound absorbing materials are usually applied over the dumping material between the skin and the finished interior. Absorption is primarily for higher frequency noise and is especially effective in reducing speech interference level noise. [2] In some propeller driven aircraft, there are tuned dampers installed in line with the propeller on both sides of the aircraft cabin. These dampers are tuned to the blade pass frequency which is the most dominant noise frequency in propeller driven aircraft. They look like tuning forks and vibrate out of phase with the incoming vibration. This has the effect of absorbing the vibration thus reducing the noise level in the aircraft cabin. Active noise control uses electronic boxes, microphones, and speakers to reduce cabin noise. There are two methods used to accomplish the reduction of cabin noise. First method measures cabin noise using microphones placed in the cabin and electronic devices to process the Power Plant Engine Bombardier Rotax 912 S3 with 1HP Propeller Hoffmann HO-V352F / 17 FQ or HO-V352F / C17FQ 2-blade constant speed propeller Performance/Consumption Max. cruise speed at 75% in 5ft 234 km/h TAS 126 kts TAS Range at 55% 916 km 495 nm ft 55% 15.6 l/hr 4.1 gal/hr ft 65% 16.8 l/hr 4.4 gal/hr ft 75% 19.6 l/hr 5.2 gal/hr Rate of climb at sea 4.2 m/s 1) 827 fpm 1) level MTOW Rate of climb in 1.9 1) 374fpm 1) 1 ft MTOW Max. operating m ft altitude Take off distance, 5 41 m 1) 827 fpm 1) ft above take off surface (ISA sea level) Landing distance, m 1) 374 fpm 1) ft above landing surface (ISA sea level) Stall speed, clean 93 km/h 5 kts Stall speed, landing configuration, full 81 km/h 44 kts flaps Dimensions / Weights Length 7.28 m 23 ft 11 in Height 2.18 m 7 ft 2 in Wing span 1.78 m 35 ft 4 in Seats 2 2 MTOW 73 kg 19 lb Empty weight 51 kg 1124 lb Fuel capacity (usable 79 l 21 gal fuel) 1) at 73kg / 19 lbs TOW Table 1. Basic aircraft data AIR-1 Page 2 of 5

Basic aircraft data including power plant, performance/ consumption and dimensions/ weight characteristics are shown in Table 1.

3 Basic aircraft data including power plant, performance/ consumption and dimensions/ weight characteristics are shown in Table THE EXPERIMENT 5.1. Measuring methods and equipment Measurements of the cockpit noise were performed by locating the audiometer between the seats within the cockpit space (Fig. 3.), both, on the runway and during the flight. applicable recommendations from ISO 5129 and AC were used The results SPL (db) 1 1 Diamond DV Katana A weighted SPL C weighted SPL IDLE CRUISE MAX T/O Fig. 4. A- and C-weighted cabin noise in various operating conditions on the runway Diamond DV Katana Fig. 3. Locating the audiometar in the Diamond Katana DV cockpit space Since the certification and noise measurement are not standardized, for the comparison reasons, on the runway, the measurements were done in typical operating conditions or flight regimes: 1) idle, with no progressive speed, 7 rpm; 2) cruise, 19 rpm, (i.e. 25 km/h); 3) maximum power, take-off conditions, 25 rpm. During the flight, for comparison reasons also, the measurement was done in various operating conditions including: idle, taxing, T/O run, take off, climb, downwind, descend, touchdown and roll off. Sound was also recorded in the typical traffic cycle duration of about 6 minutes and finally some distinctive results are calculated, presented and discussed. The noise was measured by means of Nor1 Sound Analyzer with an extensive set of functions available in its expanded version including level/time measurements, octave filters and statistics in every frequency band. The spectral weighting functions A- and C- or Z-weighting are available for all functions and the instrument functionality includes the ability to measure with all three time constants (F, S, I) applied simultaneously. The Instantaneous Sound Pressure Level (SPL), The Maximum Sound Pressure Level (Lmax), The Minimum Sound Pressure Level (Lmin), The Integrated Averaged SPL (Leq) and The Sound Exposure Level (LE or SEL) are some of the measured parameters presented in this paper. Octave band measurements and some simple statistical parameters are also included. During the planning of the measuring set and the procedures, the SPL (db) db 1 1 A weighted SPL C weighted SPL Fig. 5. A- and C-weighted cabin noise in various operating conditions during the flight Diamond DV Katana A weighted C weighted Leq LF(max) LF(min) LS(max) LS(min) SEL Fig. 6. A- and C-weighted characteristic noise parameters during the flight AIR-1 Page 3 of 5

4 The Integrated Averaged SPL (Leq) Diamond DV Katana Octave-band results in all observed flight regimes are shown in Fig. 8. Low frequency noise is dominated with the highest levels in 63 center frequency band. 6. COMPARISON WITH OTHER SMALL TRAINING AIRCRAFT Many studies have been done during the project and some of the interesting results of cabin noise measurements are shown in figures below. Presented measurements are discussed more in the paper [4]. Fig. 7. A-weighted Leq Percentiles during the flight Octave band Leq Diamond DV Katana Octave center frequency k k k k k 1 1 A WEIGHTED CABIN NOISE Piper PA 44 Seminole IDLE CRUISE MAX.T/O Operating conditions Piper PA 34 Seneca Fig. 9. Comparison of A-weighted cabin noise for two aircraft in various operating conditions Fig. 8. Octave-band results during the flight 5.3. Discussion The results of measuring A- and C-weighted cabin noise on the aircraft for the three different operating conditions are presented in Fig. 4. The greatest noise is, like we had expected, at maximum take off power, 93.7 dba (12.1 dbc). That is quite dangerous, but mitigating circumstance is of a short duration. Minimum noise is at idle and it amounts 79.2 dba (99.5 dbc) what is quite high, but acceptable. A- and C-weighted cabin noise in various operating conditions during the flight is presented in Fig. 5. The greatest noise is at takeoff, 93.3 dba, and the minimal noise is at idle, 79.2 dba. A- and C-weighted characteristic noise parameters during the flight are presented in the Fig. 6. Overall Sound Exposure Level (SEL or LE) during the flight in one school cycle is dba, A-weighted Integrated Averaged SPL (Leq) is 87.5 db, Maximum Sound Pressure Level (Lmax) amounts about 93.2 dba (15.5 dbc) and Minimum Sound Pressure Level (Lmin) amounts about dba (92.5 dbc). A-weighted Leq Percentile levels during the flight in one traffic circle are shown at Fig. 7. Interesting to notice is that the level of 82.5 dba is exceeded for 95% of the measurement time. 1 1 A WEIGHTED CABIN NOISE Piper PA 44 Seminole Operating conditions Fig. 1. A-weighted cabin noise in various operating conditions 7. CONCLUSION Today's piston engines for light aircraft have higher levels of noise because they are cooled by air and do not have exhaust silencers (mufflers). Because of that, there is large space for improving of noise performance. Most significant aircraft noise sources are propeller and turbulent flow. From the Katana's cabin noise measurement results it can be concluded that the noise level is not so high like in AIR-1 Page 4 of 5

5 the other one piston engine training aircraft in the property of Croatian Aviation Training Centre (CATC). The comparison of the results with the two-piston engine aircraft shows significantly higher noise levels in Seminole and Seneca cabins compared to Katana cabin, among the rest due to better performance acoustic insulation inside the cabin. ACKNOWLEDGMENTS This study is a part of the project No supported by the Ministry of Science, Education and Sports of the Republic of Croatia. REFERENCES [1] D. Miljković, M. Maletić, M. Obad, Comparative Investigation of Aircraft Interior Noise Properties, 3rd Congress of the Alps Adria Acoustics Association, 7, Graz [2] Jack E. Foster, The Pilot s Guide to Cabin Noise, First Edition, Tucson, AZ [3] ana_factsheet_129.pdf, June 12. [4] J. Ivošević, I. Mihalinčić, T. Bucak, Noise Analysis of a Twin Piston Engine Aircraft, 1st EAA Euroregio 1., Congress on Sound and Vibration, Ljubljana AIR-1 Page 5 of 5

AIRCRAFT FAMILIARIZATION. Some questions may not apply to the aircraft you are flying.

AIRCRAFT FAMILIARIZATION. Some questions may not apply to the aircraft you are flying. 541-895-5935 Name Date AIRCRAFT FAMILIARIZATION Note: If this information is not provided in the aircraft s flight manual give it your best guess. Some questions may not apply to the aircraft you are flying.