Prof. João Melo de Sousa Instituto Superior Técnico Aerospace & Applied Mechanics Part B Acoustic Emissions 4 Airplane Noise Sources
The primary source of noise from an airplane is its propulsion system. Principal engine types: Piston engine with propeller Turbojet Turboprop Turbofan Propfan Turbo-engines However, all these engines generate the propulsive force by accelerating air (or gas) masses rearwards
The piston engine driving a propeller is used today in small general aviation airplanes only Four or six air-cooled cylinders A mixture of air and gasoline is burned in the cylinders A reduction gear is used
The helical tip speed is the speed of the propeller blade tips relative to the air: flight speed From the angular velocity of the propeller axis, we get: rotational speed of the blade tip propeller diameter The resulting helical tip Mach number is: propeller speed (rpm) Low subsonic airspeeds: V < 700 km/h
The development of the turbojet started in the 30s, meeting the demand for higher airspeeds An afterburner may also be used for extra thrust in takeoff and climb, or transition to supersonic flight Here a relatively small amount of hot gas is accelerated to a very high speed, whereas propellers use a large mass flow of cold air at relatively low speed to produce thrust Efficient at high flight speeds only
The inefficiency of the turbojet at low airspeeds is solved with the introduction of the turboprop Enlarged turbine extracts most of the heat energy, supplying it as shaft power to the propeller avoids extreme helical propeller speeds The residual energy in the gas flow is still used to generate additional thrust Used today in many types of small commuter airliners
Bypass ratio The compromise between the turbojet and the turboprop is the turbofan (or bypass engine) ducted fan replaces the propeller Mean velocity at engine exit is lower than for an equivalent turbojet engine, while the mass flow is larger More economical but also more quiet A turbofan is similar to a turboprop but the bypass ratio is much lower. Nevertheless, the fan contributes largely to thrust
The need of fuel conservation and the limits on the increase of bypass ratio in turbofans led to the concept of propfans (under development) One or two multi-bladed propellers designed for high-subsonic speeds In addition to the noise associated to the propulsion system, the airframe noise must also be considered in the context of airplane noise sources
Airplane piston engines convert the chemical energy in the fuel into shaft power operating on the four-stroke cycle principle: intake compression power exhaust Net work per cylinder and per cycle: determined by engine design and fuel/air ratio
The power delivered to the crankshaft by the pistons is a function of W i, the number pistons and the number of strokes per second, i.e. shaft power number of strokes per second Noise is produced at a series of discrete frequencies: Cylinder firing frequency: Estimate of the overall exhaust noise of an unmuffled engine at 150 m sideline: (plus harmonics...) Exhaust firing frequency: (most intense) (A-weighted)
The propeller blades convert the shaft power of the engine into thrust by accelerating air backwards, as shown: Propulsive efficiency: Due to the dramatic reduction of thrust when the helical tip Mach number approaches unity, the propulsive efficiency of the propeller also collapses
Components of propeller noise: Vortex noise: Associated to the turbulent wake of the propeller Not the most significant source Broadband spectrum above 1 khz Rotational noise: Loading noise: Associated to thrust generation Thickness noise: Associated to the symmetrical displacement of air volumes by the blades Both contributions to rotational noise are characterized by: Blade passage frequency: number of blades Fourier analysis:
The piston engine exhaust and the propeller produce similar types of noise: 3 4 db bandwith 5 Hz A series of tone noises harmonically related to their fundamental frequencies Ungeared two-bladed propeller driven by a 4-cylinder piston engine in static conditions For supersonic propellers, the noise from higher harmonics becomes the most intense contribution Noise directivity patterns: (static in-flight)
There are several empirical methods for the prediction of propeller noise. These involve parameters such as shaft power, propeller diameter and number, rotational tip Mach number, number of blades per propeller, etc Approximation for far-field propeller noise: maximum where: This maximum SPL occurs at an emission angle of approx. 105º:
The noise sources involved in the operation of turbojets and turbofans are as follows: Fan Compressor Combustor Turbine Exhaust jet The first four are generated inside the engine, whereas exhaust jet noise occurs outside The fan radiates noise forward and aft, through the inlet and discharge more or less equally The turbofan compressor noise is emitted to the front whereas combustor and turbine contribute to exhaust noise behind the engine
At full-power conditions, the noise levels from turbojets and low BPR turbofans are almost entirely due to exhaust jet noise At low engine ratings the compressor may become predominant Strong noise directivity (max. at 45º, always) Turbojet Low BPR turbofan
High BPR engines produce lower exhaust jet velocities and therefore show reduced exhaust noise Strong noise directivity (max. at 45º, always) High BPR turbofan In the flyover of an aircraft powered by turbofans, usually two peaks occur along the time history of noise fan noise dominates exhaust noise dominates
The effect of increasing the BPR on fan and exhaust noise is shown in the figure: High noise levels Suppressor nozzles: Reduced noise levels fan design acoustic lining corrugated Less turbulence Higher frequencies (attenuated) Low BPR Less noise multi-lobe mixer mixed non-mixed
For BPR 4, exhaust jet noise and fan noise are of equal importance Subsonic tip speeds Typical frequency spectra from fan noise (forward direction) Supersonic tip speeds
Generation of exhaust jet noise: Sensivity of exhaust jet noise to jet velocity It is a function of the intensity of jet turbulence... broadband noise
Summary of current and future engine noise reduction technologies: Higher BPR Scarf inlets Forward-swept fans Swept/leaned stators Chevron nozzles Active noise control ICAO Annex 16 (Vol. I)
Airframe noise is the far-field noise from an airplane traveling through the atmosphere with the propulsion system inoperative. It is basically aerodynamic noise generated by air flow turbulence Sources of airframe noise: Boundary layer Trailing edges High-lift devices Landing-gear Other structural discontinuities (cavities,...) Mostly significant during landing approaches Also from panel vibrations
Results obtained from measurements of lowaltitude flyovers of small propeller-driven airplanes: Airframe noise peak 200 Hz broadband Tertsband spectrum (typical)
Results obtained from measurements of lowaltitude (adjusted) flyovers of jet-powered airplanes: Airframe noise 200 Hz 1250 Hz double peak Tertsband spectrum (typical)
Measurements of takeoff and landing of jetpowered airplanes show the effect of a dirty configuration : Future airplanes will require reductions in airframe noise (comparable to fan noise on approach)
Interior noise levels affect passenger comfort, crew fatigue and hearing impairment, therefore with an impact on flight safety as well Cabin noise results from the noise emitted by the various external sources, that is transmitted to the interior of the fuselage along airborne paths and structureborne paths, and also from interior sources, such as: Air-conditioning systems Jet-powered airplanes have noise attenuating structures and many have rear fuselage-mounted engines cabin windows turbulent boundary layers
The pure-tone characteristics of propellers are more annoying that the broadband noise from jet propulsion systems (same A-weighted level) Large interest for future large high-subsonic transports powered by propfans Propeller-driven airplanes use the so-called synchrophaser to reduce the noise inside multi-engine transports, thus eliminating the beating (i.e. superposition of pure-tones of different frequencies from propellers operating at sligthly different speeds
Tertsband sound pressure levels at two powersettings in a cockpit of a single-engine propeller-driven airplane: 9-cylinder air-cooled engine 2-bladed propeller DHC-2 Cruise power Takeoff power
SIL db The Speech Interference Level (SIL) is a measure of the degree to which background noise interferes with speech: Max. speech interference for face-to-face conversation: 64 db