Propeller-driven Aircraft Research Articles

Analytical model for investigation of interior noise characteristics in aircraft with multiple propellers including synchrophasing

A simplified analytical model of transmission of noise into the interior of propeller-driven aircraft has been developed. The analysis includes directivity and relative phase effects of the propeller noise sources, and leads to a closed form solution for the coupled motion between the interior and exterior fields via the shell (fuselage) vibrational response. Various situations commonly encountered in considering sound transmission into aircraft fuselages are investigated analytically and the results obtained are compared to measurements in real aircraft. In general the model has proved successful in identifying basic mechanisms behind noise transmission phenomena.

Modeling wind tunnel effects on the radiation characteristics of acoustic sources

It is pointed out that the relatively high fuel economy available from propeller-driven aircraft has renewed interest in high speed, highly loaded multiple blade turboprop propulsion systems. Undesirable features related to community noise and the high intensity cabin noise have stimulated new research on the acoustic characteristics of turboprops. The present investigation has the objective to develop a mathematical model of the essential features of the radiation of acoustic disturbances from propellers in a duct and in free space in order to quantify the success with which duct testing can be expected to approximate free field conditions. In connection with the importance of source directionality, a detailed model is considered which consists of a finite element representation of the Gutin propeller theory valid in both the near and far field.

Cabin noise suppression in propeller-driven aircraft

The fuselage (cabin) (15) of a propeller (45) driven aircraft (10) is isolated from vibration associated with the wake (46) of the propeller by vibration isolators (50, 55 and 110) in the wing and tail surfaces of the aircraft.

The determination of optimum flight profiles for short-haul routes

A method of identifying suboptimum flight profiles for short-haul propeller-driven aircraft, using multivariate optimization techniques, is outlined. These profiles are defined in terms of simple parameters which can be readily monitored and controlled by the crew without the assistance of an autopilot. A sequence of minimum-fuel studies, based on a typical modern commuter aircraft, is presented. The effects of some operational constraints are examined. It is found that a 1.5% fuel saving can be achieved relative to the current flightmanual operating techniques. The climb and descent segments are most significant in this type of operation.

Further Observations of the Production of Ice Particles in Clouds by Aircraft

Abstract Evidence is presented for the production of high concentrations of rather uniformly-sized ice crystals in a supercooled stratus cloud by a commercial, turbine, propeller-driven aircraft.

Noise control in propeller-driven aircraft

Analytical model predicts noise levels inside propeller-driven aircraft during cruise at mach 0.8. Double wall sidewalls minimize interior noise and weight. Model applied to three aircraft with fuselages of different size (wide-body, narrow-body, and small-diameter) to determine noise reductions required to achieve A-weighted sound level not to exceed 80 dB.

Fuel Efficiency of Small Aircraft

There is a basic mismatch between the amount of power installed in small propeller-driven aircraft and that required for efficient cruising, which results from climb performance requirements. It is shown in this paper that there is a way of using excess power for most efficient cruise, the resulting airspeed coming closest to the Gabrielli-von Karman limit line of vehicular performance. A survey of 111 light aircraft was conducted, and it is found that many are operated at this optimum, while many more are not. A figure of merit is developed that measures cruise performance. Rationale is presented that is directly applicable to design for cruise efficiency.

Analysis of Interior Noise-Control Treatments for High-speed Propeller-Driven Aircraft

An analytical method is described for prediction of the interior noise levels for propeller-driven aircraft, given the exterior noise signature, its harmonic spectrum, and a description of the fuselage sidewall structure and various candidate noise-control elements. The structural response is described by the theory of Koval, but simplified to consider the stiffeners as smeared elements. The incremental transmission loss (TL) due to add-on noise-control elements is derived from the Beranek and Work method. Comparisons between experimental data and the theory are presented. The method is reasonably accurate below the ring frequency, but is somewhat conservative at the normal incidence angle. This method is, however, expedient computationally, is economical, and permits rapid comparisons of noise-control penalties for various treatment concepts.

Interior Noise Control by Fuselage Design for High-speed Propeller-Driven Aircraft

An analytic study was performed to define the acoustical treatment weight penalties that are required to provide an interior noise level of 80 dBA in propfan-power ed aircraft at Mach 0.8 cruise. The prediction method, described in a companion paper, combines Koval's theory for cylindrical shell noise transmission loss (TL) with Beranek and Work's method for multilayered acoustic treatment analyses. Three fuselage diameters are studied which represent commuter, narrow-body, and wide-body aircraft. The calculated acoustic treatment weight penalties range from 1.7 to 2.4% of aircraft takeoff gross weight (TOGW) for add-on designs. Advanced noise reduction designs, those which permit structural modifications, reduce the acoustic treatment weight penalties to 1.5% TOGW for aluminum aircraft and from 0.74 to 1.4% TOGW for composite fuselage construction. The wide-body results agree with the weight penalty estimates of an earlier turboprop aircraft study.

Propeller Light Aircraft Noise at Discrete Frequencies

In-flight measurements are analyzed to study the acoustic field of a propeller-driven light aircraft. The separation of noise sources is achieved by examination of the influence on the acoustic field of the propeller and the engine. At high rotational speed, the propeller is the dominant source. Evaluating then the field due to steady/unsteady loading on the blades, one concludes that the former are acoustically the most efficient. The analysis of flyover noise measurements, using an original signal processing (Doppler effect compensation) enhances the results.

Interior noise in a propeller-driven aircraft with a cylindrical fuselage

Interior noise in a propeller-driven aircraft with a cylindrical fuselage is analyzed in this paper. The frequency range of interest is divided into a low-frequency range (0–500 Hz) and a high-frequency range (500–5000 Hz) and different noise transmission mechanisms are assumed in these two ranges. In the low-frequency range, since the interior noise is mainly due to the overall cylindrical modes, the fuselage is treated as a cylindrical shell with the properties of the stringers and ring frames smeared over its surface. In the higher-frequency range, the interior noise is mainly due to the individual panel modes of the fuselage and is treated as such in the analysis. Fuselages of different lengths (10–25 m) and different diameters (2–6 m) are analyzed.

Independence of noise levels for propeller-driven small aircraft on test altitude

Noise certification under FAR 36, Appendix F, requires the measurement of maximum A-weighted sound level during flyovers at maximum continuous power, 1000 ft above ground, at a sealevel airport. In order to investigate the effect of altitude and horsepower on sound-level measurements, tests on a Cessna 172 and Beech 33 were made at three airports having elevations of 42, 2350, and 6750 ft. Engine power was varied from approximately 50% of raged maximum continuous to the maximum available at the test altitude. The results for these two representative general aviation aircraft show that above approximately 65% power A-weighted sound level is essentially constant and independent of altitude, depending primarily on propeller tip helical Mach number. Noise certification of propeller-driven small aircraft can thus be reliably performed at a wide range of test sites irrespective of site elevation. [Work supported by FAA.]

Trace Constituents in the Vicinity of Jet Streams

Abstract An extensive series of experiments is being undertaken to determine the concentrations and transport of trace atmospheric constituents in the stratosphere and troposphere in the vicinity of jet streams during tropopause “folding” episodes. The first experiments were undertaken in March and April 1968 and the results are described and discussed. Both propeller-driven aircraft and U. S. Air Force jet aircraft were used, supplemented by ozonesonde and lidar measurements. Trace constituents investigated included ozone, sulfates, various cations, freezing nuclei, aerosol particles in general, and radioactive nuclides. A high, positive correlation was observed between the intensity of radioactivity and the sulfate concentrations. Relative concentrations were consistent with transport of air from the troposphere to the stratosphere on the anticyclonic side of the jet stream. Concentrations of freezing nuclei were on the average much lower but much more uniform in the stratosphere than in the troposphere.

Masking of speech by aircraft noise.

Word-intelligibility tests at various intensity levels were administered to a crew of trained listeners in the presence of recorded noise from jet and propeller-driven aircraft. The noise was that which would be present outdoors and in a house as the result of engine runup operations and if aircraft were flying overhead shortly after takeoff and prior to landing. According to visual inspection of the data, methods of measuring or evaluating aircraft noise predict the results of the speech tests in the following order of merit, from best to worst: (1) articulation index (AI), (2) perceived noise level in PNdB, (3) speech-interference level (SIL), (4) noise criteria (NC), (5) over-all sound-pressure level (SPL A scale), and (6) over-all SPL C scale. The differences among PNdB, SIL, NC, and dB (A), in this regard, are probably not significant according to these tests.

Speech Interference Level and Aircraft Acoustical Environment

Basic concepts and formulation of the speech interference level (SIL) measure are discussed and the implications of the use of SIL to measure aircraft cabin environment are analyzed. Intelligibility tests with both words and phrases indicate that serious interference with speech can be demonstrated by adding supposedly unimportant frequencies to the SIL criterion masking band. Innocuous effects are also demonstrated by masking with low frequencies alone. Conclusions are drawn that because of the complex nature of a propeller-driven aircraft acoustic environment, SIL criteria, without qualification, are unacceptable for measuring either intelligibility or comfort.

Aircraft Cabin Comfort Studies

Several psychophysical tests relating to aircraft cabin comfort studies are described and the results are discussed in relation to speech interference level (SIL) measures. Different conditions of subject motivation and attention are seen to affect the threshold annoyance level, with about 27 sones representing the “normal” operational threshold for both actual and synthetic spectra of propeller-driven aircraft type.

Electric drive for aircraft

AERODYNAMIC EFFICIENCY of very large highs-peed propeller-driven aircraft could be increased substantially if the power plants could be moved out of the wings, thereby decreasing the size of the nacelles. In addition, a plan then might be worked out whereby each power plant could operate at its lowest specific fuel consumption point, with some units being shut down during low load periods. However, such a scheme is not feasible under current practice because of difficulties which would be encountered in transmitting mechanical power through flexible members. Stopping engines and feathering the propellers directly connected to them is a rather unsatisfactory method of reducing power, and can be quite dangerous in wartime.