Counter-rotating Propellers Research Articles

Acoustic interference of counter-rotation propellers

The noise fields from the rotors of counter-rotating propellers having the same number of blades and angular frequency tend to cancel or reinforce each other depending on the relative phase of the two fields at the point of observation. Because of this, the total noise field at the blade passage frequency or harmonics forms a characteristic standing wave pattern. A general investigation of this acoustic interference phenomenon is carried out. Unlike previous works, the present analysis allows the front and the rear rotor to have different blade geometry and loading. Further, the effect of forward flight is included. Numerical results indicate that at high subsonic cruise Mach number the acoustic interference pattern differs substantially from that at static condition.

Advanced turboprop aircraft flyover noise: Annoyance to counter‐rotating‐propeller configurations

Two experiments were conducted to quantify the annoyance of people to the flyover noise of advanced turboprop aircraft with counter‐rotating propellers (CRP). The objectives were: (1) determine the effects of tonal content on annoyance; and (2) compare annoyance to advanced turboprop aircraft with annoyance to conventional turboprop and jet aircraft. A computer system was used to synthesize realistic, time‐varying simulations of advanced turboprop takeoff noise. For the first and second experiments, respectively, the system generated 27 noises representing CRP configurations with an equal number of blades on each rotor, and 35 noises representing CRP configurations with an unequal number of blades on each rotor. Included in each experiment were five conventional turboprop and five conventional jet takeoffs. Each noise was presented at three levels to subjects who judged the annoyance of each stimulus. Analyses of the judgments examine the effects on annoyance of blade passage frequency, tone‐to‐broadband noise ratio, and aircraft type. The annoyance prediction ability of current noise metrics is also examined.

Reduction of the noise of a model counterrotation propeller at cruise by reducing the aft propeller diameter

The forward propeller of a model counterrotation propeller was tested with its original aft propeller and with a reduced diameter aft propeller. Noise reductions with the reduced diameter aft propeller were measured at simulated cruise conditions. Reductions of up to 7.5 dB were measured for the aft propeller blade passing tone, and reductions in the harmonics were also measured. Reductions in the interaction tones were observed as the result of the reduced diameter aft blades no longer interacting with the forward propeller tip vortex. Significant reductions in the total noise at each harmonic were observed. The chief noise reduction at each harmonic came from reduced aft propeller-alone noise, with the interaction tones contributing little to the totals at cruise. Total cruise noise reductions, as much as 3 dB at given angles for the blade passing tone, and as high as 10 dB at some of the harmonies were observed. These reductions would improve the fuselage interior noise levels and represent a cruise noise benefit for using reduced diameter aft propeller blades.

A Model for Closing the Inviscid Form of the Average-Passage Equation System

A mathematical model is proposed for closing or mathematically completing the system of equations which describes the time-averaged flow field through the blade passages of multistage turbomachinery. These equations, referred to as the average-passage equation system, govern a conceptual model which has proven useful in turbomachinery aerodynamic design and analysis. The closure model is developed so as to insure a consistency between these equations and the axisymmetric through-flow equations. The closure model was incorporated into a computer code for use in simulating the flow field about a high-speed counterrotating propeller and a high-speed fan stage. Results from these simulations are presented.

Noise and performance of a counter-rotation propeller

The noise and aerodynamic performance of a 3 X 3 counter-rotating propeller scale model were quantified experimentally in anechoic environments with a velocity of 68 m/s incoming flow on. The model diameter was 400 mm and the aft blade rotational speeds and the rotational Mach numbers were varied up to 9000 rpm and 0.55, respectively, while the front blades were kept at 9000 rpm. The reinforcement of counter-rotating harmonics was observed even at high blade passage frequencies. The axial separation between two rotors had a favorable effect on the noise levels, whereas an adverse effect on the performance was found for the large spacing with axial velocity acceleration attendant. The spike due to the overturning of tangential velocity was found near the tip section. A difference of the rotational speeds yielded the spinning sound waves with a beat frequency of three times the rps difference. The split frequencies could be identified on the measured spectrum when the two rotors had such nonsynchronous rotational speeds. Nomenclature A = area B - number of blades D — diameter of propeller / = frequency H = enthalpy or total head / = rothalpy J - advance ratio k = load harmonic t = axial spacing between two rotors M = Mach number m — sound harmonic N = rotational speed P = pressure r - radius s - static T = thrust t = total U = axial velocity W - tangential velocity fl = angular velocity /3 = blade setting angle p = density oo = downstream infinity

Errata: Noise of Counter-rotation Propellers with Nonsynchronous Rotors

Errata: Noise of Counter-rotation Propellers with Nonsynchronous Rotors

A numerical method for the design and analysis of counter-rotating propellers

A numerical method has been developed using the techniques of Lock and Theodorsen as described by Davidson to design and analyze counter-rotating propellers. The design method develops the optimum propeller geometry by calculating the planform and twist distribution for each propeller disk through the use of specific inputs of engine shaft horsepower, diameter, and disk spacing. The analysis method calculates the performance of a given counter-rotating propeller system at any flight condition. Using the NACA four-digit airfoil family, the performance of a counter-rotating propeller design for a given flight condition was investigated in the design and analysis mode.

Noise of counter-rotation propellers with nonsynchronous rotors

Noise of counter-rotation propellers with nonsynchronous rotors

Noise radiation patterns of counter-rotation and unsteadily loaded single-rotation propellers

In order to understand the effects of installation on propeller noise, numerous measurements are required to define the directivity of the noise as well as the level. An experimental study was designed to map the noise radiation pattern for various single-rotation propeller (SRP) installations and one counter-rotation propeller (CRP) installation covering an area ± 60 deg from the propeller disk plane and ± 60 deg laterally. The configurations considered included an SRP at angle of attack and in tractor and pusher operations and a CRP. A firstprinciples linear theory was validated for the SRP tractor operation over the angle range mentioned above. The increases in noise that arise from an unsteady loading operation such as an SRP pusher or a CRP exceed 15 dB and depend on the observer location. In particular, the majority of the additional noise from the unsteady loading appears to radiate in the axial directions.

Noise of counter-rotation propellers

Theory is presented for noise generation of counter-rotat ion (CR) propellers with special emphasis given to the effects of acoustic and aerodynamic interference between the two rotors. New radiation formulas are given for noise caused by unsteady loading. Spinning mode characteristics similar to those of turbofans are explicitly displayed so that reinforcements and cancellations between acoustic fields of two rotors or between acoustic modes of one rotor can be studied. Mode orders and cutoff criteria are compared with well-known results from spinning mode theory of turbofans. A theoretical wake model is also presented so that the roles of viscous and potential flow can be seen. The potential wake, which is ordinarily negligible in turbofans, is more powerful for CR propellers because of the smaller blade numbers and the strong load gradient at the blade tips. Noise predictions for an existing CR propeller airplane are compared with experimental results.

Prediction of Steady and Unsteady Loads and Hydrodynamic Forces on Counterrotating Propellers

Linearized unsteady-lifting-surface theory has been applied in the study of counterrotating propeller systems with equal or unequal number of blades operating in uniform or nonuniform inflow fields when both units are rotating with the same rpm. The mathematical model takes into account as realistically as possible the geometry of the propulsive device, the mutual interaction of both units and the three-dimensional spatially varying inflow field. The propeller blades lie on a helicoidal surface of varying pitch, have finite thickness and arbitrary planform, camber and sweep angle. The inflow field of the after propeller is modified by taking into account the effect of the race of the forward propeller, so that potential and viscous effects of the forward propeller are incorporated. These additional effects play an important role in determining the unsteady loading of the after propeller. This, together with some refinements introduced in the numerical procedure, has brought the theoretical results into better agreement with experiments. A computer program has been developed adaptable to a high-speed digital computer (CDC 6600-7600, Cyber 176) for counterrotating systems of equal and unequal number of blades, in uniform flow, for comparison with existing experiments.

Turbulent Wake behind Slender Propeller-Driven Bodies at Angle of Attack

Several previously unstudied effects on the turbulent wake of propeller-driven bodies are examined. The first is the effect of body pitch angle on the wake properties. The second is the addition of an appendage such as a sail to an axisymmetric body. The third is the effect of replacing a single propeller by an equivalent set of side-by-side, counter-rotating propellers.

Pressure field near counterrotating propellers1

Abstract : Expressions for the vibratory pressure field produced by an operating counterrotating propeller system are developed in terms of first and second blade harmonics of individual propellers. The coefficients of the first and second blade harmonics of a 3-bladed propeller are expressed in closed form in terms of complete elliptic integrals. It is found that the pressure signal due to a counterrotating propeller can be obtained from the summation of the pressure fields of its two components as though the two propellers operate as separate units, since their mutual interference was found to contribute little to the total vibratory pressure signal in the near field. This study indicates that the counterrotating propeller system has vibratory characteristics much superior to an equivalent single propeller having the number of blades of one of its components, delivering the same thrust at the same RPM at optimum conditions. A study of the interference effects by means of a two dimensional analogue indicates that the effect of the forward propeller on the aft is much greater than the effect of the aft propeller on the forward propeller.