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This paper aims to analyze the propulsive performance of small-amplitude pitching foils at very high frequencies with double objectives: to find out scaling laws for the time-averaged thrust and propulsive efficiency at very high frequencies; and to characterize the Strouhal number above which the effect of turbulence on the mean values cannot be neglected.

The thrust force and propulsive efficiency of a pitching NACA0012 foil at high reduced frequencies (k) and a Reynolds number Re = 16 000 are analyzed using accurate numerical simulations, both assuming laminar flow and using a transition turbulence model. The time-averaged results are validated with available experimental data for k up to about 12 (Strouhal number, St, up to 0.6). This study also compares the present numerical results with the predictions of theoretical models and existing numerical results. For a foil pitching about its quarter chord with amplitude α₀ = 8^o, the reduced frequency is varied here up to k = 30 (St up to 2), much higher than in any previous numerical or experimental work.

For this pitch amplitude, turbulence effects are found negligible for St ≲ 0.8, and affecting less than 10% to the time-averaged thrust coefficient CT¯ for larger St Linear potential theory fails for very large k, even for the small pitch amplitude considered, particularly for the power coefficient, and therefore for the propulsive efficiency. It is found that CT¯ ∼ St² for large St, in agreement with recent models, and the propulsive efficiency decays as 1/k, in disagreement with the linear potential theory.

Pitching foils are increasingly studied as efficient propellers and energy harvesting devices. Their performance at very high reduced frequencies has not been sufficiently analyzed before. The authors provide accurate numerical simulations to discern when turbulence is relevant for the computation of the time-averaged thrust and efficiency and how their scaling with the reduced frequency is affected in relation to the laminar-flow predictions. This is relevant because some small-amplitude theoretical models predict high propulsive efficiency of pitching foils at very high frequencies over certain ranges of the structural parameters, and only very accurate numerical simulations may decide on these predictions.

This paper aims to consider the thrust force generated by two plunging and pitching plates in a tandem configuration in forward flight to find out the configuration that maximizes the propulsive efficiency with high-enough time-averaged lift force.

To that end, the Navier–Stokes equations for the incompressible and two-dimensional flow at Reynolds number $500 are solved. As the number of parameters is quite large, the case of constant separation between the plates (half their chord length), varying seven non-dimensional parameters related to the phase shift between the heaving motion of the foils, the phase lag between pitch and heave of each plate independently and the frequency and amplitude of the heaving and pitching motions are considered. This analysis complements some other recent studies where the separation between the foils has been used as one of the main control parameters.

It is found that the propulsive efficiency is maximized for a phase shift of 180° (counterstroking), when the reduced frequency is 2.2 and the Strouhal number based on half the plunging amplitude is 0.17, the pitching amplitude is 25° and when pitch leads heave by 135° in both the fore -plate and the hind plate. The propulsive efficiency is about 20 per cent, just a bit larger than that of an isolate plate with the same motion as the fore-plate, but the corresponding lift force is negligible for a single plate. The paper discusses this vortical flow structure in relation to other less efficient ones. Finally, the effect of the separation between the plates and the Reynolds number is also briefly discussed.

The kinematics of two flapping plates in tandem configuration that maximizes the propulsive efficiency are characterized discussing physically the associated vortical flow structures in comparison with less efficient kinematic configurations. A much larger number of parameters in the optimization procedure than in previous related works is considered.

An aircraft having a fuselage, a wing, twin booms spaced from the fuselage, one on each side thereof, and a stabilizer supported by the booms, further comprises a plurality of lifting fans mounted in the booms and arranged to draw in air and to discharge it downwardly to produce lifting thrust on the aircraft. The aircraft comprises a fuselage 1, high‐mounted wing 2 and a pair of booms 3 which carry a tail unit 4, 5. A cluster of three jet‐propulsion engines 6 is mounted beneath the wing adjacent each boom and each boom houses a bank of eight direct‐lift fans located in longitudinal positions that ensure that their resulting line of lifting thrust passes through the aircraft centre of gravity. Each engine 6 comprises an auxiliary compressor independent of the engine compressor, which may be driven by an exhaust gas driven turbine forward or rearward of the main engine or the auxiliary compressor may be driven by clutches from the turbine rotor driving the main engine compressor or by gearing. The air delivered by the auxiliary compressors is fed to a common balancing duct 47 from whence it passes through ducts 48 and 49 to the inlet volute 16 of each lifting fan 7 to drive a turbine 13 associated with fan blades 12. Stator blades 14 and 17 are provided and any set of blades may be angularly adjustable and the fan inlet nozzle may be of variable area. Adjustable shutters 19, 22 are provided at each fan inlet and outlet orifice respectively and the duct from each engine 6 incorporates a control valve and a non‐return valve so that when the fans are inoperative with shutters 19, 22 closed the compressed air from the auxiliary compressor may be directed to a supplementary propulsive jet. Differential adjustment of the fan blades, intake area or closure shutters may be used to give a lateral or pitching control effect. Specification 811,840 is referred to.

A Summary by Dr. Alexander Klemin of the Papers Presented Before the Fourteenth Meeting of the Institute held at Columbia University, New York, on January 29–31, 1946. AERODYNAMICS IN spite of increased wing loadings, the use of full span wing flaps has been delayed, because of inability to find a suitable aileron. The Development of a Lateral‐Control System for use with Large‐Span Flaps by I. L. Ashkenas (Northrop Aircraft), outlines the various steps in the aerodynamic development of a retractable aileron system well adapted to the full span flap and successfully employed on the Northrop P‐61. Included is a discussion of the basic data used, the design calculations made, and the effect of structural and mechanical considerations. Changes made as a result of preliminary flight tests are discussed and the final flight‐test results are presented. It is concluded that the use of this retractable aileron system has, in addition to the basic advantage of increased flap span, the following desirable control characteristics: (a) favourable yawing moments, (b) low wing‐torsional loads, (c) small pilot forces, even at high speed.

Assisted high‐lift devices which are based on the removal or the addition of air jets from the flow over the wings may be classified as follows, in accordance with their method of operation:

Under this heading are published regularly abstracts of all Reports and Memoranda of the Aeronautical Research Committee, Reports and Technical Notes of the U.S. National Advisory Committee for Aeronautics, and publications of other similar research bodies as issued

The forces on ellipsoidal bodies placed obliquely in a converging or a diverging stream can be found direct by calculation of the pressures on the surfaces. It seemed worth while to do this in illustration of the general question, as a rather plausible line of argument had led to erroneous values of the transverse force. The results are found to agree with those of the indirect, but more general, investigation by Professor G. I. Taylor in R. & M. 1166.

SINCE its inception, but a few years ago, the Institute of the Aeronautical Sciences has grown remarkably in the size and distinction of its membership, and the technical value of the papers appearing in its journal is now thoroughly well established. Thanks in a large measure to the organizing ability of its Secretary, Lester D. Gardner, the annual meeting was a great success. For the first time in its brief history simultaneous sessions were held, a method which works out quite well in the specialized atmosphere of modern aeronautics. The great difficulty with the annual meetings of American societies is that they are so huge as to render intellectual assimilation problematical. But the geographic conditions of the United States make it impossible for members from different parts of the country to attend frequent meetings such as those of the Royal Aeronautical Society.

Few modeling approaches exist for cycloidal rotors because they are a prototypal technology. Thus, the purpose of this study was to develop new models for their analysis and validation. These models were used to analyze cycloidal rotors and a helicopter that uses them instead of a tail rotor.

Three different models were developed to study the aerodynamic response of cycloidal rotors. They are a simplified analytical model resolved algebraically; a multibody model resolved numerically; and an unsteady computational fluid dynamics (CFD) model. The models were validated using data coming from three different experimental sources, each with rotor spans and radii of roughly 1 m. The CFD model was used to investigate the influence of rotor arms. The efficiency and the stability of the rotor in different configurations were studied. An aeroelastic multibody simulation was used to verify the influence of flexibility on the rotor response.

The analyses suggested that cycloidal rotors can increase the efficiency of a helicopter at high velocities while flexibility reduces it and may lead to instabilities.

These models do not consider the effect of boundary layer friction on the trailing vortices generated by the rotor blades.

These models allow a four-step aerodynamic optimization procedure. First, a range of optimized configurations is obtained by the analytical model. Second, the multibody model refines that range. Third, the CFD model detects eventual problematic blade interactions.

The models presented should serve researchers and industrials looking for a means to measure the performance of cycloidal rotors concepts. The results presented also guide an initial cycloidal rotor design.

The paper aims to present a dynamic model of flexible oscillating pectoral fin for further study on its propulsion mechanism.

The chordwise and spanwise motions of cow-nosed ray’s pectoral fin are first analyzed based on the mechanism of active/passive flexible deformation. The kinematic model of oscillating pectoral fin is established by introducing the flexible deformation. Then, the dynamic model of the oscillating pectoral fin is developed based on the quasi-steady blade element theory. A series of hydrodynamic experiments on the oscillating pectoral fin are carried out to investigate the influences of motion parameters on the propulsion performance of the oscillating pectoral fin.

The experimental results are consistent with that obtained through analytical calculation within a certain range, which indicates that the developed dynamic model in this paper is applicable to describe the dynamic characteristics of the oscillating pectoral fin approximately. The experimental results show that the average thrust of an oscillating pectoral fin increases with the increasing oscillating amplitude and frequency. However, the relationship between the average thrust and the oscillating frequency is nonlinear. Moreover, the experimental results show that there is an optimal phase difference at which the oscillating pectoral fin achieves the maximum average thrust.

The developed dynamic model provides the theoretical basis for further research on propulsion mechanism of oscillating pectoral fins. It can also be used in the design of the bionic pectoral fins.