Showing papers in "Journal of the Aeronautical Sciences in 1946"

Superaerodynamics, Mechanics of Rarefied Gases

Abstract: Zahm [1] in 1934 published an article on the aerodynamics of highly rarefied gases, a branch of fluid mechanics which he called superaerodynamics. At that time, however, with the means of propulsion then available, flight at extreme altitudes did not seem to be realizable. Therefore, superaerodynamics has been considered as a subject of academic interest rather than one of practical engineering importance. With the recent perfection of the rocket as a propulsive power plant, the situation is radically changed and there should be no limit to the altitude that can be reached by an aircraft. There are even indications that the optimum flight altitude of long-range rocket airplanes is approximately 60 miles. At these and higher altitudes, the air density is so low that the fluid must be thought of as one having a coarse structure and not as a continuous medium of conventional fluid mechanics. Then concepts of superaerodynamics are needed to guide the design of such an aircraft.

Vortex Frequency and Flow Pattern in the Wake of Two Parallel Cylinders at Varied Spacing Normal to an Air Stream

Abstract: The frequency of vortex formations in the flow about a pair of parallel cylinders, at various separations, is investigated with the hot wire anemometer. A resume of the literature on vortex frequencies is given with a view to relate the studies of obstacles of diverse shapes to the present case. The law of dynamical similitude is developed for individual solids, for the efflux through a rectangular orifice, for secondary eddies, and for the two-cylinder system. When two cylinders are separated by a gap just smaller than the diameter, instability occurs. At larger gaps the cylinders behave like independent bodies. As the gap is decreased, the main sequence of frequencies (probably generated on the outer sides of the cylinders) changes from a value corresponding to a single cylinder, of diameter d, to a value associated with a solid body of breadth equal 2d (at zero gap). A minimum frequency is reached at G/d =* 1 /n , where G is the gap distance. For gaps smaller than the critical, a formula is given to represent the main sequence: fd/U = 0.09 + ! l(G/d) y5]/[(£/<*) + 2 ] p where / is the frequency and U is the velocity. The picture is complicated by vortexes generated within the gap. At spacings less than one-half diameter, a low gap frequency is found. Between one-half and one diameter a high gap frequency is present, decreasing to the independent cylinder value at the critical spacing G/d = 1. In addition, doubled frequencies of the main sequence are also found, generally on the centerline of the wake, attributed to overlapping vortexes from the external sides of the cylinders. For all values of the gap, the dimensionless parameter fd/U is shown to be independent of Reynolds numbers greater than 15,000. Below this value, fd/U rises a t first and then drops sharply, the maximum apparently depending on the gap. The velocity distribution in the wake was obtained by traversing with a pitot static.tube. In a plane four diameters downstream from the center of the system, it was found that the wakes of both cylinders are mixed for gaps below the critical and separate for larger gaps. At G/d less than about one-half, the central pressure drop is great enough to cause the flow to reverse.

Supersonic wave drag of thin airfoils

Abstract: The linearization of the equations of motion for the supersonic flow of a perfect fluid is discussed, and methods of solution using elementary source sink solutions are developed. These methods are applied to the calculation of performance of several types of three-dimensional supersonic airfoils; in particular, the drag at zero lift of a family of almost triangular, symmetrical wings is calculated. The significance of the results is discussed.

Auditory Signals for Instrument Flying

Abstract: I 1936, DE FLOREZ DEMONSTRATED that a pilot could fly an airplane with his eyes blindfolded when two of his instrument indications were given by means of an auditory signal in his earphones. The pilot flew with rudder and elevators only, allowing the inherent stability of the Fairchild 24 to take care of lateral control. The signals employed were (1) a turn indication consisting of increase of the signal intensity to one ear and decrease of intensity to the other and (2) an air-speed indication consisting of a change of pitch of the signal. Although the plane described a wide climbing spiral rather than a straight path, the pilot was able to maintain satisfactory control and to recover from spins, thus demonstrating the feasibility of flying by auditory indications. I t is well known that the large number of operations required of the pilot of a modern airplane, and the multitudinous instruments which he must follow, tax the abilities of the pilot to the limit. His eyes are especially subject to overload, since he must keep track of the large number of instruments on the panel and at the same time look out for obstacles and observe his position relative to the ground. In fact, in the larger airplanes both pilot and copilot are sometimes kept busy, especially when flying under " instrument'' conditions. The continuously increasing speed of modern planes and the advent of new devices such as air-borne radar will, if anything, increase this critical load on the eyes. Thus it would be an advantage if some of the flight indications cbuld be furnished to the pilot through the ears. Accordingly, a further studyf of auditory flight indications has been carried out during the last 2 years, the results of which will be reported in this article. The purposes of the study were to determine (1) what types of auditory signals could be followed with

Secant Modulus Method for Determining Plate Instability Above the Proportional Limit

Abstract: The classical plate instability equation is considered in terms of strain rather than stress. Upon the assumption that the critical stress and critical strain are implicitly related by the stress-strain curve of the material, it is found tha t the secant modulus can be used in the plate buckling equation to predict critical stress above the proportional limit. The assumption is confirmed by tests on aluminum alloy Z and channel sections.

Stress-Strain Formulas

Abstract: Most of the analytical expressions for stress-strain curves that have appeared in the literature and a few others are considered critically. I t is concluded that six have general value, and, of these, two are to be preferred in seeking a fit to stressstrain data.

Analysis of Relay Servomechanisms

Abstract: Methods are developed for the transient and steady-state analysis of relay-type servomechanisms, with intermittently applied torque, inertia, viscous friction, dry friction, finite spacing of the controlling contacts, constant delay time in torque change, and stabilization by "flip-flop/' "front-lash," and anticipatory circuits. Solutions are obtained through representation of the motion of the servomechanism in the position-velocity or "phase" plane. In this plane, the discontinuous character of the driving torques, dry friction, delay time, finite contact spacing, and certain types of stabilizers, defines boundaries that are frequently straight lines. The boundaries divide the phase plane into a number of regions, within each of which the motion of the servomechanism is determinable by elementary means. Theorems and semigraphical constructions are developed for the rapid representation of the transient and steady-state motion in the phase plane, and simple constructions are demonstrated for converting the phase-plane plots to conventional time plots. Guided by phase-plane representations, exact mathematical solutions are obtained for a number of cases of interest. A method is developed for testing the physical significance of the tnathematically derived steady-state solutions. Finally, the techniques developed are applied to the comparative examination of several methods for suppressing "hunt ing" of the servo. The analysis indicates an anticipatory signal to be the most promising for control of torque application in a high performance relay servomechanism.

On the Steady Flow of a Gas Through a Tube with Heat Exchange or Chemical Reaction

Abstract: This paper is concerned with a simple phenomenological discussion of gaseous combustion through a tube and related phenomena involving hydrodynamic, thermodynamic, and chemical considerations. I t is also indicated how the discussion can be conveniently developed into a treatment from the point of view of microscopic chemical kinetics. Indeed, some interesting results are already obtained without such an extension. For instance, when there is no chemical reaction, it is found that for the range of Mach Numbers of flow l / \ / y < M < 1 in the subsonic region, the temperature of the gas is actually decreasing while external heat is being added. I t is also shown'that , by continuous heating, both subsonic flows and supersonic flows will eventually approach the sonic state. When 1

Determination and Presentation of Compressive Stress-Strain Data for Thin Sheet Metal

Abstract: Various methods of obtaining the compressive stress-strain curve of thin sheet metal are described, and their advantages and limitations are briefly discussed. The presentation of stressstrain data in dimensionless form is outlined, and the advantages of such a presentation in correcting for the effect of variations in yield strength and in Young's modulus are brought out. Attention is called to a classification of stress-strain curves by shape factors following a three-parameter formula proposed by Osgood.

The Effect of Radius of Curvature and Preliminary Artificial Eccentricities on Buckling Loads of Curved Thin Aluminum-Alloy Sheets for Monocoque Constructions

Abstract: In two previous papers, *> 2 the results of tests on thin curved aluminum-alloy sheets in compression were presented. The present paper, whose publication was restricted during the war, shows that sheets of aluminum alloy tested under special conditions offer highly increased buckling and ultimate strengths under compression loads By decreasing the radius of coldrolled curved sheets 40 to 50 per cent, a much greater ultimate buckling load can be obtained if these panels are tested under the usual radius of curvature (24 in.). Buckling and ultimate loads are distinctly higher, the usual buckling loads being exceeded by about 100 per cent. Local eccentricities of the curved panels, generally assumed to be exceedingly detrimental to the bearing capacity in compression, did not decrease the buckling load of the panels. Even holes produced by shooting bullets through the panels did not weaken materially their normal buckling strength or their ultimate resistance.

Static Displacement and Coupled Frequencies of a Twisted Bar

Abstract: T ELASTIC BODY CONSIDERED is a straight bar with constant rectangular cross section and an initial structural twist of 6 (constant) radians per unit of length. The object of the paper is twofold: (1) to calculate the static displacement components u(z) and v{z) in the direction of the principal axes u and v at each section due to the uniform loads wu and wv also in the direction of the principal axes; (2) to approximate by the energy method, using the displacement functions mentioned in (1), the first two natural frequencies of the bar. The method used is applicable to any end conditions but only the fixed-free case is considered. In the case of a fixed-free bar without twist the two fundamental frequencies corresponding to vibrations normal and parallel to the minor principal axis v are given by the formulas

Nondimensional Buckling Curves-Their Development and Application

Abstract: Basic expressions for column buckling, initial buckling in compression, initial buckling in shear, interrivet buckling and local crippling are reduced to a common parameter—namely, the ratio of tangent modulus to buckling stress (Et/F). A nondimensional buckling parameter B is plotted as a function of the parameters F/F0.7 and n, where F0.7 is a secant yield stress and n is determined by the shape of the knee of the stressstrain curve. Constants for evaluating the buckling parameters for the various types of buckling phenomena, values of the secant yield stress F0.7, and the shape parameter n are tabulated for the materials commonly used in aircraft. I t is shown tha t these nondimensional buckling curves, when entered with the proper buckling parameter, may be used to evaluate the buckling values required for design.

Aero Medical Aspects of Cabin Pressurization for Military and Commercial Aircraft

Abstract: Whenever aircraft fly at high altitudes for long periods of time and over long distances, the foremost design requirement for aircrew efficiency and passenger comfort is cabin pressurization. In military aircraft, cabin pressurization is used to eliminate completely the need for continuous use of oxygen, to prevent aeroembolism only, or simply to reduce the cabin to a level where use of pressure breathing is not required. In commercial aircraft, the health, comfort, and safety of the passengers are of prime consideration. In military aircraft, a significant hazard introduced by cabin pressurization is explosive decompression caused by structural failure of the cabin when under enemy gunfire. Criteria are presented by which safe operating differentials may be chosen to minimize this hazard. The advantages of isobaric cabin altitudes below the now conventional 8,000-ft. level for military aircraft are reviewed as they apply to passengers of commercial aircraft. Finally, emergency procedures necessary for crew and passenger safety in case of failure of the cabin-pressurization system are discussed.

Tensile Properties Affecting the Formability of Aluminum-Alloy Sheet at Elevated Temperatures

Abstract: The tensile properties thought to influence formability have been determined for 15 aluminum-alloy sheet materials tested at elevated temperatures. Complete load-extension diagrams have been obtained, and the effects of temperature, strain rate, and exposure time have been noted. An appreciable increase in the elongation over a zero gage length is found at elevated temperatures. This suggests that forming operations in which this property is a criterion may be facilitated at such temperatures. A decrease in the limit of uniform elongation is noted at elevated temperatures, suggesting that operations in which this property is a criterion may be less successful. A fiftyfold change in strain rate has an appreciable effect on properties at the higher temperatures, an increase in strain rate appearing to be equivalent to a decrease in temperature. In the case of materials subject to precipitation hardening, the time at temperature appears to be a minor factor for times ranging from 5 to 20 min. and temperatures up to at least 450 °F. In certain cases unusual load-extension diagrams were noted. I N T R O D U C T I O N In the aircraft industry aluminum-alloy sheet is often subjected to plastic forming, generally at room temperature. It has been found that some of the recent highstrength materials exhibit poor forming characteristics under these conditions and it has been suggested that formability may be improved at elevated temperatures. Previous investigations 2> 3 dealing with the elevated temperature properties of aluminum alloys have not included some of the recent materials and have not been concerned with formability. Consequently, an investigation was undertaken to evaluate over a considerable range of temperatures the tensile properties related to formability. The program comprising a portion of the ''restricted'' Project NRC-548, was financed by the Office of Production Research and Development of the W.P.B. and was supervised by the War Metallurgy Committee. This paper, which presents some of the results, has been released for publication by the latter agencies. TENSILE DATA, FLOW CURVES AND FORMABILITY The load deformation data derived in a tensile test are of interest in connection with formability since they Received January 8, 1946. * Research Engineer. t Engineer. + Associate Professor of Mechanical Engineering. describe the strain hardening characteristics of the material and the conditions for necking and fracture. It is often desirable, however, to express the data in terms of a true stress-true strain relation (flow curve) in preference to the more familiar but less fundamental engineering stress-strain diagram. This is possible on the assumption that the flow curve approximates a straight line between the points of maximum load and fracture and that the incidence of maximum load corresponds to the beginning of necking (the limit of uniform elongation). True stress and true strain may then be expressed as:** a = 5(1 + £ ) , * = log, (1 + E) up to and including the limit of uniform elongation, and subsequently as a = s (Ao/A)t

Comparison of Propeller and Reaction-Propelled Airplane Performances

Abstract: COINCIDENT WITH the development of successful turbojet power plants for aircraft, a broad new field of propulsion has come to light. Power-plant research and development has progressed so rapidly tha t a wide choice of power plants is now available to the aircraft manufacturer, who as yet has had little opportunity to demonstrate the practical applications. The purpose of this paper is to evaluate the airplane-performance potentialities of four types of widely differing power plants and to indicate their trends and particular fields of application. Conventional power plants of the reciprocating, in.ternal-combustion type, both direct and indirect aircooled, have been extended to compounding with a gasturbine wheel operated by the normal exhaust gases and delivering the power thus generated back into the crankshaft. The independent gas turbine is also available for use in conjunction with the propeller, in which case approximately 80 per cent of the energy in the gases is absorbed by the turbine to drive the propeller and the remainder is utilized in the form of reaction propulsion. Air-stream engines include the turbojet, reso-jet (or intermit tent duct)., and the ram-jet (or athodyd), all of which ar.e reaction motors depending upon atmospheric air supply. A third classification of available power plants consists of dry or liquid-fuel types of rocket motors, which are distinguished principally by the fact t ha t atmospheric oxygen is not used for combustion as in the case of other power plants. For this paper four power plants having distinctly different performance characteristics have been chosen : (1) a V-type, reciprocating, liquid-cooled engine employing water injection for emergency power; (2) a turbojet engine having about 23 per cent less sealevel stat ic thrus t than the former; (3) a hypothetical subsonic ram-jet; and (4) a bipropellant liquid-fuel rocket motor. For brevity, these power plants will be referred to as propeller, turbojet, ram-jet and rocket, respectively. Because these power plants differ so radically in performance characteristics, it is difficult to determine a

Calculations of Flow of Air and Diatomic Gases

Abstract: Measurement of airflow is frequently required in connection with aeronautical projects. Conventional practice, as exemplified by the A.S.M.E. Test Code for Flow Measurement, provides two different formulas (or sets of formulas) for the calculation of flow from pressure and temperature measurements made with standard nozzles, orifices, and the like, the proper formula for use in any particular case depending upon whether the pressure ratio across the restriction is greater or less than the critical value. In most cases of aeronautical interest the formula for subcritical (or restricted) flow is applicable. However, the exact formula for this case is quite cumbersome, and often requires much laborious calculation. Without sacrifice of exactness, the much simpler formula for critical flow may be generalized and used in all cases merely by including a "restriction factor" that is a function of pressure ratio only, and so may be tabulated once and for all. This paper proposes such a formula and presents a table of restriction factors applicable to air and other diatomic gases.

Influence of Different Factors on Buckling Loads of Curved Thin Aluminum-Alloy Sheets for Monocoque Constructions

Abstract: Aerodynamic requirements of smooth surfaces and fair contours for both wings and fuselage have made aeronautical engineers conscious of the buckling problems associated with the design of semimonocoque (sheet-stringer) construction. This paper discusses the fundamental principles involved in increasing the buckling strength of the sheet. Test results show a 50 to 70 per cent increase in ultimate buckling load obtained by changes in the configuration of the sheet, which resulted in more load being carried in the supported edges and less in the unsupported central region. Although several suggestions are made, the practical method of achieving the desired results is left to the ingenuity of the designer. As a sidelight, these tests emphasize the necessity for well-controlled end and side conditions if consistent results are to be obtained in panel testing.

Jet Propulsion Applied to Helicopter Rotors

Abstract: Some of the factors affecting jet-driven rotor performance are discussed and it is shown that the relatively high tip speeds required in order to realize a reasonable jet efficiency result in a considerable reduction in the aerodynamic efficiency of the rotor. Although this aerodynamic efficiency may be improved by decreasing the rotor solidity, a practical limit is reached, in the case of jet drives involving airflow through the blades, beyond which the specific fuel consumption increases almost asymptotically with decrease in solidity. However, the weight-saving possible with such drives, and their simplicity, may in certain cases offset their low aerodynamic and propulsive efficiencies. A rough comparative performance and weight estimate are made between conventional and jet-driven helicopters in an at tempt to evaluate their relative merits.