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Aeronautical Engineering and Airplane Design

Preface

This work, practically a course in aerodynamics and airplane design, is subdivided into two parts: Part I, Aerodynamical Theory and Data; Part II, Airplane Design.

In PART I it is proposed to deal briefly with the fundamental ideas and theories of aerodynamics in a simple yet comprehensive manner.

It is important for the aeronautical engineer and for every student of aerodynamics to have at his disposal exact definitions of such terms as lift, drag or resistance, center of pressure, wing cord, angle of incidence, and other well known expressions.

Although the exact nature of viscosity, skin friction, eddying or density resistance, stream line flow, turbulent flow, the sustaining action of cambered wing surfaces, and the principles of comparison for forces on bodies of varying dimensions still present many difficulties, it is hoped to give a simple and, above all, practical sum- mary of these points. The more difficult theoretical demonstrations will be reserved for special articles.

The authors propose also to give a brief description of the chief aerodynamical laboratories and of experimental methods there employed. Without a knowledge of such methods, appreciation, and application of the laboratory data available is certainly not easy.

Considering the comparatively recent growth of aerodynamics, the amount of material now available is extraordinary. It is unfortunately scattered through a variety of publications; English, French, German, Russian and Italian, presented in varying ways and in varying systems of units. Nor is all of it entirely worthy of credence.

In this course it has been attempted to reduce this material, particularly that of English and French origin, to one system of presentation with forces measured in pounds, areas in square feet and velocities in miles per hour or feet per second, so as to be more readily applicable in American design ; to include all the material which is trustworthy and of immediate and pressing utility to the designer, in carefully classified form.

The Economic Laws of Flight will be fully dealt with, in horizontal and ascension flight. The consideration of the performance curves of a machine will be particularly useful to those engineers and students to whom the subject is comparatively new.

Throughout, illustrative problems will be worked out on important points, especially to facilitate comparison between wing sections.

PART II will include a discussion of available aeronautical materials, timber, steel, alloys, rubber, etc. — with trustworthy values for stresses; a variety of diagrams and scale drawings representative of modern design, and a classification of the most important modern machines, with their main data.

At this stage of the art, it is impossible to say that any method in design is standard, but a systematic procedure of design will be fully developed. Particular stress is laid on the evaluation of factors of safety. The dynamic factor of safety, the material factor of safety, the worst loading possible in the air, the worst possible shock on landing ; nothing offers so many possibilities of confusion and untrust worthiness ; and nothing is in more need of definite and accurate statement. Complete strength calculations will be presented for body, chassis, wing girders, and controlling surfaces, and the design of a standard machine will be carried through, with consideration of motor and propeller problems, weight distribution and balancing. Throughout the course, the most elementary mathematics are employed, and nothing beyond a knowledge of the first mechanical principles is presupposed.

It is hoped, therefore, that the course will be easily understood by any engineer or student approaching the serious study of the airplane for the first time. At the same time it is felt that much will be of service even to the experiential aeronautical engineer.

TOC

PART ONE
Aerodynamical Theory and Data

CHAPTER 1
Modern Aerodynamical Laboratories
Early Experimental Aerodynamics — General Requirements in Airplane Design — Difficulties of Full Scale
Experiments — Towing Methods — Wind Tunnel Methods — Laboratories of the Wind Tunnel Type 15

CHAPTER II
Elements op Aerodynamical Theory
Liquid, Fluid and Perfect Fluid — Density of Air — Variation of Density of Air with Height — Principle of Relative Motion — Bernouilli's Theorem for Fluid Motion — Total Energy of a Fluid Applied to the Theory of the Pitot Tube — Definition of Angle of Incidence. Resultant Pressure, Lift, Drag, and Center of Pressure in a Plane or Cambered Wing Section — Definition of Lift and Drag Coefficients — Position of Center of Pressure or Resultant Vector of Forces — Forces on a Flat Plate Immersed in a Fluid and Normal to the Direction of Motion — Forces on Flat Plates Inclined to the Wind 23

CHAPTER III Elements op Aerodynamical Theory — Continued Skin Friction — Viscosity — Coefficients of Kinematic Viscosity — Reynolds' Number — Prandtl's Theory of the Boundary Layer — Density Resistance to a Plate Moving Edgewise — Total Skin Friction; Dr. Zahm's Experiments — Curves for Computaions with Dr. Zahm's Formula — Turbulent Flow, Eddy, or Density Resistance — Comparison of Forces Acting Upon Similar Bodies; the Importance of Kinematic Viscosity and Reynolds' Number — Stream Line Bodies — Energy Considerations for a Perfect Fluid Flowing Past a Stream Line Body — Stream Line Bodies in a Viscous Fluid — Resistance of Wires, Cables, and Cylin- ders — Fluid Motion Around Wing Surface 27

CHAPTER IV
Flat Plates. Simple Problems on Sustention and Resistance op Wing Surfaces

Coefficients of Resistance for Circular or Square Plates Normal to the Wind ; Varying Sizes — Coefficients for Rectangular Flat Plates Normal to the Wind; Varying Aspect Ratio — Coefficients for Flat Plates In- clined to the Wind — Preliminary Application of Data for Flat Plates in Rudder and Elevator Design — Problems on Flat Plates — General Considerations of Sustaining Power and Resistance of Wing Sections — Problem of Sustention and Resistance of Wing Surface 32

CHAPTER V
Comparison op Standard Wing Sections

Representative Wing Sections Selected — Complete Data Presented — Points of Interest in Considering a Wing

Section — Consideration of a Few Sections in Common Use 37

CHAPTER VI
Effects op Variations in Profile and Plan Form op Wing Sections

Effect of Variation of Position of Maximum Ordinate in a Wing Section of Plane Lower Surface, and Con- stant Camber 0.100 for Upper Surface — Behavior of Wings with Reverse Curvature at the Trailing Edge — Effect of Thickening the Leading Edge of a Wing — Effects of Thickening Wing Towards the Trailing Edge— " Phillips Entry "— Effects of Varying Aspect Ratio— Choice of Aspect Ratio— Effects of Raking the Plan Form of a Wing-Swept Back Winys — Negative Wings Tips of Swept Back Wings ; Effect on Longitudinal Stability 41

CHAPTER VII
Study op Pressure Distribution

Methods of Obtaining Pressure Distribution — Comparison of Results from Pressure Distribution and from Force Experiments — Effect of Variation of Speed and Scale on Lift and Drag Coefficients — Distribution of Pressure at Median Cross Section of Various Surfaces — Distribution of Pressure Over the Entire Surface of a Wing; Lateral Plow, Its Bearing on Aspect Ratio — Distribution of Pressure Over the Entire Surface of Wing and Curves of Equi-Pressure — Relative Importance and Interdependence of Two Surfaces — Distribution of Pressure; the Principle of the Dipping Front Edge; Why a Wing Sec- tion Is Advantageous as Compared with a Flat Plate 46

CHAPTER VIII
Biplane Combinations

Orthogonal Biplane Arrangements with Varying Gap Between Planes — Distribution of Forces Between the Upper and Lower Wings of a Biplane — Distinction Between Static and Dynamic Stability — Stable Biplane Arrangements — Results of Experiments on Biplanes with Stagger and Decalage — Comparison of Aerodynamical Losses Involved in Obtaining Stability by Reversed Curvature Wings and by Stag- ger; Decalage Combinations — Aerodynamic Comparison Between the Monoplane and the Biplane 51

CHAPTER IX
Triplane Combinations — Uses of Negative Tail Surfaces

Interference in Triplanes — Some Considerations for Tri planes — Triplanes for Fast Speed Scouts — Use of Negative Tail Surfaces — Effect of Influence of the Wash of the Wings on Stabilizer Surface — Problem on the Design of Tail Surfaces to Oive Longitudinal Static Stability 57

CHAPTER X
Resistance of Various Airplane Parts

Airplane Bodies from the Aerodynamical Point of View — Tractor Bodies — Pusher Bodies — Radiator Resist- ance — Resistance of Fittings — Resistance of Airplane Wheels — Resistance of Wires and Methods of Plot- . ting — Resistance of Stationary Smooth Wires — Resistance of Vibrating Wires — Resistance of Stranded Wires — Resistance of Wires Placed Behind One Another — Resistance of Inclined Wires — Suggestions for Stream-Lining Wires — Resistance of Miscellaneous Objects 61

CHAPTER XI
Resistance and Comparative Merits of Airplane Struts

Considerations of Comparative Merit of Strut Sections — Strut Sections Developed by Ogilvie — Another Series of Struts Tested at the N. P. L.— Tests on Struts, Length to Width Varied— Two Eiffel Struts- Effect of Length of Struts — Resistance of Inclined Struts— The Effect of Changing the DV Product for Struts 65

CHAPTER XII
Resistance and Performance

Nomenclature — Structural and Wing Resistance for British B.E.2 — Theoretical Laws for Minimum Thrust and Minimum Horsepower — Effective or Propeller Horsepower Available Curve — Minimum and Maximum Speed; Maximum Excess Power; Best Climb; Descent — The Two Regions of Control; Control by Throt- tling — Variations in Propeller Horsepower Curves — Angle of Glide 69

CHAPTER XIII
Resistance Computations — Preliminary Wing Selections

Example of Estimate for Parasite Resistance for a British Machine — Examples of Parasite Resistance Distri- bution in School Machines — Parasite Resistance Coefficient for a Sturtevant Seaplane — Allowance for Slip Stream — Preliminary Estimates for Parasite Resistance — Preliminary Selection of Wing Section and Area 72

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