Aircraft Structures & Design Top-Notch Notes 2023 PDF

Top-Notch Aeronautics Top-Notch Notes Aircraft Structures & Design Copyright Notice The notes and discussion, except the materials from other sources, are copyright of Top-Notch Aer...

Top-Notch Aeronautics Top-Notch Notes Aircraft Structures & Design Copyright Notice The notes and discussion, except the materials from other sources, are copyright of Top-Notch Aeronautics. Unpublished Work © 2023 Top-Notch Aeronautics. All rights reserved. Any redistribution or reproduction of part or all of the contents in any form is prohibited other than the following: you may print or download to a local hard disk extracts for your personal and non-commercial use only you may copy the content to individual third parties for their personal use, but only if you acknowledge the author as the source of the material You may not, except with the express written permission, distribute or commercially exploit the content. Nor may you transmit it or store it in any other website or other form of electronic retrieval system. Engr. Venice D. Portes LECTURER  Top-Notch Notes in Aircraft Structures & Design Brief Summary of Mechanics & The forces are in equilibrium only Strength of Materials when equal in magnitude, opposite in direction, and collinear in action. A set of forces in equilibrium may be added to any system of forces Fundamental Concepts without changing the effect of the original system. Action and reaction forces are Statistics – a branch of mechanics which equal but oppositely directed. studies the effects and distribution of forces of rigid bodies which are and remain at Structural System – Any deformable body rest. In this area of mechanics, the body in which is capable of carrying loads and which forces are acting is assumed to be transmitting these loads to other parts of rigid. the body. The constituents of such a system are beams, plates, shells, or a combination The deformation of non-rigid bodies of the three. is treated in Strength of Materials. Bar Elements – One-dimensional structural Rigid Body – can be considered as a large members which are capable of carrying or number of particles in which all the transmitting bending, shearing, torsional, particles remain at a fixed distance from and axial loads or a combination of all one another, both before and after four. applying a load. Two-Force Members (Axial Rods) – Non-Rigid Body – Any force applied will Bars which are capable of carrying distort the shape and/or size of the body. only axial loads. Principle of Transmissibility – A force may be moved anywhere along its line of action without changing its external effect on a rigid body. Axioms of Mechanics Trusses – Structural members composed entirely of out of axial rods. The resultant of two forces is the diagonal of the parallelogram formed on the vectors these forces. (Parallelogram Law) Unpublished Work © 2023 Top-Notch Aeronautics 1|Page  Top-Notch Notes in Aircraft Structures & Design Plate Elements – Two-dimensional extension of bar elements subjected to biaxial in- plane loading. Membranes - Plates made to carry only in-plane axial loads. Shear Panels - Those which are capable of carrying only in-plane shearing loads. Surface Loads – Loads which are produced by surface contacts. (e.g. static & dynamic pressures) Shells – Curved plate elements which occupy a space. Fuselage, building domes, pressure vessels, etc. ate typical examples of shells. Load Classification Distributed Load spread out over a large area can be uniformly or non-uniformly distributed Body loads – Loads which depend on Concentrated Load body volume. (e.g. inertial, magnetic, & can be applied at more than one gravitational forces) location on a beam multiple loading points may exist on a single beam Unpublished Work © 2023 Top-Notch Aeronautics 2|Page  Top-Notch Notes in Aircraft Structures & Design Static Loads – Loads that are either Internal Reactions constant or applied over a long period of time. Independent of time. All dead loads are static loads. Dynamics Loads – Loads that are variable and applied over a short period of time. Time-dependent. All live loads are dynamic loads. Support Reactions Hinge Support Thermal Loads – Loads created on a restrained structure by a uniform and/or non-uniform temperature change. Hinge-Roller Support Unpublished Work © 2023 Top-Notch Aeronautics 3|Page  Top-Notch Notes in Aircraft Structures & Design Fixed Support Centroid It is the point at which area (or volume or line) can be concentrated. It is the point at which the static moment is zero. Fixed-Roller Support For corresponding geometric shapes, Determinate vs Indeterminate Statically Determinate Structures – External reactions can be obtained by utilizing only the static equations of equilibrium. Statically Indeterminate Structures – External reactions cannot be obtained by utilizing only the static equations of equilibrium Section Properties Area Unpublished Work © 2023 Top-Notch Aeronautics 4|Page  Top-Notch Notes in Aircraft Structures & Design Center of Gravity Moment of Inertia (Second Moment of Area) Radius of Gyration Polar Moment of Inertia Centroidal Moment of Inertia (with respect to an axis passing through the centroid): Unpublished Work © 2023 Top-Notch Aeronautics 5|Page  Top-Notch Notes in Aircraft Structures & Design Generally shows up in two places: Column – A member which is loaded so that it is compressed axially and develops Strength of Materials compressive stress. In aircraft construction, common types of columns used are round or streamline shaped tubes; and some are built up from sheet metal. Mechanics Tie – A member subjected to tension load only. In aircraft construction, ties are round, square and streamline tubes, wires, tie rods, and cables. Parallel Axis Theorem Truss Stability – A truss is said to be stable if it is externally and internally stable. External Stability – All the reactions are not parallel to each other; and all the reactions are not concurrent. Internal Stability Note: m = number of members, J = number of joints, R = number of unknown reactions. Truss Support Truss Analysis Truss Structure – It is a structure which is composed entirely of two-force members. It is a framed or jointed structure made up of columns and ties, the whole structure being designed to act a beam. The members of the truss form a series of rigid triangles or frames. Unpublished Work © 2023 Top-Notch Aeronautics 6|Page  Top-Notch Notes in Aircraft Structures & Design Truss Loads Simple Stress Applied forces and moments Normal Stress – will occur to members that Reactions at the supports are axially loaded which could be either: Axial loads in truss members Tensile Stress – members subject to Method of Joints pure tension (or tensile force) Compressive Stress - members subject to compressive force Shear Stress – caused by forces parallel to the area resisting the force (also known as tangential stress). Method of Sections Thin-walled Pressure Vessels – A tank or pipe carrying a fluid or gas under a pressure is subjected to tensile forces, which resist bursting, developed across longitudinal and transverse sections. Force Systems in Space The equilibrium of any free body in space is defined by six equations: Strain Simple Strain – Also known as unit deformation, strain is the ratio of the change in length caused by the applied force, to the original length. Unpublished Work © 2023 Top-Notch Aeronautics 7|Page  Top-Notch Notes in Aircraft Structures & Design Axial Deformation the load must be axial the bar must have a uniform cross- sectional area the stress must not exceed the Stress-Strain Diagram proportional limit. Proportional Limit (Hooke’s Law) – within the proportional limit, stress is directly proportional to strain. Elastic Limit - the limit beyond which the material will no longer go back to its original shape when the load is removed. It is also the maximum stress that may be developed such that there is no permanent or residual deformation when the load is entirely removed. Yield Point - the point at which the material will have an appreciable elongation or yielding without any increase in load. Ultimate Strength - The maximum ordinate in the stress-strain diagram is the ultimate strength or tensile strength. For a rod of unit mass ρ suspended Rapture Strength – the strength of the vertically from one end, the total material at rupture (also known as elongation due to its own weight is the breaking strength). Stiffness – the ratio of the steady force acting on an elastic body to the resulting displacement. It has the unit of N/mm. Unpublished Work © 2023 Top-Notch Aeronautics 8|Page  Top-Notch Notes in Aircraft Structures & Design Shearing Deformation – An element subject Biaxial Deformation to shear does not change in length but undergoes a change in shape. Triaxial Deformation Shear Strain – the change in angle at the corner of an original rectangular element. Modulus of Rigidity – the ratio of the shear stress τ and the shear strain γ Relationship Between E, G, and ν (also called the shear modulus of The relationship between modulus of elasticity). elasticity E, shear modulus G and Poisson's ratio ν is: The relationship between the shearing deformation and the applied shearing force is Bulk Modulus of Elasticity (K) – is a measure of a resistance of a material to change in volume without change in shape or form (also called Modulus of Volume Expansion). Poisson’s Ratio – ratio of the sidewise deformation to the longitudinal deformation. Thermal Stress – the internal stress created that if temperature deformation is permitted to occur freely, no load or stress will be induced in the structure. Unpublished Work © 2023 Top-Notch Aeronautics 9|Page  Top-Notch Notes in Aircraft Structures & Design deformation due to temperature Angle of Twist changes: deformation due to equivalent axial stress: Power Transmitted by the Shaft thermal stress: KEY CONCEPTS IN AIRCRAFT If the wall yields a distance of x as STRUCTURES shown, the following calculations will be made: TORSION Torsional Shearing Stress The airframe is the basic structure of an aircraft, design to withstand aerodynamic forces and stresses imposed. Functions: For solid cylindrical shaft: the structures of most flight vehicles are thin walled structures (shells) resists applied loads o aerodynamic loads acting on the wing structure provides the aerodynamic shape protects the contents from the For hollow cylindrical shaft: environment Aircraft structures are generally classified as follows in terms of criticality of the structure: critical structure, whose integrity is essential in maintaining the overall flight safety of the aircraft; Unpublished Work © 2023 Top-Notch Aeronautics 10 | P a g e  Top-Notch Notes in Aircraft Structures & Design primary structure carries flight, Monocoque (French for "single shell") ground, or pressurization loads, and construction uses stressed skin to support whose failure would reduce the almost all loads much like an aluminum aircraft’s structural integrity; beverage can. These are unstiffened shells o if this structure is severely that must be relatively thick to resist damaged, the aircraft cannot bending, compressive, as torsional loads. fly. Semi-Monocoque secondary structure that, if it was to fail, would affect the operation of the aircraft but not lead to its loss; and o mainly to provide enhanced aerodynamics (e.g. fairings) tertiary structure, in which failure would not significantly affect operation of the aircraft. Truss type Constructions with stiffening members that may also be required to diffuse concentrated loads into the cover. It is a more efficient type that permits much thinner covering shell. WING STRUCTURE In this construction method, strength and The principal structural parts of the wing rigidity are obtained by joining tubing (steel are spars, ribs, and stringers. or aluminum) to produce a series of These are reinforced by trusses, I- triangular shapes, called trusses. beams, tubing, or other devices, Monocoque including the skin. The wing ribs determine the shape and thickness of the wing (airfoil). In most modern airplanes, the fuel tanks either are an integral part of the wing’s structure, or consist of flexible containers mounted inside of the wing. Ailerons (French for "little wing") extend from about the midpoint of each wing outward toward the tip, and move in Unpublished Work © 2023 Top-Notch Aeronautics 11 | P a g e  Top-Notch Notes in Aircraft Structures & Design opposite directions to create aerodynamic PART FUNCTION OF AIRCRAFT STRUCTURES forces that cause the airplane to roll. Skin Flaps extend outward from the fuselage to reacts the applied torsion and shear near the midpoint of each wing. The flaps forces are normally flush with the wing’s surface transmits aerodynamic forces to the during cruising flight. When extended, the longitudinal and transverse flaps move simultaneously downward to supporting members increase the lifting force of the wing for acts with the longitudinal members takeoffs and landings. in resisting applied bending and Skin axial loads acts with the transverse members in reacts the applied torsion and shear reacting the hoop, or forces circumferential, load when the transmits aerodynamic forces to the structure is pressurized longitudinal and transverse supporting members Ribs and Frames acts with the longitudinal members structural integration of the wing & in resisting applied bending and fuselage axial loads keep the wing in its aerodynamic acts with the transverse members in profile reacting the hoop, or circumferential, load when the Spar structure is pressurized resist bending and axial loads Ribs and Frames form the wing box for stable torsion resistance structural integration of the wing & fuselage Stiffeners or Stringers keep the wing in its aerodynamic resist bending and axial loads along profile the wing skin Spar divide the skin into small panels and thereby increase its buckling and resist bending and axial loads failing stresses form the wing box for stable torsion act with the skin in resisting axial resistance loads caused by pressurization Stiffener or Stringers NOTE: resist bending and axial loads along The webs (skin and spar webs) carry the wing skin only shearing stresses. divide the skin into small panels and The longitudinal elements carry only thereby increase its buckling and axial stresses. failing stresses The transverse frames and ribs are rigid act with the skin in resisting axial loads caused by pressurization Unpublished Work © 2023 Top-Notch Aeronautics 12 | P a g e  Top-Notch Notes in Aircraft Structures & Design EMPENNAGE STRUCTURE The empennage includes the entire tail group which consists of fixed surfaces such as the vertical fin or stabilizer and the horizontal stabilizer; the movable surfaces including the rudder and rudder trim tabs, as well as the elevator and elevator trim tabs. Stabilator o A type of empennage design that does not require an elevator. Instead, it incorporates a one-piece horizontal stabilizer that pivots from a central hinge point. It is moved using the control wheel, just as the Pylon elevator is moved. o The anti-servo tab also functions as a → connects the engine to the airframe of trim tab to relieve control pressures an aircraft and helps maintain the stabilator in → design uses air passing through the the desired position. pylon to actively disrupt the jet engine exhaust stream after it exits the engine, LANDING GEAR STRUCTURE disrupting and redistributing the axial The purpose of the landing gear in an and azimuthal distributed sources of jet aircraft is to provide a suspension system noise from the aircraft. during taxi, take-off and landing. It is Nacelle designed to absorb and dissipate the kinetic energy of landing impact, thereby → aerodynamic housings distinct from the reducing the impact loads transmitted to fuselage that surround exterior the airframe. components on an aircraft. → most commonly protect instruments POWERPLANT STRUCTURE and equipment located along an → On single engine airplanes, the engine is aircraft's wingspan, such as engines, usually attached to the front of the fuel tanks, or weapons. fuselage. → serve a variety of purposes including → There is a fireproof partition between drag reduction and directing airflow for the rear of the engine and the cockpit the purposes of engine cooling and use or cabin to protect the pilot and in the combustion reactions inside passengers from accidental engine engines. fires. This partition is called a firewall and → commonly include the following is usually made of ahigh heat resistant, components: stainless steel. o Engine cowlings such as inlet and fan cowls are designed to protect aircraft engines and reduce parasitic Unpublished Work © 2023 Top-Notch Aeronautics 13 | P a g e  Top-Notch Notes in Aircraft Structures & Design drag, including form drag and skin o Large civil and military aircraft – friction drag pressurized cabins for high altitude o Thrust reversers are mechanisms that flying serve to redirect some of the air o Amphibious aircraft – landing on flowing out of the engine to the sides water and partially forward, resulting in a o Aircraft designed to fly at high net flow that creates significant speed at low altitude – structure of drag, slowing the aircraft’s forward above average strength to motion, and aiding in braking withstand the effects of flight in procedures. extremely turbulent air o The exhaust system including the exhaust cone and exhaust nozzle typically are covered with nacelle. STRUCTURAL COMPONENTS OF AIRCRAFT → Air loads are the resultants of the LOADS ON STRUCTURAL COMPONENTS pressure distribution over the surfaces of → The structure of an aircraft is required to the skin produced by steady flight, support two distinct classes of load: maneuver or gust conditions. o Ground loads – Includes all loads → Wings, tailplane and the fuselage are encountered by the aircraft during each subjected to direct, bending, movement or transportation on the shear and torsional loads and must be ground such as taxiing and landing designed to withstand critical loads, towing and hoisting loads combinations of these. o Air loads – comprises loads imposed on the structure during flight by maneuvers and gusts. → The two classes of loads may be further divided into: o Surface forces – act upon the surface of the structure (e.g. aerodynamic pressure) o Body forces – act over the volume of the structure and are produced by gravitational and inertial effects → Aircraft designed for a particular role encounter loads peculiar to their sphere of operation: o Carrier born aircraft – catapult take- off and arrested landing loads Unpublished Work © 2023 Top-Notch Aeronautics 14 | P a g e  Top-Notch Notes in Aircraft Structures & Design → The force on an aerodynamic surface (wing, vertical or horizontal tail) result from a differential pressure distribution caused by incidence, camber or a combination of both. → The position of the center of pressure (CP) changes as the pressure distribution varies with speed or wing incidence. → The aerodynamic center (AC) is the → To produce minimum loads on the wing point in the airfoil section about which structure during ground maneuvers, the the moment due to the lift and drag undercarriage must be located just forces remains constant. forward of the flexural axis of the wing → At High Mach numbers, the position of and as close to the wing root as the AC changes due to compressibility possible. effects. → The flexural axis is a line or locus of point along which the streamwise or chordwise angle of attack does not change when a discrete load is applied there. → Other loads include engine thrust on the wings or fuselage which acts in the plane of symmetry but may, in the case of engine failure, cause severe fuselage bending moments. → While the chordwise pressure distribution fixes the position of the resultant aerodynamic load in the wing cross- section, the spanwise pressure distribution locates its position in relation to the wing root. → A typical distribution for a wing/fuselage combination is shown to the right. Similar distributions occur on horizontal FUNCTION OF STRUCTURAL COMPONENTS and vertical tail surfaces. → Ground loads encountered in landing → The basic functions of an aircraft’s and taxiing subject the aircraft to structure are: concentrated shock loads through the o to transmit and resist the applied undercarriage system. loads → The majority of aircraft have their main o to provide an aerodynamic shape undercarriage located in the wings, o to protect passengers, payload, with a nosewheel or tailwheel in the systems, etc. from the environmental vertical plane of symmetry. conditions encountered in flight Unpublished Work © 2023 Top-Notch Aeronautics 15 | P a g e  Top-Notch Notes in Aircraft Structures & Design → Semi-monocoque – thin shell structures → The shape of the fuselage cross-section is where the outer surface or skin of the determined by operational shell is usually supported by longitudinal requirements. stiffening members and transverse → Transverse frames which extend frames to enable it to resist bending, completely across the fuselage are compressive and torsional loads without known as bulkheads. buckling. FABRICATION OF STRUCTURAL → Monocoque – thin shells which rely COMPONENTS entirely on their skins for their capacity to resist loads. → For purposes of construction, aircraft are → The primary function of wing ribs is to divided into a number of sub- form an impermeable surface for assemblies. These are built in specially supporting the aerodynamic pressure designed jigs, possibly in different parts distribution from which the lifting of the factory or even different capability of the wing is derived. factories, before being forwarded to → Aerodynamic forces are transmitted in the final assembly shop. turn to the ribs and stringers by the skin → Each sub-assembly relies on numerous through plate and membrane action. minor assemblies such as spar webs, → Resistance to shear and torsional loads is ribs, frames, and these, in turn, are supplied by shear stresses developed in supplied with individual components the skin and spar webs. from the detail workshop. → Axial and bending loads are reacted by → In general, spars comprise thin the combined action of skin and aluminum alloy webs and flanges, the stringers. latter being extruded or machined and → These conditions apply to all are bolted or riveted to the web. aerodynamic surfaces such as the horizontal and vertical tails, except in cases of undercarriage loading, engine thrust, etc. → Fuselages comprise members which perform similar functions to their counterparts in the wings and tailplane. → Aerodynamic forces on the fuselage skin are relatively low but supports large concentrated loads such as wing reactions, tailplane reactions, → The ribs are formed in three parts from undercarriage reactions. sheet metal by large presses and rubber → It carries payloads of varying size and dies and have flanges round their weight, which may cause large inertia edges so that they can be riveted to forces. the skin and spar webs; cut-outs around → Aircraft designed for high altitude flight their edges allow the passage of must withstand internal pressure. spanwise stringers. → Holes are cut in the ribs at positions of low stress for lightness and to Unpublished Work © 2023 Top-Notch Aeronautics 16 | P a g e  Top-Notch Notes in Aircraft Structures & Design accommodate control runs, fuel and PRINCIPLES OF STRESSED SKIN electrical systems. CONSTRUCTION PROPERTIES OF MATERIALS Ductility → A material is said to be ductile if it is capable of withstanding large strains under load. → These large strains are accompanied by a visible change in cross-sectional dimensions and therefore give warning of impending failure. Materials in this category include mild steel, aluminum and some of its alloys, copper and polymers. Brittleness → A brittle material exhibits little deformation before fracture, the strain normally being below 5%. Brittle → Countersunk rivets are used in the materials therefore may fail suddenly forward chordwise sections of the wing without visible warning. that should be as smooth as possible to → Included in this group are concrete, delay transition from laminar to cast iron, high strength steel, timber and turbulent flow. Dome-headed rivets are ceramics. used nearer the trailing edge. → Sandwich panel Elasticity o comprises a light honeycomb or → A material is said to be elastic if corrugated metal core sandwiched deformations disappear completely on between two outer skins of the stress removal of the load. bearing sheet → All known engineering materials are, in o possess a high resistance to fatigue addition, linearly elastic within certain from jet efflux limits of stress so that strain, within these o method of construction includes limits, is directly proportional to stress. lightweight ‘planks’ for cabin furniture, monolithic fairing shells Plasticity generally having plastic facing skins, → A material is perfectly plastic if no strain and the stiffening of flying control disappears after the removal of load. surfaces. Ductile materials are elastoplastic and o prone to disbonding and internal behave in anelastic manner until the corrosion elastic limit is reached after which they behave plastically. Unpublished Work © 2023 Top-Notch Aeronautics 17 | P a g e  Top-Notch Notes in Aircraft Structures & Design → When the stress is relieved the elastic resistance. Ultimately a flat disc is component of the strain is recovered produced. but the plastic strain remains as a o For design purposes the ultimate permanent set. stresses of mild steel intension and compression are assumed to be the Isotropic same. → In many materials the elastic properties o Higher grades of steel have greater are the same in all directions at each strengths than mild steel but are not point in the material although they may as ductile. vary from point to point, such a material is known as isotropic. → An isotropic material having the same properties at all points is known as homogeneous (e.g. mild steel). Anisotropic → Materials having varying elastic properties in different directions are known as anisotropic. Orthotropic → Aluminum → Although a structural material may o A feature of the fracture of possess different elastic properties in aluminum alloy test pieces is the different directions, this variation may formation of a ‘double cup’ implying be limited. A material whose elastic that failure was initiated in the properties are limited to three different central portion of the test piece values in three mutually perpendicular while the outer surfaces remained directions is known as orthotropic. intact. STRESS-STRAIN CURVES o In compression tests on aluminum and its ductile alloys similar difficulties → Low carbon steel (mild steel) are encountered to those o The behavior of mild steel in experienced with mild steel. compression is very similar to its o Aluminum and its alloys can suffer a behavior in tension, particularly in form of corrosion particularly in the the elastic range. salt laden atmosphere of coastal o In the plastic range it is not possible regions. The surface becomes pitted to obtain ultimate and fracture loads and covered by a white furry since, due to compression, the area deposit. This can be prevented by of cross-section increases as the load an electrolytic process called increases producing a barreling’ anodizing which covers the surface effect. This increase in cross-sectional with an inert coating. area tends to decrease the true o Aluminum alloys will also corrode if stress, thereby increasing the load they are placed in directcontact with other metals, such as steel. To Unpublished Work © 2023 Top-Notch Aeronautics 18 | P a g e  Top-Notch Notes in Aircraft Structures & Design prevent this, plastic is inserted STRAIN HARDENING between the possible areas of Although the ultimate stress is increased by contact. strain hardening it is not influenced to the same extent as yield stress. The increase in strength produced by strain hardening is accompanied by decreases in toughness and ductility. → These indicate that strain energy is lost during the cycle, the energy being dissipated in the form of heat. → Brittle Materials → This energy loss is known as mechanical o include cast iron, high strength steel, hysteresis and the loops as hysteresis concrete, timber, ceramics, glass, loops. etc. o The plastic range for brittle materials extends to only small values of strain. o The fracture of a cylindrical test piece takes the form of a single failure plane approximately perpendicular to the direction of loading with no visible ‘necking’ and an elongation of the order of 2–3%. o In compression the stress–strain curve for a brittle material is very similar to that in tension except that failure occurs at a much higher value of CREEP & RELAXATION stress. o This is thought to be due to the → A given load produces a calculable presence of microscopic cracks in value of stress in a structural member the material, giving rise to high stress and hence a corresponding value of concentrations which are more likely strain once the full value of the load is to have a greater effect in reducing transferred to the member. However, tensile strength than compressive after this initial or ‘instantaneous’ stress strength. and its corresponding value of strain have been attained, a great number of structural materials continue to deform slowly and progressively under load over a period of time. This behavior is known as creep. → Some materials, such as plastics and rubber, exhibit creep at room temperatures but most structural materials require high temperatures or Unpublished Work © 2023 Top-Notch Aeronautics 19 | P a g e  Top-Notch Notes in Aircraft Structures & Design long-duration loading at moderate AIRCRAFT DESIGN temperatures. → In some ‘soft’ metals, such as zinc and lead, creep occurs over a relatively short period of time, whereas materials such as concrete may be subject to creep over a period of years. → Creep occurs in steel to a slight extent at normal temperatures but becomes very important at temperatures above 316°C. → Closely related to creep is relaxation. Whereas creep involves an increase in Requirement: strain under constant stress, relaxation is the decrease in stress experienced over Size & Configuration a period of time by a material Range & Endurance subjected to a constant strain. Take-off & Landing Distances Flight envelope (load factor, stalling FATIGUE velocity, maximum → A condition where a structural member velocity, etc.) may fracture at a level of stress Rate of climb & descent substantially below the ultimate stress Absolute & Service Ceiling for non-repetitive static loads. Cost → Fatigue cracks are most frequently Reliability & maintainability initiated at sections in a structural PHASES OF AIRCRAFT DESIGN member with stress concentrations. o Changes in geometry, e.g. holes, notches or sudden changes in section. o Designers seek to eliminate such areas by ensuring that rapid changes in section are as smooth as possible. (e.g. fillets) References: Aircraft Structures by David J. Peery and J.J. Azar Aircraft Structures by T.H.G. Megson Conceptual Design Aircraft Design – A Conceptual It is in the conceptual design that the basic Approach by Daniel P. Raymer questions of configuration arrangement, JAA ATPL Books (Oxford) size, and weight, and performance are answered. Unpublished Work © 2023 Top-Notch Aeronautics 20 | P a g e  Top-Notch Notes in Aircraft Structures & Design The first question is, “Can an affordable OVERVIEW OF THE CONCEPTUAL DESIGN aircraft be built that meets the PROCESS requirements?” If not, the customer may wish to relax the requirements. → The actual design effort usually begins Conceptual design is a very fluid process. with a conceptual sketch. This is the New ideas and problems emerge as a "back of a napkin" drawing of design is investigated in ever-increasing aerospace legend, and gives a rough detail. Each time the latest design is indication of what the design may look analyzed and sized, it must be redrawn to like. A good conceptual sketch will reflect the new gross weight, fuel weight, include the approximate of the ff: wing size, engine size, and other changes. o wing and tail geometries Preliminary Design o fuselage shape o internal locations of the major During preliminary design, the specialists in components areas such as structures, landing gear, and ▪ engine control systems will design and analyze ▪ cockpit their portion of the aircraft. Testing is ▪ payload/passenger initiated in areas such as aerodynamics, compartment propulsion, structures, and stability and ▪ landing gear control. A mockup may be constructed at ▪ fuel tanks this point. → First-order sizing provides the A key activity during this phase is lofting, information needed to develop an the mathematical modeling of the outside initial design layout. This is a three-view skin of the aircraft with sufficient accuracy drawing complete with the more to insure proper fit between its different important internal arrangement details. parts. Enough cross-sections are shown to verify that everything fits. The ultimate objective during preliminary design is to ready the company for the SIZING: INITIAL WEIGHT ESTIMATE detail design stage, also called full-scale → Design takeoff gross weight is the total development. weight of the aircraft as it begins the Detail Design mission for which it was designed. This is not necessarily the same as the During this phase, specialists determine "maximum takeoff weight." Many how the aircraft will be fabricated, starting military aircraft can be overloaded with the smallest and simplest beyond design weight but will suffer a subassemblies and building up to the final reduced maneuverability. assembly process. Actual structure of the aircraft is fabricated and tested. → Empty-weight fraction (We/W0) can be estimated statistically from historical Unpublished Work © 2023 Top-Notch Aeronautics 21 | P a g e  Top-Notch Notes in Aircraft Structures & Design trends as shown in the figure. Empty- → Thickness ratio (t/c) – maximum weight fractions vary from about 0.3 to thickness of the airfoil divided by its 0.7, and diminish with increasing total chord. aircraft weight. → Airfoil lift and drag → Fuel-weight fraction estimation: Note: 2-D airfoil characteristics are denoted by lowercase subscripts Note: whereas the 3-D wing characteristics Wx/W0 = total mission weight fraction (by are denoted by uppercase subscripts. multiplication) → The point about which the pitching AIRFOIL & GEOMETRY SELECTION moment remains constant for any angle of attack is called the aerodynamic center. The aerodynamic center is not the same as the airfoil's center of pressure (or lift). o The center of pressure is usually behind the aerodynamic center. o The location of the center of pressure varies with angle of attack for most airfoils. o Pitching moment is measured about some reference point. ▪ Subsonic airfoil – 25% of the chord → Camber – curvature characteristics of length back from the leading most airfoil. edge. → Mean camber line – an imaginary line ▪ Supersonic airfoil – 40-50% of the which lies halfway between the upper chord length back from the surface and lower surface of the airfoil leading edge. and intersects the chord line at the → NACA Airfoils leading and trailing edges. o 4-Digit Airfoils: NACA 4412 Unpublished Work © 2023 Top-Notch Aeronautics 22 | P a g e  Top-Notch Notes in Aircraft Structures & Design 4 = Camber 0.04c best lift-to-drag (L/D) ratio. It is the point 4 = Position of Camber at 0.4c at L.E. in the airfoil drag polar that is tangent to 12 = Maximum Thickness 0.12c a line from origin and closest to the o 5-Digit Airfoils: NACA 23015 vertical axis. 2 = Camber 0.02 → Fat Airfoils (t/c > 14%) Design Cl = 0.15 x First Digit o Stall from the trailing edge 30 = Position of camber at c(0.30/2) o Turbulent boundary layer increases 15 = Maximum Thickness 0.15c with angle of attack o 6-Digit Airfoils: NACA 653-421 → Moderate Thick Airfoils (t/c = 6-14%) 6 = Series Designation o Flow Separates near the nose at a 5 = Minimum Pressure at 0.5c very small angle of attack, but 3 = Cd near minimum value over a reattaches itself so that little effect is range of Cl of 0.3 above and below felt the design Cl o At higher angle of attacks, the flow 4 = Design Lift Coefficient 0.4 fails to attach, which almost 21 = Maximum Thickness 0.21c immediately stalls the entire airfoil o 7-Digit Airfoils: NACA 747A315 → Very Thin Airfoils (t/c < 6%) 7 = Series Designation o The flow separates from the nose at 4 = Favorable pressure gradient on a small angle and reattaches almost the upper surface from L.E. to 0.4c at immediately the design Cl → Ways to improve stall characteristics 6 = Favorable pressure gradient on o Changing the angle of attack of the the lower surface from L.E. to 0.7c at wing tip airfoils or a different wing tip the design Cl airfoil. A = Serial letter to distinguish different o Stall characteristics for thinner airfoils sections having the same numerical can be improved with various designation but different mean line leading-edge devices such as slots, or thickness distribution slats, leading-edge flaps, and 3 = Design Lift Coefficient 0.3 Krueger flaps. 15 = Maximum Thickness 0.15c o Active methods (suction or blowing). → Supercritical airfoils are designed to → Wing structural weight varies delay and reduce transonic drag rise, approximately inversely with the square due to both strong normal shock and root of the thickness ratio. shock-induced boundary layer separation. Relative to conventional, supercritical airfoil has: o Reduced amount of camber o Increased leading edge radius o Small surface curvature on suction side o Concavity in rear part of pressure side → Design Lift Coefficient is the lift coefficient at which the airfoil has the Unpublished Work © 2023 Top-Notch Aeronautics 23 | P a g e  Top-Notch Notes in Aircraft Structures & Design WING GEOMETRY Quarter Chord Line Sweep → It is the sweep most related to subsonic flight. → The quarter chord (25%) line is used because subsonic lift due to angle of attack acts there and, up until the introduction of supercritical sections, the crest was usually close to the quarter chord. o The airfoil crest is the chordwise station at which the airfoil surface is tangent to the free-stream direction. Mean Aerodynamic Chord (MAC) → The entire wing has its mean aerodynamic center at approximately the same percent location of the mean aerodynamic chord as that of the airfoil alone. o In subsonic flow, the aerodynamic center is at the quarter-chord point on the mean aerodynamic chord. o In supersonic flow, the aerodynamic center moves back to about 40-50% of the mean aerodynamic chord. Leading Edge Sweep → The angle of concern in supersonic flight. This is important because the leading edge has to be behind the Mach cone to reduce wave drag. Unpublished Work © 2023 Top-Notch Aeronautics 24 | P a g e  Top-Notch Notes in Aircraft Structures & Design → The shape of the reference wing is determined by its aspect ratio, taper ratio, and sweep. Aspect Ratio Taper Ratio (λ) → The ratio between the tip chord and the centerline root chord. Wing Sweep → Primarily used to reduce the adverse effects of transonic and supersonic flow. → Oblique wings are wings with one wing swept aft and the other swept forward. Twist They tend to have lower wave drag. → Improves directional stability. → Geometric twist is the actual change in → Increases the effectiveness of vertical airfoil angle of incidence, usually tails at the wing tips. measured with respect to the root → Increases Critical Mach Number. airfoil. o the lowest Mach number at which o A wing whose tip airfoil is at a the airflow over any part of the negative (nose-down) angle aircraft reaches the speed of sound compared to the root airfoil is said to → Pitch-up is the highly undesirable have "wash-out”. tendency of some aircraft, where upon o If a wing has "linear twist", the twist reaching an angle of attack (AOA) angle changes in proportion to the near stall, to suddenly and distance. uncontrollably increase AOA. Unpublished Work © 2023 Top-Notch Aeronautics 25 | P a g e  Top-Notch Notes in Aircraft Structures & Design → Aerodynamic twist is the angle Dihedral between the zero-lift angle of an airfoil → It is the angle of the wing with respect and the zero-lift angle of the root airfoil. to the horizontal when seen from the o If the identical airfoil is used from root front. to tip, the aerodynamic twist is the o Tends to roll the aircraft level same as the geometric twist. whenever it is banked. o A wing with no geometric twist can o 10° of sweep provides about 1° of have aerodynamic twist if, for effective dihedral. example, the root airfoil is symmetric → Dutch Roll (zero-lift angle is zero) but the tip o Produced by excessive dihedral airfoil is highly cambered (zero-lift effect. angle is nonzero). o Repeated side-to-side motion involving yaw and roll. o To counter the tendency, the vertical area must be increased. Wing Incidence → It is the pitch angle of the wing with respect to the fuselage. o If the wing is untwisted, the incidence is simply the angle between the fuselage axis and the wing's airfoil chord lines. o If the wing is twisted, the incidence is defined with respect to some arbitrarily chosen spanwise location of the wing, usually the root of the exposed wing where it intersects the fuselage. Wing Vertical Location → High Wing → Wing incidence angle is chosen to minimize drag at some operating condition, usually cruise. Unpublished Work © 2023 Top-Notch Aeronautics 26 | P a g e  Top-Notch Notes in Aircraft Structures & Design → Mid Wing located at the trailing edge of the wing tip; increases torsional load. → Cut-off forward swept is used for supersonic aircraft; part with little lift is cut-off; reduced torsional load → Low Wing High Lift Devices Wing Tips → A smoothly-rounded tip easily permits the air to flow around the tip. o A tip with a sharp edge (when seen nose-on) makes it more difficult, thus Biplane Wings reducing the induced drag. → Gap – the vertical distance between → The most widely used low-drag wing tip the two wings is the Hoerner wingtip. → Span Ratio – the ratio between the → Tip curved upwards/downwards shorter to the longer wing increase effective span without → Stagger – the longitudinal offset of the increasing actual span. two wings relative to each other → A swept wing tip addresses the o Positive, when upper wing is closer to condition that vortices tend to be the nose Unpublished Work © 2023 Top-Notch Aeronautics 27 | P a g e  Top-Notch Notes in Aircraft Structures & Design o Negative, when lower wing is closer ▪ Some single-engine propeller to the nose airplanes have the vertical tail → Decalage – relative incidence between offset several degrees to counter the two wings “propwash". o Positive, if the angle of incidence of Tail Arrangement the upper wing is greater than that of the lower wing o Negative, when the lower wing has the greater angle. Conventional → Lightweight → Horizontal tail is in the wake of the wing → Does not allow for aft-mounted engine → Low horizontal tails are best for stall recovery T-Tail → Heavier due to strengthening of the vertical tail to support the horizontal tail TAIL GEOMETRY & ARRANGEMENT → Allows for a smaller vertical tail due → Tails provide for trim, stability, and to endplate effect control. Trim refers to the generation of → Horizontal tail is clear of wing wake a lift force that, by acting through some and propwash tail moment arm about the center of → Allows for an aft-mounted engine gravity, balances some other moment → Most prone to Deep Stall, where the produced by the aircraft. wing blankets the Elevator causing a o For the horizontal tail, trim primarily stall refers to the balancing of the Cruciform moment created by the wing. → Compromise between conventional o For the vertical tail, the generation of and T-tail a trim force is normally not required → Less weight penalty compared to T- because the aircraft is usually left- tail right symmetric and does not create → Undisturbed flow in lower part of any unbalanced yawing moment. rudder at high angles of attack Unpublished Work © 2023 Top-Notch Aeronautics 28 | P a g e  Top-Notch Notes in Aircraft Structures & Design → No endplate effect. Other Configurations H-Tail (Twin Tail) → Undisturbed flow in vertical tails at high angles of attack → May enhance engine out control in multiengine aircraft with the rudders positioned in the propwash → Endplate effect on the horizontal tail; reduced size possible V-Tail (Butterfly) → May allow for a reduced wetted area → Reduced interference drag → Control/Actuation complexity → Adverse roll-yaw coupling Control canard → Surfaces are out of the wing wake → Negligible contribution to lift Inverted V-Tail → Used to control angle of attack of → Proverse Roll-Yaw Coupling wing → Reduced spiraling tendencies → Used to balance pitching moments → Ground clearance problems due to flaps Y-Tail Lifting canard → Avoids complexity of ruddervators → Contributes to lift → V surfaces provide pitch control only → Higher aspect ratio for reduced → Rudder in third surface induced drag Boom-Mounted Tails → Greater camber for increased lift → Allows for a pusher propeller → Pushes wing aft; bigger pitching configuration moments due to flaps → Tailbooms are typically heavier than → Canard is closer to CG a conventional fuselage → Pitch up tendencies are avoided construction Tandem wing → May be connected or not; high-, → 50% theoretical reduction in induced mid-, or low-mounted horizontal tail, drag because liftis distributed which can have a V configuration between the two wings Ring Tail → Aft wing experiences downwash and → Doubles as a propeller shroud turbulence causedby the forward → Conceptually appealing, however wing proven inadequate in application → Wings must be separated as far as possible Three surface → Theoretically offers minimum trim drag → Additional weight; more interference drag; complexity Unpublished Work © 2023 Top-Notch Aeronautics 29 | P a g e  Top-Notch Notes in Aircraft Structures & Design Back Porch / Aft-Strake → For certain types of aircraft, the → Incorporated into a faired extension fuselage size is determined strictly by of the wing or fuselage "real-world constraints." → Used to prevent pitch up but can o Once the number of passengers is also serve as a primary pitch control known and the number of seats surface across is selected, the fuselage Tailless length and diameter are → Offers the lowest weight and drag essentially determined. → Reduced wing efficiency → Fuselage fineness ratio is the ratio → Most difficult configuration to between the fuselage length and its stabilize maximum diameter. o If the fuselage cross section is not Tail Arrangement for Spin Recovery a circle, an equivalent diameter is → As a rule of thumb, at least 1/3 of the calculated from the cross- rudder should be out of the wake. sectional area. Wing → The actual wing size can be determined simply as the takeoff gross weight divided by the takeoff wing loading. o Remember that this is the reference area of the theoretical, trapezoidal wing, and includes the area extending into the aircraft centerline. Tail Volume Coefficient INITIAL SIZING → Aircraft sizing is the process of determining the takeoff gross weight and fuel weight required for an aircraft concept to perform its design mission. → An aircraft can be sized using some existing engine or a new design engine. o Fixed engine – the existing engine is fixed in size and thrust o Rubber engine – new design engine can be built in any size and thrust required Geometry Sizing Fuselage Unpublished Work © 2023 Top-Notch Aeronautics 30 | P a g e  Top-Notch Notes in Aircraft Structures & Design Note: The moment arm (L) is commonly o To minimize the weight penalty, the approximated as the distance from the balance weight should be located tail quarter-chord to the wing quarter- as far forward as possible. chord. → Aerodynamic balance is a portion of the control surface in front of the hinge Control Surface Sizing line. This lessens the control force → Wing flaps occupy the part of the wing required to deflect the surface, and span inboard of the ailerons. helps to reduce flutter tendencies. o If a large maximum lift coefficient is required, the flap span should be as large as possible. → Spoilers are commonly used on jet transports to augment roll control at low speed, and can also be used to reduce lift and add drag during the landing rollout. → High-speed aircraft can experience a phenomenon known as aileron reversal in which the air loads placed upon a CONFIGURATION LAYOUT & LOFT deflected aileron are so great that the wing itself is twisted. → Inboard profile drawing depicts in much o At some speed, the wing may twist greater detail the internal arrangement so much that the rolling moment of the aircraft design’s subsystems. produced by the twist will exceed → Lofting is the process of defining the the rolling moment produced by the external geometry of the aircraft. aileron, causing the aircraft to roll → Production lofting, the most detailed the wrong way. form of lofting, provides an exact, o To avoid this, many transport jets use mathematical definition of the entire an auxiliary, inboard aileron for high- aircraft including such minor details as speed roll control. Spoilers can also the intake and exhaust ducts for the air be used for this purpose. conditioning. → Flutter is a rapid oscillation of the Fuselage Loft Verification surface caused by the air loads. This can tear off the control surface or even → Buttock-plane ("butt-plane") cuts form the whole wing. This is minimized by the intersection of the aircraft with using mass balancing and vertical planes defined by their distance aerodynamic balancing. from the aircraft centerline. → Mass balancing refers to the addition of weight forward of the control surface hinge line to counterbalance the weight of the control surface aft of the hinge line. o This greatly reduces flutter tendencies. Unpublished Work © 2023 Top-Notch Aeronautics 31 | P a g e  Top-Notch Notes in Aircraft Structures & Design Wing/Tail Layout and Loft of the wing's mean aerodynamic chord. o For a lifting-type canard, the mean aerodynamic chords of the wing and canard should both be determined, and the appropriate percent MAC for each should be identified. Then the combined MAC location can be determined as the weighted average of the percent MAC locations for the wing and canard (weighted by their respective areas). → Tip airfoils are selected for gentle stall properties. Root airfoils are selected for best performance. → Wing rigging is the process of vertically shifting the airfoil sections until some desired spanwise line is straight. → For a stable aircraft with an aft tail, the → A wing fillet is generally defined by a wing should be initially located such circular arc of varying radius, tangent to that the aircraft center of gravity is at both the wing and fuselage. Typically, a about 30% of the mean aerodynamic wing fillet has a radius of about 10% of chord. When the effects of the fuselage the root-chord length. and tail are considered, this will cause o Typically, at rear of an aircraft, fillet the center of gravity to be at about 25% arc radius is more. This will result in of the total subsonic aerodynamic reduction of the flow separation. center of the aircraft. → For an unstable aircraft with an aft tail, the location of the wing depends upon the selected level of instability, but will usually be such that the center of gravity is at about 40% of the mean aerodynamic chord. → For a canard aircraft, such rules of thumb are far less reliable due to the canard. downwash and its influence upon the wing. o For a control-type canard with a computerized flight control system (unstable aircraft), the wing can be initially placed such that the aircraft center of gravity is at about 15-25% Unpublished Work © 2023 Top-Notch Aeronautics 32 | P a g e  Top-Notch Notes in Aircraft Structures & Design PROPULSION & FUEL SYSTEM INTEGRATION isentropic ramp inlet works properly at only its design Mach number, and is Propulsion Selection seen only rarely. Inlet Location Inlet Geometry As a rule of thumb, all inlets should be located a height above the runway equal → Any inlet must slow the air to about half to: the speed of sound before it reaches the engine. The final transition from → at least 80% of the inlet's height if using supersonic to subsonic speed always a low bypass ratio engine occurs through a normal shock. The → at least 50% of the inlet's height for a pressure recovery through a shock high-bypass-ratio engine depends upon the strength of the Propeller-Engine Integration shock, which is related to the speed reduction through the shock. → Tip speed – the vector sum of the → The greater the number of oblique rotational speed and the aircraft’s shocks employed, the better the forward speed pressure recovery. → The theoretical optimal is the isentropic ramp inlet, which corresponds to infinitely-many oblique shocks and produces a pressure recovery of 100% (ignoring friction losses). The pure Unpublished Work © 2023 Top-Notch Aeronautics 33 | P a g e  Top-Notch Notes in Aircraft Structures & Design → Pusher configuration o Can reduce aircraft skin friction drag because the pusher location allows the aircraft to fly in undisturbed air. o The fuselage-mounted pusher → Fixed-pitch propeller is called a "cruise propeller can allow a reduction in prop" or "climb prop" depending upon aircraft wetted area by shortening the flight regime the designer has the fuselage. decided to emphasize. o Reduces cabin noise because the → Variable-pitch propeller can be used to engine exhaust is pointed away from improve thrust across a broad speed the cabin and improves the pilot’s range. outside vision. → Controllable-pitch propeller has its pitch directly controlled by the pilot through a lever alongside the throttle. → Constant speed propeller is automatically controlled in pitch to maintain the engine at its optimal RPM. → Spinner – part of the propeller-engine that pushes the air out to where the propeller is more efficient. → Down-draft cooling o exits the air beneath the fuselage, which is a high-pressure area and therefore a poor place to exit air. → Updraft cooling o flows the cooling air upwards through the cylinders and exits it into low-pressure air above the fuselage, creating more efficient cooling flow due to a suction effect. FUEL SYSTEM An aircraft fuel system includes the fuel → Conventional tractor tanks, fuel lines, fuel pumps, vents, and o This location puts the heavy engine fuel-management controls. up front, which tends to shorten the forebody, allowing a smaller tail area Discrete tanks and improved stability. → fuel containers which are separately o It also provides a ready source of fabricated and mounted in the cooling air, and places the propeller aircraft by bolts or straps in undisturbed air. → normally used only for small general aviation and homebuilt aircraft Unpublished Work © 2023 Top-Notch Aeronautics 34 | P a g e  Top-Notch Notes in Aircraft Structures & Design Bladder tanks → has the aft wheel so far behind the → made by stuffing a shaped rubber CG that the aircraft must takeoff bag into a cavity in the structure. and land in a flat attitude o causes loss of about 10% of the Taildragger available fuel volume. → two main wheels forward of the CG → widely used because they can be and auxiliary wheel at the tail made "self-sealing." → conventional landing gear o If a bullet passes through a self- → more propeller, less drag and weight sealing tank, the rubber will fill in → ground loop is encountered the hole preventing a large fuel Tricycle loss and fire hazard. → two main wheels aft of CG and Integral tanks auxiliary wheel forward of CG → cavities within the airframe structure → CG is ahead of the main wheels so that are sealed to form a fuel tank. aircraft is stable on the ground and → would be created simply by sealing can be landed at a fairly large existing structure such as wing boxes "crab" and cavities created between two → Carrier-based aircraft must use twin fuselage bulkheads. nose-wheels at least 19 inches in → still prone to leaks diameter to straddle the catapult- → fire hazard of an integral tank can launching mechanism. be reduced by filling the tank with a Quadricycle gear porous foam material, but some fuel → much like bicycle gear but with volume is lost. wheels at the sides of the fuselage → requires a flat takeoff and landing LANDING GEAR ARRANGEMENTS attitude → permits a cargo floor very low to the ground Multi-Boogey → For extra heavy aircraft (200-400 kips) → Redundancy for safety TIRE SIZING The tires are sized to carry the weight of the aircraft. Typically, the main tires carry about 90% of the total aircraft weight. Nose tires carry only about 10% of the static Bicycle load but experience higher dynamic loads → two wheels fore and aft during landing. → small outrigger wheels on the wings Type III to prevent aircraft from tipping → used for most piston-engine aircraft sideways → has a wide tread and low internal pressure Unpublished Work © 2023 Top-Notch Aeronautics 35 | P a g e  Top-Notch Notes in Aircraft Structures & Design Type VII → must be strong enough to handle → used by most jet aircraft the lateral and braking loads of the → designed for higher landing speeds wheels. → To repair or replace the oleo strut, the entire wheel assembly must be SHOCK ABSORBERS removed because it is attached to the bottom of the strut Rigid axle Triangulated gear → used by WWI fighter aircraft → similar to the levered bungee gear → attached to the aircraft with strong → when deflected, an oleo shock rubber chords("bungees") that absorber is compressed which stretched as the axle moves upward provides a leveraged effect that the Levered bungee oleo can be shorter than the → common to light aircraft required wheel travel → gear leg is pivoted at the fuselage → oleo can be replaced without → rubber bungee chords underneath removing the wheel assembly the gear are stretched as the gear → wheel lateral and braking loads are deflects upward and downwards carried by the solid gear legs Solid spring gear → little heavier than the oleo shock- → used by many general aviation strut gear aircraft → tire scrubbing effect that shortens tire → simple but slightly heavier than other life types of gear Trailing link → deflects with some lateral motion → resembles the triangulated gear but instead of straight up and down with the solid gear leg running aft which tends to scrub the tires rather than laterally sideways against the runway → common for carrier-based aircraft → bounce a lot o provides the large amounts of Oleopneumatic shock strut (oleo) gear travel required for carrier → most common type landings → combines spring effect using → aft travel of the wheel is desirable for compressed air with dampening operations on rough fields effect using a piston which forces oil o if a FOD is encountered, the aft through a small hole (orifice) motion of the wheel gives it more → metered orifice having mechanism time to ride over the obstacle with varying size for maximum efficiency CASTORING-WHEEL GEOMETRY → When used as a shock-strut, the oleo → For ground steering, a nosewheel or itself must provide the full required tailwheel must be capable of being amount of wheel deflection which castored (turned). can lengthen the total landing-gear → The castoring can introduce static and height. dynamic stability problems causing "wheel shimmy," a rapid side-to-side Unpublished Work © 2023 Top-Notch Aeronautics 36 | P a g e  Top-Notch Notes in Aircraft Structures & Design motion of the wheel that can tear the → Gear in the wing reduces the size of the landing gear off the airplane. wing box, which increases weight and → Prevention of shimmy is accomplished may reduce fuel volume. by selection of the "rake angle" and → Gear in the fuselage or wing-fuselage "trail”. junction may interfere with the → In some cases, a frictional shimmy longerons. damper is also used to prevent shimmy. → However, the aerodynamic benefits of This can be a separate hydraulic these arrangements outweigh the plunger or simply pivot with a lot of drawbacks for higher-speed aircraft. friction. AIRCRAFT CERTIFICATION Type Certificate (TC) – confirms that the aircraft is manufactured according to a design approved by the regulator, where that design ensures compliance with all airworthiness regulations. Supplemental Type Certificate (STC) – further certification exercise undertaken to a Type Certificate to cover the changes