Kul Aircraft Structural Design (4 cr) Structure Design for Assignment 1 INTEGRATED LECTURE IL-2

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1 Kul Aircraft Structural Design (4 cr) Structure Design for Assignment 1 INTEGRATED LECTURE IL-2 M Kanerva 2016

2 INTEGRATED LECTURE IL-2 Review of IL-1 Risk analysis guidance for assignment 1 Concept development guidelines for aircraft structures Page 2

3 Assignment 1 Kurssin aikana oppilaat ovat yhdessä suunnitteluorganisaatio, jonka tehtävänä on konseptoida ja mitoittaa määrätty lentokonerakenne. Ensimmäisessä harjoitustyössä suunnitellaan siiven kantavalle rakenteelle sopivaa konseptia. Konseptin tulee soveltua spesifikaation mukaiseen tarkoitukseen ja täyttää sille määrätyt vaatimukset. Hyvän konseptin kehittäminen edellyttää potentiaalisen konseptin arvioimista erilaisten riskien kannalta. Tämän harjoitustyön ydin keskittyy alustavasti suunnitellun konseptin (laadulliseen) arvioimiseen. Page 3

4 Concept Development Overview To find a structure that meets the specification and can be realised without a considerable risk. To be defined: materials structure/system solutions manufacture integration Installation..in order to find reliable estimates for the mass and cost of the aircraft structure Page 4

5 Design Specification & Concept Development Design Specification Concept Development Page 5

6 Design Specification Contents Functions Geometry Design features of the structure Loads Design criteria Maintainability Methods and tools Page 6

7 Assignment 1 Tarkasteltava rakennekohta on siiven poikkileikkaus. Poikkileikkaukseen liittyvät vaatimukset on muotoiltu spesifikaatioon seuraavasti: Alustava siiven geometria asettaa rajoitteita poikkileikkauksen suunnittelulle: - Tarkasteltavan poikkileikkauksen jänteen pituus on c = 4.8 m (siiven trapetsisuus c t / c r 0.25) - Käytettävän siipiprofiilin paksuussuhde on t / c = Profiilin tulee soveltua nopeaan matkalentoon (Mach 0.82) Poikkileikkaukseen kohdistuva kriittisin kuormitustapaus on alustavasti analysoitu ja se muodostuu oleellisesti seuraavista kuormista, kun siipi(kotelo) ajatellaan palkkimaiseksi runkoon kiinnitetyksi rakenteeksi: - Taivutusmomentti - Vääntömomentti - Leikkausvoima Muut poikkileikkauksen ominaisuudet: - Siiven sisätila muodostaa mahdollisimman suuren polttoainetankin - Käsiteltävän rakenteen jättöreunalle tulee varata tila laskusiivekkeille - Käsiteltävä rakenne on kaarilla tuettu - Salkorakenteet ja mahdolliset jäykisteet ovat jatkuvia rakenteen tyvestä kärkeen - Rakenteen valmistusmenetelmien tulee soveltua massatuotantoon Page 7

8 Assignment 1 Yleisohjeistus työhön: Raportoikaa rakenteen Spesifikaatio tarkasti rakennekonseptin suunnittelua varten! Konseptin yleisen kuvauksen tulee selvittää rakenteesta kaikki riittävät tiedot, mm. - Yksionteloisen vääntökotelon poikkileikkauksen geometrian kuvaus piirroksessa, jossa alustavat materiaalipaksuudet ja eri rakennemateriaalit on osoitettu; - Selvitys erillisistä rakenne-elementeistä ja erityisesti niiden välisistä liitoksista; - Miten epästabiliteetti pyritään välttämään paneelimaisissa / sauvamaisissa rakenne-elementeissä? - Mahdollisten fail-safety / safe-life ominaisuuksien selvittäminen; - Rakenteen ja sen osien verifiointi? Luentokalvoista löytyy spesifikaation ja konseptisuunnittelun kannalta oleelliset suunnittelunäkökohdat! Page 8

9 Concept Development How to Start? Geometry, functionality and load requirements define applicable concepts: environmental requirements external geometry interfaces required fail-safe features load cases Constraints set by maintainability requirements must also be noted: interchangeable/replaceable components inspection requirements Page 9

10 Concept Development How to Proceed? Concept development typically proceeds as follows: 1. Baseline concept is selected, designed and sized, and evaluated 2. Derivatives of the baseline concept are developed, designed and sized 3. As applicable, concepts based on other architectures and/or technologies are developed, designed and sized 4. The concepts are ranked on the basis of their (1) weight, (2) cost, (3) maintainability and (4) risks Note! Development testing is performed as needed to evaluate structural efficiency and/or manufacture of a concept Page 10

11 Assignment 1 Arvioikaa konseptia suhteessa olemassa oleviin ratkaisuihin. Perustelkaa arviot selvittämällä yksityiskohtaisesti konseptisi kyseinen ominaisuus suhteessa pohdittuihin vaihtoehtoisiin ratkaisuihin. Rakennemassa: - Arvioikaa karkeasti poikkileikkauksen rakennemassa (kg/m). Miten konseptissa pyritään keveyteen? Missä yksityiskohdissa on tingitty keveydestä ja miksi? Miten olisi voitu päästä vielä keveämpään rakenteeseen? Valmistettavuus: - Suunnitelkaa alustavasti valmistusvaiheiden ja kokoonpanon prosessit. Miten konseptissa pyritään tehokkaaseen valmistustekniikkaan? Missä yksityiskohdissa on tingitty yksikertaisesta valmistuksesta ja miksi? Miten olisi voitu edelleen tehostaa valmistusta? Miten valmistusmäärät vaikuttavat konseptin valmistettavuuteen? Taloudellisuus ja toimivuus käytön aikana: - Mitkä yksityiskohdat vaativat todennäköisesti eniten suunnittelutyötä? Miten konseptissa pyritään helppoon ja sujuvaan huoltotoimintaan? Mitkä yksityiskohdat saattavat tuoda ongelmia käytön aikana ja millaisia? Riskitekijät: - Mitä asioita edellisiin kohtiin liittyen on vaikeaa arvioida etukäteen eli millaisia riskejä konseptiin liittyy? Pohdi erityisesti konseptin toimivuutta sekä huoltotoimintaa käytön aikana. Taulukoi riskien vaikutus (alla olevan mukaisesti) omaan taulukkoonsa. Page 11

12 Assignment 1 Harjoitustyö tulee kirjoittaa ulkoasultaan selkeäksi ja yleisesti tieteellistä tekstiä vastaavaksi. Varsinkin työssä käytettyjen lähteiden merkitsemiseen tulee kiinnittää huomiota Ensimmäinen versio työstä tulee olla palautettuna paperiversiona viikolla 6: 8.2. (2016) Lopullinen työ tulee olla palautettuna paperiversiona viimeistään viikolla 7: (2016) Työhön sopivaa lähdemateriaalia (kurssien Kul sekä Kul materiaalien lisäksi): Yleiset rakenneratkaisut: - Niu, M.C.Y. Airframe Structural Design. 2. painos. 1999, Hong Kong Conmilit Press. Rakennemassa-arvioita sekä rakenneratkaisuja: (Ch. 7 ja Ch. 8) - Torenbeek, E. Synthesis of Subsonic Airplane Design Delft University Press. Komposiittirakenteiden rakenneratkaisut ja valmistus: - Niu, M.C.Y. Composite Airframe Structures Hong Kong Conmilit Press. (kysy kurssihenkilökunnalta lainaksi) Metallirakenteiden perinteinen valmistus: - Horne, D.F. Aircraft Production Technology Cambridge University Press. (kysy kurssihenkilökunnalta lainaksi) Konseptisuunnittelu yleisesti: - Howe, D. Aircraft Conceptual Design Synthesis Professional Engineering. (kysy kurssihenkilökunnalta lainaksi) Valmistutekniikka kehittyineisiin komposiitteihin liittyen: - Sampe Journal (kysy kurssihenkilökunnalta lainaksi) Page 12

13 Concept Development Importance Performance and production costs of a structure are mainly defined by the decisions made during the concept design phase Page 13

14 Concept Development Risk Analyses Risk analyses help to compare concepts support final concept selection Formal techniques for risk analyses have been developed In a simple risk analysis: 1. Potential risks are identified 2. Probability (P) and severity (S) are evaluated for each concept and risk e.g. in the scale Products P x S are computed and summed 4. Concepts with high probability of a severe risk and with high sum of P x S are rejected or designed further to lower the risk Page 14

15 Concept Development Risk Analyses Example of Probability/Impact Chart Probability of the incident Impact Minor Detrimental Severe Not probable 1. Insignificant risk Possible 2. Minor risk 3. Substantial risk Probable 3. Substantial risk 2. Minor risk 3. Substantial risk 4. Significant risk 4. Significant risk 5. UNBEARABLE RISK Page 15

16 Assignment 1 Pisteytä alla olevat kohdat ja taulukoi pisteet: Rakennemassa (1 3 pistettä, kevyin konsepti saavutettu: 3 pistettä) Valmistettavuus (1 3 pistettä, helpoin valmistettavuus saavutettu: 3 pistettä) Kustannukset & huolto (1 3 pistettä, pienimmät kustannukset saavutettu: 3 pistettä) Muut riskitekijät (1 3 pistettä, pienimmät muut riskitekijät saavutettu: 3 pistettä) Riskitekijät lasketaan siten että yhden kohdan pisteet lasketaan kaavasta p r / 3, missä p r = SP S = riskin vaikutus toteutukseen, 1 3 (huomattava kohtalainen vähäinen) P = riskin todennäköisyys, 1 3 (huomattava kohtalainen vähäinen) (eli tämä riskien analysointi omaan taulukkoonsa) Laskekaa lopuksi keskiarvo yllä esitetyistä viidestä arvostelun kohdasta ja analysoikaa eri kohtien painoarvoa. Page 16

17 Concept Development Structure Objectives Low weight fully stressed structure with weight efficient buckling prevention multifunctional parts/structures as possible and beneficial Economic production decent non-recurring costs low recurring costs common parts as possible and beneficial (e.g. left/right hand parts and lugs) Maintainability damage resistant and tolerant easily reparable to the extent required Page 17

18 Fail-Safe and Safe-Life Structures Philosophies Page 18

19 Concept Development Material Selection Criteria Environmental resistance Mechanical performance: short-term fatigue strength (crack-free life) fracture toughness crack growth rate Electrical/thermal properties Cost Availability (Maturity) Page 19

20 Concept Development Material Selection Metal alloys, Composites, Hybrids Note! Materials, their properties and use have been discussed in other courses Metallic, composite and hybrid structure concepts should be developed and compared when applicable Page 20

21 Concept Development Material Selection Metal Alloys Aluminium alloys most common, titanium alloys/steels are used when required by the environment and/or by space limitations Tough materials are preferred in structures subjected to cyclic tension loading A/C B747 L-1011 DC-10 A-300 B727 B737 B757 B767 Upper skin/str 7075-T6 / 7075-T T76 / 7075-T T651 / 7075-T T T651 / 7178-T T T6 / 7150-T T6 / 7150-T6 Lower skin/str 2024 / T76 / 7075-T T351 / 7075-T T T351 / 2024-T T T3 / 2224-T T3 / 2224-T3 Page 21

22 Concept Development Material Selection Notes on Metal Alloys Material anisotropy to be taken into account Fatigue strength may not correlate with static strength Protective coatings against corrosion and wear to be used as needed A protective coating may affect fatigue properties Surface roughness highly affects fatigue strength Fatigue strength can be improved with compressive surface stresses Page 22

23 Concept Development Material Selection Notes on Composites Page 23

24 Concept Development Material Selection Notes on Composites UD prepregs give best mechanical properties most commonly used Woven Fabrics and Non Crimp Fabrics (NCFs) are used especially when injection techniques are applied Allowed strain levels or other qualified sizing criteria must be available for the fibre/matrix-combination, the criteria taking into account damage resistance/tolerance requirements the manufacturing technique effects of composite inhomogeneity Prepreg-laminate Woven fabric laminate NCF-laminate Page 24

25 Concept Development Structure Options for Lifting Surfaces Solid skin, spars, ribs and stiffeners Sandwich skin, spars and ribs Multispar (with or without stiffeners) Multirib (with or without stiffeners) Full sandwich Note! Multispar refers to a concept in which several spars and only few ribs (e.g. end ribs) are used (multirib defined analogously) Page 25

26 Concept Development Structure Notes on Lifting Surfaces Increasing number of crossing members (e.g. ribs/stiffeners) increases complexity in manufacture Constrained optimization is often performed to find the best design for each option (e.g. number of ribs or spars is optimized) Page 26

27 Concept Development Structure Assembly Options Parts manufactured separately and joined (1) mechanically or (2) (2) by bonding Fully integrated structure Semi-integrated structure Page 27

28 Concept Development Structure Assembly Options for a Composite Shell 1. All parts manufactured separately, assembly from inside out 2. Integrated skeleton + two skins manufactured separately 3. Integrated skeleton/skin, the other skin manufactured separately 4. Manufacture in one cycle, one skin removable 5. Manufacture in one cycle Page 28

29 Concept Development Structure Assembly Options for a Wing Spar 1. Parts manufactured separately, assembly with mechanical joints 2. Integral spar: composite structure; or NC-machined metallic structure Page 29

30 Concept Development Structure Notes on Assembly Options With increasing integration: + number of manufacturing steps decreases fi cost savings are possible + number of joints decreases fi weight savings - complexity of the tooling increases - complexity of the composite lay-up process increases - inspection becomes more complicated Page 30

31 Concept Development Parts Targets Main targets are (1) structural efficiency and (2) efficient production, e.g. flange angles 90 as possible sharp corners avoided Page 31

32 Concept Development Parts Design Principles Materials and manufacturing techniques have their own advantages/constraints that are utilised/respected in design, e.g. Geometry constraints Deformations due to manufacture (e.g. springback of flanges) Tolerances To provide an example, common design rules of composite parts are shortly reviewed on the following slides Page 32

33 Concept Development Composite Parts Lay-up Rules Symmetric laminates to be used; asymmetry results in structure distortion 0/90/45/-45 layer angles normally used: at least 10 % of layers in each direction is a normal practice Layers with different orientations should be dispersed through the thickness Number and orientation of layers in different parts of the structure varying in accordance with local loading (as possible) Page 33

34 Concept Development Composite Parts Geometry 1/6 Relatively tight corner radii are possible but higher radii are preferred Cavities (e.g. in joint areas) to be avoided, especially in injection: they provide easy flow paths for the resin when injection technique is applied resin rich pockets are susceptible to cracking when the component is loaded Page 34

35 Concept Development Composite Parts Geometry 2/6 Part release to be assured - negative draft angles are possible but result in more complex tooling L-flanges are preferred over T- flanges (T-flanges may, however, be required for structural reasons) Z-shaped ribs/spars often provide easiest inner tool removal in integrated composite structures Tool removal is further easier with slight tilting of webs (if applicable) Page 35

36 Concept Development Composite Parts Geometry 3/6 Deformation capability of reinforcement (and prepreg) must be accounted for in part design: UD-reinforcements/tapes deform poorly in the fibre direction Woven fabrics deform in shear, deformation capability depends on the type of weave Tape width affects deformability in automatic tape/fibre placement Note! Beyond its deformation limit a reinforcement starts to wrinkle, which is not acceptable Page 36

37 Concept Development Composite Parts Geometry 4/6 When parts with complex shapes are designed, it should be noted that: Orientation of a fabric with respect to the component coordinates has a radical effect on its formability Forming of a fabric by shear affects fibre orientations and on the thickness of the layer Formability of the reinforcement stack must be confirmed if this approach is planned to be used Possibility to inspect the part must always be assured Page 37

38 Concept Development Composite Parts Geometry 5/6 Cut and fold techniques can be used to make shapes that are beyond the deformation limit of the reinforcement Another possibility is to use lay-ups/preforms assembled from several parts In both cases, structural integrity and efficiency of the final product must be secured with efficient joints in between the sections Page 38

39 Concept Development Composite Parts Geometry 6/6 Specific commercial software are available for studying formability of fabrics, i.e. for predicting positions and orientations of fibres within the deformed fabric Such a simulation tool is available also e.g. in CATIA Simulation tools should be used, as needed, in conceptual design If necessary, formability of a reinforcement to the desired shape must be checked experimentally simulated manufactured Page 39

40 Concept Development Composite Parts Sandwich Structures Skin lay-up rules as for laminates Edges to be closed Reinforcement for local loads (joints, hails etc.) as needed Lay-up on top of a honeycomb core results in a poor laminate quality Note! Unsymmetric lay-up is often possible (e.g. top and bottom skins with different lay-ups) Page 40

41 Concept Development Discontinuities Stress concentrations should be minimised: proper shaping of holes and notches local thickness increase as needed and possible smooth thickness/structure variations flanges at hole edges? load-carrying hole cover? Page 41

42 Concept Development Joints Common Features Joints should mainly be designed for shear loading Joints exhibit stress concentrations to structures; reduces especially fatigue strength Higher FoS is applied due to lower reliability of analysis and manufacture A structure with a joint cannot be stronger than a structure without a joint Page 42

43 Concept Development Joints Notes on Lap/Strap Joints 1/2 All failure modes must be accounted for Safe bearing failure type is preferred Failure modes to be in balance adequate edge distances Minimum edge distances are different for metals and composites 3-5 D 3-4 D 2-3 D 2-3 D Page 43

44 Concept Development Joints Notes on Lap/Strap Joints 2/2 Local reinforcing to be used as needed Stress concentrations to be minimised Stepped-lap/tapered joints to be used as needed (expensive) Page 44

45 Concept Development Joints Notes on Bonded Joints Adhesives should be designed only for shear loads Bonding works best with thin structures Ductile (tough) adhesives to be used when possible Joint should always be stronger than the members being joined Proper surface treatment and material compatibility is a prerequisite for a durable bonded joint Quality control of an adhesively bonded joint is difficult Peel stresses to be accounted for: Page 45

46 Concept Development Joints Notes on Asymmetric Joints Bending & peel stresses to be minimised Joint support to be provided as needed (to avoid local bending) Page 46

47 Concept Development Joints Notes on Tolerances Joint geometry must be defined so that secondary assembly loads are avoided Trimming and shimming to be used as needed Page 47

48 Concept Development Interfaces Basic Rules Secondary loads/stresses minimised (direct load paths) Reasonable and well shared tolerances Page 48

49 Concept Development Weight Estimates Continuous weight evaluation of the concept is an essential part of concept development Weight estimates shall become more detailed with decreasing number of potential solutions Notes! If the weight target is not achieved, possibilities for weight savings should be studied starting from heaviest parts Moments of inertia are normally also needed and must be calculated Page 49

50 Concept Development Cost Estimates Cost estimates have been discussed in the course Aircraft Project: Non-recurring costs (NRC) and recurring (RC) costs to be accounted for Design costs are estimated from statistics (e.g. design and analysis hours/kg) Tooling must be pre-designed to evaluate production costs Preliminary production planning must be made for the evaluation of investments Learning must be accounted for in labour hours (and in material consumption) Page 50

51 Concept Development Results Results of concept development are collected to the extent and in the form defined by the design organisation, e.g.: geometry definition technology definition (materials, part manufacture and assembly techniques) FE models and stress reports weight estimate cost estimate risk analysis results compliance analyses Page 51