Aircraft Structures Chapter 22 Fuselages Fuselage Structures 2

Aircraft Structures Chapter 22 - Fuselages

Fuselage Structures 2

Stresses in an Aircraft Fuselage 1. Aircraft fuselages consist of thin sheets of material stiffened by large numbers of longitudinal stringers together with transverse frames. 2. The distance between stringers is usually small, so that the variation in shear flow in the connection panel is small. Therefore, it is reasonable to assume that the shear flow is constant between adjacent stringers, so that the analysis simplifies to the analysis of an idealized section in which the stingers/booms carry all the direct stresses while the skin is effective only in shear. 3

Stresses in an Aircraft Fuselage Bending Moments Shear Forces Torsion 4

22. 1 Bending We will use this equation in this chapter to calculate the direct stress in each boom. r e b m e R this from ! 6 1 r e t p a ch To find the broom area: r e b m e R this from ! 0 2 r e t p a ch 5

Example 22. 1 The fuselage of a light passenger carrying aircraft has the circular cross -section shown below. The cross-sectional area of each stringer is 100 mm 2 and the vertical distance given in the figure are to the mid-line of the sectional wall at the corresponding stringer position. If the fuselage is subjected to a bending moment of 200 k. N. m applied in the vertical plane of symmetry, at this section, calculate the direct stress distribution. 6

Example 22. 1 (Cont. ) How to solve? Boom Areas Moment of Inertia (I) Calculate the stresses 7

Example 22. 1 (Cont. ) How to solve? Boom Areas From Symmetry: ü B 1 =B 9 ü B 2 =B 8 =B 10 =B 16 ü B 3 =B 7 =B 11 =B 15 ü B 4 =B 6 =B 12 =B 14 ü B 5 =B 13 The stringers 5 and 13 lie on the neutral axis of the section and are therefore unstressed, the calculation of boom areas B 5 and B 13 does not then rise. 8

Example 22. 1 (Cont. ) How to solve? Moment of Inertia (I) Ixx =2× 216. 6× 381. 02+4× 216. 6× 352. 02+4× 216. 6× 269. 52+4× 216. 7× 145. 82 =2. 52× 108 mm 4 9

Example 22. 1 (Cont. ) How to solve? Calculate the stresses 10

22. 2 Shear We will use this equation to find the shear flow distribution. r Remembe this from 0! 2 r e t p a h c Where 11

Example 22. 2 The fuselage of Example 21. 1 is subjected to a vertical shear load of 100 k. N applied at a distance of 150 mm from the vertical axis of symmetry as shown, for the idealized section, in Fig. 22. 2. Calculate the distribution of shear flow in the section. 12

Example 22. 2 (Cont. ) How to solve? Shear flow distribution Taking moments about some center Finding the shear stress distribution 13

Example 22. 2 (Cont. ) How to solve? Shear flow distribution Open section shear flow qb 14

Example 22. 2 (Cont. ) How to solve? Taking moments about some center r Remembe this from 7! chapter 1 15

Example 22. 2 (Cont. ) How to solve? Taking moments about some center A= Π x 381. 02= 4. 56 x 105 mm 2 16

Example 22. 2 (Cont. ) How to solve? Finding the shear stress distribution 17

22. 3 Torsion We will use this equation to find the shear stress distribution produce by a pure torque. r e b m e R this from ! 8 1 r e t p a ch Lets apply it to example 22. 2: 18

22. 3 Torsion From symmetry we get: 19

22. 3 Torsion 20

22. 4 Cut-Outs in Fuselages Loads are redistributed in the vicinity of the cut-off. Reinforcement Increased Weight Rigid Frames 21

22. 4 Cut-Outs in Fuselages 1. In practice , it is necessary to provide openings in these closed stiffened shells for, for example, doors, cockpits, bomb bays and windows in passenger cabins. 2. These openings or “cut-outs” produce discontinuities in the otherwise continuous shell structure, so that loads are redistributed in the vicinity of the cut-out, thereby affecting loads in the skin, stringers and frames.

22. 4 Cut-Outs in Fuselages 3. Frequently, these regions must be heavily reinforced, resulting in unavoidable weight increases. In some cases, door openings in passenger aircraft, it is not possible to provide rigid fuselage frames on each side of the opening, because the cabin space must not be restricted. In such situations, a rigid frame is placed around the opening to resist shear loads and to transmit loads from one side of the opening to the other.

22. 4 Cut-Outs in Fuselages To find the effect of cut-outs for windows: 24

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