BÀI GIẢNG QUY TRÌNH TRONG HÀNG KHÔNG

Design Criteria – Lower Skin Panelshall be sized using tension and shear interaction criteria:  all = allowable tension stress of the material used  t = applied tension stress  s = a

Procedure of Initial Sizing

Design Criteria Initial Sizing

Design Criteria – Upper Skin PanelNo local buckling up to ultimate load They shall be sized using compression and

shear interaction criteria:

 c = applied compression stress

 all = allowable compression stress of the panel, which is the smallest value of : crippling stress

 skin local buckling stress

 intermediate (Johnson Euler) column buckling stress

 s = applied shear stress

 all = allowable shear stress of the panel, which is the smallest value of :

 skin shear local buckling stress

 allowable shear stress of the material used

0

1

2

2





cr

s cr

c









 s

c c  s

Design Criteria – Lower Skin Panelshall be sized using tension and shear interaction criteria:

 all = allowable tension stress of the material used

 t = applied tension stress

 s = applied shear stress

Please note that Lower panel is also critical due to fatigue The criteria have to be considered is :

2 3

: Mises Von

to according failure

material as

where

0 1

















t comb

comb

all

0 0 1

1  



t

G all





Design Criteria – Spars

They shall be sized using shear criteria :

s = applied shear stress

all = allowable shear stress of the panel, which is the smallest value of:

 skin shear local buckling stress

 allowable shear stress of the material used

0 1



s

all





Design Criteria – Ribs

Due to concentrated loads (attachments: engine, flaps, etc):

Due to shear loads : Due to aerodynamic loads, the ribs shall be designed as

beams simply supported at the spars The web shall be sized using shear criteria :

s = applied shear stress

all = allowable shear stress of the panel, which is the smallest value of:

0 1



s

all





Design Criteria – Ribs Cont.

Due to crushing loads : The stiffener shall be sized using compression

criteria :

 = applied compression stress

0 1





all

Initial Sizing: Upper and Lower Skin Panel

idealised depth of the primary structural box, h

and bottom surfaces required to react the

appropriate bending moment, M, at each section from: P = M/h

Initial Sizing: Upper and Lower Skin Panel

Initial Sizing: Upper and Lower Skin Panel

only by the spar caps:

 Initially assume Ab is divided equally between all the boom spar

caps on one side of the box, and fb is the allowable proof stress in this case

b b

b

hf

M f

P

Initial Sizing: Upper and Lower Skin Panel

 For a distributed flange assume initially a uniform effective thickness across the width, w, to give

 Typically this thickness will be made up of skin and stringer area The effective stringer are being about half of that of the skin area Thus the actual skin thickness is about:

b

e

hwf

M

t 

b

e

hwf

M

t  0 . 65

Allowable Stresses - Direct

(bending) Stress

 The accurate evaluation of the allowable bending stress is

complex, requiring a knowledge of the detail features of the

structure both in the compression and tension surfaces

Experience suggests that if the magnitude of the allowable

compression stress is also used for the tension surface it makes the right order of allowance for fatigue/crack propagation

requirements although this assumption can only be approximate, especially when the allowable compression stress approaches the 0.2% proof value Thus the same allowable stress level may

be initially assumed in both surfaces The main parameter in

determining the allowable compression stress is the loading

intensity

 If mass booms are used as the primary means of reacting direct load, then it is appropriate to assume that under ultimate bending loads the 0.2% proof stress may be used

Allowable Stresses - Direct

(bending) Stress

allowable bending stress at ultimate loading may be assumed to be the lesser of the 0.2% proof stress or fb, where fb may be

approximately represented by:

where L is the local rib or frame spacing

w is the width of the box perpendicular to the bending axis

P is the effective end load

A is a function of the material

FB is dependant upon the form of construction

2 / 1















wL

P F

A

 Note that the value of A are appropriate to allowable stress and (P/wL) in MN/m2 units In general the values of A give conservative values for F b at stresses below the limiting value.

 Typical values for FB are also given.

Comparison efficiency for various stringer shapes:

Ideal Practical design Panel type

Skin

Str

A

A

Efficiency

Skin

Str

A

A

Efficiency

Geometry Elements Design

practise

a

a

t

b

10 or less

w

w

t

b

18-22

f

f

t

b

6-8

skin

str

A

A

0.5

a

t 0.7t

w

f

b b

0.4

Initial Sizing: Spar Webs

i. Due to Overall Torsion Moment:

 Estimate the enclosed area, A, of the primary structural box at

representative sections across the span

 The corresponding shear flow is:

QT = T/2A

 Where T is now the applied distributed torsion, and QT will be

nose up or nose down and hence positive or negative depending on the sign convention

 Select the allowable shear stress, fs as appropriate

 The mean material thickness needed to react the torsion

moment is then:

tq = T / 2Afs

Initial Sizing: Spar Webs

Qv = V/hT,

where V is the applied vertical shear force

Where x is the chordwise location of particular web relative to the mid point of the box

T V

w

x Q

s

w w

f Q

t 

Initial Sizing: Ribs Web

i. Due to concentrated loads (attachments: engine, flaps, etc):

can be taken as a cantilever beam loaded by a vertical shear force equal

to the hinge reaction and a bending couple due to the offset of the hinge chordwise from the rear spar location The spar web will react most of the vertical shear, and in practice if the hinge fitting is perpendicular to the rear spar, the rib flanges at the spar will be loaded by direct forces given by:

Where V is the hinge reaction

x is the offset of the hinge from the spar

h is the depth of the rib at the spar

h

x V

R  

Initial Sizing: Ribs Web

ii. Due to shear loads :

R

rib (i+1)

rib (i)

rib (i-1)

Qz 1 Qz 2

R  Qz 1 - Qz 2 (1)

2

z Q Q

R  

FS RS

q 3 q 3 h q R

h

3

2

 (daN/mm) = shear flow (6)

d up upright h h FS h RS

2

h

3 2

 (daN/mm) = shear flow (6)

2

Initial Sizing: Ribs Web

iii. Due to crushing loads :

Where:

lower = normal stress at lower panel ; trib = rib web thickness

lower

rib

panel n

t h E

L

t

.

.

.

2

) (

) ( upperpanel abs lowerpanel