Page 223 - Aircraft Stuctures for Engineering Student

Problems 207

depth of web will be 750mm, and the depth between web-to-flange attachments is
725 mm.
The spar is to carry an ultimate shear force of 100 000 N. The maximum permissible
shear stress is 165N/mm2, but it is also required that the shear stress should not
exceed 15 times the critical shear stress for the web panel.
Assuming a to be 40" and using the relationships below:
(i) Select the smallest suitable web thickness from the following range of standard
thicknesses. (Take Young's Modulus E as 70 000 N/mm'.)
0.7mm, 0.9mm, 1.2mm, 1.6mm
(ii) Calculate the stiffener end load and the secondary bending moment in the
flanges (assume stiffeners to be symmetrical about the web).
The shear stress buckling coefficient for the web may be calculated from the
expression
K = 7.70[1 + 0.75(b/d)']
b and d having their usual significance.
The relationship between the diagonal tension factor and buckling stress ratio is
T/TCR 5 7 9 11 13 15
k 0.37 0.40 0.42 0.48 0.51 0.53

Note that a is the angle of diagonal tension measured from the spanwise axis of the
beam, as in the usual notation.

Am. 1.2mm, 130As/(l +0.0113As), 238910Nmm.