Text extracted from the document “minimax stress analysis”
4. Spar Analysis:
Spar analysis is done in accordance with Reference 8.
In order
to determine the maximum allowable compressive stresses in the spar caps, the material modulus of rupture is modified according to the spar cross section (per Reference
to determine the maximum ultimate stress in the compression flange. The opposite flange is then checked for the negative compressive load, and dimensions modified if necessary. These ultimate stresses are multiplied by 2/3 to determine the maximum allowable stress.
The
calculated stresses are then compared with these maximum allowables. A doubler was added to the rear spar as a result of preliminary calculations.
Figure C-2 and C-3 show the calculated
spar bending loads versus allowable loads.
Maximum shear load for the front spar occurs at the +4.4 G symmetrical load, and is calculated to be 293 pounds in the web just inboard of the strut attach position (72.75 inches from aircraft centerline).
From Ref. 10, section 2.7, the shear web
is calculated to support a shear load of 262 pounds without buckling. Since shear buckling of thin plywood is an inexact science, a test was performed (in addition to the original structural static test) on a representative shear sample identical in dimensions to the spar web.
This sample resisted a
total shear load of 375 pounds before the test fixture failed.
The sample itself did not fail. The leading edge D-section is not loaded in torsion, and is held essentially rigid (in twist) relative to the spar by the wing and strut structure.
Therefore,
the D-section can be assumed to support part of the shear load of the main spar. Based on the deflection of the spar web for a given load, the D-section is calculated to support 46 percent of the load of the shear web, or 172 pounds, for a total of 547 pounds.
This provides a safety factor of 1.87.
The rear spar shear web dimensions preclude buckling failure, therefore, the maximum shear load depends on the basic shear strength of the 1/16 inch plywood. The maximum shear load for the rear spar occurs at Va with 1/3 aileron deflection.
This
load is calculated to be 350 pounds just inboard of the strut attach position. Ultimate shear strength of this section is 637 pounds, for a safety factor of 1.82.