Adhesive Bonded Doubler for H-3 Helicopter Blade Spar

The effectiveness and reliability of an adhesive bonded boubler in the elimination of crack formation and growth due to stress concentration in a metallic substructure is demonstrated. Quantitative design information to enable optimization of the doubler is developed and the methods for the field application of doublers to the H-3 spar are described. A preliminary mechanical and thermal stress analysis has shown that a boronepoxy laminate should provide the lightest weight, thinnest, and most reliable double compared to any other available material, except perhaps beryllium sheet material. The mechanical and thermal loads induced in the doubler and the adhesive bond under the most severe conditions, operating at -65 F, are 44,500 psi compression and 1050 psi shear respectively.