Local Strain Distribution Produced by Warping: The Case of a Tail Rotor Blade of a Helicopter

Aircraft engineers in the design phase often use practical solutions and results of the elementary theory of structures under the classical hypotheses assumed for shear, torsion and bending behaviour of structural elements that can be studied as "beams". This assumption, very powerful in general, under particular situations (for example: the modification of loading conditions in the development of new aircraft versions) can be inadequate to represent real distributions of stress and strain. This is particularly true when local behaviour of structures depends on secondary phenomena such as warping produced by non-uniform bending and torsion acting on structural elements with variable stiffness along their flexural axis. In this paper the results of analytical and experimental studies carried out for a metallic tail rotor blade of a helicopter are summarised; this research activity was born during a collaboration between the Department of Aerospace Engineering of Pisa and an aeronautical industry. Local strain distributions under real loading conditions have been obtained by means of finite element analyses and by means of strain gauge measurements. To describe warping mechanisms accurately, also caused by the presence of a reinforcing plate of the blade structure, sub-modelling technique has been used and during a fatigue test, the blade was instrumented with an appropriate number of rosettes. Numerical and experimental results show, with a good agreement, that local geometry of the blade and the severe loading condition produce a considerable increase of strains that elementary theory can not predict; moreover, numerical results and above all strain measurements have proved that local yielding of the material can occur during the service life with detrimental effects on the fatigue endurance of the tail rotor blade.