Advanced Grid-Stiffened Composite Shells for Heavy-Lift Helicopter Blade Spars

Abstract

Heavy-lift helicopter blades are estimated to be large and heavy (upwards of 360 kg per blade) when the spars are constructed as monocoque structures. It is proposed to replace these conventional spar designs with lighter grid-stiffened composite shells. Composite stiffened shells have been known to provide superior strength to weight ratio and damage tolerance with a great potential to reduce weight. The design space for grid-stiffened rotor blade spar structures is new and the behavior of these structures under axial, bending, and torsion loads needs to be accurately predicted. The overall objective of the present research is to develop and integrate the necessary design analysis tools to conduct a feasibility study in employing grid-stiffened shells for heavy-lift rotor blade spars. A new analytical model was developed to accurately model various grid stiffening configurations and the results were compared with current state-of-the-art analysis, finite element analysis (FEA) and in certain cases, experiments. Parametric studies of grid density, stiffener angle, and aspect ratio of stiffener cross section showed excellent correlation (within 5-7%) between FEA and the new model. On the other hand, differences in the range of 26-60% between current state-of-theart analysis and FEA were seen for certain designs. A preliminary design study was conducted to evaluate the weight saving potential of a simple cylindrical grid-stiffened rotor blade spar structure compared to a baseline monocoque design. Discretized design variables were stiffener density, stiffener angle, shell laminate, and stiffener aspect ratio and the design constraints were stiffness, material strength, and stability. For the range of the design variables explored, a weight saving of 9% compared to the baseline was obtained without violating any of the design constraints. Nomenclature ̅ = non-dimensional shear correction parameters δ i = beam end deflections ε i , γ ij = in-plane normal strains, shear strains θ = stiffener orientation angle κ x , κ y , κ xy = curvatures a ij , b ij , d ij = equivalent stiffness compliance coefficients (i, j =1,2,6) b s , h s = width, depth of the stiffener cross section d 0 , d 90 , d θ = spacing between longitudinal, transverse, and angle stiffeners EI, EA, GJ, = bending, axial, torsion stiffness GA = shear stiffness of the beam k bc = local and global stiffness matrices of a beam-column L 0 , L 90 , L θ = length of the longitudinal, transverse, and angle stiffeners m i , n i = plate or shell moments, forces per unit width (i= x,y,ψ,z) R g , R s = radius of the grid layer, shell at mid-plane