Optimisation of composite wind-tunnel wing models for frequency, flutter and divergence

A comparison has been made between the composite beam designs produced by m inimum mass optimisation using two different sets of constraints. The first approach constrained the design to have a given separation between fundame ntal bending and fundamental torsional natural frequencies; the second cons trained the design to have a given flutter and divergence speed. The beams are modelled as a series of elements, stepped in thickness at discrete node s, with the Dynamic Stiffness Method being used for calculation of their na tural frequencies. The aeroelastic constraints are obtained from the Fortra n program CALFUN. The results show that for similar flutter and divergence speeds, the optima produced using aeroelastic constraints have a slightly l ower mass (up to 4% lower) and a less 'hard' flutter onset. However, thr ti me taken to produce these optima is significantly longer tin excess of 2 or ders of magnitude). A preliminary study discusses the merits of a combined optimisation method where frequency constrained optimisation is used to pro vide a near-optimum starting point for flutter and divergence constrained o ptimisation. In addition, a wind-tunnel model of one of the optima has been manufactured and subject to both modal analysis and wind-tunnel tests to v alidate the flutter speed calculations. This shows that when using strip th eory, CALFUN predicts a conservative value of flutter speed for this design . Further investigation has shown CALFUN's lifting surface theory to be mor e accurate for low aspect ratio models.