Structural modification of a helicopter tailcone

The structural modification of a Lynx Mark 7 helicopter tailcone is considered. The problem is especially demanding because of the almost symmetric structure of the tailcone, which in its initial configuration (prior to modification) results in a number of very weakly excited responses to certain excitations, so the corresponding terms in the full 6×6 receptance matrix are very difficult to obtain. The modification, in the form of a large overhanging mass, has the effect of coupling the initial-system responses and the weakly excited ones become important in determining the modified-system receptances using the structural-modification theory. The results are interpreted with the aid of a finite-element model. Very good estimates of the rotational receptances are obtained, the only significant difference from finite-element results being that the model is stiffer than the physical structure. Noise on the weakly exited responses is amplified in the estimated modified-system responses and it is demonstrated, using the finite-element model, that the use of a single X-block measurement to connect the modification to the tailcone is insufficient at frequencies higher than the first double peak of the modified-system receptances. It is considered that the exercise provides a demanding test of structural modification theory resulting in a very useful practical assessment of the limitations of the method.