The “displaced frequency” method is widely used in the aircraft industry for estimating the generalised masses and stiffnesses of aircraft, which are used in response and other aeroelastic calculations. The usual procedure is to measure the natural frequencies, ωi, and the associated normal modes, zi, by standard resonance testing techniques. (The number of frequencies and modes actually measured depends, of course, on the accuracy required in subsequent calculations.) A small mass, or group of masses, is then added to the structure at a convenient point or points and the resulting frequency changes, δωi noted. Now if αi and εi are generalised masses and stiffnesses respectively, then in normal co-ordinates