A semi-empirical analysis of unstable crack propagation in thin sheets is given. An effective value of the crack tip strain concentration factor is used as the criterion for instability. Good correlation is achieved with experimental results from unreinforced sheets of aluminium and titanium alloys and high-strength steels. The interdependence of geometry and stressstrain curve properties in determining the unstable crack propagation characteristics of sheet materials is illustrated in an appendix.

By consideration of a series of simple models the static aeroelasticity of conventional aircraft is first discussed and then extended to the integrated configuration combining fuselage, wing and tail. It is shown that, on the basis of linear aerodynamics, a maximum speed exists at which control is possible. Overall deformations and control forces become large at speeds approaching this critical condition. Part II, to be published, contains calculations on slender plate aircraft including non-linear aerodynamics.