1. The coefficients CMSand CVTare non-linear functions ofthe non-dimensional helixpitch andaregivenby equations (A-5) and (A-6). The core radius factor d^ depends on the core swirl velocity profile: for the `forced vortex' core usually assumed in theoretical studies of helical vortex motion d^ is1.0,whereasfor a `flat' swirl velocity profile d^ is » 0.779. Delta wing vortices typically display both forms of swirl velocity variation, with the swirl profile naturally dividing into three distinct regions14: a small 'inner' core where viscous effects predominate and the swirl velocity increases linearly with radius, an 'outer' core where viscous effects are small and the swirl velocity is almost constant, and finally a surrounding potential vortexflow with swirl velocity varying inversely with radius. The corresponding axial velocity variations give maximum swirl ratio at the edge of the outer core; as far as vortex stability is concerned it appears then that it is the outer core region that is critical. The inner core radius is typically 10-20% of that of the outer core, but it contains perhaps 20-30% of the total vorticitycontained withina leading-edge vortex.