"physics behind vertical flight is described by the author as 'momentum theory', which was developed for marine propellors in the late nineteenth century. As the name implies, this is just an application of the principle of conservation of momentum. The rotor disk of the helicopter feels a thrust created by the action of the air on the helicopter blades. It must therefore exert an equal and opposite force on the air. This forces the velocity of the air in the rotor wake to be opposite in direction to the direction of the thrust. Momentum conservation, energy conservation, and mass conservation then give a relation between the induced power loss and the rotor thrust. The author also gives details on the 'vortex theory', which is based more on fluid dynamical laws of the flow field of the rotor wake. Emphasizing the local aspects, it reduces to momentum theory in appropriate limits. The author also shows how momentum theory applies to the forward flight of the helicopter. The author also treats helicopter performance analysis, which boils down to determining the power required and available for a range of flight conditions. The rotor forces and power must be calculated, and the author details two methods to do this: the 'force balance method' and the 'energy balance method'. The use of the computer has made this analysis considerably easier for the design engineer of course. The author gives a very interesting overview of helicopter speed limitations and how the helicopter could be landed safely after an engine failure, all of this being analyzed from a physics perspective. The mathematics of rotating systems is included in the book, along with the differential equations of motion for the rotor blade. The motion of the blade is expanded into a normal mode representation and analyzed using Sturm-Liouville theory. The author though outlines other approaches to the blade dynamics, such as the Lagrangian formulation and the Galerkin method. And also, in spite of the ability of computers to solve for the aeroelastic equations of motion, the author considers their analytical solution for the cases where such solutions can be obtained. One very interesting part of this discussion was that of 'ground resonance', which is a dynamic instability involving the the coupling of the blade lag motion with the in-plane motion of the rotor hub. There is then a resonance between the frequency of the rotor lag motion and the natural frequency of the structure supporting the rotor. "