There are several well developed theories reported in literature for evaluating performance of a hot wire anemometer. On the other hand, although hot film probes play a rather useful role in the experimental liquid flow studies, no well developed theory systematically accounting for conduction and free convection losses is available, particularly, in the Reynolds number range of 2-350 (based on average flow rate and film width). This is indeed surprising since the fact that in this range of Rn viscous effects are of the same order as those due to inertia, was first recognised by Barker as early as in 1922. Thus it has been necessary to resort to careful calibration for each shape of the probe. The theory presented here represents a modest step towards minimising dependence on such tedious experimentation with reference to widely used wedged film probes.

The use of the theory of geometrical acoustics in the calculation of focussed bangs has received wide attention over the past two decades. None of these calculations however had been formulated for an aircraft manoeuvring in an arbitrary manner in a real atmosphere. Further, in most cases the results have not been presented in such a manner as to be useful to practising engineers. Recent formulation now permits the prediction of the focussing ray precisely and directly without having to compute all the rays reaching the ground from the source.