Designed by George Fletcher and introduced in around March of 1961, the FROG Viper diesel of 1.5 cc displacement was probably the best engine of any size ever manufactured by International Model AIrcraft (IMA) Ltd. under their FROG banner. It was designed as a competition engine from the outset, owing nothing whatsoever to the design of its predecessors in the FROG 1.5 cc series.
The twin ball-race twin-stack rear drum valve design arrangement of the Viper set it well apart from any of its high-performance contemporaries such as the Oliver Tiger Cub and the E.D. Super Fury. It performed at a high level as well, being found to develop some 0.162 BHP @ 15,800 RPM. The quality of its construction was of the very highest order.
A plain-bearing glow-plug version of the Viper called the FROG Venom was also produced, although this was far from a success. It performed at a remarkably low level by comparison with its diesel progenitor, being found to develop only 0.100 BHP @ 15,000 RPM despite the use of a fuel containing 30% nitromethane.
Sadly, the FROG Viper was one of the victims of IMA's early 1962 decision to abandon model engine manufacture altogether. Although the manufacture of some FROG models was resumed in 1964 by Davies-Charlton (D-C) Ltd., the Viper was not among those models. Hence this fine engine had a production life of less than a year. Examples in good condition are relatively rare today.
A detailed article about the FROG 1.5 cc models from start to finish (including the Viper and Venom) may be found elsewhere on this website.