It is obviously necessary that an electric model should have adequate installed power. The widely-recognised Kopski "Watts per pound" recommendations take account of weight, dragginess, and the speed and agility required.  In this context ,"Watts" means battery output under static conditions. Motor output will typically be 50% to 80% of battery output.

   This motor output can be utilised to drive a large slow prop or a small fast prop. What you get depends on the motor characteristics and the gear ratio.

   The large prop will give a high static thrust and a slow efflux speed. Conversely, the small fast prop will give a low static thrust and a high efflux speed. It is important to make the correct choice; very poor performance can result if you don't.

   How to choose?    The efflux speed should be roughly equal to the intended max flying speed. This flying speed must be the same as that used to evaluate the installed power.

   It is to be regretted that efflux speed is rarely mentioned in test reports. It can be measured with a wind meter. As an alternative to efflux speed measurement, pitch speed can be utilised; static efflux speed is usually 70% to 80% of static pitch speed. See "Pitch speed" topic on this website.

   You may have noticed that I have made few references to thrust. It is frequently mentioned in the magz, with the implication that "more thrust" is always better. The quest for high static thrust can unwittingly result in inadequate efflux speed, with dire consequences for flight performance.

   Summary.    In addition to providing the correct installed power, ensure that the efflux speed is appropriate to the model's flying speeds.