cor blimey - the whole point of explaining this stuff is so that studes will remember it and carry the knowledge with them for the rest of thier flying career - Can I suggest that the formula above introduces a soporific quality to the proceedings! What on earth is wrong with describing it as a build up of pressure under the disc and in the hover the helicopter sits on it (thus reducing power etc, etc) wether this description is right or wrong it is easy to understand and helps explain a lot of other things later on - Keep it simple.