212 man said.
is not true; the H/V curve is plotted for level flight, and the graph in the RFM will often state this. The published graph is only valid for one set of ambient and weight conditions whereas in reality it expands and contracts. It may even disappear given the right circumstances (how else can you do a vertial Cat A Take Off?)
I believe that the lower part of the H/V curve shaded area on the left is calculated for takeoff and the upper part is calculated for the cruise although the join is not shown, and slightly different assumptions are made for the cruise part, i.e. a slightly longer delay before reacting etc.
also on many graphs (R22 is a good example) a recommended take off profile which remains outside the shaded area is shown.
also I am talking about single engine helicopters so Cat A, is not relevent.
maybe a clever chap could give us the definitive answer on this ?
regards
CF