Originally Posted by
Torquetalk
The next part of the procedure requires lowering the collective (once VTOSS and a rate of climb are achieved) to recover 102%. This follows fairly shortly after the initial droop and acceleration and results in an increased ROC. But if more lift/rotor thrust has already been achieved by drooping the RPM somewhere below 102%, why does lowering the collective again result in an increased ROC?
As mentioned already, the droop before Vtoss may not solely be for the best L/D ratio, however the Nr for the best L/D ratio may also change as airspeed increases, so that is also something to consider (as well as the fact you're sliding down the power required curve).
I agree with ShyTorque though that a margin for some droop in the acceleration from TDP to Vtoss allows for the short lag in power delivery when an engine suddenly is asked to spool up to 100% output and gives pilots some accepted margin to work with. That's something that was pointed out to me when first flying PC1 takeoffs, as well as the need to
convert some rotational energy into added lift so that obstacles can be cleared (or the touchdown can be cushioned) while on the back end of the power required curve. I liken the second point to making mountain takeoffs in a piston; limited by full throttle, there were some sites where a little Nr droop (only between 2-3%) gives some extra lift between ground effect and translational lift so that treetops/obstacles could be cleared on the edge of the landing site.