Ronnie, Static droop is the RRPM that you get when you apply power to a rotor system;ie, on the ground with ,say 100% set and collective fully down,Tq, say 40%; now as you increase tq in 10% steps the RRPM may `droop` to 99.9,99.8 ,99.7%,etc, up to 100%tq. this can be plotted on a graph and will give you a line; most RRPM gauges are somewhat inaccurate so the gauge should have been calibrated ,or you use a data recorder.That `line` will give you the response of the governor on the engine,be it hydromechanical ,or electronic(FADEC),and should follow the engine manufacturers set-up; if not the governor needs to be adjusted.; that is the Static Droop line.
Transient droop is the RRPM that you get when you apply a rapid collective input/up/down,ie, if you apply say 70% tq in one second the RRPM may droop to say,98%,apply 90% and it droops to 96% and in each case should recover to the static figure. This gives the dynamic response of the governor system. These tests are normally carried out with the helo on a tie-down to get the system sorted and adjusted; and if you mave multiple engines they should all follow the same pattern- not always true as in service ,engines can be changed,components too.
Having sorted it on he ground ,we then go and fly it ,and then do the same tests again,only now ,we also do it from /into autorotation,and one has to be careful because we may overspeed the turbine as we get to zero Tq,or damage the freewheel units if we don`t `join the needles` before applying power.
The `transient` droop checks are important because if you have a sluggish governor,you can well droop the engine to the point that it `surges`,ie it is getting too much fuel ,but not enough air and it will `cough,splutter` may flame-out,overtemp,.
This also depends on the type of compressor in the engine.
Further,there is also `droop cancelling` which means that if we apply power, the RRPM will rise,instead of fall.
Also , on earlier helos,an `anticipator` is fitted in the system which basically measures the rate at which the collective lever is being moved up/down,and by how much; this sends a `short-cut to the governor to either increase /decrease fuel flow,before the RRPM has time to respond,thus `anticipating `the response.
Best helos for engine response,before the Fadec systems ,were the Gazelle,and Scout/Wasp,where you could pull full power in less than half a second,and would only surge if the governor was a bit worn,or the compressor was in need of a wash.. Syc