What is a speed droop?
Speed droop is the decrease in engine speed as the engine load is raised. It is a property of many mechanical governors. Speed droop can vary and is often adjustable (although not always easily). On electronic engines speed droop is often turned on by an electronic switch.
Speed droop is useful for many applications and for mechanical speed governors, it is a control that allows the engine to remain stable.
Why is droop necessary?
The best description we could find for the reason for this is from a Woodward manual located here. The extract is as follows from page 13:
In a system without droop, a load increase will cause the engine to slow down. The governor will respond by increasing the fuel until the engine speed has returned to the original speed. Due to the combined properties of inertia and power lag, the engine speed will continue to increase beyond the original speed setting, causing an overshoot in speed. The governor again will respond to decrease speed to correct for the overshoot. It will over-correct the speed in the other direction causing an undershoot. This overcorrection of speed in both directions (instability) will amplify until the engine trips out on overspeed.
And this means...?
For most engines with mechanical governors, the engine speed at no load is 2-3Hz higher than the full load speed setting, because as this droop effect takes place, the speed of the engine will fall and therefore the aim is to have the full load speed generating 50Hz (or 60Hz).