As @Zeus has pointed out correctly: Drag coefficient stay constant in my scenario, not drag -- I've updated the explanation accordingly. Sorry for that blunder. The conclusion does not change: Engines with constant PSFC only have an advantage when flying slowly and get worse much quicker than engines with constant TSFC -- but actual engines are mostly ...
The simple (and probably unsatisfying) answer would be:
The maximum headwind into which a helicopter can land will be somewhat less than it’s listed maximum operating speed.
Flying at Vmo into a headwind of equal speed should lead to a stationary velocity relative to the ground. In theory, the aircraft could gently lower itself all the way the ground like ...
The relevant section (3.2.3) of CAP 698 explains how to calculate this as follows:
Distance to Reach given height.
To calculate the ground distance travelled in order to attain a given height above
a) Convert the IAS 100 kt to a TAS, assume no position error.
b) Apply the wind component to the TAS to obtain the ground speed.
c) Determine the ...