Ground effect
puntosaurus
Gordon Leismans book gives a good account on ground effect, including the effect of forward speed.
For instance: T at height Z / T OGE = 1 / ( 1 - (R/2z)^2)
The way I modelled this in my R44 simulator is by using a similar formula to calculate the induced speed by the rotor. The ground effect blocks the downwash, so the induced speed gets reduced. I augmented the classic Glauert formula the calculates the Vi, with a similar term as above and get the same trust calculations as Leishman suggests.
The reduction in induced speed increases the rotor performance, and as a consequence the trust.
A similar reasoning can be applied on the effect of forward speed on the rotor induction speed. This leads to a behaviour where at a given (low altitude) the power first increases as the blockage disappears because the downwash escapes backwards (not blocked anymore), and this happens before translation lift starts taking over (= Glauert), that reduces induction speed which in turn increases trust.
Hope that helps
Delta3