ORIGINAL: Tall Paul
The plate attached to the landing skids prevents the lift of the rotors from making the heli climb.
The flow at the plate is no different than the flow without the plate, when the plane is on the ground.
It's just action.. the downward moving air's lift force preventing the plane from climbing because the air hits the plate fixed to the machine... reaction.
It's the old bootstrap situation.
Then if the plate had three side walls with one side open the helicopter wouldn't be forced (albeit very gently) in the opposite direction? Answer: On a frictionless surface the helicopter would creep in the opposite direction.
Rock,
I agree drag would occur in the vertical direction and could rob thrust but I'd bet the logging mod was to trim tare weight for more payload. Green wood is extremely heavy and non-essential weight in a commercial transport application can make a huge difference in total profit over the useful life of the equipment.