Hazlenuts
Perhaps the 'effective' aspect ratio is increased?
I think perhaps we are talking at cross purposes, as you are offering a reason why drag is reduced, and I am seeking a Newtonian explanation of why lift is increased.
As a straight response to your suggestion I would only remark that there is nothing in Newton's equations that relates to AR and that infinite AR (2D) wings also show ground effect.
In the case of drag reduction I think it might be very simple, and lift and drag changes are two sides of the same coin:
When we talk of a lift increase in ground effect we usually mean an increase at constant alpha. When we talk of a drag reduction we usually mean a reduction at constant lift coefficient. Invert that first statement and we have 'a given lift coefficient is obtained at a lower alpha when in ground effect' That means that at constant lift the alpha is reduced in GE, so that the resultant force vector is inclined further forward leading to a reduction in drag.
Lyman
Hence the compression, increased pressure, and lift without benefit of incidence increase?
Newton's equations as applied to an explanation of lift generation contain mass, velocity, momentum and their derivatives. There is no mention of compression or pressure - indeed such things are foreign to Newton's work.
Consequently I think that you cannot invoke such concepts in a valid Newtonian explanation of how ground effect works.
For myself, I can't get my head around anything further than:
The Newtonian explanation of lift in free air is that starting with a block of air at rest (zero momentum), passage of a wing leaves some of that air with a downward velocity and momentum. to impart that change there must have been some force whose magnitude is given by the rate of change of momentum. There must also have been an equal and opposite force acting on the wing, which we call lift.
Now it seems to me that when the wing is placed near the ground the underwing mass available for deflection is reduced - vanishing to zero if the TE actually touches the ground. In addition I would think that the average downward velocity of the mass of air going under the wing would also be lower than in free air because of the constriction imposed by the ground - in effect being half the downwash actually at the TE.
Both of those seem to me to lead to a reduction in lift rather than an increase. If anyone can point out my mistake or suggest some other Newtonian mechanism that would explain the lift increase one finds in ground effect I will be very interested!
Finally, I found these piccies that might be interesting in this discussion -
