Question about Ground Effect

[Niall]

The basic principle is that when a wing gets closer to the ground it creates mor downforce.

The principle behind this is that as the wing gets closer to the ground there becomes less space between the ground and wing for the air to flow through. Hence the air is accelerated even more than normally and hence more downforce is acquired.

What I want to know is why does the air accelerate more tan need to attain the pressure it would have been if the wing had been in open air and instead accelerates more than it needs to and hence cretes more downforce.

Niall

[ma]

Easy.

Think of the underside of the wing and the ground as forming a restriction much like the venturi in a carburettor. ALL of the air between the wing and the ground must squeeze thru this restriction, and must speed up to do so.

What governs just how much it has to speed up is the "throat ratio", which is easily visualised as the difference between the height from the ground of the leading edge of the wing, and the height between the lowest point of the wing underside and the ground. As the wing gets closer to the ground, the ratio between these two heights increases.

For a wing in free air, the ratio is the distance that the air has to travel on the underside vs the distance it has to travel on the top side.

[twin 40's]

wings closer to the ground cannot have as largea rake on them.

The air flows under the wing at a faster speed so more downforce is produced however, the wing will stall at smaller angle than wings at the present height. So the angle or rake has to be kept lower so that the airflow doesnt break off the surface of the wing which would cause it to stall.