“Ground effect creates down force”
This is correct and wrong at the same time.
Ground effect, simply it means the effect the ground creates.
When the wing gets close to the ground, the force that created by the wing increase and lose the resistance ate the same time.
As the wing gets closer to the ground it increases down force so “Ground effect creates down force” is also right answer to this.
But when a wing of an airplane is close to the ground it creates lift force rather than creating the down force.
So “Ground effect creates down force” is right and wrong at the same time.
As it gets close to the ground it increases the effect. May be that’s why many people say the force created by diffuser is ground effect. The wings that are close to the ground such as diffuser and front wings gets better efficiency because of ground effects. In summary, ground force explains why it gets better efficiency not ground force creates down force.
For example, WIG ship is an iconic example of using ground effects. WIG means Wing In Ground effect.
As you can see it on the picture, the ship is flying very close to water even thou it has small wings.
That is possible because improved efficiency of the wings.
May people are impressed when WIG ship flew very high, but WIG ship was created to fly low with better efficiency. But yes I know plans are spouse to fly very high.
Then why ground effects are created?
First, many people would know when speed of air increases it lowers the pressure and when it slows it increases pressure.
Take a look at the picture, at the curved line 3out of 4 space has decreased so speed would increase to 3out of 4. As the speed increases it loses the pressure.
Think of the curved part as bottom side of wing, loss of pressure will creates down force that pulls down to the ground.
And if the wing gets closer to the ground half of space are gone so it will increase twice of speed. So more speed more pressure that means more down force.
There are other effects too. But I will come back to you with more information next time