I'm not a believer of the pressure having an effect on ground effect theory, but I have a question to which I have no answer.
If the presence of the ground reduces the induced flow, then wouldn't the lower induced flow result in a greater pressure due to Bernoulli's principle?
Before everyone's blades start spinning, I'm not saying aerofoils produce lift because of Bernoullie (also not saying they don't)...I just don't want to get into that discussion.
_____________________
I agree that the engines don't have to work as hard IGE than OGE (actually seen this phenomenon myself
). I agree that the rotors produce the same amount of lift in a stable hover IGE and OGE. From that I deduce that the extra work done OGE is due to drag...the only one that can change is blade induced drag. This is where it gets complicated. I seem to recall that blade induced drag is a function of angle of attack, density, geometry, and airspeed. The trick here is that density is still air density, ie not affected by dynamic pressures.
The only quantity in the blade induced drag equation that changes IGE vs OGE is the angle of attack (alpha). alpha is a result of induced flow, relative airflow, blade pitch and rotation of the blade. IGE vs OGE you change induced flow and blade pitch. Blade pitch is changed by the pilot (collective increase). Induced flow would increase slightly as a result of the collective increase, but would decrease overall due to the resistance to allowing the air to move through the ground. At the tips the vortex is changed, but that also causes a change in induced flow and a subsequent change in angle of attack.
Stopping here, the OGE vs IGE argument is explained simply without resorting to pressures.
However, pressure does play a role. It is the mechanism that changes the vortex and restricts the induced flow. If you really wanted to, I'm sure the equations could all be rewritten very complexly (that a word?) so that pressure is used and induced flow is ignored. That doesn't mean pressure causes anything, its accepted that the most simple explanation is the how that things work, other explanations are useful only in specific studies.
___________________
Someone pointed out that pressure doesn't increase below fixed wing aircraft flying very close to the ocean. That's correct, but in that case the "downwash" isn't moving straight down. Instead there is a wake behind the aircraft. That wake wants to flow, but the ocean won't allow it. The restriction in flow causes an increase in pressure (this is actually Bernoulli's theorem in a different light) in the wake. The equations of lift and drag still don't need that pressure change to explain ground effect, so I stand by those that insist pressure isn't a factor.
___________________
I know, I talked too much. Pressure is the mechanism that causes some of the changes that result in the rewards of ground effect. Ground effect (at the wing/blade) is fully explained without reference to the pressure of the air.
Food for thought: What about compressibility right at the blade? We know it happens...we've all seen contrails coming off the blades in the hover. Does that change IGE vs OGE? If so, it gives a bit of ammunition to the pressure explanation enthusiasts.
___________________
Back to original question. It's been said that the pressure beneath is the same IGE vs OGE yet the induced flow changes. Why doesn't the change in velocity of flow change the dynamic and hence the total air pressure?