Why is ground effect greater over smooth hard surfaces?

Question

Helicopter Flying Handbook: Ground effect is the increased efficiency of the rotor system caused by interference of the airflow when near the ground. The air pressure or density is increased, which acts to decrease the downward velocity of air. Ground effect permits relative wind to be more horizontal, lift vector to be more vertical, and induced drag to be reduced. These conditions allow the rotor system to be more efficient. Maximum ground effect is achieved when hovering over smooth hard surfaces. When hovering over surfaces as tall grass, trees, brushes, rough terrain, and water, maximum ground effect is reduced. Rotor efficiency is increased by ground effect to a height of about one rotor diameter (measured from the ground to the rotor disk) for most helicopters. Since the induced flow vertices are decreased, the AOA is increased, which requires a reduced blade pitch angle and a reduction in induced drag. This reduces the power required to hover IGE

I understand ground effect and In Ground Effect versus Out of Ground Effect. Air pressure is increased below the helicopter, reducing induced flow, increasing AOA and decreasing drag.

But why is ground effect greater over smooth hard surfaces than over tall grass, trees, bushes, rough terrain, and water?

I've searched Aviation Stack Exchange, textbooks, and the front page of Google, but haven't found an explanation.

At first I would think friction from surfaces like tall grass would hold air below the helicopter, increasing ground effect. But we know this isn't true, since this would mean there's less ground effect over a runway.

Another guess would be the terrain disrupts the air from bouncing back toward the helicopter. Does the air reflect sideways instead, meaning the force of air bouncing back toward the helicopter is what increases lift? The Helicopter Flying Handbook contradicts this explanation with the following as the core source of ground effect:

"The air pressure or density is increased"

Don't be afraid to include complicated aerodynamics if necessary. :)

Answer 1

From a conservation of energy perspective, the rotor is transferring energy to the air, which then moves down and is deflected by the ground. A soft or drag-inducing surface will absorb more of that energy, whereas a smooth hard surface will deflect it.

Think of it like running: If you run on a hard surface, your down force creates an equal force up. If you run on sand, some of the down force is lost by moving the sand, and you are not propelled up/forward as much. This is why it is harder to run on sand than a solid surface.

Answer 2

I think it's based mostly on perception of altitude above ground level. If you're hovering over trees, or over a rough sea, what you care about first and foremost is your clearance from the tops of the trees and waves. And if the surface of the vegetation is dense, like tall crops, trees or bushes, that's what your eyes will focus on, so you perceive that as your height AGL. But for performance purposes, your aerodynamic altitude AGL is actually greater, namely the surface of the ground.

For example, if you're hovering half a rotor diameter above the tree tops, you would perceive that you're close to the ground and might reflexively expect a bit more thrust, but they wouldn't have to be very tall trees for you to be two rotor diameters above ground level, so well out of any aerodynamic ground effect.

Answer 3

Not to be "that guy" but none of the provided answers are correct.

You got close in your original question when you talked about friction and the air collecting under the helicopter. But surface friction doesn't help create a better air cushion under the helicopter (that would be good). Rather, surface friction keeps the air closer to the helicopter, allowing more of it to recirculate into the rotor system.

In a hover, the downwash is flowing vertically down, and as it encounters the surface, it transitions from a vertical downward flow to a horizontal flow spreading out in all directions. I'll use one of my corny smartboard drawings to illustrate.

Over a smooth surface, this outflowing air travels more easily with less friction, and thus gets farther from the helicopter, making less of it available for recirculation. (Magenta flow below).

The same downwash over any surface that increases airflow friction, increases the amount of air that decelerates close enough to the helicopter to be recirculated. (Blue flow below). Hence you will need more power to produce the same amount of lift. Less power than OGE, but still more power than a smooth surface.