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squishy
Ground turbulence or ground effect, cannot be stopped by the type or condition of the ground's surface, it's caused by all ground, no matter what kind of ground. It's the prop wash from the craft hitting the ground and coming back up or forming a bubble under the craft and impacting performance. It is what causes quads to wobble on descent because they fly or lower through their own prop wash. It's a problem all rotor craft must deal with, any craft which points thrust downward will have to deal with it. Water and broken surfaces can be used to reduce the effect, flat cement or roads would have the most.

Here's some good reading on it:
When an aircraft is flying at an altitude that is approximately at or below the same distance as the aircraft's wingspan or helicopter's rotor diameter, there is, depending on airfoil and aircraft design, an often noticeable ground effect. This is caused primarily by the ground interrupting the wingtip vortices and downwash behind the wing. When a wing is flown very close to the ground, wingtip vortices are unable to form effectively due to the obstruction of the ground. The result is lower induced drag, which increases the speed and lift of the aircraft.[3][4]
A wing generates lift, in part, due to the difference in air pressure gradients between the upper and lower wing surfaces. During normal flight, the upper wing surface experiences reduced static air pressure and the lower surface comparatively higher static air pressure. These air pressure differences also accelerate the mass of air downwards. Flying close to a surface increases air pressure on the lower wing surface, known as the "ram" or "cushion" effect, and thereby improves the aircraft lift-to-drag ratio. As the wing gets lower, the ground effect becomes more pronounced. While in the ground effect, the wing will require a lower angle of attack to produce the same amount of lift. If the angle of attack and velocity remain constant, an increase in the lift coefficient will result,[5] which accounts for the "floating" effect. Ground effect will also alter thrust versus velocity, in that reducing induced drag will require less thrust to maintain the same velocity.[5]
Low winged aircraft are more affected by ground effect than high wing aircraft.[6] Due to the change in up-wash, down-wash, and wingtip vortices there may be errors in the airspeed system while in ground effect due to changes in the local pressure at the static source.[5]
Another important issue regarding ground effect is that the makeup of the surface directly affects the intensity; this is to say that a concrete or other smooth hard surface will produce more effect than water or broken ground.
"Education is not about filling buckets; it is lighting fires." W.B. Yeats

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Techno
I thought about that air pocket. the plane would be so thin I don't think it would matter
-Techno
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squishy
My quad isn't even an airplane, and it effects it greatly, the craft's form does not matter either. It's the thrust meeting an immovable object (the ground) and bouncing back at itself. This will effect the wings (in the case of a multirotor, it's props, which are spinning wings). This is an advanced concept and one I learned while trying to reduce the wobbles from my descending quadcopter. So far the only solution I have come up with is to fly out of the prop wash, in any direction. If you have ever seen those large hexacopters like the S800 from DJI, they have each motor tilted a little, this forces the thrust and prop wash out to the sides so you can descent in the clean air left in the middle. I have found that by simply flying in any direction while descending does the same thing. Basically to remove ground effect entirely, you must be moving and cannot hover and rise/descend in one spot.
"Education is not about filling buckets; it is lighting fires." W.B. Yeats

http://www.youtube.com/user/squishy654
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squishy
I think I may have just solved the ground effect issue for VTOL and I am not even in this game...

The rotation of the motors for level flight require more mechanical engineering that what has been done before but this will help a lot with one of the biggest problems.

Current tilt rotor designs require flight controllers to mask the ground effect, they fly like this, usually with four motors like foamntapes design. Notice all the dirty air beneath the airplane. During hover with a wing or transitions, this will cause instability, in fact, as I explained in my post before this, it will also cause instability when simply descending, you don't need to be near the ground to feel the effects of your own prop wash.

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Here is my solution, it's not really mine it's stolen from the s800. If you are able to tilt the motors up and inward at the same time, you throw the prop wash out to the sides while in a hover and keep the air pretty clean under the wings. This will help you take off, land and descent much better and those unwanted effects will be reduced. This is just a very rough sketch, obviously, and I believe the angle or deflection only needs to be like 10 degrees or so, at least that's where I would start, then increase the angle till you get unwanted and unknown efficiency effects. This design will reduce flight times, but who cares about those anyway, you are just trying to make VTOL function well for now. This may even add a self stabilizing force similar to dihedral.
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"Education is not about filling buckets; it is lighting fires." W.B. Yeats

http://www.youtube.com/user/squishy654
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