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Because the back-draft(updraft) is rotating, even though the air is travelling away from the inlet(the ground and so called infill layer) at a high velocity, the curved path of the vortex wall means it takes longer for the updraft to travel up than the down draft to travel down. This speed difference along with the centrifugal force of the airmass pushing outwards as it rotates creates a low pressure core. The airflow direction of the low pressure core is towards the point of origin. Therefore the low pressure core of the vortex is the source of drag itself since the flow vector is in the opposite direction of vortex expansion.

With an airplane, the vortex core is flowing in the opposite direction that the plane is travelling, with a tornado, the vortex core has a downdraft jet towards the earth, while the circulating updraft moves towards the clouds.

Because the back-draft(updraft) is rotating, even though the air is travelling away from the inlet(the ground and so called infill layer) at a high velocity, the curved path of the vortex wall means it takes longer for the updraft to travel up than the down draft to travel down. This speed difference along with the centrifugal force of the airmass pushing outwards as it rotates creates a low pressure core. The airflow direction of the low pressure core is towards the point of origin. Therefore the low pressure core of the vortex is the source of drag itself since the flow vector is in the opposite direction of vortex expansion. With an airplane, the vortex core is flowing in the opposite direction that the plane is travelling, with a tornado, the vortex core has a downdraft jet towards the earth, while the circulating updraft moves towards the clouds.

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[–] • 0 pt

I don't know what you're talking about. The pressure in the center is what's important not the velocity. For sure axial velocity isn't important, and only tangential flow matters elsewhere. Vortex threads attach to surfaces since this in effect seals the end and maintains low pressure in the center. The vortices that form around the inlet of a jet engine seek out a surface. They typically find the ground and often you see an inlet vortex sucking up water off the deck, but really any surface will do. A tornado functions like this. Condensation warms air aloft and creates the normal thunderstorm uplift, then air rushes in to fill this void, and a vortex thread attaches to the ground. The tangential flow comes from inflow rotation, which is usually influenced most by Coriolis force.

[–] • 0 pt

The pressure in the center is what's important not the velocity

To whom? Pressure = velocity, because pressure is measured as static pressure or atmospheric pressure. Stillness, static, non moving, you can only reduce pressure by moving a mass of air, thus you cannot have a pressure reduction without accelerating air.

For sure axial velocity isn't important

Any reasoning behind this claim? Axial velocity is the parameter that defines the center of circulation.

Vortex threads attach to surfaces since this in effect seals the end and maintains low pressure in the center.

Vortex threads are enhanced by ground effect, the downdraft impinges on the ground and adds energy to the outward flow. The rotational momentum of the vortex prevents the outward flow from filling the low pressure. The air has to climb over the vortex wall and travel towards to lowest pressure, IE where the air has the greatest velocity.

If you move air, air will move to fill that void you tried to leave behind. The core of vortex threads are the only places in nature where laminar flow happens. Everywhere else is turbulent flow, it takes a lot of engineering to create steady laminar flows, because it takes a lot of engineering to create the vortices necessary to force your craft along.

Helicopters work better in ground effect because the recirculation vortices are being limited by the outwash of the rotor downwash impinging on the ground. For a helicopter to be like a tornado it would have to be upside down, instead of creating low pressure above the craft, you'd be sucking the craft to the ground. Tornados have their bulk flows moving upward, the downwashing jet is a small component, inversely the helicopter has its bulk flow moving downward, with a small upwashing component caused by the tip vortices, and the recirculation that causes.

In other words, without ground effect, the tips on rotors create upwashing vortices that curl around and effectively push down on the aircraft.