Hi Guys, If I could add my small piece. I am no mathamatician, but, 20 years ago one of my aircraft tied down with 1/2 inch nylon rope from the top of each wing strutt anchored to two 12" square concrete blocks, 18"s long buried into the ground, pulled those two blocks clean out of the ground and proceeded to cartwheel down the strip 30 mtrs, crossing a fence and hitting a clump of trees, smashing four feet off one wing, breaking the nose leg, and one blade of the prop, all with the two blocks still attached. No one was on site during this event, so we dont have any idea of wind speed, but must have had a lot of force.