[aerohead]

My thought is that if the shape of the envelope is such that it is "just" maintaining attached flow,that if it moves or deforms to a contour that exceeds what the flow will follow,that flow will separate,defeating the whole premise of the tail.My inflated boattail on the T-100 had an aluminum band that held the envelope to a cantilevered bottom spar,which prevented either vertical or lateral displacement.Air pressure inside the envelope provided enough skin tension that it performed as a "solid" surface.Tuft tests demonstrated attached flow over the entire surface.