Changes

A schematic drawing of a cylindrical type of total energy tube is also presented<ref>https://crgis.ndc.nasa.gov/crgis/images/7/7e/PEN00109.pdf</ref>. These resemble closely the total energy tubes found on gliders today. The working principle of these tubes is not trivial and a theoretical approach to the design of these is not available to the author's knowledge. Qualitatively, these rely on the low pressure found in the wake behind the cylinder created by the separation of the boundary layer. In 1976 NASA did extensive experiments on these devices and the optimal geometry was found and presented in the Technical Memorandum X-73928<ref>https://crgis.ndcntrs.nasa.gov/crgis/images/7/7e/PEN00109search.pdfjsp?R=19760020138</ref>.