heavy and expensive, and most probably would have required a long-time development, with every experimental change in speed ratios requiring a complete change in gears. Again, their original choice was so correct that it lasted them through essentially all their active flying years. The very substantial advantages of the chain drive were not, however, obtained at no cost. Torque variations in the engine would tend to cause a whipping action in the chain, so that it was vulnerable to rough running caused by misfiring cylinders and, with the right timing and magnitude of normal regular variations, the action could result in destructive forces in the transmission system. This was the basic reason for the Wrights' great fear of "engine vibration," which confined them to the use of small cylinders and made a fairly heavy flywheel necessary on all their engines. When they were requested to install an Austro-Daimler engine in one of their airplanes, they designed a flexible coupling which was interposed between the engine and the propeller drive and this was considered so successful that it was applied to the flywheel of some engines of their last model, the 6-70, "which had been giving trouble in this regard."[11] [Footnote 11: Grover Loening, letter of 10 April 1963, to the Smithsonian Institution.] Although flat, angled, and vertical engines had all been operated successfully, the best and most modern automotive engines of the time were vertical, so their choice of a horizontal position was probably dictated either by considerations of drag or their desire to provide a sizable mounting base for the engine, or both. There is no record of their ever having investigated the matter of the drag of the engine, either alone or in combination with the wing. The merit of a vertical versus a horizontal position of the engine was not analogous to that of the pilot, which they had studied, and where the prone position undoubtedly reduced the resistance.