SYSTEMATIC experiments carried out by the Dornier‐Werke for several years (for the purpose of investigating flight properties and the quantities of importance for flight performance) included, during 1935, point‐to‐point measurement of the downwash field in front of the tail unit of the Do 17, using twin tube pressure recorders. These flight measurements showed that the stability components of the two halves of the tail plane are not the same on account of the different downwash components produced by rotation of the slipstream. The conclusion that the stability of the aeroplane could be raised by symmetrical arrangement of the airscrews, in accordance with the more favourable starboard side, was well founded, and it was soon recognized that this precaution, in conjunction with the other well‐known advantages of symmetrical direction of rotation of the engine units, is one of the most effective methods for improving the flying properties of two‐ and four‐engined aircraft. This knowledge, which in the meanwhile had been confirmed in various places by wind tunnel tests induced the Dornier‐Werke to generally introduce counterrotating power plants for twin‐engined aircraft.
Stiesz, W. (1940), "Slipstream Rotation and Stability: German Experiments on the Comparative Stability of a Do 17 with Different Arrangements of Airscrew Rotation", Aircraft Engineering and Aerospace Technology, Vol. 12 No. 12, pp. 352-360. https://doi.org/10.1108/eb030721
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