Before revision: Purpose The cylinders of rotary aircraft engines of World War I (engines with the crankshaft fixed to the airframe and rotating cylinders) were notoriously difficult to keep cool leading to thermal distortion.
After revision: Purpose The cylinders of rotary aircraft engines of World War I (engines with the crankshaft fixed to the airframe and rotating cylinders) suffered from uneven cylinder cooling as the side racing the direction of rotation received more cooling air which lead to thermal distortion.