A phenomenon often seen in films of aircraft was often seen in the films of the Wrights: that the propellers appear to spin backwards. This effect can be clearly illustrated in the film below, made of an engine test of an original Wright engine.

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As the engine starts, speeds up, coughs, and starts again, the flat-bladed 'fan' turns at many different speeds.

The camera is filming at a steady rate, shooting a set number of images per second. If the blade came around each time the camera shot a new frame, and maintained that speed, the blade would appear to be still. As the blade whirs around, though, it is moving at a variety of speeds. Depending on the speed, then, the blades can appear to me moving backwards, forwards, or just stading still.