What is the fine-grained topic of the following text: In these model-scale tests, the sail structural properties did not show significant differences in performance, but affect the point where flapping sets in: a model with a stiffer material and a cross-cut panel layout starts flapping for a longer sheet length, compared to a lighter cloth and a tri-radial layout. Only return the topic.
Effects of sail structural properties on flapping onset in model-scale tests