Final answer:
The key factors that made the Wright brothers' wind tunnel a useful tool for wing design were their experimentation with long, narrow wings and their discovery of wing warping.
Step-by-step explanation:
The key factors that made the Wright brothers' wind tunnel a useful tool for wing design were their experimentation with long, narrow wings and their discovery of wing warping. By combining scientific theory with careful experiments, the Wright brothers found that long, narrow wings provided more lift than short, wide ones. Additionally, they developed a technique of wing warping to control the aircraft's movements. These discoveries helped them create a controllable airplane design.