What was the recurring barrier to realizing Da Vinci's most ambitious devices?

Lack of appropriate energy sources like modern metallurgy or efficient engines

The critical recurring theme preventing the physical realization of Leonardo da Vinci's grand mechanical visions was not a deficit in his imagination or theoretical grasp of mechanics, but rather the severe limitations imposed by the materials and available power sources of the late 15th and early 16th centuries. He relied on materials such as wood, leather, animal sinew, and human muscle power. For complex machines like the proposed 'tank,' the continuous horsepower needed to overcome static friction on rough terrain was far beyond what a single man operating a crank could sustain. Similarly, devices like the ornithopter were technologically sound in their aerodynamic studies but lacked the necessary power-to-weight ratio achievable with contemporary technology. The gap between his theoretical mechanics and the practical physics constrained by available energy sources—such as the absence of internal combustion or high-yield spring mechanisms—was the definitive barrier to transforming his advanced theoretical machines into built inventions.