What aerial device did Leonardo da Vinci sketch with a helical linen sail?

Leonardo da Vinci sketched an aerial screw with a helical linen sail.

In a drawing usually dated to about 1489, Leonardo proposed a large spiral structure covered with linen. He reasoned that if the screw rotated quickly enough, its surface might press against the air and lift the machine upward.

The design is often described as an early helicopter or helicopter precursor, but it was not a practical powered aircraft. Leonardo had no lightweight engine capable of supplying the required power, and the proposed craft would have been too heavy for human muscle alone.

The aerial screw is commonly confused with Leonardo’s ornithopter designs, which imitate flapping wings. The screw instead relies on rotary lift, making it conceptually closer to a modern helicopter rotor than to a bird-inspired flier.