To counteract gravity, for each kg of helicopter you need to impart 9.8 units of momentum per second to the surrounding air. You can either do that by moving a lot of air very slow, or a little air very fast. Because energy goes like velocity squared, it’s way more efficient to push down a lot of air slowly.
It’s one of them energy vs momentum things.
To counteract gravity, for each kg of helicopter you need to impart 9.8 units of momentum per second to the surrounding air. You can either do that by moving a lot of air very slow, or a little air very fast. Because energy goes like velocity squared, it’s way more efficient to push down a lot of air slowly.