Been doing some research on propeller-pitch. And it seems that a variable-pitch-propeller seems superior, with a constant-speed-propeller(adjusting pitch for constant RPM) seems the most efficient of all. A quadcopter with a low RPM will generally spend less energy on air-drag so we have more efficiency. The problem is once you accelerate the RPM increases and efficiency is lost due to airdrag, similar to a car in a low gear driving fast, causing a too high RPM of the engine itself, which wastes energy. I've seen a few variable pitch-quads, most seem to be in the development stage, there are even petrolium variable-quads being researched. My question is, are there(other than added weight) some big mechanical drawbacks to having a variable-pitch propellers over a conventional propeller? Also will all the professional high-speed quads be using variable-pitch like helicopters in the future?