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Hey! I love this problem :) The motor you shared shows >20g/W efficiency at ~40W of power and net lift of 500grams with those 28x9.2 (100g CF props). This impli

Hey! I love this problem :) The motor you shared shows >20g/W efficiency at ~40W of power and net lift of 500grams with those 28x9.2 (100g CF props). This implies that we need to produce roughly 40 Watts with under 500grams -- hard to assume negligible structural/electronics mass so more conservatively 40 Watts under 250g or 320Watts under 2kg to make like an octocopter with some power to spare. this is roughly 1 to 4 sq meters of solar area at 32% to 8% efficiency, and the swept area of eight 28" props is about 12 square meters it's might not even look totally ridiculous! I can find super-light 100Watt panes under 2kg on Amazon (probably actually 60 Watts) but it's definitely more like 2 - 4 times more mass efficiency than what is commercially available and not 18-30. Silicon mass is under 4 grams per Watt of solar so I think it should be possible to get there with silicon with a focused engineering effort. Organic photo-voltaic materials have shown 6 watts per gram in the lab (imagine for space applications!) -- though they would need even larger area due to lower efficiency and act as more of a sail .... not saying it is a practical machine to make, but again I definitely think it is in the realm of possible.

walrus01 7y ago
It doesn't scale down like that. Small motors and small props under 7 inch size have less than 5g/watt thrust efficiency. If you look at the motor and propeller setup that would go on an Armattan Chameleon Ti 7" for instance (a freestyle/gopro hero7 carrying sized FPV quad frame you add your own parts to). On the large scale you would never actually be applying only 40W to a tmotor u8 with a 28" prop on it, because the total weight of the motors and the whole craft will mean it would just sit on the ground. You'd be moving air around like a fan and not lifting off. Ideally you would design a large octo for a 1:1 thrust ratio (hover) at about 48 to 50% throttle per motor, which is why you see the thrust tables starting from 50% and going upwards to 100% throttle. Bigger and slower (lower kV) props and motors have higher efficiencies. If you google a bit about the few successful human powered helicopters that have been built in the past ten years you'll see the ultimate extent of using very big, very slow props to maximize lift per watt. I would dearly love to be proven wrong on my above post. If you can find me a super efficiency triple junction GaAs panel to mount on top of a 20 kilogram sized coaxial filming octo, I'll find a way to get it into the hands of XM2 and see if they want to try flying it.

walrus01 7y ago

It doesn't scale down like that. Small motors and small props under 7 inch size have less than 5g/watt thrust efficiency. If you look at the motor and propeller setup that would go on an Armattan Chameleon Ti 7" for instance (a freestyle/gopro hero7 carrying sized FPV quad frame you add your own parts to). On the large scale you would never actually be applying only 40W to a tmotor u8 with a 28" prop on it, because the total weight of the motors and the whole craft will mean it would just sit on the ground. You'd be moving air around like a fan and not lifting off. Ideally you would design a large octo for a 1:1 thrust ratio (hover) at about 48 to 50% throttle per motor, which is why you see the thrust tables starting from 50% and going upwards to 100% throttle. Bigger and slower (lower kV) props and motors have higher efficiencies. If you google a bit about the few successful human powered helicopters that have been built in the past ten years you'll see the ultimate extent of using very big, very slow props to maximize lift per watt. I would dearly love to be proven wrong on my above post. If you can find me a super efficiency triple junction GaAs panel to mount on top of a 20 kilogram sized coaxial filming octo, I'll find a way to get it into the hands of XM2 and see if they want to try flying it.