DIY drone, 8 rotors, 30-inch props

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spinningmagnets   100 GW

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DIY drone, 8 rotors, 30-inch props

Post by spinningmagnets » Mar 26 2018 2:51pm

I don't know the best section to put this in, so....

This guy seems to be pretty smart, and he provides persuasive arguments for each point of design. The carbon-fiber 30-inch diameter propellers from hobby King are (according to him) the best bang for your buck in large propellers, A computerized performance study suggested 8 rotors in a single-layer was a sweet spot for cost/performance...

I've seen double-layer drones with two counter-rotating motors and props at the end of each boom This was done when the drone was size restricted, but...after you built it to the largest size, you still wanted more power, regardless of the efficiency hit.

It "looks like" the motors are this model (at 15:33)
https://hobbyking.com/en_us/9225-90kv-t ... tore=en_us

https://www.youtube.com/watch?v=Y6dkRrnUGVo

craneplaneguy   10 kW

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Re: DIY drone, 8 rotors, 30-inch props

Post by craneplaneguy » Mar 28 2018 9:15am

Nice work! Little details like the very even spacing of the zip ties.....very cool.

Swinging a bigger prop slower then a faster smaller prop, though he worked it out on his own, will always be the way to "go." Kind of re-invented the wheel there, same with a single prop not being forced to operate in the screwed up airflow of a nearby prop being more efficient. Back in my early ultralight days, we used direct drive prop/engine combos, small props bolted directly onto the crank flange, spinning at 2 stroke engine RPMS. Cheap, simple, and light, but grossly inefficient as fas as converting noise to thrust, and man were they noisy! Then, we started converting the exact same airframe and engine, with reduction systems, adding weight, cost, and complexity, but the increase in performance from going from a 36" prop turning 5 or 6K RPM to a 60" prop turning 2800 rpm was HUGE, like 1,000 FPM climb versus 350, plus the noise level went WAY down. So yeah, I like this guy's thinking, and he does seem smart. At one point he may discover, like others before him, that a single (one blade only) bladed prop (with a counter weight, obviously) is the very most efficient, but they have never caught on, just too goofy looking!

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Re: DIY drone, 8 rotors, 30-inch props

Post by Punx0r » Mar 28 2018 10:30am

That's pretty cool and nicely made.

The only thing I picked up on was him describing the shape of the hollow section used for the arms. I absolutely get that square section is the easiest/best to work with, but from a technical perspective round is stiffer/stronger for the same weight. Also, since the load is chiefly in one direction (vertical) an oval or rectangular section would be more strength/weight optimised.

Practical considerations likely dominate though, you've got to work with what's available and you can incorporate into the rest of the design without introducing other problems (i.e. fasteners, pivots).
Last edited by Punx0r on Mar 31 2018 7:04am, edited 1 time in total.

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spinningmagnets   100 GW

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Re: DIY drone, 8 rotors, 30-inch props

Post by spinningmagnets » Mar 28 2018 11:50am

I wouldn't begin to start a project like this until after I had mastered a 3D printer (decent ones are now under $500). Then...I could make foam-core oval-section arms wrapped in carbon fiber (two layers, spiral one way first, then an opposing spiral). The odd-shaped end-connections wouldn't be a problem, I could 3D print them, and even bond aluminum pieces for reinforcement.

I like the 8 propellers. If one prop/motor/ESC failed for any reason, the motion sensors might not understand why the craft was dipping on one side (instead of a failed motor, it could be a down-gust of wind?) either way, it would just send more power to the neighboring props to keep balance.

I can see something like this rescuing a person off the roof of a burning building. I'm curious if it could lift 200-lb...

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Re: DIY drone, 8 rotors, 30-inch props

Post by liveforphysics » Mar 28 2018 9:26pm

The motors he selected dyno so-so, the T-motor U11-U12 makes a substantial improvement in performance. I agree those props are a great value for the money and I run them too.

On my prop dyno, I lose no efficiency having the prop circles overlap by up to about 25-30%, after that it becomes a measurable efficiency hit but still not more than a few percent even being directly over lapping.

If you picked the right upper prop and lower prop, the lower prop can actually harvest some of the energy lost to spiral air motion and recover it feeding the lower prop and improve efficiency over a non-stacked prop setup, but I never found the right combo to even get me back to breaking even, but I do believe it's possible as some combos had very minor thrust/Watt efficiency hits.
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craneplaneguy   10 kW

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Re: DIY drone, 8 rotors, 30-inch props

Post by craneplaneguy » Mar 30 2018 10:09am

It's always interesting when practicality gets mixed up with "most efficient", re; the square tube he used instead of oval or round, and why. My plane (and several other kitplanes) has round spars, I'm sure partly due to the off the shelf availabilty of them, rather then some optimized built up and labor extensive spar. But recently RANS, my plane's kit company, has came up with a purpose designed extrusion that serves as a tough leading edge (when you bash into the hangar wall/hangar rash) AND the structural main spar, it's never been done quite this way before, and it has a lot of us all abuzz. They will be offering this new wing as a retro fit for my model plane, the S-7S, besides it being the standard for the brand new S-21.https://www.rans.com/s-21-outbound-progress

Livefor: that's some heavy stuff, IMPROVING the efficiency, with the exact proper type of stacked props, very cool. My understanding, in the traditional airplane world, is stacked props is a last resort, to get more total thrust in spite of any efficiency losses, usually because they have run out of room for a bigger single propellor. My plane for example, has 3" taller gear, which allows me to run a bigger prop, without trashing it on the ground. Some WW2 planes had counter rotating stacked props, because they were making so much frigging HP they couldn't use it all up in one prop, even after making the gear as tall as they dared. This drone stuff is all new, mostly, territory and I guess a lot of the old rules and pre conceptions don't apply. My own experience has shown that all other things being equeal, a tractor prop, working in undisturbed airflow, is better then a pusher, working in the disturbed air of the fuselage.

craneplaneguy   10 kW

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Re: DIY drone, 8 rotors, 30-inch props

Post by craneplaneguy » Apr 06 2018 10:29am

I saw this 8' long pro window washer pole, all carbon fiber, that extends to 42' long the other day. It reminded me about this thread, note that it is round , not rectangular, for max stiffness in all directions would be my guess. With the pole's second additonal extention, it goes up to EIGHTY FEET! Pretty handy when doing commercial buildings, no squeege either, the tank of de-ionized water he carries, and the trick brush (lots of tech there also), delivered with a lith battery powered pump and 400' of lightweight hose, makes his setup self contained. The same guy has also built, it turns out (when he noticed my PluginPrius) an electric truck, using scrouged Tesla cells (Panasonic?) more on that in a few weeks, when I will be doing his crane work on a SIP home he is building.
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