[Tom] likes to build little helicopters and decided to build one that runs on compressed air. (Video, embedded below.) Turns out it was a little harder than he thought. Originally, he was trying for a compressed air quadcopter. He’d already worked with an air turbine, but putting on a vehicle that can lift itself into the air turns out to have a lot of hidden gotchas.
[Tom] went through a lot of design considerations to arrive at the helicopter design. He considered counter-rotating props, but there were a host of problems involved. He finally settled on a single prob with a tail rotor that resides on the far end of a long boom to allow the resulting lever arm to reduce the work required of the tail rotor.
Some of the best parts of this build are the test setups. We liked the test rig he has for testing the thrust of a propeller. It allowed him to understand that his quadcopter design wasn’t feasible with the props he had on hand. His schlieren photography setup was interesting, too, and provided some cool photos of the airflow.
In the end, the results were somewhat disappointing. However, you sometimes learn more from your failures than your successes. Also, an astute Hackaday reader might have some advice on how to make the flying machine better. We feel like it is very close.
We’ve seen [Tom] working on this in one form or another for a while. Some of his air-powered designs are quite sophisticated.
If you look at the energy density of compressed air vs pressure and then pressure vessel weight vs internal pressure for a given volume, you’ll see that this can only result in very small flight times, even when optimized and with 100% efficient everything.
Right. There actually aren’t a lot of “gotchas” in this kind of design if you do your homework before you start. It’s still cool that he tried to make it work, but anyone who’s done the math to figure out how much energy is stored in e.g. compressed air vs. a lithium ion battery or gasoline would know it’s not going to fly more than a few seconds.
Not to discourage anyone… but search the literature and do the math first, you’ll save yourself a lot of wasted time and frustration.
Same thing could be said for any form of helicopter though, Its in the nature of aircraft held up by rotating wings to need a great deal of energy to function. At small scales this compressed air concept actually works out reasonably well, it just doesn’t scale up at all well…
And these drinks bottles can take a very surprising amount of pressure if you care to push them that far, while being very light. So a double bottle and with two large rotors set up like a Chinook, even including the overlap for that eggbeater sound (and importantly to cut down on frame weight while having optimal rotor size) will probably result in pretty reasonable flight times for such a small, pretty safe, light toy, even more so if you are willing to push the bottles right up towards their rapid failure point. Sure its very unlikely to best the now astonishingly energy dense Lithium batteries, but weight for weight its not going to be that far off, even tiny motors of the right specs are pretty heavy (relatively), and there is just no way to get enough power out of a light enough ICE to really compare weight for weight, now volume for volume is a different story…
Has anyone tried to combine lighter-than-air technology with a quad drone? Could remaim aloft for a much longer time. Drone motors would only be used for maneuvering, landings an takeoffs.
Done years ago:
https://www.engadget.com/2016-04-11-festos-flying-sphere-makes-the-creepiest-drone-deliveries.html
The size of a container versus the energy it can store is quite well understood. Higher pressures mean thicker walls which is an issue. That’s why people are saying the maths needs to be done.
Hey I am more Mathematician than anything else, I’ll never oppose doing the maths. I’m just saying at this small scale toy size the maths works out that its perfectly possible to use those bottles – they can store more pressure than you may think at least for a short time (I carefully took one up to 100psi as a test and it took it without any obvious issue, though plastic creep may have been occurring it wasn’t rapid enough to notice) and they weigh almost nothing.
Its only if you want to scale up the energy requirements to make it more than a lightweight toy that it starts falling down
Since he’s using compressed air anyway maybe he should consider tip jets ( https://en.wikipedia.org/wiki/Tip_jet ) the rotor interface might be more complex but he could ditch the tail boom.
And fuel them from hydrogen in the bottle.
Pressurized hydrogen and fire, that would be a blast!
How about using one of those small CO2 cartridges instead of compressed air in a bottle? Too heavy?
He could try to pump air in the bottle while it’s in a freezer. It would get a boost when flying due to the air inside the bottle warming up and expanding. Might exceed internal pressure though…
/s/single prob/single prop/
It seems like the actual pressurized bottle isn’t doing much to sustain rotor speed. It looks like basically while it is attached to the external pressure source it spins up the rotors then the second it disconnects, everything slows down.
Maybe it isn’t pressurizing the storage bottle as much as you think.
What happens if you hold the rotors from moving, then pressurized up the bottle, then let go? Do they spin up?
A few ways to go with that information. One is, if you are bent on using a separate “spin up” vs “sustain” method modes, you aren’t actually limited to using compressed air for the spin up. You could just use an electric motor to spin up, release, which could (somehow?) activate the pre-pressurized bottle for sustain mode. He has no problem building (excellent!) valves so should be do-able.
I don’t know how much pressure a soda bottle will take, but I can tell you an 8g CO2 cartridge (the type for BB guns) holds… wait for it … 8g of liquid CO2 at about 750 PSI at room temperature. Cranking the mill, that is about 4.4L (sorry for mixing imperial/metric) of gas that will come out, roughly, constantly at that 750 PSI until the last bit. Since the 1L bottle he’s using contains… I’ll use that SWAG method and assume >4 ATM (about 60 PSI), in theory at least, the big bottle contains more volume of gas, but will not get anywhere near the amount of pressure.
Without doing the experiments, I’m not sure how useful that info is, but maybe it is somehow.
Just trying to suss out if it is a volume of gas (flight duration) issue vs. inadequate power (PSI/pressure) issue.
Very cool build none the less.