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I own a very neat little device for starting a car when the battery is flat. It weighs almost nothing and is small enough to fit in my pocket, but contains a lithium cobalt battery that can deliver several hundred amps, at 12v, for a few seconds. Easily enough to start a 2L diesel engine from cold.
It
strikes me that without too much effort, this technology could be applied to a quadcopter with a couple of handles on it. The motors would need to be designed to run at a few kilowatts, but they only need to run for a second or two. It wouldn't be a replacement for a regular parachute, but more of a solution for places a normal parachute couldn't work, such as jumping out of a 3rd story window.
Motor thrust test
https://www.youtube...watch?v=p4AjSqfnN54
[mitxela, Jul 01 2019]
[link]
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What could possibly go wrong ...? |
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// jumping out of a 3rd story window. // |
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Sure, why not ? You first ... just give us a few minutes to get the cameras set up.. |
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Presumably it will be a closed-casket ceremony ? |
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Not sure about using this as an emergency parachute, but
there must be a market for a device that make short (maybe
10-30s) hops. |
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Deliberately, or accidentally ? |
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The latter is WKTE & Baked by Eurocopter, Bell, Boeing, Mikoyan ... |
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Sure, but how heavy would that powerful motor be? Any ideas for the
motor, that would be able to consume those kilowatts? |
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Apparently you can get around 5-10kW per kg. I'd also bet
that you can do better by over-driving a smaller motor, since
it will be running for a short time. |
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Yes, it's all about the duty cycle. A very approximate rule is that a device rated for continuous duty at power rating P can be run at 1.5P on a 50% duty cycle, or 2P on a 25% duty cycle. But it's not linear, and the only way to really determine the ultimate limits is destructive testing. |
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Bear in mind that the failure rate will follow the usual bell curve, and that a device that's been significantly overdriven, just once, may look OK on normal test but be irretrievably compromised, so it tends to be a "one-shot" option. |
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There will also need to be some sophisticated die-by-wire flight control because of variations in user mass. Canopy parachutes are to a certain extent self-compensating in that they can be sized to a worst-case user mass and if the user is lighter then they just descend a little slower, plus the force of air resistance being the square of velocity is helpful. But an active reaction system needs to get the thrust right very quickly; too much, and the energy source may be depleted before landing*; too little, and the term "controlled plummet " is relevant. |
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It's why bang seats have parachutes, not extra rockets, and why rocket-braked stores pallets like Hajile have never been particularly successful. |
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The mathematics of the problem are daunting. For a 75kg body (still allive at this point) the work done to lift said body to the third floor window ledge (maybe 10m) is approximately 7500 Joules. So, neglecting air resistance and loss of mass due to involuntary sphincter relaxation, the body (again, still alive) will have an identical energy on hitting the ground from said window ledge. |
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A two metre drop is just about tolerable, so the braking system needs to dissipate 6000 Joules; but it's necessary to add in the mass if the gadget itself too, and any inefficiencies in its operation. So to keep the terminal velocity non-terminal, or at least acceptably painful, it might be wise to work on the basis of having 10kJ of energy stored and available. That's about 3kW/h of energy. |
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Gasoline has an energy density of about 45MJ/kg, so you only need about 2.5g of it to supply the necessary energy, assuming lossless conversion - even at 10% efficiency, 25g is a very manageable quantitu. But using stored electrical energy the picture is somewhat bleaker. A single use primary battery might just be able to do it. |
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*"Making contact with the ground at a rate that permits the user to walk away unaided, without undue delay, and not requring professional medical attention". Also applicable to many other aviation activities. See "The A25 Song" for further information. |
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I wouldn't bother trying to work it out from potential energy, it's much better to look at the thrust to power ratios of existing small aircraft. |
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See link for a thrust test video of a large quadcopter motor, which gives 18.4kg thrust at 100% throttle, drawing 3470W. A few of those on a quadcopter would easily function as our parachute. You'd need a few of the car-starters to run it, but that was to be expected. |
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At the other end of the scale, an R22 has a maximum takeoff weight of 622kg (so its thrust must be a fair bit more than that) and its powerplant puts out 93kW. So it clearly doesn't scale linearly, and the rotor size makes a big difference. Maybe instead of four small rotors, two counter-rotating rotors would improve the efficiency. |
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The twin counterrotating blade design is the basis for a few recent "personal helicopter " concepts. The R22 is pretty much a "minimalist" design for rotorcraft. |
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In the specific case cited - BASE jumping from a "low" window - bigger rotors have the disadvantage that the space they require may intersect the building during the early part of the descent. They also have more inertia, and in this application the spool-up time needs to be very short. For a 10m drop (which will reliably cause serious injury if not death) you have just 1.4 seconds to go from cold start to soft landing. |
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Working out the potential energy of the situation merely illustrates the idealized form if the problem - in fact, due to real-world inefficiencies it's much more complex, and rather worse. |
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//you have just 1.4 seconds to go from cold start to soft
landing// Clearly, then, the solution is to run up the stairs
and jump from a higher window. |
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I think efficiency is going to be the killer here (as well as energy density and all the other things). I don't know how efficient something like a helicopter is at converting chemical energy in fuel to a lifting force, but my instinct suggests the answer is "not very efficient". |
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The answer is also "Sufficiently efficient to achieve the stated purpose ". |
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Just because a thing can be done doesn't mean it's wise or safe to do it. |
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Actually, if this a single-use emergency device, it could be
made simpler. You want a fire-extinguisher-sized CO2
cylinder with three nozzles pointing down-and-outwards.
The person hangs from a handle underneath, so the thing
looks like a partly-opened three-spoked umbrella. |
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A trigger on the handle opens the CO2 flow, sufficient to
provide (say) 50kg of lift, slowing the descent. A 15ft rope
hangs below the whole affair with a weight on the end.
When the weight comes off the rope, the CO2 flow is
increased to further slow the descent and allow a survivable
landing. |
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Some of the early wearable "rocket packs" were in fact
powered by compressed gas alone, and were good for at
least 30s. |
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[8/7] Sure, but my point was that you not only need to calculate the energy required to power a fan to stop the fall of a body from a reasonable height, you also need to multiply this energy requirement by a not-small number to account for not all of the energy supplied to the fan being converted into a lifting force. |
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// I don't know how efficient something like a helicopter is at converting chemical energy in fuel to a lifting force // |
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Which is why I linked to the thrust test of a large quadcopter motor. |
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// fire-extinguisher-sized CO2 cylinder // |
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But then it wouldn't be electric. The title clearly states "electric" - do pay attention. |
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// bigger rotors have the disadvantage ... // |
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Very good point. Perhaps instead of handles on the thing, it could have a tether, say, ten feet or so, which would let you throw the quadcopter out of the window, let it achieve a hover, then jump out holding the tether. |
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//But then it wouldn't be electric.// |
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// out of the window, let it achieve a hover, then jump out holding the tether. // |
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<[mit] hurls unit from window, it fails to start, neglects ro release tether, is pulled from window, hurtles groundward/> |
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<Dopplering scream, satisfying THUD/> |
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Momentum extinguisher? :) Well, what about using that electricity to
rapidly heat up liquid nitrogen? |
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In terms of liquid-to-vapour expansion ratio, the best working fluid is in fact water. |
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That's why HTP is so good as a monopropellant. |
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//In terms of liquid-to-vapour expansion ratio, the best
working fluid is in fact water// |
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I suspect that is untrue. One mole of gas is about 22 litres
for any gas. So, the best expansion ratio come from the
liquid with the lowest molar volume. Water is 18ml;
mercury is about 15. So, basically, you want mercury as
your propellant. Obviously there are issues, but aren't
there always? |
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For last year's St. Rudel's Day event, the Intercalary
replaced the water in the greater north-north-east koi pond
with mercury. The fountain was beautiful (until it became
amalgam), but the koi were really, really pissed off. |
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^ their thrashings were a dying art. |
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// jumping out of a 3rd story window. // |
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I always have problems with this (I think it's because the
Americans do it differently from us? so depending on the
source of the material I'm reading it's often different) would
that be the second floor above the ground floor (so the 3rd
'including' ground level)? |
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//Presumably it will be a closed-casket ceremony ?// |
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Because a two floor drop from a second floor
(above ground level) window should normally be under
twenty feet (including allowance for window height
above floor level), which is imminently survivable as long
as you don't try anything flashy like a swan dive head first
onto tarmac. |
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//imminently survivable// only if it's just about to happen.
Otherwise it's eminently survivable. |
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If you jump from a second floor window the impact is going to be quite immanent I would of thought. |
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In fact, both immanent and imminent. |
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Is there any difference in flight of a bullet with a very strong magnetic field and one without? talking of electric and it's sibling magnetism para-shoots. |
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If the bullet is conductive, it should be slowed by an external
magnetic field due to eddy currents, I think. |
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