This is an idea that has been sitting for a while gathering dust at the back of the broom closet I use as a brain these days. To be honest, I'm posting it in the hope of getting some useful feedback, even though this is the halfbakery.
The simple aim is to make a model airplane capable of reaching stupidly high altitudes. It will be electrically powered, using onboard batteries at low altitude, and then jettisoning these and relying on solar panels alone for the latter part of the flight.
The problems of reaching stupidly high altitudes are, it seems to me, two-fold. First, the air is obviously stupidly thin, meaning that it provides very little lift. However, it also provides very little drag, meaning that you should be able to fly stupidly fast (for a given amount of thrust). I'm hoping that the lift:drag ratio (which determines flyability) will remain OK even up to very great altitudes.
The second problem is propulsion. A propellor, or even a ducted fan, is fine at low altitude. But, at very high altitude, I suspect it would have to spin impossibly fast to generate useful thrust. So, our model plane will carry an electric motor and propellor, but then jettison these to reduce weight once it gets too high (and fast) for them to be useful.
However, it is possible to get modest amounts of thrust using electrostatics. Google "electrostatic lifter" and you'll see plenty of videos. The basic idea is that you put a huge voltage between a leading-edge electrode and a trailing-edge wire. Ions flow between the two, and (and this is what gives the thrust) entrain un-ionized air molecules. You can produce about 5 grams of thrust per watt of electricity using this system.
(Please don't tell me that any thrust produced by the ions will be nullified when they hit the trailing electrode. I know. But the point is that uncharged air is entrained by the ions as they move between the two electrodes, and it's this uncharged air that is accelerated overall, providing thrust.)
Now, is this method of propulsion any use to us? Possibly. The ions themselves are accelerated to very high velocities. This means that they can potentially accelerate entrained air up to very high velocities. At normal atmospheric pressure, the air is quite dense and hence the entrainment only accelerates it by a limited amount. But, at very low pressure, I suspect that entrained air can be accelerated to a much greater velocity (relative to the thruster). On the other hand, there is much less air to be entrained in the first place. But on yet another hand, if the forward velocity of the thruster is high enough, the ion train will encounter more air molecules. What I don't know is how these various factors balance out.
That, basically, is it. A simple electric model airplane, but using electrostatic thrusters to reach far, far into the uppermost upper atmosphere. What would limit the altitude?-- MaxwellBuchanan, Jun 10 2018 SABRE engine https://en.wikipedi...BRE_(rocket_engine)being developed in the UK...waves small Union Jack [not_morrison_rm, Jun 10 2018] A paper on solar aircraft https://www.emerald...8/17488840610663648Has some interesting stuff. [neutrinos_shadow, Jun 10 2018] Energy management on solar aircraft https://www.science...i/S0196890413000277You need to keep your batteries... [neutrinos_shadow, Jun 10 2018] Qinetiq Zephyr http://www.airbus.c...nce/uav/zephyr.htmlNow owned by Airbus, holds records for altitude & endurance [neutrinos_shadow, Jun 10 2018] Like this? https://www.youtube...watch?v=vzZy1AqlenoHow to make a lifter. [Skewed, Jun 10 2018] Cockcroft–Walton generator https://en.wikipedi...%93Walton_generatorSparky ... [8th of 7, Jun 10 2018] The link says solids that are lighter than air are possible. If I read it optimistically, perhaps too optimistically, Its a honeycomb vacuum balloon https://physics.sta...te-a-floating-brick [beanangel, Jun 10 2018] Your own Vulcan bomber notion Your_20own_20Vulcan_20bomber [not_morrison_rm, Jun 11 2018] Short tribute M Farady pic https://drive.googl...xccZlhxYbyIKXwDvI7fKing of electricity [not_morrison_rm, Jun 11 2018] Altitude, where pressure is equal to Mars surface pressure (6 mbar) https://www.wolfram...ure+at+34650+meters [Inyuki, Oct 14 2018] Mars helicopter. https://youtu.be/oOMQOqKRWjU?t=43NASA Mars Helicopter Technology Demonstration [Inyuki, Oct 14 2018] ionocraft https://www.youtube...watch?v=trVf9gPZbkg [Inyuki, Oct 14 2018] Ionic Propulsion Airplane https://youtu.be/a9yYu-ZM1S0?t=15"The first aeroplane propelled by ionic wind" [Inyuki, Nov 22 2018] Erm, lifters don't seem to work in vacuum, or even a washing machine, but I digress.
You could use the helium heat-exchanger out of one of them Sabre engines (see link) to compressify the air way up there, then put it through the lifter.
But, I'd guess just using the Sabre engine 'as is' would be the best approach.-- not_morrison_rm, Jun 10 2018 //lifters don't seem to work in vacuum// I know. But we're not talking about vacuum, we're talking about air at very low pressure, but hitting the plane at high speed (ie, many molecules per second; potentially as many as in slow flight at seal evil).
That's why I said I'm interested in the balance of effects. Fewer neutral molecules getting entrained = bad; lower density of air allowing entrained molecules to accelerate more = good; fewer total ions generated = bad; lower current consumption = good.
And yes, I'm a huge fan of SABRE (although it doesn't actually have any huge fans, just compressors and turbines and rocketry). It is so far ahead of anything else out there but, equally, tricky to build in miniature.-- MaxwellBuchanan, Jun 10 2018 I would think the ionosphere would limit the ES thruster (or possibly help it...). And having no batteries means only gliding at night (so losing altitude). There is much research and experimentation in this general direction, although not with ES thrusters. Some linkys for you.-- neutrinos_shadow, Jun 10 2018 You're trying to get a vehicle to operate in a "difficult" region.
Too little atmosphere for aerodynamic lift at modest velocities, but too much atmosphere for orbit, because of drag at the necessary speed. Too much atmosphere for ion/thermion propulsion - although the little gas there is can be harvested as a working fluid, if you have the energy source.
You can't "fly" and you can't orbit. What else ?
Well, an ultralight dirigible can go a long, long way up, where there's any amount of solar energy, but we suggest an aerodynamic lifting body propelled by a deuteron ram - they're easy to make down to tiny sizes, if you have the materials science. Just stick an MHD array on the back to provide service power for the vehicle.-- 8th of 7, Jun 10 2018 Isn't there a problem with the power required by Ion lifters & the weight they can lift?
I've only ever seen fragile balsa wood & tinfoil constructs that appear to be hooked direct to the mains used to demonstrate this.
The on-board power supply you'd need to replicate that would seem to mean it will then be too heavy to lift.. unless you propose to use an extremely long micro-light mains cable of some description trailing from the craft to the ground, which I think will also probably be too heavy anyway (by the time you'd paid it out to the height you're on about)?-- Skewed, Jun 10 2018 //a problem with the power required by Ion lifters & the weight they can lift?//
I don't think so. A smallish lifter (a 1ft triangle) can produce up to 20 grams of lift for 5W of power. That's actually a much, much higher thrust-to-power ratio than a jet engine.
And I'm not asking this thing to go vertically up - the thruster is just pushing a plane. Lifters scale reasonably well (if you're sensible). I'm imagining something with a square metre of wing area built from solar panels, and harvesting about 100W of electricity and providing at least a hundred grams of thrust.
The "on board power supply" needs to be nothing more complex than an RC ladder to step up the solar voltage - it needn't weigh more than a few grams (maybe a few tens of grams at most).
At sea level, 100 grams of thrust for an ultra lightweight high aspect ratio plane weighing 3-500 grams would be wayyyyy more than is needed for climbing flight. So it really just comes down to how everything scales with increasing altitude.
//Too little atmosphere for aerodynamic lift at modest velocities, but too much atmosphere for orbit, because of drag at the necessary speed.//
Yes, but what about immodest but suborbital velocities? The man in the Internet tells me that both drag and lift vary as the square of velocity. So a plane that will fly at 1000ft ought to fly equally well at an altitude where the air density is 100 times less - it'll just need to be going 10x faster.
Re. the Zephyr - 20km is encouraging. But note that the Zephyr is designed for sustained flight, and hence carries hours worth of batteries as well as cameras or other payloads. I want to build something that will reach its maximum altitude within (say) 12 hours, meaning that it doesn't need any batteries.-- MaxwellBuchanan, Jun 10 2018 I feel like if you're depending on atmosphere, you will be hard pressed to build anything that flies higher than a hydrogen balloon.
I wonder what the thrust-to-weight ratio of a crookes radiometer is. It produces almost no thrust, but the tinfoil weighs almost nothing. It only works in a partial vacuum and is directly solar powered. Perhaps a high-alititude foil plane could be made to fly with the effect.-- mitxela, Jun 10 2018 The radiometer is a thermokinetic device; the black sides of the vanes are hotter, so gas molecules striking them depart with greater momentum. It's the momentum imbalance that makes it turn.
Photon reaction - the "solar sail" effect - is much weaker, and works only in vacuo.
// an RC ladder to step up the solar voltage //
<discreet cough/>
You can't "step up" voltages with purely passive components.
We suspect that you may be thinking of a Diode-Capacitor ladder, or Cockcroft-Walton multiplier, a simple and convenient way of "pumping" high DC voltages from a low voltage AC supply. <link>
// a plane that will fly at 1000ft ought to fly equally well at an altitude where the air density is 100 times less - it'll just need to be going 10x faster. //
Airfoils for high speed, high altitude flight have substantially different cross-sections to those which operate well in thicker air and at lower speeds. The factors that determine performance do not scale linearly, or at the same rate.-- 8th of 7, Jun 10 2018 //I feel like if you're depending on atmosphere, you will be hard pressed to build anything that flies higher than a hydrogen balloon. // But why?
Suppose you want to fly where the air is only 1/10,000th as dense as at sea level. To get the same lift you were getting at sea level, you have to go 100 times faster (say, 1000m/s as opposed to 10m/s). At that speed, the drag will also be the same as at sea level. So, if you can usefully apply the same power, you should be able to get the same lift/drag ratio as at sea level.
The Crookes' radiometer, I think, produces too little thrust (and it produces it in the wrong direction, relative to the sun).-- MaxwellBuchanan, Jun 10 2018 //a low voltage AC supply// Yes yes, and the components to convert a few tens of watts DC to AC needn't weigh much at all.
//The factors that determine performance do not scale linearly, or at the same rate.// Yes yes, again. But we don't need great performance at low altitudes. At low altitudes (and low speeds) the thing can be propellor-driven and the solar power augmented with batteries - all of which are jettisoned at higher altitude. You can get a plane up to 20km (as per your link, elsewhere), so what's to say you can't go to 40km, or 80km? And if you really can't fly a wing designed for 80km at low altitudes, then just lift the thing to 20km under a balloon and then let it go.-- MaxwellBuchanan, Jun 10 2018 The important word "vacuum" is missing but the [link] suggests a floating honeycomb is possible. This would have a fixed altitude though. You could power it upwards from there though.-- beanangel, Jun 10 2018 Lots of knobbly things are being jettisoned from a great height here.-- pertinax, Jun 10 2018 Well, if you're going to have a hydrogen balloon, you instantly have a hydrogen:oxygen interface opportunity - with the addition of a polymer electrolyte membrane.
I'd guess that graphene sheet might also do the trick?
In fact, this is the base of my own "Your own Vulcan bomber" idea punted on here Mar 27 2011. <link>
NB please excuse poor graphical pun <link>-- not_morrison_rm, Jun 11 2018 Wonder what could be done with a Van der Graaf generator - reuse the prop motor to drive the belt.
If it was a 4-blade X rotary wing (no fuselage, just wing), propulsion could be from the back of one blade to the front of the next.-- FlyingToaster, Jun 11 2018 [+] Find a good civilian use for this and it won't be "stupidly" anymore.-- pashute, Jun 11 2018 ... but find a good military use for it and that will make you rich(er) ...-- 8th of 7, Jun 11 2018 Well, it's not going to be carrying onboard batteries, so it can't loiter; and it's not going to be carrying armaments or high-power cameras, so I'm not seeing much military application.-- MaxwellBuchanan, Jun 11 2018 //so I'm not seeing much military application.//
Something to bounce radio signals off that you can deploy (relatively) covertly to provide coverage anywhere your satellite cover is inadequate or you just don't trust the nation controlling the satellite you'd normally use (because, maybe they're the ones you're at war with)?
Can't see that needing heavy equipment.-- Skewed, Jun 11 2018 // high-power cameras //
... like the sort of thing you find in top-of-the-range cellphones now ?
Gigapixel cameras used to be the preserve of the deep-pocketed military. In two years time, they'll be in kid's toys ...-- 8th of 7, Jun 11 2018 Yes, but a decent lens that'll give detail from 300,000ft is probably going to weigh a bit.
Oh, all right, you win. If it works I'll build a spare and paint it grey for the military. It'll cost them, though.
Which raises another question. If a gentleman were to launch an autonomous, 2-3m lightweight flying machine in English airspace (though not near an airport; well, maybe, but only Stansted which doesn't really count), have it ascend slowly to an implausible altitude, pick up a few GPS transmissions from it, then let it descend, do you think anyone would actually notice?-- MaxwellBuchanan, Jun 11 2018 Looks like its (nearly) baked. See link [ionocraft].-- Inyuki, Oct 14 2018 //I don't think so. A smallish lifter (a 1ft triangle) can produce up to 20 grams of lift for 5W of power.
Problem is, those lifters on youtube are running off an external power wotsits, so the real power-to-lift ratio is difficult to work out.
There are ways around having to lug a heavy transformer into the sky, and I'm working on it..-- not_morrison_rm, Oct 14 2018 Yes, agreed - but you don't need a transformer. There are transformerless circuits that will hugely upstep a DC voltage using only R/C components, I think.-- MaxwellBuchanan, Oct 14 2018 Can the surface of a propeller change state to accommodate a changing environment? less speed more grip.-- wjt, Oct 27 2018 Variable-pitch propellers exist.-- notexactly, Nov 08 2018 Indeed, and variable-geometry propellers have also been investigated.
But // less speed more grip. // in terms of changing surface finish makes no sense. A propeller is just an airfoil, and surface contamination (roughening) of airfoils is a Bad Thing; the catastrophic consequences of even a thin layer of ice on a wing can spoil your whole day, indeed make you loathe and detest ice for the rest of you life (which may not be very long at all, particularly if you've made the mistake of traveling in one of the bastard offspring of the DC9, such as the MD-82).
Smooth, laminar airflow over the airfoil is the desired situation, and a rough surface causes turbulence, which reduces efficiency and thrust and wastes energy.
Many aircraft props do have a matt finish. This is typically the result of a surface treatment to prevent corrosion, and the difference between that and a "polished" one isn't significant in aerodynamic terms.-- 8th of 7, Nov 08 2018 // particularly if you've made the mistake of traveling in one of the bastard offspring of the DC9, such as the MD-82//
Their safety record is outstanding right now. Some of that is due to them sitting around waiting for engine services that only seem to be done in New Zealand of all places, but you are unlikely to fall out of the sky in one anytime soon.-- bs0u0155, Nov 12 2018 We certainly aren't. We prefer something with better aerodynamics, like the Starfighter.
And the best you can do with those horrible little JT8-D turbojets is give them a quiet burial in an unmarked grave. Really, the Jumo 004 has a better reputation.
Grudgingly, well - apart from the slightly cramped cockpit - the standard floor model DC-9 isn't too bad, no worse than many of the other contemporary rear-engined T-tail designs. But once they started stretching the fuselage, it all went to hell. The margin between approach speed and stall speed is far to tight, they have to be flown "right down the slot" or they stop, and they drop, and they never fly again. An ideal job for ex carrier fighter jockeys, where they can relive their glory days while the punters twitch and gibber in the main cabin.
Deep stall ? Did someone mention deep stall ... ?-- 8th of 7, Nov 12 2018 The only thing I don't like about the DC-9 is the virtual absence of first- or even business-class.-- MaxwellBuchanan, Nov 12 2018 So, like demanding that the hangman uses a silk rope ? (Yes, we know your family is entitled).
The comfy chair and the complementary champers may seem a pleasant distraction when it's all going well, but when you're smeared across five hectares of JP-1 soaked cornfield it's pretty irrelevant.
About the only things in favour of the DC-9 are that it's fixed wing, and a reassuring absence of die-by-wire control linkages.-- 8th of 7, Nov 12 2018 I'll take a rusty old DC-3.-- RayfordSteele, Nov 13 2018 Ah, the DC-3 ... beautiful. Very robust, simple design with low wing loading, easy to fly.
And they don't "rust" - athough like all airframes, any corrosion needs to be very carefully monitored.-- 8th of 7, Nov 13 2018 So, to return to my pre-crash question, if one were to launch a washing-machine-or -smaller-sized drone from a rural part of country, would it be likely to attract attention from anyone whose attention one would rather not attract?-- MaxwellBuchanan, Nov 13 2018 The US military admits to having radar technology that can detect a baseball-sized airborne object across the entire width of North America. So maybe.-- notexactly, Nov 14 2018 If my drone makes it across the Atlantic, the US military will be entirely justified in being concerned. I was thinking of the fields on the far side of my village.-- MaxwellBuchanan, Nov 14 2018 Said radar is on a Navy ship, and can be put anywhere on the ocean…-- notexactly, Jun 08 2019 I know we're not at the height of the Cold War, but the US still deploys significant RADAR in the UK, the PAVE-PAGE at RAF Flyingdales for example. The UK has it's own long range RADAR, of less well known capability. Then there's RAF Milldenhall etc. that aren't going to be slouches in the RADAR dept. Add in any AWACS etc. I doubt you'd evade detection if you got to and hung around at an interesting altitude.-- bs0u0155, Jun 10 2019 Can it have Michael Gove's face on it?-- xenzag, Jun 10 2019 Indeed, //the fields on the far side of my village//, being in East Anglia, are likely to be fairly close to one air base or another in almost any direction. Have you thought of moving your village off the apron of Airstrip One?-- pertinax, Jun 10 2019 // I doubt you'd evade detection if you got to and hung around at an interesting altitude. //
Time for some drone-applicable radar stealth ideas then?-- notexactly, Jun 11 2019 random, halfbakery