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Author Topic: Capacitor powered electric vs. rubber.  (Read 2740 times)
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TheLurker
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« on: February 01, 2020, 02:44:09 AM »

Here's a thought experiment for the weekend; is it realistically possible to get a model weighing (at a very, very, very wild guess) somewhere about the 50g to 60g mark to give a reasonable flight from a hand launch with twin super-cap drive motors and a motor run in the 25s to 30s region, i.e. something like the typical rubber motor power run?

Why do I ask? Well...I have had an idea niggling away at the back of my mind for a smallish, 25" span or thereabouts, "Kit Scale" Bristol Freighter (I do so like work-horse aeroplanes) for a while now and I think if I go ahead with it, it's going to have to use electric motors.  Although the fuselage would be more than big enough to take a battery pack that'll bump the weight up markedly which will mean it needs more v for the lift and it's quite definitely a type that would look wrong zipping through the air, it's far more yer stately* sort of aircraft; which means it needs to be built light to reduce speed needed for lift which brings me to super-caps about which I know nothing other than that they exist.

Cheers,
Lurk.

*The person who said it lumbers through the air can take 500 lines now and double round the playing fields twice in full BD after school.

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Robmoff
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« Reply #1 on: February 01, 2020, 04:11:47 AM »

I have a Tomboyish (I've taken a few liberties) of about 18 inch span and all depron (I know, but it was quick and easy, and this is just a POC model). If anything it flies too fast for what you want on a single 10F cap. Matching the motor (geared) and prop to the power pack was a faff, but it does now perform very consistently, well it did until an encounter with the wall knocked the motor loose and I did not notice for half a dozen flights.
I did build a charger incorporating the required inrush resistor and a self powered voltmeter. I fly on 3.2 - 3.3 volts, any more and I'm among the rafters, you may need less.
Rob
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Prosper
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« Reply #2 on: February 01, 2020, 04:41:04 AM »

I should think it's possible although it might be marginal as you describe it - couldn't a 25" Freighter weigh less than 50g though? You don't have to make things stressed for tension as in rubber models, the nacelles are as clear from impact damage as is possible, so can be lightly built, and the capacitors are your noseweight so no lead needed. I see a lumbering tub full of air which could be pretty light. Of course it depends on how much detail and opacity you're after. Drag could be the big problem, with lightness the only answer if you want it to lumber for 25-30 sec. You'd need the biggest motor which is described variously as 0820, 8520 or even 8.520. This plus 10 farad cap weighs close to 10 grams, so your powerplant can be assumed to be 20g plus maybe a gram or two for gearing. Is that lighter than conventional electrics? I agree with Robmoff - finding the best prop diameter/pitch/gearing is a faff, but it makes the difference between a lumberer and a powered-glide project.  
 
The norm seems to be to use small very fine-pitched propellers spinning very fast, and indeed these spin for minutes on end, but they thrash up a tiny volume of air into a complete tizzy while your model subsides to the ground. Only after fitting bigger, usually geared, props with coarser pitches have I lumbered models for any duration. However the big coarse props drain the capacitors of their useful voltage rapidly. My limited experience tells me the capacitors would need to be charged higher than their rated 2.7 volts and this reduces their lifespan. I haven't yet quantified the reduction, but I hope for at least a hundred lumbers before changing the cap - I've used a simple plug-in type setup. I bought quite a few 10F supercaps from China at less than a quid each so I don't mind treating them harshly. China trade may be disrupted over coming weeks or months of course, but if you shop at places like Farnell or RS they'll chisel 3 quid out of you for the same product.

As the Freighter is a poor subject for rubber you might need to reconcile yourself to conventional lipo/speed-controller electric if your supercap experiment failed. I have made a twin-engined 'lumbering breadboard' lashup which weighs about 60g and has a 31 inch span. And a very thick wing. It does lumber but there hasn't been any suitable weather to continue testing - I fear 30 sec might be unachievable. The biggest problem I have with this form of lumbering is the noise. It can be harsh.

I hope you give this a go - it seems to me that test pilots are in very short supply where scale supercapacitor motivated models are concerned.

Stephen.
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Robmoff
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« Reply #3 on: February 01, 2020, 07:46:15 AM »

OK More on the Tomboyish.
Span is 16 inches, weight 19g with undercarriage which I usually leave off, motor run hot for 20sec + cruise for 30 sec and the prop is still turning at 3 minutes if charged to 3.2v, prop from Kyosho RTF Citabria, but the motor/gearbox would swing something a bit bigger but not for as long!
The motor is unplugged for charging, There used to be a DIL switch on the board, but the prop would catch my fingers when switching on, be in not doubt this WILL cut you. I could save weight now that I am using a plug by using less Vero board, and if you used just the one plug/socket for charging and the motor you may save even more.
The charger uses a 18650 lithium cell and sits in a holder and has the inrush capacitor in the circuit so the plane does not need to carry that around. I would not like to be without the voltmeter on the charger. I know that it is not the most accurate in absolute terms, but it does tell me when I've charged it enough to get stuck in the rafters.
I will be fitting a similar system to a hangar rat type model soon.
Go on build a POC model.
Rob
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TheLurker
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« Reply #4 on: February 01, 2020, 05:23:23 PM »

Thanks for those replies, food for thought.

As far as that 50g/60g weight range goes.  If you're looking at 20g for a pair of motors with capacitors that only leaves 30g for the finished airframe for a 50g AUW and even with a tissue only finish, which is what I would do, I can easily imagine something I build at 25" span being well north of 20g sans power.  I wonder if the thing to do is scale down, perhaps to about 20".
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OZPAF
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« Reply #5 on: February 01, 2020, 07:55:51 PM »

I have a small 16" WS sports FF model with a 720(7 dia x 20 long) coreless motor with a 65dia prop and 10F supercap that weighs 20-21g. It will easily do 45- 60 sec flights depending on charge(I have only gone to 3.5v so far).

I have done thrust tests on this motor/ prop combination at 3.5v and get around 36g initially dropping quickly to 24 and then a steady decay. I feel that 2 of these motor/prop combinations would work for a model of 40-50g as you would not need the brisk(around 60deg initial) climb of my model.

Also you could save some weight (3g ?) by possible using one 10F supercar to run both motors in parallel.

Yes they are initially noisy and the props are not scale size but the props are plentiful and cheap(they are micro quad props). The whole set up is both cheap and will take almost unlimited charges.

I have found that due to the small prop - torque is almost non existent and with the smooth steady power decay - they are dead easy to trim. I have only used side and down thrust adjustments and the side mounted supercap provides the right glide turn.

Anyway more food for thought.

John
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TheLurker
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« Reply #6 on: February 02, 2020, 03:14:02 AM »

Quote from: OZPAF
I feel that 2 of these motor/prop combinations would work for a model of 40-50g as you would not need the brisk(around 60deg initial) climb of my model.
There's nothing brisk about the about the flying characteristics of a type 170.  Smiley

Quote from: OZPAF
Also you could save some weight (3g ?) by possible using one 10F supercar to run both motors in parallel.
I rather think if I was using (a) Supercar for power airframe weight wouldn't matter. Wink

Quote from: OZPAF
Anyway more food for thought.
Aye, this starts to look like it may be just about on the right side of the barking mad, hare-brained scheme line.  Certainly worth some further work when current hare-brained schemes are done and dusted.
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lincoln
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« Reply #7 on: February 02, 2020, 03:38:57 AM »

If I'm not mistaken, I've seen video of a so called "rocket" powered by electric motors and capacitors that went straight up, 100 or 200 feet. So my guess is that the Bristol could work out fine.
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OZPAF
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« Reply #8 on: February 02, 2020, 03:50:15 AM »

I just checked the size of the props on the Bristol 170 and as they are 14' in diameter than for 27"WS  the scale propeller size matches the standard direct drive props that I mentioned - 3.5"dia.

Sound is another matter - however no power source will provide a scale sound Smiley

The coreless motors and the props also come in normal and contra rotating directions - if that was necessary.

Hmm - I'm starting to  think of a Mosquito but it would have a WS of only 15" to match the 3.5" dia props.  Shocked Just a bit much I suspect!

However - I'm sure a 27"WS Bristol is a possibility.

have fun.

John
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Yak 52
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« Reply #9 on: February 02, 2020, 05:15:41 AM »

This plus 10 farad cap weighs close to 10 grams, so your powerplant can be assumed to be 20g plus maybe a gram or two for gearing. Is that lighter than conventional electrics?

Probably not  Wink There is a sweet spot where capacitors are as good as Lipo but it is for fairly small models.

A reasonable rule of thumb is to aim for static thrust of half the model weight. A 8.5mm coreless vary in quality but a good one with direct drive on Lipo would give you 35g thrust. So two of them should fly a 140g model in a a lumbering manor  Shocked

Some none scale context: One of my E20's uses a Racerstar 8520 on a 65mm direct drive prop and 1S 200mAh. It weighs 32g and climbs out at 65 degrees to about 40 meters in 8 seconds  Grin Two of them on a 50g model would be interesting  Cool Also, Bert in our club, has a twin 7mm E20 which would weigh a bit more (50-60g at a guess) and climbs in a similar manor.

A good 7mm coreless (a.k.a. 0720) should give you around 15-20g thrust easily. They would weigh about 3.5g each and about run all day on a 160mAh lipo (about 4.5g.) Add to that a gram or so of timer obviously. So 13 grams of kit giving 40g of thrust would do you nicely with with some head room to knock the power back.

I would also be tempted to buy a couple of Ebay 7mms and test them with a single cap and see if it would work. Cost similar to a motorway service station coffee  Tongue

Have a look at Richard Crossley's Coronado by the way. It's 31", 65g and flies on 4 x 7mm Voodoo 10's throttled back to 50-60%. He uses a 3.7V 200mAh lipo. These motors are Didel's on a 2510 prop (IIRC) and give about 15g max thrust. (You could almost certainly get better thrust from a more modern motor on a 65mm prop.)

I think the Bristol Freighter is eminently doable. It's just a case of picking the right size model. A bit of thrust testing and experimenting with a quick (profile/depron?) version would tell you a lot. There's such a wide variety of quality with these little motors but  fortunately none of them are really expensive.


Jon
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Prosper
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« Reply #10 on: February 02, 2020, 07:37:57 AM »

Quote from: Yak52
Probably not  Wink There is a sweet spot where capacitors are as good as Lipo but it is for fairly small models.
Yes, that's rather what I guessed Jon. But if TL scales down as he has mentioned, the powerplant weight is the same so the wingloading increases and he'll have a model that lumbers extremely fast! I think that once the argument goes over to conventional electrics then it's a stroll in the park because you can have yer motors putting out as much power as required for as long as required, and with all that power for about the same weight, then milking peak efficiency from the prop isn't critical. Judging from John's and Robmoff's observations though, 7mm motors and supercaps ought to do it anyway, making my gloomy estimations way out - sorry if I've misled you TheLurker.

Stephen.
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strat-o
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« Reply #11 on: February 02, 2020, 10:01:26 AM »

I used to be into micro RC.  Just to give you an idea of what is available, you can get lipos with very small capacity for very little weight.  Full River lipos with 10 mah capacity weigh just 0.6g.  This has ~30% more power than the 10F capacitor weighing 10g.

That said, I think that small super capacitors could be repackaged in a way to be way more suitable for FF.  Super capacitors are made from fairly light materials like aluminum foil, paper, charcoal and electrolyte sealed in plastic.  Actually not too difficult to make yourself.  I think the main issue with manufactured ones is the weight of the aluminum canister plus the plastic shrink wrap that gets wrapped around them.

Making your own would allow you to make a couple of cells that could be run in series giving you are higher voltage of 5+ volts.

Also, super capacitors don't tend to fail over time.  When they fail they fail once with a pop which is the sound of the positive conductor breaking through the paper separator to the negative conductor and shorting out.

Marlin
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TheLurker
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« Reply #12 on: February 02, 2020, 03:05:17 PM »

Quote from: Prosper
I think that once the argument goes over to conventional electrics then it's a stroll in the park...
Aye.  Things that appeal about super-caps as a possible power source is, from what I've read here, they give a rubber power like power curve, peak, plateau and tail-off and the thing that drew me to them originally is they although they may go "pop", and I didn't know that, they aren't an airborne incendiary like LiPo batteries, which, quite frankly, give me the heebie-jeebies and although caps need recharging for each flight the recharge time is negligible cf putting 1500 turns on a rubber motor.

I think, as I said above, when my current set of hare-brained schemes have been put to bed I'll start on a plan for a 170 for supercap with a fallback position of standard electric power. So, thank you all for the info. and all being well I'll see you all back here in about a year and a bit when I'll be asking even more daft questions.

Cheers,
Lurk.
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« Reply #13 on: November 08, 2020, 05:10:12 PM »

After reading these posts...I dug out my cheap-o chinie parts. I have some 2.7volts 10F, a 3v 15F tv one, then 2 3v 15F,  and a handful of 3v 10F. The 2 geared motors are like KP 0 size I think,,,and ones a 'longer' motor. When I put 2 of the 3v 10F on the 'big' motor....and charged it with 2AA bats for 2min.....wow!!!...nice long powerful run....now the shorter motor just kinda 'puff puff's along...with any combination of batts. I'm running the supplied prop, a black 5.8"....
My flying bud is building a small 24" FF oldtime, so I'll give him the powerful package.....see what happens...
Seems that there is power there in capacitors....but the motor needs to be matched???

I'll do more and report!
Marc
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fred
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« Reply #14 on: November 09, 2020, 07:05:04 PM »

Look at these guys Utube channel Old ish Videos but their Basic info is Good and I've found it quite accurate. Enjoy.
 https://www.youtube.com/watch?v=vF1mOyZc1lk
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TimWescott
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« Reply #15 on: November 10, 2020, 03:51:39 PM »

This is fairly useless unless someone else wants to pick up the torch: there's absolutely no reason that an RC-style ESC driven from a microprocessorized timer couldn't be light as a feather and give a "rubber like" power curve.  It just take a bit of printed circuit board and a few lines of code, and it's done.  Oh, wait -- a bit of PCB, a few lines of code months of fiddling and debugging and it's done.  I knew there was a reason I wasn't diving into the project!

Unfortunately, I'm not the one with time to do this.
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« Reply #16 on: November 10, 2020, 04:58:39 PM »

This is fairly useless unless someone else wants to pick up the torch: there's absolutely no reason that an RC-style ESC driven from a microprocessorized timer couldn't be light as a feather and give a "rubber like" power curve. 

Yes you can get the same power curve with a RC like setup.  But I will be getting in three flights while you are charging the battery.

RC, electric free flight, rubber or capacitor, all are fun, offer unique advantages and limitations.  Myself, I like doing indoor duration with a capacitor.  There the challenge is getting long flights without getting into the ceiling. 
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George
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« Reply #17 on: November 10, 2020, 05:46:28 PM »

This is fairly useless unless someone else wants to pick up the torch: there's absolutely no reason that an RC-style ESC driven from a microprocessorized timer couldn't be light as a feather and give a "rubber like" power curve.  It just take a bit of printed circuit board and a few lines of code, and it's done.  Oh, wait -- a bit of PCB, a few lines of code months of fiddling and debugging and it's done.  I knew there was a reason I wasn't diving into the project!

Unfortunately, I'm not the one with time to do this.

This is pretty much what an electric profiler does. Like Atomic Workshop's Zombie (integral ESC for brushed motors) or Derek Knights KPAero timers (PPM signal to an external ESC.) Trimpots set the time, power, and decay.

Although much less sophisticated, my recent version of the Peterborough Timer has full power for about 70% of the run followed by 30% of slow ramp down. Initially I was trying to avoid this (in comps you want full power for the whole run) but for sport scale flying I have embraced it as a useful flight profile/glide transition and could probably get a 50/50 phase split. Lurk, I'll sort you out with one when you are ready to pursue the Bristol Freighter...

It's also 0.3 grams Smiley

Capacitor powered electric vs. rubber.
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TheLurker
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« Reply #18 on: November 11, 2020, 04:19:19 AM »

Quote from: Yak 52
... Lurk, I'll sort you out with one when you are ready to pursue the Bristol Freighter...
Ta very much.  I'm stalled (ha ha) in the early stages of my next topp sekrit projeck* and I have some stuff to repair so a start in Feb next year for the 170 is highly unlikely, but it will happen.  Eventually.



*If anything comes of it, it will appear on a bungee glider thread and that's all I'm prepared to say for now. Smiley
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Russ Lister
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« Reply #19 on: November 11, 2020, 06:07:05 AM »



*If anything comes of it, it will appear on a bungee glider thread and that's all I'm prepared to say for now. Smiley

My 'secret' project will appear in the bungee glider thread too .... is this a case of 'great minds' or is something more sinister occurring?!
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TheLurker
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« Reply #20 on: November 11, 2020, 06:33:20 AM »

Oh it must be sinister.  So much more fun than plain old coincidence.  As for "great" minds?  MrsLurker has views on my mental capacity which have been expressed quite forcefully at times.  They are not flattering views.  Never mind, we genuises *cough* *cough* *cough* must suffer for our art.  Now, please pass me that bottle of absinthe....
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« Reply #21 on: November 11, 2020, 06:46:17 AM »

George, and others, I'm happy with a 3v 10f cheapie chinnie and the geared motor with the orange 2.5" prop....over a minute run...with a 30sec power stage. These were off amazon, took 2 months to get here,,,,,but, they are shorter than the the ones I got from mouser.com, weigh less!
I will weigh the whole package and post a picture later today.
The yellow 15f isn't as good in my tests so far....
Now to build that 10gm buzzard bombshell!!!
Marc
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« Reply #22 on: January 28, 2021, 04:39:52 AM »

To avoid going crazy during lockdown, I decided to try out capacitor powered free flight. My main concern was if I built a bunch of models without being able to test them (flying field closed, sports hall closed), I would have no learnings to profit from. So I decided to go the route of profile models where it would be easy to change motors and capacitors.
The first idea was to buy a couple of the cheap Chinese foam free flight models as the Youtube videos show them flying quite respectably. After stiffening the boom with 3x0.2mm carbon and adding tip dihedral, the model weighed 10.67g with a span of 280mm.
The second model was sacrified to use just the power unit. I found some very nice wings I had been given for an indoor model and used those.
The model weighed 10.16g finished with a span of 320mm. So I'm pretty sure it will fly.

Chris P
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« Reply #23 on: January 28, 2021, 04:58:34 AM »

When rooting through my scrap box,to my great joy I found a 6x14mm geared motor unit that came from a Parkzone Ember. I also found a foam wing that Banggood sold as spares for an Ember clone that they had some years back. I thought that this combination would result in a model that was less whizzy and better for indoors. I found some incredibly small switches on AliExpress. I thought it might be possible to charge the Supercap with the motor attached, but it's not very satisfactory. I fitted a 10F Supercap in the model and the motor seems to run for ages. By this time I had built the charger shown on Youtube using the DC-DC converter from Banggood.
350mm span, 13.56g weight.

Chris P
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« Reply #24 on: January 28, 2021, 05:14:16 AM »

The motivation for the 3rd model came from a Youtube video showing quite a small model turning tight circles in a small hall. I made up a 254mm span wing by soaking balsa and strapping it to an old Ace foam wing core for a couple of days. A perfect aerofoil section! Weight of the model with a 5F Supercap 8.71g. I fitted a 720 motor, which I now think was a mistake. I'll fly it first, but I think that the smaller 614 would be better.

Chris P
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