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Jul 11, 2020, 06:48 PM
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Quote:
Originally Posted by springer
This one was on the FlySky thread recently: https://m.banggood.com/Flysky-FS2A-4...l?rmmds=search
Thank you, that's very helpful!

Looks like you need to solder in your own header pins? Good thing I've been practicing that sort of fine soldering lately, soldering rows of header pins to some small microcontroller boards for a friend who teaches at a local college!

-Flieslikeabeagle
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Jul 11, 2020, 11:12 PM
Registered User
Quote:
Originally Posted by flieslikeabeagle
...
I know, right? It makes no sense at all. Neither does injecting yourself with disinfectant, but that idea was touted as well...

For reference, a basketball weighs 624 grams, which is 22 oz, or 1 3/8 lbs. More than double that terrifying 250-gram "drone" weight limit. And a basketball often moves faster than a small parkflyer, too. Clearly, basketballs are deadly threats, people who play basketball are flaky lunatic terrorists, and we need laws to protect innocent civilians from deadly basketballs.

-Gnobuddy
I agree with you, and the sad thing is something like 400-500grams would have worked for most of us.

I have done some research. this maybe doable.

There seems to be an ample supply of 2206 motors. They have too high of a kv for my liking but I will try to manage. They seem to be less than 40 grams.

Depending on the kv (I hope I can get something <1800) I am thinking of using a 2s battery. even the 2s 1300mah batteries seem to be in the 60 grams range.

3-4 micro servos, say 6 grams each 20-25 grams.

10A esc (I have seen Turnigy esc's in the 10-15 gram range)

Receiver (I have seen decent ones in the 8-10 gram range).

So with propeller, rods etc we will be aroung 150 grams this way. We can probably shave another 20 grams depending on our picks.\

This leaves us around 100grams for the rest of the plane. I will try to build a Mustang with 60-65cm wingspan, using mostly re/yellow insulation foam or maybe eps. 1-2 bamboo sticks should be enough for supporting the wing. I don't think I have a margin for paint but may have some decals.

If it turns our slightly heavier(15-20 grams) than 250 so be it, I will take the risk.
Jul 12, 2020, 02:07 AM
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Quote:
Originally Posted by RedLabel
I have done some research...
...2206 motors...too high a kv for my liking...less than 40 grams.
So far, our plans have run almost exactly parallel. I'm thinking of these motors (33 grams, 1700 rpm/V): https://www.getfpv.com/iflight-xing-...fpv-motor.html

Most of the published ratings are obviously complete nonsense (885 watts and 55 amp bursts through a 33-gram motor? Sure, that'll happen! ). But if the three published motor constants are anywhere close to reality, it should be able to handle 10 - 12 amps without too much efficiency loss (I threw together a quick Open Office spreadsheet to plot the motor efficiency curve).

Those are about the lowest-Kv affordable outrunners I can find in that weight class, though I also would have preferred something considerably lower. Big, low-rpm props are much better suited to light, floaty models. So for exactly the same reason as you, I too decided to go with a 2S lipo pack, but probably closer to 800 - 900 mAh, to get the weight down to the 40-gram range. Even on a 2S pack, my initial estimate is that this little motor with its too-high Kv of 1700 will only be able to spin a 7" prop. A real pity, as a 9"-10" one would be much more efficient and produce much more thrust for the same power consumption.

I looked briefly for gearboxes that I might be able to cobble one of these motors into, without much luck. I did find 4mm bore pinion gears that would fit the motor shaft, but had no luck finding a suitable spur gear to mate with it, so a DIY gearbox doesn't seem to be an option.
Quote:
Originally Posted by RedLabel
3-4 micro servos, say 6 grams each 20-25 grams.
I found these 5-gram ones at Digikey : https://www.digikey.com/product-deta...110-ND/5154685

My current FlySky receiver weighs 15 grams, so I'm hunting for alternatives. I got a tip on a much lighter one, but it's only available from BangGood in China, and will take 2 months to get to me...by which time, Fall will be here, and the weather may not allow flying outdoors.

Last week I peeled the paper off some white Ross Craft Board from the nearest Dollarama store, cut out a 30" span wing, cut out horizontal and vertical stabs, built a little hot-wire cutter, and hot-wired out a fuselage from 1 1/2" thick white EPS foam. The bare foam weighs a hair under 50 grams, but I haven't yet figured out how to make it all go together yet, so that may not turn out to be the final fuselage. I have a lot to learn about scratch-building with foam.

If it has to be small and light, IMO it at least needs to be pretty (eye-candy in the sky), so the shapes I cut so far were inspired by Golden Age race planes, based on outline drawings tweaked by eye on 1/4" grid paper until the shapes looked good to me. No sense in saying any more unless I can manage to build it and get it to fly!


-Flieslikeabeagle
Jul 12, 2020, 05:00 AM
gpw
gpw
“There’s no place like Foam”
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Thread OP
Irresponsible governmental decisions have landed us ALL in a big Pickle … Do we ignore this foolishness ? I think , like Beagle , thinking that this may change with a NEW Administration …
Latest blog entry: Lost plans
Jul 12, 2020, 07:18 AM
treefinder
springer's Avatar
Gpw, you really think dementia Joe and any left "leaning- (more like left leaping) administration will want to involve itself less in our private lives???

Methinks we really need to change congress, they write the laws that enable the executive branch's agencies to make rules. A much tougher job to change, though. . .
Last edited by springer; Jul 12, 2020 at 07:33 AM.
Jul 12, 2020, 07:31 AM
treefinder
springer's Avatar
On a friendlier side topic, I think you guys are thinking too heavy on the motors and LIPOs for your sub 250 builds. I only use a 40 gm emax 2812 or 2822 motor on my 36-40" span planes. All the original one sheeters used 2730 blue wonders 1300 to 1700kv. They are still available. The OSG originally used a 20gm 2000 kv motor. They all performed well. I have always had a contrarian position on batteries as well, I use the smallest one I can. For instance in a plane where Horizon Hobby will Spec a 2200mah pack, I will use a 1000kv or max 1500 pack. (The 1500 number only came about when I found 3s ZOP 1500 packs the same dimensions as the 1300 zippy compacts I was using) I'm of the opinion that the law of diminishing returns comes into play here. Big lipo has to be carried around, too big and the "longer flight times" aren't realized.
Jul 12, 2020, 10:17 AM
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Captain Dunsel's Avatar
Quote:
I'm of the opinion that the law of diminishing returns comes into play here. Big lipo has to be carried around, too big and the "longer flight times" aren't realized.
Not only aren't the flight times significantly longer, but the model has to fly faster (and land faster) to handle the weight. As an example, the E-Flite Cub calls for a 3S1800 lipo. With it, the Cub flies more like a P-51; instead, I like to use a 3S1000. The model balances better, does more realistic stall turns and slips, and flies at a more realistic speed.

Side note: Mike, I've been working on sub-250 gram models, using MPF. My first, a Junkers CL1 (Build thread at https://www.rcgroups.com/forums/show...7-Junkers-CL-I). At that size, I've been sticking with the SEMFF combat-type KFM airfoil.

CD
Jul 12, 2020, 12:32 PM
treefinder
springer's Avatar
Cap, I saw your thread when you first started it, but lost it. Beautiful job on your Junkers!

When one thinks that a full sheet of MPF weighs about 5.5 oz, and some will be lost in scrap, there are several oz left for gear.
Jul 12, 2020, 12:54 PM
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flieslikeabeagle's Avatar
Quote:
Originally Posted by springer
The OSG originally used a 20gm 2000 kv motor.
Thanks! I really appreciate the tips you guys are giving me.

I started out looking for motors in the 10 - 20 gram weight class, but couldn't find local sources here in Canada. I bought one of those little SkyFly receivers from Bang Good last night - shipping is supposed to take 60 days, so it will be Fall (mid September) before I receive anything, and probably too late in the year to fly - usually it's raining all day every day by then. The 33-gram drone motors are available from Amazon.ca, so I have a chance of getting them within a couple of weeks.

I remembered another of my lightest early RC planes - a GWS pico Tiger Moth, AUW at somewhere between 7 and 8 ounces, IIRC. I used the stock brushed geared IPS motor, which weighed about as much as these 33 gram drone motors. But it only used two servos, and the (72 MHz) receiver was really tiny and light. I flew it initially with surplus-store Qualcomm 600 mAh (?) lipo packs, later with a 450 mAh, 2S pack, I think. Only 2 amps current draw at full throttle, so it would fly very comfortably for 30 minutes at a time, and I was tired of flying before it was!
Quote:
Originally Posted by springer
I have always had a contrarian position on batteries as well, I use the smallest one I can.
I always tried for maximum lightness too (except when the weight was necessary for balance in some models, like my old Haikong Spitfire), but my knowledge dates from 2010 and earlier, and little of it relates to models under 250 grams, so I still have a lot to learn.

My rule of thumb was to first pick the motor and prop (to match what the airframe needed and the flight performance I wanted), then choose lipo size for about a 10C discharge rate at full throttle. A 10C discharge would empty the battery in 6 minutes, but since we don't fly at full throttle all the time, that usually translated to about 8 - 10 minutes of flight time. Most of my batteries lasted for years, too.

On a few highly aerobatic models I would pick the pack size for a 15C discharge rate at full throttle, which usually translated to about 6 minutes of flying. I wouldn't want much shorter flight times than that.

But no more gearboxes, and small outrunners invariably have far too high a Kv to be a good match to a light slow airframe, so step 1 (match motor and prop to airframe) is already kaput...I'm in a brave new world!

-Flieslikeabeagle
Jul 12, 2020, 01:07 PM
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Quote:
Originally Posted by Captain Dunsel
...E-Flite Cub...
I'm out of touch, but I remember E-Flite being one of the manufacturers that often picked really poor power systems for their models - usually too heavy, and with much too much pitch-speed, so that the little E-Flite Jenny, for instance, wanted to zip around like a little jet fighter while sucking down lots of amps, instead of flying gracefully like the slow trainer it actually was. GWS got it right with their pico Tiger Moth, thanks to the deep-ratio gearbox they used.
Quote:
Originally Posted by Captain Dunsel
My first, a Junkers CL1 ...
Very nice!

I've always thought the Heinkel He-111 bomber would make a pretty model, with that very aerodynamic fuselage, and rounded wingtips and empennage outlines. I've never seen anybody build one though, perhaps because of the associations with the Nazi era.

If I every build a twin again, it will probably be with a pair of little brushed motors. I did build one brushless twin, and that will probably be my last; it worked very well, but at almost twice the cost and more than twice the complexity, so not worth it to me.

-Flieslikeabeagle
Last edited by flieslikeabeagle; Jul 13, 2020 at 05:10 PM. Reason: Fix a typo
Jul 12, 2020, 05:04 PM
Registered User
Captain Dunsel's Avatar
Thank you, guys, I'm pretty pleased with the Junkers. Even with landing gear, it's still under 250 grams.

The He-111 is definitely a plane I'd like to build. Using KFM wings, the rounded wings are a snap to do. If I did one, I might make it the Spanish-built, Merlin-engined version. No Nazi markings.

One nicety about brushed motor twins (or more motors) is that you can run them in series, with one ESC. Back when I was doing brushed twins, I'd put two motors on a 16 cell (NiCAD or NiMh) pack. Each motor 'saw' 8 cells. If a wire broke, etc., BOTH motors would quit -- no unbalanced power (having said that, if a prop adapter let go....).

I think the problem now would be finding brushed ESCs big enough to handle the power. I've looked at HobbyKing, but all their brushed ESCs seem to be 'out of stock'.


CD
Jul 12, 2020, 06:59 PM
treefinder
springer's Avatar
Beagle, I gotta admit that the virus has trashed banggood historically good delivery times of 2 weeks. Hopefully life there will return to "normal" eventually.
Jul 12, 2020, 10:09 PM
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Quote:
Originally Posted by Captain Dunsel
One nicety about brushed motor twins (or more motors) is that you can run them in series, with one ESC.
Funny thing about this: when I was pretty young, I wired two identical small brushed motors in series, connected them to my little home-brew "battery eliminator", and watched as one motor raced to full speed, while the other remained stopped. Neither motor had any sort of load on it.

If I gently pinched the shaft of the racing motor to slow it down, the other motor would start to turn, quickly speed up, and the motor I was trying to slow down would come to a dead stop.

This happened every time - the pair would flip from one state (motor A at full speed, motor B stopped) to the other (motor A stopped, motor B at full speed), but I could never get them to a steady state with both motors turning.

It was some years before I learned enough about electricity to understand what was happening. The faster a motor spins, the more voltage it drops across itself (more back-EMF). Two motors in series means both get the same current. That in turn means whichever one has more voltage across it is receiving more power than the other one. This will spin it faster, making it get even more power, and so on; a vicious circle that always ends with one motor at full RPM and the other stopped.

So if you put two motors in series, even supposedly identical ones, the inevitable manufacturing differences mean that one of the will inevitably start turning faster or sooner than the other, and as soon as that happens, the faster-turning one will get most of the power from the battery, so it will speed up to full rpm, while the other one slows to a stop.

Fast forward some twenty-five years, and I read about successful RC twin-motor models using series-wired brushed motors. How could this be? I had seen for myself that this could never work - hadn't I?

After some head-scratching, I figured it out. The difference is because the motors in an airplane are each spinning a propeller - they're not unloaded like my childhood experiments. It takes significantly more power to spin a propeller faster. So if one motor starts to turn faster than its partner, it is immediately loaded more heavily by its (faster turning) propeller, just like my childhood fingers pinching the shaft of the faster motor; this slows it down, reduces the voltage drop across it, shunting more power to the other motor, which then speeds up to restore the balance. And so on, back and forth - the system will always find a stable equilibrium with both motors turning at (nearly) identical RPM, as long as both motors and both props are nearly identical!
Quote:
Originally Posted by Captain Dunsel
...If a wire broke, etc., BOTH motors would quit -- no unbalanced power (having said that, if a prop adapter let go....).
That last scenario happened to me with my one and only twin-engine RC model (a Multiplex Twinstar II). I had found a matched pusher/puller prop pair that worked with my motors and battery, and had converted the model to use counter-rotating propellers. It tracked like a dream on the ground and in the air - but I hadn't realized that both prop-adapters used the same right-hand thread, which means one of them was trying to unscrew itself, while the other was trying to tighten itself (as is normally the case).

And so, after dozens of successful flights, one day I was flying straight and level on a circuit around the park when the model abruptly snapped into a spin, nose down, spinning violently as it accelerated towards the ground. I had no idea what had happened, but all that spin-recovery practice kicked in: I pulled the throttle to zero, lowered the nose even more, added a little opposite rudder and aileron to stop the spin, pulled gently to level flight, and opened the throttle gently...and all hell broke loose again!

Luckily I still had some height remaining, and the second spin didn't have time to develop, so I was able to recover to straight and level flight a second time, keep the throttle at zero, and glide the Twinstar down to a dead-stick landing. Like most Multiplex airframes, the Twinstar II has excellent glide characteristics, so this didn't take much piloting skill. It was only after landing that I saw one prop adaptor nut was gone, and the corresponding prop was missing. Aha!

I found the missing prop on the ground later, but did not dare to reinstall it. Instead I swapped two motor wires, and went back to using two normal-rotation APC E props. I flew the model that way until I eventually sold it to another RC pilot.
Quote:
Originally Posted by Captain Dunsel
I think the problem now would be finding brushed ESCs big enough to handle the power. I've looked at HobbyKing, but all their brushed ESCs seem to be 'out of stock'.
"Progress" strikes again!

-Flieslikeabeagle
Jul 13, 2020, 11:45 AM
Got shenpa?
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Quote:
Originally Posted by springer
Beagle, I gotta admit that the virus has trashed banggood historically good delivery times of 2 weeks. Hopefully life there will return to "normal" eventually.
That's the best any of us can hope for these days.

I certainly have no cause for complaint - waiting a couple of months for my unnecessary little toys is the tiniest of minor inconveniences, at a time when so many people have so much worse to deal with.

Scientists are starting to realize that the novel coronavirus is most probably also spread by aerosols we humans emit when we breathe / talk / sneeze / cough. Aerosols particles (micro droplets) are so small we can't see them with the naked eye, and because of this, they hang in the air like smoke or mist, and don't settle to the ground within two metres (six feet) of the person emitting them. A whole new ball-game, then - it's not just surfaces we have to worry about, or people closer to us than two metres.


-Flieslikeabeagle
Jul 13, 2020, 03:02 PM
flyin' fool
goldguy's Avatar
Well then .............. mask up ................


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