RC Helicopter Problem

Black Hole

Black Hole

May contain traces of nut
Anybody into RC modelling?

I've got a large-ish toy RC helicopter of the twin counter-rotating gyro-stabilised rotors variety. Yaw is controlled by slowing down one of the main rotors, pitch is controlled by a tiny vertical-axis tail rotor (and roll looks after itself). For scale, the main rotors are about 12" diameter.

I've had it a few years, must have picked it up on offer at Lidl I think.

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Pretty quickly I discovered I need a large indoor space, or a very calm day outdoors, to fly it. There's not enough room in my house to lift off and control the drift in time for it not to bump into something. Thus it doesn't get flown very often at all.

Today there's snow on the ground and no wind at all, and covid so nothing is pressing. Get the chopper out (oo-er missus).

The problem is: it wouldn't take off. With the "gas" control at max, it just about managed to skim across the ground. Yes, it was freshly charged (LiPo, I assume).

I thought that, cold air being denser, it should get more lift not less.

Do LiPos age? I don't think it was exposed to the cold long enough for the LiPos to chill, but the only idea I have is internal resistance limiting the current available to the lift motors.
 
👀Envy

It may be just as well, as if it cooled in flight, you might lose control of it?
 
Black Hole said:
I thought that, cold air being denser, it should get more lift not less.
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Yes, though at model sizes the % is probably tiny compared to all the other inefficiencies.

LiPos are notorious for needing careful storage (not fully charged, not allowed to drain - occasional top-ups) and as these are probably bargain basement ones they may be sub-optimal now.

However, the low temperatures will also reduce the battery performance as well as possibly increasing friction in gearboxes, etc. And the grease in the gearbox(as) may have dried out as well.

Give it a service perhaps ...
 
MikeSh said:
Give it a service perhaps ...
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Not sure how easily it comes apart, but I can have a look.

Pretty sure it's not a question of temperature: having brought it in for a dry out over the radiator (pictured) I had a go indoors and same problem - full blast it still barely got off the floor.

The loss of power can only be marginal, but that's all that would be needed, and I guess the cheap design was on the limit of working anyway. While I'm looking at it I'll see whether I can identify a replacement LiPo - weight will be critical. I have recent experience with LiPos for my mouse conversion.
 
I still don't get what happens if you service it, get a new battery, charge it, then fly it on a cold day. Will it not cool very quickly, then the battery cuts out your communication with it?
 
Scrat said:
I still don't get what happens if you service it, get a new battery, charge it, then fly it on a cold day. Will it not cool very quickly, then the battery cuts out your communication with it?
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I don't think my particular problem is the cold - it's no better indoors. However, regarding very cold temperatures, if the battery is cold I expect it to have a higher internal resistance and therefore the operating voltage drops when the motors are started up. I see no reason that would kill the RC though, it's not like there would be no voltage coming out of the battery.

Once running, the battery and motors will be generating heat, so that problem should go away, and any stiffness because of thickened grease also go away.

I have found the chopper does disassemble to some extent, it is held together with microscopic cross-head screws... until I get to the LiPo and the control board which are secured to each other and the chassis with double-sided foam tape and a cable tie. To make any further progress I shall have to cut through the foam tape, but the LiPo looks like a larger version of the one I bought for the mouse project, so I should be able to replace it for a few quid (and I recognise the connector as a standard in RC - not the case on the mouse battery).

I'll add photos later.

MikeSh said:
And the grease in the gearbox(as) may have dried out as well.

Give it a service perhaps ...
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I can't see anything to service; I shall do some googling though. There are two main motors, mounted directly fore and aft of the rotor shaft, both pointing down with metal pinions on each motor engaging with separate large plastic gears on the rotor shaft, one carrying drive to the top rotor and the other carrying drive in the opposite spin via a hollow shaft to the bottom rotor (and the gyro only acts on the top rotor).

I don't think the plastic gears need any grease - oil wouldn't last and grease would make them sticky. All that's left are the motors themselves and the hollow shaft. I imagine these are all dry bearings, and I doubt the chopper has even an hour of flying time on it.

I have thought of something which might be diagnostic: There is a trimmer knob on the controller, which adjusts the bias between the main motors so the operator can trim out any tendency for the chopper to yaw (with the yaw stick centred). The trim seems very sensitive at low rotational speeds (static test) - one motor runs or the other, it is difficult finding the point where both motors are running (with a low "gas" setting - what's the right word?). Maybe a red herring, but maybe indicative of low voltage / high battery resistance?
 
Black Hole said:
I don't think the plastic gears need any grease
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That's fine. These things often have an enclosed gearbox with a bit of lubricant, but if it's all open air, clean and spinning freely then it's probably meant to run dry.

I think a new battery may be all that's needed.
I wouldn't worry about the trim thing - at this point anyway. As you imply it's meant to operate at high power, not idling while planted on the ground.
 
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Can't figure out what that small can with three wires to the control board, mounted near the rotor shaft, is.
 
Maybe, they've certainly gone to a lot of trouble to mount it just there, but I'm not sure what the point is. The part it is adjacent to and appears to be aligned with isn't rotating - it's a sleeve within which the shafts rotate. If it is a tacho, it could only be sensing the hollow shaft.

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As for the battery, I've scoured 'tInternet but can't find one. I guess the weight is critical. Vital stats:

3.7V​
1000mAh​
JST connector*​
28g​
55x32x11mm overall​

* There seem to be several different connectors described as "JST", but this one has pins spaced at 0.1". The wires are chunky (ie capable of carrying a fair current).

The markings on the ribbed tape outer covering were obscured by the adhesive and remnants of the foam tape (very sticky), and although I could read 3.7V and 1000mAh (could be 4000mAh but seems unlikely), I peeled the outer cover off and was trying to clean it up but wiped out some of the print in the process.

2E726EDD-A8DF-4677-9F96-8A856B9C150D.jpeg

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So in the absence of a replacement battery, all I can do is put it back together and store it away.
 
xyz321 said:
Your battery looks like it may be emitting gas might be a fire risk.
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I admit that as a possibility, I have no experience what a "puffy" LiPo looks like. From what I've read, the main risk is during fast charging.

MikeSh said:
I'd leave the battery out.
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Yes. Anybody know what appropriate disposal would be?

xyz321 said:
This could be a possible alternative part but you might need to swap the cables and connectors over.
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Size OK, wires look too thin, weight 36g.

rpb424 said:
Does this get close? Pretty much meets the specs but also appears to be thinner and lighter. Described as 800mAh but the picture actually shows 1000mAh.
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Definite maybe - I missed that because of the wrong description. Wires still look a bit thin. 20g, might have to add weight to get the right CoG... but how is it so thin, and if it is that thin why isn't it more like 15g? However, I have a friend with Amazon Prime so I might give it a punt.

Ezra Pound said:
https://www.aliexpress.com/item/32819778785.html

9.0 x 30 x 43 mm
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That's interesting... even has the right part number (but if it is the matching part, why are the dimensions so different?). Delivery by 19th April!

Thanks all :thumbsup:
 
Black Hole said:
Maybe, they've certainly gone to a lot of trouble to mount it just there, but I'm not sure what the point is. The part it is adjacent to and appears to be aligned with isn't rotating - it's a sleeve within which the shafts rotate. If it is a tacho, it could only be sensing the hollow shaft.
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I know little about RC helis but one thing that would make sense to be installed here is a gyro as this is near enough the point about which it would rotate horizontally. There appear to be markings on it which should confirm this one one way or another.
 
Black Hole said:
Yes. Anybody know what appropriate disposal would be?
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Local recycling centre battery bin? I'd hesitate to use a supermarket one as they are inside usually and don't look very safe to me (even for 'normal' batteries).
Otherwise I'd Google some RC model plane or racing boat websites - they are major users, so must have experience.
 
This is curious. The 'copter is advertised as having gyroscopic control, but I assumed that meant the mechanical balance bar thing which provides stabilisation of the top rotor by gyroscopic action. I can see that some kind of rate of turn feedback in the vertical axis is useful for trimming the speed differential between the rotors... but that leaves a question as to what the manual trim control is for?!

My guess is that the gyro output is very coarse and the zero point offset has to be trimmed-on-test.
 
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