Plane Losing Power - what's most likely cause?

[Tony Smith 7]

Hi,

 

My electric glider is definitely getting less and less powerful.   Its a brushed "380" type motor with a 9.6v 1100mAh power pack.   

 

What would be the most likely cause for this, do you reckon?  Do these motors wear out and reduce their performance?

 

I'm not sure what I should check first.

 

Thanks, Tony S

[Marc Humphries]

Depending on use and abuse - brushed motors suffer from armature and brush wear.  How many flying hours has it had?

 

Is it geared or direct drive? 9.6v through a 380 (rated at 6.0v or 7.2v) eventually takes it toll.

 

Alternatively - your battery may be playing up.  Check it out under load on a Watt Meter if you have one.

 

Cheers

[Romeo Whisky]

Well yes, brushed motors do wear the brushes out, especially if the battery voltage is well above the motor voltage rating which can wear them out quite quickly.  On the other hand replacement brushed motors are "dirt cheap", (much cheaper than the battery pack for instance), so that would be one idea to check, although it would be easier to replace the battery pack first.

 

The fact that you also quote an 1100mAh 9.6V suggests you are using an 8 cell NiMh pack, and I don't think you can buy 1100mAh packs anymore as they were replaced by 1300, then 1400, then 1500 and now 1600 capacity packs - which might indicate your battery pack is getting pretty old too and they don't last forever.  If your battery is more than 1 year old, I'd suggest you buy a new one.  If that doesn't improve performance it must be the motor.  An extra battery pack is always a good idea anyway.  You can't have too many!

[Tony Smith 7]

Thanks for the quick (!!) responses.  The battery and motor are only a few months old, supplied with a cheap RTF model so their quality could definitely be questioned. 

 

I don't have a watt meter, but I have a Fluke and a DC clamp meter.   What sort of voltage and current should I be seeing at full load?   That might be a sensible first check, see if the battery's holding up and giving what's needed.  

 

I know its going to sound tight, but I don't want to buy bits unnecessarily, especially as brushed motors and NiMH are clearly old hat.   Unfortunately it doesn't seem straight forward to find a suitable brushless motor otherwise I'd convert.

 

Thanks again.

[Marc Humphries]

Freshly charged - an 8 cell pack will show over 12 volts with no load.

 

Under load - you should be looking at 8 - 9 volts or so.  Current should be around 10 Amps max.

 

See what reading you get with your Fluke and clamp meter.

Edited By Marc Humphries on 10/08/2010 18:59:57

[Simon Chaddock]

Tony

In my experience brushed motors tend to work ok until the brushes give out then they fail completely (mine did).

With the symptoms you describe the battery is the more likely culprit. You may have a failing cell in your pack.

A failing cell does not accept or hold its charge properly so it gets exhausted well before the others, the the pack voltage falls and you get less power. Running the pack in this state causes the failed cell to get over discharged, which does it no good at all, and it gets worse and worse after each use.

 

Try measuring the pack voltage after a short flight. If you have a failed cell it will show a low voltage even off load, say about 8V. 

 

If this is the case a new battery is the only answer.

[Tony Smith 7]

Hi all.

 

Looks like the battery I think.   Straight from the charger it reads around 11.5V.    On full power this drops to 9 or 10 immediately, with a draw of 11 amps, but over the next 30 seconds or so the voltage drops further to 7V or even 6V, with the current dropping to 8 or 9 amps.    That power drop is really obvious on the bench, as I can hear it slowing down.  Maybe explains why its not obvious in the field, as it feels like full power at launch.

 

Off load the voltage goes back up to 10.75V, suggesting that there's not a specific dead cell (I think).

 

Next problem - as mentioned above where on earth do I get a NiMH flight pack nowadays.

[Erfolg]

I returned to modelling at the end of the "Brushed Motor Era". All of my models were 480 powered.

 

I built a Tomboy, which would only do a powered glide or a most impressive stall. The solution was to draw a heavier current. The out come was more power, but it was apparent that the motor performance deteriorated rapidly. Even 80w in, shortened the life.

 

I found the same with all the models I had at the time. You pushed the motor, its life was shortened. It seems that the magnets and brush/commutator deteriorates.

 

The life of a brushed motor is short, could this be your issue?

 

Give me a brushless every time, by a country mile.

 

Erfolg

[winchweight]

Batteries don't last forever. I had a pair of good quality 3S packs that were flown and flown, but gradually they lost their impetus and have now been replaced.

[Simon Chaddock]

Tony

Definitely sounds like the battery.  

You can get a 9.6V 1500mAh here but you may have to change the connector.

[Stephen Grigg]

For some strange reason,I bought a load of Batteries at the bring and buy,and dont use them.Ill photograph and list them and if they are suitable you can have them.Ill do that tonight

[Tony Smith 7]

Thanks everyone. 

 

Given the way the battery behaves - voltage seems OK but doesn't hold up under load, and the fact that its pretty new,  I reckon it  was just not a very good battery and maybe not up to delivering 10A without damage.

 

I note that the cells of that size on sale at Component Shop are only rated for 4A, you have to go up to 2/3AA size to get high current rating.

 

I'm interested in any batteries or cells available!

 

I'll do some tests with the airframe, see whether it can be arranged to accommodate a 200g battery and still balance out.  The original battery was 160g but it looks like I'll have to go heavier unless I jump ship to Lipos.

[Erfolg]

Ohh my Gawd

 

Still using Nicads or Nmh?

 

I Caaaaaaaaaaaaaaan noooootttttttttttttt believe it!

 

Brush less and Lipo is the way to go. Particularly if costs and replacement is the driver. If performance is the issue, brushed Nicad is dead, long live the brushless lipo.

 

 

[Tim Mackey]

LIPO.

[Tony Smith 7]

I hear you.   Unfortunately its structurally hard to fit a brushless motor into this plane, I've already spent countless hours chasing down specs and asking suppliers whether or not the spindle can be reversed (most either don't know, or just don't answer).   When it was all new the battery + brushed motor gave all the power and endurance I needed.

 

Lipos, again more conversion costs and hassles - new ESC needed (and will that work with my receiver?) then I'll doubless need a new battery charger and finally risk of burning down the house.

 

All that's a bit tongue in cheek, but I'm really a glider person so I'm a bit impatient with all this power equipment.    I suppose I'll need to get used to it if I continue to live on the flat.

 

By the way, its NiMH not Nicad - I'm not quite that old fashioned.

[Erfolg]

Tony

 

Do not worry about the house!

 

It is saving your models, all those memories!

[Stephen Grigg]

Any of these batteries any use to you Tony

[Frank Skilbeck]

Tony you don't need a new ESC with Lipos, you existing one will work but it won't have the correct low voltage protection, so you'd have to make sure you don't fly until the battery is run down to below 3v per cell.

 

Having said that I've converted a couple of electric gliders from brushed/Nimh to brushless/Lipo for some club members and the step up in performance is pretty impressive.

[Tony Smith 7]

Posted by Stephen Grigg on 12/08/2010 23:42:49:

Any of these batteries any use to you Tony

Sorry, been away on the boat for a few days.    I don't think any of those would suit in my case, but thanks a million for trying.  

[Tony Smith 7]

Posted by Frank Skilbeck on 13/08/2010 22:35:18:

Tony you don't need a new ESC with Lipos, you existing one will work but it won't have the correct low voltage protection, so you'd have to make sure you don't fly until the battery is run down to below 3v per cell.

 It was that low voltage protection that concerns me.  As I understand it a Lipo gets destroyed if you over discharge it.

[sparks59]

Posted by Frank Skilbeck on 13/08/2010 22:35:18:

Tony you don't need a new ESC with Lipos, you existing one will work but it won't have the correct low voltage protection, so you'd have to make sure you don't fly until the battery is run down to below 3v per cell.

 

Having said that I've converted a couple of electric gliders from brushed/Nimh to brushless/Lipo for some club members and the step up in performance is pretty impressive.

 Just to clarify, He will need to do the esc if he wants to go brushless.

 

I had a similar problem with an artf cessna, with nimh and brushed motor (like a car starter motor!) I tried with 2s lipo, the esc lasted about 3 very steady flights before starting to release the magic smoke.

n the end, I junked all the gear and put in  a new (cheapy)brushless motor, esc and 2s pack. It knocked about 6 oz off the AUW and flies better than ever.

sparks

Edited By sparks59 on 15/08/2010 11:36:03

[Tim Mackey]

Thats true, but one should either use timer to ensure you land before that point is reached ( its how I manage all mine ) or there are after market devices that can be wired in simply to give the protection feature. ( Lipo saver )

[Frank Skilbeck]

Sparks

 

I've run a Twin Star and Twin Jet on a Lipo/brushed motor combination with no problems at all with some old (Jeti) ESC's, the power increase was pretty good but the motors on the Twin Star didn't last too long and one fell out on the Twin Jet and and proceeded to chop the wing up in flight .

 

 

[Tony Smith 7]

Thanks all.    I think what I'll do, now that I know that the battery is the issue, is to just replace the battery with something as near original as possible.  

 

But I get the message on brushless, and if I can finally find a sensible not too expensive way to convert then I'll do that as well.   Since that will mean a new ESC, I can look at Lipos at that stage since all new ESCs seem to support either.

 

Losing some weight would definitely be good.   My current motor weighs 100g, and the battery 165g, but all that weight is in front of the C of G, so just substituting a lighter motor and battery might mean having to add ballast (which is a hateful thing).

[Tim Mackey]

Tony have you seen the ridiculously low prices of modern brushless equipment?

Motor and speed controller to suit your model £12

and LiPo batteries from around £9

 

Come on Tony, get with the modern age - you will be pushed to find a decent NiMH pack for that!