Nicad to Lipo upgrade?

[Yorkman]

Hi all

As a very part time model flyer, I decided to dig out my Avro York (scratchbuilt by my dad-true scale) after a couple of years in mothballs, and get it in the air again.

Although flown successfully over a couple of years (you may have seen it in the air at Lordshill a couple of years back) it was always a mighty struggle to defeat gravity-a wingloading around 26oz/sq ft and speed 400s on 7 cells-I decided that weight loss was the order of the day and junking the two 1900 nicad packs for a single 4100mah lipo pack did that nicely! My concern had been the motors-7.2v-coping with the voltage of a 3 cell lipo. I was told that they should be fine, but testing (and an abortive test flight) seems to be indicating otherwise...

Basically, at full power static runs I'm getting fantastic thrust but the motors are worryingly hot (as are the speed controllers)-the battery seems to be barely breathing heavily...

The setup is-the 4 speed 400 7.2v motors are wired in two parallel pairs,  driving 9x6 props through 2.25-1 reduction gearboxes,two 30amp speed controllers,4100mah lipo pack.

What expert opinions can I get on this? I assume there are some experts on here?;-)

Thanks in advance

Martin

[David Ashby - Moderator]

They will get hot Martin, for the reason you surmise. You may get away with it if you use sensible throttle but tell me what's the span and weight of the model? and do you know the current drawn?

[winchweight]

TIMBO!!!!!!!

He'll be along in a minute. have you considered changing the motors to brushless?

[Tim Mackey]

As Shaun suggests....brushless motors would be  sensible move - you will however havre to change the ESCs as well ( 4 will be required )

If however that is not feasible, then brushed 400 can motors are ususally OK on 3s Lipo, but as you have the motors in parallel the inductance will be lower than noral, and current therefore higher. The expected life of a brushed speed 400 on 3s Lipo is around 10 - 15 flights. If you will consider brushless, then post again, and we will recommend some set ups - not expensive either really....reckon on around maybe sub £100.

[Guest]

On the basis that the york is flying much faster than you prevoiusly did with the nicads - why dont you throttle back once in the air and see how hot motors are when you land 

 this has got to be a more cost effective solution than changing to 4 brushless and 4 esc's

[Yorkman]

Thanks for the responses guys.

The model is 6' span, the weight about 5 1/2 lbs (weighed it the other day-have forgotten!) wingloading now 20 oz/sq ft. I don't have any means of measuring the current drawn.

I did consider going brushless, but the cost is prohibitive cosidering the use the model gets/will get-the decision to resurrect it was based on the info that brushed would be ok with the lipo pack!

Timbo-if what you say-15 flights only before motor burn out-what do you think about these motors http://www.puffinmodels.com/product.php?prod=1072 which appear to answer the problem?

Also, if the parallel wiring setup is contributing to the problem, rewirng to four separate motor/esc is feasible-brushed 30amp esc cheap as chips from giantcod.co.uk

ps-think I may have already burnt out 1 motor!

[Tim Mackey]

Yeh they look OK - but I would still personally put the 20 quid toward a brushless set-up - One of your problems is the parallel wiring -  could you try 2 in parallel, which are then put in series with the other two in parallel ? Hmm....that sounds confusing try this description instead

2 in parallel ( twice ) - with one pair in series with the other pair...that would sort of equate back to 1 x motor resistance 'cos 2 in parallel halves the resistance and doubles the current, but then put that in series with the other pair doubles resistance back up again and halves the current, so back where you started IYSWIM I think you may need to increase the voltage though to get decent pitch speed. You really ought to be using a whattmeter

[Guest]

wont work Timbo - each series motor will only see half voltage and result in 1/4 of power.

I still think cutting throttle back to reduce amperage on motors when cruising is the easy answer

[Tim Mackey]

I did say he would need to increase the voltage.....

However, I think I agree with you John - if brushless is definately not on, then throttle back, and get some spare can 400s for the resultant inevitable "maintenance"

[Yorkman]

Hmmmm

why are you opposed to those motors wound specifically for 3s lipos?

To me seems the obvious answer?

throttling back obviously takes place anyway, but static testing is getting motors very hot on approx 70% power and i've put smaller props (8x4) on as well...

[Tim Mackey]

I am not - I agreed that they looked OK

[Guest]

Yorkman

you are going from nominal 7 volt nicad to 10 volt lipos .  almost 50% increase in volts will give similar increase in amperage and therefor  over twice the power  - assuming straight resistance type calculation - but motors are inductive type resistance and not straight resistance and my ancient electrical theory fails me when it comes to inductance - anyone care to put up the appropriate equations ??

it is possible that your estimate of 70% is still giving too much voltage into the motors - try it in the air.

the 480 3 cell motors look ok to me as cost effective next step - I might even get one to replace my geared 480/ nicads in my 100" glider - altho that would give me serious cg problems with no space for dead weight and I hate dead weight

got to go now - her indoors just got back and needs my valuable assistance ( labouring ) in the garden

[Yorkman]

Timbo-apologies. 'Opposed' somewhat overstating the case-less than enthusiastic, perhaps?

John-interesting that you reckon I now have twice the power to previously. Power is not an issue, it was weight loss that was needed, so I don't need super-duper brushless or these brushed ones running at their absolute limit....

I think I will be giving those 3 cell motors a try-has anyone on here any experience of them?

Thanks for your thoughts folks.

Martin

[Guest]

Yorkman,

let us know how 3 cell motors do - thanks

 re power  increase

volts = current x resistance   and power (watts ) = volts x current

for 7 volt nicad and assume resistance of 1 ohm then  7 = current x 1 gives current = 7 and power =49

for 10 volt lipo  10 = 1 x current gives current = 10  giving a power raise to 100 ( motor resistance assummed same)

As I indicated above motors have inductance resistance rather than straight resistance and my meagre electrical tng never covered inductance but if inductance behaves same as resistance then the above will be a reasonable indication of the power increase.

[Yorkman]

will certainly let you know how I get on.

[Yorkman]

ps-anyone going to be at middle wallop this weekend? (i know the forecast is abysmal!)

[Guest]

I'm torn between electric at Wallop or vintage at Warminster or looking after new Grandson while his mother goes off to see   Dolly Parton at Cardiff  -  the abysmal weather looks like it is making the decision for me.

I wonder if I could talk my daughter into giving me her ticket ??

[Yorkman]

Warminster? Is that Lordshill? I'm going to need something to entertain my kids as boating is definitely out...

Perhaps you can get a few more tickets and we can all go see Dolly?  

                                                                                                                              

[Simon Chaddock]

Just a point about Volts.

A volts increase from 8.4 to 11.1 in itself is not going to hurt your brushed motors BUT the motor speed will increase, roughly in proportion to the volts, for any given current. So with 3 cell LiPos your motors will want to turn nearly 30% faster. Using the existing props the current will thus increase, not directly in proportion to the volts but still significantly and note the heat generated in an electric motor is proportional to the square of the current, so just 20% more current  = 45% more heat!

To keep the same total power as before (volts x amps) you will need either finer pitch or smaller diam props to let the motors rev higher. If you want to keep the exisiting props then either use higher voltage rated (3 cell) motors or, as suggested, find out what throttle setting gives the equivalent to the original cells and treat full as an overload, to be used sparingly - a bit like "take off power" on the civil Merlins!

[Yorkman]

thanks Simon

as puffin models didn't have a shipset of motors, and I can't find them anywhere else-am going to go for plan 'b'-

as I don't own a wattmeter, but I do own some kitchen scales, the plan is to chuck a couple of nicad packs back in, measure the  static thrust , then put the lipos back in and find an equivalent throttle setting-than, as you suggest, leave the rest for 'take off power' or 'war emergency power'! will just have to organise a 'gate' for the throttle lever........and wait for some test flying weather....

certainly still intend to change to the 3s motors when available tho.

btw-did donwsize from 9x5s to 8x4 props but still waaaaaaay too hot

[Former Member]

[This posting has been removed]