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Lipo for trainer

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andy paterson08/11/2019 13:22:50
60 forum posts

I have a wot trainer fly,s well at half throttle all the time on a 4s 3300 lipo the motor is 4 max 960 KV motor , can I use 3 s 3300 lipo and still fly it .

Nigel R08/11/2019 13:45:07
3711 forum posts
582 photos

easier answer perhaps - go down 1" in prop diameter

Frank Skilbeck08/11/2019 14:01:45
4664 forum posts
101 photos
Posted by Nigel R on 08/11/2019 13:45:07:

easier answer perhaps - go down 1" in prop diameter

If you are going from a 4s to a 3s then you'd normally increase the prop dia or pitch. Without changing the prop the motor will generate around 60% of the power on a 3s compared to a 4s at 100% throttle. I'd try your existing prop and if it's not powerful enough then either increase the diameter or pitch.

gangster08/11/2019 15:34:14
1008 forum posts
17 photos

You need to be very careful playing around with cell count and prop size. You need a suitable meter to check the power and to ensure you do not exceed maximum motor current. You say it’s a 4 max motor. Suggest you give them a ring they are helpful

Don Fry08/11/2019 15:43:48
4557 forum posts
54 photos

Andy, is there a reason why you want to do this?

Making the flight pack 25% lighter is not necesssily an easy alteration. And the planes recommended 4s pack is bigger than you are using. Sooner or later, losing the weight off the front either has consequences, or needs to be replaced with lead. A big flight pack, working a bit easier, might be better in the long term.

But again, why do you want to do this? I dabble in electrics, but the experts may advise what is possible might not be optimal.

andy paterson08/11/2019 15:59:17
60 forum posts

Hi Don

I mistakenly brought 2 off 3s 3300 instead of 3s 2200 for my wot4 foam e , 3300 are to large for the plane hence asking about them for the wot trainer which I use as well .

Simon Chaddock08/11/2019 16:33:05
5672 forum posts
3011 photos


You really need a Watt meter to find out if you can.

With a Watt meter measure the full power amps and watts it uses on a 4s (take care!)

Find out the maximum allowable amps for the motor.

If the full power amps on 4s is already close to the motors maximum amps then a 3s is a positive "no".

If the full power 4s amps are significantly less than the motors maximum by say 30% then you can increase the prop size and/or pitch in stages measuring each time with the Watt meter until the full power amps are close to the motor's maximum.

It will fly if the full power 3s Watts is close (85%?) to the 4s figure. Only you can decide if the resulting performance is "good enough".

Edited By Simon Chaddock on 08/11/2019 16:33:21

Peter Beeney08/11/2019 19:04:23
1587 forum posts
59 photos


Looking at this from a slightly different angle, if I started from the other way round and was changing upwards from a 3S to 4S then with all other things being equal I’d reduce the prop size down a couple of notches to keep the current flow remaining within permissible limits. For instance: a particular motor on 3S and a 12 x 6 windmill performs in an perfectly adequate manner; changing to 4S and a 9 x 7 tug keeps the current flow in the same sort of area but the performance is now being remarked about… Such as “ How do I get some of that?”

So all you have to do now is just reverse this; however, the action is bound to slow down somewhat - all other things being equal. I would be inclined to raise the pitch size for starters, this might improve the speed a bit more on 3S. A case of trial and error perhaps - or as in my case, poke and hope… It looks as if the 3S will be lighter than the 4S, every little helps…CoG back a smidgen at the same time - floats my boat! It’s always useful if you have some means of checking the various points of interest, current flow, motor revs etc., too.

As you remarked that it fly’s well on half throttle this implies (to me that is) that this is at half battery voltage and half current flow and thus half revs. So maybe a bit more more wellie may be required on 3S but still very flyable. Well worth a try.

Good luck.


Don Fry09/11/2019 06:48:34
4557 forum posts
54 photos

I would guess that Peter is right. There is no cost involved, you have the batteries. Have a go. But make sure you don't alter the centre of gravity with the change. One way to sort it is to attach weight to each 3s battery, so you are not messing about with the airframe as you go from 3s too 4s in a session.

Trevor Crook09/11/2019 08:10:33
935 forum posts
67 photos

Certainly worth trying the 3s without changing anything else apart from a cg check. You don't need a wattmeter to do that, and if the performance is still ok, it means you can swop between batteries at a flying session with no fuss. I got around cg issues when using different weight packs by taping lead to the lighter ones.

Interestingly, a few years ago I tried to inject some performance into a model by going from 3s to 4s, keeping the same powertrain. To keep the current within the esc's 40A spec, I had to go down from a 12x6 to a 10x6 prop. This consumed more power from the battery than the 3s setup, but although the model made more noise the performance was about the same, demonstrating the higher effeciency of a bigger, slower turning prop.

andy paterson09/11/2019 08:33:35
60 forum posts

Thanks for the answers , I have a meter and see the motors draw and I also have a rev checked, time to play.

Thank you all,

Frank Skilbeck09/11/2019 08:59:19
4664 forum posts
101 photos

If it;s this motor then the recommended prop on a 4s is a 10 x 6 and on a 3s a 12 x 8, but if you don't need the power then using a smaller prop is the way to go.

I did this on a SFM Fokker DVII, I initially flew it on a 4s and it was overpowered, so I put in a 3s without changing the prop and this is much more suited to the model, the 4s made the throttle feel twitchy as a small throttle movement gave a big change in speed, where as the 3s feels less responsive more in keeping with the model.

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