Factors effecting 2s vs 3s power system choice.

[David Hall 9]

I am looking for a power system for my 2 meter electric sailplane. It is for use for F5J/ height limited thermal soaring.  I guess that it will weigh around 750-850g. A smart rate of clime is needed, but not vertical performance.

There is a desire to save weight (provided everything balances). I notice that many designs use a 2s battery. The favourite folding prop will be somewhere around 10x6. Whilst checking over available (and inexpensive) motors, I can see that, say, a 1100KV motor will give a nominal 8140 rpm on 2s and a 750KV motor will give similar rpm (8325) on a 3s battery (given the motors are not under sized). I expect that the 2s system will draw more current than the 3s system for a similar power output, so there is the battery rating to consider.....but, are there any other aspects that affect the choice?

I have a few 3s batteries to hand, so if there is not much between the systems, would not really want more (2s) batteries.

Edited By David Hall 9 on 03/11/2016 16:18:29

Edited By David Hall 9 on 03/11/2016 16:19:23

[Former Member]

[This posting has been removed]

[David Hall 9]

I was wondering if the higher KV motor is capable of more power output on 2s than (given the same range of motor, say Aerodrive 2830) a lower KV motor on 3s?

[Biggles' Elder Brother - Moderator]

Assuming you want similar performance (ie the same power over the same time duration) you will find there will be absolutely no difference in weight between the 2s and the 3s battery required.

This is because the energy in each case (power times time) is the same. The energy density you can achieve in a battery (energy/weight) is dictated by the chemistry - in this case both are Lipo batteries so have the same chemistry - therefore their achiveable energy density is identical. Hence they will weigh exactly the same! The 2s will have fewer cells, but more need a greater capacity - one trades off exactly against the other.

So, on balance (no pun intended!) I would go for go for the 3s - lower currents, same weight battery. you might possibly (as SteveJ suggests) save a tiny amount of weight via a lighter ESC possible because of the lower currents.

BEB

[David Hall 9]

Thanks for your ideas. Perhaps it's connected, a higher rated (KV) motor appears to be capable of higher power output than a lower rated motor from the same range (possibly related to the lower number of turns used in the windings of the higher KV motor). I wonder if that has something to do with the choice of 2s over 3s in these systems.

 

Edited By David Hall 9 on 03/11/2016 17:23:14

Edited By David Hall 9 on 03/11/2016 17:28:40

[PatMc]

The power ratings quoted for motors are meaningless unless the power is given with the voltage applied.
The limiting parameter of any motor is the max current. Which means that using 2s with whichever motor you choose will limit the power to 2/3 of the power when using 3s.
IMO it's best to choose the lower KV & 3s.

BTW which model do you have ?

[David Hall 9]

Perhaps the choice of a 2s system over a 3s is not so common. Steve J's observation, that with high voltage radio gear and servos on 2s, the BEC can be eliminated sounds to be a plausible reason for this choice. I recall (but can't remember where) reading about the risks to the model when using a BEC and running the batteries near to their output and discharge limits. Many of the models in use are very expensive, so as a safety feature and for weight reduction, perhaps this is why.

For my own choice, my first electric thermal model cost less than the servos in a top tier glider. I use my Phoenix 2000 as a sports model. This is a little limited in thermal flight, but has got me liking the style. As a more able practice model, I'm using foam wings from an ARTF Radian kit glider (ready made and cheap). I'm building a pod and boom fus to allow me to fit the gear of my choice and get it to balance. I am thinking of a more able model for the comps next season (I'm not sure that my fellow competitors would be very impressed with my foamie).

I'm fairly new to electric flight, thanks for your help in understanding the choices available.

Dave.