SJM-400, that kind of motor? 4s-Lipos possible?

[Danyboy]

Hello fellow enthusiasts!

This week, I became a happy owner of a SJM-400...

The build was really a blast!

But now I am somewhat stuck at the question of battery.

On the webpage, they talk about 1100Ah 4s-Lipo 12C or 1800Ah 3s-Lipo 15C minimum...
Anyone got expirience with the 4s-setup?

I could get some descent batteries (2100Ah 4s1p) for a descent price (about 60$), which would also enable to use the original idea of mounting the battery on its tray without falling too much tail-heavy.

I know the ESC will hold up (actually its a rewrapped Sonix, at least it works and looks and programms suspicously similar... ;) ) for sure, but what about the motor?
I'm not really looking forward to burning motors...
But on the other hand, it is tempting indeed: Higher voltage means higher RPM, means smaller pinion, means less current, means longer flighttime with same battery-capacity... :)

So in short:
Tell me please, does any one have tried or heard or seen a SJM400 on 4s?

Thank you very much!

Cheers from switzerland,

Daniel

[darkside212]

Hi Daniel,
I would stick with 3S--and go with a larger capacity or higher discharge rated pack

the 4S 1100 pack would be too small for discharge capacity as you increase voltage the discharge capacity doesnt change so a 12C 1100 4s pack would be good for about 13A discharge....I'm not sure WHY they would advise that pack...other than it fits in the airframe

[darkside212]

Post some pic :D

[Danyboy]

Pics not yet taken but will come...

I heard it once that any photos taken before maiden flight will bring bad luck.
Not that I believe in such things but I stick to it nevertheless... If I can't prove such a saying wrong, I don't want to prove it right... ;)

I would definitely go with 2100Ah 4s1p, tested those and found CG to be well manageable...
If I go with same type of cells but with 3s, I need to do some modification since the battery needs to go pretty much forward, off the original mounting plate...

Well, let's see...

[darkside212]

I still think a 4s pack is overkill for voltage...:D

[Coolice]

Hey,

I agree, 4s on a small SJM400 / T-Bataar is over kill.
I used to have an SJM T-Bataar (similar to 400) and I used a Poly PRO 1200mah LiPo (20c) which worked great.

Stick to the 3s pack and keep weight down so as to keep the model nimble. With 4s I am sure you will have to do a lot more homework motor wise inorder to run without risks.
.

[arclite5]

The reason they say that a low capacity 4s pack would work is because of a simple law: More Volts = Less Amps. The 4s 1200mah pack would not be discharging at the same Amperage as the 3s 2100mah pack would be, because it is putting out the Amps at a higher voltage.

I'd go with the 3s packs 'cuz we all know them and don't have to learn anything new to use 'em =]

-Kyle

[Coolice]

Hey,

Oh yeah a valid point, that is the main reason for 4s as like you say the more volts you have the less resistance there is in passing the current. Hence the setup runs more efficiently. Having said that this would be more true for say a mid-size model like the Trex, the SJM is a micro and so your not going to pull anywhere near the amps of a Trex.

The other thing as you know is the higher Mah of the pack the greater the current draw it can handle without any problems.

As you have decided now, stick with the 3s to save a little money and weight, then buy another pack to allow you to fly longer :wink:
.

[Danyboy]

That's exactly the point I was aiming at:
For the same power, if you have more voltage, you need less current... :)
Also, If the motor spins faster, you get higher efficiency and more power (on electric motor, power equals torque x RPM).

Nevertheless, I will stick to those trusted 3s1p 2100mAh-apcks I already do have.
I need to modify the frame a bit to obtain correct CG, but I already have a plan...
And I love it, when a plan works out... ;)

As of right now, with the battery installed in the forward-most position, it's only secured on the aft-half with rubber-bands, the forward-half hangs free. If I would move more forward, I couldn't catch the battery with the aft-hooks (having them moved already one hole forward).
Right now, it is about half an inch tail-heavy: Still trimmable, but you can definitely feel it in transition from forward-flight to hover...
It sucks...
One of the things I will *never* understand, is how the manufacturers always get away with a certain suggested configuration, that will *never* achieve CG...
At least, nowadays, the builds work fine out of the box, so you don't need to mod before first flight...

Power with 3s is just ok with the recommended 10° pitch (12T pinion, power-curve according manual), though you can hear her bogging...
After hovering, the motor stays cool, battery gets just a bit over hand-warm after 10mins flight, after which I've putten 1700mAh back in, so I'll stick with 10mins...

Cheers,

Daniel

[Coolice]

Also, If the motor spins faster, you get higher efficiency and more power (on electric motor, power equals torque x RPM)

Hey,

I wouldn't have said youd get more power by spinning a motor faster, efficiency is good yes due to the voltage increase but not power.
Most in/out runners are more powerful with a lesser Kv rating than a higher one, in this case they generate more torque and hence we can swing more pitch to make the models faster.
Having said that it's all a combination of Kv, gear ratio and available power.

3s will be lighter compared to 4s and hence more fun I am sure once the best setup is found for your equipment.
.

[Danyboy]

What a blast...

I cannot say I do have extensive expirience with model-helis, owning the Zoom400 and BladeCP, this is my third heli...

But damn, one can really say I'm proud to have bought it... :)

It really just does what you command it to do... As it's supposed to be... :)

As seen from many other SJM-users, I have now mounted the 3s-battery underneath the forward-tray. CG is now almost spot-on, just a tad aft (about 5mm), but a *lot* better than with battery under the lower and hooked tray, which holds now the BEC...

I have found that with forward-flight (with some steep/fun-turns), in 9 minutes, I put between 1400 and 1500mAh back in the battery, not bad at all... :) 9 minutes of hovering use an average of 1900mAh...

Motor is indeed bogging (still have to measure headspeed, though), but for the moment, I stay with the setup: Though its around 0°C outside, the motor doesn't even get handwarm, so I suppose it won't get very hot in summer too...

At the moment, I wish I had a lot of money, so I could buy severy soccer-fields, have a glass-complex built around it, some nice, bright lights and a good heating installed, so I could go fly day and night... ;) :D

Ian, mechanical power of electric motors is measured in torque x evolutions. So if you keep the torque and increase the RPM, you will get higher power. Electrical power consists of current x voltage, so if you stay with the same current and increase voltage, you'll get more power.
Since torque its quite closely related to current and voltage to RPM, if you spin an electric motor faster due to higher voltage, for the same lift (you only increase motor RPM, not rotor!) (lift equals produced power, which equals the models weight which does not change in a theoretic example) you will use less current. While still having more power available... And since you need less current, you can use a smaller battery for the same flighttime, so you get closer to the 3s-battery.
The major question is though, whether you could really lift the RPM of the motor for another 3,7Volts and still keep it in its most efficient RPM-range. I suppose (now after some flights) the motor - with the actual power demand - is already on the upper end of its efficiency range. And that was the answer I was seeking with the initial question: Can that motor be used with 4s or not?
Topic of battery weight is on that specific model irrelevant in my opinion: I rather carry a heavier battery around than some lead, since when I hook up the motor to the lead, it'll never turn... ;) I think I could still use about 30grams extra on the nose...

Cheers,

Daniel

[Bonsai]

are you flying version 1 or version 2 ? How about posting some pix I will try and post some of my version 2 ,,, As soon as i stop having fun with it. :glasses2:
Dean

[Danyboy]

Flying version 2...

Same here, will post some pictures after having fun with it... :)
It is sooo amazing, after flying Shogun400 and BladeCP, this one is definitely a different size of calibre, though being in the same size... :)

Seriously, I've got a friend of mine, which is quite expirienced in taking inflight-photos. Unfortunately, he's quite busy with his new family and all, but we'll see... :)

Cheers,

Daniel

[busted blade]

my SJM 400 3s has a ton of tinker time in it and it will outfly a trex 3s mainly because its 70grams lighter. setup is - v1 frame,v2 head,trex main gear, trex tail rotor, trex front tr drive pulley, 3s 2070 25/40c packs, medusa 3400, 11t pinion running 28-2900 head wide open, 9 min mild/8min hard flying, mah 285 blades. if youve flown yours with the stock motor this setup is night and day better. i considered 4s but a big enough pack will be hanging out of the canopy and i personally beleive the small diameter mainshaft will not handle the load of 4s and hard 3d