Lipo batter y help

It's better to use series rather than parallel batteries if at all possible. If not balance diodes need to be used to isolate the parallel batteries. Next, You should consider using a seperate 5v output regulator (BEC) for the servos. Finally, you could use 11.1v instead of 7.4v, however you do run a greater chance of wiping out the motor and ESC if the propeller is not chosen properly. Conversely, the 11.1v gives you more "headroom", thus allowing for more reliable radio/servo operation, particuliarly towards the end of the battery charge.

Thanks Chuck. You said "Next, You should consider using a separate 5v output regulator (BEC) for the servos" So, does a BEC give 5v to the servos or do I need a separate regulator? Howard

With reservations, like dropping 20% off the prop diameter and pitch, yes.

Probably because when th spec was prepared suffucient capacity in a single cell wasn't available.

Confuses everybody till they understand teh rather complex way you nd to go to arrive at a suitable power train.

Its no longer a 40 2 cycle with a 10x6 ;-)

The main thing to understand is the relationship between prop size, KV and voltage to achieve a given power, and to match the power source to the motor and prop you want to use. And to use a large enough pack and motor - don't try to squeeze the ultimate out of a small motor. Go up a size.

now his model is quoted at 3.4lb AUW..to gt vertical performance probably needs around 120W/lb, which means you need about 400W.

To get at last 6 minutes flat out - probably 10-12mins normal flying, you need around 40Watt-hours in th pack. On a 2s pack thats a 5400mAh pack, on 3s its about a 3600mAh pack.

I would go for a 3600 mAh 3s pack. Peak current will need to be around

40A, so you need a 40A SC or better.

Now 400W is about a 25 sized nitro equivalent, so the bird probably likes a 9 or 10" prop.

That equates to about a 1300KV motor with a 40A at last peak capability on a 10x7 sort of prop.

I note that the power 32 is only 770kv, and prefers props in the sort of

13" size on a 3s pack. The 25 is a slightly higher KV and wont actually do a full hover type performance.

To be honest, I wouldn't want to go much over 12" on a 50" model espcially for potential hovering, and at 400W I actually think you would be better going up to 4s or even 5s,with a separate BEC.

You DEFINITELY need a separate BC with that sort of servo load. I recommend dimension engineering as the best there is.

Given you need a BEC anyway, or a battery, you might as wll go up in pack voltage

Id personally use a 4s or 5s 2500 pack. And a motor of similar kv spec (920)to this

Plus a 13x6 prop.

That should give you unlimited vertical, but not a very high top speed and plenty of prop braking if you need it. Ideal for smooth pattern style ship.

The built in units are usually adequate for about four "normal" servos and the receiver. With 6 or more servos, I'd suggest using an external unit for the servos. In some cases, this permits use of higher voltage rated servos if the external unit output voltage can be adjusted. One problem is that servos vary widely in current draw, by use, design, and setup. Another possible issue is related to signal ground (receiver to servo), and a power return ground from the servo to the external unit. (Noise) Signal ground wiring should not carry current related to the power ground. In an ideal world, a servo would have a seperate power and signal ground. The seperate battery idea has been used, but it does increase weight significantly. If you use two regulatiors, the servo positive lead must not be connected to the receiver if you intend to run the servos on an external unit.

You can try an alternate scheme that uses parallel regulators with blocking diodes. This may work properly with some regulators and not with others.

Wow! Lots of good info. Thanks.

What you said above normally applies to the standard run-of-the-mill linear BECs built into the ESCs. There are quite a few ESC one the market today that have switching-type BECs built into them. The above limitations, as stated, normally don't apply to built-in switching BECs.

In fact, the original poster clearly stated that he's using "Eflite 60 Amp Pro Switch-Mode BEC Brushless ESC". This _is_ an ESC with built-in switching BEC, so what you said above does not apply to his ESC.

To the original poster: check the ESC manual and/or manufacturer's recommendations about using the BEC in your ESC. Most likely, unless your are doing something relatively exotic, you have absolutely no reason to install a separate BEC. And if you decide to use an extra BEC anyway, make sure that it is of better quality than the one in your ESC, so as not to defeat the very purpose of installing an external BEC :)

Here's the product page on your BEC

This is a switching BEC, to which most of what was said in this thread about built-in [linear] BECs does not apply. According to the manufacturers recommendations, this BEC can provide 2.5A to up to 6 standard servos from 3-6 LiPo. Since you are planning to use 6 HS-81 from 3S battery, you are within the specifications provided by the manufacturer. So, if you trust these specifications, then NO, you don't need an external BEC. (Unfortunately, I have no personal experience with this controller/brand).

Once again, if you decide to stick with an external BEC, make sure that it has higher specifications than the built-in one. CC BEC would be a good choice, since it is rated to 10A. But anything rated at 2-3A would simply defeat the purpose of using an external BEC in this case.

I would agree that your ESC will handle 6 hs-81s. Please make sure no servo binds at full stick throw. mk

Again, lots of good info. Thanks to all for their 2 cents.

"5V switching-mode BEC circuit capable of 2.5A continuous current on any recommended input voltage Drives up to 7 analog or 6 digital standard sized servos, with the BEC, on any recommended input voltage."

The 81's draw 220-280ma running, no load. What they draw full or 1/2 load running is not specified. Due to cooling and other issues, a 2.5amp BEC may or may not be adequate. A 10 amp external CC BEC is a bit of overkill, but, that's what I'd use to drive the servos, I'd use the internal 5v BEC for the receiver. And, possibly up to the 6v version of the CCBEC with these servos.