Can someone explain the current (electric servos) changes as airplanes scale up?
Please don't hit me if this has been asked in some other forms elsewhere... But I can't seem to find it.
The question I have is how would I tell some things about RC planes power packages (for SERVOS not props) to know when I need to take additional steps?
EX: Prior to recently - All my planes were foamy ARFs / PNFs. Either the standard $2 9 gram servo, micro servos, or pre-packaged with the plane.
So for say a foam Micro - 1S with 4xLinear servos is fine...
As you go to 2S and larger planes, 20-30" span, you tiptoe into 9 gram land. (2S and as many as 6 9 grams I have had work fine.)
Then you get into larger park flyers. 3S, 4S come into play and you still use the ESC / BEC and I have used up to 17 gram servos here....
As you get bigger or stronger servos, there are as I understand dangers of:
* Brown outs - where you temporarily kill your receiver.
* Hangs and unexpected issues there.
* Also as you step up above X voltage, you can't rely on a BEC to step it down and could fry your "brain" mid-flight.
All that I know and understand, but what are the rules of engagement here? What drives those decisions on middle of the road (say 6S setups where there are BECs that do fine?)
How would I figure out that say my 4 digital servos will be ok with a 3 Amp BEC, but adding one more would cause me to need a larger one? Or how would I know if there would be enough power from any given BEC for the radio, and servos?
Lastly - Say I added a separate battery for the radio and it was 2S 1000 mah, how would I know how long that would last for? (without finding out the hard way?)
Pay attention to the voltage rating of the servos you choose and to the voltage rating of your RX.
Higher voltage will require lower current for the same servo power.
Then add up the stall current of the servos.
I tend to be a bit paranoid and anything above 4S LiPo gets a separate RX battery. I have seen higher power ESCs go up in smoke. Having a completely different RX power source at least lets you try to choose where it lands.
Using a separate source for the RX you should think about if you need a "power expander" or not. A power expander is a board to help you deliver current to many servos without using as much low current capacity wiring. A normal servo wire isn't good for more than 5 amps continuous demand. Heavy duty servo wires are only good for 7 to 10 amps depending on length and quality. similarly a common RX switch or heavy duty RX switch can prevent you from delivering adequate RX and servo power.
What setup you will need or want to use depends on the size of your model and its demands for the way you want to fly it.
There are no good rules/guides on this yet because people have not bothered gathering the data. I did prove that with 2 X 3200 mah 5 cell NiMh packs feeding power to a model's RX through 2 "heavy Duty" switch harnesses you can pull the voltage at the RX down to the level where the RX reboots with just six 168 in-oz digital servos by "stirring the sticks"... not even flying.
The way you test your RX system's duration is by flying. Do an appx 10 min flight and measure what it takes to recharge the RX pack. Continue checking your actual demand and adjusting your expected total flying time available, since this does change as you become more aggressive after getting used to a model. Then only fly for 50% to 75% of the time calculated to give a large safety margin. (also you are more likely to cause a low voltage issue when the pack is discharged below 20% capacity remaining)
Some of the newer radios with telemetry can have a low RX voltage alarm set. I have mine set above the minimum resting voltage for the RX pack (way above where LVC would be set if used for the aircraft's motor battery)
OH MAN! That was awesome advice!
It makes sense too. Follow on question:
I have heard and may have witnessed once (not an expert so not sure) a Spektrum radio doing a brownout.
Happened on my daughter's 55 inch trainer. We went to OrangeRX and have yet to see that again... But considering my radio, I have several options.
If you were going to fly large electric aerobatic planes (48-73 inch) would you use a Full range OrangeRX setup or a FlySky / Turnigy receiver setup?
Is one or the other more prone to blackouts?
Brown-outs are simply evidence that you have inadequate RX current supply capacity somewhere.
Something you installed is not letting the RX and servos get the current they are demanding.
Spektrum chose to use components that put their reset voltage at appx 3.85v (80% of nominal for the common 4 cell NiCd RX pack). This is down low enough that your servos are not delivering anywhere near rated torque.
If you were pulling a LiPo down to 80% of nominal you would be below LVC of any ESC set appropriately for that LiPo...
And you wouldn't be complaining about the ESC tripping off your power system at 2.97 v/cell. You'd be looking for how to deliver the current your motor was demanding.
Don't blame the RX for its telling you that you don't have the ability to supply the radio system's power demands. You are the one who chose the RX's power supply.
As another says - there are no hard and fast rules about this ...
I have a Cessna 182 that uses 2 aileron, 2 flap, elevator + rudder servo - that's 6 x 9gr standard budget servos. In fact they are Tg9e from Hobby King ... less than $2 a piece. The power is the BEC in the stock ESC as supplied by factory ... 3A. Rx is the FrSky with FlySky 9x. LiPo used - 1300 up to 1800 3S. Never one glitch or loss of control.
I have a F16 70mm EDF ... again 6 x 9gr servos, but with 5S LiPo pack through a Red Brick 70A ESC with BEC. No problem.
I've had 3A and 5A Switching UBEC's separate to ESC ... and had failures just same as using ESC inbuilt BEC's.
But generally it's advised that up to 4 analogue servos of small to medium can operate from a good ESC BEC. Digitals though can pull hefty current and above 3 would be advised to use separate BEC. BUT to be honest majority of models do not need digital and analogue are fine.
If you go up in servo size above the 9gr ... then advised to find separate power supply whether SBEC or battery pack. Personally - I prefer the separate battery pack as another says - at least you stand a chance of keeping Rx control even if motor is dead.
My biggest separate power pack job is my 60 powered Biplane of 4kg ... with 3 x 40gr, 1 x 12gr servos in ... she carries a standard 4 cell NiMH pack of 2200mAh - which gives me a full weekends use ... and despite the scare-mongerers who decry such packs - NEVER had any problem at all, even when hauling her round in crazy stunts.
Think on this as well ... standard 450 heli ... 4 servos, often with tail using a high speed digital. How many do you see with separate BEC ? Near all I see use the ESC built in BEC ... and a Heli is using all servos constantly + the gyro.
How to decide ? Sounds wrong - but to be honest it often comes down to what you can fit in the model space.... and weight.
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