BEC, a draw back???

[Clifford Stone]

I have been wandering about the web . . . bumped into 'dont use BEC' a couple of times where performance is paramount in relation to ESC's. However, no one says why or what the advantages are. I can guess, but guessing is not good enough.

Anyone got any hard and fast facts??? The setup is a 3536 1400k Turnigy motor, 8x6 prop, 2200ah, 4S, 14.8v, battery not to weigh more than 270gs inc., plugs and cables, use any 'c' ratting you wish. The ESC is an open choice, I use a Hobbywings 100a Plainum Pro with BEC. Hitec Rx + two Hitec MG85BB servos, thats the setup.

The ESC is way over rated . . . I was told it delivers more raw power to the motor, but I can see the logic of; is some of that power being filterd off by the BEC, if it is, how much in terms of potential performance?

This set up, drags 1025gs of balsa, foam and plastic covering around the 10 lap course in around 80 seconds, give or take a few, according to me having a good or bad day . . . recently, mostly bad!!!

There is leeway to carry an Rx battery, as I carry 70gs of balast to meat the 1022gs minimum weight.

CJS

[Chris Bott - Moderator]

I would think that powering the Rx and servos usibg a BEC will just affect duration a tiny tiny bit.

What does a Rx battery weigh? If it's inside 70g then you could go with one and have no performance penalty. I really can't see a BEC making any difference though.

[Welshboy]

You dont mention what servos you are using which might help.
At 25c your battery is capable of delivering about 56 amps. A standard size digital servo is capable of using 2 amps at max torque. Doing flat out pylon style turns with 3 such servos could be drawing 6 amps . Thats 6a not going to the motor, a power drop of nearly 90 watts from 750w down to 660w.
From memory my Eneloop 4.8v 2000mah weigh 95g or thereabouts.

Not so much hard and fast facts as back of envelope scribbles, but I hope they make some sense.
(I'm not a pylon racer or performance fiend so I may be totally on the wrong track. Apologies if so)

[Biggles' Elder Brother - Moderator]

You're right that a servo can draw 2amps - but its extremely unlikely to be drawing anything like that in practice - even given the tight turns in Pylon racing. I've done quite a few calculations on this over the years and in practice, even working hard, servo current draw tends to be tiny - milliamps not amps.

The main reason our servos are capable of so much torque is to acquire control linkage stiffness which gives us control fidelity and reduced risk of flutter - actually aerodynamic loads on flying surfaces are quite small - so the in-flight effort required from a servo is not generally anywhere near its maximum.

If it bothers you, then carrying a Rx/servo pack is at least useful weight (as opposed to ballast) and you will get enhenced power - but my guess is it would be miniscule and the effect would be measured in a few thousands of a second over a whole race! But maybe that's significant in pylon racing?

TBH - polishing the model further would probably have more effect! Or maybe a turbulator to trip off the boundary layer and reduce skin friction?

BEB

Edited By Biggles' Elder Brother - Moderator on 28/10/2012 23:47:48

[Chris Marshall]

As I see it the key thing is the internal resistance of the battery. I would guess at 10mohms for a 4S 35C pack which is the max allowed for e2k unless it's changed. The elevator servo may be peaking at 2A through the turns, assume worst case and say an average of 2A. Assuming a switching BEC this means a current drain from the battery of about 1A This will result in an additional voltage drop of 10mV. The motor is probably pulling about 40A flat out resulting in a terminal voltage of say 14 V The servo drain is therefore causing an additonal reduction in voltage and hence prop speed of less than 0.1% Because the power is proportional to prop speed cubed this means a reduction in power of just over 0.3%. You would have to be pretty good to notice that! A better reason for a separate battery is the possibility of failure of the main battery or the BEC, Because you do not have much weight to play with the choice of battery isimportant. It needs a high C rating- or to put it another way low internal resistance, so NiMH cells particularly small ones are a poor choice. two LiFe cells or if your servos will take the voltage LiPos would be my choice.

[Clifford Stone]

Posted by Welshboy on 28/10/2012 23:24:30:
You dont mention what servos you are using which might help.
At 25c your battery is capable of delivering about 56 amps. A standard size digital servo is capable of using 2 amps at max torque. Doing flat out pylon style turns with 3 such servos could be drawing 6 amps . Thats 6a not going to the motor, a power drop of nearly 90 watts from 750w down to 660w.
From memory my Eneloop 4.8v 2000mah weigh 95g or thereabouts.

Not so much hard and fast facts as back of envelope scribbles, but I hope they make some sense.
(I'm not a pylon racer or performance fiend so I may be totally on the wrong track. Apologies if so)

I do, they are 2 Hitec HB85MGBB jobies.

CJS

[Chris Marshall]

Hitec (USA) support tell me that all their digital servos will happily run on two LiFe cells. I don't think yours are digital but they may still be happy if they are rated at 6V. 2 LiFe cells are 6.6V fully charged but drops very rapidly to 6V

[Chris Marshall]

Thinking about it some more I think I would just continue to use the BEC in the ESC. I have a couple of Platinums myself which have a switched BEC and I think would happily power the two servos. The most likely failure is probably the motor which is being pushed to it's limit or beyond withthis setup. I have heard that the motors specified are quite variable in the magnet strength which translates into higher or lower top speed and hence power consumption. This is probably going to cause more variation that the little change caused by using the ESC BEC

[Clifford Stone]

Mmm . . . I doubt my servos work hard, in race trim on 60% rates, full up elevator give 3mm of deflection, I have a 'smooth feed in style' with a final pull of about half stick movement. I personaly do not use expotential as a lot of racing pilots do, so I feel, literaly, I have far more direct control over how much elevator and alieron is use, the alieron also has a max of 3mm deflection but only half, or less in most casses, is ever used.

Batteries are now limited by weight in ek2 too 270gs, alowing any 'C' ratting the pilot wishes. 'C' ratting is so wildly over stated on many battery packs, it was felt the amount of chemical that gives a battery its potentcy and therefor the weight of said battery. This is not my finding but that of far more clued up individuals in ek2. It has been confirmed to me indipendantly this is a fact.

So it seems, the use of an RX/servo battery would be a very small advantage . . . but I need to improve my course flying, which looses me 'seconds' and not wory about the 10ths and 100ths of seconds gained from fiddling with an extra battery . . . for now

CJS