So how much current DO servos use in flight ?

[Tim Mackey]

As part of an experiment to electrify my old heavy Wot 4 - I monitored actual flight results of the powertrain and servo current using an eagle tree data logger - there is always discussion about just how much servos take with people estimating anything from way too little - to far too much ! Below is one set of graph'd data from a typical wot4 flight with spins, loops, flick rolls the lot. The servo current is shown by the dark blue line. This was a fairly spirited flight so servos were worked quite hard. It was also quite windy so I reckon this was a fairly typical loading for the surfaces. She is fitted with full size servos ( 4 in all ) 2 digitals in the wing, and analogue 3003s for rudder and elevator. It confirms my beliefs and earlier bench testing results that properly fitted servos, with free moving hinging and pushrods etc take little current in flight - note the average draw was under .25 Amp, and even peak was under .5A The model weighs around 5.5 lbs.

 

 

If interested there is a full set of data graphs for the flight including pack amps etc over in the thread on the last page or so ....

Edited By Timbo - Moderator on 05/07/2009 10:36:50

Edited By Tim Mackey on 11/01/2012 12:10:09

[David Ashby - Moderator]

Tim - sorry if I'm a bit slow but does the red line represent cumulative current drawn from the battery?

[Tim Mackey]

Hi David...Yes

Interestingly ( as described in the thread linked to ) in the 4s 2P configuration the battery did NOT give its all, yet in the 8s config it did !

[birdy]

Hmmmmm... So why do 2A becs sometimes give up under four or five servos I wonder... Very interesting...

[Tim Mackey]

Because many so called 2A becs are not really capable of 2A at all ....except for incredibly short bursts.  Linear BECS are far worse than switching ones, and switching ones cost more so are rarer on cheapie ESCs - although I have to say, there are some excellent cheap ESCs around these days.

[Simon Chaddock]

Great - somebody doing some real testing!

Obviously when compared to the flight motor the servos requirement is small but its not insignificant, so perhaps we need to think more carefully about linkage mechanical efficiency and ways to reduce the aerodynamic loads on the servos.

Just a thought but if you scaled the full size control loads on something like a P47 Thunderbolt to model size (say 6 foot span) we should not really need anything more than a miniature size servos! 

[Tim Mackey]

I actually believe ( after many years of seeing peoples idea of good building ) that excessive current drain and subsequent voltage depression of servo supplies is down to exactly that.....poor / stiff / binding / glued / gunged / hinges and often too big a servo driving too long an arm connected to too big a surface for the job. poor aerodynamics is surely a part of it too....we have all seen / heard the dreaded flutter from poorly fitted control surfaces.

[Tom Sharp]

to me all this theory is like asking how much energy it takes to eat a sticky bun        

[birdy]

Well if we spent enough time making the surfaces perfect, then they would also probably come out hevier or very weak. After all I imagine most aircraft don't have to do blenders or like... Also you'd think they would correctly specify bec... Darn producers!

[Tim Mackey]

Well its a bit like many other things......take car petrol consumption figures; the figures claimed by the manufacturers are pretty well impossible to achieve, and were obtained by a specialist driving the car in a way that is completely unrealistic and unachievable on the roads in normal conditions - they can be achieved in special controlled circumstances, but the reality is you will never achieve them yourself. So its not an out and out lie, but its stretching the truth shall we say !

I have to say however, despite the issues that some folk seem to have with BECs letting them down, I have never had a single issue with any of mine, and I have been flying electric models for many years, then again I do tend to err on the safe side when planning an installation.

[Former Member]

[This posting has been removed]

[Steve Hargreaves - Moderator]

This is interesting.....I test flew my Yak54 last Sunday which happens to have a very visible Rx battery monitor. The Rx battery was fully charged & I had the top green light on. After a succesful 20 min test flight I taxied back...& cut the motor.

The monitor was showing the next to top green light. I don't know why but I decided to "stir the sticks" ie rotate each stick through its full movement thus getting every surface moving. The monitor immediately shot down to the lowest "oops you've just crashed" red light but recovered to its second from top green once the servos stopped. Obviously this is down to the current drawn by all 5 servos but did shock me a bit. The battery is a 1700maH AA pack & the model wasn't flying hence no load on the control surfaces so I would have expected better than that......

Obviously you would never do that with the sticks during a real flight (although people who've seen me fly might wonder sometimes!!) but it does maybe show the sort of current draw our batteries have to accomodate sometimes. I understand it was this sort of issue that caused the early Spectrum Rxs to "brown out" & re-set themselves as the battery voltage got too low......

[birdy]

Well thats the brilliant thing about lipos I'spose!

[Tim Mackey]

Steve Hargreaves wrote (see)

This is interesting.....I test flew my Yak54 last Sunday which happens to have a very visible Rx battery monitor. The Rx battery was fully charged & I had the top green light on. After a succesful 20 min test flight I taxied back...& cut the motor.

The monitor was showing the next to top green light. I don't know why but I decided to "stir the sticks" ie rotate each stick through its full movement thus getting every surface moving. The monitor immediately shot down to the lowest "oops you've just crashed" red light but recovered to its second from top green once the servos stopped. Obviously this is down to the current drawn by all 5 servos but did shock me a bit. The battery is a 1700maH AA pack & the model wasn't flying hence no load on the control surfaces so I would have expected better than that......

Obviously you would never do that with the sticks during a real flight (although people who've seen me fly might wonder sometimes!!) but it does maybe show the sort of current draw our batteries have to accomodate sometimes. I understand it was this sort of issue that caused the early Spectrum Rxs to "brown out" & re-set themselves as the battery voltage got too low......

Yes it is, but take into account a few factors here.

1) How good is your onboard voltage monitor - EG: will it really respond visibly between a good condition 4 cell pack at say 4.8V and at 4.7V - one of the "problems" if you like with NiMH is their flat discharge curve, holding good nominal voltage until the end, when it drops off very quickly. This means in practice that the monitoir could well show it as being "in the green" one minute, and literally next minute in the danger zone.

2) How sure are you that the pack was fully charged to start with? Many chargers have a devil of job doing the job properly - see my short piece here about NiMH charging.

3) AA size packs are not terribly good at delivering high current anyway - I dont know how big your model is or what loads are on the servos....but for large stuff, I always go either LiPo and switching regulator, or larger cells such as sub C etc.

[Chris Channon]

Timbo, what a good article.

I think that now we have digital servo's,Li-po's, etc people ae missing the point in how powerfull a standard servo is.

I have been using standard servo's in models up to one 1/4 scale for years with no problems, as you say it is all down to correct instation of the linkages, servo's and flying surfaces.

Perhaps it is a " Bling " thing? ie, "I have this so and so servo so it must be better?"

Yes i have flown very high performace models with all singing and dancing servo's fitted, to me, it is the pilot who makes the Aeroplane fly nicely, not the servo fitted !

Keep them coming Timbo

Regards

Chris C

[Tim Mackey]

Cheers Chris

[Mal Quelch]

Timbo - Hi !

I know this is only my first post, but I'm very much a 'lurker' here so hello to everyone I seem to know so well!

Yes I suppose I will have to fill in a bit about myself but for now just let me say I am very surprised at the current drawn by your servo's. As you mention four servo's, 2 digital and 2 analogue I assume your figures are a 'total' current draw. What would be interesting to know is where you were measuring it.

If before the UBEC ( assuming you were using one ) It would show the benefits of a switching regulator and its draw from the flight battery as opposed to the actual current drawn by the servos. If this is the case, it would also be of interest to know what the servo current actually is as that is what this thread is about  !!  How about that then - first post and smiley's too !!  I could get to like this!!

In a case like this, it would almost be better to consider pre and post regulator consumption in terms of Watts?

Anyway, in Steves stick stirring example above, if he was using  a series regulator in his ESC and the actual servo current was known all could be revealed! The truth is out there !!

Mal 

[Tim Mackey]

Hi Mal and welcome to the forum

This was actual servo + radio gear as would be seen if using a "normal" battery, ie: AFTER the regulator. Many people still use a seperate supply for their gear whether its electric or IC so it seemed a better way to measure it.

Of course, in "power used" terms then if it was fed via a regulator ( which this actually was ) then the current taken from the battery would, as you state, be proportianaly less, ( for instance, when using a 3-cell LiPo  pack the current drawn from the battery pack will be around half of what a 4-cell nickel pack would be required to deliver ) but that particular factor was outside the T.O.R of this experiment. It would be tricky to measure Steves pure radio draw distinct from total draw in actual power terms if he was using an ESC BEC, because that would involve seperating out power in versus power out at the BEC circuitry which is integral to the ESC itself, being of course fed by the same supply.

Standalone UBECS of course, are easy to do.

Besides....I suspect that Steve was flying IC - why ? - havent a clue...just a gut feeling

I have done a bit of work already to demonstrate the adavantages of a regulated high voltage radio supply versus a direct feed over in another thread HERE.

Thanks for your input Malcolm.

[Mal Quelch]

Hi again Timbo.

So your figures are for 'realistic'  current draw and that is amazing, 0.5A Peakcurrent !

 it was the place of measurement that would have helped initially.

As for I/C flying   I guess the word motor must have confused me! I know I should have checked but everything seems to be electric these days  !! And as you were using A123's in your WOT4 a separate rx battery is a must,  unless you use very well timed flights due to the low discharge voltage possible from these cells and the abrupt cut off associated with them.

Once again-truly amazed!

Mal

[Steve Hargreaves - Moderator]

Hi all....back to the fray after a few days "working" & blimey a few interesting comments there....

 Just for the record Timbo is correct this was an IC Yak.....typical club model, 52" span, 5 standard analogue servos 60 4-stroke power. The battery had been fully charged with a dedicated NiMh charger (interesting article there Timbo...) the night before & had put in about 500maH if i recall (which is about right from previous experience....a flying session....a couple of weeks in the shed & then a charge before flying) & the battery is 12 months old.

I take the point about the accuracy of the monitor....who knows what green light number two means in terms of voltage but I would hope that it is at least relative (ie bottom red is a considerably lower voltage than top green). More testing needed methinks but sadly I don't have a variable DC supply to see what voltage each light represents.

What I can do however is put a constant load on said battery using my charger/cycler up to 1A & see what happens....I will try this & report back.

Personally I think LiPos & a regulator are the way forward & when my fleet needs new batteries this is what I will do. The main attraction to me is the way LiPos hold their charge. Recharge when you get back from a session & they will still be ready to fly several weeks later...no way I would chance that with NiMhs. I'm amazed that no-one yet offers a "box" containing a Lipo, suitable regulator & cut off circuitry for if the voltage of the LIPo drops too far (if you'd left the radio switched on say)

[Former Member]

[This posting has been removed]

[Tim Mackey]

"I'm amazed that no-one yet offers a "box" containing a Lipo, suitable regulator & cut off circuitry for if the voltage of the LIPo drops too far (if you'd left the radio switched on say)"

The tr Lipo packs have this feature ( PCM / DPM ) which does completely isolate the output if voltage gets too low.The nearest thing to a similar device for the tr packs would be the cheapy  regulator gizmos that I have used a lot of, which are from H/City and combine a regulator with a loud alarm screecher which sounds off as the pack input volts approch the low safe level ( approx 6V ). These units will work on any input voltage up to around 20, but the LV feature is obviously only useable with the normal 2s LiPo input.

[Phil Claridge]

hi timbo i have an orca tx lipo 2600mah its about two years old but never been used when i tried to see if there was any thing left in it, it showed no voltage at all it says on the battery low voltage protected but as its done nothing for two years is it worth trying to see if it will take a charge the battery has only the futaba style connector no balance leads regards phil

[Tim Mackey]

The PCM ( protection circuit module ) should have protected it.... so give it try and see how it goes. As its for the TX though, be absolutely certain that all is well before committing it to use again!

[Bladerunner]

Posted by Steve Hargreaves on 17/06/2008 09:50:00:

Hi all....back to the fray after a few days "working" & blimey a few interesting comments there....

 Just for the record Timbo is correct this was an IC Yak.....typical club model, 52" span, 5 standard analogue servos 60 4-stroke power. The battery had been fully charged with a dedicated NiMh charger (interesting article there Timbo...) the night before & had put in about 500maH if i recall (which is about right from previous experience....a flying session....a couple of weeks in the shed & then a charge before flying) & the battery is 12 months old.

I take the point about the accuracy of the monitor....who knows what green light number two means in terms of voltage but I would hope that it is at least relative (ie bottom red is a considerably lower voltage than top green). More testing needed methinks but sadly I don't have a variable DC supply to see what voltage each light represents.

What I can do however is put a constant load on said battery using my charger/cycler up to 1A & see what happens....I will try this & report back.

Personally I think LiPos & a regulator are the way forward & when my fleet needs new batteries this is what I will do. The main attraction to me is the way LiPos hold their charge. Recharge when you get back from a session & they will still be ready to fly several weeks later...no way I would chance that with NiMhs. I'm amazed that no-one yet offers a "box" containing a Lipo, suitable regulator & cut off circuitry for if the voltage of the LIPo drops too far (if you'd left the radio switched on say)

 

New post to old post...you could also try sanyo eneloop as a direct replacement for existing AA packs. Hold their charge for months, no regulators required and directly compatible with existing NiMh charge gear. I've tested these and they have good load performance too