ESC maximum current draw - difference between static and flying?

[Jonathan M]

I'm trying to determine a power-train for a lightweight 500g electric-launch 2m RES glider, so keeping weight down is important.

 

My static tests with a watt-meter and a spare but heavy 20A (max 30A) ESC produce a peak draw of about 9A (at roughly 10v so 90W power) from a fully-charged 3s 450mAh.  Ideally I'd like to fit a much lighter 10A (max 12A) ESC to save weight.

 

(This is with a 9x5 folding prop; waiting for the 10x6 to arrive to see what the figures are for that; and don't yet know what I'll use in practice as I need to find a balance between enough power for an efficient climb to 60m within 10-20 secs max and conserving capacity for several launches plus actual thermal flying etc.  Also my test battery is quite old and tired with a much poorer C (and burst) rating compared to modern ones, so not convinced its full charge is as 'fit' as a new one's would be.  Needless to say, after three 20s runs at full tilt the peak figures drop somewhat!)

 

So my basic question is this:  what can I expect the maximum current draw to be in actual flight?  A bit more than the 9A static figure... or a lot more?!

 

If a lot more, can anyone recommend a good but light 15A ESC?

 

Finally, although I expect only experiments in the air will give me the answer, is 80 or 90 W/lb going to be enough power for this sort of model?

 

 

[Gary Manuel]

Current in flight should be less than static, not more. The motor is doing less work because the air through the prop is already moving.

[Dad_flyer]

The C rating of a battery is related to the internal resistance. A better battery will then have less voltage sag under load and drive the prop faster, drawing a higher current. The bigger prop will also make the motor draw quite a lot more current.

So with better battery and bigger prop you will be well over 10A static. I have no Idea how much power you will actually need for the performance you want. You could try flying with the big prop and bigger ESC and see where the throttle needs to be for the climb you want, then measure the current/power at that setting, and the mAh you need to put back into the battery after a flight. Then you should have the information to make your ideal setup?

[PatMc]

Jonathan, I wouldn't expect there to be a lot of difference in current between static & flight since the model will be climbing steeply throughout the power run.

I don't have a 2mtre glider that's as light as yours but have 3 of around 1kg each, the power/weight ratios are 134, 119 & 80 w/kg. The two most powerful have been measured over quite a few flights consistently achieving circa 200 metres in 30 secs. The performance of the least powerful, an electrified vintage Graupner Amigo II, hasn't been measured but I'd estimate that it would reach 60m in under 20secs quite comfortably. Battery used is a 700mAh 3s, prop 9x5 folder, motor is a 1050kv Keda 2217/16.

PS power 165W, current drawn is 14A   

Edited May 18, 2021 by PatMc

[Simon Chaddock]

Johnathan M

Gary is quite correct.

Static give the highest amp draw. The battery voltage can only fall as the capacity of the battery is used up. The amp draw falls away faster then the voltage drops.

If you are running the ESC very close to its maximum for many minutes then ESC heat build up is a possible issue with either an auto safety shutdown or a complete failure.

 

The maximum power you need depends what you want the glider to be able to do.

An efficient 2m glider should be able to maintain level flight on 20W/lb or less but if you want a steep climb then 100W/lb is the sort of figure required. If you want 'unlimited vertical' then you are into the realms of 150W/lb but don't use that sort of power in level flight as you would be quite likely to break something.

 

[Jonathan M]

Thanks for the replies - all very helpful.

 

Ran another static test with the 9x6 but with a larger slightly less ancient 3s on 50% charge, which gave me another volt on top and therefore a 100W peak.  On a full charge (or better still fully-charged brand new 3s 450mAh batts for the model) this should be even smarter, say 12v, so power of about 110W peak, which equates to 100W/lb which sounds just right for the sort of 45°-60° launch needed.  Clearly a bigger prop will eat more Amps and therefore Ah, but the rules of the game (F5-RES) are for a max 20sec motor run and an altitude limit of 60m for the launch, so more than happy now with this as a starting point.

 

Also now found a good quality 20A ESC that's much lighter than my existing one - more sensible to have a much safer margin than risk a 10/12A unit.

[Martin Harris - Moderator]

It’s a long time since I measured it but the typical reduction at full throttle in flight seemed to be in the order of 10% or so - I do remember that it was less than I’d anticipated. 

[PatMc]

Martin, I think the current reduction from static is very little with a glider that's climbing very steeply for the entire power run. The motor is providing virtually all the lift the wings are mainly only contributing drag. ?

[Peter Jenkins]

I have fitted telemetry to my 2 F3A aerobatic machines principally to measure height and motor temperature.  However, I also get volts, amps and a few other things.

With a freshly charged 10S pack my Hacker Q80 14XS pulled 3600 watts at 98 amps static - I very quickly shut it down as it's limited to 2800 watts and 80 amps.  The aircraft weights 5 Kgs or 11 lbs in old money.  In the air it's a different matter.  From take off into a vertical climb with full power produced figures of just under 2800 watts and 78 amps.  Based on my findings I would not place much faith in static runs to produce the ideal prop setup for electrics.  In future all testing will be with telemetry while airborne.  For me that translated into an over reading of between 22% and 28% depending on whether you take the power or current figures.

[Jonathan M]

That's very revealing and useful - gives a rough rule of thumb of between a fifth and a quarter less.

 

It would also suggest that a smaller ESC could in fact be safely used with my own comparatively tiny power-train, but (i) there'll still be a large peak draw at hand-launch with the motor at full tilt but before the airframe has picked up speed, and (ii) although the motor run is short there isn't the same ESC cooling as on power models.

 

 

[MattyB]

37 minutes ago, Jonathan M said:

That's very revealing and useful - gives a rough rule of thumb of between a fifth and a quarter less.

 

It would also suggest that a smaller ESC could in fact be safely used with my own comparatively tiny power-train, but (i) there'll still be a large peak draw at hand-launch with the motor at full tilt but before the airframe has picked up speed, and (ii) although the motor run is short there isn't the same ESC cooling as on power models.

 

As per @PatMc’s post above I’m not sure that rule of thumb is applicable in this case - in a power model yes unloading to that kind of degree can occur, but when you are trying to drag a glider up very steeply at low speed that’s a very different situation that is much closer to a static run.
 

The simple solution is to fit the bigger ESC initially along with a current sensor and monitor it in flight - that will give you a conclusive answer. I suspect you may just be able to get away with the 10 or 12A ESC if you limit power on to 20sec bursts, but I’d be worried about running it that way in the longer term; it will run hotter which makes it more likely that the BEC could fail.

 

Edited May 19, 2021 by MattyB

[Dickw]

1 hour ago, Jonathan M said:

That's very revealing and useful - gives a rough rule of thumb of between a fifth and a quarter less.

 

It would also suggest that a smaller ESC could in fact be safely used with my own comparatively tiny power-train, but (i) there'll still be a large peak draw at hand-launch with the motor at full tilt but before the airframe has picked up speed, and (ii) although the motor run is short there isn't the same ESC cooling as on power models.

 

 

While I do have models that show current drop of 25% or more on acceleration, they tend to be the ones with considerable excess power and higher pitch props.

 

For an example of a glider launch, have a look at the attached graph of current v altitude. Admittedly this is for a larger and heavier F5J type glider at 3.7m span and 2.5Kg weight, but it should give you some idea of what happens during an 18 second climb to 200m.

You can see the current peaking at 54.6amps as I allow it to reach full power static just before launch. It then falls by 7.3 amps (13%) once a steady climb is established. Some (but not all) of this fall can be attributed to an initial drop in battery volts, so a current reduction of 10% due to model acceleration seems a reasonable figure for a glider.

 

Dick

 

[Jonathan M]

Thank you - very clear and illuminating!

[PatMc]

Jonathan, I remembered I had this video of my Amigo taken in 2018 so uploaded it to Youtube. It may give you an idea of the rate of climb achievable with 80W/lb (typo in my previous post has it as 80W/Kg)

Sorry, I didn't have time to edit the original so the whole thing is 14minutes long but the first climb. from hand launch, is 20secs & starts at 1min 24sec. Second climb is from fly by at about 15ft, starts at 5min 33sec & 25 secs of power. 

 

  

Edited May 19, 2021 by PatMc

[Gary Manuel]

?

Edited May 19, 2021 by Gary Manuel

[Jonathan M]

6 hours ago, PatMc said:

Jonathan, I remembered I had this video of my Amigo taken in 2018 so uploaded it to Youtube. It may give you an idea of the rate of climb achievable with 80W/lb (typo in my previous post has it as 80W/Kg)...

  

 

Cheers Pat - lovely to see, and a respectable enough climb on just 80W/lb.  So if, my calcs for the model of about 100W/lb on the 9x5 are right, then it should easily get to 60m in 12-15 secs (the current generation of 2m RES models have much more slender fuselages and thinner wings so less draggy all round).  Else have a 10x6 on order and will run the static tests again when the new batts arrive - and repeat in the air.

 

Its an X-RES which I bought second-hand a year or so ago and flew as a bungee-launched glider until I got caught out in a boomer much too far downwind.  I deployed spoilers but then had difficulty in regaining orientation at that great distance (a 2m span looks very small indeed at 300m high and 1km away!) and lost signal when it came down the other side of a hill.  After an hour's search with the dog and a pair of binos I found it - nose smashed in the earth with a broken back and some wing damage - so set it aside. (I resumed flying off the bungee with a used PuRES I bought shortly afterwards, with which I've had great enjoyment and some good duration success, most recently about 33mins off a modest 45m launch).

 

Now I've decided the X-RES will make an excellent candidate for conversion to electric-launch... after all the nose is already 40% shorter!!  I love the simplicity of the bungee, but electric will open up a wider choice of thermal flying sites.

 

But the challenge is to keep the extra weight right down, as these F3-RES and F5-RES models are only good for relatively light wind days (say 5-8mph, 10 max) and there's no point in carrying unnecessary ballast if it can be avoided.

 

Pics below are of the model as originally re-covered, and then the fateful day...

 

 

 

 

Edited May 19, 2021 by Jonathan M

[Peter Jenkins]

Wow, a genuine tent pegging!  Sorry Jonathan!?

[Jonathan M]

Thanks for the commiserations Peter.  It hurt a bit at the time, but also taught me a big lesson about what's an acceptable horizontal range for these light fixed-camber gliders!  And more importantly its now given me a fun repair/conversion project... ?

[Jonathan M]

New 3s 450mAh Lipos arrived, as did a third prop for the static tests, and this was the outcome:

• 8x4 peaks at 67W/lb on the first 15sec run, dropping to 63W/lb on the fourth 15sec run; and after resting Lipo for ten mins it's down to 79% capacity.
• 9x5 peaks at 96W/lb down to 90W/lb on the fourth run; after resting down to 70% capacity.
• 10x5 peaks at 115W/lb down to 100W/lb on the fourth run, 67% capacity.

Obviously the weaker prop combo will take longer in a shallower climb to reach the same 60m altitude as the medium/powerful prop combos and use up more capacity in doing so, but until I get it airborne and practicing F5-RES stuff, I won't know exactly which prop will give the right balance between climb performance, max number of launches per Lipo, and remaining capacity for soaring etc.

 

Also of interest is that the 8x4 prop only peaked at 6.4A (the 9x5 at 9.4A and the 10x5 at 10.8A) which means that the punier prop would allow a much lighter 10/12A ESC to be fitted.

 

 

Today's work-in-progress on the Six Million Dollar Glider (the now shortened nose needed widening from an internal 22mm to 25mm just in the area of the 23mm motor diameter for clearance, which was ever so fiddly):