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Maths was never my strong point!

Please help me choose a LiPo.

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David Davis11/07/2018 05:58:47
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3685 forum posts
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I am planning to build a lightweight special for my assault on next year's Coupe Des Barons **LINK**

This is a competition for three-channel French trainers in which there are four rounds or flying tests. Each round of the competition lasts for 5 minutes but I know from experience that I need to be able to fly for 10-15 minutes to allow for delays at the start of the round or the Contest Director stopping the clock on safety grounds during the round itself.

The rules permit an electric motor of up to 825 watts. I have a 800 watt 790kv motor, a 13 x 8 propeller and plan to fit a 4S LiPo.

What size Lipo will I need to be able fly for 10-15 minutes with this set up?

Please show your calculations!

Cuban811/07/2018 07:39:24
2911 forum posts
1 photos

800W turning a 13X8 sounds a lot for what you describe as a lightweight model, as at full throttle this will be giving a static thrust of around 5.5 lbs - can you give a description of the airframe, a large & floaty vintage type perhaps?

Dave Hopkin11/07/2018 07:43:46
3672 forum posts
294 photos

An "800watt 790kv" motor will draw X current depending on the prop load you fit, bigger the prop the higher the current

What you need to do is set the motor and prop up on a test rig and measure the current it draws turning your chosen prop

Once you know the current draw from the bench test you can divide the battery mAH capacity by the current draw to give you a decent idea of the duration

If you find you are drawing 20Amps at cruising speed then

A 4000 mAH will give 4 Amps for an Hour, 20/4 = 5, 1 Hour /. 5 = 12 mins

Estimated Current Draw / Battery Capacity = X

1 Hour / X = Duraction

David Davis11/07/2018 07:53:25
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3685 forum posts
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Posted by Cuban8 on 11/07/2018 07:39:24:

800W turning a 13X8 sounds a lot for what you describe as a lightweight model, as at full throttle this will be giving a static thrust of around 5.5 lbs - can you give a description of the airframe, a large & floaty vintage type perhaps?

This is the model. It has a 1.55 metre or five foot wingspan. Sorry I don't know the weight because I don't have a set of scales, besides I intend to build a lighter model for next year's competition..

baron ready for the coupe (2).jpg

...and this is a picture of all of us at the start of the competition on 2nd June.

group photograph.jpg

Edited By David Davis on 11/07/2018 08:02:05

David Davis11/07/2018 07:56:01
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3685 forum posts
688 photos
Posted by Dave Hopkin on 11/07/2018 07:43:46:

An "800watt 790kv" motor will draw X current depending on the prop load you fit, bigger the prop the higher the current

What you need to do is set the motor and prop up on a test rig and measure the current it draws turning your chosen prop

Once you know the current draw from the bench test you can divide the battery mAH capacity by the current draw to give you a decent idea of the duration

If you find you are drawing 20Amps at cruising speed then

A 4000 mAH will give 4 Amps for an Hour, 20/4 = 5, 1 Hour /. 5 = 12 mins

Estimated Current Draw / Battery Capacity = X

1 Hour / X = Duraction

Thank you Dave, I'll rig something up, but high speed is important in at least two of the rounds, the pylon race and the streamer cutting event.

Cuban811/07/2018 08:21:42
2911 forum posts
1 photos

An interesting and fun project. A few back of fag packet calcs give a WOT power on a 13X8 of something like 750W on a 4S lipo, so you'll have plenty for the fast stuff, but I'd wager that the model will be using a quarter of that when correctly trimmed and just cruising around for duration - maybe as little as a couple of hundred watts so 15A perhaps less given flying conditions on the day..

A very useful resource here http://www.godolloairport.hu/calc/strc_eng/index.htm

Plug a few figures in and it'll give you an idea of where you are - saves a lot of time and the results are quite accurate, usually within 10 to 15% of real life conditions, I find. Saves buying a lot of props and blown ESCslaugh

Be interested to see your measured figures.

 

Edited By Cuban8 on 11/07/2018 08:31:19

Nigel R11/07/2018 09:00:31
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3750 forum posts
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"I have a 800 watt 790kv motor, a 13 x 8 propeller and plan to fit a 4S LiPo"
 
The motorcalc website suggests a 3548-790kv will draw about 850W on a 4S. You might want to try a 13 x 7 or 12 x 9 to drop the power a little, but that's something you'll troubleshoot with a wattmeter when the setup is ready (right!?)
 
As for lipo size, I wll go out on a limb here and suggest you will need a big unit to get your 15 minutes. I would guess a 6000mAh or even larger. It'll be a big lump of a battery, for sure.
David Davis11/07/2018 10:03:55
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3685 forum posts
688 photos
Posted by Nigel R on 11/07/2018 09:00:31:
"I have a 800 watt 790kv motor, a 13 x 8 propeller and plan to fit a 4S LiPo"
 
The motorcalc website suggests a 3548-790kv will draw about 850W on a 4S. You might want to try a 13 x 7 or 12 x 9 to drop the power a little, but that's something you'll troubleshoot with a wattmeter when the setup is ready (right!?)
 
As for lipo size, I wll go out on a limb here and suggest you will need a big unit to get your 15 minutes. I would guess a 6000mAh or even larger. It'll be a big lump of a battery, for sure.

I used a 2200mAh 4S battery for the event in June and ran out of electricity with a minute to go in the pylon race when I was doing quite well.

boris in flight.jpg

I was befriended on the day by a guy called Phillipe who won his heat in the pylon event. His model did indeed feature a young housebrick of a LiPo!

Don Fry11/07/2018 10:43:34
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4557 forum posts
54 photos

As I recall, the tests are

Hitting meter long soft targets, planted in the ground, most wins

Most passes under a bar, limbo style

Combat ribbon cutting

A pylon race.

It strikes me you need 2 batteries. A small one for the first two, with a low pitch slow fly prop, to get you a slow precise light machine.

And a big battery for the power hungry last two. So I would reason your design puts the batteries on the centre of gravity. Your trannies has two flight modes, one for a floater, one with a load of down elevator trim.

Sorry can't help with the maths.

Nigel R11/07/2018 10:49:56
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3750 forum posts
587 photos

There's a plan.

2200mAh in current location.

Add another, dead on the CG, which you can plug in, in parallel, when needed.

Is prop swapping allowed?

14x6 for the slow fly rounds, 12x10 for the pylon round

Geoff S11/07/2018 11:15:04
3588 forum posts
14 photos

It's very difficult to work out duration because it all depends on how you fly - or how the demands of the competition require you to fly.

For example - My scalish DB 58"ws Gypsy Moth weighs 3kg and will fly for 10 minutes on a 4S 4AH LiPo but I'm rarely at full throttle. The motor (an EMax GT3526 05 710rpm/v) turns a 13x6.5 prop at 8900 rpm for a current of 37 amps which is a power consumption of around 550 watts. There's still around 25/30% of the energy left in the battery but I prefer not to push my batteries too hard.

It's not so much maths as measurement. The maths is trivial eg (theoretically) a 4AH battery will deliver 4 amps for an hour or 40 amps for 6 minutes but those calculations only work if you're flat out all the time. The power is simply current x voltage and you need around 80 watts/lb for good performance (the old rule of thumb was 100 watts/lb but that was with brushed motors and I find the lower figure more than adequate).

In other words you need to get in the right area, fly the model (after checking you're not drawing so much current that you're in danger of cooking anything - motor or esc) then see how much energy you have left in the battery after a set time and go from there.

An electric motor is a very flexible device and that's why those not used to them find them a bit tricky to understand. The power they deliver (to a first approximation) depends on the applied voltage and the load (ie the prop) and the real killer is current rather than voltage.

Geoff

David Davis11/07/2018 11:52:30
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3685 forum posts
688 photos
Posted by Nigel R on 11/07/2018 10:49:56:

There's a plan.

2200mAh in current location.

Add another, dead on the CG, which you can plug in, in parallel, when needed.

Is prop swapping allowed?

14x6 for the slow fly rounds, 12x10 for the pylon round

I like the idea!

Chris Walby11/07/2018 13:07:20
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1200 forum posts
299 photos

IMHO, set up the model within the normal limits of the motor/ESC/prop, then head out to the field.

Practice each discipline and record how much energy you have used from the battery pack and then you will get an idea of duration for each event. My son and I can fly the same model and he can reduce the flight time by 50% through youthful use of the throttle stick, my point is that unless its a flying brick the energy used is almost completely dictated by the throttle usage.

The parallel battery is good as it adds duration and if on the C of G won't upset the balance + in the heat of the moment you won't forget to change the prop back etc The alternative is change the cell count for performance (if the motor/ESC are up for it) and them lower cell count where you don't need the flat out performance (sounds like a bit of a faff + having twice as many lipo's about with similar weights).

I would expect a lot of the competitors have learnt from experience and a lot of previous practicewink

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