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Electric Stunt => Gettin all AMP'ed up! => Topic started by: paul winter on October 12, 2012, 11:36:04 AM
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hi
as im new to this sparky lark
answer me a question
im using a 750 kv motor
castle ice lite 50 esc
will hubin timer
and hobby king 5s 3000 batteries that weight 13.79 ozs/391 grams
but a 4s 3000 battery is 10.54ozs /291 grams
can i use 4s or ??????? to save alot weight
paul
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IF a 4S 3000mah battery is big enough, then you don't need a 5S 3000mah battery. You need a smaller 5S battery to deliver the same amount of "power". A 5S 2400mah battery would equate to a 4S 3000mah battery.
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You neglected to say how much the rest of the plane weighs. You need to match the total energy available from the pack to the plane weight -- get it wrong, and you either don't have enough battery, or you don't have enough oomph to finish a flight.
But, let's analyze.
First, let's ignore Kv:
One 3000mAh 3.7V cell will deliver (approximately, and at the very most) (3A) * (3600 seconds) * (3.7V) = 39960 Joules. Round that to 40kJ.
Four of 'em will deliver four times as much: 160kJ.
Five of 'em will deliver five times as much: 200kJ.
If you need 200kJ to fly your plane, you need 200kJ to fly your plane. Going to a 4s pack would mean you'd need to increase the battery capacity by 5/4, to 3750mAh.
How heavy is the plane going to be? A 60 ounce plane would need those 200kJ, so it'd need the extra cell. A 48 oz plane could get by with the 4s pack.
Now, let's pay attention to Kv:
That 750kV motor could be made to turn up to 10k RPM on five cells (really, you'd want it slower, but 10k max). On four cells, you would only be able to ask for about 8300 RPM. That's kinda slow: you could maybe make it work with a high-pitch prop, but speed regulation would suffer.
Now, let's pay attention to battery pack capacity:
If you need to use that motor, and if your plane isn't going to weigh 60 ounces, then don't use fewer cells: use five smaller cells. You weight will go down roughly proportionally to the cell capacity, and your motor will turn fast enough.
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hi
40 with out battery
pw
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Then you want something like a 2500mAh 5s pack, or 2600mAh.
Multiply your total airplane weight in ounces by 50 to get the capacity needed for a five cell pack. This means the weight of the plane with the battery in it, so you'll need to do some cut-and-try with catalog battery specifications.
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Tim
I fly a 52 ounce plane on a 4S 2600mha battery. I am assuming you 50 x (weight in ounces) to get the require 5s battery is using a standard heli ESC with governor and a non governor timer. I use a non governor ESC with a timer/governor that has a variable punch up and no punch down. This allows me to fly level laps quite a bit below the power required to do maneuvers but as soon as I pick the nose up I get a kick from the governor that is programable. In other words I save lots of energy in the level laps and during the times the plane is heading down (unloaded), and use what power I need to do the maneuver whenever the plane is under load. It seems to work.
Andy
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Tim
I fly a 52 ounce plane on a 4S 2600mha battery. I am assuming you 50 x (weight in ounces) to get the require 5s battery is using a standard heli ESC with governor and a non governor timer. I use a non governor ESC with a timer/governor that has a variable punch up and no punch down. This allows me to fly level laps quite a bit below the power required to do maneuvers but as soon as I pick the nose up I get a kick from the governor that is programable. In other words I save lots of energy in the level laps and during the times the plane is heading down (unloaded), and use what power I need to do the maneuver whenever the plane is under load. It seems to work.
Andy
Interesting. I use the "knowledge" gained from going through all of the entries in the "list your setup" thread about two years ago, and doing a curve fit against weight vs. total energy. Most of those were with Hubin timers/Heli governors. My numbers matched a published rule of thumb at the time to shoot for 7W average, 11W peak.
You innovators. Always going and screwing things up for the guys that try to go by historical data. Do you use a KR timer? How much charge do you have to put into the battery at the end of a flight? I suspect your peak power needs are the same.
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Hi Paul,
I created a spreadsheet some time ago which may help you determine Pack sizes and ESC requirements. It's only designed to give a ball park figure but seems to stack up quite well as long as you don't have a particularly badly trimmed or high drag airframe.
It's simple to use, just enter the model weight in oz the flight time in seconds and a wh/llb figure. (It also shows the equivalent wh/oz alongside). [112 wh/llb = Tim's 7 wh/oz and 176 wh/llb = his 11 wh/oz].
It should then populate the rest of the sheet from those inputs.
To determine how much battery you need use around 112 wh/llb and read off the Suggested Minimum Battery Capacity for your required cell count. (Lipo or A123). The Lipo figures are based on 70% usage so have a considerable safety margin.
To determine a suggested ESC rating enter around 176 and read off the Average Current row (Again for your cell count).
It's as simple as that.
It also has a section where you can enter the motor kv for different cell counts (Again Lipo or A123) and this will suggest the minimum and maximum RPM you could consider running that particular motor at. This is loosely aligned with the limitations of the Governor in the CC Phoenix range. The most efficient RPM is probably in the range of 500 rpm inside those extremes
Here's the link https://docs.google.com/spreadsheet/ccc?key=0AkULk7WbzmeMdHIyV2JhMkVwV2ZZVkRwUzlkYnZodVE#gid=0
If it's of any use I suggest you download it into your favourite spreadsheet program as I have found the on-line version has been tampered with in the past and I have had to re upload it several times.
As I said it only provides ball park figures so if you end up with a smouldering pile of assorted Balsa and plastic no warranty is extended or implied. H^^
TTFN
John.
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I've been using these in my 67 ounce Sultan @1850mah per flights. They are holding up well so far with about 25 cycles each on four of them. http://www.hobbyking.com/hobbyking/store/uh_viewItem.asp?idProduct=21351
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That's awfully good. Flight duration ?
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Mike Haverly,
Thanks for posting that link. Those look very interesting! I've been using some Flightmax 20c 5s 3000's that weigh about 13.5 ounces. These are two ounces lighter. I will have to get a few to try.
Thanks,
Jason
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Tim
I fly 5 minute duration flight with 5.3 second lap times at a launch RPM of 8100 with the vari-gain set to four (it is adjustable from 1 to 10). At the end of the flight I have right at 20% left in the battery. With the gain set to one, maintain 8100 RPM through the flight no additional boost under load, I cannot safely do a wing over. With the gain set to 4 the governor adds enough additional punch to fly any maneuver without a problem. I am using one of Dennis Adamisin 12x6.5 WEP props.
Vari-gain takes the reference RPM in this case 8100 and subtracts from that the RPM under load. What ever the difference is it multiplies that by the gain parameter, in this case 4, and adds that back into the reference RPM. I have been wanting to actually take the reference RPM down another 100 RPM and up the gain to 5 and see what the results are. I know from experience that going too far with this will lead to motor RPM hunting during flight. A little hunting is not a problem too much can be a distraction to the flier and the judges.
Andy
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My flight duration is 5min. 25sec. E-Flite Power 32, Hubin FM-9R timer, Phoenix 60 ESC. 8720 RPM, gain is at 50 (wide open). Prop is APC 13 x 5.5, 5.3 second lap time. I bought those particular batteries because they were pretty close to the same weight as the Thunderpower 5S2600's that I also use. Never had a problem and they seem to have plenty of power.
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After witnessing Howard's early flight last Sunday AM (cold, dense air), I would ask if Andrew maybe gets by with less battery because of warmer, less dense air. Altitude might be about the same; I think Salem is about 550' ASL, but I don't know exactly where Andrew flies, either. A slicker airplane could also be a factor...thinner wing or film covering? ??? Steve
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If I remember Howard's comments, the flight was in cold, dense, still air. I've heard comments (by either Howard or Paul Walker) that the plane uses less energy in wind (presumably until you have to turn power up). And I know that turning takes less energy in dense air (but I'm not sure about level flight).
So the "still" may have trumped the "dense".