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Picking a power system

YelKiteFlyer

New to GSN!
[H=1]LUCIEN MILLER'S WAY[/H]
(Taken from ATTF podcast # 207)

Note:
An electric motor is a constant rpm machine. As you put on a larger prop the motor will try to spin the prop at the higher constant rpm but the current will increase. The propeller determines how much power the electric motor produces.

  1. Find out how many watts are needed.
  2. Pick the battery
  3. Pick the motor
  4. Pick the ESC needed


[H=1]Watts Needed[/H]
  • Motor Glider = 50 watts per pound
  • Trainer = 75 watts per pound
  • Sport flight = 100 watts per pound
  • Pattern or warbird flight = 150 watts per pound
  • 3d planes = 200 to 250 watts per pound
  • Scale Helicopters = 150 watts per pound
  • 3d helicopter = 400 to 500 watts per pound
  • Speed flighing helicopter = 700 to 800 watts per pound

I picked a 3DHS Super Vyper to do the calculations.

Super Vyper = 1000 grams = 2.2 pounds
2.2 x 250 = 550 watts needed

80% Rule with electric systems
Always use only 80% of an electric systems.
If you use more, you are shortening the life cycle of that system.

(watts with 80% rule)
550 / .8 = 687.5 watts

[H=1]Picking the Battery[/H]

Picking the cell count of a battery
# of cells = 2 to 5 times voltage, preferably 3 to 4 times.

3 cell = 11.1 = 22 to 55 amps
4 cell = 14.8= 30 to 74 amps
5 cell = 18.5 = 37 to 92 amps
6 cell = 22.2 = 44 to 133 amps

Watts / volts = amps
3s battery
550 / 11.1 = 49.5 amps (a little to close?)
49.5 / 11.1 = 4.46 (a little to close?)

4s battery
550 / 14.8 = 37.2 amps
37.2 / 14.8 = 2.51 (that is between 2 to 5 times)

[H=1]How Big a Battery?[/H]

Amps pulling from motor / capacity of battery = C rating
4s 2700
37.2 amps / 2.7 = 13.8 C rating

60 minutes / C rating = minutes of flight (full power, total discharge)
4s 2700
60 / 13.8 C rating = 4.38 minutes of flight.

80% Rule
4.38 minutes x .8= 3.50 minutes (full power, total discharge)

REAL LIFE (Usually fly at 2/3 throttle = 1/2 current)

3.50 minutes x 2 = 7.00 minutes flight time

80% RULE
Never pull 80% of C rating out of a battery. Personally Lucien does not like taking out more than 1/2 the C rating out a battery. This saves the battery's life cycles.
C rating of battery: Our calculated C rating / C rating of battery = %
25C: 13.8 / 25 * 100 = 55.2%
30C: 13.8 / 30 * 100 = 46.0%
45C: 13.8 / 45 * 100 = 30.7%

[H=1]Picking a Motor[/H]

Prop recommended by 3DHS with watts;
3s = 14 x 7 or 13 x 6.5 with 500 watts
4s = 12 x 6 with 700 watts

So what I need is a motor with a 12 x 6 prop on 4s producing 687.5 watts weighing in the neighbourhood of 130 to 150 grams.

[H=1]Picking the ESC[/H]

The speed controller should handle the highest number of amps the motor will put out, with the prop you pick out, plus the 80% rule.

Max amps (of motor with prop) / .8 = Number of amps your speed controller should have.
 

YelKiteFlyer

New to GSN!
The Weight should be the considered as the all up flying weight, including a
battery. Some times you have to just have to take an educated guess at the
expected battery weight for the first order calculations. Once you have the
actual weight, you can run through it one more time, but typically the watts
per pound will only vary by about 10% at the most, unless you end up using a
much heavier battery.

Lucien Miller
Innov8tive Designs, Inc.
 

Steve_B

70cc twin V2
Ok.. in that case I'm pretty sure that the Super Viper example is incorrect as the weight used was excluding battery. If you used the realistic flying weight (in the order of 46oz / 2.9lb) then I think you will end up with very high power figure. For instance substituting the realistic all up weight of 2.9lb then you end up with about 900W as the 'needed' power. That is significantly higher than even the most powerful motor option recommended by 3DHS for the Super Viper.

As another example I tried the rule on my 60oz 51" AJ Slick. The 3DHS recommended Hacker for the Slick is officially rated for 700W, people commonly fly the Slick on between 700 and maybe 1000W. If I use the 200-250W and 80% rules I got a required power prediction of 940-1170W, which is very high compared to reality.

Do you think that maybe the application of the 80% factor is pushing the numbers up too high, I think you get more realistic numbers for 3D models without it.
 
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YelKiteFlyer

New to GSN!
You have two different issues here.

1) I was not aware of the answer to the question you asked. And you are right, the example of the Super Vyper is wrong. I will have to recalculate the example. Maybe I should have used the example Lucien used in the ATTF podcast 207, but at this moment I am working on finding out what I needed.

2) The 80% rule has nothing to do what you actually need to run the model.

The 80% rule is the overhead you are building into the electrical system.

The 80% rule is there to insure that the electrical system is not overused.

You can run an electrical systems at 100%, but it will not last. You can do it, but components will start to break down.
How long will they last?
A) They will last the minimum amount of time the system was rated at.

B) The electrical system will last as long as you do not exceed the rating of every component within the system.

Do manufacturers overbuild their electrical systems, probably. By how much? Unknown.

So the question is, what percentage of overhead are you going to build into your power system?

Me, I have no idea, Lucien says build in a 20% overhead. Sounds like a reasonable plan to me.

Robin
 
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Steve_B

70cc twin V2
Robin,
I think maybe with 3D planes that don't use full throttle for long periods and where weight is especially critical it's safe enough to dispense with the 80% safety factor.

My point is that if you use the 'safety factor' for 3D models you end up coming up with a larger, heavier and more powerful motor than any of the manufacturers recommended motors for the model in question. As per my last post, you would end up calculating 900W for the Super Viper which would rule out all the officially recommended motors because they are rated for much less than 900W.

Steve
 
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YelKiteFlyer

New to GSN!
You would have to contact the manufactures to find out how they calculate their recommended systems.
It would be interesting to know how they do it!
Do they use a formula, or calculate it differently for each plane they put out. If it is a formula can they share it?
Now that would be interesting information,
Robin.
 

Steve_B

70cc twin V2
Robin,

I'd guess they originally arrived at the recommended motors by experience and by test flying. Once they came up with a system that worked well for a certain size/weight/type of model then for subsequent similar models that same power combo would be the starting point for flight testing of the new design... or at least that's my guess.

Reverse engineering from the recommended power combos and model weights published on the likes of 3DHS and EF web sites usually comes up with a watts per lb figure in the 200 - 250 range, exactly as per the guideline, but excluding any additional safety factor.
 

gyro

GSN Contributor
I can tell you first hand that the Omega 130g motor on 4s with a 12x6 pulls about 750w, and is PLENTY of heat to pull that plane around VERY nicely.
 
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