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Old Nov 07, 2011, 07:40 PM
They Call him Dead!
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Originally Posted by Aeroplayin View Post
One of the things we can do is use the RC Web Club Web site to maintain a document library and refer to it from here. I can manage the documents so they are all in one place, on one page, all the time, and we can store them in chronological order. Let me know if this sounds like something that could help.
This sounds good to me! Should we ask people to post their system in a certain format so that you can copy and paste? If so tell me how you want it and I will stick it in the first post.
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Old Nov 07, 2011, 07:47 PM
VOLTS > AMPS
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United States, MN, Buffalo
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Since we are on the subject of power systems, I have a beater sport plane that has a KD 36-16M, apc clone 12x6e, and a turnigy K-force 40amp. I needed to discharge some batteries last night and I played around with it.

The acceleration on it was much faster then the airboss/torque combo. At first I didn't believe it but I tested it over and over again and the yak just spooled up and was almost instaneous on the accel. The airboss/torque lagged a little bit.

I remembered I had some youtube videos when I was trying to figure out the braking difference between the K-force and Airboss.
here they are
http://www.youtube.com/watch?v=u2lott-VDC0
http://www.youtube.com/watch?v=9iAQB4Um4Wc

You can hear it in the pitch of the prop, but do smaller motors really spool up faster then the big ones or am I just hearing things? The little motor pulls about 10 more amps then the torque also and puts out about 120 more watts. Don't know about thrust as I don't have a fish scale.
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Old Nov 07, 2011, 09:42 PM
c/f
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Originally Posted by _OZ_ View Post
c/f, I don't understand the battery weight to watts relationship. Also, how do you know how many watts you want through a prop? Finally, can you post your formula?
The battery is actually the power not the motor, the motor is basically a convertor to get the horsepower of the battery to turn a propeller. To me as long as the motor is rated for a given watt range of the battery potential, its a go for me.

People have long wanted the e quation to be as simple as nitro powered, it actually is for me when you consider the battery is the powerplant producing the horsepower. ie 749watts equal 1hp, so my formula is 10g battery produces 25 watt power or .015HP

So in the end 3D performance preffrence for a given RTF weight range will show a needed battery for a preffered prop, which in turn will show me a given HP into a specific prop. Using the EF models is a great place to start as everything is built to exacting weights with only the prop prefference as a variable.

Want to do an interesting spreadsheet concerning HP needed for given weight range models? Go to Horizon Hobby and you will see that starting in the ultra micro series that for every 35grams increase in RTF weight they increase the battery size/weight/KV/prop while in some cases its the same motor. You can go all the way up thru the model weight range and see what gram weight of the battery recommended and using my formula you will see how it compares in HP to its equivalent nitro counterpart which most are dual rated. The e motor specs would merely be capable of handling the battery wattage produced.

Its probably best to PM with any more questions as to not hijack/confuse thread anymore than needed..02
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Old Nov 07, 2011, 10:27 PM
Not as Good as The Kid
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Originally Posted by c/f View Post
People have long wanted the e quation to be as simple as nitro powered, it actually is for me when you consider the battery is the powerplant producing the horsepower. ie 749watts equal 1hp, so my formula is 10g battery produces 25 watt power or .015HP
But the fact is that this is the same over-simplification that we're trying to move beyond. I know that I need about 1500 Watts of power (2 HP) to turn a 16x8 APC propeller 8800 RPMs, and your formula says I need a 600g battery, when a 450g battery will easily give me the volts and C rating I need. There are also heavier batteries that will actually deliver less volts at WOT and therefore not provide the power I need, and also make it impossible to balance the plane properly. I wish it were that simple. I also wish it were as simple as multiplying Amps time volts, or volts time Kv, but no such luck.
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Old Nov 07, 2011, 10:51 PM
c/f
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Well on the EF page concerning the 4016 motor and 1400W into a 15X8 it is recommended 5s 3850-5000 MAH so here is a quick spreadsheet, while my formula is not an exact lab formulation it gets me close to the bullseye from 1oz micros all the way to 12S 40% models, without looking for anomally batteries as you have referenced.
ie

Have a look:

http://www.hobbyking.com/hobbyking/s...5000&location=
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Old Nov 08, 2011, 06:38 AM
Not as Good as The Kid
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Originally Posted by c/f View Post
I understand where you're coming from, and your comment is very valid with regard to battery selection and what you said about power. But the issue here is that the different data from the six different props were all taken with the right battery, the same battery, the same battery charged to the same capacity, the same motor with the same Kv, and each prop had the same length and pitch. So once the battery selection is made, and even the prop size is selected, there are still propeller options, and Kv options relating to the prop options that complicate things beyond the battery selection.
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Old Nov 10, 2011, 09:00 PM
Not as Good as The Kid
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I was just doing some flight time calculations on the 48 inch Edge thread and trying to make a linear relationship, so I went back to Lee’s data for some insight. I suddenly realized something interesting that we're going to have to deal with eventually....

The comparisons between the half-throttle settings across the board show something interesting. I'll probably have to plot the data and stare at it for a while, but here's a quick synopsis...

Half throttle settings for all six samples generates between 22 and 24 percent of the WOT Amps and between 67 and 69 percent of the WOT RPMs. Any thoughts?
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Old Nov 10, 2011, 09:10 PM
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Originally Posted by c/f View Post
Well on the EF page concerning the 4016 motor and 1400W into a 15X8 it is recommended 5s 3850-5000 MAH
Not sure what you mean.

But the Torque 4016 + 15x8 prop + 5s 3850 do not give 1400 W, not even 1100 W.

-Battery is NOT power, it's the energy source, and you need the motor to convert electrical power to mechanical power.

Mechanical power = force x velocity = work per unit time

Electrical power = voltage x current.

The battery weight alone cant do anything helpful.

Vien
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Old Nov 10, 2011, 09:39 PM
3D Hack
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United States, IL, Glen Carbon
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Originally Posted by Aeroplayin View Post
I was just doing some flight time calculations on the 48 inch Edge thread and trying to make a linear relationship, so I went back to Lee’s data for some insight. I suddenly realized something interesting that we're going to have to deal with eventually....

The comparisons between the half-throttle settings across the board show something interesting. I'll probably have to plot the data and stare at it for a while, but here's a quick synopsis...

Half throttle settings for all six samples generates between 22 and 24 percent of the WOT Amps and between 67 and 69 percent of the WOT RPMs. Any thoughts?
The power curve is exponential.

Similar to driving a car down the road. It takes much more power at higher speeds as you loose efficiency.

JC
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Old Nov 10, 2011, 10:16 PM
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Originally Posted by JC Spohr View Post
Similar to driving a car down the road. It takes much more power at higher speeds as you loose efficiency.
JC
Most power consumption at high speed is due to aerodynamics drag though, just a little fraction is due to the decrease in engine efficiency. The drag power of a car is proportional to (cruising velocity)^3

In electric motor, the story is different. The power loss increases because resistance of the whole power system increases at higher operating temperature. Friction also increase btw.

Vien
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Old Nov 11, 2011, 08:19 AM
3D Hack
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Originally Posted by vienquach View Post
Most power consumption at high speed is due to aerodynamics drag though, just a little fraction is due to the decrease in engine efficiency. The drag power of a car is proportional to (cruising velocity)^3

In electric motor, the story is different. The power loss increases because resistance of the whole power system increases at higher operating temperature. Friction also increase btw.

Vien
Different cause but same effect?
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Old Nov 11, 2011, 08:35 AM
Not as Good as The Kid
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Good stuff, and also falls in line with the fact that the higher magnetic field, reverse voltage, harder working FETs, etc, will contribute to resistance as well, making it an uphill climb between half and full. And since heat is a function of current^2 and resistance, all negative components increase at higher throttle ratings.

So while the Amps are increasing more than 300%, representing the increase in electrical Watts, the RPMs, which represent the mechanical Watts, are only increasing by 33%, but the output is the same using the two different formulas:

Electrical Watts at Half-Throttle = volts * Amps
23.62v * 14.3A = 337.8W

Mechanical Watts at Half = pK * p-pitch * p-lenth^4 * (RPM/1000)^3
1.08 * 0.67 * (1.25^4) * (5.76^3) = 337.6W
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Old Nov 11, 2011, 08:39 AM
They Call him Dead!
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Originally Posted by JC Spohr View Post
Different cause but same effect?
Well, there is also proportionally more drag on the prop just like the drag on a car increases...right Vien?
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Old Nov 11, 2011, 10:01 AM
Not as Good as The Kid
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Originally Posted by dead View Post
Well, there is also proportionally more drag on the prop just like the drag on a car increases...right Vien?
Also, the mechanical formula tells us the amount of power in Watts it takes to turn a 15x8 propeller with a 1.08 pK 5,760 RPMs, which was the APC's RPM reading at half-throttle from Lee's data. So it takes 338 Watts to do this, while it takes 1,100 more Watts to go from 5,760 RPMs to 8,560. That's 338W to produce the first 5,760 RPMs and 1,100W to generate the remaining 2800 RPMs.

So for the APC prop, that's 17 RPMs per Watt for the first half of the throttle curve and 2.5 RPMs per Watt for part-2. When I have some time, I'll run the numbers for the rest of the prop samples from Lee's data so we can compare to the reproduced data that should be coming soon.
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Old Nov 11, 2011, 10:06 AM
3D Hack
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Originally Posted by dead View Post
Well, there is also proportionally more drag on the prop just like the drag on a car increases...right Vien?
That is essentially why I was hypothesizing in my first post.

JC
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