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Habu 2 6s power system calculation

#1
First I would like to say this is my first post. I have made the formatting as organized as I can, hope you enjoy.

I recently bought all these components with the intention of building and flying the Habu 2.


I will also be using some equipment I already have.
Any other equipment is miscellaneous things like glue, etc.

When I decided to invest money into this, I had the intention of trying to fly faster than any plane I have owned in the past. For this reason I bought the BL32 motor since it could handle 6s voltage over the BL15 motor that could only handle 4s voltage. Link to BL15 motor: http://bit.ly/1O2HJrC

Looking back at this decision, I see how naive I may have been. I didn't bother to crunch the numbers before I made the purchase and it may have costed me.


Calculations


RPM
KV of motor multiplied by the volage of a full charge 4s and 6s battery for each respective motor. 4.2v per full charge cell x4/x6.

BL15 Motor 3600KV x 16.8V = 60480 rpm

BL32 Motor 2150KV x 25.2 = 54180 rpm


Wattage
I don't have the BL15 motor, thankfully E-Flite has the current draw of the BL15 motor on the same fan that I am using on it's website, it's 46 amps.

BL15 Motor 46 amps x 16.8V = 772 Watts

This current draw for the BL32 is my own measurement I took
BL32 Motor 32 amps x 25.2V = 806 Watts

(Both these measurements are using the same fan unit)


Conclusion
So does this ultimately mean that I would have been faster with the 4s BL15 setup since I have lower rpm with the 6s setup?

Will I even be fast? According to this source http://bit.ly/1Xxl8hl, I need 180 Watts/Pound for the higher performance an EDF jet needs. It also says 200 Watts/Pound will give me unlimited ballistic performance.

The weight of my model is 1.6kg or 3.52 pounds. With this simply do 806 Watts / 3.52lb = 228 Watts/Pound

This seems to indicate my model will still have great performance. Either way, I will fly it and measure the speed. If it is not satisfactory I can still change the fan to something with more blades.

Right now my fan has 5 blades and I'm only drawing 32 amps. My ESC is rated for 60, so I can probably go for 10 blades and just push my ESC harder than 32 amps. That would be the cheapest way to increase my performance.

If you have made it this far, thanks for reading. If anyone has any other ideas to help me increase performance or if anyone can already look at my numbers and know I will be fast, please let me know. I'm looking for at least 120 mph.

Thank you for reading my first post. :)
 
Last edited:

Craftydan

Hostage Taker of Quads
Staff member
Moderator
Mentor
#2
Accelangel, Welcome to the forum!

I applaud your effort to grossly overpower your airframe to irresponsible levels ;)

At 32A expected WoT draw, you've also grossly underpropped that motor. Although the ESC limit is something to pay attention to, it's really the motor's current limits that's more telling -- that motor is rated to 62A continuous and 75A burst, so that expected draw is only using about half what that motor can do . . . waste! Your ESC is small for letting this beast run full open while properly loaded, so yeah, the 60A ESC will probably overheat first . . . best to target WoT current around 55A, if you can . . . or get a bigger ESC.

Yes, she will probably fly as is, and fly well, but you're right -- she'll probably be happier with a better fan. How many more? not as simple as x2, but I'm not that familiar with EDF trends since it's quite a different region than open air props. It probably should go without saying, bench test with the new fan first.

Positive side . . . if you can get your current up to 55A WoT, you're looking at 1400W, or 400W/#, all for a better sized piece of plastic. 75A burst => 1750W, or 500W/# if you upgrade the ESC too . . .

. . . At that point, you'll also want to look carefully at your batteries -- At 30C, they won't have much headroom as they discharge and your off-the-ramp punch will be dramatically different than it-flies-OK sag you get at landing . . . not to mention the puffy packs :eek:
 

quorneng

Well-known member
#4
I supposes the question also is do you have any experience with such a high powered EDF?
Not the flying side so much (although important) as handling a high power set up.
Get it wrong and its expensive in an instant!
 
#5
New habu from a hobby shop, it was used. Which means it came with parts that are NOT STOCK.
It’s amazing what you can discover if you got the right pick of the litter.
I popped her open, discovered an 80amp esc. A wemotec 70mm fan with an unknown inrunning core motor with cooling heat reducers rotating a 9 blade edf Fan. Everything runs great. Used a 3s to test everything. But I need a recommendation for a dual connection of 3s LiPo batteries
In series to add up to 6s connection
I wonder if it’s possible to get longer flight times much like the E-flite havoc. Kinda what I’m trying to as close as possible

New paint job with color shifting paint!!!
 

Attachments

DamoRC

Well-known member
Mentor
#6
If you want to use 2 x 3S in series you will need to make or buy a connector like this one.

When you connect two batteries in series (and they should be identical batteries) you will double the voltage but the effective C-rating and capacity (mAh) will be the same as only one of the batteries. I would think that for this model you will need 2 x 3000mAh, 45 C minimum, for good performance.