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Electric Stunt => Gettin all AMP'ed up! => Topic started by: Duke.Johnson on July 25, 2013, 03:30:20 PM
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I'm planning to build my next Akromaster and convert it to electric. So here's my questions so far.
What set up should I use?
How did you decide on that set up? So I can start figuring this stuff out.
Should I build compartments in the wing to hide the electronics? Putting the battery in the outboard wing as tip weight?
Please figure that I know very little about this topic. Thanks for the help.
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Hi Duke, Go to the "List Your Setup" Sticky Thread at the very top of this forum and scan through it for models of a similar size to your Akromaster. Any of those setups should work for you - you just have to decide which one you want to use. ;D
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Hi Duke!
Here's some pix that might help you along. My brother converted an IC Akro to Electrons, here are some pix of the work in progress. Pieces used:
* Motor: Brodak #BH-1836: Arrowind 2210
* ESC: Brodak #BH-1861: Arrowind 18A
* Battery: Brodak #BH-1887: 3S x 1800
* Timer: Brodak #BH-1869: Hubin FM-0c
Use the above (or similar sized parts from others) to drive an APC 7x5 prop. It is a light power setup that goes like.... heck on 52' lines, easily dialed back for PA speeds.
The pictured one was a conversion, I started a purpose built electric Akro that will have the pack in a pocket on the inboard side of the fuse, with a cut-out into the LE.
Hope Andy B checks in here too, he designed the power system for the RSM electric P-51 (roughly same size as the Akro) that uses larger parts and is somewhat heavier, but is also capable of turning larger props. With the larger parts you would likely want to push the battery aft either inside or under the wing...
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This is starting to help. I'm interested in how to cut out the LE to make this pocket, do you have a picture of that? I would like it to run PA speeds.
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Hi Duke;
Dennis previously did a Brodak Shark 402 build where he installed the battery into a pocket in the wing. He posted a few photos here: http://stunthanger.com/smf/index.php?topic=28485.msg275550#msg275550
John Cralley also posted photos of his battery install in a Ringmaster at http://stunthanger.com/smf/index.php?topic=32155.msg318985#msg318985
all the best,
Dave
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Thanks
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What size Akromaster do you have?
MM
Specifications:
Wingspan 34 in 796 mm
Wing Area 250 in² 16.1 dm²
Engines .15 - .19 in³ (2.5 - 3.1 cm³) 2-stroke
I'm still hoping someone might be able to teach me how to size this stuff. Like the set-up on the Akromaster above, it looks great but I don't know how to decide on which motor.
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Start reading the List Your Setup section. Or better yet contact our own Denny Adamisin for details.
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You might want to consider this motor at the USA warehouse of HobbyKing. They have it in stock as of now for $23.60 plus shipping. you can use a 2 or 3S battery depending on the RPM and prop you want to use.
http://www.hobbyking.com/hobbyking/store/__23910__Turnigy_Aerodrive_SK3_3530_1340kv_Brushless_Outrunner_Motor_USA_Warehouse_.html?strSearch=Turnigy%203530
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Duke, you may find this thread useful: http://stunthanger.com/smf/index.php?topic=19275.0 (http://stunthanger.com/smf/index.php?topic=19275.0)
To distill this down a bit, and give some electrical rules that hopefully a plumber can understand :) :
Note that we'll do everything off of airplane weight. Unfortunately, we don't know the airplane weight!! So getting on the open forum and asking people how much their Akromasters weigh would be a good idea -- you can figure that the all-up weight for electric will be about the same as for slime power, possibly for slime power with a full tank.
I'm going to assume a 20 ounce airplane weight. I hope that's in the ballpark.
First figure out what motor to get:
- You'll need about 11 watts per ounce of airplane weight. So for a 20 ounce airplane, that's 220 watts. So look at motors that are rated for no less than 220 watts peak power
- For this sized airplane you want to spin the prop between 10000 and 12000 RPM (that drops to about 9000 for a "full sized" stunter)
- For this sized airplane you probably want a 3-cell pack (bigger airplanes = more cells). This isn't a general rule -- you could probably make this work with two cells, or with four, if you size your motor and ESC right
- A 3-cell pack is an 11.1 volt pack (there's 3.7 volts per cell, and it adds up -- 3.7V/cell * 3 cells = 11.1V)
- You want to spin 12000 RPM at the end of the flight and at full current -- this means you can only count on 8.3V or so (that's 75% of full voltage -- (11.1V)(0.75) = 8.3-ish volts)
- The motor speed is the motor Kv times voltage. So you want to find Kv from Kv = (target speed) / (voltage). In our case that's (12000 RPM) / (8.3V) = 1450 RPM/volt
- Now you know enough to go motor shopping -- woo hoo! Look for motors that can handle 220W peak, and have a Kv of 1450 or better
- If you just can't find a suitable motor, go back to the step where we decided on the number of cells, and try it again with four
Now figure out what ESC to get:
- power equals current times voltage. We can turn that around: current equals power divided by voltage. So when we're asking for 220 watts from the motor, we need a current of (220W) / (11.1V) = 19.8-ish amps -- call it 20A (more cells means higher voltage, which means less current for the same power -- this is why bigger planes have more cells)
- So you know that you need a 20A ESC that can handle three cells
- If you are using a KR timer, or if you don't care about a "perfect" run, then almost any ESC will work (there's some really cheapo ones that the KR timer doesn't like)
- If you are using a Hubin timer then you need to get an ESC that has a governor mode (the KR timer has a governor -- the Hubin timer depends on the ESC)
- Go select an ESC
Now you need a battery pack:
- You're going to use about 0.6 or 0.7 watt-hours per flight if you do a full six minutes.
- Watt-hours can be calculated like power: watt-hours = amp-hours * volts. You can turn that around: amp-hours = (watt-hours)/(volts)
- You have a 20 ounce plane, so you need about 14 watt-hours. You have an 11.1V battery, so that works out to (14W-h)/(11.1V) = 1.26 amp-hours, or 1260 mA-h (1 amp = 1000 mA, so 1 amp-hour = 1000 mA-hour)
- You only want to discharge the battery by 75%, so you need more than 1260 mA-h: (1260mA-h) / (0.75) = 1680 mA-h
- So you need a 3-cell pack with better than 1680 mA-h capacity. 1800 would be plenty. You could probably squeak by with 1600, because the Acromaster will probably fly the whole pattern in less than six minutes
- You want at least a 20C discharge rate, and mo is betta
- Go buy a 20C or better 3-cell pack with more than 1680mA-h of capacity
And, buy a timer.
There. Done. I'd say "isn't that easy?" except -- well, it's sure a lot of math! Maybe this is why people direct the math challenged to the "list your setup" page.
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Thanks Tim, why didn't you say this sooner ;D This made a lot of sense to me. Even a plumb dumber can do that figure-n. LL~ One thing I didn't get is where you came up with 10k-12k RPM's for the smaller Akromaster. Really though, thanks for the equations
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Thanks Tim, why didn't you say this sooner ;D This made a lot of sense to me. Even a plumb dumber can do that figure-n. LL~ One thing I didn't get is where you came up with 10k-12k RPM's for the smaller Akromaster. Really though, thanks for the equations
The 10-12k is mostly from seeing that number quoted for similar sized planes, not from any great insights on my part.
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FINALLY got a roundtuit.
Attached pix show installation of a Brodak EPS-15 motor & pack in a Sig Akromaster fuse. Basically the front end is built pretty much normal, then a big pass through slot is cut for the battery box. THe battaery box is 1/16" ply top and bottom with a 1/8" lite ply oter wall. It is the same width as the firewall which of course it supports. The battery is installed from the inboard side, and a velcro stap holds it in, but in flight the velcro pretty much does nothing. It is screaming out for some balsa fairings above & below the box - like a cheek cowl.
THe pack is on the thrust line so the weight is above the wing, but that is the same place it would be with the CG. THe payoff is that the wing can be built normal, without any cut-outs for the battery. THe battery box also supports the firewall, which would not be the cas if the battery box was lowered to the wing.
Based on prior builds I think this will be spot-on for the CG.
There is a small tray installed under the nose, this is where ht ESC will be mounted. It is somewhat less visible there and will get great cooling. But it will also be first on the scene of a crash...
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Dennis -- looks good. Should I copy this mounting scheme I will endeavor to always crash inverted.
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Hey Duke:
I'm getting enthusiastic enough about my own Akromaster that I just priced out all the 'lectric stuff from Hobby King. Have you done anything on your project?
Size sure makes a price difference with the electric stuff, unlike good engines.
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I've been thinking about it some. What did you come up with for a set up? It would be a quick little project. I really need to build a Gentle Lady glider next. I'm going to have the plans printed today. I'm going to make it a power glider, probily with an OS 480.