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Old May 02, 2005, 11:39 PM
Old Tyme Electric
steve crewdog's Avatar
Joined Mar 2005
666 Posts
HOW and WHY do you choose a power system for an aircraft? (Antic e-conversion)

Hi all,

It would be easy to say ďHereís what I want to do and how do I do it?Ē and someone will provide the answer. But Iíd like to learn HOW and WHY you choose a power system for an aircraft. It used to be so simple, .40 2 cycle = .60 4 cycle and the bigger the fuel tank the longer the flight. But electricís obviously have a different system.

I looked for a FAQ or a Sticky about how to convert but I could not find one. If I overlooked it please excuse me and point me to it.

Iím not made of $$ so Iíd like to find a system I could use in my current project (Proctor Antic) and if/when it rekits itself I could then throw it on another similar size aircraft. I am leaning to the axi outrigger motors and lipoly batteries but am open to ideas. I'd like to stick with good solid tech so this aircraft will last.


Current project I want to put this electric in: Proctor Antic. 6 sq. feet of wing, recommended flying weight: 6.5# =17oz/sq ft wing loading. I realize itís a REAL draggy airframe so would that make a difference than if I was going to put the same engine in, say, a gunk powered 4*60.


If you do have a suggestion for a motor/battery/esc combo, would you mind taking the time to explain how you came to the decision so I could learn too?



Many thanks,


Steve
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Last edited by steve crewdog; May 02, 2005 at 11:47 PM.
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Old May 03, 2005, 12:38 AM
Use the 4S Luke
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USA, TX, Euless
Joined Aug 2003
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Looks like it is spposed to fly slow right?
There is a lot to motor sizing and some of us as use gear boxes which adds to the options. For that plane an AXI 4120 would work fine. Look around at Espritmodel.com
There is a table of planes with glow engine sizes as well as several electric setups. Also good data on the AXI motors.
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Old May 03, 2005, 02:27 AM
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Power system selection can be complex - and expensive if you get it wrong.

You can start from many different points depending on the criteria - budget, flight characteristics desired, parts already on hand, fuse size/width, duration, speed, max weight, hover/3D ability etc etc.

One of the oldest rules of thumb is still a good general starting place - watts per pound. There are several different versions with slightly differing break points but it goes aproximately like this:

ROG and scale flight = 50 watts/pound
mild aerobatics = 75 watts per pound
good aerobatics = 100 watts/pound
3D = 125 plus watts/pound.

I prefer to err on the hi side. The planes I have that are the most fun are all over 100/pound. But thats just me. I think 50 watts per pound is toooooo looooow tho. Too close to the edge with no excess power for recovering, go arounds etc.

I usually start with the batteries because that is the basic limiting factor in how much power you have to work with. They are also often the heaviest and largest and most expensive (for lipos) parts of the power system. All those factors come into the design as well - size, weight, budget.

For example - if you are useing lipos - say 2000 mahr 10C cells - that means you can on average get a max of 20 amps x 3.5 volts from each cell = 70 watts per cell. If your plane weighs 7 pounds and you want to do aerobatics 70 X 100 = 700 watts. At 70 watts per cell that means you need a pack with a total of 10 cells. You now have choices of 10S1P or 5S2P if you want to stay exactly at you target number. Or you could go 5S3P for more duration OR more power. I prefer a little head room power/capacity wise unless weight is critical - which it often is.

Then you need to look at motors that can handle 700 watts comfortably. Again I prefer a little head room. While you are looking at motors you also need to think about prop sizes and rpms - and here it gets even more complicated.

Say you picked a 5S2P pack above - that means a voltage of 17.5 volts average under load. You need to pick a motor that will stay within max rpm specs and not over rev on 17.5 volts. A motor with a Kv of 4000 for example at 17.5 volts would be spinning at 70000 rpm. Probably toooooo much for most large motors .

Say you find one with a Kv of 1500. That still equals an rpm of 26250. Too hi to spin a decent sized prop direct drive - now you need to look at gear boxes to get the rpm down. But how far down? Do you want fast speed for pylon or lots of thrust for 3D? For pylon you want a small hi rpm prop. For 3D you want the biggest diameter you can get.

Mabey the motor you want is an outrunner and wont work well on a gear drive or you just dont want a geared setup. Now you go back to the batteries and try a different pack configuration to get the voltage down - mabey 3S3P (9 cells = 630 watts) or 3S4P (12 cells = 840 watts). One is a little low one is a little over kill.

Then you are back to checking Kv and rpm and prop sizes and then back again. Somewhere in there I do some rough calcs for thrust and prop speed, compare it to my other setups.

One of the toughest things is trying to determine how many amps a particular prop on a particular gear box/motor will draw with a particular battery.

I run the whole mess thru MotoCalc. Then do a search on here for similar systems and ask questions if in doubt. Lots of searching. Some motor mfg's publich specs for different props at different voltages, some dont - some are even vaguely accurate. There are several websites with published specs/test results also.

A good one I cant find at the moment is The Great Electric Motor Test. Anyone have that lynk?

Eventually you come up with something you think will work and you spend the bucks and keep your fingers crossed.

Let us know how it turns out

Good luck!

Larry
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Old May 03, 2005, 02:31 AM
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Ofcourse you could also start at the other end - find a motor and come up with a battery pack that give enough power/volts to spin the prop of choice.

Or mabey you HAVE a motor already or mabey you have packs already or mabey some other criteria is more important.

Lots of ways to solve the problem.

larry
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Old May 03, 2005, 08:52 AM
Old Tyme Electric
steve crewdog's Avatar
Joined Mar 2005
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What would you think about the axi 4120? I've heard nothing but good about them. This combo comes wih a jeti 70 amp speed control and the price looks ok. http://www.hobby-lobby.com/brushless-axi4120.htm

Now, what would REALY be cool would be a mod that would let you attach dummy cylinder heads to the outrigger brush to get that complete rotary effect as the motor spins.....
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Old May 03, 2005, 11:27 AM
Use the 4S Luke
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Great idea! I've got a DR1. I'll give it a try.
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Old May 03, 2005, 12:37 PM
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Great Electric Motor Test link

Larry,
The link to the Great Electric Motor Test data is http://www.flyingmodels.org/motortest/index_right_e.htm motor
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Old May 03, 2005, 01:37 PM
BEC
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Randy Smithhisler in Puyallup has had one of those flying for many years on a geared Astro Flight brushed 25 and probably 16 cells (NiCd). More than enough for Antic-like flight. So the AXi 4120 idea is a good one. Lets you avoid all the added variables of a gearbox and with a rear mount kit it could just bolt onto the firewall you already have.

Some idea of budget and whether you want to go lithium or nickel would be helpful, too.
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Old May 03, 2005, 04:11 PM
Old Tyme Electric
steve crewdog's Avatar
Joined Mar 2005
666 Posts
Quote:
Originally Posted by feathermerchant
Great idea! I've got a DR1. I'll give it a try.
Then we also need to figure out a way to get the sound effect of a coup switch....
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Old May 03, 2005, 04:18 PM
Old Tyme Electric
steve crewdog's Avatar
Joined Mar 2005
666 Posts
Quote:
Originally Posted by BEC
Randy Smithhisler in Puyallup has had one of those flying for many years on a geared Astro Flight brushed 25 and probably 16 cells (NiCd). More than enough for Antic-like flight. So the AXi 4120 idea is a good one. Lets you avoid all the added variables of a gearbox and with a rear mount kit it could just bolt onto the firewall you already have.

Some idea of budget and whether you want to go lithium or nickel would be helpful, too.
That's what I'm also thinking. I thought about adding the extension and cowling the engine for streamlining but that would take away the spirit of the aircraft and with all that other drag out there.....


Budget... Hmmm.... esc/motor are already at 240, would trying to hold the batteries to under 100 but still going for decent flight times be stoopid? Also with the short nose moment I think I'm going to need something heavy up front anyway so that make Lipos redundant. Or does it? This learning curve with electrics is almost straight up for us converting pilots.


One mod I did figure out this morning: I am making a swapable tail wheel/skid so I can fly from both kinds of runways if I need to. Or so I can just taxi around in front of my house in the street for fun when it's unflyable.



TAIA,

Steve
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Old May 03, 2005, 04:25 PM
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Quote:
Originally Posted by Larry3215

ROG and scale flight = 50 watts/pound
mild aerobatics = 75 watts per pound
good aerobatics = 100 watts/pound
3D = 125 plus watts/pound.

Larry

EDF jet????

200-250 watts/lb for decent performance
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Old May 03, 2005, 05:39 PM
BEC
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Batteries under $100 means no lithiums, maybe not even nickel - say GP3700s or such - and no way to charge the battery.......but remember that a well cared for battery is good for several hundred flights.

Yes, the learning curve is nearly straight up and it's worse now than it was, say, 5 years ago when all we had were nickel batteries and the basic motor choices were Astro cobalts (brushed) and Aveox or MaxCim brushless - and there are viable motor/gearing choices from each of these folks for the airplane today. It's just that there are many many more choices besides those now.

As for cowling, doesn't the Antic have some sort of Eindecker-like cowling - or is it just that relatively ornate engine beam mounting setup?

I've two places I'd suggest you call: One is Pete Peterson at Model Electronics in Renton, and the other is Kirk Massey at New Creations RC in Willis, Tx. Between the two of them, especially Kirk, they should be able to get you set up with several viable alternatives.
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Old May 03, 2005, 06:08 PM
Old Tyme Electric
steve crewdog's Avatar
Joined Mar 2005
666 Posts
Quote:
Originally Posted by BEC
Batteries under $100 means no lithiums, maybe not even nickel - say GP3700s or such - and no way to charge the battery.......but remember that a well cared for battery is good for several hundred flights.
Ok what price range would be realistic for a decent battery setup for a Sunday flyer? Need to revise the budget obviously....

Quote:
Yes, the learning curve is nearly straight up and it's worse now than it was, say, 5 years ago when all we had were nickel batteries and the basic motor choices were Astro cobalts (brushed) and Aveox or MaxCim brushless - and there are viable motor/gearing choices from each of these folks for the airplane today. It's just that there are many many more choices besides those now.

As for cowling, doesn't the Antic have some sort of Eindecker-like cowling - or is it just that relatively ornate engine beam mounting setup?
Ornate beam, but I'll probably just mount it directly to the firewall and eliminate the ornate beam.

Quote:

I've two places I'd suggest you call: One is Pete Peterson at Model Electronics in Renton, and the other is Kirk Massey at New Creations RC in Willis, Tx. Between the two of them, especially Kirk, they should be able to get you set up with several viable alternatives.
Wilco. I've got a great LHS-Performance Hobbies in Burlington WA-that always has time for my dumbarsed questions when I come in. They know their stuff and are patient about it too.


4120/18 is on the way....
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Old Jun 19, 2005, 10:04 PM
New Jersey USA
Joined Jul 2004
76 Posts
I'm wondering if you are going with a 700 watt set up and the need for a ESC that will handle those high amps, if a 3 speed mechanical speed control would make more sense? would have to be a whole lot cheaper.
D
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Old Jun 20, 2005, 08:09 AM
Don't cut the Yellow Wire
dr.E's Avatar
Ridgewood ,NJ
Joined Jan 2000
3,532 Posts
I have both the Antic Bipe and Mono. The mono flies with a Maxcim geared set up and 24 cells (2400 cp's). The Bipe has a Hacker B50 10l 6.7 gear on 18 cells.

As a refference. Bob Benjamin featured an Antic mono on floats with an Astro 40 geared on 22-24 cells.

These models are notoriously tailheavy and need plenty of nose weight when flown glow. It is a perfect candidate for Nimh's rather than Lipos because you need the weight.

It has plenty of wing area to carry the extra pound or so.

Simple conversion:

Astro 40 marine on a superbox (little higher Kv than the sport wind but lower than the FAI)

24 cp 2400 cells

Astro ESC


Motor and batteries will run you 250.00 (street price)
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