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sizing electrics?

Old 06-05-2003, 04:56 PM
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turbo-RCU
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Default sizing electrics?

Hi,

I am starting to get the electric 'bug'. I find myself needing a guideline to match airframe size/ weight to motor size/ speed controller.. Kind of like the prop for engine( glo counterpart) size charts and the wing area/ plane weight for the more common sizes. I'm just would like some starting points.

Is there a website with this info? I would appreciate any information on this issue.

Thanks,
turbo
Old 06-05-2003, 06:42 PM
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Default sizing electrics?

Try this site. It is free for 30 days. lets you determine what size motor/ prop is best for your plane.

Motor Calc
Old 06-06-2003, 03:50 PM
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Matt Kirsch
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Default sizing electrics?

turbo,

Motocalc's a useful tool, but you need to understand a fundamental concept about electric planes: The size of the motor has nothing to do with it.

See, in a glow plane, the chemical reaction that releases the energy happens inside the engine. The amount of power released is directly proportional to the size of the combustion chamber. The size of the combustion chamber is what dictates the size of the engine, so we subconsciously relate physical size with power output.

With electric motors, all that goes right out the window. The chemical reaction that releases the energy happens OUTSIDE the motor, in the battery. It's the battery that dictates how powerful an electric power system is, not the motor.

That said, it's impossible to come up with a simple chart that says a plane with X wing area and Y weight needs Z motor, because by itself, a motor has ZERO power. One needs to look at the electric power system as a whole, starting at the battery, when powering an airplane.

Even then, it's impossible to make any sort of chart without getting horrendously complicated, because even though a .40-size trainer and a .40-size Extra 300 both recommend a .40-size engine, they have very different power requirements. By using gearboxes and larger propellers, you can make more efficient use of the energy from the battery, and fly planes using signifigantly less Horsepower, without sacrificing performance.

If you were to simply build a power system that spun the same prop at the same RPM as a glow engine, you'd end up with a very heavy, very inefficient power system, and a plane that flies poorly.
Old 06-15-2003, 10:29 PM
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smallglow
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Default sizing electrics?

I have recently tried to fly a few electrics, and the experience is that every one of them is seriously underpowered. They just don't fly! Tried a Clancy Stagger Bee weighing 640g with a geared Graupner 280 and a 8x6 fixed prop and it just will not fly. Tried a friends sailplane with a folding prop the other day and that did not fly either (it did fly OK a few weeks ago with a fixed prop) Another annoyance factor is that you have to remove the wing and connect the battery pack. Has anybody tried a hatch for easy replacement of battery and a main switch so one does not have to pull at these damn battery connections before and after every flight. Is there no easy to connect connectors or is the Tamaya car connector or the still little gold plated connectors the only choice? Maybe Cobalt or brush-less motors is the answer to get enough power. Any opinions out there?
Old 06-16-2003, 10:33 AM
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Mike Wiz
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Default sizing electrics?

If your experience with electric powered planes is that they don't have any power then it's most likely that you've been playing with a setup that hasn't been optimized. You're either using the wrong number of cells, wrong type of cells, wrong prop, wrong gear ratio, wrong motor or a combination of these.

I'm sure someone recommended something to you, but I personally wouldn't think you should be having trouble with that Stager Bee. Andy Clancy designs and flies a lot of electric planes. He wouldn't have become such a popular airplane designer if his products didn't perform well.

Usually what I do before putting together any electric airplane is key into Motocalc or ElectriCalc my airframe specs and performance requirements. Then I can pick and choose cell counts, motors, gear ratios and props until I get an answer I like.

For a beginner I recommend ElectriCalc because it has a simpler user interface, in my opinion and the cause and effect of each change is reflected immediately on the screen. ElectriCalc helped me when I was first trying to understand the interdependency of these different variables in an electric power system. It is different than glow and a little more thinking is required, but it's really not very hard to get a handle on.

You can find ElectriCalc here. http://www.slkelectronics.com/ecalc/
You can find Motocalc here. http://www.motocalc.com/

Wiz
Old 06-16-2003, 03:55 PM
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Matt Kirsch
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Default sizing electrics?

smallglow,

The right power system means everything, and unless you're just copying someone who's already been successful with a particular plane and power system, you really need some knowledge of how to put together a power system for a particular plane. You might say, "I don't need to know this watts volts amps turns gearing shtuff for glow. All I gotta do is slap an engine on the plane and go." Well, that's because when you put a glow engine on a plane, you're just copying what someone else (the kit's designer) has already done. Sound familiar?

Unfortunately, the cost of higher-end power systems often makes people balk at the prospect of buying electric planes, so many larger manufacturers recommend marginal low-cost power systems for electric planes, and don't recommend any electric power system at all for glow planes. This means that unless you have substantial anecdotal proof from an impartial third party (end user) that a given power combination works on a given plane, you're on your own.

How was the Stagger Bee's power system designed? You can't just slap parts on the plane willy-nilly and expect it to work. To properly power an airplane with electricity, you need to ensure enough horsepower (watts, which come from volts and amps generated by the battery) gets to the propeller. With its maximum number of cells at the maximum current, a Speed 280 simply cannot handle enough horsepower to fly a 22oz (640g) airplane. The Hobby People listing for the Stagger Bee recommends a Speed 400 system as the minumum, and judging by the size and weight of the plane, it will be barely adequate.

I suspect your saiplane problems are another case of randomly changing parts with no thought as to how the system will be affected as a whole. If you switched from a fixed prop to a larger folding prop in the hopes of getting more power, you overloaded the motor so it was generating more heat than thrust. Motors have limitations.

I think you'd be much happier with your electric experiences with proper power systems.

Try this on the Stagger Bee: 6V Speed 400, 3:1 gearbox, 9x6 APC Electric propeller, and an 8-cell battery pack.
Old 06-19-2003, 07:48 PM
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Default sizing electrics?

Is there no easy to connect connectors or is the Tamaya car connector or the still little gold plated connectors the only choice?
Try Astro Zero Loss (50A continuous, 75A short term) or Sermos connectors. Both are easy to connect / disconnect - but because of that, would not use them in a high-vibration environment without a locking device.
Old 06-19-2003, 09:03 PM
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Default sizing electrics?

Thank you all for the advise, I guess it is with this as with averything else, one has to read up on the topic and experiment! I will try a Hacker B20-15 with 1:4 planetary gear. Best regards, Lars
Old 06-20-2003, 12:45 AM
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Default sizing electrics?

I'm having the same trouble finding info as turbo. I will try ElectriCalc, but in the mean time, has anyone had experience powering Old Timers around the 64" mark? I plan to build the TD Coup (see my Avatar) and I am considering making it electric powered. I have a good quality 540 - is this suitable and any ideas on gearing/battery combo?
Old 06-20-2003, 11:42 AM
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Mike Wiz
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Default sizing electrics?

I will try ElectriCalc, but in the mean time, has anyone had experience powering Old Timers around the 64" mark? I plan to build the TD Coup (see my Avatar) and I am considering making it electric powered. I have a good quality 540 - is this suitable and any ideas on gearing/battery combo?
First off, think of your project as needing a particular number of cells to fly properly and then think of the motor second. A 64" wingspan old-timer isn't enough information for me to evaluate it. If you could tell me the wing area and what it might weigh if you were to build it as a glow plane then I can make some assumptions that will get us in the ballpark.

Say, if you were going to fly this plane with glow power what motor and prop would you use?

Also, simply stating that you have a 540 size motor isn't enough information about the motor. You need to know a few other variables......most importantly, how many RPM/Volt it will spin, also what its internal resistance is and it's no-load current. If you don't know these they can be determined.

I recommend a book called "The Electric Motor Handbook" by Bob Boucher. The book is a little dated, but it's still a good source of basic information about electric motors.

Incidentally, Bob owns AstroFlight. He and his brother Roland invented electric powered flight.

Anyway, with these motor variables plugged in one can begin to assess weather or not the motor will be suitable to convert the batteries stored energy into enough thrust to fly the plane.

I know this all sounds complicated, but believe me once you get a feel for it you'll be able to just look at a plane and tell about how many cells you'll need to fly it......just like you do now when you decide to use a .40 or .60 in a plane just by looking at it. After awhile, you'll also get a feel for which motor/gear box combos you like best for a particular cell count.

It gets a little more complicated than this because the prop size comes into play too. Electric motors are very versatile in that they can swing a very wide range of prop sizes and still be within there normal range of operation....much more so than a glow engine.

Really.....playing with some numbers in Electricalc will put you miles ahead of the game, but you'll need to find out those motor specs, or play with a motor in E-calc that these variables are known. Ecalc and Motocalc come with hundreds of electric motors in their database. Maybe your 540 is already one of those a known motors. Is it one of the popular car motors out there?

Lastly, Ill plug numbers into Ecalc or Motocalc for you if you want and I can give you some recommendations.

Wiz
Old 06-21-2003, 10:57 AM
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Brinesharks
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Default sizing electrics?

Wiz,

Thanks for your reply - I realise my question is pretty open ended! In answer to some of the questions:

The TD Coupe has a wing area of apprx 704sq inches. I can't work out a weight - I have the original 1936 article scanned by Garry Hunter into Word/PDF format but strangely it doesn't seem to give the prototypes weight (although it does say it cost US$7.00 to build with commercial wheels and silk!). It's of typical stick balsa old timer construction so should not weigh much more than a glider of the same wing area.

I would normally power it with a .36 to .40 four stroke IC (at the most!) and use maybe an 11/8 prop. An engine run of about 10 min a flight is all I'd size the tank for.

The elec motor is a Mabuchi RX-540VS buggy motor I was given a few years back (never ran it - but used one in an elec helo). Max input 8.4V DC and max output 280W. I haven't bought a battery pack yet as I thought, as you say, this would have a large bearing on performance.

Thanks for the offer of using the software. I'll give it a go and see what I come up with. Feel free to make any recommendations though! :-)

Thanks again for your help.

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