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Old 02-01-2009, 09:27 PM
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electric formulas, how to know what i need?

Hi, i have have had plenty of glow and gass powered planes. you've probably been asked 100times... i have looked, but cant find what i need.....

Q. how do you work out what equipment you need for any given airframe? (motor and esc)

Q. how do you work out what size prop would be good for a given combination?

Q. how do you get the same size motors from brand to brand? E.G. some seem to use size, other nitro equivalent eg. .10.

Q.how do you work out what kind of battery power you need?

Q. how do you over power an airplane? e.g. i use a .75 in a plane designed for .50 engines

Q. why do engine manufactuers use different terms for the same thing????

Sorry about all these dumb questions, but i have no knowledge at all on electric plane other than the foamie i have.

Anyone got the patience to tell me how to begin?

Thanks
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Old 02-01-2009, 10:25 PM
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Re: electric formulas, how to know what i need?

does anyone know the answers to the above?
many seem to feel that electrics is a bit of a black art! i hope not!
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Old 02-01-2009, 10:53 PM
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Re: electric formulas, how to know what i need?

Quote:
Originally Posted by Bam Bam View Post
Hi, i have have had plenty of glow and gass powered planes. you've probably been asked 100times... i have looked, but cant find what i need.....

Q. how do you work out what equipment you need for any given airframe? (motor and esc)

Q. how do you work out what size prop would be good for a given combination?

Q. how do you get the same size motors from brand to brand? E.G. some seem to use size, other nitro equivalent eg. .10.

Q.how do you work out what kind of battery power you need?

Q. how do you over power an airplane? e.g. i use a .75 in a plane designed for .50 engines

Q. why do engine manufactuers use different terms for the same thing????

Sorry about all these dumb questions, but i have no knowledge at all on electric plane other than the foamie i have.

Anyone got the patience to tell me how to begin?

Thanks
Hey Bam Bam

A good place to start is Motocalc or drive calculator, if the motor and ESC you plan to use is listed, the calculator will tell you how heavy the plane can be and what type of performance you can expect. Also what kind of flight times based on the type of battery and Mah.

Another good resource is one of the major brands like 3D Hobby Shop or Extreme Flight. I'm sure they have packages that will fit almost any plane you want to put up there.

I sure hope that helps.

Ernie
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Old 02-01-2009, 11:33 PM
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Re: electric formulas, how to know what i need?

Quote:
Originally Posted by Bam Bam View Post
Hi, i have have had plenty of glow and gass powered planes. you've probably been asked 100times... i have looked, but cant find what i need.....

Q. how do you work out what equipment you need for any given airframe? (motor and esc)

Q. how do you work out what size prop would be good for a given combination?

Q.how do you work out what kind of battery power you need?

Having these 3 answered will answer the other ones. The best place to start is determining how heavy your airframe is going to be. Using the manufacturers advertised flying weight is the best place to start. Then use this rule of thumb to determine how many watts your system needs to make, depending on how you want it to fly:

"Watts per pound" range from 50W/lb for floaters and trainers, through 75-100W/lb for sport, to 150W/lb for good aerobatics and 3D, to 200W/lb for ridiculous power!

If you have an airplane that is advertised to weigh 6 pounds RTF and you want good 3D power, I like to go with 175 watts per pound or 175 watts/lb, which will be 1,050 watts.

The prop you use is usually determined by the motor itself. The bigger the prop the higher the power, but their is a limit to what a given motor will produce and a relative point at which the motor will fail. The higher the load (bigger the prop) you put on the motor, the more current it will draw (i.e. amps). There is a basic calculator you can use online for determining what motor/battery/prop/esc combo to use, but I recommend buying something like electri-calc or moto-calc.

Try these:

http://adamone.rchomepage.com/calc_motor.htm

&

http://brantuas.com/ezcalc/dma1.asp

Motor manufacturers will give you some general idea as to what size esc and prop to use, and usually info on power output, whether they list it in watts or a glow comparison. I prefer knowing what the motors ratings are up front, so I don't have to dig. As long as you know 5 things about a motor, you can accurately calculate the power of a combo.

1. Kv or RPM's Per Volt
2. Sustainable Amp Draw
3. Internal Resistance or Ohm rating
4. No load current
5. Max voltage

You have to know how many RPM's the motor will turn, to calculate the thrust of a given prop.
You have to know the amp draw it can handle, so you don't over do the prop size.
You need to know the internal resistance to calculate efficiency (i.e. a close guesstimate on amp draw/flight time with a given prop/battery).
You need to know the "No Load Current" to calculate what the motor will pull in amps with a prop on it and the calculators need it for accuracy.

And amp or watt meter is a good tool to have as well. It will take all of guess work out of the equation when trying different props/batteries. They will normally show you watts, volts and amps, which is critical to be mindful of to avoid cooking things that shouldn't be cooked.

Back to our example model...needing 1050 watts, I would start with an AXi 4130/16. They recommend a 70-77 amp esc, so we will go with a 70. If you ran a 16x10 APCe prop with a 6s LiPo you would be pulling around 45 amps at full throttle, so that gives you plenty of leg room with the esc.

Now, to figuring out the proper battery, you have to look at the battery itself. For example, if you are looking at a pack that is rated at 20c, this means it can handle current 20 times it rated capacity. If the battery is a 3300mah pack it can handle 3.3 amps times 20 or 66 amps continuous.

This would be plenty safe for our setup. Now to figure out how long it will fly.

3300mah = 3.3amps
3.3amps divided by 45amps (what our setup pulls) = .073 hours
.073 hours times 60 minutes = 4.4 minutes

This at full throttle, so you would likely get around 6 minutes safely with throttle management.

If you wanted more time, you would need to bump up your batteries capacity from 3300mah to say, 4000mah, etc..

Hope this helps.

Chris
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Old 02-01-2009, 11:53 PM
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Re: electric formulas, how to know what i need?

hey thanks to both of you as you both explained very clearly how they work!
what i need to do now is start looking around at what kind of plane i wanna get then see if i can figure it out.

I guess that after a few goes i will start to make better sense of it all, like anything there is no subtitute for experience!

also i imagine putting more power isn't going to fly better, as weight is still our enemy! yes u can chuck a 1.60fx on a .46 profile, but it wont fly like a fly!

Thanks.

I want as much info as i can get, so if others have more to share.....
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