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Old Feb 15, 2010, 11:37 PM
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J-3 Piper Cub

So I bought this J-3 Piper CUb off of ebay for a good price. Here's a link to the same one from the same seller, (mine was cheaper).

http://cgi.ebay.com/37-4-ARF-RC-EP-J...item19b4bf2e96

http://myworld.ebay.com/gemstraveller/


It's an empty plane and I want to make sure that what I plan to put in it is suitable and will work. This is new for me so I would appreciate the advice.

It does not have ailerons but I plan on adding them.

Here's what I plan to drop into it:

RX - spektrum
TX - My waiting to arrive DX6i
Motor - A Skyartec BL 250 - 4500KV Outrunner motor
ESC - I have a 12A that came with the motor but I also have an 18A and a
20A. Which should I use?
Lipo - with the 12A esc I'd use a 7.4V 15C 900mah
- with the 18A esc I'd use the 11.1V 800mah
- with the 20A esc I'd use a 11.1V 800mah (I'd like to use the 11.1V)

Prop - stock is a 7 x 4" but I have five 6 x 4" APC props from a goofed order from Grayson hobbies that I'd like to find a use for.
servos: - rc-timer micro servos from ebay

Here are the specs for the plane listed on the ebay listing:

Length : 720mm
Wing span : 950mm
Flying Weight : 500g
Motor : 370A with gearbox
Propeller : 7 X 4"
Battery : 9.6V AAA 600mAh (not included)
Radio : FM4-Channel proportional radio (not included)
Package Weight : ~1285g


SO my questions are:

Which prop to use? Can I use my 6 x 4s ?

Which esc should I use and is my 11.1V suitable for it?

Can I put ailerons on this and use one servo or does each aileron need its own servo?

Thanks for the help, this will be fun!

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Old Feb 16, 2010, 12:43 PM
Always Electric!
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You asked a lot of questions. To start, you can set up one servo to run both ailerons, and you can set it up to eliminate some of the adverse yaw too.

Do you have a wattmeter or some device to measure currant? 4500kv seems awfully high, and the only way to find out the current draw is to measure (or you could ask here somewhere and someone might know. Which esc will depend on the current draw, but I'll bet the battery won't like putting out 20A. Otherswise I think the pack is big enough if it can handle the amps.

Good luck!
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Old Feb 16, 2010, 07:49 PM
Vanadu
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Thanks nikolaus, I'll find out the current draw from the guys on the thread that deals with the heli that the motor comes from.

More in a bit.

Quote:
Originally Posted by nikolaus_c View Post
You asked a lot of questions. To start, you can set up one servo to run both ailerons, and you can set it up to eliminate some of the adverse yaw too.

Do you have a wattmeter or some device to measure currant? 4500kv seems awfully high, and the only way to find out the current draw is to measure (or you could ask here somewhere and someone might know. Which esc will depend on the current draw, but I'll bet the battery won't like putting out 20A. Otherswise I think the pack is big enough if it can handle the amps.

Good luck!
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Old Feb 16, 2010, 08:32 PM
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Ok, Cut and pasted from a forum here, this is what I found in regards to this motor. I believe they were using an 11.1V lipo. Also, they had it set up as a heli:

" That Skyartec motor has got some real power in it but it's a bit of a hungry beast. The average current draw was 8.32A and the max was 13.43A. I'm using a CC25 but according to an email from CC they say the CC10 could handle short bursts up to 14A as long as the average is below 10A. It would be intyeresting to see if the CX200 pulls more or less Amps with this motor. Being able to use a CC10 would be a nice weight saving bonus. "

Does this make it easier to figure out which esc to use? Hope so

THanks for the help. When it's all up and running, there will be flight videos, promise!!
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Old Feb 17, 2010, 09:06 AM
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What prop size were those numbers from, or was it turning a rotor in a helicopter?
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Old Feb 17, 2010, 08:12 PM
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Quote:
Originally Posted by nikolaus_c View Post
What prop size were those numbers from, or was it turning a rotor in a helicopter?
I can't confirm that.

I posted the question of what the motor draws on the wasp v3 helicopter thread and the response was this:

'Hovering about 7-9A, Full pitch climbs about 13-15A'

That probably is a bit more accurate than what I had before.
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Old Feb 18, 2010, 08:35 AM
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Ok, that is for a helicopter. Electric motors are funny things in some ways. For airplanes (I looked at you blog and you look like you have some experience with helis). For any given motor with any given voltage applied the amps are decided with the size of prop. You can think of it as a motor tries to deliver a certain RPM for a certain voltage, higher voltages yield higher RPMs, but bigger props require more energy than smaller ones for the same RPM (more watts) so the amps go up.

What is the reduction of your gearbox?
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Old Feb 18, 2010, 07:51 PM
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Quote:
Originally Posted by nikolaus_c View Post
Ok, that is for a helicopter. Electric motors are funny things in some ways. For airplanes (I looked at you blog and you look like you have some experience with helis). For any given motor with any given voltage applied the amps are decided with the size of prop. You can think of it as a motor tries to deliver a certain RPM for a certain voltage, higher voltages yield higher RPMs, but bigger props require more energy than smaller ones for the same RPM (more watts) so the amps go up.

What is the reduction of your gearbox?
Yipes! I have no idea what that even means.........lol!!




But I'm willing to learn, that's why I bought an empty plane.

So what does that mean?
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Old Feb 19, 2010, 09:14 AM
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Which part?

Before I start, I wont assume that you know that power is measured in watts, with is calculated with amps X volts. 10 volts at 10 amps is 100 watts. If your battery has 10v and there is no load (i.e. it isn't connected to anything) than you have 10volts X 0 amps, which is 0 watts being delivered.

You said your motor has a KV rating of 4500. That means, that with no load (prop, rotor, or any other resistance, just the shaft spinning) it will turn at 4500 X the volts applied by the power source (or battery). For the sake of theoretical example, lets say with 10 volts you get 45000 rpm, and your motor is drawing 0 amps once it has sped up to speed.

I've heard it explained that motors don't produce power, but rather torque and RPM. If you have no prop, you are drawing 0 watts (still theoretical) and it is useless for your application. You overcome this by putting on a prop, say a 7X4 (I'm just making up numbers for the sake of example). Your motor will try to turn it at the 45000 RPMs but there will be considerable resistance, so the motor will start sucking in amps to keep turning at the original speed. Now you have 10 volts but 50 amps (still making up numbers for the sake of example) which you multiply to get 500 watts (finally a measurement of power!). If you try a bigger prop, like an 8x6, your motor will still try for the 45000rpms, but now it takes 60 amps (bigger prop trying to spin at the same speed), and it your system will put out 600 watts. Make sense?

However, in real life because of resistance, motors with no load really draw around 1-2amps, your controller steals some currant, battery voltage drops under load (more amps make you pack voltage sag) and as you put on bigger and bigger props the RPMs will come down a little but you will see the amps climb.

If you have a gearbox, say 4.5:1, I think it then makes it like your Kv rating is 1000, but I'm not sure, I'll have to check. Your gearbox may say, or (unfortunately) you can count the teeth on the gear on the motor, and the one that runs the prop adapter, and they'll form a ratio (30 teeth on the prop line and 10 on the motor gear gives you 3:1)

Let me know if this helps.
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Old Feb 19, 2010, 08:52 PM
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^^ Yes, makes sense.

By the looks of some more pics of the same plane in other listings, it doesn't seem to have a gear box.
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Old Feb 20, 2010, 08:54 AM
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Yes, it doesn't have a gearbox. But that motor doesn't have a Kv rating of 4500. Maybe some time tomorrow I'll have a chance to look at some of the numbers for you. That motor may be around 1000, so your motor will try to spin the same prop 4.5 times as fast, so it would pull way more amps.
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Old Feb 20, 2010, 09:30 AM
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Here's some stuff that isn't very scientific but should ball-park it for you. If you like that brand Skyartec (stuff seems pretty cheap-but don't buy the propellers they're $10! Personally, I end up using the APC-E on everything), they sell a Cessna 182 ARF (RTF?) that is about the same weight as your plane, so the motor they sell with a Kv rating of 1500 should work better than what you have. That should about be ok for your 2 cell battery, but a 1000kv may be more suitable for your 3 cell pack. That is where if you could find some kind of watt meter (a device you put between your battery and the rest of the system to see how many amps the system is drawing) you could be exact.

Here's the motor. http://www.rc-fever.com/ce1018-brush...kv-p-2646.html
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Old Feb 20, 2010, 09:54 AM
Vanadu
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Quote:
Originally Posted by nikolaus_c View Post
Here's some stuff that isn't very scientific but should ball-park it for you. If you like that brand Skyartec (stuff seems pretty cheap-but don't buy the propellers they're $10! Personally, I end up using the APC-E on everything), they sell a Cessna 182 ARF (RTF?) that is about the same weight as your plane, so the motor they sell with a Kv rating of 1500 should work better than what you have. That should about be ok for your 2 cell battery, but a 1000kv may be more suitable for your 3 cell pack. That is where if you could find some kind of watt meter (a device you put between your battery and the rest of the system to see how many amps the system is drawing) you could be exact.

Here's the motor. http://www.rc-fever.com/ce1018-brush...kv-p-2646.html
Thanks! That helps to put things into perspective for me.
I was going through some rc stuff in the closet and I found two motors that I had forgotten about. They are rated 2900, these seem like they are more suited for this project. I originally bought them to do a brushless conversion on my storm launcher a while back but never did.

Here is a picture.
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Old Feb 20, 2010, 03:43 PM
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Did you ever figure out what reduction your gearbox was?
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Old Feb 20, 2010, 06:58 PM
Vanadu
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Quote:
Originally Posted by nikolaus_c View Post
Did you ever figure out what reduction your gearbox was?
No I didn't, I dont' know how I would do it. Is there a formula? RC projects for me have usally been about trial and error

Upon going through more rc stuff, I have found that I have a bunch of 370 size motors from my cybird last summer as spares. I also have a SPF-5-ESCspecially made for parc flyers. I'm starting to wonder if I should not try to run before I know how to walk. Maybe the best idea would be to use the
370 motors and the SPF5 and burn through some motors while learning to fly. Then I could add ailerons later and eventually buy an RTF kit.

I don't know what to do anymore. My skyartec motor now seems like overkill and the 370s and SPF5-ESC seems to make the most sense.

*Sigh*

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