HobbyKing.com New Products Flash Sale
Reply
Thread Tools
Old Jun 06, 2007, 05:03 AM
Registered User
Joined May 2006
77 Posts
Discussion
CG Question

I have Blue Arrow Flix 3d. In the manual it says: CG 30%. Does that mean 30% of the wing from the leading edge? Or 30% of the plane from the prop?

Also, I have the Blue Arrow 18amp ESC and Blue Arrow BL29OS brushless... what Prop and lip battery should I use? I have a Blue Arrow 11.1v 3S 2200mAH battery, would that be alright with this setup?
aliqamar is offline Find More Posts by aliqamar
Last edited by aliqamar; Jun 06, 2007 at 05:10 AM.
Reply With Quote
Sign up now
to remove ads between posts
Old Jun 06, 2007, 05:26 AM
Registered User
Letchworth, Great Britain (UK)
Joined Jul 2004
11,544 Posts
Tried to download the manual from http://www.ltair.com:81/en/download.php , but it crashed my PC

Anyway, the position of the c of g is usually measured from the leading edge of the wing adjacent to where it mounts to the fuselage. It's quoted as an actual measurement (e.g. 60mm back) or, as in your case, as a percentage of the wing width (the "chord") at that location. So, if your wing is 6" wide where it joins the fuselage, your c of g should be 1.8" back from the leading edge (6 x 30% = 1.8) That means the model should balance when you try to balance it on your fingertips, with your fingertips positioned 1.8" back from the wing leading edge.

So far as the battery is concerned, what does the manual call for? So long as it specifies a 3S LiPo (11.1v nominal), what you're proposing should be okay. But if yours is a different mAh from the suggested one, it's weight will probably be different so you'll possibly have to mount it further forward or backward to get the c of g right.
abenn is online now Find More Posts by abenn
Reply With Quote
Old Jun 06, 2007, 09:38 AM
Oldie but goodie
Popsflyer's Avatar
United States, CA, Orange
Joined Jun 2003
3,890 Posts
I'm by far no expert, but a 3-2200 size pack seems HUGE for what the typical 3D plane might use.

Usually the smallest, lightest pack that will provide the amps needed to fly the recommended set-up is used in 3D capable planes. It will fly with more power and weight, but it won't be pretty.
Popsflyer is offline Find More Posts by Popsflyer
Reply With Quote
Old Jun 06, 2007, 02:53 PM
An itch?. Scratch build.
eflightray's Avatar
South Wales U.K.
Joined Mar 2003
13,190 Posts
I managed to download the manual but it look a long time and didn't like scrolling before fully downloaded.

It recommends a 1500 2s lipo, and the CG at 1/3 (one third) back from the leading edge.
eflightray is online now Find More Posts by eflightray
Last edited by eflightray; Jun 06, 2007 at 03:00 PM. Reason: Added picture
Reply With Quote
Old Jun 07, 2007, 12:08 AM
Registered User
Joined May 2006
77 Posts
previously I burned 4 brushless motors using the 30amp ESC and 3S 2200mah battery. Now I have bought the 18amp ESC with the recommended Motor, so I want to know if teh 3s 2200mAH is technically compatible with an 18amp ESC or not regardless of the weight factor, and what prop size should be used, as I recall IvanC mentioned before that I overamped it by using the wrong prop size and battery size. Can somebody shed some light on this
aliqamar is offline Find More Posts by aliqamar
Reply With Quote
Old Jun 07, 2007, 02:46 AM
Registered User
Letchworth, Great Britain (UK)
Joined Jul 2004
11,544 Posts
Quote:
Originally Posted by aliqamar
previously I burned 4 brushless motors using the 30amp ESC and 3S 2200mah battery. Now I have bought the 18amp ESC with the recommended Motor, so I want to know if teh 3s 2200mAH is technically compatible with an 18amp ESC or not regardless of the weight factor, and what prop size should be used, as I recall IvanC mentioned before that I overamped it by using the wrong prop size and battery size. Can somebody shed some light on this
So long as you're using the recommended volts, it's the prop size that primarily determines the number of amps a motor will draw: Bigger prop, or greater pitch, or both, means more amps You really need a wattmeter or clamp-on ammeter if you're going to experiment with props, to make sure you don't draw too many amps for the motor.

The ESC must be good for as many amps as you expect your motor to draw. Ideally you should add on a 20-30% margin to make sure the ESC is working comfortably. Apart from cost and weight, putting in a larger ESC than necessary will not alter your current draw. Putting in a smaller ESC than necessary will not alter the current either, until it gets to the point where it burns up

Every battery has a limit to what amps it can deliver: Generally, smaller capacity battery (less mAh) can deliver less amps. You get the amps the battery can deliver by multiplying its mAh by its C rating and dividing by 1,000 so if your battery is rated at 10C it can deliver 2,200 x 10 / 1,000 = 22A Provided your battery is large enough to deliver the amps you want, using a larger capacity battery (same volts) will not change your amps but will give you longer flight durations. If the battery is too small to deliver the required number of amps it will (a) keep the amps down to what it can deliver and (b) possibly overheat and eventually destroy itself. In those circumstances, using a larger battery will increase the amps.

But, provided you're using the recommended battery volts, the main thing is the prop size. If you've burned up 4 brushless motors it suggests that your prop is too big, so you must use a smaller one. What size do the instructions recommend, and what size were you using when you burned up the motors?
abenn is online now Find More Posts by abenn
Reply With Quote
Old Jun 07, 2007, 09:38 AM
Oldie but goodie
Popsflyer's Avatar
United States, CA, Orange
Joined Jun 2003
3,890 Posts
If you're burning Motors (4 ???), then you are pulling way too many amps. If excessive enough and you've went to a smaller ESC, now you may be going through ESC's too.

Other than the voltage, the Motor and ESC do not care what size Battery pack you use. If you are over propped and the pack is capable enough, it will supply whatever the motor wants until it cooks itself. If the pack or ESC is too small, then you'll burn up one or both of them.

You need to check the manufaturer's recommendations for prop size @ a particular voltage (ie: 10x4.7 on 2S- Lipoly, 9x3.8 on 3S, etc).
Popsflyer is offline Find More Posts by Popsflyer
Reply With Quote
Old Jun 07, 2007, 11:15 AM
Registered User
Joined May 2006
77 Posts
This is what the manufacturer of the plane recommends in the manual:

Battery: 2s(7.4v) 1500mAh Li-Po battery
Motor: 29 os Brushless motor
ESC: 18A Brushless ESC
Propeller: 10X5 --- 10X8 slow flier prop

PS: I used a 10X4.7 and 10X5 prop with the above setup, but I had a 11.1V 2200mah battery and 30A brushless ESC
aliqamar is offline Find More Posts by aliqamar
Reply With Quote
Old Jun 07, 2007, 01:34 PM
Registered User
Letchworth, Great Britain (UK)
Joined Jul 2004
11,544 Posts
Quote:
Originally Posted by aliqamar
This is what the manufacturer of the plane recommends in the manual:

Battery: 2s(7.4v) 1500mAh Li-Po battery
Motor: 29 os Brushless motor
ESC: 18A Brushless ESC
Propeller: 10X5 --- 10X8 slow flier prop

PS: I used a 10X4.7 and 10X5 prop with the above setup, but I had a 11.1V 2200mah battery and 30A brushless ESC
By using a 3S battery (11.1v) you overloaded the motors by about 50% -- maybe more Lucky you had a 30A ESC, otherwise you would have burned that out too. The fact that your battery is 2200mAh instead of 1500mAh is not an issue -- it's just the volts.

I don't know your motor, but it may be able to take 11.1v -- but only if you use a smaller prop to get the amps back down within limits. Best advice is to stick with the recommended 2S (7.4v) battery pack until you find someone with experience of your motor who can tell you whether or not it can take 3S
abenn is online now Find More Posts by abenn
Reply With Quote
Old Jun 07, 2007, 01:49 PM
Registered User
Joined May 2006
77 Posts
okay, i'll then go and get 2s 7.4v battery pack. manual says 1500mah, if i want i can get 1800 or 2200mah, that wouldn't be a problem, right?
aliqamar is offline Find More Posts by aliqamar
Reply With Quote
Reply


Thread Tools

Similar Threads
Category Thread Thread Starter Forum Replies Last Post
CG Question - Wingo Radek Z Electric Plane Talk 17 Oct 19, 2001 03:39 PM
Miss 2 CG question and intro n3jpu Parkflyers 1 Aug 03, 2001 10:27 AM
CG Question Dave Hederich Power Systems 4 Jul 02, 2001 12:57 AM
"Mini" completed.....CG question..... Danny Troy Parkflyers 4 Jun 06, 2001 02:43 PM
Terry CG question???? garyc Foamies (Kits) 2 May 27, 2001 11:03 AM