Electric General Discussion General Discussion forum about rc electric related aircraft, accessories, flight, tips, etc.

Motor selection

Reply

Old 09-17-2016, 09:59 AM
  #1  
gphil
Senior Member
Thread Starter
 
Join Date: Oct 2011
Location: Elberton, GA
Posts: 217
Gallery
My Gallery
Models
My Models
Ratings
My Feedback
Default Motor selection

I have a 4 - Star by SIG that I am going to put a electric motor into. . . . The plane can be either one has EG designation. I have been searching for the proper combo for this plane to the point it get confusing . Even some hobby shops will not help one of which said "good luck" So here is what the things calls for. Brushless motor @ 500 - 800 Watts.. 80 - 1000 kv ESC to mach, 75 I believe. Lipo Batt 3 - 4s Prop is APC 12 x 6 elec. Those are the numbers. If I was putting a glow on it would be close to 60cc I am sure some guys know the plane. Even look at Hitec motor on ebay but sort of affraid of a used unit. Nothing exotic for me just a good motor reliable and so on. If you can give me some input ti would be great. Thanks and I will be looking for some news, Gary

Last edited by gphil; 09-17-2016 at 12:52 PM.
gphil is offline  
Reply With Quote
Old 09-17-2016, 01:34 PM
  #2  
aymodeler
My Feedback: (3)
 
aymodeler's Avatar
 
Join Date: Feb 2010
Location: Hartland, CT
Posts: 286
Gallery
My Gallery
Models
My Models
Ratings
My Feedback
Default

Take a look at Cobra motors sold by Innov8tive Designs. They publish performance charts for different combinations of propellers and batteries.

A C-3520/12 might be close to what you need: http://innov8tivedesigns.com/parts/brushless-motors/cobra-c-3520-12-brushless-motor-kv-820

According to the propeller chart with a 4S battery you will get 576 watts at 39 amps with a 12x6 prop. You should size for at least 100 watts per pound, so depending on your final airframe weight, you may want to look for a combo that will get you bit more power. However, if you need more than about 700 watts you might find that you need to go up to 5S or larger battery as most motors in this size range are limited to around 50 amps for continuous operation (for a 4S battery - 50 amps x 14.8v = 740 watts).

Al
aymodeler is offline  
Reply With Quote
Old 09-18-2016, 05:26 AM
  #3  
gphil
Senior Member
Thread Starter
 
Join Date: Oct 2011
Location: Elberton, GA
Posts: 217
Gallery
My Gallery
Models
My Models
Ratings
My Feedback
Default

Thanks guy for all the good information. I have looked at that site mentioned and it seems good. I do not know anything about this side of modeling. Old nitro flyer here and not too good at it. Built a lot of models in my time and control line flying so this all is a challenge for me. . . As mentioned my SIG 4-star has a flying wt. of 5.5 lbs. Do I consider that the guide for motor selection. Don't know the empty wt, without engine that is. Ok and contact me , if you like , at gphilflier@yahoo.com Older senior here and that I can operate. lol Need a teacher like you. I find most of the fellas know what they have but not how to recommend . . . Even some of the well known shops said search for your self. So I may pester if you let me. Thanks once more..... Gary ( aka gphil )
gphil is offline  
Reply With Quote
Old 09-18-2016, 03:07 PM
  #4  
aymodeler
My Feedback: (3)
 
aymodeler's Avatar
 
Join Date: Feb 2010
Location: Hartland, CT
Posts: 286
Gallery
My Gallery
Models
My Models
Ratings
My Feedback
Default

With a 5.5 lb flying weight you should be good with something like the Cobra motor I mentioned in my last post. I did not look through all of the options, but look for something that will give you 550 - 750 watts with a 4S (14.8v) battery and something close the the 12x6 prop.

The other thing to think about is the all up weight of the motor, battery, ESC as compared to the glow setup. At 5.5 lbs, I am guessing that this is the 4* 40, correct?

Assuming so, a typical glow 40-46 sized engine would be somewhere between 16-18oz, plus another 3-4oz for a fuel tank, throttle servo, throttle linkage, etc. The Cobra motor is 7.6oz, a 3000mAh 4S battery is about 10oz, and another 2oz for the ESC and you are at just under 20oz total for the power system. So this compares pretty well to the glow setup.

If you have not flown electric before, do you have a charger for the batteries?

There are all sorts of preferences out there for batteries, connectors, chargers, etc. Truth is that they are all fairly comparable, but once you settle on a specific solution, then it is usually best to stick with it. For example, I prefer to use the EC3 style battery connectors. If I always stick with these connectors, I never have to worry about adapters. Other guys I know prefer "T" connectors, while others prefer "XT" style. They all work fine and it's mostly a matter of preference (unless you are running a real high power set-up drawing 100+ amps).

There are other considerations too. The golden rule is to never run you batteries below 3.7v per cell. Doing so will likely damage the battery pack. I try to limit my flight times to never run my batteries below 30% charge capacity. This will keep me above the 3.7v limit. Also, NEVER overcharge a battery pack as that can lead to a fire.

I know it all seems daunting at first, but once you get past some of the jargon it really is simple stuff. Do some searching here and at other sites and talk to others at your field and you will get the hang of it quickly.

Please don't hesitate to contact me directly with any specific questions you might have. I will send you a note via e-mail so you can have my contact info.

Al
aymodeler is offline  
Reply With Quote
Old 09-18-2016, 03:57 PM
  #5  
gphil
Senior Member
Thread Starter
 
Join Date: Oct 2011
Location: Elberton, GA
Posts: 217
Gallery
My Gallery
Models
My Models
Ratings
My Feedback
Default

Thanks once more and my plane may be sightly heaver. The motor suggestions I sent was from the kit. I will have to really get the numbers correct. Yes I have a new charger for the batteries. I purchased a kit that was incomplete so got a charger for that and batteries. However the next batteries will have to be larger. Thanks once more gphil
gphil is offline  
Reply With Quote

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off


Thread Tools
Search this Thread

Contact Us Archive Advertising Cookie Policy Privacy Statement Terms of Service