Coax brushless conversion help - Page 2 - RC Groups
Nov 06, 2012, 11:57 AM
Registered User
Quote:
 Originally Posted by Luvmyhelis Get the Turnigy 1811-2900 at hobbyking. Vastly superior motor. They use copper windings whereas the ones you are using have cheapo aluminum in them. Half a dozen different issues can cause the motors to cut out.
They look nice, but out of stock, damn.. I know Ill get flamed for asking this but I've not had time to read up on how to determine the KV size of brushless motors I need..

anyone have a quick primer?
Nov 06, 2012, 02:19 PM
OlliW

# how to select a motor primer

First, the most important piece of information you need to know about a motor is the maximal mechanical power it can deliver. Unfortunately, most often you won't find good values for that (and sometimes you even won't find any), although I guess it makes sense that the motor must be "big" enough to deliver the mechanical power to rotate the rotors at the desired rpm. I mention that since the power requirements are often overlooked in the sense that the KV value is given the most attention.

The second most important piece of information you need is then indeed the KV value. It tells how fast the motor would spin at a given voltage - if (and only if) the motor is running free without ANY load (no rotor attached). The rotational speed is then calculated as

rpm (@no load) = KV x Voltage

Another important piece of information is the efficiency. However, unless you really got a dude the differences are not that big for motors in the price range most of us find acceptable. The important piece to understand here is that the lower the actual rpm WITH load is, the lower is the efficiency. The rpm region where a motor runs most efficient is about 80-90% of the no-load rpm (which we get from the KV value and voltage). Secondly, one should never ever have the actual rpm get below 50% of the no-load rpm (the motor would then be more useful for cooking eggs). So, as a rule of thumb

A final piece of information is the gear ratio. What it does is I guess obvious.

To answer your question: If you are confident that a motor can handle your power requirements, the KV value of the motor is then choosen such that the possible gear ratios allow to match the desired rotor rpm to 80-90% x KV x voltage.

In order to piece this together you need to know what rpm of the rotor you maximally will need. Unfortunately, this is usually (also) not known from advance, and one has to add in "experience" (e.g. from forum members).

As an example. To hover a lamaV4/CX2 coax the rotors have to spin at ca. 1800 rpm. The gear ratio is 69:8 (if I am not mistaken), hence the motor has to spin at 15500 rpm. With a KV of 2900 and 85% rpm drop this would be achieved with a voltage of 6.3 V. With a 2S battery of 7.4 V this is achieved at about 4/5 of throttle, i.e., for hover you would have to move up the throttle to 4/5. With a 3S battery of 11.1 V this is achieved at about 1/2 throttle (roughly).

You don't want just hover, so you pull up throttle to 100%. Obviously the 2S setup would give you some agility, but only slightly more, and maybe comparably to the stock setup (which also hovers at roughly similar throttle). Using a 9-tooth pinion could be more appropriate here. With the 3S setup you obviously should have plenty of power.

Please note, and this is important, this is achieved ONLY if the motor is capable of delivering this power. Obviously, if the motor is too weak, it will stall and can't deliver the required power. It will tell you though (unfortunately only after you may have bought it): You will not reach the desired rotor rpms and/or the motor will run hot.
Last edited by OlliW; Nov 06, 2012 at 02:42 PM.
 Nov 08, 2012, 08:40 PM Professional heli wrecker Great post Olli!
 Nov 10, 2012, 09:41 PM Registered User so you guys weren't exaggerating about how bad the combo upgrade kit was. Mine completely stopped functioning right after 5 flights. I upgraded the escs to 10A hk ones but I will try to get those motors that were suggested.
Nov 12, 2012, 07:42 PM
Registered User
Quote:
 Originally Posted by Luvmyhelis Get the Turnigy 1811-2900 at hobbyking. Vastly superior motor. They use copper windings whereas the ones you are using have cheapo aluminum in them. Half a dozen different issues can cause the motors to cut out.
So they are back in stock now a couple of quick questions b4 I order 4 (have to have a few spares :-))

1. What so I use to fasten the 1811's to the frame? Can I use the motor mounts that came with the turbo ace's? Or is there a better mount on hobbyking you recomend.
2. are these the escs you use/recommend?
3. If I wanted to go to a 3c (11.1v), can I use that with some turnigy 10a esc's but with the stock 4 in 1? It won't fry at 11v? lol
 Nov 13, 2012, 07:35 AM Registered User My 1811's are mounted using the C-10 bases. The C-10's didn't work out that great and the frame was already set up to use them. In my situation, the C-10's and the 6amp ESC's wouldn't lift the heli on 2 cells and just barely got it off the ground even on 3 cells. The 1811's, 10 amp ESC's and 900 mah 3 cell have been great.
Nov 13, 2012, 09:44 AM
Registered User
Quote:
 Originally Posted by kodak_jack My 1811's are mounted using the C-10 bases. The C-10's didn't work out that great and the frame was already set up to use them. In my situation, the C-10's and the 6amp ESC's wouldn't lift the heli on 2 cells and just barely got it off the ground even on 3 cells. The 1811's, 10 amp ESC's and 900 mah 3 cell have been great.
What frame are you using? And what tooth pinion are you using? 9 tooth, 11 tooth?
Nov 14, 2012, 07:46 AM
Registered User
Quote:
 Originally Posted by gearwolf What frame are you using? And what tooth pinion are you using? 9 tooth, 11 tooth?
The frame is the MicroHeli one and the gearing is the stock gears. I was able to get a frame from Ebay for \$30. I also have a Lama 4 and that frame is \$45!!!!!!!!! Mine also has 400 size skids for more stability on take off and landing. I was able to stuff all of my components under a police canopy.
 Nov 22, 2012, 05:26 PM Registered User So got mine to fly with carbon fiber blades. It takes a lot of power since the chord is smaller but now they won't break apart because of a blade of grass.
 Nov 23, 2012, 07:22 AM Registered User What carbon fiber blades did you find for your heli? Have a link?
 Nov 23, 2012, 05:24 PM Registered User I actually just modified these ones http://www.hobbyking.com/hobbyking/s...dProduct=20394 to accept the bumpers for the main blade grips. If you haven't seen the Walkera Lm130 or the Lm180 they already introduce the angle of attack in the blade grips but it has it's drawback. So unlike most other coaxials these don't have asymmetrical blades to begin with but they use conventional blades at a set angle of attack. Unfortunately they made the stock blades with a large chord compared to most other blades. With a 3s and the turnigy 1811 2900kv motors at half throttle I was hovering about 4 feet off the ground. Not great but I have a lot more weight than stock on here.
 Nov 24, 2012, 07:30 AM Registered User Interesting. I use the Xtreme 55 Degree blades right out of the package. The intent is to get less balooning so you can get speed without a lot of lift. I have to contend with a couple of large maples that stick into my yard and they help me stay below the lower limbs. There's a guy on the regular CX2 thread talking about making new canopies out of CF and even Kevlar. I told him he might have more interest in doing some blades, especially for larger coax's like the BOL and the Walkera 400D = 400 size coax.
 Dec 16, 2012, 08:58 AM Registered User Anyone doing a brushless conversion should think about a programmable transmitter to go along with it. If nothing else, it lets you put all of your Spektrum equipped heli's on one transmitter. I couldn't believe the price of this thing! I just wish it was plane jane black: http://www.hobbyking.com/hobbyking/s...y_Mode_2_.html