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Hello there! - and a motor question

#1
Hello all the way from Norway!
I'm a noob, and have only been doing some rc car back in my youth. I’ve now got the interest for both multirotors and planes. I’ve been learning a lot from Flite Test’s vids and planning on building a quadcopter and plane this summer. I’m also hoping to get into fpv as soon as possible.

I’ve started looking/planning the parts but are kind of stuck on the motor section: If I’m building (let’s say) a versa wing, it seems like it gonna weigh around 1kg (2,2 pounds), and for that it seems like I need 300-350 watts. BUT, (and this is where I’m stuck or something) motors are sorted in any way other than max watts… :confused: HobbyKing sort the motors after size. Why does size matters?:rolleyes: Weight may matter, but I can’t see why size matters for most builders. Another data that is very accessible is “kv”, and the kv is for sure important, but if I need around 300 watts and want around 1400 kv then it’s not interesting for me to find a 1400 kv that can push 80 watts. The only way I think make sense is to first find the power the motor can give me, and then find the “right” kv I want within the power range. One web shop that have done this right is headsuphobby.com which sort the motors like 100-200watts etc.. What are your thoughts about this? Do you have any tips or tricks on how I can find motors with the right power, without going headless through endless lists of motors by size?

I don't know if I should post this in any other part of the forum, but I feel like this maybe a noob question, and it's also question shared for all aircrafts so... :)
 

PsyBorg

Wake up! Time to fly!
#2
Along with Flite Test I have used Painless360's videos where he gives Awesome tutorials and teaches a ton of stuff about anything RC related. If you are new I suggest using his videos as well as Flite test to ease the learning curve.

Here's a link to his video on how the motors are marked and what they mean.

https://www.youtube.com/watch?v=uLutMoh4Ttg

Here's the link on how to choose the correct motor for your application.

https://www.youtube.com/watch?v=HSQGI6u2DlM
 
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rockyboy

Skill Collector
Mentor
#3
One web shop that have done this right is headsuphobby.com which sort the motors like 100-200watts etc.. What are your thoughts about this? Do you have any tips or tricks on how I can find motors with the right power, without going headless through endless lists of motors by size?
I agree with you on Heads Up RC has a great organization method - and they are a very good and reputable company to order from. But they don't offer shipping to Europe due to some problems they have had in the past. :(

My method so far has been to search the web for recommended setups for other planes in the same weight class. However, I'm stepping up my game now by trying to use some of the online power system calculators. A handful of links below are the ones are recommended by some of the most incredible builders around here.

http://www.ecalc.ch/motorcalc.php
http://adamone.rchomepage.com/calc_motor.htm
http://www.motocalc.com/tutorial/

I'm still learning how to use them :)
 

rockyboy

Skill Collector
Mentor
#4
I was looking into this more as I have a "mystery motor" on the bench at home and I need to figure out it's power input/output specs. In the process, I ran across this website http://rcadvisor.com/carlos-power-rule-explained with a very interesting weight rule:

Motor Sizing
The key to my power rule is the observation that our outrunner brushless motors can handle about 75 watts of input power per ounce of motor weight (2.65 watts/g). This of course depends on the shape of the motor and how much cooling air it gets. But as a rule of thumb, it was reasonably accurate.

Then I concluded that a sport electric model airplane needs about 75 watts per pound to have enough power to fly as it was intended. This was based on the old power system rule of thumb and my own observations of successful power systems.

From there, some simple calculations told me that the target weight of a brushless outrunner motor should be 10% of the total airplane weight.

I studied a lot of successful power systems to confirm the validity of the rule. The rule works well across a large number of different types of model airplanes. I noticed that very small model airplanes generally need bigger motors than predicted by the rule. Similarly, very large electric model airplanes can get away with smaller motors. But overall, the rule works extremely well.

I like this rule of thumb a lot. You can pretty much count on a product spec sheet telling you the weight of an electric motor. How much power it can produce is a lot less obvious.

Using this rule of thumb, sizing a motor up or down depending on the power requirements is easy. Do you want a fully aerobatic model? Make the motor 15% of the total weight.
 
#5
I'm grateful for the links! All seems very usefull :D I guess I just have to live with the weird/wrong way of sorting the motors ;) I wish Heads Up RC would still ship to EU.

I think I'm gonna order some parts now and build me myself a versa wing just to get started. I think I'll start by building it as a tractor and then go for a blunt nose as a pusher later. Because of the big shipping cost hobbyking adds to the package I'm forced to look for a other place to order from. Do you got any experience with myrcmart? I'm looking at RCX BE2814-5 1400KV, thoughts? I think I'm looking for 300-360 watts motor, 40amp esc, 3s lipo - I see many uses 2200 mAh, thoughts in 2650 or 3000mAh? I also think I'm wan't to buy the Turnigy 9X. Thoughts? Any tips on small things I should add to the order as a first order? I'll add a charger and some servos too..

When I get the parts I'll make a "my first plane" post in the plane section!