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What Motor an Electronics do I Pick????

jd7792

Senior Member
#1
Would it be possible for a video article to be made on how to select motors and electronics for a scratch build?

I am still relatively new to the hobby and I am very impressed with the knowledge that is past to and fro between members, but I have to admit that I can't get a full handle on the basics of choosing the running gear for a plane. How do you decide what motor will deliver the power that is needed and then from there what is the process for choosing the electronics to go with the motor?

I am certain that the process is quite obvious to those who have been making these choices for a while, but more I read the more questions that I come up with.

Why are there so many different size motors? How big does the ESC have to be? What determines the voltage needs of the battery? What's the difference between digital and analogue servos? As you see I still have a lot to get my head around, and I bet I'm not the only one with these questions!

To have a video reference to turn to that explains these basics would be invaluable for newbies like me that are still riding along the early stages of this massive learning curve that this hobby puts in front of RC Pilots.
 

Hamdhan

The Expert Newbie
#2
There is a video on picking a prop and there is a video on picking your battery.
However, for first plane, I highly recommend buying a PNF aircraft.

But if you insist:
Motor = Buy motor depending on how much thrust is required and how much thrust the motor produces
ESC = Check the motor for Amp rating of ESC
Battery = Look up the motor specs, usually for medium-small or trainer aircraft, 2S or 3S are used.
Propeller = Check the RECOMMENDED prop under motor specs.
 

jd7792

Senior Member
#5
I wrote this thread a couple of months ago, as you see from the date. Since then I have read a few more articles, watched a few more videos and asked more questions. I still don't know how you pick motors, I can't get past the "bigger the Kv the better" feeling, even though I know that isn't correct. I am still to find clear answers on how to make the choice of motors and why????

I fly the Bixler look alike, the Sky Surfer (Bananahobby version) and I have just finished building my second scratch build, a 30" flying wing, and am starting to understand the other pieces such as ESC, Battery etc, the first build was a F16 3D. I asked for advise and went with a couple of suggestions. The F16 has a 2200Kv RcTimer motor and I had a spare 1950Kv motor from a second Surfer buy, so I am going with that on the wing, just to start with until I get a second 2200. I finally got hold of a Turnigy Tx so I have spent a while getting that sorted....I think! so the maidens wont be far away.

I still would like to see a good video on choosing motors. The whole Kv thing really still spins my head! If you know of any be sure to let me know, in the mean time I will just have to keep reading and asking questions.

Thnx for revisiting this thread.
 

colorex

Rotor Riot!
Mentor
#6
I tend to choose my motor from the thrust I need. If my empty plane weighs 200 grams, then I get a motor with 100 grams more thrust more than the weight of the electronic system.
 

Hamdhan

The Expert Newbie
#7
Kv is not everything. You could get a 15000Kv motor that is for a 30mm EDF... its basically useless with a 5x5 prop. Just look up the specs and the "Thrust" section of the specs.
 

Ak Flyer

Fly the wings off
Mentor
#8
KV=motor rpm x Battery voltage.

I have a park 300 that's 1080 kv and it's a complete dog on a 2S battery. 1080kv x 7.4vdc = 7992 max rpm

Add a 3S battery and it turns into a rocket on the same plane. 1080kv x 11.1vdc = 11988 max rpm or 3996 additional or roughly 50% more rpm.

Think of the batteries in cells. Each cell is worth a certain rpm to a motor, when you add a cell (2S to 3S) you add a block of motor speed. Same thing when you go from 3S to 4S or 4S to 5S. That's why larger motors which handle more voltage (higher cell rating) they have a lower kv rating so that the end result is about the same prop speed.

The reason they shoot for about the same prop speed is that props have a maximum rpm they can achieve before they stop working. This is mostly based on diameter of the prop. As the length of the prop increases, you reach a higher speed at the tip of the prop. Once the tip of the prop exceeds the speed of sound it stalls the prop due to the turbulence created. Once the prop stalls it will produce little to no thrust so the max speed must be kept below that.

Long story short, your motor and battery have to be matched with your prop to achieve the most thrust with your combination without over speeding your propeller. There's more to the story ie pitch etc but that's the goal. Your esc is then matched to the load created by the rest of the package.
 

jd7792

Senior Member
#9
Thanks Ak, this is the first time I have heard this. Another piece of the puzzle!
So does this mean that you pick the prop first, then depending on the thrust you want you pick the motor and battery combo to deliver the optimum RPM (in order to reach the desired thrust)?
 

Ak Flyer

Fly the wings off
Mentor
#10
No, you pretty much start with a motor. For instance, my Hobbyzone Supercub LP is pretty heavy with a larger battery and a bunch of mods I've done. I wanted to go brushless and went with a Turnigy Easymatch 480 and now I have unlimited vertical and it will hover with floats on. I'm still experimenting with props but right now I have a 10x7 to give me better clearance on tires. That's another thing to consider is space limitations. I could have easily gone with a 450 or 400 for a little bit lighter weight or more scale performance but I wanted to make a rocket ship out of it. I went with the recommended 40 amp esc and don't have any reliability issues.

There's a million combinations I could have gone with but the recommendations from the manufacturer are a good place to start. The easy match line has a lot of good data that make it easy to get going. They tell you esc, battery and prop data that they've tested along with thrust numbers that each combo makes, for most motors anyway.

Like colorex said, one way to go is to figure out your weight and look for a thrust combo that will get you where you want. Now 1.5 times the weight is a lot. That's in 3D range. Most real planes don't produce anywhere near their weight in thrust. For scale flight you can go quite lower. theoretically a 1:1 ratio will allow you to hover but only on a full battery if all things are right.

I personally look at planes with the performance I want and compare a few different ones to see what setup they are using and do something similar. Those are proven setups that I know will work. For instance, I know from looking at similar sized 3D planes that a 480 , 40 amp esc and 12x6 prop are a good combo with lots of power so I knew that it would work for my application. They also give weights.

E-flite from Horizon Hobby is a good resource because they list the entire power setup of all of their models and they have videos of most everything so you can see the type of performance you're looking at. They list everything the same way Turnigy easymatch does. A power .25 from e-flite is the same as a .25 Easymatch motor. Park 300 e-flite = park 300 easy match.

As you compare you will start to see the differences and be able to translate them to other lines.
 

jd7792

Senior Member
#11
I can't thank you enough for your help. I understand the idea and method you use and it sounds logical at this stage. I still have a lot of reading to do and a world of figures to grasp but at least I understand the theory now, at least I think I do!
I am sure that I am going to come up with questions but for now I feel I am starting to get somewhere. There are still a lot of grey areas but I'll tackle them as they pop up.
If you don't mind I'll be sure to contact you in the future with some of those questions.
I think there would be a lot of pilots, both newbies and experienced, out there that will benefit from some of this thread so once again thanks for your time and effort.
 

Ak Flyer

Fly the wings off
Mentor
#12
Not a problem. We all learned from somewhere and please do feel free to contact me, directly if you like with any other questions you may have. I'll answer them to the best of my knowledge or help direct you towards those who know better.