General Electric Motor selection

joeosborn

joeosborn

New Member
Ok, here we go. I have found a basic idea for my foam plane, {Sorry Tony found another that I kinda like better :apologetic:}. The problem is, the guy doesn't tell you what size motor he used or how to select one.

What I would like to know is there a formula to come up with this? I know this is going to be regarding size and weight, and style, but not sure how or how much.

Can someone share some enlightenment on this please?

Thanks

Joe
 
You posted this in JUL 2013....here it is JAN2014 and no one has given you a reply.....
something wrong here....
The question isn't a very hard one either........I too would like to know what equipment to get for a given scratch built airplane.
 
flaper said:
You posted this in JUL 2013....here it is JAN2014 and no one has given you a reply.....
something wrong here....
The question isn't a very hard one either........I too would like to know what equipment to get for a given scratch built airplane.
Click to expand...

The question is more complex than you think. No reason to get nasty with your reply though. We are all busy and with it being just me, I do miss threads from time to time. There is only so much that I can do.

The way that I calculate what size motor to use is I go into my box and pull one out that looks like it will fit on a plane well. Meaning, I would not get a .32 size electric motor and put it on the RCH Trainer, it's obviously too big.

Most times, for most scratch built planes, something in the 60g/1000kv range is going to work well. That is what I have spec'd on the RCH Trainer. I think it's actually a little heavier than that, but still works.

It's all about trial and error. You can look through the different motors that are out there and find the thrust numbers per given prop and compare them to the AUW plus the calculated weight of the motor and ESC which you have yet to purchase, and get a number that is at least a 1:1 thrust to weight ratio.

That is how I do it. And again, I apologize for not seeing this thread when it was posted. All it takes is for someone to bump a thread up so that it goes back into the new posts and I have a better chance of seeing it. I don't miss many, but I do miss some.

Hope this helps.
 
If I may be a bit snarky.... The question is an easy one to ask... but that doesn't make the answer an easy one to explain. If it were so easy to answer... why then would someone need to ask the question instead of just look up the answer which has been given millions of times?


The first step... find out how much the airframe will weigh with all of it's components installed. Next, determine the type of flighing it will be used for.. ie a trainer, a sport flyer, 3D flying, precision or other. Then match the amount of thrust and speed a given motor/prop will provide for that size/weight and type of flying you expect to do with it.

That is it in a nutshell really... There is no one answer simply because each airframe will differ as to their size, weight and the performance specs you want to see out of it. I wouldn't look for a mathematical formula... there isn't a need for an engineering degree if you look at what is already provided elsewhere and just borrow from what has already worked on a similar aircraft.

I'll usually start by looking at existing kits/arf's that are similar to the one I'm trying to outfit. I look to see what motor/prop/battery they are suggesting then I go to the websites of the various motor manufacturers that I prefer and look through the specs they give for their motors using the suggested equipment they have tested with to see which most closely matches what the kits/arfs had suggested. I also take into account the amount of power to weight that the motors have and try to select the lightest one that meets the power/speed requirements I am looking for.

If you really really want a formula... you need to start by going and getting an aeronautical engineering degree... I prefer to do it in a simpler manner and just look at the existing aircraft for a place to start.
 
Tony said:
The question is more complex than you think. No reason to get nasty with your reply though. We are all busy and with it being just me, I do miss threads from time to time. There is only so much that I can do.

The way that I calculate what size motor to use is I go into my box and pull one out that looks like it will fit on a plane well. Meaning, I would not get a .32 size electric motor and put it on the RCH Trainer, it's obviously too big.

Most times, for most scratch built planes, something in the 60g/1000kv range is going to work well. That is what I have spec'd on the RCH Trainer. I think it's actually a little heavier than that, but still works.

It's all about trial and error. You can look through the different motors that are out there and find the thrust numbers per given prop and compare them to the AUW plus the calculated weight of the motor and ESC which you have yet to purchase, and get a number that is at least a 1:1 thrust to weight ratio.

That is how I do it. And again, I apologize for not seeing this thread when it was posted. All it takes is for someone to bump a thread up so that it goes back into the new posts and I have a better chance of seeing it. I don't miss many, but I do miss some.

Hope this helps.
Click to expand...

My apology ..if it sounded like I was being nasty, did not intent it to sound that way.
Thank you for your feed back...it is somewhat helpful ....
But I can't help thinking there is something else missing.
Wing span ....Weight of airplane...
I hope more people read this thread and can offer some input.
 
You can get a rough estimate of a plane if you know wingspan, total drag in the air among a million other variables. This is one of the reasons why I have not released 3 other planes that I have been working on. I just flat don't have the time to experiment with motors, wing size, control surface size and so on. It can take hundreds of hours to get a plane flying the way it should. Just look at the RCH Trainer. I have went through about 20 different revisions, with most being small, and only 3 being large changes, and that has taken place over the last two years. If I had the time, yes, I could get everything done in about a month, but life takes away from that lol.
 
Ok joeosborn Im going to try to explane how to choose a moter and prop for your rc planes, so stick with me , here we go. There is 3 basic things you need to know to figuer this out,
1, The total weight of the plane ready to fly as it would sit for take off.
2, The amount of ground clearance for the prop.
3,The wingspan and wing area of your planes wing.
Thats it in a nut shell,since one and two is self explanatory, I'll walk you through the third one.
There is three things that the wing can tell you,
1,The stall speed of your plane
2,Prop diameter
3,How fast to prop the plane for (How fast the plane will fly)
So lets figure out your wing loading (How much weight your wing can carry or lift,
when we talk about wing loading we use terms (Weight per squar foot of wing area)
so measure the wingspan wingtip to wingtip, then measure the chord(Average wing width is your chord)
1, multiply the wingspan by wing chord
this will give you your wing square inches.
2,convert to square feet by dividing the answer by 144.
3,To find your wing loading divided the plane weight by wing area in square feet this answer will be your wing loading.
so every square foot of your wing can carry that much weight.
Now with this info we can figuer out the stall speed of your plane(the slowest speed possible with out falling out of th sky) To do this find the square root of your wing loading anwser.To do this just google it, put this in google,square root of blank is( the blank is your wing loading anwser. Take that anwser and multiply it by 5, and then you have your stall speed.Now you want you top speed to be 2.5 to 3 times your stall speed you can even go up to 4 to 4.5 times for some planes. then you want about 75% of your planes weight in thrust.pick the bigest diametet prop as possible (up to 25% wingspan) now sipn it fast enough to get your speed you went .So look at the spec for moter and mach up prop size with KV and on back through your power system it all should just fall into place for you just take you time. to figuer out the watts of a moter just take max amps multiply by the volts from your battery to get the watts for that moter, (a 3s lipo is 11.1 v)
I know alot of info take your time a you will get it. Hope this helps out...... .
 
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