Rules of thumb for matching motor, controller, batteries and Props

[Captain Corsair]

I am new to electric. Looking for a simple listing guide for insght into matching prop, controller, batteries, chargers, motor to model . We have something similiar for 2 cycle, 4 cycle engines where prop suggestions are provided for type of models and engines.

Thanks for your assistance

[Ron]

I generally try to fly on 50 to 60 watts to the pound for most sport applications.
when building a model, I try to keep the power system radio etc. to 1/2 the total weight of the model or less. However, on some of my larger models, I find that the 50 watts per pound rule kind of goes out the window because larger models can fly very well on less. My 103" Catalina flies on less than 40 watts to the pound so.....I calculate this way..... scale and most sport models..... Calculate stall speed....calculate prop rpm.....keep less than 30 amp draw.... make propellor pitch speed 2 to 2 1/2 times the models stall speed,swing as large a dia. prop as you can, and you will be happy for most sport flying as long as the thrust from the prop is also minimum 40 % of the models weight. At 65 % you will have a pretty aerobatic model. No hovering of course, but lots to do the obligatory loops, rolls, stall turns immelmans, reverse cuban eights etc.
..... hovering type models require lots of thrust, but less pitch speed, and a 3 pound model can easily hover on 360 watts.
Watts equals...volts times amps ....10 volts times 30 amps equals 300 watts
A standard 10 cell nicad or nimh cell is calculated for my purposes at 10 volts under load. all my models fly on either 10 or 20 cells as this makes things simple for calculation purposes.... this gives a range of models from a 48" fully 3D model on 10 cells to 120" span scale models on 20 cells at the upper end.
With this method I can get away without purchasing many packs of cells, as every pack can be used in every model....yes, the models are designed so that cells are easy to put in and remove at the field for charging. Hope this helps instead of confuses you. One other thing...Brushless motors and lithium polymer cells are not required to do this...my 80" Eindekker flies on 10 cells and a cheap brushed motor on a gearbox swinging an 18 X 10 prop @ 3600 rpm......
1/3 scale DR1 flies on 20 cells...swings a 30 X 14 prop at 2400 rpm full throttle, but cruise rpm is more like 1400...flies for about 10 min and is very aerobatic ( for a DR1 that is ) . after you have done a few electrics you kinda get a " feel for it " and you'll be off to the races........when you buy batteries and especially a charger.
buy GOOD QUALITY......I can't stress this enough...this is the heart of your electric models and cheaper stuff generally won't work as well if at all.....good batteries will last you several seasons easily, and a quality charger will work well for you forever.

[Air-Gar]

Captain,
My suggestion to get started in e-flight is for you to download ElectriCalc, electric flight software, study the manual, and play around with it. When you are familiar with E-calc it will answer just about any question you can think of regarding designing electric power systems.
Air-Gar

[flypaper 2]

If you pick a model that weighs, say 14 oz., pick a system that will fly that much weight. For instance a GWS zero weighs roughly that much and uses a Himax 2025-4200., CC 25 amp ESC, TP 2100 batt., and 10-8 prop, and you build one of your own design that weighs that much, then you have a power system that would fly it. Hope this helps. There are to many variables to pick out parts separately unless you've been in it for a while.

[JPriami]

Captain,
My suggestion to get started in e-flight is for you to download ElectriCalc, electric flight software, study the manual, and play around with it. When you are familiar with E-calc it will answer just about any question you can think of regarding designing electric power systems.
Air-Gar

I found the page for this program but can only find an upgrade not the whole program?

[stanger89]

Captain,
My suggestion to get started in e-flight is for you to download ElectriCalc, electric flight software, study the manual, and play around with it. When you are familiar with E-calc it will answer just about any question you can think of regarding designing electric power systems.
Air-Gar

I use motocalc personally but I assume Electricalc is similar in this: You really need to understand a bit of the theory behind what these programs are telling you before they can be really useful. The reason is these programs can give you configurations that look like they will work, but in reality won't for any number of reasons. Look at Ron's post, he covers most of what you need to understand. Start with how much power you need, move on to voltage and current (try to limit current). I will disagree with Ron on power requirements. I'd say shoot for at least 70W/lb, but that depends on the model. A Cub could fly on much less power than an Extra. Probably 50W/lb for a floater, 70W/lb for something sport, but not aerobatic, probably 90W/lb for aerobatics, and probably 100-110+ for 3D.

[Ron]

Power requirements are given as a starting point for your models...re watts required for different types of flying, I know fellows who think that 45 watts to the pound is plenty, and I know guys that think anything less than 125 watts to the pound is not enough. you have to try a couple models to sorta get a feel for it.
Guy that won the CDN E scale nationals in 2000 sport scale..............................( I was a judge at this event)...
was flying his large multi motored model on 38 watts to the pound.....After the event, he was doing loops....rolls...inverted...immelmans etc etc.....with this same model.....I fly a pattern model on just about 100 watts to the pound.....most say it will be underpowered, but if you actually fly it, you will find that it has more power than required for the job at hand. NO it's not enough power for FAI routine, but then I'm not good enough to fly it, have no intention of getting good enough, and don't need that type of power system to be competitive in my class. what I do need is to be a good enough pilot, and more power won't make you a better pilot, but practise will..... motocalc/e calc are only tools that give results from information entered.... results are ....GIGO....( garbage in....garbage out ) you really have to know what you are entering, for example....If the program says you will have a max climb rate of 980 feet per min at 40degrees.....do you know what this looks and feels like? ....Have you flown a model with this climb spec? If you are flying a scale model ( cub/WW1 fighter/ most civilian aircraft) ...this climb rate is way over whats required. If you're flying a 3D type, it isn't enough..That's why I mentioned the formula I used in the first post...it WILL work, the models will fly, and you can adjust for personal flying preferences from
there..... I have been flying exclusively electric for a dozen years or so, and these
" formulas" have served me well over likely close to 100 different models of every type... sport/pattern/3D/ scale/on wheels and on floats/ flying boats and every size from 18" wing span to 108" wing span....BUT don't ask me about gliders :-)...I think I last flew one of them in the late 50's or early 60's

[stanger89]

Power requirements are given as a starting point for your models...re watts required for different types of flying, I know fellows who think that 45 watts to the pound is plenty, and I know guys that think anything less than 125 watts to the pound is not enough. you have to try a couple models to sorta get a feel for it.

Yup, a starting point, and one which will depend on what and how you fly. For the record, I'm a member of the "more power is better" club, power can get you out of problems. The trick with electrics is to not sacrifice too much weight in the persuit of power.

Guy that won the CDN E scale nationals in 2000 sport scale..............................( I was a judge at this event)...
was flying his large multi motored model on 38 watts to the pound.....After the event, he was doing loops....rolls...inverted...immelmans etc etc.....with this same model.....

Got any links, that sounds very interesting. Honestly, I'm surprised 38W/lb can do aerobatics, but I am still learning, and as with everything there are exceptions to every rule.

I fly a pattern model on just about 100 watts to the pound.....most say it will be underpowered, but if you actually fly it, you will find that it has more power than required for the job at hand. NO it's not enough power for FAI routine, but then I'm not good enough to fly it, have no intention of getting good enough, and don't need that type of power system to be competitive in my class. what I do need is to be a good enough pilot, and more power won't make you a better pilot, but practise will.....

Sounds about like where I am, I'm pretty good, but non enough to be competitive, and I'm aiming for 100+ W/lb with my Somethin' Extra.

motocalc/e calc are only tools that give results from information entered.... results are ....GIGO....( garbage in....garbage out ) you really have to know what you are entering, for example....If the program says you will have a max climb rate of 980 feet per min at 40degrees.....do you know what this looks and feels like? ....Have you flown a model with this climb spec? If you are flying a scale model ( cub/WW1 fighter/ most civilian aircraft) ...this climb rate is way over whats required. If you're flying a 3D type, it isn't enough..That's why I mentioned the formula I used in the first post...it WILL work, the models will fly, and you can adjust for personal flying preferences from
there.....

I have the perfect example of GIGO with Motocalc. On my journey into electrics, I started with my Sig Wonder. (prepare to laugh) My first power system, based on limited understanding of the conepts and Motocalc, was an Astro 15 spinning (IIRC) a 10x7 on 10x 1700s. I have two words do describe how it flew: "Space Shuttle" The thing flew like a brick, but it did fly. It was a rather nerve racking experience.

Second power system was a Speed 400 geared (don't remember the specifics, something like a 8x6 3:1) on 7 600's. This also flew, but was not enough power nor duration to be fun.

Finally, after my studies in error, I've got a Mega 16-15-3 spinning an 8x4 (IIRC) on 7x1700s. This thing is a blast. It moves out, goes vertical as much as I care to take it, gives good duration. Comes out to be about 210W and roughly 3 lbs, for about 70-80W/lb.

I have been flying exclusively electric for a dozen years or so, and these
" formulas" have served me well over likely close to 100 different models of every type... sport/pattern/3D/ scale/on wheels and on floats/ flying boats and every size from 18" wing span to 108" wing span....BUT don't ask me about gliders :-)...I think I last flew one of them in the late 50's or early 60's

And I agree with most of them (wish I'd followed them more earlier :)). I just think that, especially if you don't have that "feel" yet, that aiming in the 70-80W/lb range is probably a better place to start.

[Daniel G]

This program was created mainly for scratch builders, but I use it to find reletive power specs for all of my models.
http://flbeagle.rchomepage.com/software/webocalc_0.3.0/webocalc.html
Then I use http://brantuas.com/ezcalc/dma1.asp to fine tune the power settings. :)

[Matt Kirsch]

By far the most difficult part is taking those Volts and Amps, and turning it into a useful prop, motor, and sometimes gearbox combination.

How do you choose? One of the main criteria, especially for a beginner in electrics, is what you can find the most information for. When you've got a motor manufacturer that gives you a bunch of vague specifications, and another that provides detailed application charts that show how the motor performs with different props and batteries, the choice is obvious.

[SargentM]

Ill add to what Matt just said.

This is why so many people use GWS gear. You want a motor and prop to have X thrust at Y amps? Its all there on their website. If I want a brushed power setup thats where I look first.

Some forumula: Runtime in mins = (C*60)/A

C= Capacity of battery in AH
A= Current draw of motor.

approximate starting points for wing area for various types of models:

Superlight indoor: 3oz/sqft 9gm/dm2
3D parkflyer 6oz/sqft 18gm/dm2
Sport flyers ~30-50" 12oz/sqft 36gm/dm2
Pylon racers: as high as you dare.
Scale: from 6 to infinity oz/sqft!

[shauntot]

Hi Ron,

My name is Shaun from the Philippines. I just read one of your posts , this one below, and you sound like you're exactly the guy I need to ask for advice.

Power requirements are given as a starting point for your models...re watts required for different types of flying, I know fellows who think that 45 watts to the pound is plenty, and I know guys that think anything less than 125 watts to the pound is not enough. you have to try a couple models to sorta get a feel for it.
Guy that won the CDN E scale nationals in 2000 sport scale..............................( I was a judge at this event)...
was flying his large multi motored model on 38 watts to the pound.....After the event, he was doing loops....rolls...inverted...immelmans etc etc.....with this same model.....I fly a pattern model on just about 100 watts to the pound.....most say it will be underpowered, but if you actually fly it, you will find that it has more power than required for the job at hand. NO it's not enough power for FAI routine, but then I'm not good enough to fly it, have no intention of getting good enough, and don't need that type of power system to be competitive in my class. what I do need is to be a good enough pilot, and more power won't make you a better pilot, but practise will..... motocalc/e calc are only tools that give results from information entered.... results are ....GIGO....( garbage in....garbage out ) you really have to know what you are entering, for example....If the program says you will have a max climb rate of 980 feet per min at 40degrees.....do you know what this looks and feels like? ....Have you flown a model with this climb spec? If you are flying a scale model ( cub/WW1 fighter/ most civilian aircraft) ...this climb rate is way over whats required. If you're flying a 3D type, it isn't enough..That's why I mentioned the formula I used in the first post...it WILL work, the models will fly, and you can adjust for personal flying preferences from
there..... I have been flying exclusively electric for a dozen years or so, and these
" formulas" have served me well over likely close to 100 different models of every type... sport/pattern/3D/ scale/on wheels and on floats/ flying boats and every size from 18" wing span to 108" wing span....BUT don't ask me about gliders :-)...I think I last flew one of them in the late 50's or early 60's


If possible, could I bother you and take a bit of your time, I've just purchased a built up 84 inch Cessna 182 from a fellow flier back here in the Philippines, it's currently running a .90 Thunder Tiger engine. Made of balsa (fuse), foam and wrapped-balsa(wings) and some fiberglass, AUW is around 13.5 lbs. (incl. the .90 engine, 7 servos, RX, etc.)

I'm planning to buy it to convert to EP which would be a nice project, as it would be the biggest EP plane here in the Philippines when done.

What would be a nice power pack for this plane for it to do scale-flying? Some are suggesting around 100 watts per pound would definitely do it (1400 watts), but I would like to get your opinion. I know more power is better, but I don't want to waste good money on it if it's not going to be used. If possible, I know this is going to be a costly project, but could I ask you for your tips - what would be a budget power system for this plane and how many watts are you recommending per pound for the type of flying i'm looking for.

I'm planning to run it on NIMH 3300 GPs. No Lipos as it would go way past the budget.

ESC recommendation would be great as well sir.

And to wrap it all up, if you could also post how much the parts will run and where available online, that would be very much appreciated.

Thank you very much for your time..
Shaun Bondoc
Philippines

[Air-Gar]

Shaun,
What is the wing area of your plane?
Air-Gar

[shauntot]

hi air-gar, sorry but I don't know as this was a scratchbuilt and the guy i bought it from didnt really care, all he wanted to do was flying it.

is there anyway of measuring that (i could get a tape measure and do some math :)

cheers
shaun

[Air-Gar]

hi air-gar, sorry but I don't know as this was a scratchbuilt and the guy i bought it from didnt really care, all he wanted to do was flying it.

is there anyway of measuring that (i could get a tape measure and do some math :)

cheers
shaun

Sure, it's easy.
First measure the wingspan. Then measure the mean chord - this is the distance from the leading edge to the trailing edge. With a Cub it should be the same from tip to tip. Then multiply them. For example, if the wingspan is 50 inches and the chord is 10 inches, the wing area would be 50in x 10in = 500sq in.

As soon as you can get this to me, I'll show you how to design a power system for your plane.

Air-Gar

[shauntot]

Hmm..sounds easy..the wig tapers a wee bit..so where do I measure on the chord? Near the fuse or near the wing tips?

Shaun

[Air-Gar]

Hmm..sounds easy..the wig tapers a wee bit..so where do I measure on the chord? Near the fuse or near the wing tips?

Shaun

measure half way between the fuselage and the wing tip.
Air-Gar