How to choose brushless + esc + prop + lipo

by vuoladodo | October 1, 2013 | (0) Posted in Projects

Hello everyone.

I build scratch planes. This winter I plan to build a foam Polikarpov Rata and a bunch of the swapables though probably without swapping pods. After flying for years and buying motors and esc on recipes online I am looking for some fairly easy way to choose the entire righ. I would really really really like some step by step advice.

Thanks in advance.

I have two examples of planes for which I need engines:

This is a foam version of the GP Slowpoke. It is now running on a RC A2212 2200KV Outrunner Brushlessmotoren w/30A Regler. The prop is a foldable 7.5/4 and it usually runs on a 1250 11.1v.

I am not very happy with the performance of this plane. I flies well but if has no pop and it sounds like a fly. I have at least 4 or five other planes that need to be rerigged.

This is a small foam j3 typeathing. It is basically finished. It has a 1m wingspan.

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COMMENTS

vuoladodo on October 15, 2013
Hey guys. Thanks a lot. I was away so I did not get to see all the responses till today. To be honest, I few RTFs and that made it easy to deal with motors, etc. Then I flew glow which I honestly still love. My entirely unscientific explanation is that I like the way that heavier planes behave in the air. For instance, between the spitfire and the racer that these guys built, I find the racer and the cruiser to be exactly the kid of thing I like to see in the air. Nitro planes have a tendency to fly more heavily and for that reason, I think, to track better. With electric I am always in a guessing game. So I will try to put this thing together step by step.
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bacibear on October 23, 2013
WebOCalc is a free program that can really help. Ever since discovering Webocalc, I have used it for all my planes with great results. If you enter a few numbers into WebOCalc (things like wingspan and chord, the weight of your model, the battery voltage, and the desired current draw), the program will tell you how the model will fly, in simple English. It will also suggest some propellers that will work with the model, and appropriate gear ratios matching each propeller. For direct-drive, pick the propeller(s) with a gear ratio near 1.0
http://flbeagle.rchomepage.com/software/webocalc_1.7.6/webocalc.html
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RedBaron1 on October 14, 2013
Step 1: You can determine the power requirements of a model based on the “Input Watts Per Pound” guidelines found below, using the flying weight of the model (with battery):

• 50-70 watts per pound; Minimum level of power for decent performance, good for lightly loaded slow flyer and park flyer models
• 70-90 watts per pound; Trainer and slow flying scale models
• 90-110 watts per pound; Sport aerobatic and fast flying scale models
• 110-130 watts per pound; Advanced aerobatic and high-speed models
• 130-150 watts per pound; Lightly loaded 3D models and ducted fans
• 150-200+ watts per pound; Unlimited performance 3D models

Step #2: Divide the total desired watts by battery voltage to determine necessary amps.

Watts / Voltage = Amps

Step 3: Determine the Voltage to /Currant ratio by dividing the given Amp by the Voltage.

Amps / Voltage = V/C Ratio

Voltage/Currant ratios should be fairly low; 2 or 3 to 1 being ideal.

Examples: 900w / 11.1v = 81a 7:1 ratio
900w / 14.8v = 60a 4:1 ratio
900w / 18.5v = 48a 3:1 ratio
900w / 22.2v = 40a 2:1 ratio

Step 4: Determine the battery discharge rate (C-rating) by divide the desired Amps by the battery pack capacity, expressed in Amps (A).

Required amps / Capacity (A) = C-discharge rate

Example: 40amps / 4 (4,000mAh) = 10C

Step 5: Determine the Max. full throttle flying time, full discharged battery time by dividing 60
minutes by the C-rating.

60min / C-discharge rate = Max. full power time

Example: 60min / 10C = 6 minutes

Step 6: Determine the 80% discharge flying time by multiplying the Max time by 0.8.

Example: 6 minutes x 0.8 = 4.8 minutes.

Clear as mud?

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JasonL on October 14, 2013
RedBaron1: Thanks very much for this! Of all the reading and searching I've been doing about motor/esc/prop/lipo combinations, this has been the most helpful. As a kid, I flew fuel/glow motors and am getting back into the hobby now that I've discovered the affordability of the electric motors. I'm waiting for a shipment from HobbyKing to start on my first FT swappable.
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RedBaron1 on October 14, 2013
The information comes directly from Lucian Miller with Innov8tiveDesigns (http://www.innov8tivedesigns.com) and All Things That Fly Podcasst (specifically episode #207) (http://www.allthingsthatfly.com/)

The math, and path through it is good. A lot of guys will argue that you NEED to start with the proper size prop, but that's easily taken into consideration once you runt the numbers and actually start looking for specific motors.

I have a worksheet in Excel that does all the calculations automatically, and allows you to try different options to compare and find an efficient system I make the worksheet available through email, just send a message (redbaronrc@gmail.com) asking for the e-power worksheet.
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chaydock on October 12, 2013
it's a little bit on the advanced side, but I would recommend paying around with eCalc.
http://www.ecalc.ch/motorcalc.htm?ecalc&lang=en
If eCalc is all gibberish to you, then go to this article about picking props/motors.
http://oddcopter.com/2012/02/06/choosing-quadcopter-motors-and-props/
It's geared towards quads, but down lower on the page is a series of links for "understanding the science" and the articles it links to cover all the things you'll probably want to know about motor construction and ratings to set you on the right track and how to better use eCalc.
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chaydock on October 12, 2013
Oh... one word of warning when using eCalc... don't get wrapped around the axle about static thrust numbers. Unless you are doing 3D and need to hover, your plane will only need to worry about static thrust during takeoff. When you fly through the air, actual generated thrust will in all likeliness be lower. And, the higher the rate of speed, the steeper the pitch that your pop needs to be in order to take sufficient bites out of the air at that speed. You mention you are running a 7.5x4 prop now... that's not a very steep pitch if you are trying to fly really fast. So, how steep of a pitch do you need???? Well, eCalc won't tell you that. A lot of variables come into play which are very plane specific and would practically need a full render of your model to work out properly. So, it comes down to a guessing game at that point. That's where talking to other's who fly the same planes comes in really handy. But, eCalc at least gives you a good starting point to work from.
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webbhm on October 12, 2013
A general rule of thumb I have seen (and use):
50-75 Watt/lb - glider or trainer
100 Watt/lb - aerobatic
150-200 Watt/lb - 3D

Calculate your total weight (battery and motor included), and figure what type of plane you want, then determine your watts. Remember, you can always throttle back, but you cannot make a weak motor give more power (when in doubt, use more power and deal with the weight).


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alibopo on October 13, 2013
Absolutely - that's it. Forgot I had this sitting around on my desktop - here's the same info in metric with a few more in-between details.

50-70 watts/pound: 11-15 watts/100g Minimum level of power for decent performance
70-90 watts/pound; 15-20 watts/100g Trainer and slow flying scale models
90-110 watts/pound: 20-24 watts/100g sport aerobatic and fast flying scale models
110-130 watts/pound: 24-29 watts/100g Advanced aerobatic and high-speed models
130-150 watts/pound: 29-33 watts/100g Lightly loaded 3D models and ducted fans
150-200+ watts/pound: 33-44 watts/100g Unlimited performance 3D models
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alibopo on October 11, 2013
Hi, I don't like those fast revving motors either. Very generally speaking - the slower the motor the bigger the prop. But even more basic than that is the actual power of the motor. It all comes down to the amps and the volts - multiply those together and you'll get the Watts (energy available for a given interval of time) - forget about KV that's just the speed the motor can turn at - it's all about those watts. The Blue Wonder can handle about 8 amps (amps is current - flow of electricity) on a 3S (11.5v) battery that's 8 x 11.5 = 92 watts (in a perfect world) probably more like 80 watts. A Blue Wonder will just manage to fly a plane of about 500g, but you'll not have much performance. And if you don't want planes that sound like a fly buzzing choose a motor that runs about 1200-1500 KV and put a bigger prop on it - 8 or 9 inches. Weigh your plane - remembering to include extra for battery and motor - and choose a motor that has enough POWER (Watts) to drive it through the air. A lot of motor sellers specify the model weight range of their motors "suitable for planes 200-600gms" etc. The one your holding up will probably come in about 500-600 gms so you want a motor of about 12-14 amps or a little bigger. That should give you plenty to get it moving. Trawl through the Flite Test articles on prop adventure etc. and you'll start to get the hang of things. At the end of the day, there's no hard and fast rules, but there are combinations that work. Check out the Flite Test builds and compare them to your model - there's some motor/prop/battery/ESC combinations they advise for these builds - they're as good a guide as any. Is yours a light model or will it need the 'beef' combo? Hope that helps. Regarding those fly-buzz motors, they tend to go on pushers that don't have much room to swing a prop, or on smaller scale models where a larger prop would just look ridiculous, and probably hit the ground if the model has scale landing gear.
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chaydock on October 12, 2013
I totally agree that Watts fly planes, not kV. However, you have to pair it with a prop that will take advantage of the available power too. In the latest FT pod cast (#3) David and Josh talk about the difference between Gas and Electric motors and how they produce power differently. They also cover what happens when you put too big or too small of a prop on either of those power plants.
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How to choose brushless + esc + prop + lipo