Mutli rotor building and design tips

   I am writing this article as an aid to someone that is looking to begin their first multi rotor build. When I decided to design and build my quad I did tons of research on the web to find answers to my many questions, so I think this would be a great to share what I have learned to give a new builder a place that he can visit and get an overall understanding of what it takes to build one.

 THOUGHTS FOR DESIGN: I knew I wanted to build a quad, my main goal is to eventually build one to use for aerial photography.  I started out with a dromida quad to learn how to fly, but it has limitations on outside flying because it only weighs around 4 ounces and its brushed motors are not strong. However I believe it is a good quad to learn on as it is very durable, inexpensive and replacement parts are easy to find at a local hobby shop or online. 

   I determined that my first build would be around 400mm in size, one that I could use outside, in the wind and still be able to fly.  I also wanted it to be powerful and quick so that I could use it to have lots of fun as my flying skills grow.  My intent is to keep the power dialed down on my radio and the KK board for the first few months, then dial it in faster and more responsive as my flying skills get better.

1)   The first step in design is to figure out what type of multi rotor it is you want to build.

 Quad or tri copter?  large and stable for aerial photography?  small and nimble for FPV and acrobatic flying?

  If you’re like me you probably want more than one flying machine, lol, but you must narrow down what it is you want out of your quad and design it to fit your needs/wants.  You can look at what may be your future use of this machine and design some adaptability into it.

 For example, I wanted my quad to have plenty of power so that I could possibly add FPV to it later on.  I wanted it to be made of wood, so if I crash it I can easily fix it.  Also I wanted to keep the money spent as low so I wouldn’t be afraid of crashing hard.  I didn’t want to fly scared, I wanted to be able to fly knowing if I crash hard and I know I will at some point, that I can repair it fairly easy.

2)      Then decide on the size you want it to be.

    I chose 400mm in size to make it easier to see, as I have old eyes.  I wanted it big enough to easily add things on later as I learn more and want to try other things.  Also I figured this size would help it not be pushed around by the wind.

   250mm quads are a hot item now that people are choosing for FPV and acrobatic flying.  I see some people go large, like a 600 to 700 mm machine.  Choose the size that fits your needs/wants. Keep in mind that the size of your quad will determine the maximum size of props you will be able to use.

3)      Next choose the configuration, type of frame you want.

  Whether you want a quad, hex, x type, knuckle H quad, dead cat, bat bone, which ever type you think looks cool.  There are many to choose from on the KK board. Dualcopter, Tricopter, Y6, Quadcopter +, Quadcopter X , Hexcopter +, Hexcopter X, Octocopter +, Octocopter X, X8 +, X8 X, H8 ,H6, V8.  Choices will vary depending on which version of software you have loaded in your KK board and which FC board you decide to use.

  My next build I will be using the electro hub to make my build, with that base there are many different looks and configurations you can go with from that starting point.  You can use one of the flite test hubs already made or use wood and make a similar hub out of that.  Wood can be found at local hardware stores, hobby shops, and hobby stores like hobby lobby and online as well.  Poplar wood 1/2" x 1/2" x 36" pieces, cut to length are what I used for the main frame.

4)      The next step is to determine your electronics.

    I decided to keep it simple and chose a power pack combo from altitude hobbies that was recommended by flite test.  This gave me motor size, proper props for that motor and proper ESC’s.  I see now that flite test is offering their own power packs for all of their quads and tri copters and this is a real blessing to help someone decide on what motors, props and ESC’s to buy for their design. It will make things so much simpler and easier and we know we will get great customer service from those guys !

 ELECTRONICS SELECTION GUIDELINE:

1) The size of your multi rotor dictates the MAX size props you can use

2) Total WEIGHT of your mulit rotor will tell you minimum size motors needed

3) Use Max Amp draw of your motor + 20% safety margin to determine ESC size

4) max motor amps x (# of motors) + 20% of total amps = C rating of lipo battery needed

 TOTAL WEIGHT:  You should estimate what the TOTAL weight of your multi rotor will be and determine a maximum weight goal.  You can get the weight of the power pack parts from their specs.  Pick a standard size battery from flite test recommendations for the size multi rotor you are building, find out how much it weighs as well.  

   Add up the weight of motors, ESC’s, props, FC board, battery, receiver, frame parts, landing gear and wiring.  Then add 20% to that total for a little insurance.

 Having an electronic scale to weigh your frame and eventually the gear you use will help in this step. As the frame parts you use, what size, how much they weigh, is the unknown at this point. You can build your frame first, or you can weigh your frame parts before you assemble them to determine their weight. It doesn’t have to be exact, just fairly close.

  You can also add into that figure, a little more for larger batteries that you might want to try, or extra weight for a camera or FPV gear, it all depends on how you intend to use your multi rotor.

    I wanted to know my frame weight before I made a final determination of electronics to order. This required some patience on my part I had to resist the urge to quickly build it !  Which I think in the long haul is best, go slow, learn, ask questions, don't be in a hurry, enjoy the whole build process!  

    Quad size dictates the max prop size you can use.  Determine this by the distance you will have between your motors and the distance you will have from your motor to your FC board and the gear in that area, which on a quad is the center of the machine.  I laid out my design size on paper and determined that I could use the 8x4.5 props with no problem, but they would also be the largest size props I could use.

  Total weight of your multi rotor will determine the motor size you will need.

Check out this link for Dr. Kiwi's test data & prop charts for this motor - http://www.flybrushless.com/motor/view/27 - The motor specs referred me to this site which gives me several prop options and info on lift, watts, volts, amps.  I find this very helpful.  

  I know I am going to use 3S lipo based on my motor specs, so 12.6 volts.

Propeller              Volts        Amps      Watts    RPM     Thrust (g)      RPM as % of Kv*V

GWS HD 8x4         10.8       11.9          128     11190        686                     71%

  This chart did not give me exact prop and voltage, but it does give me useful information, telling me at 10.8 volts, the thrust will be 686g of lift.  This seems to be more than what the actual motor specs indicate.  On this website, the data you see is actual data measured by using a RC motor thrust stand.

 DETERMINING PROPS, MOTOR, ESC:  1)  You have determined your max prop size based on the frame size of your quad.

2) you now have a total weight calculated.

3) Ensure the motor has enough lift capability (2.2:1 thrust/lift ratio) and that it will work with your max prop size.  More if you want acrobatic or are going to carry camera gear.

4) Choose an appropriate ESC based on max motor amps , plus 20%, then choose next size up available.

 Example: My goal was not to exceed 1000g

 Looking at my motor specs:

Suppo 2208/14 (28-26) 1450kv Specifications

 ● Number of Cells: 2-3s lipo

● Max Efficiency Current: 5-9A (>74%)

 ● No Load Current (10v): .6A

● Max Current: 12A/60s ● Max Efficiency: 80%

 ● Shaft Size: 3.17mm

 ● Max Watts: 133W

 ● Weight: 36g

● Minimum Recommended ESC: 18A*

● Recommended Model Weight: 200-500g 

  I see the recommended model weight is 200 to 500, so having 4 motors I know that my max lift capability is  2000 grams.  Knowing that hover on a quad is supposed to be around 50% throttle, so mid-range of my motors is 1000g.  This was my max weight goal.

 rules of thumb to build by:

Goal is to hover at 50% throttle

2.2 x weight (of quad or tricoptor) = 2.2:1 thrust /weight ratio, this is the minimum amount of thrust needed to have a well flying machine. So a 1000g quad should have the ability to lift 2200g. 4 motors on a quad, each motor should be capable of producing 550g of lift.

3 x weight of the quad, 3:1 thrust ratio, to have a good acrobatic quad.

  Motor size and max amp draw of the motor will determine ESC size and battery C rating needed. 

CHOOSING THE RIGHT BATTERY:   Motor specs tells me max amp draw of my motors is 12 amps, 4 motors= max total amp draw of 48 amps.  You need this figure to ensure you use a large enough C rating on your lipo battery.

battery size in Mah x C rating = max amp discharge rate

2200mAh = 2.2A

2.2 x 25C = 55A

2.2 x 40C = 88A

  So a 2200mah /25C rated lipo battery is capable of a constant 55 amp draw.  Here you want around a 20% safety zone as well .  20% of 55 is 11 , so a 44 amp constant draw on the battery is very safe.

 When determining the size of your ESC's, you should build in a 20% safety factor as well.

EXAMPLE::  my motor max amps are 12.  So the minimum ESC size for this motor would be 14.4 amps.      Choose the next size ESC you can get, which for me was 20 amp ESC, the power pack that I purchased had a 22 amp ESC.    Choosing a 30 amp ESC would not give any additional power or safety, it would only add extra weight to the quad.  

  Concerning ESC choice, everything I read about ESC's says to make sure they use simonK software on a multi rotor.  As this software gives them the fastest refresh rates available, which means your motors respond to stick input faster.

Ipeaka ESC specs:

Input Voltage: 2-4s Lipo

Output Current: 22A Continuous, 25A Burst (10 Sec.)

BEC Output: 2A, 5.0v

Pre-loaded SimonK firmware; allows use up to 400Hz refresh rates

Extremely low output resistance for efficient operation

Plug n' play -- no programming required

Smooth, linear throttle operation

  At this point you will have decided on the type and size of your mutli rotor. You will have the information you need to ensure your power pack will fly the machine in the manner you want, by looking at the motor/prop/ESC specs, determining their lifting capabilities and using this information in conjunction with the overall weight estimation of your machine. 

FLIGHT CONTROL BOARD:  I chose the KK board based off of much research saying that it is easy to tune because of the onboard screen. You can’t go wrong with this board. In my research I stumbled on the naze board and purchased one of those, as it looked fairly easy to program using a computer and usb port. I do not want to use any board that is difficult for me to program, or has tons of choices to make via free ware software, I want to keep it all as simple as possible to keep the FUN in this adventure.

Eric of flite test has been playing with the naze board and they will be doing articles on it, so I think it will be fun to watch that and try things that they suggest.

   Hopefully I have guided you through choosing your electronics in a helpful manner.  I tried to keep it simple and in order.  When I got to this stage I had purchased all of these items and they were in the shipping process. I had my wood and began to paint and build the frame.  I began to think about how all of this stuff would connect together.  What I realized was that some of these items did not came with all of the connectors and wiring leads that I would need to hook everything together.  I ended up ordering a lot of different things, silicone wire, heat shrink, battery connectors,  receiver wires, FC board wires, to hopefully ensure that I could figure it all out when the parts arrived and have enough stuff to complete the build.

   So I will attempt to take the mystery out of this part of the building process as well, because I could not find a one stop source on the web, that would give me this information.  I ended up getting a lot of things I didn’t need, but this is ok because I do intend to build more multi rotors and perhaps build a few air planes from the flite test store in the future.

  Flight Control Board layout:

  The KK board has the ability to use CPPM, as well as the Orangerx receiver that I use. CPPM is Combined Pulse Position Modulation.  CPPM allows you to be able to use only one signal wire from your receiver to the KK board.  Plug one end into the batt/bind slot on the receiver, the other end plugs into the aileron channel on the KK board.

  Power up the KK board and go to mode setting, scroll down to CPPM and enable it by selecting YES.

This is the type of wire to use to connect the receiver to the FC board.

 Brown = negative  Red=power  Orange=signal

  If your receiver and FC board is not CPPM capable, you will need 4 of these wires for 4 channels on the FC board.  If you will use your radio to control any other functions, you will need more of these wires for the extra channels.

  On the right side of the KK board are the motor pins.  Here you will plug in the wires from your ESC’s.  Label the wires from each ESC according to your type of multi rotor and how your FC board numbers the motors, then plug them into the proper slot of the board.  The KK board gets its power from the red wire on M1, on the other ESC wires you should remove the red power wire and tape them around the other wires to keep it from shorting out.  This eliminates any electrical interference that may come from these wires into the KK board.  It will not damage the KK board if you don’t remove them.

 Black wire = negative  Red=power /5 volts  White=signal   This is the lead that comes from the ESC that plugs into the KK board.

  Most of the ESC’s used on multi rotors have a BEC (battery eliminator circuit), which supplies 5 volts @ 2 amps to power the FC board.  ESC’s are pretty amazing devices.  They take the DC voltage from your battery and convert it to 3 phase AC power to run your motor.

  BUZZER plug:  the KK board comes with a buzzer with a that has a JST connector. The buzzer gives you audio cues as to the status of the board when powered up as well as acts as the low battery alarm.

  The red port on the KK board is where you plug in wires from your main power spider rig or the power distribution board. Here you need a JST connector as well.  You can buy a cable that has the jst connector on it or solder one on wires and make it yourself.

   This is the completed wiring on my quad.

  And this completes the wiring of your receiver and KK board.  This was very confusing to me at first for many reasons, mainly I didn’t know the names of the wires or the connectors so it was hard to determine what I needed to buy.

 Power Distribution :  I purchased a power distribution board for this build, it turned out to be faulty.  Luckily I found this out during the motor testing phase on the table.  One of my motors stopped working while I was balancing them.  I decided to wiggle the main wires on the distribution board to see if the fault was there and sure enough I completely lost power.  So if you’re not using a high quality distribution board, such as the electro hub, I would recommend doing a spider rig.

https://www.youtube.com/watch?v=Tn2Nt3sjsI0   This video will show you how. I watched this video and did this after I took out the cheap Chinese distribution board that I used at first.  I would recommend using the same size wire as the wires on your battery, for the connection to your battery.  And then use the same size wire that is on your ESC for those connections.  Same goes for anything else you will be powering from the main battery voltage.  I used silicone wire , which is what store bought electronics comes with and it is easily purchased online.   And get some heat shrink in various sizes to go with the wire.

Spider rig wiring

Basic wiring layout showing connections bewteen ESC’s, battery , FC board and reciever.

  TOOLS NEEDED:   This brings us to soldering and a soldering iron.  There is a good flite test video on soldering. They recommended getting a good soldering iron and I read it in other articles as well.  I have done soldering in the past with a cheap soldering iron and it usually turned out to be a frustrating event.

 Rcmodelreview on youtube has a great video on soldering as well.  I ended up purchasing a soldering station to use instead of the pencil type irons I have previously used and I can say that it is well worth the money !    You can find a decent solder station for $50 to $75.  

  I purchased this one from Radio Shack and it makes a huge difference in being able to solder fast and well.

 I recommend getting one that has a temperature dial that goes up to 800 or 900 degrees F.  I use mine at a temp of 750 degrees or so and everything flows well.  Use 60/40, lead/tin rosin core solder and some rosin flux as indicated in both videos.

  Prop Balancer :  you will need to be able to balance your props to eliminate vibrations and allow your motor to run as efficient as possible. Un balanced props can over heat your motor and cause stability issues.

  I use Gemfan 8x4.5 nylon props.  They are inexpensive and fairly durable.  They do break in certain crashes which is actually a good thing , better to break a prop than bend a motor shaft or tear up a motor.  You will need lots of extra props and I recommend having an extra set of motor shafts as well.

I purchased a Top Flite Power point prop balancer and it works very well.  It uses magnets on the side to hold the shaft that you use to balance the props.

  I recommend that you balance the shaft with the rubber cones that are used to hold the prop on.

    Push the cones together on the metal shaft with no prop on it, then spin it.  It will spin freely and fast at first, then as it comes to a stop you will see that the cones have heavy sides to them like a prop.  Sand the cones on the heavy side , while still on the shaft, using 100 grit sand paper.  Keep checking it for balance and sanding the heavy side until it balances out fairly well.  It doesn’t have to be perfect, but you want it to spin slowly towards any heavy side it may have.  A fast spin around indicates you need to sand some more.  Once you get it balanced, draw a straight line across both cones with something you can see and will be permanent.  I used red finger nail polish, and no it wasn’t mine, lol.  Then when you put a prop on, you align the lines on the cones up to each other each time to ensure the shaft remains balanced each time.

  There are many videos on how to balance a prop so I wont go into detail here.  Since the props I use are fairly cheap I wont spend 20 minutes balancing them.  I start off sanding the top side of the heavy wing, once I get the prop to turn slowly to the heavy side I apply tape or heat gun glue to the light side wing.  Scotch tape works well for this, I prefer to use a thin strip of hot glue on the under side of the light wing.  If I get too much on it I scrape it off with my thumbnail.  Once you have balanced a few props it becomes fairly fast and easy to do.

  I also balance my motors like I saw on a youtube video. https://www.youtube.com/watch?v=RItntpZZH4g  In this video the guy recommends a phone app that you can purchase to use to balance your motors.  I found a free app that I use and I get very good results with that.  I used pieces of black electric tape on my motors as weights.  Then I also purchased some small stickers at walmart to use the next time.

  Naturally you want to balance your motors WITHOUT your props on !  I have seen a video where a guy balances his motors with props spinning, I would not do this ever, your taking to big of a chance of getting hurt and tearing up stuff. 

  In time I want to build a small stand where I can mount a motor to it and my phone and get them finely tuned.  Im not so sure that they need to be finely tuned, but I feel they will run better, more efficient and cause less vibration for your FC board and other electronics on your machine.  Also if you put some sort of rubber material under motors when you mount them, it help nullify vibrations as well.  It depends on how you mount your motors.  I use wire ties, so its easy for me to do.

  Battery Charger :  This is a must if you are using lipo batteries.  Read flite test articles and watch the videos on lipo battery storage and charging.  This is one area you cannot be lax in because of the existing danger that comes with lipo batteries.  I highly recommend storing your lipo batteries in lipo bags and then inside of ammo boxes.  In time I hope to have a battery bunker like the one on the flite test video.

  I purchased this one from Motionrc and I purchase all of my batteries from them as well.  I wanted one source for my batteries because they all come with different type connectors on them, (each store uses different connectors)and I didn’t want to have to worry about changing the leads on various batteries.  This charger comes with mostly everything you need to charge lipo batteries. Plugs into the wall, here in the USA.     Has clips that can be used on a car battery in the field to power it.  The leads are not expensive and you can purchase the connectors and make your own if you want to.

  Be sure a check out many articles about lipo charging, storage and safety. Other than what I have already stated I would recommend to always give them storage charge after returning from a day of flying. The charger has a setting for that and it doesn’t take long because you should stop flying your machine when the battery voltage gets to 10.7 to 11 volts. I set the battery alarm on the KK board for 11 volts. This gives me a minute or two to safely fly it back to me and land without having to rush.  If your FC board does not have the battery alarm feature, you can purchase one and it’s a simple install on the machine.  Most RC radios have a timer that you can use as well to aid in ending your run before you do damage to the lipo, which happens if you go below 3 volts on any cell.

 Radios and Receivers :  I chose to use the Orangerx transmitter and recievers.  They are inexpensive and reliable.  I got mine from Hobbyking.  The shipping cost is kinda high, but you can load your order up with other items you need and get all you can for the shipping price.  It changes with the total weight of your order, I don’t know the levels of change, I just added stuff until the shipping price went up ,then withdrew the last item.     This way you can take full advantage of the cheaper prices that hobbyking offers.  Be aware that it will take around 4 to 6 weeks to get your items from them, so get the order in early and you can be building your flying machine while your waiting for the radio to come in.

  There are many different opinions on what radio to get for your first purchase.   I chose this because of price, features,simplicity,6 channels and other options it has.  You buy it and use it , no need to update its software or change software or change modules, just learn it and use it.  It took me a day or two to gain an understanding of how to use and setup the switches and dual rates and expos and such, but it finaly clicked in my head and now its very easy and intuitive to use.  It does everything I need it do.  Later on down the road if I need more features from a radio, I will have more RC knowledge and can make a purchase that is right for me at that time.

  I decided to setup the throttle kill switch and as it turns out it is a very good thing to do !  On the maiden flight I would hover and move the quad around a bit to see how it flew, sit it down, walk over and pick it up and change a setting, then try again.  Using the throttle kill button ensures you don’t throttle up your motors accidently when picking it up and it keeps you from having to constantly unplug the battery when picking it up.  Setup a throttle kill switch for your flying machine !  On the OrangeRx radio, on the settings list choose the throttle kill option, then activate it.  There is a switch on the top, left side designated for throttle kill, easy as pie.

  List of various tools and supplies needed:     Small screw driver set , small nut driver set, cheap digital calipers, magnifying glass, can of air, wire cutters, exacto knife, razor knife, black electrical tape, helping hands soldering stand, black sharpie marker, hot glue gun and glue, double sticky tape, velcro and velcro straps, 4” to 6” black wire ties, CA glue, extra electrical connectors and wires, scissors.

   These are items that I keep in a storage box that are dedicated to my RC hobby.  Most of these items were purchased at local flea markets and were inexpensive.  Also I have a small back pack to carry tools and parts that I may need when I go flying.  So hopefully I can make any needed repairs and continue flying.

  Building Tip:   You can use dollar store clothes hangers to make useful things for a multi rotor.  I used them to make my landing gear.  You get 6 hangers for $1, plastic is light weight but sturdy.  Cut to length, bend after heating it mildy with a lighter, hot water, heat gun if needed.  It glues well with a hot glue gun too.  It doesnt hold paint very well as is, try roughing it up with light sandpaper.

  Blade Guards I modifyed and landing gear that I made from plastic coat hangers

  I hope this helps someone in their quest to build a multi rotor machine.  I have tried to put a lot of the information needed to aid in building here into one article in hopes to save people some time of researching so much on the internet. 

  Here is my Quad after a few changes I have made. The blade guards didnt look good or work as planned.  I used plastic coat hanger pieces on all sides and created "tip over guards" to keep from breaking props when landing a little roughly.

   It is very gratifying to fly a machine that you thought of, designed, and built on your own, I highly recommend it !  You can get the quad you want that fits your budget. Repair it easily when needed or modify it later on down the road.

 Thanks for reading my article !

  Happy Flying !