Droning On: The Anatomy of A Drone

 

drone3

These last few weeks I’ve been ordering parts for the Hackaday Testbed, a basic quadcopter to be used here at Hackaday. The top question I see when surfing multicopter forums is “What should I buy”. Which frame, motors,  props, speed controller, and batteries are best?  There aren’t easy answers to these questions with respect to larger quads (300mm or more) . There are a myriad of options, and dozens of vendors to choose from.

Advice was simple in the pre-internet days of R/C planes and helicopters: just head down to your local hobby shop, and see what lines they carry. Hook up with a local club and you’ll have some buddies to teach you to fly. This advice still holds true to a certain extent. Some hobby shops carry the DJI and Blade lines of multicopters. However, their flight control systems are closed source. If you really want to dig in and adjust parameters, you have to either buy a combo package with an open source flight control system, or buy every part separately. Unfortunately, very few local hobby shops can afford to stock individual parts at that level.

In the online world there are several “big” vendors. The classic names in the USA have always been Tower Hobbies and Horizon Hobby. Some new US-based companies are All e RC and ReadyMadeRC. Several Chinese companies, including HobbyKing and RcTimer, maintain warehouses in several parts of the world. I’m only listing a few of the big names here. If I’ve left out your favorite site, drop some info in the comments section.

The killer with many of these companies is supply. A popular component will often go out of stock with no hint as to when it will be available again. When it comes to single parts like batteries, it’s easy to just order a different size. But what about motors or speed controls? These components need to be matched on a multicopter. Changing one for a different model means changing all of them, so it pays to buy a spare or two when ordering! Click past the break for a breakdown of some multicopter parts.

Motors

HK 28-30s Brushless MotorPower systems have been a tough problem since the early days of radio controlled planes. Picking the right power system is a lot like picking the correct microcontroller or op-amp for a circuit design. There are a seemingly endless number of parameters to be taken into account. Once you break it down though, it isn’t too hard.

Motor part numbers are often encoded in terms of stator or can sizes. A DJI 2212 motor means it has a 22mm diameter stator which is 12mm tall. This isn’t a hard and fast rule though, so don’t live by it. The first and foremost parameter in a motor is KV - Thousands of RPM per Volt. Theoretically, an 800KV motor will turn 800 RPM on a 1V power source. A 14.8V 4S LiPo battery will spin the motor at 11,840 RPM. It’s important to remember that this is a no load rating. The motor’s RPM will drop significantly when it’s spinning a prop.

The next number to look at is the voltage the motor is rated at. Sometimes this value is represented in volts, and sometimes in cells, which usually refers to the 3.7V nominal voltage of LIPO cells. A motor capable of handling a voltage of 4S means it’s good for 4 LiPo cells in series, or 14.8V.

For the Hackaday Testbed, I went with the NTM Prop Drive 28-30S 800KV / 300W motor. These motors are low cost, and should have plenty of power for our quadcopter.

Speed Controls:

In the old days, R/C plane throttles were controlled by a servo moving a switch. It was full throttle or nothing. Thankfully those days are gone, and we have cheap MOSFETs around to give us digitally controlled throttles for our brushless motors. Speed controls are generally rated by maximum voltage and current. Figuring out which to buy is simply a matter of matching one to the motor you plan to use. Sometimes manufacturers overstate their speed control’s capabilities.  Therefore, it’s often a good idea to leave a bit of overhead, lest you let out the magic smoke.

One more parameter to look at is SimonK firmware. Simon Kirby figured out that many cheap speed controls were running Atmel microcontrollers. He reverse engineered a few boards and wrote his own open source firmware with features like soft start, calibration, and easy parameter updates. Make sure the speed control you choose is compatible with SimonK firmware. Even if it is compatible, check how easy it is to flash the controller. Some controllers have easily accessible testpoints, others require soldering directly to the micrcontroller.

For the Hackaday Testbed, I went with the HobbyKing Blue Series 30Amp controller.

Batteries:

LiPo Battery AssortmentMuch like speed controls, batteries will depend on the current and voltage requirements of the prop and motor combination. Lithium Polymer battery packs are sold with three basic parameters: Cell count, capacity, and maximum current. Size and weight are also important – don’t buy a battery so big your quadcopter can’t lift it!

3S and 4S batteries are common in multicopters these days. Maximum current is usually stated in therms of “C”, or the capacity of the battery. A 4000mAh battery with a rating of 45C is stating that it can supply 4 amps for 1 hour, or a maximum current of 180 Amps. It goes without saying that drawing  180 Amps is not a good idea for the overall longevity of the battery.

For the Hackaday Testbed I went with Zippy 35C constant / 45C burst 4S 4500 LiPos.

Propellers:

Multicopter Propellers can confuse even veteran pilots. Props are rated with two numbers. 9×6, 8×4, or 10×4.7 are all common propeller sizes. But what does it all mean?

The first number is diameter. The second number is pitch. Diameter is simple. A 10×4.7 propeller will have a diameter of 10 inches. Pitch is a bit harder. The pitch number of a propeller describes how far the propeller will move forward in a single revolution. If you spin a 10×4.7 propeller it will theoretically pull your plane (or quadcopter) forward 4.7 inches. A 10×6 propeller would pull forward 6 inches. Things get even more complex – how different is a 10×4.7 vs a 9×6? The answer is not much.

For the Hackaday Testbed, I’m going to test several propellers, starting with a conservative 10×4.7.

Simulating the drive system:

While propeller and overall drive system performance can be calculated by hand, a computer can be a big help. There are several software packages available to aid with propeller and drive system selection. eCalc is a website based calculator with a free trial and a subscription pay version. Make sure you select the correct calculator, as they have different versions for planes, helicopters, electric ducted fans, and multicopters. A second option is MotoCalc, which is downloadable software. MotoCalc also has a time limited trial. Freeware options are PropCalc and DriveCalc.

Nothing beats real world tests though. Years ago, the Astro Wattmeter created a small revolution in the market. The Wattmeter plugs inline with the battery and measures total current draw and voltage of the drive system. From these values it can calculate power, mAh used/remaining and several other values. One thing to remember is that a propeller will perform differently on a test bench (Static thrust) vs in the air (dynamic thrust).  To this end, there are on-board Wattmeter style systems which can either send telemetry data back, or store it for later download.

If I only had a frame

X650F quadcopter frameMulticopter frames come in a multitude of shapes and sizes. The most basic difference between them is the number of motors they carry. Tricopters use three motors in a “Y” formation. A servo tilts the motor at the bottom of the Y to achieve yaw control. One nicety of the tricopter configuration is that all propellers turn in the same direction.This means standard airplane props can be used all around.

Moving up the chain is the quadcopter, which uses four motors and props. Quads (and all sizes beyond tricopters) use matched sets of clockwise and counterclockwise rotating propellers. When all four motors are rotating at the same rate, yaw due to torque effect is canceled out. Increasing throttle to one set of motors while decreasing to throttle on the other set allows for yaw control.

Moving beyond the basic multicopter types, we have the hexacopters and octocopters. The advantage to Hex and Octo format is of course more power! Hexes and Octos can lift more, and in some cases can keep flying if a single motor fails. Hexactopers can be laid out with six arms from a central point, or they can be in the Y-6 formation. Y-6 is essentially a tricopter with stacked counter rotating props at each point of the “Y”. Similarly octocopters can be an 8 pointed star, or an X-8 formation. Y-6 and X-8 formations offer reduced weight due to less arms, however stacked propellers are always somewhat less efficient than two propellers operating in clean air.

Another decision to make is materials. In this case the sky is basically the limit. Multicopter frames can be as simple as a couple of crossed sticks or as complex as interlocking plates with tubes of woven carbon fiber. Kits can be purchased, or designs can be hand made. If you’re scratch building, the local hardware store is often a great source of parts. I’ve seen everything from towel rod to PVC pipe pressed into service as part of a multicopter frame.

The Hackaday Testbed will have a few frames, but I’m starting out with an aluminum and fiberglass model from HobbyKing.

Flight Control System:

Stay tuned for this one. Flight control and radio control systems will get their own columns. Suffice it to say, we’re going to try several, starting with the MultiWii and the KK2 multicopter boards.

Wow, this turned out to be quite the long post.  Until next time, keep on Droning On!

Title Photo by [Alexander Glinz]

Comments

  1. greenbacks says:

    I got a hex carbon fiber frame with flip flight controller, just need the time to put that bad boy together.

    By my estimates should be able to lift at least 6 kg payload.

    ;)

  2. HV says:

    Nice intro article with lots of detail in the right spots.

    Tricopters can run with all three motors going in one direction and having 3 standard props as you mentioned, or you can have one motor reversed and use a CW rotating prop on that motor. It reduces the overall torque that the tail motor/servo combo has to counter-act. With all three motors going the same direction the tail motor will constantly be tilted to one side to counter the torque, with the tri only dealing with torque from 1 rotor it will have to tilt much less and equalize control throw in both directions a bit.

    Some of the mini quad and hex frames are pretty powerful, fast and tiny, and they’re great for doing FPV type flying in tight places and small yards, it’s worth looking at frame sizes which match your requirements as well. Look at some YT vids for the QAV250 or BlackOut quads and hexes or some of the other models to get an idea for the little ones.

    I’ve built a Tri, Y-6, Hex and Quad and they all fly great with tuning. The tri with a Afrotech Naze 32 was by far the cheapest. In your section flight controllers It’d be nice if you could cover what the PID loop is, how it works, and how it affects flight, but in detail with quality diagrams/drawings. It’s kind of the black magic section of multi-rotor flight controllers that would help a lot of people.

    • Adam Fabio says:

      It sounds like the tricopter setup would be slightly more efficient with the tail running neutral. I really have to build one up this summer.
      I’ve been drooling the BlackOut and similar mini-multicopters. Those things look like a heck of a lot of fun with a good FPV setup. talk about nape of the earth flying!

      I’m definitely going to cover PID loops and tuning, as well as what happens when the loops aren’t well tuned. That should be an interesting video…

      • SirPoonga says:

        Look at what the microcontroller supports. Most support a tricopter with the single reverse prop. There are some, especially older versions, that only support all normal props. Actually, I should have a Fortis Airframes tricopter sitting at my mailbox when I get home. Can’t wait to try out a tricopter.

        I will be trying the MultiWii AIO with MegapirateNG APM firmware and the Naze32. APM supports the new X series FrSky telemtry when used with a Teensy 3.0 hack to translate MAVlink to FrSky S.Port. The MultiWii code in the Naze32 can support the older D series FrSky telemetry. I am going to see which way works best right now. I like the fact that the MultiWii AIO can either be flashed with MultiWii firmware or APM firmware. I can try both out and see what works best for that microcontroller.

        I will be using SunnySky motors and Afro ESCs.

        I have a FrSky Taranis for my Tx. What Tx is the test bed using?

        • HV says:

          You don’t need to reconfigure your flight controller or have ‘new’ versions to reverse one of the rotors on a tri-copter, it just reduces the torque the tail has to compensate for. The FC doesn’t know the difference, it just works less hard on yaw stabilization.

  3. Spork says:

    Total cost looks like ~ $180 for this build. In case anyone else was wondering.

  4. peabody124 says:

    I’d recommend checking out the Tau Labs flight controllers (there are a number that run the code). The flight performance is great and it is all open source. Of course I’m biased – I’m a developer there.

  5. MS3FGX says:

    Will be interested to see more details on the MultiWii. I’ve been considering building a quad recently, and really want to use a controller that’s open source.

    • Mike Szczys says:

      Do you have any quads at all? Wonderful wife of mine got me a Syma X1 for my birthday because I’ve been lamenting for years about the quad build I want to do. I think it’s great to get a bit of flight time. It really gets me thinking about hacking this one as a way to ease into a more complex build.

    • Adam Fabio says:

      You’re in luck, the MultiWii is one of the first board which will go on the Hackaday Testbed

      • mastrogippo says:

        Having started with a KK, moved onto a Multiwii and then to an APM, I’d say you should skip the Multiwii and review the APM directly. It is far more advanced and flexible, at a similar cost (both for clones and for the original).

  6. Farbro says:

    This is exactly the level of detail that is needed for someone like me. Enough detail and overview to let me know what I’m looking at, and give me a toehold for further research so I don’t spend all my time googling basic terms!

  7. TechnoMan says:

    Had a horrible crash with my XAircraft 650 quad. Complete disintegration. Just rebuilt it using SK-enabled ESC, KV750 motors, NAZA 2 motor controller/GPS, but using the original aircraft frame (replaced some legs with CNC machined aluminum). It’s very stable, very fast and takes great videos (using a Gold gimbal) on the GoPro. Fatshark FPV makes it seem like I’m in the quad. Wicked. Building your own is very rewarding and saves money.

  8. terrapin says:

    Have a look at the Afro series of ESC’s. Pre flashed with SimonK and they a specifically designed for Multi rotors. Pre configured so you can just plug and go. They also allow firmware updates via the servo cable so they require absolutely no soldering.

  9. Lazyzero says:

    Great article. This could be a very good point to start with multirotors.
    But just one point on the flight controller, don’t use the old kk board it is not really state of teh art any more go with the KK2 or multiwii. They are both open source software and for the multiwii also a kind of Open Hardware.

    If you like I can contribute to the column about the KK2 board and ESC flashing with SimonK firmware. I am the author of the kkmulticopter flashtool, which is the main used software to flash the ESC and KK boards.

    Best Christian aka Lazyzero

  10. Marty says:

    Quadcopters are brilliant fun! I started off with a little hubsan X4, which didn’t last long!
    I was flying in my backyard, I went a little to high & the wind caught it, lifted it straight up in the air because its sooo light and away it went! that was that lost,

    soon moved on to buying an x525 frame with all the bits & bobs with it, (70bux on ebay minus the radio gear) including the kk multicopter board! which only flys in acro mode ( no stabilise or anything ) the full build log is on my website…

    http://www.martin-gardner.co.uk/x525-quadcopter-build-setup-test/

    I’ve since moved on again to an Alien 500 FPV frame along with the open source Multiwii Pro AIO (All In One) from hobbyking! I have to say, running megapirate version 3.0.1r4 was the turning point for me….. the options & parameters are vast to say the least! along with the ground station software (APM Planner 2.0 or Mission Planner) like most new things the learning curve is there, but its not so bad!

    The GPS return to home feature works great when you get in a tight spot!!
    just remember to change a few parameters before you try it, ie: setting the height the quad will go to when RTH is activated, the speed it returns to home at! the loiter time it spends above the home position before it lands etc…

    With megapirate being open source also, the amount of features & flying modes you get are amazing! the guys have did an amazing job with it,

    If money is not an issue, then I would recommend the PixHawk flight controller, its possibly the most advance piece of kit on the market from the guys that brought you the original APM flight controller, there already working on a new piece of kit which is a 32bit system running linux! :)

    Good luck with the build… :)

    • Adam Claassen says:

      +1 for the PixHawk based FC. NuttX (A POSIX compatible RTOS (with C std. lib. built in) for embedded devices) is much easier to deal with than interfacing a BBB-esque controller with something that can handle time sensitive loops, pushing the commands back and forth through an ungodly number of level shifters.

  11. arachnidster says:

    “The first and foremost parameter in a motor is KV – Thousands of RPM per Volt. Theoretically, an 800KV motor will turn 800 RPM on a 1V power source.”

    Shouldn’t that either be “RPM per volt”, or an 8KV motor?

  12. kerimil says:

    I am totally building of getting one. I already have an RC plane and I plan to equip it with ardupilot/pixhawk one day but quadrocopters are awesome too

  13. Pedant says:

    s/a myriad of/myriad/g

  14. geebles says:

    Multiwii is great, been using it on all mine for nearly 2 years now! Especially if you’re familiar with arduino (it has moved over C headers and CPP files now, i use Visual Micro (a plugin for Visual Studio) and have to say its really great! Also consider the ‘Naze32′ (NOT naza) which is an ARM port of multiwii (just bought one, not tested it yet)

  15. debuti says:

    Whoaa!! Dude this is like the most explained post ever. Thanks a lot for all the info researched here!!
    Next time i would apreciatte more info on the rc radios
    Thanks!!

  16. staticdet5 says:

    I really appreciate this article. A good primer on some of the basics. I’ve wanted one of these for awhile. Given the cost and complexity, I really wanted to build one myself but didn’t know where to start. This article is a nice breakdown of material I need to get started.

  17. Andrew says:

    “But what about motors or speed controls? These components need to be matched on a multicopter. Changing one for a different model means changing all of them” — that’s not really true. The flight controller has to account for different throttle characteristics on each arm anyway, so it doesn’t make much difference. This is due to many factors (wear, the location of the center of gravity, …). Mine flies just fine with different size propellers or differing motors on each arm.

    It does pay to buy spare ESCs though. This is mainly due to the research&development cost (even in a hobby project), especially if you’re going to customize the flight controller code or the SimonK firmware code. Better to only have to do it for only one specific model.

    +1 about AfroESCs, they’re a generation further than Blue Series and such. Using their I2C communication support (instead of PWM) simplifies communication and cabling and allows for two way comms if you’re willing to support I2C in your flight controller.

  18. wolf says:

    it would be nice to post a picture depicting all the common parts used, kinda like a schematic looking paper, like an exploded view showing the components http://site.nitroplanes.com/magpie1.jpg

  19. Adam Claassen says:

    I really could have used this about 2 years ago when I built my first multirotor. It would have saved me a boatload of money on iterating.

    I would also say, that based on my experience, this might be a bit too big for a 1st build for most people. Having no real RC experience, learning to fly this big, slow to respond machine with whirling sharp blades on it was daunting, even in my 1/2 acre backyard. If I could do it over again, I would have built a 300mm quad first, with a simple controller board like the flip 1.5 (witespyquad). This way, you have a small, light frame that you can fly rather aggressively to get gains and sticks adjusted properly and won’t break a prop EVERY TIME you crash, which gets expensive either due to high prop prices in the US or high shipping prices from Hong Kong.

    I suppose if you wanted to go big the first time, I would at least recommend getting one of those $30 micro-quads and getting some stick time first. It will help to not be learning to fly on your big, expensive ‘copter, because crashing is unavoidable at first.

    • Adam Claassen says:

      Plus, mini/micro ‘copters are easier to travel with, and the batteries are cheaper so you can get more flight time in without a) charging or b) spending insane amounts of money on batteries ($30 for a 3000 mAh 3S 30C pack on Amazon, giving around 7 minutes of flight time on my quad)

      • Lazyzero says:

        7min@3S3000mAh sounds like a quiet heavy copter. I guess about 1.5-2.0kg. My favorite copter in the moment flies about 10min with 3S1300mAh at a size of 320mm motor-to-motor diameter and about 0.5kg. It is a simple wood frame, but solid and crack proofen. I build it with cheap Keda 20-50s and T-motor MT2206.
        Some pictures and details can be found here: http://lazyzero.de/modellbau/woody8 (sorry it is german, but there is a google translate button on the top of the page).

        Christian

  20. LK says:

    Protip for the first test flights: Have some wooden sticks stick out from every quadcopter arm a bit longer than the propellers stick out, so the props won’t make contact with walls / the ground if the quad topples over or something.
    This way I only had to replace four or so props in the year I am flying my quad now. It will cost you 100 gram or something, but I think thats worth it, especially in the beginning when you don’t need the fastest quad.

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