FPV Drones

Drones!

I started out with a DJI Mavic mini. Puchased in advance of a camping trip to try to stitch together a cool video of our time in the woods, but never made the video. The problem? It was all B-roll! The mavic is a great drone, with incredible range and a variety of features that make it feel safe to fly in pretty much any circumstances. After that camping trip I took it offroading a few times, but mainly used it to inspect things that were high up.

This post is about FPV drones though. With a Mavic drone, you connect your phone and the drone streams video to your phone. If you take your hands off of the remote control, the drone stays where you leave it. FPV is like flying an airplane. Instead of streaming to a screen in your hands, you strap a screen to your face (FPV goggles) and recieve a realtime point-of-view video feed from a forward-facing camera mounted on the drone.

FPV drones are difficult to fly. They crash a lot. They tend not to have features like “hover in place” or “return to home”. They occasionally lose reception and fall out of the sky. They can be heavy and require an FAA registration. They are not really dew/snow/canal/shore resistant. The propellers are sharp, exposed, and spin at 40k RPM. Once, my drone caught fire. The batteries are high capacity / high discharge, and using them safely is a discipline.

All that said, the rush of FPV flight is like nothing else. These drones can go 99mph. They can punch 100’ into the air in the blink of an eye. They can change direction and cut a turn like nothing else. They put me in the treetops, with the birds, over the edge, off the roof, above the city. They are absolutely incredible little machines. And I don’t have to risk my life for the thrill.

Getting Started

I got started on a simulator, which is a fancy way of saying ‘video game’. I used Velocidrone, which set me back $20, and bought a crappy remote control from NewBeeDrone for another $25. After a week getting used to the controls (while still being pretty terrible), I felt ready to fly a real drone.

In Denver we’re fortunate to have a drone store. Full Send FPV. What they lack in inventory they make up for in heart. I’ve visited a lot of other hobby stores in the course of this hobby looking for components or advice, and what I’ve found is that drones in general, and FPV drones specifically, are looked down on by the rest of the RC community. Feels like gatekeeping.

At any rate, Full Send sold me the Emax TinyHawk II. There are others, but this has served me well. I bought a kit that came with the drone, a slightly better remote control, and goggles for around $200. I also purchased around 15 1S batteries to go with it, and an ammo can to safely contain the batteries.

Flying in the park it takes me about an hour to go through all my batteries. I can fly this drone indoors and even have a little circuit where I leave the back door and side door of my house open and fly through the kitchen, out the back, around the tree, into the side yard, and back into the house. Flying indoors is a lot harder than flying outside. Outside, there’s a lot more space, and therefore time, to perform acrobatic tricks. Dogs like to chase the drone, and young children are often captivated by it when they see it, and very amused to see it crash.

To go past this point, I decided to buy HD equipment and start building my own drones. Building drones reminds me of building computers. There are a bunch of components that you buy together and assemble into a machine with a purpose. What follows is my recollection of what I’ve learned to this point, organized in a way that hopefully helps someone unfamiliar with the hobby orient themselves.

There’s also this incredible resource:

https://www.getfpv.com/learn/

And this comprehensive 1-hr video by Paul Nurkkala:

https://www.youtube.com/watch?v=QuUlXBJo-Xw

A bunch of FPV drone terms

Basic Components

There are four major components to FPV.

The Receiver

This is the remote control. These are commonly configured in “mode 2”, which means that throttle and yaw are controlled using the left stick, and pitch and roll are controlled using the right stick. For an FPV drone, the throttle action stays in position, while pitch/roll/yaw controls spring back to the center. FPV receivers tend to have a bunch of auxillary switches in addition to the control sticks.

I went the HD route and both my video transmission and reciever are handled by the “air unit”, and it’s all bundled to my DJI reciever. This is the simplest approach. There are other reciever protocols, mainly for long-range control, that let you use an open source reciever and require you to install a separate transmitter and antenna on the drone.

The Goggles

You strap a pair of goggles to your face. Without the goggles, these drones are really hard to fly. There are HD goggles and Analog goggles, and they aren’t really compatible. Right now, racing uses analog almost exclusively because the image latency between drone and goggle is better. Analog goggles, like analog receivers, have a modular system where a video receiver can be installed in the goggles. You screw antennas into the goggles in order to receive signals from the drone.

The Drone

This parts obvious but I’ll go into some details. There are two different ‘kinds’ of drone- HD (or digital) and Analog. There are a bunch of different types of frame style (or build purpose).

Batteries

Drone batteries are weird. More on that later.

A Comparison of HD vs. Analog for FPV

There are two key “kinds” of FPV drone. HD, and Analog.

Analog drones transmit NTSC or PAL video back to the goggles, which recieve that signal and decode it for display. When an analog drone starts to drift out of range, the pilot experiences static or color distortion. The image is still visible and the goggles are still decoding it, but the quality gets worse and worse gradually until you’re looking at white noise.

HD drones transmit an image that is consistently high quality by encoding it to a packetized digital format (like h.264/mp4) prior to transmission. As an aside, I don’t actually know what kind of encoding is being done, but getting an SDR that recieves at these higher frequencies and attempting to decode the video is on my list of things to do. When an HD drone starts to go out of range, the onboard encoding reduces the quality level of the video and even blurs the sides of the field of view. This reduces bandwidth required for a picture and maybe allows the transmitter to send more frames or ECC bits. But when you go completely out of range in an HD drone, the picture goes black. That’s alarming when it happens!

HD adds a bit of latency because it’s encoding the video at 120fps in realtime. In the three months I’ve been involved in the hobby all the major manufacturers have made strides in optimizing this. Analog will likely always be lower latency than HD.

In either case, most of the videos you see posted to Youtube are filmed using an action camera strapped to the top, and not using the drone’s builtin video feed.

FPV Frames and Purpose-built Drones

There are different kinds of drones form factors that all have to do with what you’re using the drone for. The key ones that I’m aware of are whoops, racers, freestyle, and long-range. There are also larger form factors for commercial use, but that’s out of my wheelhouse. Drones tend to be differentiated by propeller size and purpose, using millimeters for the smaller ones and inches for larger.

Whoops are small drones that have propeller guards. There are tinywhoops, which are suitable for flying indoors, and cinewhoops, which are large enough to mount a GoPro. Tinywhoops tend to be useful for acrobatics, but a cinewhoop is likely to be underpowered (mine certainly is). Tinywhoops will have a 65 or 75mm prop size. Cinewhoops, 3-4 inches.

Racers are drones built for racing or acrobatics. Because drone racing right now is an acrobatic sport, these things have a lot of power and a lot of maneuverability. They are typically built in a five-inch form factor. Propellers are exposed on arms that extend out from the drone in an X pattern. They will likely be visible in the live feed. I believe racing and freestyle drones are effectively the same, but it’s possible they’re not.

Long-range drones are built to fly long distances, or to have long battery life. These used to be 7" form factor, but more recently some folks have figured out that lightweight drones perform really well at this, and a 4" form factor is drawing a lof of attention. One differentiator with these is that the prop arms are arranged in a “dead cat” configuration, where the front arms are nearly a straight line, and the rear splay out in an X pattern. For weight-optimized drones, this configuration puts the FPV camera in front of the propellors so that footage can be used and an action camera doesn’t need to be mounted to the top.

The frames tend to be made out of machined carbon fiber sheets. Most frames require assembly to connect the arms to the body, and to hold a top plate and bottom plate together to shield the electronics. The thickness and shape of the sheets represents a tradeoff between durability, weight, and vibration damping. There are a couple companies (Switchback is one) that publish intesting data on the efficacy of their frames, but no standard to use to compare different frames.

Types of Antena & The Importance of Good Antennas

Good antennas provide range and signal. A good signal means low latency and high picture quality, and range is just good clean fun. There are antennas on the drone as well as on the receiver and the goggles. The same antennnas can be used for transmitting and receiving. On analog drones, you’ll have separate antennas for the reciever as you do for video transmission. An HD drone will have just one antenna (or a module with multiple).

One tip that gets repeated a lot is importance of ensuring that antennas are connected before powering on your transmitter. This is because transmission pushes a lot of current and heat will build up and damage the electronics if an antenna is not connected. I think the equipment is getting better at detecting these conditions and lowering output power.

Antennas can be omnidirectional or patch, dipole or polarized. The drone itself will always contain omnidirectional antennas. Receivers tend to have omnidirectional antennas as well. Goggles can be outfitted with both omni as well as patch antennas. This allows for some reception when the drone is in any alignment relative to your head, and extremely powerful directional reception when the patch antenna is pointed at the drone.

Polarization is required at some racing events. I believe it reduces interference when many drones are in the air at the same time. There is left-hand and right-hand polarization, LHCP and RHCP. The “CP” means circular polarization which implies there are other kinds. The polarization needs to match between the drone and the goggles or signal strength will be really bad. I don’t know if the receiver also needs polarized antennas! Upgrading my antennas led to a noticeable improvement in video quality.

One other thing to keep in mind is there are different types of antenna connector. SMA connectors are screw-in connectors, most commonly used in goggles and receivers. MMCX connectors are used by the DJI air unit. Those are a brass fitting that pressure-fits into place. The Caddx HD air units use a U.FL connector that snaps in, but is also held in using a special bracket.

I prefer the U.FL connectors to the MMCX because of the way they break in crashes. With U.FL, the antenna tends to be ripped out of the connector. Replacement cost is a new antenna, $10-20. With MMCX, I’ve had several crashes where the antenna does not disconnect and rips the attachment point on the printed circuit board. That creates a challenging repair requiring specialized tools, or else you have to replace the air unit.

Caddx ships LHCP antennas with their air units, and I upgraded the antennas on my goggles as well to a set of omnidirectionals that can be stowed in the case without needing to be removed from the goggles.

Batteries

Drones use LiPO batteries, which stands for lithium polymer. Batteries consist of one or more cell wired together in series. A battery with one cell is a 1S battery. A battery with four cells is a 4S battery. The defacto standard is for each cell to be charged to 4.2 volts, and to discharge to 3.3 volts. Cells tend to contain between 300 and 450MaH of power. If you discharge below 3.3V per cell, the battery will cease to deliver power and your drone will fall out of the sky.

The batteries used by FPV drone support high rates of discharge. Discharge is denoted by the “C” rating, such as “80C” or “100C”. The C rating can be divided into the milliamps to determine the supported discharge rate. 1350Mah / 100C = 135 amps. So there’s a lot of power available to the motors, if you need it! Probably the most practical inital application of the C rating is to determine the charge rate. That same 1350Mah @ 100C battery can be charged at a rate of 1.35 amps.

Charging Infrastructure

Charging LiPo batteries is a bit of a discipline. I use the ISDT 60, which supports input from either an AC or a DC power source.

This charger can charge a single battery at a time. Batteries have two groups of wires coming out of them, a “balance lead” and a “current lead”. I made up the latter name. The current lead tends to be an XT-60 or XT-30 connector. Smaller batteries will have a different conector.

Both leads need to be plugged into the charger. The balance lead allows the charger to ensure that each cell of a multi-cell battery is at approximately the same voltage as the rest of the cells. It also allows the charger to measure the “internal resistance” of each battery cell, which can indicate a potential issue with a battery.

Because of the high C rating, these batteries charge pretty fast. 45 minutes to a full charge. But if you’re flying eight packs that can be a pretty big time commitment. So there are parallel charging boards that can be used to charge multiple batteries at the same time using the same charger.

Parallel charging connects multiple batteries in parallel so that they look like one battery to the charger. If using a parellel charger, you need to ensure that all the batteries connected to it have the same number of cells and the same MaH capacity. If they have different C ratings, set the amperage to the lowest charge rate. If you were going to charge one battery at 1.3 amps, but are charging four in parallel, you can set the charging amperage to 5.2 amps and they’ll finish in the same amount of time.

The pros invest in complicated setups that chain multiple independent chargers together to a single power supply. They do this for a reason! Parallel charging can age batteries faster, hide issues related to internal resistance, and might introduce some other risks that I’m not remembering right now.

Fire Safety

The cells in a LiPO battery are essentially a long sheet of lithium that has been wound around itself into a rectangle. If a cell is punctured or damaged, a charge can cross between this wound sheet in a way that short-circuits the cell. A squished cell might be okay, a pinched or bent cell is definitely not. Cells are heat-tolerant to 135 celcius, but beyond that temperature they will spontaneously combust. If a cell is short-circuited it can easily generate this amount of heat. Once a lithium battery has caught fire, the fire is exceptionally difficult to put out. A battery fire is fed by the battery itself, like thermite, and does not require supplemental oxygen to burn. There are no fire extinguishers on the market that can extinguish a lithium fire. So about the only thing you can do is move it to a safe place and wait for the fire to burn itself out.

Also: The fumes from a battery fire are extremely toxic. Close the door and call the fire department.

Traveling with Batteries

If you’re driving with batteries, keep them in the passenger cabin and not the trunk. And never leave your batteries in the heat. If you’re flying with batteries, bring them in your carryon, never stowed luggage. Airlines have begun to outfit some aircraft with lithum fire containment bags. These are rated to a specific level of energy. To be on the safe side, don’t store more than 10,000MaH in a single carryon. If you’re traveling with more than 10,000Mah, package and stow units of 10k in different overhead compartments. Charge batteries to storage voltage and individually wrap them in ziploc bags (to prevent them somehow shorting each other). Put them into a larger bag that can easily be isolated from other items in your carryonn. Never fly with a damaged battery!

Some good habits to have about batteries:

  1. Store them in an ammo can or similarly hardened container. Removing the seal from the ammo can will help prevent it from exploding in the event of a fire.
  2. Store batteries at 3.8 volts. Check on your batteries periodically.
  3. Dispose of damaged batteries. This is not an area to save money.
  4. Always attend to batteries while they charge. Do not leave the room. Do not leave your house.
  5. Charge batteries away from flammable objects. Secondary fires are preventable.
  6. Check in on the cell resistance while charging. A big difference between cells means something is wrong.

Building a Drone

I mentioned earlier that building drones is a lot like building computers.

A computer might have the following BOM:

  • Case
  • Power Supply
  • Motherboard
  • RAM
  • CPU
  • NVME hard drive
  • GPU

Every HD drone has the following BOM:

  • Frame
  • Motors
  • ESC - Electronic Stability Controller
  • Flight Controller
  • VTX Air Unit
  • Camera

Analog drones replace the VTX Air Unit with two separate components, a radio reciever, and a video transmitter. You can optionally add additional components to your drone:

  • GPS
  • Barometer / magnetometer / compass
  • Buzzer
  • Cool LEDs
  • Optical Flow Sensor

Building a drone is pretty easy, if you have all the right parts.

  1. Assemble the frame
  2. Mount the motors and ESC to the frame
  3. Cut and tin the motor wires, tin the ESC motor leads, and solder them together
  4. Mount the flight controller to the frame
  5. Mount the air unit to the frame, possibly using zip ties. Some soldering may be required.
  6. Mess around in betaflight.
  7. Go fly

It’s not that easy. My first drone took a couple weeks to build. I bought extra components in case more things went wrong and used them to build a second drone. My third drone was stalled by the wrong sized motors. My fourth (a replacement for the third, after I lost it) went really smooth, but I ran into an issue with a loose component on the PCB that caused sudden power loss (self-repaired). My fifth (replacement for the second) was going well until I bent a pin in the air unit attachment, then damaged the PCB wiring while removing the attachment for direct soldering.

Frames

Decipher the diagram and screw the frame together! There are several important details for frames.

First, what is the mount pattern for the motors. Some motors use M2 screws and narrow spacing, others use M3 screws and wider spacing. Make sure they match.

Second, what is the mount pattern for the electronics. I’ve seen 30x30mm, 20x20mm, and 25x25mm. Most frames support a variety but it has to match your flight controller or you’ll be building two drones.

Third, where the heck are you going to mount the air unit? Some frames have thoughtful placement, but many don’t. I’ve had to zip-tie air units to the top plate of the frame in my most recent builds, and that seems to be an acceptable approach.

Fourth, where are you going to mount other components? (if you are) You can tape a GPS chip to a motor arm, or you can buy a 3d printed housing to squeeze over your standoffs and hold the chip. Ideally the frame comes with 3d printed parts.

Fifth, what size camera does the frame support? I ran into this recently where my frame was build for a “nano” camera but I had a “mini” camera. I had to cut up the camera mount to get it to fit. Like everything else, check dimensions first.

Motors

Drone motors are brushless DC motors with a wound copper core and an outer shell of rare earth magnets that spins with the propeller attached to the top. They have three wires that map to three terminals on the ESC.

Motors have several key details - the mount pattern, the screw size, the prop mount type, the stator dimensions, and the Kv, and also the magnet count.

I mentioned motor mounts in the preceding section. Props can be either T-mount or ?????. The t-mount uses M2 screws to hold the prop in place, while the other mount type has a cap nut that tensions the prop to the bell. The t-mount are common for smaller motors and drones.

A motor might be identified as “1408 2205Kv”. 1408 is the stator dimensions, 14mm high, 8mm deep. This is the thing that copper wire is wound around. A larger stator adds weight and torque. It can move a larger propellor. The Kv rating indicates how many RPMs will be output for each volt applied to the motor. Multiply voltage by Kv to get RPMs. If I powered my 2205Kv motor at 16.8 volts, the props will spin at 37,044 RPM.

https://www.rotordronepro.com/understanding-kv-ratings/

When selecting motors you have to consider the batteries your drone will use. All else being equal, a 4S battery at 16.8 volts will spin the propellors slower than a 6S battery at 25.2V. So if you intend to use larger batteries, you need motors with lower Kv ratings.

Flight Controller & ESC

I combine the flight controller and ESC into one section because they’re commonly sold as a “stack”. More recently they have been built as All-in-one (AIO) units that combine both functions onto a single board, in order to save weight.

The ESC is like the drone’s cerebellum. It’s where the power leads are wired in for the battery, and it has MOSFETs and other components that help to translate signals from the flight controller into power in the motors. So the ESC generally has two pads, a positive (+) and a negative (-) for power input, and 12 pads for motor control.

ESCs have an amperage rating that is per-motor. So a 40A ESC can drive 40A * 4, or 160A, across all four motors. Recall the discharge rate of the batteries - 1350Mah @ 100C = 135A. A larger battery or a higher C rating might test the limits of your ESCs.

Most ESCs will come with power leads and a capacitor included. The capacitor is optional, used to “clean” the input from the battery. Install it as close to the ESC as possible, ideally directly connected to the pads, but if that’s not possible you can install it in the XT-60 connector.

The motor leads are fairly simple. Each motor has three wires, which are soldered to the ESC in order. Inside, middle, outside. If the motor direction is incorrect that can be fixed in software after the fact. I’ve never not had to reverse motor directions in a build.

The flight controller (FC) connects to the ESC most commonly through a prepared cable with plastic clips. There are generally four signal wires, one for each motor, power, ground, and ESC telemetry. But the included ribbon cable will ensure compatibility within a stack.

The FC also has little sections, called UARTs, for things you might want to connect to it. A UART generally has a 5v or 3.3v pad, a ground, tx, and rx. Note that when connecting a peripheral to the FC, the tx pad from the peripheral maps to the rx pad on the FC, and vice versa. The peripheral transmit pad sends data to the flight controller’s reciever pad. There are “HD” FCs that come prepared with a ribbon cable to connect a digital air unit, to reduce soldering.

Internally, the flight controller has an accelerometer and a processor that runs a firmware suite (most often Betaflight but there are others) which controls the drone. When the drone is in flight, the firmware is keeping the drone stable, interpreting signals from the receiver and instructing the ESC to drive power to the motors, sending telemetry to the goggles on-screen display, writing telemetry to flash, checking for failsafes, and a whole lot more. There are different types of FC chips that have different capabilites, with the F7 being the state of the art in 2021.

HD Air Unit / VTX / Camera

The HD air unit kind of does an “all in one” with video transmission and reciever control. They’re pretty straightforward to connect to the flight controller. The air units have six pads - 9v, ground, tx, rx, sbus, and ground. It’s pretty common to get a ribbon cable with a plastic connector on one end and a bunch of loose wire on the other.

One really important thing is never to trust the ribbon cable that comes with the flight controller. I ruined a Caddx Vista and had to mail it to China for repairs after solding the cable and powering on, sending 9v to my sbus pad and frying a diode. (They graciouslly informed me it was a diode when I sent them a picture). Confirm that the wires do what they’re supposed to do before powering up.

Anyways, these things do the heavy lifting of transmitting the video feed from the camera to the goggles, and transmitting RC signals from the receiver to the flight controller. There isn’t too much of a difference between different units. The DJI units have a larger, higher-quality camera, multiple antennas, and an SD card slot for DVR. The Caddx has a smaller housing with bolt holes, suggesting that one day you’ll be able to mount it cleanly to your frame.

Cameras for these tend to be interchangeable. Some cameras (like the Caddx Nebula Nano) don’t support inverting the image, which might be inconvenient if the ribbon cable for the camera is too short to support mounting in the correct orientation.

Other Peripherals (GPS, Buzzers, LEDs)

I really enjoy having a GPS and barometer on my drones. The GPS can give you speed and exact positioning, while the barometer provides altitude. Lots of flight controllers have barometers built in now. The suggestions I’ve read have indicated that GPS chips need to be mounted away from the ESC, ideally with line of sight to the sky. A few centimeters is fine in practice.

The way GPS works is a little strange. When powered on the chip gets a fix on as many satellites as it can. Those are cached in onboard volatile memory, which the gps chip supplies power to using a small battery. If you power the GPS off, those positional fixes will remain in storage and will be relevant for several hours, so that if you power back on and you’re still in the same general area (give or take 100 miles), your GPS “lock” will be fast. If the lock is lost, the GPS chip will take a few minutes to re-acquire satellites, even if you have line of sight to the sky.

GPS is really good at determining speed and precise Lat/Lon, but not so good at determining altitude. For altitude, a barometer is best and the firmware will tare it to zero whenever you take off.

Buzzers are useful in case the drone crashes. I have one that’s mapped to an auxillary switch that I can toggle on demand, that also goes off if the receiver signal is lost. That way if I’m out of range or crash, I can find the drone pretty easily using my ears as well as my eyes.

Drone culture is similar to gamer culture so LEDs are an important feature. LEDs on a drone are wired in series and the flight controller firmware provides a lot of options for controlling them.

Optical flow sensors are the sensors that DJI drones like the Mavic series use to stay in position, or loiter, over a fixed location. These aren’t really in wide use outside of commercially available drones, and they aren’t supported by Betaflight today (iNav, an alternative drone firmware, does support them)

Tools & Supplies

Fortunately the tools required for this hobby are not excessive.

Hex wrenches - 1.5mm, 2mm, and 3mm Fine point tweezers Wire cutters Pliers DC-powered Soldering iron Voltmeter

Other Stuff

This is the end of the article. Some notes for the future:

Skills

  • soldering
    • tinning your soldering iron
    • beware cheap solder from china
    • importance of adding flux
    • role of solding iron temperature
    • pre-tinning wires
  • basic electronics troubleshooting
  • reading manuals & transcribing wires
  • running sketch software on your computer
    • betaflight
    • dji
    • the esc firmware stuff
  • adjusting firmware

What to bring

Flight safety