When I power up my flight controller and ESC's, I hear a series of beeps. What do all of the beeps mean?

The normal blheli startup tones are similar to beep beep beep... boop. boop

The first 3 beeps are the ESC's powering on and initializing then there's a pause followed by two more beeps which means the throttle signal has been received.

https://youtu.be/_hkzm8ZXsxE?t=58


recommendations for a DIY/dev platform lifter drone

To answer your main question, I'm afraid there's nothing as widely used the Ender3.
It sounds like you want to build something with more lifting capacity and space than a race quad, with GPS.
There are two main styles of frame that might suit you.
One is based on the old 'DJI Flamewheel', which has plastic arms attached to hub of two flat plates. You can build these with as many arms as you want but the arms are only available in one size.
The other option is the many designs that use carbon fibre tubes. The tubes are very light and strong, and fairly cheap. Commercial designs use injection moulded or CNC aluminium fittings, but these would be perfect for 3d printing. Here's one I found but I'm sure there are others.
Unlike a 3d printer, the mechanical design of a drone really isn't all that important. You could use bits of broom handle and plywood, and it would still fly. That's lead to a wide range of designs and no clear market leader.
I'll have a go at the other questions:

sorry, I don't know much about the options here.

more blades is less efficient, which is probably why you can't find any multi-blade folding props. I'm not sure that more blades are quieter, unless you're comparing it to the same size 2-blade prop spinning faster.

most people use Velcro straps, which are lighter than sliding mechanisms, and work with a wide range of battery sizes. Batteries need replacing every few years, and you can't guarantee getting the same exact size.

the DJI video unit is the only real option for high quality digital video at the moment.

This is different to the video used in quads like the Mavic, that transmit via wifi to a phone.
Racers use much cheaper analogue gear based on old CCTV signals, but the picture quality is limited to old PAL/NTSC TV resolution. It's easy to send an analogue signal to a screen, and you can buy screens with built-in receivers. You can't always see screens in strong sunlight, which is why people use goggles.

Systems that transmit to a phone are generally proprietary and only available as part of a complete drone. They also have a noticeable lag, due to the video compression/decompression required, and other processing on the phone. That's why they're preferred for taking video, but not for high-speed flying. At the moment you can't have both (although the DJI air unit is close).

betaflight is the firmware run on race drones. It's not aimed at autonomous flight like the Pixhawk.

No, because there's no common wifi module for drones.


Help with finding a motor to rotate the plane of a propeller

I think what you are looking for is a servo. At least, that's what it's called in the hobby, officially it's a servomotor. Each unit consists of a motor, gears, a sensor to measure the position of the output shaft, and a comparator circuit. The electronics compares the requested angle to the actual angle, and drives the motor as required to minimise the difference.

They're commonly used to control the steering for RC cars and boats, or the control surfaces in RC planes. They are specified by their movement speed and torque. Better servos have ball bearings on output shaft, and you can get metal gears for tough applications.
You might want to look at the servos used for retractable landing gear on RC planes. These servos only have two positions, and are generally geared a bit lower for extra torque.
Servos generally run at 4-6v and are controlled by a pulse width modulated signal from an RC receiver. Alternately you can buy a 'servo tester' for about $5 to generate this signal, or it's easy to generate from an Arduino.


Why is 900 MHz better for long range than 2.4 GHz?

The strength of an electromagnetic signal will attenuate (decrease in strength) over long distances between the receiver and the transmitter due to the inverse square law, but this phenomenon applies equally to all wavelengths/frequencies.

The main difference in transmission distance capability between long-range RC systems like TBS Crossfire and FrSky R9M (which use the 900 MHz ISM band) and run-of-the-mill RX systems that use the 2.4 GHz band is due to the diffraction (and reflection) of the signal by obstructions in and around the line of sight between the transmitter and receiver. This process is explained well in this Electronics.SE answer. Essentially, lower frequency (longer wavelength) signals are less susceptible to signal strength losses by reflection and diffraction because they require larger objects to affect the signal in the same way.

As the Electronics.SE answer points out, this becomes easier to conceptualize if you look at two different frequencies like 1 MHz (wavelength of 300 m) and 100 MHz (wavelength of only 3 m). If there's, for example, a house or other large object in the vicinity of the path between the transmitter and receiver, the 1 MHz signal with a wavelength of 300 m will find it a lot easier to avoid being disturbed and reflected than the 100 MHz signal with a wavelength of 3 m for the simple reason that it is just so much larger in comparison to the house.

At extremely long ranges (like over the horizon), other effects also come into play like the ability of lower frequency signals to more easily reflect off the curved surface of the earth and the ionosphere, but these kinds of transmission distances aren't an issue for RC aviation for more practical reasons. :)

2.4 GHz RC solutions are far more ubiquitous because the antennae required to transmit and receive signals at a frequency of 2.4 GHz are far smaller than those required for 900 MHz. This is due to the change in wavelength between the two frequencies and is evident in the size difference between 2.4 GHz receiver antennae and those for 900 MHz:

TBS Crossfire Immortal-T antenna: (The active section runs horizontally in the picture)

Futaba 2.4 GHz receiver and antenna: (The active section is silver/white in the picture)


Spray painting a drone, what to look out for?

If you have the technical know how, I would suggest you remove the shell from the drone and spray paint that separately to avoid contamination any of the electronics or mechanical components.

If you aren’t able to do that, then apply masking tape and/or plastic or any other protective layer over any exposed electronics, cameras, ports, motors etc. Basically anything functional. Also make sure not to point the propellers as this could lead to them being unbalanced.

Before you paint the drones, you may want to scuff the surface with some sand paper. This will provide better adhesion for the paint - if you’ve ever tried writing in marker on a smooth surface, you know how difficult it is to stick.

After you have applied your paint, you can add a clear coat and a fixative to protect the paint.

Here is a video showing how to spray paint a drone: https://youtu.be/5aSsd3tCJ-c, notice how the person completely disassembles the drone before applying the paint.

You may also want to consider a vinyl wrap of hydro dipping as decent alternatives to spray paint.

Here is another example of how to spray paint a more premium drone: https://youtu.be/mP4oyjipwhQ


What does the TVL rating mean for FPV cameras?

TVL (TeleVision Lines) is a measurement of the "resolution" of an analog camera. Digital video systems use pixel resolution to specify how many pixels wide and tall the image is (e.g. 1080p = 1920 pixels wide by 1080 pixels tall), but analog video systems don't technically have pixels so there has to be some other way to represent their resolution capabilities.

 (cit.)

The horizontal resolution power of analog video (TVL) measures how many distinct vertical bars can be depicted by the device (either a camera or display screen) across a horizontal length equal to the height of the picture. The example diagram above shows analog video with 6 TVL.

Cameras with higher TVL ratings will be able to capture smaller details than other cameras, although this is also dependent on the capabilities of the other equipment in between the camera and your FPV goggles or screen.


What do I need to do to get OpenTx 2.3.X to include D8 on a Taranis QX7 model?

This depends on your version of QX7.

If it is ACCESS, then, as far as I know, there is currently no way to enable D8.

If it is ACCST FCC, it should already have D8 enabled by default, so it is unlikely to be this model.

If it is ACCST LBT, then when updating the radio, when selecting the features, you can enable D8 mode by selecting the appropriate checkboxes as shown in this video by Whirly Bloke: Video


When evaluating the weight of a multirotor, is there a standard way that the components are grouped?

To me the only weight that matters is the total weight of the craft that is airborne.  Or commonly referred to as All Up Weight (AUW).  That is what performance depends on.  That is what regulations are based on ( mostly ).  Anything else is just the weight of individual components in my opinion.


What does the full scale range in a IMU MPU9250 datasheet signify?

TL;DR:
The configurable scale ranges for the accelerometer, gyroscope, and magnetometer on the MPU-9250 IMU affect the maximum resolution and range of the reported values.
As you increase the full-scale range setting, the maximum value/rate that can be reported by the IMU before the IMU is saturated increases, but the resolution of the value/rate decreases.

Section 3.1 of the datasheet for the Invensense MPU-9250 has a table that states the "Gyroscope ADC Word Length", which represents how large the values reported over the I2C or SPI interfaces are. In this case, the gyro word length is 16 bits. A 16 bit two's complement binary number (explained wonderfully in this video by Ben Eater) can range from -32768 to +32767.

Take the example of the ±500 °/s full-scale range. The unit resolution of the two's complement value reported by the gyro (aka Least Significant Bit/LSB resolution) in the ±500 °/s full-scale range is: $\frac{32767}{500} = 65.534$ LSB/(°/s) (degrees per second per LSB). This is not coincidentally labeled in the graph as the "Sensitivity Scale Factor" for the FS_SEL=1 condition.


Solidworks works great and is very intuitive for beginners for a CAD software. Given that you have the entire simulation suite it can perform stress analysis and automatic optimization for the frame, air flow simulation for the propellers, etc etc (nice video to start with here: https://www.youtube.com/watch?v=6y_G6om74Mc). If you want to go deeper in the analysis of a custom flight controller, then you should investigate solutions through Matlab - a bunch of mechatronics students have produced a lot on the subject and you can buy drone focused video courses like this one to get up to speed: "Model, Simulate and Control a Drone in MATLAB & SIMULINK" on Udemy.
But unfortunately I'm yet unaware of a dedicated piece of software to do all this; my approach is to use engineering "general purpose" CAD softwares and model everything because I've learned these tools at work.


There are a lot variables when it comes to building your own drone, so there will be a variety of motor sizes and Kvs that will work.
I think the recommended motor size for the ZD550 is 3508 (like this motor) but that doesn't mean you can't use something different, like a 2814 motor, because a 2814 is fairly similar in stator volume to a 3508. This answer should help you figure out what motors you want. Torque is mostly related to motor size, so you shouldn't have to worry about not having enough torque with either a 3508 or 2814 motor. And these motors should easily be able to carry some extra weight with no problem.
As for Kv, the prop size greatly affects the recommended Kv. Here's a good explanation of Kv and how to choose the correct Kv. The ZD550 frame can fit 10''-15'' propellers, so I would recommend lower Kv (~600) for 15'' and higher Kv (~850) for 10''. Also, Kv will greatly depend on what battery you intend to use. Generally a higher voltage battery = lower Kv motor, and vice versa.


Beeper beeps if transmitter is turned of even if it shouldn't

After making sure that you have the RX_Loss beeper disabled, the next port of call would be to check your DShot config.

It may not be the beeper that is making the noise - there is a DShot beacon configuration that allows you to set your ESCs to beep in the case of RX_Loss  - Id recommend you make sure you have that setting correct.

Edit: from your Diff All, it seems that beeper on RX_Loss May still be turned on, though I’m not sure.


How does one hold the motor bell still when tightening prop nuts?

You know those rubber wristbands that are given out at lots of events?

You can wrap one of those around the motor bell to get better purchase on it, and then torque down the prop nut.

There are also specialised tools to hold motor bells, such as the one pictured below which can be found here, on Thingiverse.


How to debug a non-working Eachine E010?

The beep from the transmitter indicates that it has linked successfully with the receiver on the drone, so at least part of the board is working. However without detailed knowledge of the design and a range of test tools it will be very hard to trace the fault accurately. Even if you did, you need specialist soldering tools to replace tiny surface mounted components, and the fault may be in a chip with proprietary firmware (i.e. not available for sale).
Really the only type of repair you could do is to replace the entire board, but I'm not sure they're available and given the price of these things I think you'd be best to buy a whole new drone and keep the frame, motors and props as spares because they are the parts that usually break.

What could have gone wrong

If it was wet when the battery was connected, it's likely that the water formed a galvanic cell between the copper traces of the circuit board and some of the other metals in the solder and component legs and cases. Galvanic corrosion is much faster than ordinary corrosion, and has probably caused an open circuit somewhere.
The other option is a short circuit that generated enough heat to melt some solder and break a connection.
If you dried it thoroughly before reconnecting the battery, then you did everything you could have, and that's normally enough - but small drones are made with the cheapest components and quality control is minimal, and sometimes they do just die unexpectedly.


There is actually no such thing as a 6-axis gyro; the correct term for this is "IMU", which stands for "Inertial Measurement Unit". An IMU is a device that combines several inertial sensors that measure the craft's orientation and position in space.

The 6-axis IMU is a composite sensor, which contains two different types of 3-axis sensors on one chip. Usually it is a 3-axis gyroscope (which measures the rate of rotation of whatever it's strapped onto) and a 3-axis accelerometer (which measures its linear acceleration). A 3-axis gyro, on the other hand, is just the gyro and nothing else, as a standalone chip. There are also standalone 3-axis accelerometers¹, but accelerometers are almost never used in flying vehicles alone, without gyroscopes.

The third type of sensor that an IMU can contain is a magnetometer, which measures the earth's magnetic field² and can be used to tell which way is north. A magnetometer can either be present as part of a 9-axis IMU, of another sensor (e.g. in a GPS unit) or an entirely standalone unit.
Do you need the accelerometer and magnetometer? Well, it depends.

For rate mode (or acro mode, as it's more commonly known), the aircraft only needs to know its rates of rotation along all three axes and matches them to the desired rates set by the pilot via stick positions. For this, just the gyro is sufficient.
However, in attitude, angle or autolevel mode (also sometimes called stabilized mode), in which the pilot defines the desired attitude(orientation) relative to the horizon, which the drone will match, a gyroscope alone is insufficient. The flight controller will need to know not only how fast it is currently spinning, but also which way "down" is relative to itself. Due to a phenomenon called integration drift³, the flight controller can't maintain this knowledge precisely enough with just a gyro. It needs an accelerometer on board to correct the gyroscope, so if you want to use these modes, you need both a gyroscope and an accelerometer.
A magnetometer (sometimes referred to as a compass) is required for navigation modes, where besides its attitude relative to the horizon the craft has to also know its heading relative to the geographical north. Note that since navigation modes also require GPS, you could forgo the builtin magnetometer in the flight controller, and get a GPS unit with a magnetometer in it instead. Or you can get both, one in the FC and one in the GPS, which will increase the accuracy of the readings. (actually, you can double the other two sensors for the same effect; some flight controllers are manufactured with a twin IMU for that purpose.)

¹: There are also single- and dual-axis versions of each type of sensor — which only means you'll need several of those to make up a complete IMU. They are rarely used nowadays, as the integrated versions are usually better in every way.

²: Or any other magnetic field, for that matter, which is why you need to keep it away from magnets and steel structures, otherwise you'll get rubbish. There's usually little metal in the air other than what's on your aircraft, though, so magnetometers tend to work quite well in UAVs unless you've got a magnet-latched canopy and put the FC next to it or something. Minimizing interference is also why magnetometers are usually integrated into GPS units and mounted as far away from the rest of the aircraft as possible.

³: Integration drift is an issue that plagues inertial navigation since the beginning of time, and is present everywhere where you can't measure something directly, but you can measure how fast it's changing instead (in other words, you can measure the derivative of that thing). So you integrate it (add up the changes over time) to get the original value. However, every measurement comes with an error, and the errors add up as well, so your total error will grow all the time and at some point it will get bigger than the original thing you were measuring.

Intuitively, integration drift is like walking in a forest. When you enter the forest, you (hopefully) have a sense of which way north (or some other reference direction) is. As you walk, you can maintain a sense of direction for some time ("I was walking towards the north, so it was in front of me, now I turned left, north must be to my right now"). However, since you don't know precisely how much degrees you turned, every turn you make will introduce a small error into your direction estimate, and those errors will add up over time. A couple hours later your sense of direction will have drifted far enough from the original that you might be heading in the opposite direction from where you think you're going. Even if you tried to just walk in a straight line all the time you would probably stray off course eventually, because without external reference you have no way of telling whether you've actually been walking in a straight line (this is known as zero drift).

The only way out of this is to have an alternative source of direction knowledge (e.g. a compass or the combination of the sun's position and time of day, or some other observable directionality in your environment like a distant mountain) which you check regularly and use that knowledge to correct the error in your "internal" sense of direction.

A quadcopter in angle mode has the same problem: to stay upright it needs to know is where "up" is, and a gyroscope can only measure (as an approximation) how much degrees it's been rotated from its previous orientation; it does not know (or care) what that previous orientation was. And even if the FC did know its previous orientation, the estimate might have drifted far off from the original. That's where the accelerometer comes in: its job is to measure the earth's gravity, and therefore the direction of "down", and use that information to figure out the initial orientation when the quadcopter is armed (like knowing where north is when entering the forest), as well as correcting possible drift of the orientation obtained from the gyros in flight.


DIY wireless mesh network to extend drone's operational range

You can do this with a configuration known as a 'repeater'; there are a number of examples of people using this to convert their favourite controller's RC system to that of a specific aircraft, but you could do the same with a single mode:

FrSky to Dragonlink Repeater
2.4ghz to EZUHF repeater

I'm not aware of an open-source implementation or Arduino examples but the above links show you may be able to use off-the-shelf parts; this is likely to depend on how open your radio hardware is.

Range-wise, the aircraft and controller need to be in range of the repeater but not of each other - so you could theoretically add half as much again but in practice you want a healthy overlap. You could add more than one node but the aircraft still has to stay in range of the last one, as the drone (and each node) can only pair with one other transciever. In theory this same setup should be possible for video too - chain a receiver into a transmitter. It is worth remembering that some VTXs can get very hot if you don't have airflow from the drone, so you may need to add a fan.

With regards power-consumption, the transmitter will be limited (legally, at least) to tens of milliwatts and the microcontrollers won't take too much - it should be possible to power such a system from a smallish solar panel, but you will need to compare the panel output to a measured system draw; then add a margin for safety and a small battery to smooth out demand.


Propeller tip losses are a function of, or perhaps we could say it is a side effect of the propeller design itself. The designer of a propeller can influence the tip losses, but a user is unlikely to find a practical way to accomplish it for a propeller they already own. So this question is theoretical.

The overall size and geometry of a propeller can influence the tip losses, but for any particular multirotor, the size or length of the propeller is most likely to be at its maximum size already. Or, if it is not at the maximum size, the length of the propeller is at the optimum size for the multirotor it is installed on. Making the propellers any longer than the airframe can accommodate makes the question morph to one of: "What changes to a multirotor influence the tip losses of the propeller." Ok, now the answer can include making the prop longer.

In the real world, the answer is: "Buy a better propeller".

In the theoretical world, redesign the propeller. And one could also add a well designed shroud with a very precise gap (very narrow) between the tip of the propeller and the shroud. And to accomplish this, the tip of the propeller would then need to have a shape that is different than one designed for free air. It would fit precisely with the shroud just a fraction of a millimeter away. This prevents the tip vortexes from rolling off the end of the propeller. But if this theory were practical for multirotors, one or more drone maker would have employed it already. And so far, it is not being done. I have not yet seen it. They are also claimed to be quieter and more efficent, but that too is not proven with any experimental evidence on an entire drone. It is all laboratory results. So there must be trade-offs that make that solution unattractive.

And to actually add a shroud, we are back to changing the entire multirotor to reduce the tip losses. I am not sure that is what the question was.

Most prop manufacturers make claims that their props are better, but provide little proof or clues regarding how they did it. We can see unique shapes used on the tips, but there is little published data that helps us replicate it for ourselves.
Was this question one of theory or a practice?


So long as your additions are very lightweight compared to the drone - like stickers or paint - this should not have a significant effect the balance of your drone. However, you want to make sure that you do not cover any ventilation holes or heat transfer surfaces as this might affect the lifespan of those parts.

In the case of metallic additions (shiny stickers, sequins) it is good practice to keep these clear of the antenna as they may affect the radio performance.

You should avoid adding anything to the propellers or motors as these do need to be precisely balanced.


How to program drones that have old flight controllers?

You can always run an older version of betaflight. Also, if you have older F3 controllers you may also like to like to look at https://github.com/spatzengr/betaflight/releases which are maintained by UAV Tech.

I have used these on some older F3 boards and it breathes new life into these old boards.


XM+ bound to FRSKY x9 lite, but commands not showing up in betaflight

Since you connected power to the SBUS pad it is possible that the inverter that sits between the SBUS pad and the UART pin on the microcontroller is fried.
To get around this, I would suggest connecting SBUS to another RX pad. However, since SBUS is an inverted protocol, you will first need to get the uninverted signal somehow. There are two ways to do this with the XM+ (other SBUS receivers have similar methods). The first option is to solder the wire that goes to the RX pad to the leg circled in red in the image below.

source
The other option is to build your own inverter using a transistor and two resistors.

source
The first option is probably the easiest if you feel confident enough to solder to that small pad, but it does come with an increased risk of damaging the receiver.


Will connecting my Mavic Pro to a an iPad Litchi app try to update my Mavic Pro firmware?

An updated or new Litchi installation will not work with outdated firmware.  The drone will not connect until there is internet communication.   Once connected it seems it will not request updates or nag.


How do certified drone operators ensure they're not harassed by the police while filming from their drone?

Look the part - wear a high-visibilty jacket with a company logo, "UAV Pilot" or other markings that makes you look like a professional.
Put up a sign which explains who you are and what you are doing - "Drone Filming In Progress - please wait until aircraft is landed before approachng pilot. Operator: SuperDronePics Ltd, Permit number: OP-4ME33NK."
Where possible, fly with an observer/helper who can intercept interested people (police, public, etc.)
If you are flying somewhere particularly likely to attract attention, it may be worth contacting the local non-emergency number in advance. I don't expect they'll post an officer with you thoughout the flight, but it could help to cover phone calls from the public - and perhaps a police officer who radios in before approaching you.

Finally, always have all your paperwork ready to show - if you are interrputed, it looks more professional to have a folder with your permits, insurance, etc. at hand. While police interest might become annoying (especially when repetitive!) you are almost always better off just answering their questions and being helpful.


Estimate safe ESC curerent for 4108 600kv motors?

Myriad factors contribute to the current draw of a drone motor including supply voltage, motor kV, the propeller geometry (i.e. diameter, number of blades, pitch), ambient atmospheric conditions, etc.
One can make educated guesses and estimations of the current a motor will draw under known conditions, but the best and most accurate/reliable method is to test the desired setup and measure the current draw under the expected operating conditions. If you can, you should try this by assembling the motor and propeller on a thrust stand and powering the motor while measuring the current draw with an ammeter.

From the Banggood listing linked to in the OP:

The motor of interest here is the 600 kV variant, which appears to be rated for up to 6S voltage, but power draw is only displayed for 4S voltages. If we take the rough assumption that the motor's current draw will increase linearly with the supply voltage (also assuming all other conditions are unchanged), then we can estimate the current draw for the listed propellers running on a 6S batery:
$$\begin{align}
\text{New Current Draw} &= \frac{\text{New Voltage}}{\text{Old Voltage}} \times \text{Old Current Draw}
\\ &= \frac{6s}{4s} \times \text{Old Current Draw}
\\ &= 1.5 \times \text{Old Current Draw}
\end{align}
$$

APC1238 (12" prop, 3.8" pitch): 25.8 A
APC1447 (14" prop, 4.7" pitch): 33.75 A
1555CF (15" prop, 5.5" pitch): 34.5 A

NOTE: These are incredibly rough estimates. I highly encourage experimentally determining the true current draw of your intended setup.
NOTE: These calculations will be irrelevant if you're not using the same propellers as the ones cited in the experimental performance data.
As @Kralc mentions in his answer, you should add a roughly 20% safety margin on top of the expected current draw when spec'ing out an ESC to account for any unforeseen situations.


Performance is unlikely to be improved, in terms of speed or agility, or even load-bearing capacity, as they are heavier, slightly less efficient aerodynamically, and more expensive.

What you do get is a degree of fault-tolerance.

With a quadcopter, losing one rotor means your drone will fall out of the sky. With an octocopter (or hexacopter, but an octocopter can lose more motors and still be able to fly) losing one rotor is not fatal - it will still fly, however, it will have a reduced control authority.

This makes them very desirable for carrying expensive loads - such as a £50,000 camera - you really don't want to drop that out of the sky if you have a fault.


Does the performance of a brushless motor decrease over time?

Like most things, it depends on how you treat it.

As far as I know, you should never need to worry about a brushless motor wearing out, especially for our purposes where we will likely demolish them before they have a chance to wear out.

You do have to watch out for the bearings and magnets, however.

With the bearings, like any mechanical component, they will eventually wear down, however this will likely not be for a long time. It is still a consideration, though.

You also have to watch out for the magnets.

Under normal circumstances, the magnets are fine to last a long time, however excessive heat can eventually demagnetise them, leading to reduced performance and eventually the motor not producing enough power for us to use.

If anyone has any other ideas or wants to correct me, please do!


ESC power up beeps sound twice: du-du-du...du-du-du beep-beep

From what I can tell, this occurrence sounds like something causes the flight controller to reset after a second or so, triggering it to reconnect with the ESCs as it initializes itself again.

The "du-du-du beep-beep" noises you point out are generated by the ESC (through the movement of the motors) to signify that they're receiving control inputs from the FC and are ready to go. More specifically, I believe that the "du-du-du" sounds (with ascending pitch) signal that the ESC is receiving signal pulses from the FC and the "beep-beep" sounds mean that the ESC is done with its own initialization process and ready to fly. (some ESCs also use the number of these pulses to indicate the number of cells in the battery they think they're connected to)
If the ESCs are detecting that the FC connects and then reconnects to them after a second or so, this likely means that something on the FC caused it to hard reset and reinitialize itself. This could be caused by anything from broken solder connections on the board to a voltage regulator that finally kicked the bucket. (or has begun to)
With respect to troubleshooting the issue and verifying that the issue is with the FC and not something else, diagnoses aren't very clear cut because the symptoms can vary from undetectable to glaring. Some dead giveaways are:

Something on the FC is getting abnormally hot
There are solder connections on the FC board which are visibly broken/intermittent
As you say, a particularly hard crash can kill/degrade FCs
Something just looks off with the FC during startup

As @Kralc helpfully mentions: (slightly paraphrased from original comment)

it can help to try unplugging parts to see if the problem changes or goes away. (Remember, it might also be a combination of parts that cause the issue, so it is good to keep notes of what you've tried) If the symptoms remain even with nothing else connected, it is more likely to be the FC.

Other symptoms are far more subtle and might not be reliably detectible by the eye. If you are able, I'd recommend trying to replace the FC if none of the above symptoms are present.


Is there an open source video processing/sensor based follow me technology?

You may want to take a look at dronecode.org, or their supported projects. Especially, PX4 (which was also mentioned in another answer):

Getting Started with PX4 Autopilot guide
PX4's Follow-Me Flight Mode

MAVLink's Follow Me plugin

You may also find some interest in some other open sourced drone projects, e.g. ArduPilot's Follow Me Mode. Another one would be DRONEKIT's Follow Me Example.
If you are interested in NVIDIA hardware, take a look at Redtail - autonomous visual navigation components.


What does the C rating on a LiPo battery mean?

The C rating is a multiplier that dictates discharge. You can figure out the maximum theoretical discharge by multiplying the C rating by the capacity. For example an 80C, 1300mAh LiPo can output 80*1.3 = 104 Amps. There is no consensus as far as I am aware as to what the letter C stands for.

It is worth noting that C rating is dubious - comparing the C rating between different manufacturers is unlikely to produce useful results as to which battery is better.


How do laterally folding propellers affect multirotor flight?

In theory, you can't accelerate and decelerate the props as quickly. If the motor accelerates, the props lower angular velocity keeps it from accelerating too quickly. Same for deceleration: if the motor decelerates, the prop's momentum makes it want to stay fast. It is only when the prop's position exceeds the motor's position, that the centrifugal force will slow it down.

But you have to consider that the angular velocity is quite a lot - therefore the centrifugal force is very high and the prop will only slightly move in its mount, thus not really affecting the flight all that much. Especially, when you consider that most drones with foldable props (e.g. DJI Mavic series) are "smart drones": They probably considered that during development and made it so that the motor speed is only raised and lowered slowly, so the effect isn't too noticeable. Plus, these drones are quite heavy so a tiny little change in prop speed will probably not have a big impact on the position in the air.

As you can see below, the propellers have different angles relative to the motor position. Obviously, propellers can only be designed for one direction of movement. Since the direction changes however, depending on whether it is accelerating or decelerating or at a constant speed, the uplift will be slightly reduced during speed changes of the props.


Why did a quadcopter in vertical descent appear to lose thrust and descend rapidly?

It sounds like you've encountered what's called "vortex ring state", when a rotorcraft descends into its own wake -- essentially, a self-created downdraft.

There are two ways to escape from such a situation:

Power out of it.  Climbing out of the craft's wake vortex requires about twice as much power as hovering, so while most full-sized helicopters can't do this, most quadcopters and other small-scale craft can.
Move forwards (or backward, or sideways) out of the craft's wake.  This is the standard procedure for helicopters.

You can also avoid it entirely by not descending too fast, or by not descending straight down.


How can I configure a servo in Betaflight?

There are multiple ways to enable servos in BetaFlight, but since you need only one servo it is recommended to use SERVO_TILT.

For servo to work you need a free pin on your board and this pin must have timer function.

Go to Configuration tab in Betaflight Configurator and enable SERVO_TILT and hit "Save and Reboot"

Then setup the switch you want to use on your transmitter and in Betaflight. Let's say it was AUX4. Make sure it shows in the Receiver tab and that the signal is received.
Then go to the CLI tab and type resource (resource list for older BF) and hit Enter.
What you're looking for is resources with MOTOR or PWM function. For example on my board there are 6 MOTOR outputs. I assume you fly 4-motors copter and you also have some spare MOTOR pads.
You need to remember the MCU pin and MOTOR number and of an output you want to use (C09 and MOTOR 5 for me).

Then you need to type:
resource MOTOR 5 NONE
resource SERVO 1 C09
save
Use your own MOTOR and MCU pin number instead of 5 and C09!
This will remap MOTOR5 pad to be able to work with PWM Servos.
Servos numbering in CLI starts from 1
After reboot you can go to the CLI tab and type resource again to make sure everything worked as expected.

Go to the Servos tab in Configurator and hit the checkbox with with your AUX channel on Servo 0 row (AUX4 for me).  If you can't see the servos tab check "Enable Expert Mode" on the top right.
Save.
Servo numbering on the Servos page starts from 0. So Servo 1 from CLI tab is Servo 0 here.

You can now power on transmitter and enable the live mode on the Servos tab.
Go to the Motors tab (remove props if you have LiPo connected!). There are servos output there.
If you switch the switch you set to AUX channel you should see Servo 1 output to change the value.

Now you've remapped MOTOR 5 pad to act as a servo output. You can connect your servo's signal wire to it and find any spare +5V and ground pads for it.
If the servo moves automatically as the quadcopter moves check that the CAMSTAB mode is disabled. The screenshot below shows it active, which you won't want if trying to manually control the servo.

If you don't have any spare MOTOR or PWM pad you can remap LED_STRIP 1 resource if you're not using it for LED.
Here's the guide how you can do this: https://youtu.be/jiD6aC0l1VQ

If you need more than 2 servos to control check Sources for how to use Channel Forwarding instead.

Sources:
1: https://github.com/betaflight/betaflight/wiki/Servos-&-SERVO_TILT-for-3.1
2: https://github.com/betaflight/betaflight/wiki/CHANNEL_FORWARDING-for-3.1
3: https://youtu.be/jiD6aC0l1VQ
4: https://youtu.be/cHi1fKP1LRI
5: https://oscarliang.com/betaflight-resource-remapping/


Why isn't my manual DSHOT600 programming of a HobbyWing ESC not responding to direction change?

I would recommend taking a look at the source code for Betaflight to get some hints, since the DSHOT commands should be the same. More specifically the code for enabling turtle mode (flip over after crash), which reverses the motor directions.
It looks like the code that activates turtle mode is:
dshotCommandWrite(ALL_MOTORS, getMotorCount(), DSHOT_CMD_SPIN_DIRECTION_REVERSED, DSHOT_CMD_TYPE_INLINE);

Where DSHOT_CMD_SPIN_DIRECTION_REVERSED is the value 21 in an enum.
typedef enum {
    DSHOT_CMD_MOTOR_STOP = 0,
    DSHOT_CMD_BEACON1,
    DSHOT_CMD_BEACON2,
    DSHOT_CMD_BEACON3,
    DSHOT_CMD_BEACON4,
    DSHOT_CMD_BEACON5,
    DSHOT_CMD_ESC_INFO, // V2 includes settings
    DSHOT_CMD_SPIN_DIRECTION_1,
    DSHOT_CMD_SPIN_DIRECTION_2,
    DSHOT_CMD_3D_MODE_OFF,
    DSHOT_CMD_3D_MODE_ON,
    DSHOT_CMD_SETTINGS_REQUEST, // Currently not implemented
    DSHOT_CMD_SAVE_SETTINGS,
    DSHOT_CMD_SPIN_DIRECTION_NORMAL = 20,
    DSHOT_CMD_SPIN_DIRECTION_REVERSED = 21,
    DSHOT_CMD_LED0_ON, // BLHeli32 only
    DSHOT_CMD_LED1_ON, // BLHeli32 only
    DSHOT_CMD_LED2_ON, // BLHeli32 only
    DSHOT_CMD_LED3_ON, // BLHeli32 only
    DSHOT_CMD_LED0_OFF, // BLHeli32 only
    DSHOT_CMD_LED1_OFF, // BLHeli32 only
    DSHOT_CMD_LED2_OFF, // BLHeli32 only
    DSHOT_CMD_LED3_OFF, // BLHeli32 only
    DSHOT_CMD_AUDIO_STREAM_MODE_ON_OFF = 30, // KISS audio Stream mode on/Off
    DSHOT_CMD_SILENT_MODE_ON_OFF = 31, // KISS silent Mode on/Off
    DSHOT_CMD_MAX = 47
} dshotCommands_e;

Note that DSHOT_CMD_SPIN_DIRECTION_NORMAL has the value 20, and seems to be the command used to set the motor direction to normal. There is also DSHOT_CMD_SPIN_DIRECTION_1 and DSHOT_CMD_SPIN_DIRECTION_2 with the values 7 and 8 respectively, but I can't find anywhere where they are used.
Examining the definition for dshotCommandWrite we find that a variable repeats is set to 10 with the current input and that delayAfterCommandUs is set to 1000.
I would assume that this means that you should send the command 21 to the motors 10 times with an interval of 1000 μs. So I would try doing as you did before but with a delay of 1000 μs (1 ms) between each repetition instead of 100 ms.


How does Amazon's new drone land itself into a station?

Remember - it only takes 3 points to get a solid fix on location in 3 dimensional space. I have been thinking about this a lot, I will also have a unit on day 1 and possibly the unit disassembled day 2 and posted on line (maybe)
For the Amazon device there is most likely a sonar/echo or similar system that creates a feedback loop for position. I also suspect use of RFID or similar "near proximity tech" to get the last few inches.
There are a lot of systems available and affordable, LIDAR in particular is really cheap (https://www.getfpv.com/matek-optical-flow-lidar-sensor-3901-l0x.html)
The tech is pretty standard and accessible, Roomba has been doing this in 2D space for almost 20 years with self docking features for the floor cleaning system.
Once you have experience with auto-leveling and GPS technology the task becomes trivial. The hard part is universality and compliance - will it work anywhere and also be legal/interfering.
There are still a number of unanswered questions - but form what I know here a few of the approaches that come to mind:

Echo location indoors (bouncing signals to determine position, similar to how the blind or bats can "see" using echos and reflection of sound)
LIDAR with some GPU strength and the addition of hyper- or hypo-visible light can create an invisible beacon.
Various systems for mapping, used commonly in drones for recovery operations such as mineshaft exploration, possibly offset to cloud computation.
Wifi stereo has a lot of research in phasing and image control of sound worth considering.
RFID or Bluetooth beacon technology, possibly with other AWS devices in house?
Augmented indoor GPS (using what signal is available + other systems)
Final landing could be simple optics targeting a unique pattern, key for orientation (downward simple camera looking for landing beacon)
Sonar, like a bat, can help with direction, distance, and obstacle avoidance beyond aforementioned echolocation.
Additional off-device processing - such is the case for Alexa, Siri, and other services.

Also note how the female receptacle is wide and allows the unit to dock with +/- 1 inch accuracy.
Also important, the FAA claims to have NAS authority indoors, but they have never been able to asset any enforcement action.
This device is a HUGE blow to the FAA's weakening grasp on NAS policy. The laws that regulate this device are NOT settled and the legality is open for debate. Per the Congressional charter, the FAA controls all airspace "0inch and up" and there has never been any official clarification for indoor operation.  A lot of questions remain unanswered.
FWIW, if f you fly this unit out of the house it is $40k minimum fine for violation of key BVLoS rules such as remote operation w/out spotter and BVLoS operation without an exemption.  There are many other rules, each $20k each per violation should anyone take this out to the porch. See and avoid is not met by the device and that one is huge and is the massive thorn FPV faces for proper recognition.  I will be following this close.


Budget drone capable of lifting 0.5 - 1 kg and flying ~2km radius

The flight you describe is 100% illegal in the US requiring 107/commercial license + BVLOS waiver.  BVLOS waivers are very rare, 61 issued searching the DB for "107.31" here: https://www.faa.gov/uas/commercial_operators/part_107_waivers/waivers_issued/.  The rules are similar in most countries.
Your budget is off by a zero as well.  Depending on your duty needs, I think minimum \$1k for the core + $ for peripherals.
A Phantom could probably limp that payload - but you need to dig into the powertrain math:

What ratio of thrust do you want? Are you carrying a load form point A to B or will you require some performance?
Weight of unit AUW desired? Payload and performance are correlated to thrust and other performance features.
Flight time desired?

Camera for control at 2km requires high power VTX (FAA HAM license required). A 1 watt VTX in an urban environment will struggle to get you signal without a lot of extra gear (signal tracking from a ground station). Also, 1w is the legal max for a HAM / FCC licensed person, else you are 25mw (0.025 watt) limited and can't mod anything legally.  To get more watts you need to dip into FCC Part 15 regulations, not cheap.
I see you are in Armenia - I don't know the local there laws but my international flight experience informs me that most of the world is either unregulated (3rd world) or as strict or more strict than the US.


DShot is a protocol that flight controllers use for communicating with ESCs. It stands for 'Digital Shot'. It is a new digital communication protocol developed by Flyduino in collaboration with Betaflight as an alternative to Oneshot and Multishot.

Oneshot and Multishot (and standard PWM) all use analogue signals. The length of an electrical pulse determines the value being sent from the flight controller to the ESC. This can lead to problems with accuracy if there are slight differences between the speeds of the clock signals in the ESC and the flight controller (which is why ESC calibration is required with analogue protocols). Also, these signals have a fixed length range of 1 to 2 milliseconds, which means they can't be sent more often than once every 4 ms or so, or 250 times a second.

Digital signals can also be more resilient in the presence of electrical noise.

The number associated with the protocol indicates how much data is sent per second.  There are several speed options available:

DShot150 – 150,000 bits/Sec
DShot300 – 300,000 bits/Sec
DShot600 – 600,000 bits/Sec
DShot1200 – 1,200,000 bits/Sec


Analog Graupner MC-12 with R-700 receiver

AFAIK servos are usually driven by a PWM signal (which the Graupner RX seems to put out as well) with usually 50Hz. So you could basically check if the RX is putting out PWM signals on the 7 outputs. Just connect your scope to one ground pin as well as one data pin. Then you should see a square wave on the scope. If the square wave has a duty cycle of 1000µs, it means 0° (so the left most position of the servo). 2000µs means that the servo is supposed to rotate to the right most position, usually 180°.

There are some exceptions with some servos (continuously rotating servos - similar to stepper motors, or some servos have more than 180° of rotation), they probably don't affect you though.

If you are unsure about the waveforms or something else, I'm happy to help. I wouldn't say that the tiny movement of the servo you described means anything. It's probably just the servo "snapping" into it's position after getting power or something to help the servo to find out its current position.


Recommendations for a BLDC motor and battery for a 28x20 propeller?

The first issue with your question is that you can't spin a 56" prop at 6000RPMs without it tearing itself apart. The tip speed of a 56" prop at 6000 RPMs is 1000mph, well over mach speeds. Practically speaking a 56" prop will probably be good up to about 3500 RPMs safely, with an absolute maximum of about 3700 RPMs (0.80 mach).

56x20 is massive, so you're going to need a significant motor. Based on the specs and what you're looking to accomplish it could vary some, but this motor is probably very close to what you're looking for: https://www.getfpv.com/lumenier-lu15-ii-80kv-professional-motor.html  The 40" props they've tested are only hitting about 3500 RPMs, and barely within the motor's wattage limits, so you will probably not quite be able to hit the maximum RPMs for the prop, but should be workable.


How tight should the screws at the bottom of the motor shaft be?

There are a few things that may be happening.
Tighter and flying bad screams bad mounting.  A prototype I was working on last week had this problem. As I tightened the motor, the c-clip on the base was rubbing on the arm and slowing down. Loosen the screw and it spins faster. To check for this, take the motor off and spin it with your hand and get a feel for how much it will "free spin". If you mount it and this "free spin" decreases then you want to look for friction.  Leave the leads disconnected when you are going to do this, as some motors will retain residual electricity and until drained they will spin slow (this is a false positive, so test everything unplugged)
Generally speaking motors are built to be bolted down REALLY TIGHT, so there is probably operator error going on.
Another very common problem is the mounting screw length.  There are 2 things I have run into related to mounting motors and screw length. First is the possibility that the screw is too long and is actually hitting something in the internals. If so, look for damage and/or compare amp draw with other motors, watch for higher heat, and be careful. The other more mystical problem is when the screw is too long, somewhat metallic or electromagnetic, and alters the magnetic field of the motor.  I have had his happen a few times and it is often not obvious.
Another possibility is that the motor and/or mount is warped. In this case, tightening can further disfigure or warp it and decrease performance.  Many elements on a motor should be parallel/perpendicular to other parts and that can help you see if a crash bent something.


What is a ground loop and how does a common ground reduce video noise?

If enough current flows and two ground pads are far enough apart, there might be a voltage difference between them because of the resistance in the PCB:s traces. That's called a ground loop.
As for why that causes noise; if the camera and the VTX have different reference points, a signal sent from the camera won't be received correctly by the VTX, and since a PAL or NTSC (The types of video that is used for analog FPV systems) signals use less than one volt, even a small in potential can cause video noise that will appear as horizontal bars moving vertically.
Here is an some more information if you would like to read more about it


In the UK a drone would almost certainly be considered WEEE waste (Waste Electrical and Electronic Equipment.)

You can take WEEE waste to your local waste handling/recycling facility; alternatively, there are "Take back Schemes" where some distributors will accept WEEE waste. This is often geared more towards white goods (fridges, washing machines, etc.); unfortunately you will need to check locally.


At what voltage per cell should I store a LiPo?

This question seems to be quite the debate in many places. However, the general consensus falls between 3.7 and 3.85v per cell. You can find a similar inquiry here: Storing at 3.7 or 3.85v and here: LiPo storage voltage

It seems that generally, the difference between storage charging to 3.7v, 3.8v, or 3.85v is negligible for most people. However, I would recommend 3.7-3.8v per cell. I store my LiPos at 3.8v per cell and all of my year-old LiPos perform almost like they are brand new.

The exact voltage you storage charge LiPos is important, but it is even more important that you just take good care of them and do storage charge them. It's better to storage charge LiPos to a close enough voltage than to not storage charge them at all.

Here is a pretty useful guide to LiPo batteries: Guide to understanding LiPo batteries


What should I look for when choosing a battery for my multicopter?

When looking for a battery, there are a number of factors to consider: the voltage (or S rating), the capacity, the C rating, the type of connector, and the weight:
Voltage
You will often see batteries having an ‘S’ rating on them, for example 6S or 4S. This is the number of cells, which directly impacts the voltage output of the battery. Just to be clear, I am specifically talking about LiPo batteries here as nominal voltages vary between battery chemistries.
Each cell will add a maximum voltage of 4.2V, and a nominal voltage of 3.7V to the battery, so to figure out the nominal voltage of a battery, just multiply the number of cells by 3.7, so a 3S battery has a nominal voltage of 11.1V and a maximum voltage of 12.6V. This is very important using a battery with too low an S rating will likely make the aircraft more sluggish, but using a battery with too many cells is likely to make the aircraft more on fire.
You may see batteries written in the format 4S1P or similar. This means that there are 4 cells in series that are treated as one parallel cell - i.e that there are no other parallel cells.
Capacity
The capacity of a battery is measured in mAh, or milli Amp-hours. This is the number of milliamperes the battery can output at the nominal voltage if the battery is discharged over the course of one hour. For example, a 1300mAh 4S battery could output 1.3A at 14.8V for one hour.
In general, a greater capacity will mean more flight time, however there are caveats, an important one being the battery’s weight (which I will talk about later).
C Rating
The C rating is the maximum discharge rating, and it acts as a multiplier of the capacity. For example, if I have a 1300mAh LiPo with a C rating of 100, the maximum current output will theoretically be 100*1.3=130A. Obviously, a higher theoretical discharge rate should give greater performance.
These metrics are very variable between brands, however. What Turnigy considers to be 100C may only be 75C for another brand, so there is in reality very little point comparing C ratings between brands.
C is also used when charging. You may have heard people say ‘charge at 1C’ or something similar, and this means the same thing. For example, charging a 1300mAh LiPo at 1C would be charging at 1.3A.
Connector Type
The way you get the power from your quad is also very important. There are many connectors currently in use, but here is a rundown of the most common used on a multicopter:

JST PH2.0: small white connectors most often used on Tiny Whoops, sometimes used in series on a toothpick.
BT2.0: small white connectors most often used on Tiny Whoops, sometimes used in series on a toothpick.
XT30: small yellow connectors, most often used on toothpicks and 2/3 inch quads.
XT60: medium-sized yellow connectors, pretty much the standard for 4-7 inch quads.
XT90: large yellow connectors, often used in parallel or series on larger Beast Class or X Class builds
Deans: not commonly used in multicopters, however many batteries you will see do have this style of connector, which often has burgundy plastic.

Weight
As mentioned before, more capacity usually brings more weight, which introduces the problem of balancing capacity and weight.
For example, a toothpick with a 5000mAh should be able to fly for an hour, but would in reality be unlikely to last a minute if it could even take off due to the large weight of the battery. Some people say that your battery should not make up more than a third of the weight of your drone, however, you can huddle this based on your motors’ thrust and your build’s weight.
Conclusion
There are many factors when choosing what battery to use for your drone, however, all of the above aspects are vital in ensuring good performance and longevity of your aircraft.


Can I use LiHV batteries as normal batteries?

Yes, you can. The primary difference between LiHV and LiPo batteries is the ability to change them to ~4.35 V/cell instead of 4.20 V/cell with a nominal cell voltage of 3.8 V instead of 3.7 V.

Just like how there's nothing stopping you from only charging your LiPos to 4.1 V/cell instead of their rated full charge @ 4.20 V/cell, there's nothing wrong with charging LiHVs to a "full" charge below their rating.


It might be a too high P-gain in the PID control loop. I assume you use Pixhawk so that document might be useful: PID tuning guide
The oscillation is from what i see on the roll axis so the MC_ROLLRATE_P might be a parameter to look into.
Regarding the Debug port: Your escs have a  Pixhawk Debug Mini connector and a serial console (with baudrate 115200 at 8 bit no parity 1 stopbit) should be available at pin 2 (tx) and 3 (rx) (pin 1: Processor voltage, pin 2: Console TX of ESC, pin 3: Console RX of ESC, pin 6: GND). See:  Pixhawk Debug Mini
You will need to connect your serial interface with the voltage.matching your controller (probably 1.8 or 3.3V) and the out pin of your interface with the in pin of the ESC and vice versa.
The commands you have available are described in the Sapog Reference Manual
Please provide more information about your setup to help others to help you with your question.


Is there an advantage to having more LiPo cells in a low-power drone?

The advantage of running at higher voltage is that you can use thinner wires and lower current (i.e. cheaper) ESCs, as power lost to resistance is proportional to current (squared) and not voltage.

Another reason can be to reach higher prop RPM when you can't find motors with sufficiently high kV, or merely want to continue using the motors you already have.

Much of the discussion about increasing cell count is confused because people increase the total battery size, for example, going from 4s 1500mAh to 6s 1500. With 50% more battery it's not surprising that both flight time and the ability to provide bursts of power without voltage sag have increased. Similarly pushing more power through the same motor is not a free upgrade. Either it's using up a safety margin or the motors were larger and heavier than necessary in the original design.


Kk2.1.6 falling in mid air and losing control

The loose nuts that you hear may be the prop nuts. If this is the case, the props are not held down with enough force and so will be slipping, not providing the torque or the RPM to be able to lift the drone.

I would suggest tightening all of the prop nuts and all of the frame fixings and seeing if the issue persists.


Why don't multiprotocol transmitters work with Radiolink receivers?

The Jumper T16 multiprotocol module doesn't support Radiolink receivers, as you know. This is because the module doesn't have any hardware to recreate the Radiolink protocol.

As can be seen on the product listing for the T16 Pro here,

  RF Chip Data

  Cypress Semiconductor CYRF6936: DSM/DSMX, Walkera Devo

  Texas Instruments CC2500: FrSky, Futaba SFHSS

  Amiccom A7105: FlySky, FlySky AFHDS2A, Hubsan

  Nordic Semiconductor NRF24L01: HiSky, Syma, ASSAN and most other Chinese models

The reason for this is likely either that:

The necessary hardware to create the Radiolink protocol either isn't available to third parties or isn't cost-effective to license
Radiolink receivers aren't popular enough to warrant the R&D required to reverse-engineer the protocol or include in a radio


Is it possible to transmit a high-resolution image over an analogue signal?

Bandwidth is the main issue. An HD signal has about four times as many pixels as an ordinary TV picture, so you'd need most of the range of FPV frequencies just for one pilot. I think that would also require four times the transmission power to get the same range.
Then the problem is that all the gear we use is based on old CCTV chips rather than having been developed specifically for FPV (which is one reason the channel allocations are such a mess). I guess no one built a CCTV system for just one HD camera - multiple cameras are important in CCTV, and lag isn't an issue, so they could use digital compression.
So, I guess the cost to develop a custom system is the big issue, especially if it might have a limited market, or become obsolete in a few years as faster, cheaper processors allow digital systems to overtake it.


How does having bent antenna impact reception?

Antennas are weird and the answer isn't really clear cut.

As long as the wire within the antenna isn't damaged, it should work fine (1, 2).

Sometimes bends in antenna can improve it, like with NASA's Evolved antenna for the ST5 spacecraft.


The Betaflight firmware has very few GPS assistance modes. AFAIK, the only one is GPS Rescue Mode which allows the quad to return to where it took off in an emergency.

For more GPS modes, I'd recommend looking into the iNav firmware, which has a lot more options for you and supports the Mamba F405 flight controller. The iNav GPS mode you'd be looking for is the NAV POSHOLD mode, which allows a multirotor to maintain a constant 3D location in space.


Datasheets for lift of a 27x12x2 inch propeller?

There are too many factors to be able to give you a precise plot. All the below have major impacts on propeller performance:

Activity factor

A blade with a thicker chord will generate more thrust, but have less efficiency

Airfoil selection

Prop airfoils change as a function of radius. Some airfoils are good (bad) at low Reynolds and/or low Mach, but are bad (good) at high Reynolds and/or high Mach

Proximity to ground

If you're close to the ground, pushing air at the ground, then you have a good efficiency bump from ground effect.

However, https://www.apcprop.com/files/PER3_27x13E.dat is a good first step. The change in pitch from 12" to 13" is not going to dramatically change matters, and probably has less of an effect than flying in ground effect or airfoil changes.

You can also compare to https://www.apcprop.com/files/PER3_26x13E.dat, which reduces diameter by an inch, while increasing pitch by an inch. You will likely find that a theoretical APC 27x12 will be in between the two in terms of performance.

I hope this is helpful. Ultimately, once you get close enough to your desired theoretical performance there's no option but to try a few blades out for empirical results.


Multirotors are inherently a very noisy environment for sensors like the gyroscope which try to measure the actual motion of the drone. Mechanical vibrations and oscillations are present all over, primarily generated by the motors but also by parts in the frame. These vibrations manifest themselves as fluctuations in the gyroscope values.

 (cit.)

Flight control firmware relies on a clean measure of how quickly the drone is moving in order to function properly, so they employ a number of techniques to "filter" the gyroscope signal and remove as much of the noise as possible. Unclean signals can result in more inflight oscillations and hot motors. Several different kinds of filters are used, often simultaneously, to accomplish this. (e.g. static & dynamic notch filters, low/high pass filters, Kalman filters) The below image depicting the suite of filters employed by Betaflight helps get across how serious an issue this is, although it isn't quite accurate today. (the image was accurate in some 3.x versions)

RPM filtering (new in Betaflight 4.1) seeks to remove motor noise, one of the most prolific sources of vibrations on a multirotor. The frequency of motor vibrations are directly correlated with their rotation rate, so the Betaflight RPM filter takes input from the ESCs about motor speeds (using bidirectional DSHOT) and uses it to target a series of dynamic notch filters which selectively remove noise from the gyroscope values occurring at a specific frequency. There's a whole page on the Betaflight wiki about this.

Articles like this from Oscar Liang and this on the Betaflight wiki
provide instructions for how to set up and configure the RPM filter, as well as this video from Joshua Bardwell on the topic.


What is the established drone racing etiquette?

I run a small MultiGP chapter, so things are slightly more relaxed in our chapter compared to some of the big chapters. But here are the official MultiGP rules for pilots:

Fly only in designated areas.
Fly only at appropriate times.
Keep your multirotor at least (30) feet away from the pilot area. Flying any closer can
cause your video transmitter to interfere with other pilot’s reception.
When finishing a race, land ON the course (at least 30 feet away from the pilot area)
When flying on the course, you must be positioned within the designated Pilot Area.
DO NOT fly from any other area.
Only power up your multirotor/video transmitter while in the Start/Finish area on the course. DO NOT
power up in the pits or while walking from the pits to the course
If you need to check your video channel or change frequency, this MUST be completed between heats. You
MUST also ensure you have the appropriate Frequency Card for the frequency you wish to check.
If you are flying before or after the event you MUST ensure you have the appropriate Frequency Card for the
frequency you are using.
You MUST ensure you have a Spotter in the Pilot Area while flying. A Spotter observes the course for
dangers such as members of public, animals, other model fliers, low flying aircraft or vehicles. Spotters
stand in the Pilot Area to ensure clear communication.
You should help with spotting for the heat before yours.

And some more rules we incorporate:

If video channels are assigned ahead of time, be sure to show up with all of your drones set to that channel (though changing after arriving is not unusual).
All transmitters (video and remote) should be set to 25mW to prevent someone knocking others out the air.
Never enter the race coarse unless the safety officer gives you permission. But keep an eye out for people who might fly during inappropriate times.


I really like the way the Joshua Bardwell does it.

He puts one on each rear arm facing forward with a zip-tie and heat-shrink over the top. It places the antennas at 90 degrees to each other and so you get good coverage from at least one antenna.

On the picture below, the yellow line is a zip-tie and red curve is the antenna from the XM+

Remember not to put too much of a tight angle on the antenna wire as it can deform the inside of the cable and have a negative effect on the signal receiving ability.


How to reset connection between drone and RC for MAVIC Air 2

According to page 36 of the DJI Mavic Air 2 manual the aircraft should be linked to the remote controller on arrival.

If not, you should follow these steps:

  Power on the remote controller and the aircraft.
  Launch DJI Fly.
  In camera view, tap (three dots)  and select Control and Connect to Aircraft.
  Press and hold the power button of the aircraft for more than four seconds. The aircraft beeps once indicating it is ready to link. The aircraft beeps twice indicating linking is successful. The battery level LEDs of the remote controller will glow solid.

I think the issue was you didn't try to link it through the DJI Fly app.


All LiPo batteries face a process called electrolyte decomposition. This means that all LiPo batteries will produce oxygen, some carbon dioxide, and some carbon monoxide through normal use. This process, however, is greatly sped up when certain things happen to a LiPo.

These are the things that will cause noticeable "puffiness" in LiPo batteries:

Over-Discharge. Don't let a LiPo go below the minimum safe voltage (Example: if you fly your drone too long and the battery gets below ~3.3v per cell) and don't pull too much current from the LiPo. (Example: using a low C-rating LiPo, like 20c, on a high performance race drone.) Make sure you pay attention to battery voltage when flying and make sure you have suitable C-rating.
Overheat. Exposure to heat can cause battery puffiness. Make sure to let LiPo batteries cool off before charging and don't fly when it is too hot. Also, make sure to store LiPos in a cool location.
Overcharging. If a LiPo is charged higher than 4.2v per cell, it can cause puffiness. Make sure you use a good balance charger to avoid this.
Improper storage. Storing LiPos at full capacity is a major cause of puffiness. Make sure to storage charge LiPos before putting them away for any significant time. If you have a good charger, storage charge is one of the options. Also make sure to store LiPos in a cool/cold area.

More in-depth information here.


Are there specific laws or regulations in the USA barring weapons on drones?

I’m not a legal expert, but according to the following link to an FAA page, it is illegal to attach any dangerous weapon to a drone: https://www.faa.gov/news/updates/?newsId=94424


How to determine the largest LiPo I can use in a quadcopter?

I've heard that you want to keep your battery weight no heavier than 1/3 your drone weight.


If it were me, there are two different kinds of drones I would want. A tiny whoop and a cinewhoop.

Tinywhoop is a loose definition of a very small drone that's about 3 inches wide with guards/ducts around its props and weighing anywhere from 20-50g. They are very durable and fly in places nothing else could. You can assemble them yourself (very easy) or buy one pre-assembled for around $100. You can add a small, powerful LED that allows some pretty fun exploration in dark and very tiny spaces without the risk of loosing a larger, more expensive piece of equipment. Example video of someone flying in their house with the lights out

A cinewhoop is a larger drone, more like 6-7 inches wide and weighing 200-400g, also with ducts. Typically it's called a cinewhoop because it's about the smallest thing you would want to fly that can comfortably carry a GoPro. You could equip this with a much more powerful light and have a real blast flying around. Like tavis mentioned the DJI FPV system would be an excellent setup for caves as it has great video quality and is very resistant to signal multipathing which is a very common affliction of analog video systems like what you would install on a tiny whoop. Multipathing is likely to be a very real issue inside a cave. If you could live with the analog break up though there are few 'starlight' type analog cameras that can operate in very dark environments. However, I don't think that would be nearly as enjoyable or as useful as a 1000+ lumen flashlight strapped to a DJI equipped cinewhoop. That would be awesome. Video here of DRL pilot NURK doing a better job of explaining what a cinewhoop is.

I would say your typical DJI consumer drone would not be well suited to a dark cave. Without GPS they would rely on downward-firing cameras for position hold which doesn't work in the dark. You could possibly rig up light to assist the cameras but I would guess that it still wouldn't work very well.


I have found where the simulator went, no where. The problem I was having was related to a common theme I'm seeing with many drone apps.
The simulator seems to be available only with iOS versions of the DJI Go app. We converted over from Android for to an unrelated app offering more features with the iOS version than the app's Android version.
So if you're using an Android device and want to have the DJI app, you need to make the transition to an Apple iOS device.


This has already happened to me, the problem was ESC configuration.
I was using a LiFe battery (3.3V/Cell) and the Low Voltage Cut-Off Threshold ESC configuration was about to cut the current with 3.2V/Cell. Look for the programmable settings of your ESC and check how it is.


Motor will not go above half thrust. Why and how to fix?

This sounds very much like like a PWM/analog ESC miscalibration issue to me. PWM ESCs required signal calibration to ensure that both the RC receiver and the ESC are on the same page about what kind of pulse length constitutes both 0% and 100%, as there can be clock speed discrepancies and other issues with the protocol.

I can't exactly find a source I like for this procedure to explain it better, so here's my rendition: (modified from those listed here)

With ESC power unplugged, connect the ESC signal to the receiver's throttle channel.
Turn on the transmitter and receiver and move the throttle to 100%.
Enable power to the ESC and wait for it to beep a few times indicating it's in calibration mode.
Briskly lower the throttle channel to 0% and wait for the ESC to beep to indicate it's now calibrated
Try to operate the ESC like normal and see if the full-throttle range has been restored


Is there a benefit to using multiple capacitors on FPV drones?

Capacitors come in various sizes and values, so one big capacitor could equal two smaller ones in value; I do not know for certain if this offers a weight saving, but can reduce PCB footprint size by being 'taller' (although, you can stack capacitors of the same footprint.)
A reason to have more than one would be to adjust the frequency response, as a capacitor's reactance is related to the frequency and the capacitance:
$$X_c=\frac{1}{2\pi fC}$$
where $X_c$ is the capacitor's reactance, $f$ is the frequency of the spike/noise and $C$ is the capacitor value.
To this end, you often see multiple capacitors for decoupling or filters; however, in an application where the aim is to decrease voltage sag it is less crucial. There are almost certainly more capacitors on the board, which may be in parallel to the larger capacitors (design dependent), but positioned close to the ICs and other parts so that each component gets the voltage it requires.


What are 'Modes' of a transmitter / controller?

The Mode of a transmitter refers to which stick on the transmitter controls which movement on a drone. There are 4 modes (named Mode 1 to 4) but the most common are Mode 1 and Mode 2.

The image below shows which control is associated with which aircraft axis, for each mode. The black text refers to fixed-wing aircraft and the blue text is the multirotor equivalent.

Mode 1 and 2 are the most common flight modes used.  Most multirotor pilots fly mode 2 (although there are exceptions, one of the OG FPV pilots FinalGlideAus notably flew mode 1).  Almost all "toy" multirotors are Mode 2.  In fixed-wing RC mode 2 is very common in the US, while Mode 1 is popular elsewhere.

When starting out, find out what mode the people you're likely to fly with use.  Having a more knowledgeable pilot being able to fly your drone, or being able to watch their fingers and see how they're doing a particular trick, or for them to be able to coach you easily is enormously beneficial.  Once you're comfortable, if you want to explore the other modes, and see if something else feels better, then go for it.  If you don't plan to fly with anyone else and have no strong opinion, go mode 2.  Virtually all of the stick cam tutorials you'll watch online are mode 2.


Technically, you could connect 2 motors if they're exactly the same and it will probably work. However, there is a good possibility of various weird things happening due to this.
The problem with connecting more than one motor to a single ESC stems from how these motors are driven. An electric motor works by using electromagnetic coils to pull permanent magnets and create a force on the rotor. However if you just supply current to a coil and leave it as is, at first a closeby magnet will get attracted to the coil, rotating the motor bell, but then it will just sit there in the nearest position after oscillating back and forth a bit like a pendulum.
To create a continuous rotation, you need to stop the current as the magnet passes by and then reverse it to now repel the magnet, adding more rotational momentum to the system. In a brushed motor this switching is done mechanically using contacts (brushes) inside of the motor. A brushless motor, on the other hand, does not have this kind of mechanical switching mechanism inside, so the switching must be done externally, and this is the brushless ESC's job.
To determine when to switch the currents around, the ESC must detect when the magnets are passing the coils. To do that it constantly measures the voltages on its three terminals (as a magnet passes by an electromagnetic coil, it creates a voltage in it), and uses that information to switch the currents in the motor in perfect sync with the motor's rotation, speeding up the current pulses as the motor speeds up and slowing them down as it slows down. Real-world ESCs and motors are not perfect, though. sometimes the ESC is incorrect in its assumptions about what the motor is doing right now or how it's going to react, and sends pulses that don't correspond to the motor's actual position. This is called desynchronization, or, colloquially, desync, and results in the motor suddenly losing almost all of its power for a short amount of time (usually less than a second) while the ESC tries to determine the correct state of the motor and reapply power in sync with its rotation. With quadcopters this condition is most common when the motors are spinning at very low RPM, loaded by a reverse airflow (e.g. falling out of the sky bottom-down) and need to suddenly speed up, all of which mess with the ESC's expectations, and can lead to a violent uncontrolled spin.
Now, if you connect two motors, the ESC will still think it's one motor. If these motors are sufficiently different, the most likely thing is that one (or both) of them won't work. Different motors will both create voltages of different magnitudes when rotated, and react with different speed to currents flowing through their coils, so one of the motors will invariably get far ahead of the other in response to a current pulse, making synchronization of the two near-impossible, and the ESC very confused. Depending on the ESC and motor combination, it can either give up entirely, or try to drive this weird "motor" anyway, likely leading to one of the motors spinning at reduced efficiency and the other just jerking.
If the motors are identical, though, e.g. of the same brand and model, and with identical propellers, they are likely to want to rotate at about the same speed, in which case they will produce more or less the same induced voltages in their wires and react the same to what's supplied by the ESC. So this setup will probably work, at least on the ground. Even then, nothing is perfectly matched. One motor will likely be slightly ahead of the other, or otherwise slightly off, which is probably not enough for triggering a desync, but means a more confused ESC and less efficiency overall, as one of the motors (or both) is driven suboptimally.
Further, when you mount the motors on an actual airplane, conditions may be even more unequal. Suppose that one of these "identical" propellers is chipped, or the plane is flying in a crosswind, or even that it's just in a banked turn, so one wing moves through the air slightly faster than the other. Then the load on the motors can differ considerably even though they are the same. Because of that differing load, one of them might get just enough ahead of the other to confuse the ESC into either giving up entirely, or making erratic assumptions about this weird combined "motor", leading to wrong signals being sent, which might, for example, slow down one motor and speed up the other, possibly ending in one of them desyncing and stopping entirely. This, in turn, can potentially lead to very unpredictable behavior of the model in flight.
So, to conclude this long-winded explanation, the answer to your question is:
Don't. Just don't.


How do DJI drones achieve such long flight times compared to traditional FPV drones?

There's no magic technology, it's just a difference in design goals. FPV drones are designed for speed, performance and robustness, DJI drones are designed for duration and carrying a gimbled camera.
Larger, slower turning rotors are more efficient at producing lift but have a lower top speed. That's why helicopters can hover and propeller planes with the same size engine can't - but the plane is faster. Photography drones (such as DJI) use large rotors and exploit that efficiency for duration. Race drones use smaller rotors because they want to go as fast as possible.
The other factor is that photography drones typically carry larger batteries for longer duration, while race drones want to keep their weight down for maximum agility and acceleration. A photography drone will fly so long as it can produce more thrust than its weight, typically they have less than a 2:1 thrust-to-weight ratio and can't maintain altitude when angled more than 45 degrees from horizontal. Race drones can have a 10:1 thrust to weight ratio, and commonly fly at pretty extreme angles.
So long as a race drone has enough duration to complete the race, any extra battery capacity is just dead weight, slowing it down. For their pilots, changing the battery every few minutes is a small price to pay for the performance.


How can I take aerial photography without leaving my house?

When flying in the UK you need to keep your aircraft within 500m horizontally of yourself, below 400ft above ground level and within visual line of sight. If you are able to satisfy these conditions, then you should be able to safely fly the aircraft from your home to the location, take the pictures, and return home.

If, however, you cannot meet these criteria then unfortunately you are unable to complete this flight safely during the lockdown.

Information on UK flying regulations, including distance rules, are available from the Drone Safe website - this is well worth a read, especially the "Drone code" page, and knowing this information will help when/if registering as a drone user.

You should also give thought to what you would do if you had an in-flight emergency - if your drone crashes or you have to land away from home, will you be able to recover it?

Edit: You mention in a comment to this question that you live in an apartment building; this complicates things a little more, specifically due to CAP 393 Article 95 which states

  95.—(1) The SUA operator must not cause or permit a small unmanned surveillance aircraft to be  flown  in  any  of  the  circumstances  described  in  paragraph  (2),  and  the  remote  pilot  of  a  small  unmanned surveillance aircraft must not fly it in any of those circumstances, except in accordance with a permission issued by the CAA.
  (2) The circumstances referred to in paragraph (1) are—
   (a) over or within 150 metres of any congested area;
   (b) over or within 150 metres of an organised open-air assembly of more than 1,000 persons;
   (c) within 50 metres of any vessel, vehicle or structure which is not under the control of the SUA operator or the remote pilot of the aircraft; or
   (d) subject to paragraphs (3) and (4), within 50 metres of any person.
  (3) Subject to paragraph (4), during take-off or landing, a small unmanned surveillance aircraft must not be flown within 30 metres of any person.
  (4) Paragraphs  (2)(d)  and  (3)  do not  apply  to  the  remote  pilot  of  the  small  unmanned surveillance aircraft or a  person under the control of the remote pilot of the aircraft.
  (5)In  this  article,  “a small  unmanned surveillance aircraft”  means  a  small  unmanned aircraft which is  equipped to undertake any form of surveillance or data acquisition. (b)

(Emphesis mine)

Therefore, unless the building is under your control (as a house might be) you should keep 50m away. What exactly constitutes a building under your control might be another question...


How to record footage from AcroBee Lite?

I just scanned the manual for "record" and nothing. I looked at the build and I see no indication of recording capability.
There are 3 options (unless there is more information):

Record on goggle. Some goggles have a DVR built in, generally low quality.
Record on a dedicated DVR on device, goggle, or another receiver.  I have used all of these options, DVR's start at $20.
Mod it and put in a recording camera (if possible).  This will get you to 1080p fast (if possible).  RunCam, Caddx, and Foxeer all have models. I was working on this spreadsheet last week for our sub-250 units researching mounting trends - it is not complete but has a lot of the key players.

I know the New Bee guys and they should be able to help you out.


This will be very hard to do.

If the shaft is very bent, I advise ordering a new one, if slightly bent, you can lay it flat on a table and try to straighten it with a hammer or vice.


Estimate position of a balloon attached to a quadcopter

The simple answer would be a potentiometer, as used for position feedback in a servo. You apply a voltage to the outer two contacts and measure the voltage at the moving, middle connection.
The next step would be a rotary encoder. These should give you less friction and more accuracy, if you need it.
A camera would require significant processing power, but might be viable on a larger drone. Measuring α would be tricky with a camera on the drone, but you could calculate it from the distance from the centre and the length of the tether.


ESCs loaded with recent versions of BLHeli (BLHELI_S and BLHELI_32) will support reversible motor direction, either through Dshot commands (obviously only supported when using Dshot signaling) or with zero-throttle centered around the middle throttle position, 1500µs (supports Oneshot/Multishot in addition to Dshot).

These modes can be set up in the BLHELI Suite configurator.


Is WiFi drone control protocol standardized?

Since no one has answered this question in pretty much a day, I'm gonna spread my half-knowledge now :)
I have never heard of a standardized wifi drone control protocol. However, I wouldn't be surprised if both chinese manufacturers inofficially use the same protocol (be it because they have a contract or because they just copy from each other or because they use the same control chips). I encountered this with a lot of LED strips: Most of these strips can be controlled with the same IR remote - across brands and so on. Even though, IR and WiFi are vastly different things, I think this could be the case here as well.
As for using the same control chips, this is also often the case with toy RC cars (27 and 40MHz). Most of the time, you can just use your remote to control your friend's car. I discovered that in kindergarden ^^.


How can foreigners fly non-nano drones in India?

Micro drones are between 250 grams and 2 kilograms:
Micro drones
These need registering, so therefore a foreigner will not be able to fly them. What you stated is for commercial purposes only. If you do want to fly commercially you will have to do some research before you travel, contacting an Indian entity and asking them to register it for you. This can be quite a hassle so I wouldn't do this unless it was really important.


This is done by placing streaming video data (RTP) directly into the Wi-Fi frames. Checkout https://github.com/svpcom/wifibroadcast/ project for more details.


How to update the firmware of a T16 Pro's internal multiprotocol module?

The first thing to do is to check that you have the bootloader. Newer radios should have it installed. (Just check that you have Multiprotocol Module firmware v1.2.1.85 or later) If you don't have the bootloader installed, here is a video on how: Jumper T16 "Update Recommended" Fix (how to flash internal multiprotocol module).

Assuming you have the bootloader, the next step is to go here: MULTI-Module Firmware Downloads. Here, you can get the latest firmware for any multiprotocol module, so this works for more than just the T16. On the left, find the module that you want to update. In this case, the Jumper T16 Pro Internal module.

After that, most of the information is filled out so just fill in the rest (in this case just channel order)

Get the file that corresponds to the channel order of your radio:

Put the file onto the FIRMWARE folder of the micro SD card from the Jumper T16 and then put the card into the T16 and power it on. Long press the system key, press the page button once, scroll down to [FIRMWARE] and click it, long-press the new file, and then Flash Internal Multi.

If you need a more comprehensive guide, check out Joshua Bardwell's video: Jumper T16 "Update Recommended" Bootloader Method (how to flash internal multiprotocol module).


For this drone, a 3s 1500mAh-2200mAh LiPo battery should work. (example 1)(example 2) I'm not completely certain that this will be the right battery, but it is extremely likely that this will work, because it would make sense with the KV of the motors.
There are several things to understand when it comes to choosing a good battery and I'll simplify a few of those things below:

Voltage: Voltage is measured in V (volts). The voltage is determined by the number of battery cells connected in series. So 3s = 11.1V and 4s = 14.8V. For your drone I would recommend 3s because the ESC may not be able to handle a higher voltage.
Capacity: Capacity is measured in mAh (milliamp hour). More capacity means more power that can be used by the drone, but also heavier. It's best to balance this so that you have enough power for a decently long flight but not so heavy that the drone can't fly. For the size of that drone, 1300mAh - 2200mAh should work well.
Type: Most drones use LiPo batteries. LiPo batteries are used because of their great power to weight ratio. LiPo would be the best for your drone. There are, however, alternatives.


Betaflight attitude update rate only 100hz?

I haven't quite dug into the source code to confirm my suspicions, but this file lends credence to the idea that this task facilitates accelerometer-based attitude calculations and not gyroscope-based calculations.
From line 274 to line 280 in the file you link to (tasks.c), the TASK_ATTITUDE task is said to be enabled if the accelerometer device is enabled (in the Betaflight Configurator):
#if defined(USE_ACC)
    if (sensors(SENSOR_ACC) && acc.sampleRateHz) {
        setTaskEnabled(TASK_ACCEL, true);
        rescheduleTask(TASK_ACCEL, TASK_PERIOD_HZ(acc.sampleRateHz));
        setTaskEnabled(TASK_ATTITUDE, true);
    }
#endif

Given that these are the only times that the TASK_ATTITUDE task is mentioned in this file, I doubt that this 100 Hz task is related to gyroscopic sampling.


Do laws on 'drones' in Australia apply to fixed wing model aircraft?

In case you would prefer an answer with citations (as different regulators have different views on this), at the following link is CASA web page regarding "Recreational unmanned aircraft (including models)". There is a link on this page to the  "... rules for flying model aircraft and drones." - this phrasing demonstrates CASA consider model aircraft and drones to be subject to the same regulations.
However, there are some differences in Australian regulation between commercial and personal/recreational use - follow the links above for the latest information.


ESCs for these tiny motors are always built into a single board that also contains the flight controller (and often the receiver and video transmitter). I'm afraid you won't find them individually. The quadcopters that they're designed for just don't have room for separate components, and everything is so tiny that the plugs and wires would be a significant weight penalty.
That said, there's no reason that a larger ESC won't work. The smallest generally available is about 6amp, but you might find 10 or 12 amp units for the same price and size - small stuff is rare and doesn't get updated as often as the more common sizes.
FWIW, I think that's a 3 pin JST 1.25mm connector. Measure the pin spacing to check.


What size screws do I need for this motor

4-M3 means that you need four screws (on each side), each with a metric M3 thread. M3 is 3mm diameter.
The length of the screw obviously depends on the thickness of the thing you're bolting the motor to, plus a little for the base of the motor.
The amount of bolt in the motor is important. Obviously you need at least a couple of mm so that it grips, and there's another couple of mm of clearance, but any more and it'll touch the coils. It won't stop you spinning the motor by hand but it'll break the insulation on the coils, short the coil to the case and the motor won't work.
So if you're bolting through a 5mm thick frame, order 8mm long M3 screws. They tend to only come in 2mm steps. The length quoted is the specification is the length of the thread, not counting the head.
If you don't fancy cutting and filing a slightly long screw, you can put extra washers under the head.
Also, when ordering screws, you want 'cap head' not 'dome head' or 'countersunk' in order to use the largest possible Allen key. The small ones tend to round off and slip.


Series or parallel connection for drone batteries?

Batteries in series = add the voltages, ie two 3S 1300 30C packs becomes 6s 1300 30C.

Batteries in parallel = add the capacities, as above you get 3S 2600 60C.

There's no particular pros or cons for either, many larger commercial drones use combinations of packs in series and parallel to get the power source they need.

For smaller quads though the overhead you get in extra weight from the leads, connectors and wiring is greater so it's generally less practical.


How useful is knowing the torque generated by a motor prop combination?

Not very useful.

If you're designing at the absolute limit of what a frame can handle, such that it's as light as possible, then maybe you'll want to know the maximum torque, but you should be designing for orders of magnitude more strength than that (It's not hard, small BLDC motors aren't very torquey).

There are control algorithms that take into account frame torque and how the forces are applied. I forget off-hand, but it was a matrix-solving control algorithm, as opposed to PID. Unless you're writing your own flight control software, that's not an issue.

When changing motors, it may give minor insight into tuning the yaw axis (these motors have more torque, so less P-Gain may be needed), but yaw is done by applying torque to a propeller, so again unless the motor can barely spin the propeller (this should not be the case in a design), then this isn't a concern.

Where you may see an advantage is when computing efficiency - ie how much torque do I have to apply to propeller x to get y grams of thrust? However, this is generally done by using current figures since those are more relevant to power system design.

Another possibility is how much braking/accelerating torque a given ESC can provide - high performance machines needs rapid changes in motor RPM, so a given ESC may be better (due to FET latency, resistance) at control algorithms than others. This would be interesting data to see, but I don't think any ESC is necessarily better than another, since braking is just "FETS on" and accelerating is just pushing the control algorithm to its max. Noisiness is much more important than acceleration torque in practice.


What is this 4th row of pins on some receivers?

Modern RC receivers that support PWM/servo output will also support other output modes, which is what the fourth row of pins are usually for.

The extra row of pins you notice on the FlySky FS-iA6B receiver is for extra features of the iBUS signal output. The regular three-high 6-channel PWM outputs are present too.

The FS-iA6B has three different major output modes:

PWM: Each channel (1-6) is output on its own servo channel
PPM: Each channel (I think either 1-8 or 1-10) is output sequentially on one servo channel in accordance with the CPPM (combined PPM) protocol

If enabled, CPPM is output on the PPM/CH1 servo channel

iBUS: Each channel is output over a serial UART connection

If enabled, serial iBUS is output on the PPM/CH1 servo channel
Special auxilary features of the iBUS protocol are available on the 4th row of pins, but these are rarely used. The iBUS signal channel provides all the features usually required fy RC aircraft.

Of the three, iBUS is preferred for use with modern hardware if its supported because of the error-resistant and precision qualities of digital signaling protocols over "analog" ones.


How to avoid a drone going out of range?

The first stage is to make sure you read your local laws about altitude and VLOS restrictions.

Once you know what you can legally do, the first stage is to set up a failsafe. Do this by setting failsafe conditions in your radio and in whatever firmware your quad uses. Choose what you want the quad to do during a failsafe, whether it should drop out of the sky or slowly lower.

You can also install GPS and set the failsafe as a return to home, and depending on your firmware there are different ways of doing this.

Another way to make sure you stay in the range is to set up RSSI or LQ depending on your system. This tells you how strong the signal is so you can monitor it.

You can also get an RF meter to make sure there is no interference where you intend to fly.

Finally, you can use a specialty long-range system like TBS Crossfire or FrSKY R9, as long as it is legal in your region.


How should one chose the correct motor KV for a build?

I think it’s learned through experience and it can also be roughly figured out using this table based on frame and propellor size:

    ╔══════════════════╦═══════════════╦═════════════════╦══════════════════╗
    ║ Frame size       ║ Prop size     ║ Motor size      ║ KV               ║
    ╠══════════════════╬═══════════════╬═════════════════╬══════════════════╣
    ║ 150mm or smaller ║ 3" or smaller ║ 1306 or smaller ║ 3000KV or higher ║
    ╠══════════════════╬═══════════════╬═════════════════╬══════════════════╣
    ║ 180mm            ║ 4"            ║ 1806            ║ 2600KV           ║
    ╠══════════════════╬═══════════════╬═════════════════╬══════════════════╣
    ║ 210mm            ║ 5"            ║ 2204-2206       ║ 2300KV-2600KV    ║
    ╠══════════════════╬═══════════════╬═════════════════╬══════════════════╣
    ║ 250mm            ║ 6"            ║ 2204-2208       ║ 2000KV-2300KV    ║
    ╠══════════════════╬═══════════════╬═════════════════╬══════════════════╣
    ║ 350mm            ║ 7"            ║ 2208            ║ 1600KV           ║
    ╠══════════════════╬═══════════════╬═════════════════╬══════════════════╣
    ║ 450mm            ║ 8", 9", 10"   ║ 2212 or larger  ║ 1000KV or lower  ║
    ╚══════════════════╩═══════════════╩═════════════════╩══════════════════╝

(table source)