What are the benefits to different types of frame arrangements?

Unfortunately it really seems to be a personal preference thing.
I tried logging the same move on both pitch and roll on a stretched x frame to determine exactly how much difference there was and I couldn't find any difference in the results.
Stretched x are generally preferred for racing, but Thomas Bitmatta decided on squished x for his JS-1 frame.
For definitions:

True X - Symmetrical in both X and Y dimensions
Squished X - Wider than it is long
Stretch X - Longer than it is wide
Deadcat - Wider on the front than the rear
Hybrid X - Wider on the back than the front (See XBEE MCK frame).

The primary advantage of squished X and Deadcat is that you can keep the props out of the frame of the camera if you are running low camera angle.


Can circularly and linearly polarized antennae be used together for FPV?

The compatibility of mixed polarity antennae depends greatly. For a little background in RF polarity, this diagram shows the difference between the four main types of antenna polarization available: (mixed/elliptical polarization do exist)

 (cit.)

In general, it is best if the TX and RX antennae match in polarization, but some other combinations are possible while minimizing signal strength loss.

Linear → Linear

Linearly polarized antennae are interchangeable, as long as they are oriented in the same direction. Signal reception between linearly polarized antennae oriented with an angle between them of θ will fall off with a polarization loss factor (PLF) of PLF = cos^2 (θ) and reception will be zero when the antennae are perpendicular to one another.

Circular → Circular

As long as the circular polarization is identical (RHCP or LHCP) between the two antennae, no signal loss will be observed due to the polarization of the antennae. Signal strength isn't dependent on the angle between the orientations of the circularly polarized antennae. However, if you aren't using omnidirectional antennae (e.g. mushroom)¹ and instead have a directional antenna (e.g. patch, helical), you must point the directional antenna in the direction of the other antenna to have the most gain. (see "What is antenna gain?"). Within the beam width of the directional ciruclarly polarized antenna, orientation doesn't matter.

As pointed out by @FlashCactus, only circular → circular polarization matches benefit from the multipath interference reduction effect. This is because circularly polarized waves switch polarization when they bounce off of an obstruction (i.e. RHCP → LHCP and vice versa), so duplicate signals taking an odd number of bounces to reach the receiver will be ignored by the antenna. (they will be recieved as the opposite polarization with which they were transmitted)

For more details on how circular polarization works, I (and @FlashCactus) recommend having a look at this question: What is circular polarization in antennas?

¹: Omnidirectional antennae are also not truly omnidirectional; they have a region of low gain along their axis, and will transmit/receive poorly in that direction. It is best to face them sideways towards the other antenna for best reception. The same is true for linear antennae; they receive&transmit best from their sides and worst along the antenna.

Circular → Linear

Because a circularly polarized wave can also be represented by two 90° out-of-phase perpendicular linearly polarized waves, the linear antenna will receive with a signal strength reduction (PLF) of half (or -3dB). Signal strength isn't dependent on the angle between the orientations of the linearly polarized and circularly polarized antenna.

This configuration can be useful if you want to spectate someone who has the opposite transmitting antenna handedness from you (LHCP while you use RHCP, for example), or to scan the radio spectrum that might contain signals of different polarization (RHCP, LHCP or linear) without switching the antenna: A linear antenna will receive both RHCP and LHCP signals equally well.

Linear → Circular

Likewise from before, because the circularly polarized antenna is built to receive both perpendicular components of the circularly polarized wave, it will receive a linearly polarized wave with a signal strength reduction (PLF) of half (-3dB). Signal strength isn't dependent on the angle between the orientations of the linearly polarized and circularly polarized antenna. As with the opposite configuration, RHCP and LHCP antennae will receive linearly polarized signals equally well.

This configuration can be commonly seen in the case of micro-quads like Tinywhoops which only come with linearly polarized transmitter antennae. Here, the orientation-agnostic benefits of receiving a linearly polarized signal with a circularly polarized antenna (and the convenience of not replacing the antennae on your goggles) are often worth the signal strength reduction.


There isn't an easy solution, because this isn't a common thing to do.
The main issue is that real-time control for FPV requires very little lag. Wifi requires several layers of processing, on a computer where it has to compete with other processes for processor time. Your video needs to be encoded and compressed, and still work if some data is missing. Most digital video isn't real-time, and relies on a buffer to smooth out these delays - but you can't fly with images that are a few seconds old.
That said, it's pretty simple to stream video over wifi with a small computer like a Raspberry Pi, and there are lots of guides on how to do it. Similarly there are lots of ways to send other data over wifi - it's just networking, and there are lots of guides on how to use UDP or TCP/IP.
You probably can't use the same computer to actually fly the drone. Again it's a real-time system that needs to adjust the motors hundreds or thousands of times each second. I think you would be best to use a normal flight controller running iNav, and to send it control signals from the Pi, via S-Bus. Effectively the flight controller attempts to hover at a GPS position, and you send it a new position occasionally.
I've not actually done this myself, or even read about anyone doing it, but I suspect it has been done.


What should I look for when choosing an FPV camera?

When choosing an FPV camera, there are several things that you need to look out for:
Sensor Technology
Most modern FPV cameras use a CMOS sensor. These are digital sensors that generally tend to have a higher resolution (measured in TVL or TeleVision Lines) than the alternative, which is CCD analogue sensors.
Pretty much every modern FPV camera uses CMOS tech, however, there are pros and cons to each:
CMOS positives:

They are generally higher resolution
They are very common so finding support should be easy

CMOS negatives:

They can introduce a shimmering effect in the image, especially around the edges of objects or in places of high contrast. Some people dislike this effect so strongly that they won't even consider CMOS cameras.

CCD positives:

They are less prone to jello, or warping of the image due to vibrations
They can have better dynamic range, although modern CMOS cameras tend to surpass CCD in this
They tend to create a "softer" image that is more pleasing to the eye, without any "digital" shimmer.

CCD negatives:

They are more expensive to make
They may have slightly higher latency
Very few new CCD cameras are being made, so there is less to choose from.

There are many more pros and cons, so I recommend reading the source of this pros and cons list.
Sensor size
The sensors used in FPV cameras can have different physical sizes.
Bigger sensor sizes perform better optically by capturing more light, so cameras with larger sensors are generally considered to produce a better image, but are also more expensive.
Format
There are two formats in our FPV cameras: NTSC and PAL. To know which camera you should get, make sure you check what format your goggles can use. Most modern cameras can be switched between NTSC and PAL, so
this isn’t too much of a big deal, but make sure you can check.
Size
Make sure you pick a camera that is an appropriate size for your drone.
The size classes commonly used today, defined by the camera enclosure's width, are Mini (22mm), Micro (19mm) and Nano (14mm). (The "full" size was 28mm).
Adapter brackets are available for mounting smaller cameras in frames with larger camera mounts, but there's nothing that would help you to fit a camera that's too wide for your frame.
For example, a CADDX Ratel which is 19x19mm would not be appropriate for a standard Tiny Whoop.
Aspect Ratio
Most modern FPV cameras will also have switchable 16:9 or 4:3 aspect ratios. Check which your goggles is designed for and which you personally prefer, though if you get a switchable one this likely isn’t a big deal.
Lens
The lens on your camera will affect the field of view. In general, a shorter lens will give a wider field of view (source). According to the same article, a wider lens can give better image quality.
Weight
If you’re going for an ultra-light build, every gram counts, and some cameras will be lighter than others. If this matters to you, be sure to check out the camera’s weight.
Software and other intangibles
There is more to a camera's performance than just the optics. The software that the camera uses to "prettify" the image is no less important, and can make or break the flying experience. Camera manufacturers each have their own WDR, sharpening, denoise, auto-exposure and other algorithms in their cameras, that all affect the picture. There are also some manufacturer-specific bugs in this software, which you may or may not care about. All of that can only be felt by looking at the actual imagery, which brings us to...
Personal Preference
This is the most important section. There are many videos and reviews online that let you see the actual image produced by the camera. Watch them and see for yourself. Do you like the look of its image quality? If so then you probably want that camera. If you don’t like the image then you probably don’t want it.


Drone simulators for designing and flying aircraft?

I am not quite sure what kind of drone you'd like to build. However, here are the ones I use for a variety of multirotor and fixed-wing drones:

I love CoppeliaSim (formerly V-REP) for multitorotors. It makes it very easy to make certain assumptions about forces and torques, and to see the results in real-time.
X-Plane is great for fixed-wing drones. X-Plane simulates flight dynamics on a fluid-dynamics level so it's actually simulating the plane as it would fly based on its geometry.


OpenTX can be built from source for your receiver using various compile options which may be useful.

Enabling USB_SERIAL and CLI will allow connection to your pc using a serial connection, which you can use for bidirectional communication.

You will need to add code to the OpenTX firmware to accept commands from the pc, to control the radio. It may be simplest to utilize the trainer variables to control the outputs.

The changes required are:

in cli.cpp, connect the serial to the trainer variables:

int cliSet(const char ** argv)
{
  if (!strcmp(argv[1], "trainer"))
  {
    int thr;
    int chan;
    if ( (toInt(argv, 2, &chan) > 0)
       &&(toInt(argv, 3, &thr) > 0))
    {
        ppmInput[chan] = thr;
        ppmInputValidityTimer = 240;//PPM_IN_VALID_TIMEOUT;
        serialPrint("!");
    }
  }
  ...cliSet continues
}

in mixer.cpp, enable all trainer channels by default:

// comment out this check
//        && isFunctionActive(FUNCTION_TRAINER_STICK1+ch)

in trainer.cpp, clear all trainer signals after timeout:

void checkTrainerSignalWarning()
{
   ...
   else if (!ppmInputValidityTimer && (ppmInputValidState == PPM_IN_IS_VALID))
   {
      ppmInputValidState = PPM_IN_INVALID;
      AUDIO_TRAINER_LOST();
      memset(ppmInput, 0, sizeof(ppmInput));   //clear all trainer inputs
   }
   ...
 }

You should then be able to use commands over serial such as:

set trainer 1 500

to control channel 1 to a value of 500


Can a receiver get damaged if you power it up without an antenna?

Running a receiver without an antenna should not cause any damage to the receiver, as no RF power is sent to the antenna (if your receiver has an active antenna there may be a DC bias present, but this does not cause problems - you will often see this in GPS receivers.)
However, for completeness, it is worth noting that some modern RC receivers actually do transmit, to send telemetry and other data back to the controller; in this case 'receiver' is a misnomer, used to mean "the radio part in the aircraft" - and it is possible for damage to occur. The correct term would be 'transceiver', a portmanteau of 'transmitter' and 'receiver'.
The reason a transmitter can be damaged when run without an antenna is because the power sent to the antenna has nowhere to go. When a (matched) antenna is connected, the power is transferred to the antenna but without it the energy is reflected back down the transmission line to the output transistors. These transistors are not designed to handle that much power coming back, and can fail. The likelihood of this happening generally increases with transmit power, but it also depends on the rating of the output transistors so this is just a crude approximation.
(Transmission line theory deviates somewhat from the normal models of power transfer, where having nowhere for the power to go would create an open circuit, and no current flows.)


Can Reinforcement Learning be used for UAV waypoint control?

Reinforcement Learning can be used for UAV control. This video details a method to use RL for quadcopters.

In summary, you program a model of your UAV that effectively simulates your drone.
You then create a policy and value neural network to control your drone and train it using your simulation.

I'm pretty sure similar methods are used in zipline's autonomous drone delivery network that manages 65% of the entire country of Rwanda's blood deliveries.


My ESC in my stack just randomly exploded, any ideas why?

From your description I assume that the ESC in the stack blew up, not necessary the FC. This makes most sense as that is the part where the biggest currents are flowing.
Check if the electrolytic capacitor on the battery connection of the ESC still is properly connected as that absorbs the voltage spikes your motors generate during flying. If that capacitor is loose or located too far away, the voltage spikes can destroy the FETs on your ESC and you will burn up the ESC and the motor connected to that ESC.


You mention that this will be in the future. While the other answer covers the current situation well, you should note that the rules are due to be harmonised across Europe and this might introduce some changes.

https://dronerules.eu/en/recreational/eu_regulations_updates

The following image summarises the requirements for the new regulations:

This table refers to the new classes, which are intended to make it easier for a consumer to tell what they can do with their drone (new drones should show their class on their packaging) [source]:

Class C0 - (can be flown in all subcategories) Very small unmanned aircraft, including toys, that:

are less than 250g maximum take-off mass
have a maximum speed of 19m/s (approx. 42.5 mph)
are unable to be flown more than 120m (400ft) from the controlling device

Class C1 – (can be flown in all subcategories) Unmanned aircraft that:

are either:

less than 900g maximum take-off mass, or;
are made and perform in a way that if they collide with a human head, the energy transmitted will be less than 80 Joules

have a maximum speed of 19m/s (approx. 42.5 mph)
designed and constructed so as to minimise injury to people

The standards also cover other aspects such as noise limits, height limits and requirements for remote identification and geoawareness systems.

Class C2 – (can be flown in subcategory A2 [close to people] or A3 [far from people]) Unmanned aircraft that:

are less than 4kg maximum take-off mass
designed and constructed so as to minimise injury to people
are equipped with a low-speed mode’ which limits the maximum speed to 3m/s(approx. 6.7 mph) when selected by the remote pilot

The standards also cover other aspects such as noise limits (but different from C1), height limits and requirements for remote identification and geoawareness systems, plus additional requirements if it is to be used during tethered flight.

Class C3 – (flown in subcategory A3 [far from people] only) Unmanned aircraft that possess automatic control modes (such as found in typical multicopter ‘drones’) which:

are less than 25kg maximum take-off mass

The standards also cover other aspects covering height limits and requirements for remote identification and geoawareness systems.  There are also additional requirements if it is to be used during tethered flight, but there is no specified noise limit (because the aircraft is intended to be flown ‘far from people’).

Class C4 – (flown in subcategory A3 [far from people] only) Unmanned aircraft that do not possess any automation, other than for basic flight stabilisation (and so are more representative of a ‘traditional’ model aircraft) which:

Are less than 25kg maximum take-off mass


Is the Experimental Aircraft Association (EAA) an appropriate association for those interested in building RC aircraft?

As for EAA - No. They seem to be politically on the good side of drone (anti-RID / Docket ID FAA-2019-1100). They do not mention any CBO (community based organization) direction searching the site.  They are Oshkosh based, a legacy aviation mecca for stunt flight.  Their agenda is legacy (general) aviation and like the AOPA they are likely looking for money in a time of decline and diminishing interest.
Regarding CBO and recreational flight in the USA (current), the AMA is the closest, but they are not a CBO (yet). The CBO regs and the recreational flight test are late (and required).  Per the way the regulations work, until this is settled all recreational RC flight is in a "non-legal" (and not not totally illegal) state. If you do not fly under a CBO you need to be commercial (generally speaking).
Simply put, in the USA the technical requirement for all non-commercial 250gram+ recreational flight is to be conducted in accordance with a CBO.  You need the CBO rules but not the membership.
Look at the details surrounding the repeal of 336 and the following pivot in regulation and pay attention to the NRPM for RID (docket from above). I posted an anti-RID comment creator that goes over a lot of the drama under the hood.
There is a Federal requirement for CBO ruleset for rec flight. A CBO has a number of complex and limiting criteria that makes these orgs rare.
As a reminder on our dire situation, the FAA has missed all of the critical timelines for this work. The way the legislation is written the FAA recognized identification areas (FRIAs) will spell the doom of all rec RC flight in the USA due to a technicality still not addressed (short version: FRIA applications have a fixed window for application, after that no more can apply, so when a field has to move it has to close).
Right now there is no organization I would recommend joining and this is why I became a commercial (107) pilot. I fly under aviation rules, I insure myself and I am only beholden to the FAA and am entitled considerably more rights and privileges.  Anyone serious about this hobby should consider a commercial license, join that organization.
As for Canada, their laws are similar but more strict and actually enforced (unlike the USA).  You need some special permitting to fly more then 250 grams.


Can something be mounted underneath a quadcopter rotor?

If you block the airflow into or out of a rotor, you reduce the lift it can generate (approximately) in proportion to the area you block. That means the motors will have to work harder, and your flight time will be reduced.
Work out what percentage of the total rotor area your camera will block. If it's more than about 20% you should probably look for a larger frame.
Obstructions above the rotors are less critical when hovering (as the rotors are sucking in stationary air from all sides) but in forward flight the airflow is more linear - and rotors don't like working in disturbed air so it's worse for high speed flight.


4 in 1 ESC and FC won't turn on when connected together

Your description of the dramatic voltage drop (from 5V to 0.5V) upon the connection between the flight controller (FC) and 4-in-1 ESC makes me think that the issue you're seeing is most likely due to a short or low resistance path to ground. There are other possible failure modes that could cause this, but a ground fault sounds most probable here.
Why?
When a ground fault is created in one of the components attached to the 5V power rail generated by the ESC's BEC (Battery Eliminator Circuit), a substantial current greater than the BEC's rating for continuous power delivery flows. This is due to Ohm's law, $I = \frac{V}{R}$ which states that a large current (I) will flow if the circuit resistance (R) is tiny.
The usual mode of operation for power supplies when they experience surges in power demand is to drop their output voltage in an attempt to prevent catastrophic failures of themselves or downstream components by decreasing the power output. (Power $P = \frac{V}{I}$) This is a near-ubiquitous safety feature.
Confirming a Diagnosis of a Ground Fault
Since ground faults manifest themselves as low-resistance paths between the power rails, the way to test for the existence of one is to (use a multimeter) measure the resistance between the +5V and GND lines on the FC and the 4-in-1 ESC independently.
It would be best if you could fully disconnect all peripherals from the FC before measuring for a ground fault, as it is possible that the issue is with one of them and not the FC, but this can also be done once a problem with the ESC is eliminated to possibly avoid desoldering and resoldering the components back together.
NOTE: remember to take resistance measurements with no power attached to the device being measured!
If the measured resistance for the FC isn't high (the exact threshold differs between circuits, but the difference between a ground fault and a safe circuit is significant and should be noticeable), then there is a ground fault somewhere inside the FC. Likewise, if the measured resistance for the ESC isn't high, then there is a ground fault somewhere inside the FC. If both components have low resistance paths between the power rails, then they both have ground faults.
Resolution
You could either attempt to correct the ground fault in the affected component(s) or replace them. Sometimes the cause of the ground fault is some SMD (Surface Mount Device) component that is too small for you to comfortably work on or broken solder mask in the PCB, in which case fixing it may be more trouble than it's worth.
However, the only cost of trying to fix and not buy new is your time. And hey, you never know when a troubleshooting venture like this will teach you something new about electronics!


I am not a legal expert by any means, so I just want to say that disclaimer.

As far as I am aware, you can check in the drone itself, but due to their volatile nature, most airlines will require you to carry LiPos in your carry-on luggage so you can react quickly in case the worst happens.

As always, check with your specific airline and the laws specific to your country of origin and your destination.


What are the available solutions for creating complex user-defined UAV behavior in autopilots?

I think your best bet would be Ardupilot. The communication protocol it uses, called MavLink, is documented and easy to use. Connect to the UART with an Arduino and you will have full control over the UAV.

There is even an Arduino library and a bunch of examples, like this forum post, of people doing exactly what you are looking to do. I did this once because I needed to grab the GPS and Compass information for a work project. You can research the various mission control programs that use MavLink to see the extent of control the protocol has. If I remember correctly you can even send direct stick commands over that interface to manually pilot the craft. This solution makes for lots of flexibility, especially when you consider the huge community writing Arduino libraries. It would be simple to add just about any sensor you could want.


Why is an MPU6000 more well suited for drones than an ICM IMU?

It's not superior, it's just good enough, cheap enough and may be more noise resistant than ICM20602.

There's a bunch of flight controllers with dual IMUs onboard. For example, Lumenier Lux F7 has MPU6000 and ICM20602, you can choose which one to use. SP Racing F7 and H7 series use two ICM20602s fused together.

Also, there's BrainFPV Radix which uses BMI160 from Bosch Sensortec. It should, in theory, provide a more clean and noise resistant signal without the need for complicated software filtering.

Refresh rate is another thing in itself. Things start to get worse with higher refresh rates: you get all sorts of noises internal and external to the IMU, which can negatively affect motor speed regulation, which in turn might make motors run hot or even fry. Also, higher refresh rates require more processing power to filter the noise. Running IMU at 8+ kHz with RPM filtering gobbles up CPU pretty quick to the point that the quad even refuses to arm.


Can I use the trainer port on a DJI Phantom 2 controller for a buddy box connection?

I’ve done some digging to try to find you an answer.

As far as I can tell, the trainer port on the back is only for use in a simulator as evidenced in links: 1, 2, 3.

According the links 4, 5, there is no known way to get buddy-box functionality without modification. You could also install your own receiver and use a transmitter system that is compatible with buddy-box technology.


What's the difference between F4 or F7 flight controllers?

There are a few differences:

Firstly, the F7 chip used in F7 flight controllers operates at a much higher frequency, so can handle more operations per second.

This is useful when you want to start adding lots of dynamic filters, for example RPM filtering, alongside features such as antigravity.

Whilst an F4 can handle almost anything you will want to throw at it in this respect, it will not be able to do so forever (the F3 used to be standard but is now lacking features). Therefore, F7 is better for future proofing.

F7 flight controllers also have extra UARTs. This can be very useful if you want lots of extra features, for example SmartPort telemetry, Smart Audio, GPS and camera control. However, it is not the end of the world if you don’t have as many UARTs as you want as you can use softserial to add software UARTs to your FC. Of course, this also takes up processing power so an F7 may cope better.

Finally, F7 flight controllers have built in inverters on their UART ports, while F4's do not. So an F7 FC can handle any inverted signal "as is" on any of its numerous UARTs, while F4 flight controllers have a dedicated external inverter chip on the PCB connected to one of the UARTs.

Because of that, F7's make it easier to use UARTs for SBUS or other inverted signals as it means you can hook them up anywhere without any inversion mods, while on an F4 controller you can only use the special inverted pads (of which there is usually only one, labeled "SBUS").


What types of cameras don't have wide angle view? Are those a good fit for a racing drone?

I don't know if there is a special name for cameras that don't have a wide angle view, but such a camera is not the best fit for an FPV camera.
If you do want a "small, lightweight camera without the wide angle lenses" anyway, and you don't care about the quality, look for an analog FPV camera with a long focal length. As I understand it you want to record footage as well, and depending on your needs and use cases there might be better options.
Why should you get a camera with a wide FOV, and what options are there if you want to record as well?
All the information about where you are flying comes from the FPV camera (unless you fly line of sight), so you will want to get as much information as possible from it. A wider field of view will give you that.
Most FPV cameras are not well suited for recording (high quality) footage, and most of them can't record onboard (meaning that you will have to record the lower quality footage that is sent to your video reciever). If you only want one camera, and you want to record acceptable quality footage, the DJI FPV camera is probably the best option since it can record onboard the drone. That camera (like a lot of other FVP cameras) has a FOV of 150°, so the fish eye effect isn't that noticeable. There are a few drawbacks with the DJI camera if you want to race though, since it is a digital system (See the question Difference between analog and digital FPV systems?), so an analog camera might be better if you are okay with recording low quality footage.
If you want to record higher quality footage than what the DJI camera or an analog camera provides, you will need to mount a GoPro or similar on your drone, alongside the FPV camera. This will significantly increase the weight, so you might want to avoid that, or just remove it when racing.


I agree, it looks like a Micro Deans plug. They're good for about 10amps. A red JST plug is good for about 5amps. A speed 400 is pretty inefficient over about 5amps.
Personally I'd junk both the ESC and motor and go brushless. You'll save weight, gain efficiency, be able to use a bigger prop and it won't wear out. And that ESC probably doesn't have a lipo-friendly low voltage cut-off.


What are the conditions for LiPo battery damage from undervoltage?

Your questions are challenging to answer because (my opinion) I think there is a lot more that goes into a batteries longevity than voltage staying between a certain threshold.  In addition to keeping the voltage in specified ranges, the following also apply.

Do you fly in hot or cold or humid conditions?
Do you fly hard and fast all the time where your batteries come off the drone warm or hot, or do you take it easy and just fly for leisure?
How long do you let your batteries sit below or above storage charge (3.8V/cell)?
Do you charge your batteries immediately after using them or do you let them sit to cool down a bit (15 minutes or more)?
What brand of battery do you have?  Some are known to be junk, others have a great track record, but some bad batches now and then.
What charger do you use?  Is it reputable?
Do you balance charge?
How old are the batteries?
How often have the batteries been crashed?

My opinion is, and I've been tracking battery usage since 2011, is that despite the claim of 300+ cycles, most LiPos will only last about 100 cycles due to the factors that I mentioned above, which also includes the occasional oops moments when you fly your battery below 3V/cell or on the rare occasion when you forget to put them in storage charge.  On top of that, the number of cycles is highly dependent on the quality of the LiPo.

To make a long story short, a few mistakes with your batteries every now and then might "damage" them, but it shouldn't affect them as much as other factors involved in flying, like crashing and the others that I've listed above.  Generally speaking, if you take care of your batteries, they should last you a while.

I know it's a vague answer that doesn't really hit on your voltage questions, but it's based on many years of experience, so I hope it helps you out.


Reasonably priced FPV Drone for beginner

let me clarify a few things that may help you decide on direction, if any, to take.  Your question is not really something anyone can objectively answer because you need to consider what your level of dedication to FPV is first. And of course rand loyalty is nothing short of a cult like relationship (#kiss)
Off the shelf "racing drones" don't exist.  The term "racing" is applied everywhere, but very few units will compete against real custom drones.
A racing drone is a bad teacher I would not recommend that path for a first unit.
First - you must learn to fly FPV (rate/acro format) first, and a real racing drone with a high camera angle and a drivetrain for racing is a recipe for a bag of broken drone parts.  A mellow drone to learn on, including LoS (line of sight) is key.  Generally speaking the learning curve generates a lot of damage.
But if you want an RTF "racing drone" buy one form GetFPV or a similar larger vendor.
As a pilot with over 10k flights and the distinct opportunity to coach and train many people on FPV flight over the years I highly recommend using a small repairable RTF and learning how to fly, land, gap, and not get hurt first.
Next is your level of dedication. To fly FPV effectively you need to be able to scratch build and repair.  Drones, especially racing units, break a lot and you need to have a pit crew (usually yourself).  Anything other than full dominion on your build is a waste of time.
The #1 problem in FPV is "going cheap".  If budget is a concern walk away right now.
I DO NOT RACE but each of my 5" builds are similar in spec to a racing unit. Here are my operational costs as a dedicated FPV pilot.  Each 5" unit is $500-1200 in the air + my goggles (modded, high end) $700, controller (upgraded+customized) $500.  Batteries are minimum $40 per and I am lucky to get 20 solid flights before sag hits, then another 20-30 before retired. So $1/flight in battery, when I factor in other year to year decay it costs me about $2-3/flight.  I keep around 100 batteries active,  a minimum 10 is required for a typical session.  With the exception of new builds I spend about $200/month in upkeep for a modest fleet - comparable to any racer.  I mention this because the hobby (not a sport) is quite expensive, lots of breaks, tons of crap gear and advice.
I don't mean to discourage you, but to race or fly with any serious intent is a legit commitment.  The #1 path to success is having a friend teach and support you.  I blew 3 ESCs on my first build's first power up - an experience like that is very common.  I buy motors in multiples of 5 because they go poof.  Everything I do is aimed to have high quality builds and minimal down time and repair.
As for the learning curve - the path I suggest is getting the right controller first and connecting USB to a simulator.  If you ever bail, the transmitter will not loose a lot of value.  After that get a cheap $90 nano unit and learn to fly LoS, exercises like "walking the dog" for LoS flight will teach you how the drone reacts in the world, knowledge critical for safe and sustained FPV flight.  From there, grab either a nice RTF or start building.  5" drones are more expensive, but smaller units are a more difficult build from scratch. I consider 5" @500-700g AUW to be the sweet spot.
I may not be the norm, but despite an aviation background, all of my childhood and adult life playing video games, and having a lot of RC experience, FPV flight was very hard for me to master.  I made a lot of mistakes and my post here is intended to make you, or anyone, think about the reality of FPV.  There is a lot of peripheral gear and disposable elements plus a lot of breakage all the time (if racing or pushing the envelope). I knew the first time I saw FPV (via Trappy in 2012) that this was the hobby for the rest of my life. If you feel that way too - jump in.


Do low temperatures damage LiPo batteries?

Low temperatures do affect LiPo batteries in a number of ways, first and foremost being the increase in internal resistance as the battery gets colder. This is why LiPos need to be warmed up before flying in cold weather, as otherwise they will be able to sustain a much lower discharge current than usual and will not give their full rated capacity:

In flight the current flowing through the pack itself will hopefully generate enough heat internally to keep it warm, though for longer-flying platforms with low current draw (i.e. long-range fixed wing) an onboard heater or thermal insulation may be required to keep the cells warm.

As for damage, it does not occur in storage or while discharging even when a LiPo battery is brought below freezing; the rated temperature range for discharging is -20° to +60°C, and the electrolyte itself does not freeze until around -40°.

In fact, storing lithium batteries at 0°C prolongs their life compared to when you store them at higher temperatures. Some owners of Li-Ion batteries even claim to "revive" dead cells by placing them in a freezer for a week or so.

The one thing you should be really careful about when storing batteries at low temperatures is the possibility of moisture condensation on the battery's terminals when you bring the cold battery into a warm room afterwards or simply if the weather changes. This can cause a short circuit, which, while unlikely to damage the battery (as the water will likely evaporate from the current before anything truly bad happens), it is still unsafe and will corrode the battery's contacts through electrolysis.

So if you decide to store your batteries in the cold, and especially outside, put them in a sealed bag together with a baggy of desiccant (e.g. silica gel) so that the air around the battery is always as dry as possible, and when bringing them inside, do not remove them from the bag until they've warmed up to the ambient temperature.

However, if you try charging a lithium battery at a below-zero temperature, the battery will be rapidly and permanently damaged via the same lithium plating process that damages cells whose voltage is brought too low. The rate of damage is proportional to the charging current (i.e. more current damages your batteries faster), and the extent of it is such that manufacturers state that the battery is completely dead and should be disposed of if it's charged below zero even once.

The allowable charging temperature for lithium batteries is 0-45°C, but I'd recommend sticking to the middle of the range and only charging at room temperature.

Please note that the exact numbers (i.e. rated temperatures) are for Li-Ion cells, but they should be close for the whole lithium rechargeable family.


Would the FAA's proposed Remote ID really ground older drones?

The FPV Freedom Coalition has an excellent summary page at this link.

To sum up the answer to your question as simply as I understand it: older drones that are not able to broadcast their information will NOT be grounded, however they will be required to only fly within FRIAs (FAA Recognised Identification Areas).

Under the current proposal, after one year no new FRIAs may be registered, so as they disappear over time it may become more difficult to legally fly non-compliant aircraft.

I am not a legal expert - if anyone feels my understanding is incorrect then please do let me know.


What does BEC, SBEC, UBEC, and OPTO mean for brushless motor ESCs?

BEC: Battery-eliminating circuit. Basically just allows you to use the ESC to power the controls circuit, servos, etc. (hence eliminating the need for a battery)

SBEC: Switched battery-eliminating circuit. Just an improved version of the BEC which makes it more energy efficient.

UBEC: Similar to SBEC, just more energy-efficient at the expense of being slightly heavier.

Opto: Uses light (optocoupler) to segregate control and motor circuits, to reduce interference. Therefore requires two sources of power, one for the controls and one for the motor.


Good beginner 6-ch radio with configurable controls under $80

From what I've seen and experienced, the FlySky FS-i6X (or the i6 version for a little less) is the "gold standard" ultra-budget RC radio. I own two (funny story for some other time), and have no complaints apart from some difficulty fine-tuning failsafe parameters for communication with Betaflight.

The radio normally sells for ~$50 USD and is bundled with the FS-iA6B, a 6-10 channel receiver. This is a valid Amazon listing and this is a valid Banggood listing at the time of writing.
Principal headline specs:

FS-i6X

Max 10 channels: 4x analog sticks, 4x toggle switches (3x dual-position, 1x triple-position), and 2x analog dials
Diversity antennae at right angles within transmitter body (I'm unsure of the exact configuration of the two antennae in software)
Trims on all analog stick axes
Not feather-light, not brick-heavy
"Buddy Box" trainer cord functionality

FS-iA6B

Output modes: 6x PWM channels, 8-channel PPM (I think this is correct, although it may be 10-channel), 10-channel iBUS
Diversity antennae

Depending on space constraints, you may want to spring for the (usually sold separately) $15 USD FS-X6B receiver. This one is a lot smaller, retains all of the same features as the FS-iA6B, and has (I think M2) holes in the PCB for easy mounting.


Difference between T700 carbon frames and 3k carbon frames?

First of all, the 300 or 700 is the grade value of carbon fiber and generally, a higher grade means more strength and better performance. The main differences between T300 (3k) carbon and T700 carbon are that T700 carbon has almost 40% more tensile strength than 3k carbon, T700 carbon is slightly denser (and thus heavier for the same volume of carbon), T700 carbon fiber has a higher carbon content and a lower nitrogen content, and T700 carbon fiber is smoother looking while T300 carbon looks more bark-like.

So, overall, T700 carbon is better for making a drone frame because it is significantly stronger with only a small increase in weight.


The earliest recorded use of a UAV for military use was in July 1849, but if we include non-military UAVs, was there anything earlier?

I have recently written a dissertation on non-military drones and the first mention that I could find for what we would consider modern drones used in a context other than direct military action was when, in 1994, they were used as overhead surveillance for NATO envoys in Bosnia.

In terms of any UAV, not necessarily guided aircraft, the incendiary balloons are the earliest example I can think of, however, carrier pigeons were used as early as 1150 according to Wikipedia, so this may qualify.


What is the advantage of a stretched-X frame over a true-X or squished-X design for racing?

I'm not a frame designer, but one distinguishing characteristic comes to mind as to why stretched-X frames are more common.

As you mention, a stretched-X frame has a longer front-to-back dimension than its left-to-right dimension. In addition to providing the rear propellers with cleaner airflow when in forward or backward motion, this results in the moment of inertia about the pitch axis being higher, which means that it requires more work for an external force (e.g. wind) to change the pitch of the quad inflight.

 (cit.)

This also means that, as you noted, the pitch axis has greater control authority (due to the longer moment arm) which allows the quad a more quickly accelerated response to the pilot's commands. The differences in control authority accessible to the pilot can be smoothed over with rate profiles and PID tuning so that the quad maintains the stability it gets from the stretched-X design while responding uniformly to pitch and roll commands.


Why do higher voltage batteries need lower KV motors?

They don't necessarily, however a given amount of power will be required to rotate a motor at a certain RPM with a given propeller.

kV, the voltage constant of a motor, is the maximum RPM it can turn at per volt of power supplied. Example:

A 4S (15V to make math easy) battery will turn a 2000kV motor at 30,000RPM (with no load, inefficiencies, etc)
a 6S (20V) battery will need a 1500kV motor to achieve the same RPM

In theory, the result is the same amount of power (watts) required to achieve the same RPM/power output/etc. A 4S 2000kV motor will draw say 20 amps, or 300 watts. To get the same power output, a 6S 1500kV motor will draw 15 amps (still 300 watts), and spin at the same RPM. Higher voltage but lower current generally results in less heat, because of ohm's law. Current creates energy loss over resistances (ESC components have internal (small) resistance, which is why they heat up, which is why they have a current rating in the first place).

A lot of Bardwell's videos go into the details of 4S vs 6S, but they work out to be pretty much the same. The reason larger models go with higher voltages (and lower kV to achieve the desired RPM) is because they are much higher wattage, and achieving that higher wattage strictly with higher current is unrealistic (Large scale planes would be drawing 500-600 amps at 3-4S voltages, dangerous and expensive).

To reference an earlier comment, the voltage "rating" of the motor is just the suggested voltage. The actual rating of the enamel coated wires would be much higher, but you will destroy components from drawing too much power under load if you went up to those voltages.


Difference between XM and XM+ receivers?

According to Oscar Liang, the XM+:

Is larger and heavier than XM (21.5 x 12 x 3.5 mm – 1.6 g)
Has diversity antennae, unlike XM
Has solder pads in addition to pin headers
Doesn't support Smart Port telemetry
Has RSSI channel support on a dedicated pad and 16th channel
Has a wider input voltage range than XM (3.7v-10v)


There are several things that can be causing this with different likelihoods.

The first thing to check is - are your propellers on correctly? It is an easy mistake to make and always worth a check.

The next thing to check is that all of your motors are working correctly. Start by spinning each of the motors by hand and see if anything feels obviously wrong, for example, a magnet out of place. If nothing feels wrong, remove the props and individually spin up the motors in Betaflight to test them. Make sure that they spin the direction that betaflight shows in the configurator.

The next port of call is your gyro. Plug your quad into Betaflight or whatever firmware you use and move it around - does the virtual representation of the quad match up with your movements in real life? If not, that is almost certainly the cause.

If all of these have failed, swap two of your motors and see if the quad flips in the same direction. If the problem follows the motor, you have a faulty motor. If the problem stays in the same place, you have a faulty ESC.


Does anyone have a manual for this radio?

So the radio arrived, and I plugged in some servos and a motor and here's what I think everything is;
Ch1 is rudder, right stick right+left(I think, there was something weird with it)
Ch2 is elevator, right stick up+down
Ch3 was motor, left stick up+down
Ch4 was ailerons, left stick right+left
Ch5 and CH6 were switches


My esc cuts the motor when I move control surfaces a lot

I think it's a combination of the BEC and stiff controls.
1 amp BECs are often just linear regulators, effectively a transistor that acts like a variable resistor. It takes in battery voltage and turns some of the power into heat so that the remainder is 5v. If you were running a 2 cell battery, it would have to dissipate 7-5=2v but with a 3 cell battery it's 11-5=6v. That's three times as much, so it can only manage a third of the current.
2 servos are normally fine with that set up, but if the stiff control surfaces cause the servos to draw more power than normal they could cause the BEC output voltage to drop to the point where the receiver or ESC shuts down. Or maybe it overheats and there's a thermal cut off - I'm not really sure of the failure mode, but at least it didn't catch fire!
If you're curious, you could check to see how hot it gets, and measure the voltage at the receiver when the motor cuts. There are also servo power meters, or you could, HobbyKing seem to have discontinued theirs.
You may be able to solve the problem by freeing up the hinges and pushrods, but a better BEC is cheap and it's not a good idea to fly with a set up that you know could fail.
The better BECs are known as 'switching' because they switch the power on and off rapidly and then smooth the output, so there's much less power wasted as heat. Sometimes these are known as UBECs, because some early ones were sold under the name 'Ultimate-BEC'.


Quality of transmitted image and electrical problems

There’s a lot to unpack here, so I’m going to give a general kind of advice that should be helpful for anyone trying to get cleaner video:
Electrical Power
There are two main aspect to the image transmission: the camera and the VTX.
Whenever you have noise, the first step would be to increase power filtering as much as possible.
The easiest way to do this is to add a capacitor to the battery pads to filter out the voltage spikes.
If that doesn’t work, try powering your VTX from a BEC on your flight controller instead of VBAT (if that’s what you already have).
Also try powering your camera from a BEC on your VTX if it has one.
Then try adding a common ground between the camera and the VTX to avoid what is called a ground loop.
There are other sources of power noise, which could include oscillations that cause voltage spikes in the motors.
Antennas
The antenna you use also has a big impact on video.
The best way to ensure good video link quality is to have two circularly polarised antennas with the same polarisation, for example RHCP with RHCP and LHCP with LHCP.
You can also use a patch antenna to increase the reception of your goggles at the expense of the number of directions from which you can receive a signal.
VTX
Have you ever powered on your VTX without an antenna? This can sometimes cause the VTX to become damaged.
If you have a spare VTX, try using that and see if quality improves.
VTX Settings
If you’re flying in a wide area, you can try increasing your VTX power (as long as it is legal).
If you’re flying in a tight space like a bando, sometimes having a higher power output can actually make the video signal worse due to multipath interference. If you experience that, you may want to try lowering the power output.
Frequency
Make sure your goggles are on exactly the same frequency as your video transmitter.
People often use the auto-scan function on their goggles, however this is known to be inaccurate and often goes to a close channel, but not the exact one.
The wrong frequency will introduce static, but it is unlikely to create the lines you describe.
It is also possible that lines may be caused by RF interference from WiFi, power lines etc. You may be able to get around this by changing your frequency.


Brushless motors (aka BLDC) are typically[*] more efficient than brushed DC motors. Higher efficiency equates to longer flight times for a same battery pack.

Amongst other reasons:

The inrush current at the moment of commutation can involve sparks (which is an inefficient waste of energy), and in any case is a free-running operation without the possibility for optimization.
The carbon brushes have a somewhat higher resistance, which is an electrical energy loss
The carbon brushes involve a certain amount of drag, which is a mechanical energy loss
The EM field is not easily controllable to reduce torque ripples, resulting in mechanical energy losses. Anecdotally, you can hear these ripples, especially when comparing power tools using universal motors (i.e. brushed AC wound in series) vs BLDC. Sounds represents energy, all other things being equal a noisier drive is a less efficient drive.

Brushless motors also are easily built into outrunner applications, which is appropriate for high-torque direct drive applications, such as a propeller. An inrunner needs an inefficient gearbox to develop the torque required to spin a blade (which might also be complex, costly, and weighty).

An oftentimes overlooked consequence of brushless efficiency is heat generation. Fundamentally, what limits a motor's power is heat, and producing less of it means that more energy can be supplied from a smaller package. The smaller package means lighter weight, which equates to longer-runtime.

Each of these on its own is small, but summed together they result in a far superior drone operation from brushless than brushed.

[*] There are some extremely efficient (>95%) brushed motors out there. However, the engineering required to get that high efficiency is costly. Cheap mass-produced brushed motors will never have those kinds of tolerances, but brushless motors are much less sensitive to manufacturing imprecision.


Picking the right prop pitch for my plane

The key here is to understand pitch speed. The pitch of a prop is how far it will advance in one rotation if there's no slip, so a 5" pitch prop spinning at 15,000rpm will advance 75,000 inches per minute, or about 70mph. That means that when your plane reaches 70mph, the prop cannot produce any thrust.
Now, exactly how much thrust is required to reach any particular speed is very hard to estimate, and how thrust drops off with speed is also hard to estimate, but it's safe to say that the above plane will have a top (level) speed of under 70mph.
The first step is to look at the recommended prop for your motor. Often it will say something like this:

Use on 2s with 9×4.7 slow fly or the GWS 9×5 direct drive propellers.
Can also be used on 3s with 8×4 or 7×3.5 propellers.

Here you can see how increasing battery voltage spins the prop faster and requires a smaller prop to keep the power within the motor's limits.
A larger, lower-pitch prop (or a smaller, high-pitch prop) will have roughly the same power requirements. The normal rule of thumb is to add an inch of diameter when you reduce the pitch by an inch, or the other way around. So if a motor can handle an 8x4, it could also handle 9x3, 7x5 or 6x6.
The larger prop will have more thrust, but the smaller, higher-pitch prop will have a higher pitch speed, and thus a higher maximum possible speed. Prop pitch is a lot like gears on a car of bicycle. Too low and the top speed will be very limited, too high and it'll be hard to get going.
Here you need to go back to rules-of-thumb. Props are made in a range of sizes that are useful for most models. Generally the highest pitch option is the same as the diameter (a so-called 'square prop') and is only useful to very fast, sleek models like pylon racers. Models with more drag will need more thrust, and won't need the high pitch speed.
For that motor, you'd use the following props on these types of model:
6x6 - pylon racer.
7x5 - 'sports' aerobatics or flying wing
8x4 - trainer
9x3 - 3D aerobatics (i.e. prop-hanging) or slow 'vintage' flight.
You can use the same proportions for other motors: a 'square' prop for speed, diameter = 2 * pitch for a trainer, etc.
If you're not sure, order a prop either side of what you think the ideal will be and see which you prefer. Props are relatively cheap and you'll probably need replacements at some point.
Ideally you should also test a new motor/prop combination on the bench with a Watt-Meter, so check it's not pulling more current than recommended (or the ESC can handle) but you can also just check how warm the motor gets when run for 30 seconds or a minute. Warm is fine, but if the motor gets hot, you'll have to be careful in the air until you're sure the extra airflow is enough to keep it cool.


Is there an international competition that yearly chooses the best drones in the world?

I don't think there is any international competition for "Best Drone". The closest thing to that would be recommendations that experienced pilots make and it would depend on what that person liked.

I imagine that if there was an international "Best drone" competition it would have categories such as fastest racing drone, best freestyle drone, best photography drone, most popular drone, etc.


A BEC is simply a way to create 5v power from the much higher voltage batteries. When you know how many amps your electronics can draw, you can choose an appropriately sized BEC.

To run two motors at full power for half an hour you will need 200Ah. The maths is simple, it's just amps * hours. At 24s (90v), that's 18kWh, which is as big as the battery in some electric cars. If you were to use hobby style batteries, you could use 40 of these 6s 20Ah lipos

I don't think I'd recommend any hobby-grade flight controller for a manned aircraft. The FC is flight-critical, if it goes wrong you will crash. Manned aircraft typically have at least three redundant systems, which compare their outputs so if one develops a fault the other two can 'out vote' it. For really important stuff, there is even more redundancy. Needless to say, there's nothing like this in the hobby world.

That should be relatively easy.


You need to be specific about what IMU hardware you are using, like a general purpose IC like MPU6050 (gyro + accelerometer), or flight controllers like  Pixhawk, pixRacer, etc ..
In Order for you to achieve a fine tuned, linear input-output response, you need to take care of few things firsts and accept some constraints :

you cannot fully get rid of vibrations, but hopefully you can reduce it to an extent by using  anti-vibration absorber like this

You cannot achieve a linear relationship between input and output signals. This is mainly due to limitations of The flight hardware you are using
In the end, the goal is get proper output signals rather than reduce vibrations. this can be achieved  by

1)Research more about your IMU hardware, like manufacturers Datasheet, or asking with the community

The reason why you are getting a change in pitch angle is due to its internal offset of various parameters associated with its IC manufacturing.

Coming to UAVs, you need to implement your own PID algorithms by using digital filters like Kalman Filter, or you can choose Open Source flight firmwares for your IMU hardware.

After implementing and testing your filter, you need to fine tune it by hooking the output to a DSO and modifying the filter coefficients to suit your needs. A lot of trial and error goes into finding the tuning parameters like Kp, Ki, and Kd.


What are the typical methods of constructing a large airframe?

While I'm far from knowledgeable enough to give you a chart like you requested, I can do a general overview of the methods commonly used in RC modelling for airframe fabrication.

I must begin with this: Almost all of these are applicable to an aircraft of about any size. It's even hard to tell which is better, they're all just different. So, without further ado, the list:

Extruded polymer foam. This is what most commercially manufactured models are made of. The manufacturing process involves CNC-machined molds, special extruding machinery, and a lot of money spent upfront. However, the models can be of any shape imaginable, are quite durable, light (although not the lightest) have excellent aerodynamics, and the unit cost is cheap once you get going. The load-bearing parts are usually reinforced/stiffened with carbon fiber inserts, although wooden reinforcements are not uncommon. There are several types of foam that can be used, each with different characteristics.
Hotwired polymer foam. This is the DIY method of making foam planes, although some commercially-available planes use it as well. It involves a resistive wire which is heated by a current and is used to cut a block of foam into the desired shape using a bow or other device that keeps the wire under tension. As with extruded foam planes, load-bearing sections are usually reinforced with carbon fiber for stiffness. The same types of foam can be used as with extrusion. The available shapes are limited by what can be achieved with straight cuts, although complex shapes can be created by gluing simpler sections together. See this video for an overview of the process. Sometimes (most of the time?) only the wing airfoil is wire-cut, while the rest of the airplane is fabricated using other methods. Planes made using this method are often covered with heatshrink film to add stiffness and durability.
Folded flat materials. Airplanes can be made from a variety of sheeted materials by cutting, creasing and folding them into shapes which are then glued together. The materials include (but are not limited to) foamboard (foam between two layers of paper/thin cardboard), depron (flat sheets of foam), corrugated cardboard or even paper. The very simplest models can even be made without folding, just by cutting the required outline out of a flat sheet, while the advanced builds feature smooth curved surfaces. While usually not especially sleek-looking, airplanes made using this method fly surprisingly well and are very durable! Most of this comes from the fact that they are also extremely light for their size, though. In any case, a foamboard (or cardboard, or depron) plane is very easy to build or fix and requires minimal tools (knife, ruler, glue) to do so. This type of construction is extremely popular in the RC community due to the simplicity and cheap price. The most prominent proponent of folded models is Flite Test, who publish free plans for all of their models. There are also countless plans for folded planes on various RC forums.
Wood and film. This is the "oldschool" method. It is very similar to the methods used for construction of early full-scale aircraft, and involves building a wooden "skeleton" for the plane out of precisely-cut thin pieces of wood (usually balsa) and covering it with a special heat-shrink film to form the surface of the airfoil. This method takes a lot of time and work, and involves cutting and gluing together hundreds of wooden parts. The method creates light, strong and stiff airframes, which can nonetheless be brittle if hit the wrong way in a crash and time-consuming to fix. While less common nowadays, this construction method remains moderately popular, with a great variety of plans available online.
Glass and carbon fiber composites. While carbon tubes, strips and sheets are used extensively together with all of the construction methods discussed above to add mechanical strength and stiffness in critical points to a weaker or more flexible material, some airframes are constructed entirely out of composites. This is arguably the most high-tech and definitely the most high-performance (if done properly) construction method of the ones discussed here. It is also the most expensive one, per unit, though it doesn't come anywhere near extruded foam in terms of upfront setup costs and thus is within reach of hobbyists who want to make only a small number of copies of their design. Creating a glass or carbon fiber airframe requires making a mold of the desired shape, carefully laying up the fiber, soaking (or impregnating) it with resin and curing under pressure, usually with a vacuum pump, to make sure that the absolute minimum of that resin actually stays in the part. Then comes sanding, polishing, and assembly of the parts. Carbon fiber, when properly done, is extremely strong yet light, and is used in high-performance thermal gliders and other planes which must withstand extreme forces while being as light as possible. While strong, such planes are often hard to fix when they do get broken (at least without compromising their original strength or lightness). Glass fiber, though somewhat outdated, is also still a popular choice for larger planes, as it is significantly less expensive and easier to fix. Sometimes DIY molded glass fiber reinforcements can be found on planes constructed from other materials, and usually made by laying the fiber directly on top of the fuselage, eliminating the need for a mold.


How to build a buddy box cable for FlySky FS-i6X RC transmitters?

Yes, on both counts! That is an S-video (aka. Mini-DIN-4) connector, and you can use it to make a buddy box cable. The port carries a 4-channel CPPM signal that can be used by cheap USB adapters as a joystick HID input to your computer or for buddy box mode, which FlySky calls "Trainer mode". I've made one for my own use, following the specifications laid out by the user gcartled on the RCGroups forum.
Making the Cable
It appears that there are sellers with listings for ready-made FlySky trainer cables, like this one on eBay, but I haven't tested them to confirm that they work. Note that @JohnRix in the comments has mentioned that the adapter he purchased for the i6 was incompatible with the i6x.
After obtaining a spare S-video cable (or creating your own from Mini-DIN-4 male terminations), chop it somewhere in the middle and reorder the connections for the 4 internal wires such that the pins 1 and 2 are connected to the opposite number on the other end of the cable. i.e.:

pin 1 → pin 2
pin 2 → pin 1
pin 3 → pin 3
pin 4 → pin 4
metal housing → metal housing

(cit.)
This creates a crossover cable that allows the CPPM signals from the "slave"/trainee transmitter to be received by the "master"/trainer transmitter.
Configuring the Trainer Transmitter to Enable "Trainer Mode"
In order to use trainer/trainee functionality, the appropriate settings must be configured on the trainer's transmitter so that it knows what switch will be used to toggle on/off control inputs from the trainee's transmitter. In this setup, the trainer's radio is the only one bound to the aircraft.
These instructions are confirmed functional for the FS-i6X transmitter, but it is likely to also work with the FS-i6 and other derivatives:

Turn on the transmitter.
Hold the OK key to open up the menu.
Scroll to System Setup and select with the OK key.
Scroll down to Trainer mode and select with the OK key.
Scroll using the OK key so that the horizontal arrow points to Mode and then use the UP key to change from Off to On.

If you want to disable buddy box functionality in the future, come back here and change the Mode back to Off.

Scroll using the OK key so that the horizontal arrow points to Switch and then use the UP and DOWN keys to select the switch that you'd like to use to enable/disable buddy box functionality.

(WARNING: you shouldn't have this switch bound to anything on your aircraft, as this could cause issues inflight when something is accidentally triggered in the process of enabling/disabling the biddy box)
Note the position of the switch you selected that changes the text Inactive to Active at the bottom of the screen. The Active position enables the buddy box and vice versa.

Hold the CANCEL key until you hear a beep to save the changes and then press CANCEL twice more to escape the menus all together.

Now, go and fly!


How fragile are the SMA connectors used in FPV equipment?

According to electronics-notes.com, the mating cycle rating for the SMA/RP-SMA connector is 500 cycles. However, it is important to note that this rating assumes that the connection and disconnection process happens as smoothly as possible, with the body of the connector not moving at all while the screw connection is tightened sufficiently to not allow the joint to shift while connected. Under nonideal conditions, the actual lifespan of the SMA connectors on FPV goggles is likely to be far shorter.

Under the best of circumstances, the connection would likely last: (assuming one mating cycle per flight session)

~19 years @ 2 sessions/month
~9.5 years @ 1 session/week
~5 years @ 2 sessions/week
~2.5 years @ 4 sessions/week
~1.4 years @ 7 sessions/week

From this, we can see that the answer likely depends on how often you fly and how long you would like your goggles to last. Although the attenuation (signal strength reduction) of the connector will increase with every mating cycle, if you fly infrequently enough it is possible that you may be ready to upgrade to a new goggle by the time that the SMA connectors begin to become a serious problem.

If you take care to make and break connections cleanly, casual pilots who don't fly very often might not see too much of a problem, but more serious pilots who fly more than once a week definitely shouldn't be removing their antennae on a regular basis.


Difference between analog and digital servos?

Analog Servos
Analog RC servos use a form of pulse width modulation (PWM) to control the speed of the servo. It's basically switch the motor on and off quickly (Commonly 50 times per second). The motor rotates a certain angle depending on the width of the on-pulse.

Digital servos
Digital servos also use PWM, but at a much higher frequency (6 times that of a analog servo). They contain a microprocessor that received the signal, and then send the PWM to the motor
Comparison
The digital motor will be smooth and react faster, but it's more complex and therefore more expensive


Why do most drones have upward facing motors?

One practical advantage is increased ground clearance, reducing the likelihood of a prop strike on landing or take-off.

Aerodynamically, a pusher1 design can be less efficient because the propeller is spinning through the wake of the fuselage - this results in 'lumpy' air, causing vibrations. I don't have exact figures, but I've heard anecdotally that this can be a 10 to 15% drop for fixed-wing designs and the exact fuselage configuration will make a difference!

1: Pushers are often defined as '[motor/engine] shaft under compression'; which seems to apply to the 'upside down' motor configuration.


why do drone frames have spaces and holes

A major consideration is weight reduction; if material can be removed without significantly reducing the strength of the item, this will result in an increased payload or flight duration. This is particularly evident in the motor arms of the product you linked, as the triangular shapes are renowned for their relative strength.
Another advantage of this is that it allows additional equipment to be attached easily, with cable ties or similar; in some cases I expect the cut-outs will be designed with this in mind - for example; a selection of parallel cut-outs can allow for a wider range of battery sizes to be securely attached to a frame.


FrSky receiver snap-on connection problem

This type of connector is called a U.FL connector.

Replacing the connectors will be super fiddly, but is possible if you are skilled with a soldering iron. The cable side is the more difficult bit but also cheaper, so you might try a new antenna and replace just the PCB connector.

You may be able to use something small, like a pin, to carefully nudge the connector back into shape - it may make it worse, but if it doesn't work anyway what's to lose?!

Finally, you could unsolder the connector from the board, cut the wire and solder it directly. The wire will be a coaxial cable, so you need to take care not to short the inner to the outer - especially when soldering as you may melt the insulator away. The inner goes to the centre pin on the connector; the outer part to ground (the circle of the U.FL connector is connected to ground so you can use this pad).

Before you trust the receiver again you should do some range tests. You can put the transmitter in the range testing mode and check what kind of range you get.


Should Crossfire transmitter antenna orientation match receiver antenna orientation?

A dipole antenna is linearly polarised, which means it radiates most of its energy in one 'plane' relative to the antenna. For maximum efficiency, the transmit and receive antenna should be orientated such that their plane of radiation is the same, or very close to.

It is important to note that as the aircraft flies around, the orientation of the antenna changes with the aircraft. The reason a vertical orientation is recommended is that aircraft - particularly multirotors - generally1 fly with one side 'up'; therefore, for whichever direction the aircraft is travelling the antenna is still vertical (or, mostly so) compared with the transmitter.

With a horizontally orientated antenna, when the aircraft turns through 90° relative to the transmitter, the antenna is now 90° out of alignment to the transmitter; it may help to think of/visualise this as "North/South" and "East/West", and the loss of signal will be comparable to vertical vs. horizontal as the angle difference is the same. Some receivers have multiple antennas and recommend orientating them at 90° to each other to mitigate this scenario.

What you may consider, if your transmitter permits it, is to have the antenna at a 45° angle as a compromise for signal strength. Most drones have a built-in lost comms failsafe so a loss of signal should not be a huge concern; for a fixed-wing model aircraft, I would be more concerned about matching antenna orientations.

[1] The main exceptions probably being stunt aircraft


I personally have never used a Deadcat frame, but I found this forum thread asking a similar question, the answer is:

  I was more interested in how the dynamics of a dead cat might affect efficiency.  But based on what some of the long-range guys have found, it doesn't appear to be an issue.  One thing that did come up with the SG+ at its genesis was instability while in steep dives, like down cliff faces, etc.  Guys were setting up in BF as a straight X design.  Once they started setting up as a custom in BF and used the program that Project Blue Falcon explains, the instability went away.


Grinding sound on FPV motor but no apparent damage

It looks like a bad phase to me. Check the resistance of each phase, wire 1 to wire 2, wire 2 to wire 3, and wire 1 to wire 3. If one of those is much higher resistance or shows no continuity, then you've got a bad phase. I suspect this is the issue.

Also try swapping the motor to a different arm. This will eliminate the ESC as the issue. If the problem follows the motor to the new arm, then the the issue is the motor, if it stays put then it's the ESC. A bad FET or gate driver in one phase on the ESC can cause the same symptoms as a damaged phase in the motor.


Drones capable of smooth plane/bird-like flight

You might be able to write custom flight control software for a multi-copter that would simulate this. You might get close just by radically limiting the pitch and roll rates and being gentle on the throttle. In either case it wouldn't be intrinsic to the design.
If you want smooth, soaring flight, you'd be best to build a fixed wing model with a fairly large wing span and low weight, similar to a soaring bird. A basic electric motor-glider like a Bixler is quite a popular FPV platform because they fly fairly slowly and smoothly, and are easy to fly (which is important when you're trying to make sense of the view through goggles).


Serial ports are hardware based, and cannot be remapped. You can however remap software serial ports. Keep in mind not every pin supports softserial either, as an available hardware timer is required. Typically you can use:

UART TX & RX pins
PPM Input pins
LED pins
Motor PWM output pins

If you have one of those free, proceed.

Enable softserial from the Betaflight Configurator "Configuration" tab, then click "Save and Reboot".

Go to the "CLI" tab and type "resource SERIAL_TX 11 A09", hit enter, then type "save" and hit enter.

Go to "Ports" tab and assign the feature you want to "SOFTSERIAL1".

Keep in mind that software serial is limited in terms of how fast it communicate information and the load it puts on the CPU. If you have a very intensive low-latency connection, typically try to avoid use softserial. Softserial is best suited to things like VTX control (smart audio and tramp protocol, etc), GPS, and smart-port telemetry, where latency, speed, or loss of signal during flight isn't an issue. While it can be used for other more system critical links like RC control, I would generally recommend against it, though it is theoretically possible.

Sources:
https://oscarliang.com/betaflight-soft-serial/
http://www.multirotorguide.com/guide/guide-how-to-set-up-the-softserial-uart/


Do drones with inflight API access for control inputs exist?

All multicopter drones use a flight controller. This flight controller is usually a small board with a micro controller like an ARM F4/F7 on it:

One job of the flight controller is to receive and decode the control inputs of the receiver. This is another board with antennas:

Nearly all receivers use a serial protocol to communicate with the flight control. So you can send commands via the serial protocol.

Serial interface

This would be a very simple way to send commands to the flight control but the downside is that you need to build your own radio receiver or find a way to send commands to the receiver directly.

MSP

Another simple option would be the MSP protocol that is used by a number of different open source projects. It is another serial interface but with this protocol you can read and write information from/to the flight control. See https://github.com/iNavFlight/inav/blob/bcd4caef92bff67e91350456903a5cdd1d779840/src/main/msp/msp_protocol.h for a list of available messages in the protocol.

Warning: it's not recommended to control the drone via this interface alone but it is possible.

Ground control software

Other open source projects offer a ground control program that allows to send commands to the drone during flight. One example is QGC http://qgroundcontrol.com/ with the MAVLink protocol: https://mavlink.io/en/ that communicates with PX4 Pro or ArduPilot flight controllers.

So you would need a drone with the flight controller, a telemetry radio ( https://ardupilot.org/copter/docs/common-sik-telemetry-radio.html ) and a PC to run the ground control software.

I don't know what the DJI drone can do but you have many open source options to start with. Maybe it's a good idea that you start with a simple drone setup to learn the basics and then move on to the more advanced stuff like controlling the drone via software commands.


Gain, as a general term, is a measure of how large or strong something (a voltage, a radio wave, a sound wave, or whatever else) is relative to some baseline value of the thing being measured. A gain is a ratio: If a device takes a voltage on one end and makes it 20x larger (amplifies) it 20-fold, it has a gain of 20. In practice they are represented using decibels, a special unit that makes it easy to work with very large or small ratios. See the note at the end of this answer for more information about them.
Consequently, the gain of an antenna is the ratio of how much stronger the radio waves generated by the antenna are, relative to those generated by a standard "reference antenna" when fed by the same signal.
This reference antenna used as a baseline is considered to have a gain of 0 dB, or 1:1 (equal to itself).
The reference antenna is usually one of two types: An ideal isotropic antenna, which radiates equally in all directions with zero losses, or a dipole antenna. To indicate which one is used as the baseline, a letter is added to the end: dBi represents gain relative to an isotropic antenna, and dBd relative to a dipole.
The dipole itself has a gain of 2.15 dBi (and 0 dBd), so to convert a dBi gain to a dBd gain you subtract 2.15 from it, or add the same number to convert the other way around.
Symmetry
Every antenna receives the same way as it transmits, so an antenna's reception gain (how strong of an electric signal is generated in it by a radio wave, relative to the chosen reference antenna) is always the same as its transmission gain.
An antenna with 3 dB (2x) gain in a certain direction will transmit 2x the power in that direction, but also radio waves coming from this direction will generate 2x stronger signals for a connected receiver to detect.
If an antenna's gain is 13 dBd (a 20:1 ratio), its signal will be 20 times as strong as a dipole's (and its reception will be 20x stronger as well)! Wonderful, right?
Well, there is a caveat to high gain, and it basically boils down to: There ain't no such thing as a free lunch.
Gain and directionality
True isotropic antennae do not exist in the real world, i.e. no real antenna radiates equally strong in all directions. There are directions in which it will radiate the strongest, and others in which it doesn't radiate at all. So the gain of a real antenna is not the same in all directions either: in some it is stronger, in some it is weaker. The easiest way to show this is a graph showing the antenna’s gain in every direction around the antenna. That graph is called a radiation pattern and looks like this:

The bold line represents the gain: the further it is from the center, the bigger the gain. Above is the radiation pattern of a directional antenna: the gain in one direction is a lot more than in other directions.
So when we speak of an antenna's gain, we really mean its gain in a certain direction. And that direction is usually the one in which the antenna's gain is the strongest.
Also, you can't get more out of an antenna than you put into it. The sum of all power radiated in all directions is always equal to the power that goes in (Minus a small percentage of losses); Antennae create gain by radiating this power such that the created waves add up in the preferred direction and cancel in others.
So if an antenna has a higher than baseline (positive) gain in some direction, you pay for that by having a negative gain in other directions. Gain only focuses power — it does not create power.
For our imaginary 13 dBd antenna it means that only a small sector will get this much gain, and all the other directions will have almost zero radiated power to make up for it.
So gain is not only the measure of how well an antenna receives or transmits, it's also a measure of the antenna's directivity: If the antenna focuses all of its power in a tight cone, it will have a huge gain within that cone, and a huge gain rating on the box... But its gain outside of that cone will be almost nonexistent. So when you are choosing a directional antenna, the gain is also an indication how directional it really is. Lower-gain patch antennae will give you a soft, forgiving cone but lower gain in it, while high-gain directionals will have a very tight zone in which they could transmit or receive at all, but within it they will be super powerful.
Omnidirectional antennae (which, as you now know, are never truly omnidirectional) have a radiation pattern that's more or less shaped like a donut:

They have an equal (and positive) gain in any direction perpendicular to the antenna, which gets lower as the direction gets closer to parallel to the antenna. These antennae also have their own kind of directivity: if you mount the antenna vertically, the "horizontal" gain will be the same in any direction, but it can be higher or lower relative to the "more vertical" directions, making the donut flatter. In other words, the antenna can prioritize gain towards the horizon versus overhead gain more or less.

A note on decibels
In many engineering fields ratios can easily get extremely large or extremely minuscule and tend to multiply a lot. Radio is one of the most prominent of those. At the input to a receiver, signals are frequently
smaller than one ten-billionth of a watt. When they come out of a transmitter, they’re often measured in kilowatts! Antennas, propagation and electronic circuits change signal strengths by many factors of ten.
To manage these vast differences in value, engineers measure ratios in decibels, or dB, which represent the ratio of
two quantities as a power of 10. The formula for computing decibels is:
dB = 10 * log_10 (ratio)
This is a logarithmic unit, meaning that when two ratios are multiplied, their representations in dB are simply added up: 10 decibels represent a ratio of 10:1; 20 dB is 1010=100:1, and 30dB is 1010*10=1000:1.
Positive values of dB mean the ratio is greater than 1 and negative values of dB indicate a ratio of less than 1: -10 dB represents a ratio of 1/10, and 40dB is 1/10000.
Here's a quick cheat sheet of dB to actual ratio for numbers smaller than 10:

1dB corresponds to about 1.25:1

3dB is almost exactly 2:1 (this is the most important one to remember)

7dB ~= 5:1

10dB is 10:1 (exactly)
For everything else, you just add up the numbers you know:

6dB (=3dB+3dB) is ~ 4(=2*2):1, and 9dB is ~ 8:1

13 dB is 10 dB + 3 dB, so 10*2=20:1.

46 dB is 40 dB + 6 dB = 10^4 * 4 = 40000:1

15 dB is 10dB + 5dB; and 5 dB is somewhere between 3dB and 6dB (2 and 4), so it's probably three point something. Thus our guess would be 15dB is somewhere in the 30-35 to 1 range.


Do propellers layered on top of each other have twice the thrust of one propeller?

At least at the small/model scale, the answer is no. There is significant efficiency loss due to the stacked propellers not receiving the same clean air as they would if they were horizontally separated.

This is demonstrated experimentally in a Youtube video by rctestflight which compares the efficiency/thrust production of three different propeller configurations sharing the same propeller type and motor type:

Coaxial — What you're talking about where two props are stacked vertically so that they share the same rotational axis.
Overlapping — Where two props' discs overlap each other by roughly half a prop diameter
Normal — Where two props are positioned side-by-side without any overlap

To quote Daniel from rctestflight in his video:

  Any prop overlap is significant in efficiency loss. Traditional side-by-side
  motors are 25% more efficient than coaxial and 8.4% more efficient
  than overlapping props.

  To me, losing 25% efficiency is not worth the small form-factor and increased ease of transportability that a coaxial system produces.


Is voltage sag safe if the voltage isn't too low?

Yes, voltage sag is perfectly normal and safe for the battery, so long as it remains above the recommended minimum voltage levels.

The C rating is important as the construction of a cell affects how quickly it can provide current. The internal surface area of the positive and negative terminals is related to the current it can provide - a larger surface area lowers the internal resistance and this causes less heating when discharging. This is why higher C packs are bulkier.


How to visually inspect the magnitude of PID gains?

This comes with an understanding of how the PID algorithm works and what it actually does. Bardwell videos can be good, but non-quadcopter reference on what a PID controller is will be the best way to visualize this.

Look at the individual P, I, D components, and see how they react to the gyros. A properly damped (critically damped) system is what you're aiming for. I don't remember the math, but you also can't just apply straight math to this, because some of it is by feel (looser = underdamped, tighter = overdamped).

Here's a photo stolen from quora (https://www.quora.com/What-are-over-damped-critically-and-under-damped-systems):


Can I power the VTX and FC from the same vbat pad on the ESC?

Yes, you can.  Generally, you want to have your VTX and Camera on the same ground (in order to avoid video noise), so this approach will definitely work.  There is continuity on pretty much every VBAT pad, as well as ground, so having a single pad or multiple pads don't really matter much.
If you're seeing EMI noise effects, I would still highly recommend running a large capacitor directly on the pads of the ESC.


Please recommend a soldering iron for hobby use

Experience: I worked for HP R&D for 31 years soldering and building boards as an Electronic Engineering Technician.
First you need to decide if you are working on surface mount parts or through hole wire boards. The irons for each are completely different.
I always used WELLER soldering irons.
I used two irons, one was micro tip for very small parts and a standard iron for large parts and wiring.
The temp control is in the tip. It is magnetic and controls a switch in the tip holder.
You can buy tips from 600 to 800 degrees.
The standard temp is ~700 degrees. There are any tricks to use in soldering. the most useful trick is to use DISTILLED Water in the tip cleaner sponge. If you use tap water it leaves calcium on the tip and the tip won't wet properly.
Get D'water @ target for $1/gal and always use it for the sponge. I used a siphon bottle for the sponge.
Use the largest tip to do the job. It holds more heat and will speed the soldering process. I did lots of work with an 800 degree very fat tip, especially on IC's. It heats and solders quickly and very locally so you don't damage the board or parts. Do not cool the joint with water, it crystalizes the joint.
It takes lots of practice to learn to solder properly.
There are videos on proper soldering technique.
If you learn and practice you will move to larger tips and hotter tips but start with medium size 700 degree Weller.
READ: https://www.weller-tools.com/what-type-of-soldering-iron-do-i-need/


What happens if i select the wrong number of motor poles in BetaFlight?

This is a setting that is needed for RPM sensing and RPM filtering. An ESC can not sense the RPM of your motors directly, but can sense how often the magnets in the motor bell pass by the coils in its stator (this is called eRPM), and will report that number to the flight controller if you've got bidirectional DSHOT enabled. If your motor bell has x magnets (or poles) in it, then they will pass a coil x times per full revolution of the bell. Thus,
    eRPM = x * RPM.

In other words, to calculate the RPM of a motor, the flight controller will read the eRPM value reported by the corresponding ESC, and divide it by the number of poles. This means that if you set the wrong number of poles, your flight controller will read the wrong RPM from the motors, and the RPM filter will not work properly.

If you don't use RPM filtering, you probably should not care. If you do, it won't work as intended (and may cause your copter to fly worse) unless you put in the correct number.


What aviation regulations govern the use of drones in Ireland?

The use of drones in Ireland is covered by the regulations in the Small Unmanned Aircraft (Drones) and Rockets Order, 2015 (SI 563 of 2015). There are also Aeronautical Notices that apply to the use of drones only.  These notices can be checked on the website of the Irish Aviation Authority.

Broadly, the regulations require that any drone flown in Ireland may not be operated:

if it will be a hazard to another aircraft in flight
over an assembly of 12 persons or more
farther than 300m from the person operating the drone
within 30m of any person, vessel or structure not under the control of the person operating the drone
within 5km of an aerodrome
in a "negligent or reckless manner" so as to "endanger life or property of others"
more than 400ft (120m) above ground level
over urban areas
in civil or military-controlled airspace
in restricted areas (e.g. military installations, prisons, etc.)
unless the person operating the drone has permission from the landowner for take-off and landing.

Note that the situation in Ireland will change later this year (2020), when the new European Regulations on the rules and procedures for the operation of drones (or "Unmanned Aircraft Systems (UAS)”) come into force.

The situation is explained in Aeronatical Notice U.06, with the catchy title: The pending introduction of (EU) Regulations in the area of Unmanned Aircraft Systems (UAS) in Ireland, available to read or download from the website of the Irish Aviation Authority.

The most up-to-date information can always be obtained from the website of the Irish Aviation Authority.


What are the usual engineering considerations for a UAV current sensor?

Is the shunt resistor usually mounted high-side (i.e. at the positive battery terminal), or low-side (at the negative terminal), and why? Does it matter?

There should be no significant difference to the measurement if the shunt resistor is on the high or low side. As the resistor is in series the absolute voltage across, and current through, the resistor remains the same. However, there may be a small advantage to one or the other depending your sensor design - for example, having it on the low side makes the measurement relative to the circuit GND, which may reduce the component count of your amplifier/signal conditioning design.

What class of amplifier, in terms of precision, is used for the sensed voltage?

This will depend slightly on your design, and you may find the precision of the ADC to be the limiting factor (e.g. an 8 bit 5V ADC has a resolution of 19.5mV.) That said, the job of the amplifier is to match the range of values from your sensor to the input range of your ADC. While a linear relationship is intuitive and straightforward, it may be more desirable to have a curve to give greater sensitivity around the expected ranges but still report invalid values with slightly reduced precision. You would need to characterise the curve and program it into your flight controller for correct reporting.

What range of output voltages does the FC expect?

This will depend entirely on the flight controller - but typical ranges for ADCs are 0 to 3.3V or 0 to 5V. Remember that if you want to report a negative reading, you will need to bias your zero value sensor ourput for mid range on the ADC.

Is there any smoothing (lowpass filtering) applied to the sensor's output to reduce aliasing from possible high frequency current ripple?

Filtering is a good idea to help reduce unwanted values. The response time of the filter needs to be fast enough to capture valid changes while rejecting noise. You can compare the filter response rate to your sample rate - your filter response can be slower if you are checking the voltage every 0.5 seconds instead of every 0.05. Alternatively you may decide to do some filtering in software, sampling several times and averaging, but this takes CPU time away from other tasks.


Quadcopter return to home function not working

Having Googled the S49 drone, it appears that your drone uses ‘headless rth’.

This means that instead of using GPS, the drone goes into ‘headless mode’ then pitches backwards towards you.

Unfortunately, due to the nature of how this works and the inaccuracies of the accelerometer, this process is rarely accurate.

I don’t know of any way to fix this as it is a hardware limitation as far as I know, so I suggest you think of it more as a way to move the drone roughly towards yourself to recover it if you lose orientation, rather than a way to have the drone return and land accurately.


Why do multirotors usually have four propellors?

With aircraft, there are 6 degrees of freedom (DoF) we want to control (roll-pitch-yaw, and x-y-z), but for hovering vehicles (i.e. rotorcraft) we can get away with controlling two degrees of freedom (x-y) by combining the 4 other degrees (roll-pitch-yaw + z).

There are many ways of controlling those four DoF, typically:

helicopters, which use variable pitch blades and some combinations of main and tail rotors, to result in a combination which results in control over roll, pitch, yaw, and thrust
tricopters, quadcopters, hexacopters, octocopters, etc..., all of which use their blades to control the roll, pitch, yaw, and thrust.

Somewhere it is inescapable that for stabilized flight on all four DoF you must have the ability to control each DoF. Mathematically, the quadcopter provides the lowest number of simple actuators that accomplish the job. Each actuator can work by simply speeding up or slowing down, whereas helicopters and tricopters require some other kind of actuation, usually a complex one. It turns out from a pragmatic viewpoint speeding up and slowing down motors is much more robust than changing angles of spinning blades.

Quadcopter

Formally, it looks like this:

So quadcopters reign over helicopters and tripcoters because they're simple. Each axis is controlled by a certain combination of motor speeds and all axes are independent, i.e. when you increase thrust you don't have to also worry about changing the pitch.

So why not hexas, octos, etc...?

Hexacopters

Hexas can lift more, but provide no additional safety margin because there is no way to stabilize all four axes when an arbitrary motor goes out (there are certain configurations which can continue to fly if one motor out of a set of four fails, but cannot fly if either of the other two fail).

Hexas also are less efficient, since they use smaller propellers. (Without diving into blade theory, and simplifying somewhat, the most efficient blade is a single blade which is infinitely large and moving infinitely slowly.)

Hexas also have 6 motors for four degrees of freedom, so when they're all flat you get what is called an "overconstrained" system. You can actually angle them to interesting effect, e.g. CyPhy Work's LVL 1 drone.

Octocopters

Octos theoretically can continue to fly if any arbitrary motor fails, but in reality, you'll likely encounter blade stalling effects or other pathologies that will not allow the octo to sustain flight. This happens because the typical octo is loaded past the point where 4 motors can carry it safely. However, certain high-value vehicles, such as those carrying expensive cameras, are octos because it's cheaper to over-spec the octo's batteries and motors than it is to replace a $50k camera.

Octos are also less efficient, for the same reason as hexas.


Betaflight - Angle mode - motors spins once I arm the quad

Airmode is a great feature if you race or fly freestyle. As you found out it spins up the motors at 0% throttle. You usually want this to be enabled since lowering the throttle to 0% would stop the motors and this results in a fast death roll of the quad.
But Airmode also helps during fast rolls when certain motors cannot spin faster or slower to get more consistent behaviour independent of the throttle position.
The default setting has airmode always actived. But you can disable the permanent airmode and enable it with a switch in the modes tab. I usually do my setup this way: One AUX channel controlls both angle mode and airmode in a way that DOWN  enables angle mode and UP enables airmode.
I take off and land in angle mode without active airmode. That way the props can stop and do not spin after arming the quad. At the same time this setup ensures that airmode is always active in acro mode.