Why do most multirotors use outrunner brushless motors?

The difference between outrunner and inrunner BLDC (brushless DC) motors is that an outrunner's rotor is outside the stator while an inrunner's rotor is inside the stator.

 (cit.)

In general, outrunners create more torque and rotate slower than inrunners, which usually have less torque and rotate significantly quicker. The two different designs are possible because it is relatively easy to conceptualize how to flip the order of the stator and rotor within the motor due to the fact that no mechanical commutation is necessary, unlike in a brushed motor.

Whichever side of the motor (outside or inside) that is the stator is far thicker than the other because the orientation of the coils on the stator contributes a lot of depth. Outrunners trade increased torque for decreased rotation rate because the electromotive force between the magnets and coils acts roughly at the air gap between them, which is further from the center of the motor in an outrunner, resulting in an increased moment arm and more torque. (remember that τ = F x r where τ is torque in Nm, F is the force, and r is the distance from the rotational axis)

This increased torque is critical because the BLDC motors have no gearboxes between them and the propellers, which require decent amounts of torque to spin. In other systems, motors could use gearboxes to trade speed for torque, but the requirements of multirotors to be as efficient and lightweight as possible don't allow for them to be installed.


How to measure the pitch of a propeller?

Theoretically the pitch of a prop is supposed to be the distance it would move in a full rotation (in an ideal world).

As most modern props vary pitch along their length, a single number is almost impossible to measure. Manufacturers tend to give a number which is an indication of how the user would expect the prop to perform.

Most propellers will work on any 5" drone, they usually have the pitch printed on the upper surface of the blade near the leading edge.


Suitable ready-to-fly micro copter for a beginner

Looking through that page, the F450 FlameWheel style kits are still available, and a reasonable choice for a DIY Ardupilot drone. These are about twice the size of a typical FPV quad and will give you more room for sensors and/or a gimbaled camera. Alternately, if you think everything you want will fit on a 250mm FPV frame (they can carry GPS and a fixed GoPro), there are lots of cheap components due to the popularity of FPV.

I would agree with the advice to also buy a cheap toy quad to learn the reflexes you need to fly, and for when your friends want to try flying. They're so cheap now that it's a tiny fraction of the cost of a bigger multi-copter. You won't find any running Ardupilot though, as the flight controllers are too expensive.

The Hubsan X4 H107 is still available - watch out for the different versions, there's a basic version, one with a recording-only camera (that's a bit pointless) and an expensive one with a low quality FPV camera.

The Blade nano has been replaced by the Inductrix - they're quite expensive but their customer service is good.

Eachine's range of toy quads are pretty good and very cheap. The E010 or E011 both work well and can often be found for $10-15. The E012 is smaller but only flies for a minute or two. The protected props make them strong enough for beginners to learn to fly indoors, but they can be flown outdoors in light winds. I would suggest ordering a few spare batteries - one is never enough.

Some of Eachine's range (like the E016) have altitude-hold, which seems like a great idea, as holding a constant altitude is one of the first challenges to learning to fly (especially indoors) - however I've found these need extra trimming to hover accurately and really aren't suitable for a beginner unless you've got an experienced helper to trim it each flight.


How do you build and use a smoke stopper?

There are different ways, but a common one is to put a light bulb in series with the positive battery lead.

This works because it will allow low currents through to power the flight controller, but if there is a surge in current from a short the bulb will light up and sap away the energy.

You need to make sure you have a light bulb rated for the voltage you intend to put through it - for example if you have a 3V light bulb and use it as a 6S smoke stopper, the filament will melt.

This guide by Joshua Bardwell is great to build a switchable smile stopper: https://youtu.be/I5a0TAmEwLE


Free simulators to use when actual RC flying is not possible

For quadrotors, there are simulators for racing drones, such as the DRL one Velocidrone.

For camera platform drones you have droneSimPro, Liftoff or DJI's own one. There are loads of them, and the physics engines are all improving in all. A fair few are free or very cheap or have a free version.

At the other end of the scale are programs like Phoenix R/C Pro - which has models for quads, helis and fixed-wing - and to be honest, if you plan to fly in any commercial aspect, at $200 it's worth paying the money for this one.


Does the RC transmitter control power consumption?

Yes, sort-of.
The speed controller switches the motor on and off in very rapid pulses, and the width of these pulses (and the width of the 'off' period) is controlled by the throttle. It's so fast that the motor just sees it as a varying voltage.
So if you cut a piece of card and stuck it on the transmitter to prevent full throttle being used, the speed controller would only run the motor at partial power. The car would accelerate more slowly, have a lower top speed and the battery should last longer. It should break less when you crash too.
However, voltage supplied to the motor is only part of the story. The current drawn from the battery is what's actually important, as battery capacity is measured in Amp-Hours (i.e. a 1AH battery can supply 1 amp for an hour, or 2 amps for half an hour, etc). The current decreases as the motor speed increases. A motor running at full throttle with no load will spin quickly but draw little current but the same motor struggling through grass will be running more slowly and drawing more power.
So if you want longer run time, drive on a smoother, lower-grip surface or change gear ratio to a lower gear so the motor can reach its top speed with less power. Or buy some extra batteries...


What propellers should I get for my drone?

The first step to getting the right propellers is to figure out what size propellers will fit your drone. What you can do is measure the distance from the center of the motor to the closest thing (probably the frame) that would block the propeller. Multiply that by two, and a few millimeters smaller than that is the maximum prop size/diameter. (smaller props can also work)

For example: if the center of a motor is 2.6 inches away from a frame, I would get 5'' props.

The next thing to look at is the pitch and number of blades. This is where most of the numbers come in for props. Let's use the example of 5143. the first part, 51, is the prop diameter (5.1'') and the second part is the pitch of the prop, 43, which would mean that the pitch of the prop is 4.3''. A higher pitch generally uses more power but can result in faster flying to some extent. (sometimes the prop pitch is not entirely accurate, as it depends on what the manufacturer thinks) The number of blades of the prop will be listed separately when the dimensions of the prop are listed like this.

Another way you may see prop dimensions formatted is like this: 5.1x4.3x3. That essentially says the same thing except it also includes the number of blades, which in this case, is 3. The 5.1 is 5.1'' diameter and the 4.3 is 4.3'' of pitch.

What I would recommend is to try many different types of propellers because many props that have the same measurements will perform differently.


How to revive an over-discharged DJI Mavic battery?

Short answer. Buy a new battery.
I heavily researched this and although there may be another solution I will share my experience.
The Mavic batteries have complex circuitry in them to maintain stability and longevity.  This is partly why the Mavic charger will not revive the battery.
Is there a way to manually charge it?  Well the answer is yes and no.   The cells inside the battery are probably okay if the battery did not deplete this far while in storage.  The problem is the circuitry will not allow a manual charge above failsafe levels restoring to a functioning DJI battery.   There are some videos and reports of people cracking open their batteries and doing various physical things to the circuitry to fix this issue.  I do not recommend this.  I found that opening the Mavic battery was not easy and left the battery case in disrepair due to it being glued or affixed with single use fasteners.  If there is a tool that can open the battery case gracefully i don’t believe it is something in a standard tool box.  On top of that the Mavic circuitry is precise to avoid radio interference and modding that could increase radio interference.
Once the battery was open, poorly, I was able to charge directly to the main leads.  The battery stack read about 10.5 to 10.6 volts on 3 cells.  Very low.  My hobby charger pulled it up to 11.1 volts static.  I was still unable to charge using the Mavic charger.  This led me to believe the circuitry remembers the charge state of the battery and will not continue if that battery has inconsistent untracked voltage changes.
I saw another video that shows desoldering the positive terminal while the balance lines are disconnected.   Then soldering it back.  I tried this and it did seem to refresh the charge level readout, but i still could not charge using the Mavic charger.  With my battery split open and looking like it would never be allowed in checked or carry on baggage i gave up and decommissioned the battery using a method that worked for me.  Once the battery became inert it could be disposed of properly.
I suggest contacting DJI before opening the case. Maybe they have a trade in policy or something when the Mavic depletes the battery too low?
Best bet is to get a new battery.


LiPo battery charger remotely controlled?

There are a few chargers with data ports but they may not work the way you want. These chargers have so many options that a remote interface would be quite complex, and the manuals often don't give much detail about the features available. Complex chargers always wait for you to confirm that it is set to the right type of battery before charging, and require you to press a button.
I think the way to go would be a really dumb charger that is set up specifically for your battery size. A charger like this will just start as soon as the battery is plugged in.
Lipo charging is pretty simple, you just provide a constant current until it hits a maximum voltage, then the charger reduces the current to the voltage constant.
You could build a simple circuit to do this, but you can also buy basic boards that do it for you. They're cheap and designed to be integrated into electronics projects.
You can monitor the pack voltage and individual cell voltages with an Arduino, Raspberry Pi or other device, depending what you want to do with the data. These all have inputs that are capable of measuring voltages sufficiently accurately to detect imbalances or tell when the battery is charged.


How do I calculate thrust for a quadrocopter?

The usual way to calculate exact thrust is actually experimentally, through testing on a thrust stand. There are countless factors that may influence the performance of your particular motor and propeller combination, some of which are hard to measure and quantify, so the exact number has to be found out just by testing it.

That said, if you only need a rough ballpark, testing your particular combination is not required; you can look up test results for other motors and props of the size you want. 20 inches is very uncommon, so the data may be scarce, though; if you don't find any, doing it yourself is the best bet.

As for dynamic thrust, it is even harder to predict, especially as it depends on the quadcopter's configuration in addition to its attitude and speed; if you want precise enough numbers, you'd probably have to employ either a wind tunnel or a numerical simulation (which is not that unreasonable of a cost for the size of drone you're planning). That said, I'm not entirely sure you actually need it, it's usually sufficient to ensure that the drone's static thrust to weight ratio is high enough (2.5+ is a good number for slow flyers such as camera drones; 4 is good for sporty flying, 8 is good for competitive flying).


Video breakup problems with high power using The Force VTX?

This can be caused by too much heat building up in the VTX causing the electronics to suffer. Try mounting the VTX to have more airflow around it or put a heat sink on it. Sometimes a VTX just can't handle its own output power.

Another issue might be that electronic noise from the VTX is getting fed back into the video signal at the camera or FC connection point. Adding capacitors to various voltage rails sometimes can help.

Still another issue could be that the VTX is drawing too much power from the regulator that is supplying it. Try connecting the VTX directly to Vbat if it can take the higher voltage.

In general, noise in the video signal is the bane of FPV video problems. It can be a bit of black magic to fix.


This is an age old problem, there is no solution, only methodology.
First, your budget is a HUGE constraint.  To build a drivetrain, without years of experience, is very difficult, I doubt you will nail it.
The approach you use to decide motor is critical, more on that as we progress.
Some things to consider FIRST:

what are your power train expectations? 10+:1 power:weight is what I use and my drone get's the hell out of town, but I also cook batteries; able to set them on fire punching out if I choose.
What is your desired type of flight, the spectrum goes from freestyle/race to sit-n-spin (hover and take photos).
What flight time would you like (short or long?)

As I am sure you know already, pitch and KV are related. High KV leans lower pitch (faster spin with blades that push less air). Low KV leans higher pitch (low KV should have more torque and allow steep pitch to push more air).
High KV build tend to have shorter flights but are more maneuverable.  Lower KV builds stay up longer and are amenable to cinematography (generally).  There are tuning specific problems across the spectrum.
The characteristics of motors generally in XXYY configuration vary greatly. xx is the rotor diameter and yy is the motor/rotor height.  Wider diameter motors have a different inertial footprint than narrow, carrying more weight to help keep the big props swinging.  Narrow can stop/accelerate/decelerate faster, again inertia is a big part of the formula.  Height of rotor is less impactful from my experience in the context of this discussion, generally a product of the diameter and desired KV (more height to wrap more wire, for example).
A 2000kv motor [example] ranges from 15xx to 25xx sizes, so KV is a hard indicator to use alone.  KV is key for calculating the target max rotational speed in conjunction with your voltage.  I have used the exact same KV motors in a small and large format and the difference was astonishing.  The takeaway is that KV helps determine relative spin rate and is not helpful as a stand alone metric.
Keep in mind that rotational speed of the propellers is huge. Bigger slower moving props do not provide the same gyroscopic byproduct as a fast spinning motor, this may be something to consider.  There are all sorts of RPM specific problems along this spectrum.
On to props.  Generally props use a AABB format where AA is the diameter and BB is the pitch. Pitch, generally speaking, is how much "screw" the prop will move along the motor shaft axis per rotation.  When teaching new pilots I explain pitch as "how far a prop will move in a block of jello spinning 360 degrees".  So a steep prop will move far while a shallow prop will not move very far [in this mental exercise].  A good internal understanding of pitch takes a while.
Battery choice is difficult.  Decision 1 is cell count (voltage). Decision 2 is mah (amount of energy stored) and generally an indicator of weight for the battery. Last is C rating (abuse factor). Mellow units can fly "well" on 25c, high performance need 75C+, my 5" cinema/chase drones need 95c+ (usually 120c).If you miss the C rating (too low) you will puff battery, decrease life dramatically, and generally be pretty upset with performance.
So to answer your question, there are a few ways to go, but first you will need to have budget to fail and try again, that is the nature of scratch builds, especially for someone lacking extensive power train building experience.  Drone building is a constant repair cycle, that is why people who take this seriously travel with mirror+ builds, I usually carry 4+ units for a shoot or session.
The easy route is to buy a pre-built where payload and performance as well as demo videos are available.
The next route is to join a flying club or commercial group and check out other builds and get some local friends to guid the choice.  This is my #1 recommendation to new pilots - let a friend help. Otherwise you will be flailing in places like here listening to a lot of armchair pilots pushing their agenda and favorite builds.
The next approach is to copy a build that fits your needs. The largest repository of builds is RotorBuilds (https://rotorbuilds.com/), nice filters, tons of videos, and possibly contact info to ask questions directly.
The last way to go is take a stab at it. That is what I did in 2013, I failed and essentially smoked a whole drone worth of parts before my first flight. I copied Charpu's build (he was #1 at the time) and it was a painful experience without any coaching.  Granted times have changed, but the information on the web is mostly garbage.
It feels safe to assume that you wanted XXYY motor and AABB prop on target voltage. I can't deliver than information, I doubt anyone without intimate knowledge of your use case will be much help.  Ergo, the approach and methodology to solving the problem becomes key.
DroneTrest has a good reference page (https://www.dronetrest.com/t/brushless-motors-how-they-work-and-what-the-numbers-mean/564)  if you have not been here already.
If you want to dig in then seek out motor specification sheets.  There are many sites dedicated to this (I have no favorite).  Look at motor+volt+prop mah draw (avg/max) and heat if available.  This information is great, but unfortunately it will not help a lot without drivetrain building experience.


How to enter DFU mode on my flight controller?

There are three ways you should know to enter DFU mode.

The first, as mentioned before, is to push down the bootloader button and power on your board.

The second method is to be used on boards that don’t have a bootloader button (which is increasingly rare). You will need to bridge two bootloader pads on your flight controller to enter DFU mode. Check the specific diagram for your flight controller to find out exactly what pads you need to bridge.

The last method is to enter the CLI and type either ‘DFU’ or ‘BL’, which will then put the FC into DFU mode. Obviously this method won’t work if you’re entering DFU mode because you ant connect to your computer.


What happened to DSHOT1200 support in Betaflight?

As is explained in this forum post, support for DSHOT1200 signaling was indeed removed from Betaflight due to its primary use case being devalued starting in version 4.1.

  DShot1200 is officially removed from Betaflight 4.1:

  Dshot1200 is only needed for 32khz looptime, and 32khz looptime isn't supported any more in Betaflight. The highest looptime in Betaflight (in BF4.1) is 8KHz, and Dshot600 is enough for 8khz looptime
  Dshot1200 was not stable when used with bidirectional DShot which is required for RPM filter, which has a higher priority than DShot1200

This GitHub issue also points out that DSHOT1200 was poorly supported by BLHELI_S and BLHELI_32 ESC firmwares and wasn't really necessary to maintain.


Optical sensors vs Ultrasonic sensors on a drone

Keyence has a useful comparison table (appologies the table doesn't look good on mobile):

╔════════════════════╦══════════════════╦══════════════════╗
║        Item        ║     Optical      ║    Ultrasonic    ║
╠════════════════════╬══════════════════╬══════════════════╣
║ Detectable target  ║ Affected by      ║ Unaffected by    ║
║                    ║  colour/material ║  colour/material ║
║ Detecting distance ║ 1000mm max       ║ 10m max          ║
║ Accuracy           ║ High             ║ Low              ║
║ Response speed     ║ Fast             ║ Slow             ║
║ Dust/water         ║ Affected         ║ Unaffected       ║
║ Measuring range    ║ Small            ║ Large            ║
╚════════════════════╩══════════════════╩══════════════════╝

So basically it depends how and where you want to use it... Since you say you are trying to avoid colliding with objects, my guess is you'll want the ultrasonic.  It gives you a much higher detecting distance (10 m vs 1m).  At most speeds, 1 m could be insufficient to avoid a collision.

Also, if you plan on using your drone anywhere where the sensor could get dirty/wet, you'll definitely want to go with the ultrasonic sensor.


How to restore LiPo-powered aircraft that have not been flown for years?

You may need new batteries.

When a LiPo battery fully discharges, thin whiskers of lithium metal form between the plates of the battery.  Attempting to recharge the battery will cause these whiskers to overheat, which may cause the battery to catch fire or explode.

Commercial batteries from reputable manufacturers have self-monitoring circuitry that will permanently disable the battery if the charge level drops too low.  If the batteries are of this sort, it's safe to test them by recharging -- if the battery has been damaged by over-discharge, it simply won't take a charge.

Cheap third-party batteries and homemade battery packs don't have this protection.  A battery that's only been sitting for a year may still be good -- I'd try recharging it in the middle of a concrete driveway or some other well-ventilated, fireproof location.  With a battery that's been sitting for three years, I wouldn't risk it.


Is it safe to fly LiPo batteries after they get "puffy"?

You should dispose of them.

A LiPo battery has three parts: the anode (negative plate), the cathode (positive plate), and electrolyte (sandwiched in between the two plates). Electricity is formed by electrons moving from the cathode to the anode through the electrolyte (which helps the electrons flow).

The electrolyte decomposes over time, which results in the formation of elemental lithium and oxygen. Some of it combines into lithium oxide and sticks to the anode and cathode, but some of the oxygen remains a gas. This gas is what causes the puffiness you see.

Oxygen likes to burn, so if you use a puffy battery, the likelihood that the moving electrons will ignite the oxygen is much higher, so once it becomes puffy, you should stop using it and dispose of it.


I think prop wash is not entirely eliminable by tuning. If you have really good gear and tune you may be able to reduce it even further, but it will be there.

When you are doing sharp turns, you are flying into your own turbulent air stream and the effectivity of your motors compared to each other changes unpredictably and rapidly. The flight controller tries to compensate, but neither the calculation nor the the motors' reaction is instantaneous, so most probably the compensation will come too late and will not achieve the required effect, which will trigger further and further micro adjustments until the quad reaches more predictable air.

So the easiest way to eliminate prop wash is to move out of your own turbulent air as quickly as possible. When doing split-S don't just descend vertically, but start moving forward before arresting your fall so your motors will be over clean air.


What's the advantage of props in VS props out?

As far I know the only difference is on the whoops. They fly much better on props out.
On 2,5-5 inch quads, it doesn't matter. Props in: our camera is dirty, you can stack easier on the tree. Props out: Your flight controller is dirty and then you damage your prop it's a higher risk that pice of the prop damage your flight controller or battery (it is a much higher risk).
Greatings form DE.


Does frequently changing the amount of power to a bldc motor wear it out?

No, changes in speed should have little effect. The only wearing part of a brushless motor is the bearings, which are generally rated for a certain number of rotations and a certain load (and are reasonably easy to replace).

With IC engines there are complex issues piston and bore temperature, expansion, lubrication and heat cycling - but electric motors are much simpler.


Why do drone propellers have blades with thin chords?

Overview

The most efficient propeller is single bladed, has infinitely narrow chord, has infinitely thin airfoil, has an infinitely long blade, spins infinitely slowly, and requires infinite torque. So at the risk of stating the obvious, propeller design is a study of the tradeoffs and compromises required to make something practical. Here's a list of some compromises:

length: landing gear height (or water depth) restricts how long a blade can be.
material constraints: thicker airfoils resist forces better. Stronger materials allow for thinner airfoils while still meeting required blade strength
power-plant constraints: the torque required to spin the propeller should match the motor/engine's efficiency and power curves.
number of blades: more blades equal more thrust for a same diameter
increased activity factor (aka blade width): wider blades produce more thrust for the same diameter
balancing: one prop is very hard to balance without causing lots of drag by the opposing mass.
vibration: three props give fewer vibrations than two
sound pollution: smaller props spinning more slowly are more pleasing on the ear
consumer perception: consumers like to see big blades in their fans, regardless of real-world efficiency and performance.
density of the fluid:

dense fluids such as water can cause cavitation, which can quickly  damage the propeller. One solution is to change tip shape so as to minimize cavitation likelihood.
on the opposite scale, with low-density fluids such as air, the blade tip can spin so quickly it stalls.

As it relates to drones

Rotorcraft are a little special, so let's break out by type:

Vertical propulsion

Multirotor drones and helicopters have tons of space compared to boats, and so don't particularly need to restrict their diameter. They choose their rotors based primarily on power-plant performance.

Longitudinal propulsion

Fixed-wing drones with landing gear obey the same rules as full-scale aircraft. If  the prop is going to hit the ground on takeoff it's not very useful!

Fixed-wing drones without landing gear, e.g. hand-launch gliders, can use very large folding propellers. These are chosen similarly to multirotors, where it's about efficient power-plant matching.

As pointed out by @RobinBennett, "You don't want it stalled when the aircraft is stationary, so there's a maximum pitch angle - and that sets the maximum speed. If you want to go faster you need to spin the prop faster, and if you've got a fixed amount of power, you need a smaller prop just to spin it faster."


What is the analog video input pinout on the DJI FPV goggle?

There is a project on thingiverse that shows the pin connector for the video input of the DJI goggles.

It's a 4 pin 3.5mm jack with the following connections

sleve - GND
ring 2 - Video
ring 1 - Right audio channel
tip - Left audio channel


To clarify, the 48kHz is NOT running on the gyroscope or the FC firmware.
What 48kHz is referring to is the PWM Frequency in the ESC, namely the frequency at which the power MOSFETs on the speed controller switch on and off in order to provide power to the brushless motors and therefore thrust to the craft.
The reason in whoops and small/light craft 3" and smaller will see a flight time improvement is that the RPm ranges these crafts fly at, the switching down-time at lower PWM frequencies means the motor/ESC is having very small active braking events in the switching cycle, then overcompensating by driving higher throttle values to compensate (both of which cost energy).

In some cases, 96kHz will also work very well, although this is often limited to smaller whoop class quads, and I would caution against it on specific instances where the ESC design has a higher dead time value because these tend to suffer failures more often at higher PWM frequency when pushed hard.


Would it be possible to inductively recharge a drone battery?

Given how close you would need to be to the charging system, it makes little sense to keep flying rather than simply land on a charger.
The question then becomes if the inductive power transfer scheme is worth the trouble vs having some sort of electrical contacts, possible with some kind of cam-driven capture in the base.
On the one hand a wireless power transfer scheme is weatherproof and may require somewhat less positioning precision; on the other, it adds weight to the aircraft, and a lot of complexity.


Implementing object detection in DJI Mavic 2 pro in real time

That's a fascinating project, but I'm not sure it's possible on a DJI drone. The firmware is closed source and not designed for easy modification.
I don't know if it's possible to replace the DJI firmware with an open source project like iNav or if you would need to build a new drone from scratch around an iNav capable flight controller.
You might also find that you need a more powerful processor to run real-time image analysis.


What's the purpose of airmode and idle up?

Airmode and idle up both seek to increase a control authority and stability during ultra-low throttle moves.

In normal flight, when the pilot commands the multirotor to pitch or roll (cyclic), the flight controller responds by increasing thrust on one side of the craft and decreasing thrust on the other side by the same amount proportional to the cyclic rate commanded. This symmetrical change in thrust allows for quicker rotation than would otherwise be possible if the flight controller only increased power on one side.

The trouble comes when a pilot tries to change the attitude of the multirotor when the throttle commanded by the pilot is near zero (min_throttle). The flight controller obviously can't decrease the speed of a motor below zero (aka. idle speed), so it has to do something to ensure it can achieve the cyclic rate the pilot wants. Without anything to fix this behavior, the flight controller would only respond with as much as it could without making any motors increase thrust more than the others can decrease thrust. This results in massively decreased control authority at low throttle because the flight controller can only provide minuscule adjustments to motor thrust.

This is where idle up and airmode come into play, although they solve the problem a bit differently.

Idle Up

Idle up is setting configurable in the RC transmitter which changes the throttle curve at the low end so that it the bottom-most throttle position doesn't actually correlate to the transmitter sending zero throttle to the multirotor. Instead, it would send some low value that you configure, usually below 10%.

This would allow the flight controller to speed up and slow down motors by the amount you configure for the increased idle speed, which is a lot better than what it could otherwise accomplish at low throttle.

Airmode

Most modern flight controller firmwares support airmode, including Cleanflight and Betaflight.

When enabled on the flight controller and the pilot commands a large rotation while at low throttle, the flight controller temporarily pretends that the pilot's throttle command is high enough for it to slow down motors to zero throttle and still achieve the cyclic rotation rate the pilot wants. When the pilot commands less cyclic rotation rate, the flight controller stops pretending that the throttle command is higher than it actually is.

For a graphical explanation of this functionality using graph paper, I recommend taking a look at Joshua Bardwell's video about airmode.

To see what this looks like visually during low-throttle maneuvers, I recommend watching Le Drib's video about idle up vs. airmode. (link starts at relelvant section)


Transmit video from drone to laptop via WiFi

You have a few options. One is to switch to a digital video system such as the fat shark bytefrost and do something like this to convert the video to a format compatible with your laptop.
Essentially the above link documents how to convert a digital FPV feed into your laptop for doing low latency video calling, but you don't have to do the video calling part. You can skip the portions about the transmitter unless you want to control the whole drone from your PC.
Or, you can do something hacky that I have done before... which is buy the display and then pop it into a cheap pair of FPV goggles. Then put a USB webcam into the goggles where you would normally put your eyes. Then boom! You can see what the drone sees via a USB camera.
You can also utilize a analog to digital converter as other users have replied, but you are going to have at least 1-2 seconds of latency unless you get a converter in the 1000's of dollar range.


I think yes, this is doable. In this diagram you can replace the ESC Motor 1 with your 4 in 1 iflight ESC. Note that you will probably need to redirect the PWM outputs to your 4 in 1 ESC signal inputs.

Could I provide power to the motors by wiring all of the positive outputs together of the power module

No, no need to do that. Use a single power output of the power board (it is simpler and in certain cases more reliable).
EDIT
Following the discussion in comments, your 4 in 1 ESC already acts as a power distribution board. Depending on your situation, you may not need the power module at all. For example, one could split battery power output into two: one to go directly to 4 in 1 ESC, and another one to an UBEC to get 5V (to power the flight controller).


How long can a LiPo pack be kept fully discharged before storage charging?

The most appropriate answer I could find comes from this study which states that a charged LiPo should be returned to storage charge if you do t plan to use it within the next 12 hours, and leaving it longer will cause a buildup of damage. The study is very interesting, though it may not explicitly answer your question I recommend giving it a read.


What do the numbers mean on a brushless DC motor?

A four-digit number (e.g. 2212)

This number describes the dimensions of the stator inside the motor. The first two digits are the diameter (in mm) and the second two digits are the height (again, in mm.)

A Kv number (already explained on this site)

Kv - or Kv - has some really good answers at the provided link so I won't go into detail here, but it is approximately the RPM per volt.

A number with N and P (e.g 12N14P)

This describes the magnets present. The number before N is the number of electromagnets and the number before P is the number of permanent magnets. More magnets means more torque, but also higher cost and size.


I fixed it by adding a common ground between the 5v regulator and the flight controller. I can't explain the science behind this, but I took the answer from this video from Painless360

The flickering is completely gone, even when I punch the throttle.

New wiring diagram:

EDIT: I will leave the post unanswered until someone intelligible explains this, and maybe other OSD flickering causes just to keep the post general to the question and not specific to my hardware and wiring.


How can I set up appropriate motor mixing for a frame of a custom layout?

Once you've performed the necessary resource remapping commands to assign the extra motor functions (e.g. M5, and M6) to alternative pads on your flight controller, you can then set up custom mixer rules to tell Betaflight what to do with the additional motors.

However, if one of the predefined mixer types works for your build, you can just load it using the Betaflight Configurator or via the CLI by sending the command mmix load <name> where <name> is one of the mixer types from the link above.

Otherwise, you'll need to configure your own mixer rules. Following the instructions from the Betaflight Wiki, you'll need to use the CLI in the Betaflight Configurator to formulate and send mmix commands.

First, you'll need to send mixer custom to enable custom mixing.
Send the mmix reset command to remove any existing mixer rules.
For each of the motors you need (starting with M1 and ending with M6 for your hexacopter) send an mmix command with the following space-separated parameters:

Motor number for the rule (in the range 0-5, so subtract one from the M motor port number).
Number from 0.0 to 1.0 indicating how much throttle should be mixed into this motor. If this motor shouldn't be affected by the throttle, enter 0.0.
Number from -1.0 to 1.0 indicating how much roll should be mixed into this motor, where positive numbers mean that a roll command over 1500µs should increase the motor's power and vice versa. If this motor shouldn't be affected by the roll axis, enter 0.0.
Number from -1.0 to 1.0 indicating how much pitch should be mixed into this motor. This value obeys the same conventions as the roll axis parameter.
Number from -1.0 to 1.0 indicating how much yaw should be mixed into this motor. This value obeys the same conventions as the roll axis parameter.
An example mmix command is: mmix 0 1.0 0.0 -1.0 -1.0

Remember to send the save command to save your new mixer configuration.

This is all quite confusing, so here's an example from the same Betaflight Wiki page showing what the mixer commands would be like to use the M1, M2, M3, and M5 ports if te M4 port were broken on your flight controller.

mixer custom
mmix reset
mmix 0 1.0, -1.0, 1.0, -1.0
mmix 1 1.0, -1.0, -1.0,  1.0
mmix 2 1.0, 1.0, 1.0, 1.0
mmix 3 1.0, 0.0, 0.0, 0.0
mmix 4 1.0, 1.0, -1.0, -1.0
save


What is the advantage of SBUS over IBUS?

SBUS and IBUS are both digital protocols and for a digital protocol, you define two voltage values to represent an active state and one inactive state. This is done for every digital protocol.

You can specify that 0V is inactive and 1V is active (this is called active high). But you can also define that 0V is active and 1V is inactive (active low).

Many bus protocols are active low and SBUS is also such a protocol. The reason for this choice is that active low signals can handle noise better. This is probably the reason why Futaba designed SBUS this way.

The ARM chips used on the flight controllers have the hardware to decode serial signals (both SUBS and IBUS are serial protocols). This decoder also is built for active high or active low signals. If you want to decode an active low signal (SUBS) with a decoder that is built for active high you have to add an inverter between so that both "talk the same language". This is the case for the F4 chips since ARM did not include an inverter on the chip. For F3/F7 this problem does not exist since ARM included the inverter in the chip and the software can enable/disable the inverter.

You get similar performance from SBUS and IBUS since both are a serial protocol with comparable frame times.


What makes mushroom antennae so good at receiving and transmitting omnidirectionally?

The primary purpose of using circularly polarised antennas is the rejection of out of phase video reflections. They tend to have lower gain compared to linear antennas but linear need the antenna alignment to be maintained. On aircraft that flip and roll in all axis and also fly near buildings ETC circularly polarised antennas with the analogue transmission is very advantageous however with digital transmissions reflections can help.


Is there a list of milestones to check off before claiming one can "fly"

In the UK, the British Model Flying Association offer an Achievement Scheme which recognises a few levels of competence:

The Basic Proficiency Certificates provide a measure of competence for pilots operating fixed and rotary wing aircraft fitted with
stabilisation technologies.
An FPV extension is also available to holders of any Basic Proficiency or ‘A’ certificate.
The ‘A’ Certificate, which may be equated to a basic standard of safety and flying competence.
The ‘B’ Certificate, which is designed to recognise a more advanced pilot’s increased ability and knowledge, and demonstrates a high level
of safety.
The ‘C’ Certificate, Aerobatics.
The ‘C’ Certificate, Scale
The ‘C’ Certificate, Gas Turbine

As well as covering theory (air law/safety) questions, the flying skill required increases as the candidate progresses through the levels - from assisted flight, through holding level figure 8's and other manoeuvers, to being able to fly in aerobatic displays.


Finding a ready-to-fly Taranis compatible, Eachine compatible, S4 compatible under 200€ drone

I think the Cidora SL5-E is a very good choice. What you need to look for is the "BNF" suffix in the description which stands for "Bind and Fly". In that case a receiver is already built in.
On the iflight website you can select this option.
If you have the choice, it's best to select the r-xsr because it comes with telemetry.
If you are browsing shops look for these acronyms:
BNF: Bind and Fly, you can pair it to your existing remote and goggles
PNP: Plug and Play, you install your own receiver
RTF: Ready to Fly: a complete kit with everything included such as remote and sometimes goggles.


How are Hall effect gimbals different than standard gimbals?

The difference is in how the gimbal position is sensed and reported to the processor in the transmitter. The two main types are potentiometer and hall effect gimbals, but in both types, the fundamental mechanism holding the gimbal and allowing it to rotate is the same.

Potentiometer (aka. pots)

Potentiometers are resistors which change their value depending on the location of the wiper along the resistive strip inside them. In a RC transmitter, the wiper is connected to each axis of the stick. The processor in the transmitter reads an analog voltage from the potentiometer and translates that to the axis' value.

 (cit.)

Hall Effect

A hall effect sensor also reports an analog voltage, but this time based on the strength of the magnetic field it senses. A magnet is attached to each axis of the gimbal and a hall effect sensor is placed on each side of the frame of the gimbal. The movement of the magnets either closer or further from the hall effect sensor registers as a changing value for the axis.

Comparison

Potentiometers are generally less precise than hall effect sensors and tend to drift over time, leading to a need to calibrate the center point of each axis every once in a while. This doesn't need to be done with hall effect gimbals.


Flight controller or ESCs on Eachine Viswhoop not booting properly

There are a few things here. An empty Modes page isn't a problem, it just means you're in Acro mode all the time. You may want to set up Angle or Horizon for a whoop that will be flown indoors. Add beeper mode here too, if you want it.
There are a few things that could stop it arming. The easy way to tell is to turn on 'warnings' in the OSD, and see what it says in your goggles.
Common problems are:

Not getting a signal from the receiver - check on the radio tab of BF.
Incorrect lower bound on the throttle channel, because it wants to see the throttle closed when it arms.
If you've set a maximum angle for arming, and the quad isn't sufficiently level.


Where can I find a list of all the model aircraft flying fields in the USA?

Does not exist, a few reasons.

AMA lists most of their fields, but that information is spotty and I have almost been arrested trying to fly at listed AMA sites that are not valid.
FAA has this list, but they have openly admitted that infrastructure and management problems have led to this data being in decay and not very useful.  This problem was exposed as 107 was coming into law and many commercial pilots raised a stink because airports on flight maps were never (EVER) updated, this led to LAANC and the apps becoming more of an authority.
No one shares spots. There have been several apps that try to get people to share and they all went belly up real fast.

This information ALL exists in the FAA, but their data is horrible (I work with this info, total nightmare).  Most of the good fields are private and invite only in my world. We run a few spots and you will be shot dead if you fly there with out permission (not an exaggeration, I often carry when flying).
Airmap and Kittyhawk will have reasonably good current AMA type fields.  That data is the best for "free".  You may want to check with Snowflake, but their data feed (last time I was involved) starts at $5k/year.
It should be noted that the FAAn NTSB, FBI, and other orgs all recognize the unstructured, uncoordinated, and non-unified nature of RC flight and use this to control airspace.  I mention this because there is considerable history on the record of the FAA trying to wipe all drone use from NAS and this is a declared strategy and we have seen it in action.  Dig into the DC restrictions of 2014-15 to understand why the FAA is an advisory and not an ally.


What is this ring attached to the bottom of this rotor shaft?

That is a circlip (or C-clip) - it looks like it is used here to prevent the shaft from coming out of the bearing.
From the Wikipedia link above:

A circlip (a portmanteau of "circle" and "clip"), also known as a C-clip, Seeger ring, snap ring or Jesus clip, is a type of fastener or retaining ring consisting of a semi-flexible metal ring with open ends which can be snapped into place, into a machined groove on a dowel pin or other part to permit rotation but to prevent lateral movement. There are two basic types: internal and external, referring to whether they are fitted into a bore or over a shaft. Circlips are often used to secure pinned connections.


"Rogallo wing" balsa rubber-band model plan - do you have one?

I found this plan online, which might be similar to what you want. The wingspan is about right, it uses a Rogallo wing and the text in the bottom-right suggests it was in a magazine called the "American Aircraft Modeler", if that rings any bells? (Your profile indicates America, so it sounded promising.)


You'd likely be looking to run a firmware like iNav on your flight controller to enable the path-following features you need like altitude hold and mission/waypoint flying. This is really the most capable and readily available autonomous flight firmware for hobbyists.
The Naze32 FC board is based on the older and less capable STM32F103CBT6 processor (cit.), which hasn't been supported in iNav since the 1.7.2 release in July 2017 because the pace of feature development outstripped the computational power of the STM32F1xx processors.
Although you wouldn't be receiving feature/bugfix updates for iNav running on the Naze32, you would still have access to the features added prior to the 1.7.3 release that dropped support for F1 boards and almost certainly includes some sort of altitude hold and waypoint following capabilities, even if they're more rudimentary than what is available in the latest versions of the firmware.
You can install and configure iNav using the iNav Configurator and by reading the getting started documentation on the iNav wiki pages.


FPV camera and beeper seems to be interfering with each other

The problem was indeed the 5V regulator. I bought an external 5V 1.2A regulator and hooked up the camera and VTX to it, and all my problems have disappeared.


What can I do with a LiPo whose balance lead has been destroyed?

If you're up for it, the fix you're looking for is to repair the connector with a new housing and new crimps (if the current ones are broken). Joshua Bardwell has an excellent YouTube video documenting how to do this, but I'll summarize here.

Most LiPo balance leads are terminated with the JST-XH connector:

 (cit.)

In the above picture, the balance lead is configured such that the left-most pin is ground/GND and then are in ascending order of voltage from left to right when the flat surface of the connector housing is facing upwards. For example, a correctly made 4s LiPo balance plug would be wired up like so from left to right:

GND
Cell 1 (4.2 V when fully charged)
Cell 2 (8.4 V when fully charged)
Cell 3 (12.6 V when fully charged)
Cell 4 (16.8 V when fully charged)

Repair instructions:

Purchase a JST-XH connector kit like this one on Amazon and a JST-XH crimper tool like the Engineers PA-09 on Amazon if you know or anticipate you'll need to replace the crimped pins (there are cheaper crimpers available, but I've had little trouble crimping JST-XH with this one and it's a nice-feeling tool)
Pick out a new connector housing that's the same size as the broken one
Remove the lowest voltage pin (left-most pin in the above picture) from the broken housing using a pointy tool to lightly press down on the edge of the silver retention tab
If the pin is broken/bent/deformed, cut/pull off the JST-XH crimp and re-crimp a new one on
Insert the pin into the new housing with the little retention tab facing upward until the retention tab on the pin clicks into place
Repeat steps 3-5 for each other pin in order of ascending voltage

I'd recommend only having one pin free of either connector housing for as much of the process as possible to avoid inserting them in the wrong order or accidentally letting two of the pins tough each other, which would short the cells together (be bad).

Afterward, you should check your work by using a multimeter to confirm that the voltages between GND and each pin are in ascending order from left to right.


I did increase tpa about 10%. This did fix my problem!

tpa will lower pid-values at higher throttle.


Power systems should be balanced. If you fit motors that can handle more power than the battery can provide, you're just adding unnecessary weight. That weight will reduce performance in all areas.
With electric power systems the battery is the source of the power, and everything else is little more than the transmission. If you want a 3 minute flight from a 4s1500mAh battery, that's an average of 60/3 * 3.7*4 * 1500/1000 = 444W, or 111W per motor. There will be higher burst loads but the motors have the mass to handle it. They'll heat up a little in the bursts and cool down in between.
If you switched to more aggressive motors or props you could increase performance in the bursts, at the cost of reduced flight time - and you might be pushing your battery too hard. If your battery is hot at the end of the flight, you're reducing its life.
If you push really large bursts through a small motor, there's a point where it will be less efficient (something to do with the maximum magnetic flux density of the stator, IIRC) so there is some benefit to larger motors but if your motors are cool at the end of a flight, you probably won't see any benefit from larger motors.


So this method will vary based on whether they are brushed or brushless motors.

For brushed:

Spin the propeller with your fingers - is anything actively resisting the motion?

If yes, remove the prop and check for something not immediately visible, like a hair or carpet fibres caught around the motor shaft.

Remove anything you find and try the prop again.

If nothing is resisting motion, check the connections between the FC and the motor.

If all else fails, swap the motor with the one diagonal from it. If the problem travels, the motor is at fault. If not, there is trouble with the board.

It is worth noting that brushed motors are a consumable - the brushes wear out, and most aren’t rated for more than a number of hours’ use - see the diagram below:

(Image Source)

If the motor is brushless, start by again turning the motor, and check no screws are touching the windings.

Check all motor connections and see if there are any breaks.

If there is not active resistance, swap over two of the motors. If the problem follows the motor, it is likely a problem with the motor. If not, there is a problem with one of the ESCs.

If you do encounter active resistance, look inside the bell and see if any of the magnets have slipped. If they have, use epoxy and tweezers (though a toothpick also works well) to glue the magnet back in place. I’d recommend also buying a new motor as it is now likely to be slightly unbalanced which may cause oscillations.

See diagram of brushless motor:

(Image Source)


How do I wire up this transmitter and camera

These components can be split into two functional groups, one for transmitting a picture and one for receiving it, which aren't connected to one another by wires. The Runcam camera and Eachine VTX are both parts of the transmitting section, while the receiver doesn't need to be soldered to anything else in the receiving part.
Required Soldered Connections:
You'll need to make the following wire connections between the different components: (splicing together the components' connector pigtails is recommended as opposed to trying to solder directly to the camera)

Battery+ → VTX 7-24V wire
Battery- → VTX GND wire (there are a total of two available, choose either one)
Battery+ or VTX +5V wire → Runcam 5-36V wire
Battery- or VTX GND wire → Runcam GND wire (in 5-pin camera connector, not the 2-pin camera controller connector)
Runcam Video wire → VTX VIDEO wire

Essentially, both the camera and VTX need to be connected to each other for the analog video signal (one wire) and each need to have connections to ground/GND and a positive voltage source within their requirements.
Battery Setup:
In this setup, the VTX is actually the component that constrains the kind of power sources/batteries you can use. It requires 7-24V to operate, which means you can use 2S-5S LiPo batteries with it. (or another battery pack within the same voltage range)
Most drone/RC LiPo batteries these days come with either an XT30 or XT60 connector, but can be purchased with other terminations. See this page for a list of the most common ones. Once you know what your battery's connector is called, you can purchase a set of connectors online and solder them to the VTX and camera as required.
It's unlikely you'll be able to match the connectors that come with the parts together. While the connectors are all off-the-shelf parts, their usage is unfortunately not really that standardized between manufacturers. You're probably better off just cutting the cable that came with the camera in half and soldering it together with the pigtail wires that come with the VTX.
Once this is done, you can hopefully power on the VTX, camera, and receiver without issues, (remember to screw on the VTX's antenna before powering up so you don't damage it!!!) set the VTX and receiver to the same channel, and view the picture on the connected Android phone/tablet.
Did I leave something out? Please let me know if I did and something doesn't make sense...


In what situations do puffy batteries pose a risk of ignition?

The risk of a puffy battery exploding during storage is still comparatively low, you shouldn't underestimate it however.
During charging the risk of explosion is at its peak. This is also the reason why you charge your batteries slower than you discharge them (i.e. to lower the risk of puffing and explosions, as well as to keep wear and tear low).
During use, the risk is quite high as well, though I'd say it's lower than while charging.
Always, always get rid of them as quickly as you can. Exploding LiPos are not a joke. Besides, a puffy LiPo has a far lower total capacity, rendering it basically useless for flying anyway (though don't be fooled - the explosion is still more than dangerous).


Why are micro pusher-prop quadcopters becoming more popular?

I have a working theory for why this may be the case; some micro-quad designs support this theory more than others.
Due to Newton's third law, the thrust force created by the propellers also has the effect of trying to lift the propeller up off of the motor in traditional multirotor designs where the propeller is on top of the motor. Aside from ensuring that the propeller can't spin independently of the motor, this is the reason why drones have propeller nuts (or a similar mechanism) to securely hold the propeller to the motor.
Many micro-quad designs, including some of the ones you link to in the OP (e.g. the Diatone BNF TinaWhoop and the FullSpeed BNF Cinewhoop) instead feature the pusher-prop configuration you mention, with the propeller underneath the motor, and forego the inclusion of propeller nuts or bolts screwed through the propeller into the motor.

(FullSpeed BNF Cinewhoop cit.)
I don't have one of these quads to confirm my observations from looking at the images, but it appears like these designs have the propeller attached to the motor with a simpler mechanism that likely doesn't provide as much strength in the vertical direction. If my hypothesis is correct, this would mean that the propellers could theoretically fly off more easily if the motor wasn't in the way of the path the propeller wants to take during flight.


How to create a motor startup tone with a BLHeli ESC?

First, remove your props, plug your quad into the computer and plug in a battery.

You then need to go into BLHeli Suite 32 and read your ESC setup.

You will then have options to sync ESC music and you can upload your tones.

A more detailed guide with lots of detail for each step can be found here: https://oscarliang.com/blheli-32-custom-startup-tone-music/


Is flying a drone in Puerto Rico the same as flying one in the continental USA?

Yes.

  The FAA proposed to limit the applicability of part 107 to small UAS operations that are conducted entirely within the United States. [source]

Puerto Rico is considered as within the United States.


I have found that the simplest and probably one of the safest ways is to use a compressor (alternatively a compressed air can will probably work), to lightly blow away the dirt. This works especially well if you just want to get rid of grass straws or similar.

If you want to more thoroughly clean something Joshua Bardwell has a video on how to clean a motor, and if you want to clean a PCB or other electronics, alcohol > 90% and some kind of brush or cotton swab works. And finally, if it's the frame you want clean, soap and hot water works.


Why don't solutions exist for a 3-axis helicopter flight controller?

Beta flight has helicopter modes ("Heli 120", "Heli 90", etc. depending on your swashplate layout), as does pretty much every other programmable flight controller software.
The outputs from the flight controller are the inputs for the ESCs, which have the same signal protocol as servos. The exception are the flight controllers with integrated ESCs, where the outputs actually drive the motors directly.
This also allows most flight controllers to be used in a fixed wing, where most of the outputs drive servos.


Does motor KV requirement scale linearly with voltage?

Assuming all other variables stay the same, yes, The Kv should scale with voltage completely linearly. There are some other changes in the electrical system due to the voltage change that have some impact on efficiency, but when Kv is changed along side voltage, that is largely mitigated. Based on the tests we've run and comparing the resulting data on the Mini Quad Test Bench motor explorer efficiency is usually within a couple percent if you scale Kv properly. If you got to crazy extremes you might see a bit more variation, but I don't have data at those extremes. For most practical applications you can assume a linear relationship.


Serial Communication Not working in APM 2.8

There are a bunch of problems which need to be overcome when using the RPi GPIO serial port.

The RPi normally runs a login process on the serial port, this will need to be disabled.

sudo systemctl stop serial-getty@ttyS0.service
sudo systemctl disable serial-getty@ttyS0.service

you will also need to edit /boot/cmdline.txt and remove this part: console=serial0,115200

On a RPi 3/4 the port name in linux if different to the port name used on older RPi's.
If using a recent image, the serial port is disabled in the config.txt by default. To fix this you need to edit /boot/config.txt and add the following line.

enable_uart=1

This page Configuring The GPIO Serial Port On Raspbian Jessie and Stretch Including Pi 3 and 4 has a very good writeup on how to get it all working.

Also make sure that you are connecting RX on the RPi to TX on the APM and vice-versa.


What should I look for when buying FPV goggles?

There are several factors that are important for what FPV goggles are right for you:
Type
The first thing you’ll notice is that there are two main types of FPV goggles: slimline and box.
Slimline goggles are usually more expensive, but they are also lighter and many people find them to be more comfortable. Their screens are also smaller, which means that getting good resolution makes the displays more expensive. There are two screens in this type of goggle, which means that some goggles (for example the Skyzone Sky02X) can support 3D video if you have the required camera and VTX hardware.
Box goggles, on the other hand, are larger and have a larger, single screen. This means that some people find these goggles less comfortable, but this brings the advantage that some goggles can fit a set of glasses inside.
Screens
There are different types of screens used in goggles. OLED is generally favoured as looking better with deeper colours, but LCD is usually cheaper.
Receiver
Many goggles have receivers built-in, and some don’t. There are also different kinds of receivers.
When looking for goggles, you really want a ‘diversity’ receiver. This means that the goggles receive two video signals and either pick the strongest or merge the signals to create a stronger one. This means you can add an omni and patch antenna, two patch antennae or two omni antennae.
Most goggles will have a receiver built-in, but the more expensive goggles, such as Fatshark HDO2 and Orqa's FPV.one, don’t come with a receiver, which means you need to buy your own. This gives you more choice to pick a receiver that best serves your needs, for example adding a Rapidfire module which is widely regarded as one of the best receiver modules out there at the time of writing.
IPD and Diopter settings
Most slimline goggles will have an adjustable inter-pupillary distance. When picking your goggles, make sure that the IPD is suitable for your face.
If you have glasses, it is also useful to make sure your goggles either have adjustable diopters (for example the Fatshark HDO2) or can take diopter inserts (for example the SkyZone Sky02X).
Battery input
There are three ways that people commonly power their goggles.
The first of these is a built-in battery that you charge through USB. Counterintuitively, it is usually the cheaper goggles that have a built-in battery, as this means that it is either more difficult to replace the battery, or you can’t bring spare batteries with to fly for longer.
The next kind is a Li-ion pack. These generally take 18650 Li-ion cells and are held in the head strap. These usually have larger capacities than the third type of batteries: LiPo.
Many people use LiPos to power their goggles, either through a specialised battery to hold in the head strap, or with a cable to a battery in the pocket. The latter of these has a distinct advantage: you can use your quad’s batteries (as long as your goggles support the voltage) and you can charge them on your balance charger.
Brand
I usually would say that brand loyalty shouldn’t play too much of a part, but when looking at goggles you want to buy from a brand that you know will offer technical support if you have a problem with your goggles. So I don’t introduce bias, I’m not going to say which brands I prefer.
Features
DVR is important in goggles, which is helpful to find your quad if it crashes.
Another ‘feature’ is a power button. It sounds odd, but many FatShark goggles don’t have a power button. If a power button is important to you. Make sure to look for that.
Another useful feature is a fan (mainly for slimline goggles) as this de-dogs the goggles during a long flying session.
Light-leak resistance is very important. If light gets into your goggles, this can break the immersion. Take a look at the reviews online to see what people think.
Screen Quality
Finally, before buying, take a look online to see if you actually like how the screens look. People such as Joshua Bardwell will actually put a GoPro in the goggles so you can see what they look like in real life.


Differences between fiberglass-reinforced plastic and polycarbonate propeller materials?

The main differences are:

Fiberglass-reinforced plastics: Very stiff for their weight which allows the propeller to keep the correct shape and be more efficient regardless of how fast it spins (less prop flattening), but when these props hit something hard, that stiffness means it will shatter and the drone won't be flyable. Fiberglass-reinforced props are also not very susceptible to heat which is good and means that they will continue to be rigid and efficient in very hot weather. The brittleness of these props means that it is not likely you will be able to use Turtle Mode successfully very often.
Polycarbonate: Good stiffness and will bend rather than break if it hits something hard. This means that when you crash, a blade may bend rather than break and can often be bent back into shape. Polycarbonate props, however, are susceptible to temperature and will become brittle and can shatter if it is very cold and can become soft and not efficient of it gets too hot.


Can I mount my RX and VTX antennae next to one another?

The distance is dependent on the wavelength of the lowest frequency. You can calculate the wavelength $\lambda$ (m) from $\lambda = c \div f$, where $c$ is the speed of light in a vacuum (ms-1) and $f$ is the frequency (Hz.) Taking the centre frequencies of the ISM bands we get:
2.4 GHz: $$\frac{299 792 458 }{ 2 450 000 000 } ≈ 0.12 = 12 cm$$
5.8 GHz: $$\frac{299 792 458 }{ 5 800 000 000 } ≈ 0.05 = 5 cm$$
(Quick 'sanity check' tip: 5.8 GHz is about twice 2.4 GHz, so we expect the wavelength to be about half; looks good.)
The smallest separation is usually recommended as a quarter of the longest wavelength, which for 2.4 GHz is only 3cm - quite manageable. That said, greater separation helps so put them as far apart as your frame practically allows.


Why are most multirotors built with an even number of rotors?

Yaw authority and ease of mixing is generally the answer - since yaw response isn't necessarily the most linear, being able to have N number of motors contribute to yaw in one direction with the same number in reverse is the simplest motor output mixer configuration.
Tricopters do work and are relatively common among odd-numbered multicopters, but once you've exceeded four rotors, there aren't many meaningful performance improvements possible for the added complexity of requiring an odd number of motor outputs and doing a custom mix for that.
There are some slightly exceptional cases such as a forward puller motor mounted perpendicular to an otherwise conventional even-motor copter frame (see FliteTest MilleniumFalcon), but these aren't really a convention odd-numbered motor copter configuration.


How can I reverse the direction of a brushless motor?

Swapping any two of the wires connecting a brushless DC motor to its ESC will reverse the direction it spins.

Despite the name BLDC which implies that the motor runs on DC power, each of the three wires (connected to one of the three phases in the motor) is driven by the ESC with a phase-shifted AC waveform. Each wire's waveform is shifted by 120 degrees from the other two.

This offset is what allows the swapping of two wires to reverse the direction of the motor's rotation.

Because nothing special is required to reverse the motor direction, it is also possible to fix this by editing the software configuration, which will do the same thing but in software. e.g. in the BLHELI32SUITE software for ESCs with the BLHELI_32 firmware installed


I don't think what you're asking for exists. There just aren't many props even available in that size. T-motor and Xoar both have props in that diameter range, but most of them are lower pitch around 8ish for that diameter. You could potentially use an airplane prop, but not sure if you could get the reverse pitch prop easily. My guess is you'd have to have something custom made.

Check these links for some commercially available options:

http://www.helipal.com/index_tag.php?tags=Industrial,Drones,Propellers&tags_type=Products%20Type,Parts,Accessories,Features

https://www.getfpv.com/propellers/commercial-industrial-propellers.html

And it looks like this company can create custom carbon fiber props to spec: https://www.mejzlik.eu/

It's also worth noting that a 27" diameter prop has a maximum RPMs of about 7700 RPM. You can't run a prop higher than about 0.8 the speed of sound without the prop essentially tearing itself apart, especially with these larger props. You can use this calculator to plan the exact target RPMs.  Keep in mind though, even reaching 80% the speed of sound takes a huge amount of power, and there may not be motors large enough to dissipate that kind of wattage. You'll probably have to end up settling for an RPM target a bit more reachable.

If you're looking at transonic or supersonic propeller designs, you're looking at a completely different system approach in both the motor and power system, and normal commercial/hobby grade components are not going to be suitable. I'd look towards custom engineering and development through a contractor with specific experience in that area.


It's relatively easy (for you or a factory in China) to buy a set of components and put together a drone, but just looking at the raw specs doesn't tell you the whole story.
For a start there are some very cheap 4k cameras, with poor dynamic range, colour balance and low-light performance. Image quality is quite hard to quantify, and even if they do give you any figures, they are hard for a beginner to understand.
DJI drones have a set of sensors to detect obstacles and prevent you flying into them. Cheaper drones generally don't, but even if they can claim more sensors that doesn't necessarily translate into better obstacle avoidance.
Then there are various software modes, usually with fancy but not very descriptive names. These may be really useful for whatever sort of video you plan to take, or you may never use them. There's no kind of industry standard so it's hard to tell if it's just a gimmick.
In this sort of market, where there's one big established make, there may be a small, innovative company with a superior product at a lower price that is struggling to be noticed - but there are also a lot of copycats who will throw together the cheapest possible thing that will look the same on the advert (and with no after-sales support).
Be careful not to "judge a book by it's cover" - look for reviews and instructional videos, and try to work out which are just thinly disguised adverts. See if there's a big fan/user forum, and if the manufacturer ever answers their questions. Or just buy a popular product that you can see works well for lots of people.


What does it mean for SBUS to be an inverted protocol?

A serial signal is composed of a series of High and Low electrical signals sent down a wire.  In the standard serial protocol, a high signal is 0 and low is 1.  In an inverted signal this is reversed so that a high is 1 and low is 0.  You might think it makes more sense that low should always be 0 and high be 1 but electronics engineers decided that standard serial protocol should be the former.

To convert a standard serial signal to an inverted signal you run it through an inverter chip (which can be as simple as a single transistor) which will simply change all high signals to low and vice versa.

F3 and F7 chips have inbuilt inverters on their hardware uarts, which enable them to transmit or receive serial signals in any polarity. For some reason, F4's were designed without that feature and thus do not support inverted signals on their uarts.


I think this problem had two different reasons; two screws that held the flight controller were a loose, and there was likely a problem with the connections between the gimbals and mail board on the transmitter.
The screws that were loose were both on the same side, which caused the FC to "lag behind" the movement of the quad and thus giving false values to the PID-controller. At least that is my theory.
The problem with the transmitter was not as obvious, but I whan looking at the reciever tab in Betaflight Configurator, I noticed that the roll channel acted almost as if it was smoothed. Even if this wasn't visible in the radio I still tried to unplug and replug the connectors to both of the gimbals, something that was recommended on a few forum threads I found.


What LiPo C-rating is best for a small 65mm quad?

C rating is very much a non-science. There’s no real way to compare between brands, so just go off the idea that the higher the C value, the better.

If you want to do it a little more mathematically (though it still won’t be completely accurate), find out the maximum current draw of each of your motors and other electronics, add them up and then add a bit (maybe 20%) for some extra safety, and this will be your maximum current draw.

Divide this by the capacity of your battery in amp-hours (so 0.45 for a 450mAh battery) and this will give you your ideal C rating.


Modeling heat build-up in air-cooled outrunners

My experience is that motor cooling does change enormously depending on how they are installed, but not three orders of magnitude.

My Funfighter motor initially got hot enough that a drop of water would sizzle off the outside, yet when flown without the cowl it could be merely warm at the end of a flight. Similarly I have a helicopter where the standard motor was too hot to touch at the end of a flight but a replacement with a built-in fan stayed cool. I've also noticed that you can put 100W through a 2204 sized motor on a pylon racer but not on a 3D model that prop-hangs a lot.

It's also worth considering that our flights are fairly short. Using a heat capacity of 0.4 J/g-C as an average of copper and iron, a 50g, 100W, 80% efficient motor would heat up 20*0.4/50 = 0.16 degrees C per second, or 10 degrees per minute. Even with no cooling, that would only raise the motor from cold to warm in a 3-4 minute flight. It's not "excruciatingly hot" because it has so much more mass than a light bulb and takes longer to heat up.

I guess this variability is bad news for anyone trying to estimate how much cooling their motor will experience, but it's good news for your average modeller who can just improve the airflow over an over-heating motor to fix the problem.

FWIW my rule of thumb for fixed wings is 50W for a 2204 motor, 100W for a 2208, 150 for a 2212 (based on the stator size, not HobbyKing's overall size numbers).

For quads, the peak power can be a lot higher because you only use it for short periods but the average power over the whole flight should be similar. 4s1500mAh battery over 5 minutes is 200W, or 50W per motor - just right for 2204 motors.


Beginner looking to try a DIY plane, any suggestions on a kit?

Probably the easiest way to get started building planes these days is with foam board models, such as those popularised by FliteTest. Foam board is a 5mm thick non-beaded polystyrene foam sheet with a layer of paper on either side for added strength. You can buy sheets from craft stores for a dollar or two.  It's easy to cut with a sharp knife (small box cutters are ideal) and it can be glued with hot glue or UHU POR (but some other adhesives will melt the foam). The paper layer makes it easy to paint, with almost any paint.
You can build a good starter plane with two or three sheets of foam, and with practice it only takes an afternoon or so. Your first attempt may be a bit scruffy but you will get better with practice, so plan to make several. Smallish, light models are fairly tough and easy to patch up, but knowing you can completely rebuild for very little cost is a huge confidence boost and will help avoid the common story of buying an expensive model from a shop, crashing on your first flight and giving up.
Download and print out a 3-view drawing of your favourite plane and stick it to the foam. Cut out a profile of the fuselage, and the plan view of the wings and tail. Stick them together, add some nose weight so it balances about 25% from the front of the wing and it will probably fly. Experiment with the nose weight, the angle of the tail and dihedral. You'll also learn a lot about the location and weather required for flying.
Next find a plan for a beginner RC plane in foam board, such as the FliteTest Tiny Trainer, Simple Cub or Scout. These models are relatively light and simple, and all have the wings angled up (called dihedral) which causes them to self-level, vastly increasing your chances of success.
These all have free plans under the 'resources' tab in their shop. Print them out, lightly glue the paper to the board and cut them out. Feel free to simplify the shapes slightly as they were drawn for a laser cutter.
Now for the electronics. There are some toy planes under $20 that are quite fun for a 10-year old under adult supervision but they're quite limited and not a good way to start the hobby. You don't have much control and they're very slow, so will blow down wind. You also can't reuse the gear in anything else so they're not a good investment.
A basic radio will cost at least \$30, but you'll want to upgrade that within a year so it's not really a good investment. For \$40 you can get a cheap computer radio with multiple model memories (for when you have more than one plane) and channel mixes (for flying wings, deltas, V-tails or flaps) - that's all the features you really need for a lifetime in the hobby. Most people will spend more for a 'nice' transmitter, but you don't need to. Transmitters generally come with a receiver.
The recommended sizes for the various bits are listed with the plans. You'll need to do a bit of research to work out what all the numbers mean.
Next you'll need a motor and Electronic Speed Controller (ESC). These are \$5-10 each.  You will break props, so get some spares.
9 gram servos are about \$3 each. You'll need 2 initially, but it's worth having a couple of spares for crashes or more complex models.
You'll need multiple batteries, as flights are typically 10 minutes or so.
Basic chargers start at about \$20. Get one that can handle a range of sizes and cell counts, and it'll last you for years.
You won't get that lot for \$50 but you should manage to keep it under \$100. I suppose the stuff that's in the plane should be under \$50, as the expensive bits are the transmitter, charger and spare batteries!
The good news is that the gear can be used on multiple models. When one gets too damaged, or you want to try something new, just move the gear across. The transmitter, charger and batteries can all be shared between multiple models.
It's worth shopping around a bit but avoid anything that's super cheap with no reviews.


How do I set up the RSSI readout on an XM+ receiver channel?

For the sake of the random visitor I must first point out that the XM and XM+ receivers do not include telemetry, as in they don't transmit anything back to your remote over the radio link. What the XM/XM+ do offer, however, is an onboard RSSI readout, i.e. supplying RSSI information directly to the flight controller.

These receivers have two ways of communicating that readout.

The first is the RSSI pad, indicated on the schematic in the receiver's manual.

You can solder a wire to this pad, and it will output the RSSI value as a voltage which can then be read by your flight controller via an ADC pin (usually labeled RSSI). This feature is, as far as I know, always on.

The second, which is the one you're asking about, is transmitting the RSSI readout as one of the RC channels, either channel 8 or channel 16.

To enable the latter option (if it's not enabled) or change the channel, you'll have to flash the corresponding firmware to the receiver. If you go to the FrSky site, to the page that offers firmwares for your receiver, and download the archive, you'll see six files, labeled something like:

XM+FCC170313.frk
XM+FCC170313-RSSI16.frk
XM+FCC170313-RSSI8.frk
XM+LBT170313.frk
XM+LBT170313-RSSI16.frk
XM+LBT170313-RSSI8.frk

The files ending in RSSI8/RSSI16 are the ones you need to flash to enable RSSI readout to the corresponding channel; the ones without any suffix have this feature disabled, if you want all the channels as actual channels and use the analog RSSI readout. Also remember to flash the RF option (FCC or LBT) that matches your transmitter, to avoid possible issues when binding.

After flashing, you'll have to rebind the receiver and configure your flight controller to correctly read the RSSI. In betaflight you need the RSSI_ADC option to be enabled to use the first option (analog readout), and you need it to be disabled to use the RSSI-on-a-channel feature. As long as you have the RSSI_ADC option enabled, the RSSI channel setting in the receiver tab won't be saved when you set it.


There is a CAA run scheme to help return lost drones to their owners called ‘drones reunited’, however you have to have registered your drone with the CAA to use it. If you have, then you may find their website helpful.

There are also other ways in which you can try to locate your drone:

Firstly, have a look around the Facebook groups local to your area in case anyone has posted about it, and if they haven’t then you can put out a post asking if anyone has found it.

You can also put up posters offering a reward if someone finds your drone and returns it to you (assuming you get the appropriate permissions).

There are also several lost and found schemes across the UK, one of which is the Metropolitan Police Lost and Found scheme.

If this is the S49 you posted about, it may have flown for a few minutes after it started to fly away (assuming you had already run down the battery a bit). You can look on Google Maps and form a search pattern to cover the area around where you lost your quad - if you can remember the direction it flew then that is especially helpful.

Finally, try to learn from the flyaway to make sure it never happens again. Many environmental factors, such as power lines, can disrupt your control of the aircraft, so always scout where you are going to fly to make sure it is safe to do so. I also put my contact details and the phrase ‘reward if found’ on my drone so if it is lost, the finder can easily contact me to return it.


Crossfire, Unify VTX settings are being overwritten

The crossfire OSD has a “my VTX” setting on the transmitter settings pane: I had “XF TX LITE > MY VTX > ACTIVE” set to “Yes” and the channels it would flip back to were set there. It seems the “MY VTX” section overrides whatever the unify is trying to do. Once I set that to “No” the changes I made stuck.


Methodology and sources for obtaining the correct replacement batteries for low-end RC aircraft

Most low-end aircraft use small LiPo batteries (lithium polymer batteries). If you want to find a replacement battery you can look at the voltage and capacity of the original battery and purchase a similar one. For example, if the low-end aircraft used a 100mah, 3.7v LiPo battery, you could go to anywhere that sells various batteries such as Amazon or hobby shops such as RaceDayQuads and buy a battery that is similar. You should buy a battery with the same voltage and the same capacity. You can also buy a slightly (but not much) higher capacity battery for longer flight time. Also, know that different LiPo batteries may be better or worse. Cheap batteries may be low quality and not last long or cause fires while more expensive ones tend to last longer and perform better.


What frame are you running?  Which propellers?  What craft AUW?

I have very similar setups built as racers, that perform brilliantly on both BF3.5.7 and BF4.1.1.  Even super-notchy 1408 motors on aggressive 3052F props can fly quite cleanly.

In terms of tunes and filters, both are nearly stock tunes and fly well, at most some filter adjustments to compensate for the higher RPM that naturally comes with 3" craft.


Freshly built quadcopter/drone flips on takeoff

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 fella 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.

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.

Now make sure the motors spin in the correct direction and the correct position to match the graphic on your configurator by using the motors tab with the propellers off

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 likely have a faulty ESC.

If your ESC is not faulty and your motors are also fine, it is also possible that your resource mapping is incorrect. In the Betaflight CLI, type ‘resource’ and ensure that the correct pin from the microcontroller is assigned to the correct motor.

You should also check that your motor direction is correct in Betaflight, and your motors are spinning in the expected directions in real life. If not, that can be fixed either in BLHeli or by swapping round any 2 wires from the motor to ESC.


What is the difference between a patch and helical antenna?

It is my understanding that in general a helical antenna will perform better than a patch antenna.  I should really find a source for that though.  However a patch antenna is flat or nearly flat and thus easier to pack and transport.  The bulkiness of the helical is probably the main drawback.

RcModelReviews has some great instructions on building your own helical antenna.  Another nice thing about the helical is that the number of turns determines how directional they are, so it is easy to see exactly what you have, know that they are polarized and all that.  With a patch, it is not so easy to tell exactly what they are.


I start my landing approach when the average cell voltage reaches below 3.5V on a stable flight (not counting voltage dips on short, aggressive maneuvers).

At that point, you should still have enough authority to land safely. You can go somewhat lower without damaging the cells, but at that point, the voltage curve starts to be very steep and it is increasingly easy to slip below dangerous levels.

I wouldn't advise landing on 3.2V simply because it's too easy to overshoot and to me, it doesn't give meaningful airtime over landing at 3.5V.

Also, note that there can be a low voltage cutoff set up in the ESC that you probably don't want to trigger while in the air.


Why do brushless drone motors tend to be more expensive than brushed?

Brushless motors are typically CNC machined for a large part of their manufacturing process. The top part of the bell and bearing race are machined from aluminum. The shafts typically steel or a titanium alloy and are CNC machined in a lathe. The steel flux ring is typically pressed, and the stators are stamped from existing templates for the most part.  The machining is somewhat expensive comparatively, the stamped and pressed parts are relatively inexpensive compared to the other costs involved. In general, it's a fairly complex manufacturing process, and the materials are not necessarily the cheapest. The cost goes down as volume goes up, and a lot of what we use in the DIY drone industry is fairly small batch production, which helps drive up the cost. There's also the supply and demand question and to a certain extent, the cost is set by what the market is willing to pay. That being said I personally know a lot of brushless manufacturers, and the race to the bottom on prices that we're seeing in certain parts of the hobby (low-cost FPV motors) are pushing the absolute limits of the margins.

Brushed motors, on the other hand, have essentially no CNC'd parts, almost everything is pressed or stamped including the can. Average quality brushed motors with average tolerances and efficiency are cheap and easy to produce. The cost with brushed motors comes when you need to increase precision, performance, and efficiency. However the advent of reasonably priced brushless motors has pretty much nullified that market, so all we're left with us the first category except in very narrow niche applications, none of which I've had the occasion to explore yet.


When flying FPV in a group, what are the best video transmission channels/frequencies to use to avoid interference?

To answer this question, there are two things you need to know.  First you need to be able to map the various channels and bands to their frequencies.  Second, you need to understand Intermodulation Distortion (IMD).  The quick explination of IMD is that two or more radio frequencies can combine to cause interference on a third frequency.

Using the above chart you can see how the different bands and channels overlap.  In general, when flying with just one other person, you are fine to choose any two channels that don't overlap frequencies.  However as you add more people to the mix, you have a higher chance of causing IMD.  To help with that issue, many races now operate using a group of channels called IMD5 or IMD6.

If you are in a group of 5 people, the recommendation is to use R1, R2, F2, F4, and E5.  If you are in a group of 6 people, use R1, R2, F2, F4, F8, and R8.


Why do the motors on my drone keep spinning up and max out when I run up the throttle on my radio on the bench?

TL;DR;

This is a result of I term windup from the PID controller. It's completely expected and normal with props off, and will be fine when you put props on and go fly! Almost everyone new to RC multirotors asks this question on their first build, so don't feel bad!

Detailed Answer

The detailed answer to this question is related to how multirotors maintain stability in flight. Multirotors use a control system called a PID controller. This is a feedback loop control that takes information from the sensors onboard the flight controller and then executes change via the motors based on how the commanded input is different from what it is sensing. For more information check this white paper from the national instruments lab.

To understand this problem, the component of the PID controller we need to examine is the I constant, often referred to as Ki. Essentially this is based on the integral calculation between the sensed value and the setpoint or target value. What this means is that I get stronger the longer there is a difference between the sensed value and the target value. Hence change over time. What you're seeing as the motors slowly spin-up is the buildup of that Ki value getting stronger and stronger as flight controller is trying harder and harder to execute a change that it can sense, but can't impact. No props means no force to execute change, hence the windup.  This is often referred to as I term windup.

Here is an excellent video from Joshua Bardwell on the topic:


What are good specifications for a intermediate medium priced quadcopter?

Okay, as someone who has just recently passed through this stage, here's my take on it. Almost all of the considerations below stem from the maxim that the best drone for you is one that you will fly the most. Both progression and enjoyment in this hobby (and many other hobbies as well) are directly linked to how much time you spend doing it. Whenever you're unsure about a decision, besides other considerations ask yourself which option will keep you in the air more.

Now, to the list itself. Your "second" drone must be:

Indestructible. Durability is valuable for all quadcopters intended for acrobatic or racing flying, and you'll likely never stop bashing your drone into stuff no matter how high your skill, but unlike the pro pilots who probably break more drones a month than you will ever own, you don't have spares lying all over the place for when you do. For a long time, this is likely going to be the only "real" drone you'll own, so it better be durable. There are two main ways you can achieve that:

The "classic" approach is to have massive unbreakable arms, a titanium cage around the components, etc. That's pretty self-explanatory, though you'll have to pay for it in added weight (which is not that bad if you don't carry a gopro).
The other way is for the drone to be so light that it simply doesn't
have enough inertia to break anything, including itself or even its
own props. This is the idea behind the "toothpick" class of
quadcopters, which some consider an excellent choice for
beginner-intermediate pilots, in part for this reason.

Basic, but high-quality. What I mean by that is that you don't need the fanciest F-infinity processors, top-end performance motors, the lowest-latency camera, etc. What you do need, however, is for all the components to be high-quality and well-built by a reputable manufacturer. i.e. an F4 FC made by a well-regarded manufacturer is better than an F7 FC by a knockoff noname brand. Not only will this reduce your cost (by picking basic components) and add to reliability (by picking trusted brands), it will also make things simpler for you and less heartbreaking when you do break something.
Easy to maintain. However durable your drone, you will crash it, break something and need to fix or replace it eventually. And when it happens, you'll want your drone to be easy to disassemble and reassemble, have replaceable parts, etc. First, this rules out the non-DIY type of drones, but the DIY ones are also not created equal. Some frames make accessing the guts as easy as unscrewing three bolts, while others need more disassembly. A bigger drone is significantly easier to work on, with less tiny fiddly parts, while a small one (3" and smaller), while frustrating to work on at times, will likely break less. For larger (5 inch +) frames, replaceable arms are a very good feature to have, and a simple replace procedure (i.e. unscrewing one bolt versus disassembling the whole frame) is important as well. Toothpicks are an interesting class in that regard, in that they have a tiny whoop-like architecture, with only 4 parts: the frame, motors, camera, and an all-in-one FC+ESC+VTX board which the motors just plug into with pin connectors. This makes them way easier to maintain than other quads of this size, but also less upgradable, since three of the main components are on one board. Which brings us to...
Upgradable. This is closely related to the previous point, but slightly different. You want to be able to swap any of the major parts of the drone (FC, ESC, motors, camera, VTX, frame, etc.) for a different or better model without also having to swap the others or encountering any compatibility issues. This is mainly a jab at some prebuilt quads that have proprietary components in them which would not work well or at all with other off-the-shelf components.
Fit for your usual flying location. This is actually important for any quad, but as this one is likely to be the only one for some time, here it is especially important. The best quadcopter is one that you fly a lot, and so you want yours to be well-suited for the size, configuration, weather conditions and other considerations at the place where you fly the most, as this will maximize your flying time, frequency of flight sessions and enjoyment of the process, all very important contributing factors to how fast you learn and how much you enjoy the hobby. If you buy "the best-flying" copter but it won't fit in your flying field, you won't fly it much at all. The most important parameter here is likely the size class of your copter, which must fit the size of your location. Loosely, it's something like this:

if you mostly fly indoors, you need a tiny whoop (although it's likely you'll later want a separate outdoors quad anyway)
for a playground/backyard-sized outdoors location with lots of small obstacles (i.e. literally a children's playground) and gaps a larger (75-85) brushless whoop or ~2 inch quadcopter with detachable propguards will offer the most
enjoyment out of flying in such a tight space while still being able to rip in larger locations.
For something like an uncrowded street, large backyard or small park a toothpick-class 2.5 to 3 inch quad would be optimal;
for larger parks you can go either the toothpick or the 4-5 inch route, depending on how crowded the place is; the former is safer for everyone, less intimidating, and slightly slower, while 4-5 inch quads are noisy, heavy and can do some damage, but are still the undisputed leaders when it comes to flight performance.
finally, if you've got huge fields, shorelines or even mountains to fly over, you probably want a 5 or even 6 inch quadcopter that is able to cover the distance well. However don't fly too far without educating yourself on proper long-range operation first, there's a lot of stuff that you need to know (and buy :) in order to not lose your precious quad a couple km away in the first few flights.

Of the quadcopters that fit all the previous points, choose (or build) the one that you like the most subjectively. Maybe it looks cool, maybe you really like the manufacturer, maybe it's elegant. The factor of likability should not be underestimated. Whatever the reason for you to love the quad, the more you do, the more love, time and effort you'll put into it, making it a better quad and you a better pilot!

As a final word, I must reiterate. Whichever quad you pick, in the end the most important thing is that you fly a lot. Buy a good amount of batteries, a nice flight backpack, and make a habit out of going out and flying. You'll love every moment of it and of the rest of your life as well.