The Matek PDB has a 5V regulator and this can be used to power the fc.
Just wire the PDB to one of the unused ESC plugs on the FC. The FC has solder pads for 6 ESCs (right on the picture) and for a quad you usually only need 4. So the connectors 1-4 are used for the escs and you can use 5 or 6 for the PDB.
The center pin (with + above) is 5V and the outer pin is ground (- above). Connect these two pads to the according pads on the PDB.


DJI FPV Goggles Max Power option not available. Location: USA

You won't know if the "hack" has worked until you also have a Vista or Air unit bound to the goggles.

As a quick test, I powered up my goggles and left everything else off.  If I go to where you can adjust the power output it only shows 25mw and is grayed out.

As soon as I power up the rest of the system and the goggles connect to my air unit, I can change power output and other settings.  There are a number of options that require the air unit or vista to be powered on before you can change them.  I assume that information must be stored on the air unit side, not in the goggles.


Ok, so this could turn into a very long answer!

The short version is, Kv stands for the velocity constant, and it has to do with the rotational velocity it takes to generate one volt of back EMF. Contrary to a lot of popular belief, it actually isn't a direct measurement of the RPMs at any given input voltage. There is an ancillary relationship between the velocity constant and the forward voltage relationship to RPMs, but there are a lot of other factors, such as bell rotational inertia (and anything else being driven), friction, and drive mechanics in the ESC that can impact the RPMs when the motor is actively being driven.

That being said, the no-load RPMs per volt is a decent estimate of Kv if you aren't concerned with being extremely accurate. If you're always using the same ESC and power supply, to drive the motor it can still be a reliable comparative metric between specific motors, even if it might have some offset to the actual electrical Kv of the motor when measured via back EMF. For a thorough discussion on the different methods of measuring Kv, check out this article on Bavaria-Direct.

Also, it needs to be pointed out that the torque and Kv don't have the relationship you might think. You can read a full article on the details over at miniquadtestbench, but essentially the torque constant or Kt which is directly proportional to the velocity constant (Kv) has to do with how much current a motor draws to produce torque, not how much torque a motor is capable of producing. This is a super important distinction. The torque the motor is capable of producing is defined by the internal resistance and the magnetic field strength at the stator. The field strength is defined by things like magnet strength rating (field density) thickness, and width/height, as well as the air-gap between the magnets and the stators (remember the inverse square law). The winding resistance is defined by the length and gauge of the wire used, as well as the number of strands. Assuming we can create a winding of equal resistance at different Kv by tweaking the gauge wire or the number of strands used, we would have equal torque capacity on both motors, only the torque would cost more current to produce on higher Kv motor.

The reason why there is a belief that high Kv motors produce less torque than low Kv motors is that the maximum torque output is limited by the maximum output rating defined by the heat dissipation capacity of the motor, and usually listed in the manufacturer specifications as a wattage limit. Essentially the higher Kv motor will burn out sooner than the lower Kv motor as the torque load increases at maximum output.  However given a load that is within the power limits of both motors, a lower Kv motor will not produce better results (faster RPM changes or better handling of a given prop for instance) than a higher Kv motor with equivalent magnetic field strength and winding resistance. Also, throttle limiting the higher Kv motor to equivalent RPMs as the lower Kv motor will produce extremely similar results, albeit with higher current spikes on rapid transitions where the difference in Kt shows itself on higher torque demand. For more details on the math behind these calculations check out this article on MathWorks


Working on a Quadcopter. Unsure if this flight controller supports an FS-IA6 receiver

So, I don't believe the components you have are entirely compatible with one another. The FS-iA6 RX only has PWM/PPM outputs, while the BetaFPV F4 AIO 1-2S FC requires some kind of serial RX. The "SPI RX" module mentioned in the spec sheet appears to just be a small FrSky RC module that BetaFPV sells and not something that would allow you to connect to your RX.
I think your cheapest mode of recovery is to get a <$15 USD FS-RX2A receiver, like this listing on Amazon. This model is featured on your FC's Amazon listing, including a wiring diagram:

It has a serial port output (Flysky iBus), which should be entirely compatible with the FC and Betaflight.


How to tell if you have a brushed or brushless motor?

As a general rule-of-thumb, brushed motors usually have two wires, while brushless motors generally have three.

Image source


Visual references when driving a Quad Copter through gates

When you're racing around a course, you're not just using visual references. You're using your knowledge of where everything is from having flown the course before many times.
If you watch videos of other people flying through gates (or around trees, or whatever) you often can't tell where they're planning to fly, yet the quad still curves around smoothly and the gate/gap only appears at the last moment. There's no last minute wobble as they hurriedly adjust their course because they've flown that same course so many times.
If you just set up a few gates and try to fly around 'freestyle', it's really hard. However, if you fly the same sequence of gates repeatedly, you'll find yourself getting smoother and faster with every battery. Don't try to fly faster, aim to fly as smoothly as possible and hit the gates as central as possible, and then your speed will naturally improve. You'll learn that after some gates you need to turn hard for the next one, and after others you set up a smooth curve that naturally takes in one gate while lining up for the next one.
If you set up the same course each session, you'll notice even quite small differences in the position of the gates because a path that would previously have lined you up nicely for the next gate no longer works and needs to be re-learnt.


What does Feed Forward do and how does it work?

The way the PID controller works is whenever the actual rate of rotation of the quadcopter does not equal the setpoint, or desired rate of rotation (which you set by moving the sticks), it sends a signal to the motors to compensate and bring the rate closer to the setpoint.

This signal is a sum of four terms: The P, I, D and FF. Since it's a sum, you can think of them as each pushing the quad in a certain direction independently, some terms adding up and others counteracting each other.

The P term is the difference between the setpoint and actual rate, multiplied by the P gain. So the amount of force applied by the P term is directly proportional to that difference. You can think of it as a spring that tries to move the quad back whenever it's bumped out of position. The further it's moved (bigger difference between setpoint and actual rate) and the stiffer the spring (bigger P gain), the stronger it will pull.

The reason why the FF term is needed is because the signal that the PID controller outputs influences the acceleration, not the speed directly. This means that the quad will not get up to speed immediately, but take some time depending on how hard it pushes itself. So when you want to sharply begin a maneuver you want the quad to apply a lot of force in that direction to get going and then sustain the rotation with a smaller force.

The PID controller, however, is meant for stabilizing. It does not distinguish between you bumping the stick and the quad being bumped out of position by an obstacle or gust of wind. So when you move the stick it "stabilizes" itself at the new speed as if you didn't move the stick at all and it was bumped by something instead. You can think of moving the stick as moving the other end of the spring that the quad is attached to. The spring doesn't care which end was moved, just that its length changed.
This means that the PID controller won't apply as much force to start the rotation as it potentially could, which means it is somewhat slow to speed up when you move the stick, and, more importantly, slower to slow down when you move the stick back to center (and can even overshoot).

You can make the reaction faster by increasing the P gain, but to a point, as higher P gains lead to oscillations, when the quad's movements lead to big P terms which in turn cause bigger movements and so on.

So instead the FF term was added. The FF term does not consider the current rotation rate at all, so it does not contribute to the PID loop's stabilizing action and is thus unable to cause oscillations. The only thing that it looks at is the sticks. The force applied by the FF term is proportional to the rate at which you move your stick (multiplied by the FF gain).
So when you, for example, initiate a roll and bump the stick to the right, the FF term will detect the fast stick movement and give a burst of force to get the quad going. Then, when you finish the roll and return your stick back to center, the FF term will detect the opposite fast stick movement and apply a corresponding force, helping the PID controller stop the quad's rotation faster and avoid overshooting.


What sort of license would I need to fly a small drone (e.g. DJI Spark) in the Israeli Golan?

The rules for drones in Israel are listed in this government document, no license is needed for personal flights but these rules must be followed (minor grammar corrections):

  Read carefully the flight manual of your drone
  Fly with a full line of sight visual contact
  Do not fly over 50 meters altitude
  Do not fly closer than 2 km from any airport or airfield
  Join Israeli flight club for model aeroplanes http://www.aeroclub.org.il/

  Do not:

  fly nearby any aeroplanes
  Do not fly above people and buildings, at least 250 meters range from them
  Do not fly in a no-fly zone
  Do not fly carelessly in a way that will cause danger for human life
  Do not fly commercial flights without a license

As to if you can fly in the Israeli Golan, you should be fine. Specific no-fly zones are listed on this map.


The decision between CW (clockwise) and CCW (counterclockwise) propellers has to do with the direction your motor/engine will spin, not really in regard to where you're putting the engine on the plane. CW props will create thrust behind them when spun in a clockwise direction, and CCW props will create thrust behind them when spun in a counterclockwise direction. It's that simple. :)
For an electric motor driven plane, it probably doesn't matter which kind you get because it's highly likely that you can reverse the motor direction in the ESC (electric speed controller), but I'm not aware of any ability to do this with model plane gas engines. If you're using a gas engine, inquire to figure out either what kind of propeller the specs say to use (if this is listed) or what direction the engine spins and get the appropriate CW/CCW propellers.
If the model airplane plans/kit you have calls for a specific kind of propeller, like for example if the plane was designed to fly better with one propeller rotation direction, I'd recommend abiding by their recommendations. There are quite interesting aerodynamic and physical phenomena like P-factor which the designer may have taken into account in the aircraft layout, resulting in the design requiring a specific kind of propeller.


How much air-intake space do I need in front of my DJI Spark's propellers?

Single-rotor model helicopters, flown indoors, will "suck" themselves onto the ceiling when the rotor is within about half a rotor diameter if you're not quick to reduce throttle.  The sucking won't happen until you're that close, though.

That gives you an upper bound for how far away an intake shield should be from a rotor without strongly affecting handling efficiency.  More generally, the farther the shield is from the rotor, the better.  Stiffer is also better because the shield is on the intake side and is thus being pulled toward the rotor.  A grid with thick beams is of course stiffer than a floppy fabric.  So if what you're guarding against is tree branches, not tiny debris, then use the most widely spaced mesh ("roll cage") that you can tolerate.

If you want to experiment with netting over such a grid, just feel how strongly it's being sucked against the grid.  The greater the sucking, the less efficient the rotors.  That may be the most practical way to convince yourself of a particular setup's performance.


A "mushroom" is just a clover-leaf antenna. Generally they have a plastic cover that makes them look like a mushroom, hence the name.


Okay, I solved it: I had installed the EU version of OpenTX, but I hadn't flashed the internal module on the transmitter. When I did that it worked flawlessly.


DJI FPV Air Unit mistakenly declares "slow card"

It appears that this issue where perfectly good and speedy SD cards are declared "slow" in DJI Air Units, as is documented on the DJI product forums. This is an issue for many DJI FPV system users.


Taranis X9D Plus 2019: Switches SE,SF,SG,SH, and SI are not working/responding on Betaflight

The issue was on my Crossfire Nano receiver. I had to change the operation mode from 8 channel to 12 channel. Now switches from SE - SH work. Even though I defined all my knobs, switches, and button on my Taranis, I am limited to 12 channels from my crossfire nano receiver(4 channels for the sticks, 4 for front switchers, and 4 for top switches).


If AUW is 90 grams, will 4500Kv motors be better than 7500Kv?

Lower kV motors will spin more slowly but with more torque. Spinning more slowly (with the same prop) will give you less thrust and less power (and thus lower current) at full throttle.
However, a well setup quad doesn't spend much time at full power, especially when you're considering your total flight time. The amount of thrust required to fly is still the same because the weight and drag haven't changed, so if the props are the same they will still need to spin at the same speed. To reach that speed, you'll need more throttle. Top speed will be reduced and if that leaves you flying more slowly than before, your flight times will increase - but you could probably get the same effect just by flying more slowly.
There may be some difference due to the relative efficiencies of the two motors, but it will be small. I don't know which would be better and you probably won't notice it.
However, that assumes you keep the same props. A lower kV motor really needs larger props, so it can generate the same thrust at lower RPM. Larger props should be more efficient because they act on more air and don't have to move it as fast. That should give you longer flight times and more thrust at low speed, with a possible reduction in top speed.
Of course, all that is irrelevant if your frame doesn't have space for larger props.


Transistor junction temperature (aka junction temperature or Tjunction) is the rated maximum operating temperature for a semiconductor before it will either permanently fail or produce possibly damaging outputs to other components in the circuit. Desktop computer CPUs often have a Tjunction of ~100 °C, and this site claims that most commercial-grade semiconductor parts have a max operating temperature of ~70 °C.

(cit. ROC = Recommended Operating Conditions and AMR = Absolute Maximum Rating)

Abiding by the manufacturer's specs for thermal limits is usually a good idea assuming they're published, but staying below 70-80 °C is always good.

Redlining temperatures in between the ROC and AMR will result in decreased efficiency and abnormal behaviors of the transistors in the ICs and switching components in the VTX, like the power regulators (most of which use inefficient linear regulators which dump a lot of wasted energy as heat) and signal processors. Some of these abnormal behaviors may cause the kind of irrevocable damage to the VTX mentioned in the OP and will for sure decrease the VTX's power efficiency.

This can cause the VTX to fail mid-flight and leave you high and dry without a video feed. High VTX temperatures could also have adverse effects on other parts of your drone if in close proximity by heating them up as well.

Possible solutions

As Oscar Liang points out, you could try:

Replacing the (usually metal block) heat sink on the VTX with something better

Take load off of the voltage regulator in the VTX by installing a more efficient switching regulator to step down Vbattery before it gets to the VTX.
Move the VTX to a location where it sees more airflow.
Try replacing the VTX; it could be dead/dying.


Will LiPo batteries deteriorate outside with the temperature changes due to day-night cycle?

The typical temperatures experienced across Europe shouldn't be a problem for LiPo storage; although depending on exactly where you are you may also want to check out this question: Do low temperatures damage LiPo batteries?

If your storage location may be subject to direct sun then this could cause extra heating which may be harmful - think of a dark colour car on a hot day. Having a light (e.g. white) box or keeping it in shade would help here.

Also, you do want to avoid condensation wherever possible as the moisture may cause corrosion or (if it pools/collects) may short the battery connectors. Many LiPo storage containers have vents to let pressure out if a battery fails and you don't want to seal these, so add the waterproof layer inside - something as simple as putting each battery in a zip-lock bag would work, for example.

Finally, storing the batteries at the appropriate storage charge level will help maximise their life wherever you store them - see At what voltage per cell should I store a LiPo?


Where do we download Betaflight Configurator and Blackbox Explorer?

Betaflight software is available from the Betaflight GitHub pages. These give the option of downloading pre-compiled binaries or the source code if you prefer to compile yourself.

Betaflight Configurator
Blackbox log viewer


How do I successfully stabilize or trim a TinyWhoop Nano?

Often the really small/simple drones don't trim to completely stable, as their controllers are very basic and the 'step size' on the trim isn't fine enough. And, if you get them to trim, then they will usually drift again as the battery voltage changes - again, due to the simple design.
I usually just get it as close as possible, and accept there will be drift - its good object avoidance practice.
Finally, a tip - if such a small drone is going to crash, just stop the props ... this usually results in less (if any) damage to the drone or whatever it hits.


The primary thing that high torques affect in a brushless motor is the current flowing through. It obviously affects your ESCs, but also the motors themselves: high currents have been known to melt/burn the insulation on motor windings, which then short out and wreck the motor itself and possibly the ESC as well.

If the source of this torque is just a prop, the best way to see the actual effects of it on your particular motor would be to find a datasheet/spec sheet for it that lists thrust and current consumption for different combinations of supply voltage, prop, and throttle percentage, like this one:

Not all manufacturers provide this kind of data, however. If yours doesn't, or you desire additional information, it's a good idea to browse the site miniquadtestbench.com. They publish the same kind of data, gained from thrust-testing a big range of motor-prop combinations in the lab, and have a handy search tool to find and compare test results for different motors, props and throttle settings. The graphs might be somewhat confusing at first, though:

MQTB tends to use a greater variety of actual props, but the selection of motors is understandably limited.

If you don't find data on your particular motor in either of these sources, and you only need a ballpark, looking at specs of other motors of the same size and KV should be reasonably close, likely to well within ±10%.


How low can I safely discharge a LIPO battery during flight?

The short answer is that you don't want to run batteries below 3.0V ever, and ideally want to avoid large current draw below 3.5V. For a 3S battery, once you start seeing voltages in single digits (3.33V/cell is 10V, below that), it's time to land as a quick reference point - although you can set more precise values of around 10.6V as the warning point through telemetry at a transmitter and/or OSD on an FPV display.

The real issue you're trying to avoid here is that in the Lithium-CobaltOxide (LiPO battery chemistry), below 3.0 volts (can be 2.8V or so in Li-Ion batteries), the battery starts plating lithium permanently onto the anode reducing the capacity and discharge performance of the pack.  Doing this greatly harms the battery, so avoid that at all costs.

The reason I reference the 3.5V point is that typically you have another 10% of battery capacity left in order to affect a safe landing approach pattern without being under time pressure to land quickly, and make a good safe landing - also under load, the battery that is at 3.5V in flight will tend to recover to 3.7-3.85V depending on craft and setup, which is more appropriate for storing a battery as you travel back home to the bench charge the battery to its proper storage voltage (3.8V).


Should VTX ever be lower freq./higher power than the control transmitter?

There's a few factors that you're missing. The first is that the RC world is dominated by traditional line-of-sight control, and 2.4GHz provides more than enough range to fly a model plane to the limits of vision. There aren't many long range system because few people need them. Even for FPV, long distance flying means a long walk or losing your model when you crash.
The other is that control is very low band-width compared to video. A typical 2.4GHz protocol includes error detection and can reject a lot of packets and you won't notice so long as one or two get through every second - maybe even less when you're at extreme range and not manoeuvring hard. A video signal needs most of the data to get through to be flyable. Analog video has no error handling or rejection, and while digital systems do, there isn't enough spare bandwidth to ignore any data. Even a corrupt picture is better than black screen!
Good RC systems these days do include RSSI (Received Signal Strength Indication) in their telemetry, so you know when you're reaching the limits of your range - and models with GPS can be set to return-to-home automatically if they loose the RC connection.
Finally, it's entirely possible to use 72MHz on a quad, with a FC that takes individual inputs for each channel - however the range is no greater than on 2.4GHz, and there was rarely any interference rejection on the old analog radios, we used to use the glitches as an indication that we were flying too far away!


The features add the yaw during rolls. If the camera is pointing straight forward a roll of the quad also looks like a roll in the camera. As soon as the camera is pointing up the roll of the quad moves the camera in a circle and the roll does not look like a roll in the camera. You have to add YAW inputs to correct the image.

This feature adds the according yaw input. But this only works correctly if you fly at the given angle. If the quad is flying at a different angle the roll will not look perfect in the camera and you would have to apply yaw to correct this again. Sometimes it is used for whoops in level mode but I'd also say that this setting isn't well suited for freestyle.

The feature can be active in acro and angle mode (it does not work in head-free mode). To activate the feature you have set the camera angle and to go to the modes tab and activate the "FPV ANGLE MIX". You can activate it with a switch only on certain modes.


What should I keep in mind when picking my first drone?

An important first step to take when getting into drones is learning to fly them. The very first drone I would recommend to someone is a cheap drone that can be used to learn to fly. It can be disheartening to buy an expensive drone only to crash it and have lost hundreds or thousands of dollars which is why learning to fly with a cheap drone first is a good idea so you can master the skills of controlling the drone. Your very first drone should be any drone cheap enough to not feel bad when it crashes. The Eachine e010 is a basic drone you could learn to fly with.

If you are looking to get into making videos with drones there are two major types to consider: Freestyle/acro drones or professional video drones. The videos you see of drones going really fast, avoiding obstacles, and doing stunts are high-performance freestyle or racing drones. Videos of scenes and landscapes are often from drones designed to carry a professional camera and are used to get extremely steady, smooth videos.

If you want a relatively inexpensive freestyle drone I recommend the iflight Nazgul5 or iflight Cidora SL5-E because it is a quality drone that performs very well for a very reasonable price. This drone has a camera on it for FPV but a GoPro would be needed to make good quality footage.

A good inexpensive camera drone that comes with a stabilized, quality camera is the DJI Mavic mini. It is not too expensive and it is light enough to not need registration. After that, there are many more good camera drones that are more expensive.


Can I replace the 433MHz modules on my homemade remote controller with an NRF24 transreciever?

Building your own radio system is a good exercise for understanding and appreciating the technology, but always remember to include a failsafe.
Regarding swapping modules, it looks like you are using the 432MHz modules which use On/Off Keying (OOK.) The work essentially by sending the state of a digital line, and support some serial formats like UART but also arbitrary pulses such as Pulse Width Modulation (PWM) and Pulse Position Modulation (PPM.)
However, the nRF boards communicate locally using SPI which is incompatible with the above. Without knowing more about your design it is difficult to say for sure how difficult this would be to integrate, but you will need a microcontroller at both ends. I can’t make out what the existing chips are – if you currently have a MCU you may just need to tweak a few lines of code but if you don’t, a more fundamental redesign may be needed.


Can the DJI fpv remote be used with a 2nd radio in a buddybox setup?

According to this thread, the answer is that the DJI FPV system doesn’t natively support BuddyBox.

However, they do mention that you may be able to use an R-XSR, and feed the SBUS output of the Air Unit into the SBUS in pad to use it as a backup receiver.


"Hot quadcopter" is a bit broad of a term, a lot of things can be hot in there.

Maybe it's the VTX, then it's normal, they can get hot up to 80C. You might want to use a heatsink though, depends on your VTX.
It might be the Flight Controller's CPU itself. For example, F7 processors can get hot up to 70C
It also might be motors. If you can barely touch them then:

You might have a bad tuning on your quad:

High D gain
Not enough filtering

Conductivity problem, you might have too long motor screws and they are touching the windings.

Sources and additional info on hot motors:
https://oscarliang.com/check-motor-screws-touching-winding/
https://oscarliang.com/mini-quad-motors-overheat/
https://youtu.be/eo2sjTNS0pc


This table provides guidance to propellor size:

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

(table source)

From this, your propellor size should be 5” for 2207/2750 KV motors, I wouldn't recommend going much smaller/bigger than that in order to not cause damage.


How can I set up differential thrust for a model airplane?

Some good info at Flite Test with video content as well. If you can create a "mix" in opentx (which takes some getting used to) you can apply the same methodology to setting this up.  There are lots of video tutorials etc.. for this.  Once you can do mixing on your Taranis, the content below may help.

In the example, they are using a Graupner radio (the same to dislike opentx??), but the idea should be the same.  What you are doing is attaching the thrust of the two motors to the rudder channel input so it will change each motor as you yank the rudder.

Flite Test demo of differential thrust


I figured this out rather quickly when I tried to arm the quad and it said that the throttle was not low. It turned out that throttle and pitch was switched in iNav....
So after changing that, everything worked...


Finding the velocity and position for small distance from IMU sensors

This is the best damned reference on rotations: Representing Attitude: Euler Angles, Unit Quaternions, and Rotation by James Diebel of Stanford University (dated  2006-10-20).
You didn't specify, but you're almost certainly using the Tait-Bryan convention-- which is first yaw, then pitch, then roll-- for rotation axes. You're also possibly using NED for translation axes, but that's less clear.
You can see all the mappings in Sec. 5.6.1 in the above link. You might check that you have correctly mapped your quaternions to the rotation matrix. It's very easy to mess up a sign value. Pay special attention to the fact that the inverse of a rotation matrix is its transpose, so it's very easy to have the right rotation matrix values but in the transposed order.
To get to your specific question, using eqn. 35, you can see how to map an inertial frame acceleration into the body frame, using the rotation matrix. It's a simple matrix multiplication, and in your case, and assuming NED, the vector z is [0;0;9.805].
Lastly, I would suggest getting this to work in simulation first, before trying empirical. Rotations are tricky because they're so pedantic, and it's easy to approach them backwards.


Is it possible to hijack a quadcopter?

A)Yes
B)No

If so, what would be the technical steps?

Find a flying drone. Intercept the communication. If encrypted go and get a quantum computer before they switch to quantum encryption.
I will not detail the step-by-step process to hijack a quadcopter. It's already on the internet. If you didn't find it yet, you didn't ask yourself the right questions.


What is the difference between FPV or LOS, angle or acro drones?

So FPV and LOS are ways to fly the drone, with FPV being First Person View and LOS being Line Of Sight.

In FPV, one generally has a fixed camera on the aircraft and controls the aircraft’s movements based on what they see on the screen/goggles. This is analogous to VR, however not exactly the same.

LOS, however, means watching the aircraft from a third-person perspective, for example how you would watch a bird fly.

Both of these methods have their advantages and disadvantages, however many people in the hobby find FPV more exciting (though not everyone).

Angle and Acro, on the other hand, are the flight modes.

In Angle, when the sticks return to zero deflection, the quad will self-level, so the entirety of its thrust is vectored down - there will be no acceleration in any given direction (part from vertically based on the throttle). In Angle mode, the drone is not able to tilt beyond a caretaking angle, so flipping, rolling, etc is not allowed.

However, in Acro, the drone has no angle limits so it can roll and even hang upside down with sufficient momentum. The drone will also maintain the same attitude once the sticks return to zero deflection, so needs to be manually leveled.

There is a combination of these two modes called Horizon, in which the drone behaves as if it is in Angle mode during normal flight, however it is still able to flip and roll.


When to replace bearings in a brushless motors?

Learning how to replace bearings is a skill worth having.

It doesn't take much slop in a bearing to add a lot of noise to the gyro signal, and that will be amplified by the PIDs, causing hot motors or reduced performance.

If the quad is a high-performance or cinematic machine, its best to replace bearings as soon as they sound rattly or get free play in the bearings.

For everyday park fliers, it matters a lot less.  The correct answer is that you change them according to your ability to do the job, and the need for it.


What is the proper method of tuning PID filters for recent BetaFlight (4.x) on micro-sUAS WITHOUT Blackbox?

OpenLager is one solution.
Tuning with auditory inputs is another (it's usually possible to hear oscillations caused by excessive P or D gains), but this is more challenging.  This is made significantly more challenging because often props are poorly balanced, and this can imitate the same sorts of errors.

Unfortunately, trial & error tends to be the best answer to this.  As far as general trends, in the sliders most often stock master gains, 0.8-0.9 P/D Ratio, and slightly high P&D Gains (1.1-1.2) will work well on quads with good motor authority (1202.5, 1103 >10,000KV for 1S quads).  Thrust_linear = 50 is also something that produces pretty great results.
The harder one is trial & error on filters - quite often you can work iteratively towards the right with the gyro all the way out to 2.0, and the D term filtering will end up somewhere in the 1.4-1.8 range with these PID settings.


Are real-time OSDs for HD FPV video systems available for drones?

One of the most popular HD FPV systems out as of writing is from DJI. The DJI Air unit, which serves as the VTX on the multirotor, has integration with Betaflight over a UART port on the flight controller which it uses to implement the Betaflight OSD.

I don't have one, so I can't confirm, but I believe most of the OSD elements available for the analog OSD are also available in the DJI system. (cit. UAVFutures) Joshua Bardwell on YouTube has a video listing out which OSD elements the DJI OSD supports.

Angle: Pitch/Roll
Battery average cell voltage
Battery current draw
Battery current mAh drawn
Battery voltage
Disarmed
ESC temperature
Fly mode
GPS latitude/longitude
GPS stats
PID pitch/roll/yaw
Power
Profile: PID and rate
RSSI value
RTC date and time

More element support may have been added since Bardwell's video was released in December 2019. Configuration of the layout of elements on the screen is supported through the OSD tab in the Betaflight configurator.


Just about anything will block/interfere with radio signaling if there's enough of it in the path between the transmitter and receiver or in the immediate vicinity of them. (sometimes referrred to as the Fresnel Zone) However, these are some of the most notorious offenders:

Tight spaces with a lot of multipath interference
Concrete and brick structures
Bodies of water
Soil and earth
Large structures
Forests and large amounts of trees
Lots of human bodies
etc.

One could go on forever listing out different materials, but these are some of the more common items to encounter for FPV.

In general, the rule is to maintain as much line-of-sight as possible between the transmitter and receiver. This is true because obstacles will force the radio waves to take indirect paths, bouncing repeatedly off of stuff until it hits the receiver antenna. Multipath interference comes from this, because often many different (multi)paths will converge on the receiver at different times and cause interference with each other, distorting the image that your goggles/FPV receiver will pick up.


What happens when a multirotor runs out of battery mid-air?

Assuming that you ignore all of the warning signs that your battery is low, while also assuming that you can completely deplete the voltage in your battery, the drone will ultimately reach a point where it is still providing power for the motors to spin the props, but the props won't provide enough lift to keep the drone in the air.

The drone will start to descend slowly and will increase descent the longer time goes on.  If you are a few feet off the ground, it might look like a soft landing.  If you are a few hundred feet off the ground, it might be a hard crash.  There will also be a point where the voltage cannot provide enough power for the flight controller and receiver and other electronics on the drone, which will shut them down and at that point, if the drone is still able to fly, it will drop since motors will not be spinning at this point.

The last scenario is not likely to happen since the main electronics tend to run at very low voltages, so they will have power for a while after the motors stop spinning fast enough to provide lift to the drone.

The drone won't just give you a warning when the battery is low.  You, or the maker of the drone, need to set that up.  If it's already set up, then you should have all you need to avoid a low voltage scenario.


Disclaimer: I don't know anything of the following for sure, so be careful. Be very, very careful.

The PID controller itself does not care whether it's a 5 or 10-inch quad. The PID gains, though, do care, and the Betaflight default preset numbers are for something around 5" size. For instance, for a tiny whoop-size quad the gains must typically be raised 3x or more!

I'd imagine this means that for a larger quad the gains should be several times lower than the default, although I have no way of confirming this. In any case, having low PID gains doesn't promote dangerous behavior from the quad, so it's safe to start by cutting the default gains in half or even 3x and working from there. I'd also predict that a bigger quad, having more inertia in both the motors and the frame itself, will need larger feedforward gains to feel responsive than a typical 5".

Another option would be to try and google some published PIDs for a similar-size quad, although that might be a dangerous proposition if you just copy them.

In any case, start from a safe number (it's a 10 inch, so be safe. be very safe) and slowly work your way up. Analyzing the step response in BlackBox logs to gauge the direction in which you need to tune is probably a good idea as well.

The other place you should look at is the filtering tab, as a larger quad naturally has lower characteristic frequencies. It's great that you run RPM filtering, but take a look at the regular lowpass filters as well, you might want to lower the cutoffs for them. You may want to look at your BlackBox logs here as well to decide.

Other than the radically-different gain numbers and possibly playing with the filters, though, the configuration shouldn't be all that different. Good luck, and stay safe.


Let's work out some numbers.

Picking a DJI Phantom 4 as a 'typical photo drone' example, it's battery is 4s 5350mAh and weighs about 500g, and provides about 20 minutes of flight time.

That implies that the motors are using (5350/1000)x(60/20)=15amps (converting mAh to Ah, and minutes to hours). At 15v that is 225W for flight.

The question talks about using 'static electricity high voltages' - but the most noticeable feature of static electricity is that it can spark across gaps. 10,000v will jump 3mm or so in dry air, and 3000v will jump 0.5mm. However I don't think we need to go that high. Stepping the voltage up 100 times to 1500v would reduce our current to 0.15a.

That means we could use 29AWG wire, which is 0.3mm diameter - at that point the insulation may be heavier than the wire. According to that chart, 29AWG has a breaking strain of 3.6lbs, which is much more than the weight we have saved from the battery so we don't need to worry about the wire breaking under its own weight.

So how long could the tether be? 29AWG weighs 0.3863 lbs per 1000ft (or 175g for 300m). We will need two strands (350g), so 1000ft (300m) of cable uses up most the weight we've saved by eliminating the battery, before adding any insulation or the transformer. 1500ft of wire would be heavier than the battery.

TL;DR:
* yes, you could hover at normal photography drone heights for an unlimited time using a tether.
* The weight of the wire prevents you going much higher than 1500ft.
* The weight of the insulation prevents you using lighter wire.


Throttle scaling is not training wheels so don't feel back about using it.  It's never fun to fly a quad with a throttle that is too touchy.

Another option is throttle expo which you can turn on in the rates area of Betaflight.  Find the hover point for the quad and set that to be the center of your throttle curve. Then add some expo to soften the rate and give you some more resolution at the hover point.

Also keep in mind that with a 3" inch size, matching your motor kV and battery cell counts are very important. If your motors are up at 6000kv area you probably want to stay with 3s batteries.


Sounds like a hardware issue, you might want to RMA it to FrSky.

The best way to get advice for troubleshooting and replacement parts is directly from FrSky.


If you must use dshot then try this library https://github.com/gueei/DShot-Arduino
However, for just playing around, it would be much easier to use PWM protocol and the servo library for Arduino.  Most ESC's are backwards compatible so I recommend trying using PWM first and if you must, use DSHOT.


Differences between brushed and brushless motors - advantanges and disadvantages of each

They are very different.
Brushed motors
Brushed motors are good for toy grade drones for a couple of reasons: they are cheaper and don’t require ESCs, which would add extra cost and complexity.
Brushless motors
However, in almost every way brushless motors are better: they produce much more power, are more efficient, produce more torque and, my favourite part, they don’t have physical brushes and so, unlike cheaper brushes motors which are only designed to run for a number of hours, brushless motors can take a lot of abuse and can last for years. Additionally, they are very water-resistant.
There are downsides: to a brushless motor - they require ESCs and, due to the torque, can do more damage in a crash.
For info on how each works, see my answer here: https://drones.stackexchange.com/a/87/50


What is the most thrust dense copter configuration?

Assuming you can only use up to four motors (because if you used six, eight, or even more you could just lift something with brute force) the most thrust dense configuration would probably be a ducted fan quadcopter.

The reason for that is ducted fans essentially stop a lot of air from being propelled sideways and more air goes down. Ducted fans can provide more lift and are more efficient than open-prop designs.

A helicopter would only have one motor and would need to be much larger to provide the same thrust as a quadcopter. Those other configurations mentioned would also not have nearly the thrust of a quadcopter of a similar size.

So, the best option would be a ducted-fan quadcopter (or hexacopter/octocopter if you can use lots of motors) with oversized motors. A good propeller would be a large prop as it gives more thrust at the expense of maneuverability and acceleration. Wider motors give you more torque but taller motors give you more RPMs so that depends on the prop. (A heavy aggressive prop should go with a wide motor and a lighter prop should be paired with a taller motor)

If you don't really care about longevity, get a powerful ESC, high cell-count battery (probably 6s), and get a motor that is higher than normal KV for maximum thrust and power at the expense of the motor's longevity. But be careful to not go too crazy with the high KV as it could potentially burn out an ESC or destroy the motor.

Also, as durability is not really that important for the purpose of this specific build, make it as light as physically possible (but still rigid, you don't want vibrations or flex) so that it can lift as much weight as possible.

Hopefully, this helps, as I'm not really sure what all of the requirements of this drone are.


What should I use to cover this plane with?

How about the thin white polythene/polyethylene from carrier bags or bin liners? I think you could stick it on with a foam-safe contact adhesive such as UHU POR.
Alternately, you can save weight from your Tiny Trainer by peeling the paper off the foam.
It sounds like what you really want to build is an old fashioned balsa model, covered in tissue and dope. They're really no harder than the model you have just built, and the balsa for that size of model is only $5-10. The tissue shrinks when you wet it, and then the dope makes it rigid, adding significant strength to the airframe.


I can't speak for your specific drone, but I have experience with electronics and cold weather.

I would definitely store the batteries in the warm, and check the manufacturer's guidelines for the correct storage voltage (for a LiPo I think this is about 30% to 40% charge, or 3.85 volts per cell) It also doesn't hurt to store them in a fireproof container for extra safety.

The main hazard I've found when storing and operating electronics in freezing conditions is the risk of freeze-thaw cycles when the temperature is hovering around 0°C. Once it is cold, it's generally OK.

Additionally, if you do decide to use the aircraft during the winter take care when moving it from a cold, dry environment into a warm one - condensation will form and can cause damage or corrosion. Let it warm up thoroughly before use - if possible, seal it into a watertight bag or box before bringing it in to preserve the dry air around the drone, and wait for it to warm up before opening to minimise condensation.


What is the point of a dual-gyro setup for a FC?

A major advantage is redundancy, so that the aircraft can continue flying if one fails. It is also common to see multiple GNSS (e.g. GPS) receivers, and sometimes even a redundant flight controller.

Of course, the problem with only two is that the software can't necessarily tell which of them has failed; 'at least three' is common for high resilience applications as it is much less likely for two failures at the same time (barring external factors, such as a power spike.)
Going further, you can also use different brands and/or technologies to remove the risk of a design or manufacturing flaw causing them all to fail at the same time.

The typical disadvantages apply - increased cost, weight and power consumption - but also software complexity increases as you need to have a method for handling the potentially erroneous data.


How to use an XM+ receiver without a flight controller?

Yes, you most certainly can. The device you will need is called an SBus Decoder, which splits the SBus serial signal from your FrSky XM+ into several different PWM channels.

Example of an 8-channel SBus decoder
Example of a 16-channel SBus decoder


How to secure the screw on a BLDC motor shaft

Use red or better. Blue and sub-red are explicitly designed to loosen with low to medium heat. (in the manual)
FWIW, thread lock is little more than an increasingly heat resistance paint, close in composition to latex paint (depending on the brand).  I have used latex paint and also nail polish in a pinch (an old trick from a car mechanic).
If you want a screw to stay, and are okay with possibly never getting it out again, there are a number of advanced and possibly no bueno tricks. All of these are from decades of mechanic work and caution is advised.

Toothpick:  Shred it, thin it out, stick it in and screw away and grind the fibers in.
Tooth pick with thread lock, glue, or paint.
Metal shavings, grind them in
Damage the correct end of the screw or thread so you get a fused connection (may never come loose >> aka >> danger zone).
In some cases, you can cover the screw with a proper cap to stop it from spinning and falling out, similar in theory to the use of a c-clip.  For example, drop solder on the top and let it "cast" into the head, then put electrical tape over it (not recommended, many reasons like motor may be off balance).
Copper braid or similar, just jam it up.
Re-tap and get a new screw

Your problem is most probably a combination of these factors:

heat
hot/cold expand and contract cycle
vibration
possible weakened threads (cut wide from friction).

The trick is context, experience, knowing the tools and materials, and not buying cheap stuff.
Now to the meat of the issue - BLDC motor, that is a broad designation (https://en.wikipedia.org/wiki/Brushless_DC_electric_motor), so you could be using an in- or out-runner. Big/low Kv versus smaller high Kv demands a different solution possibly. Each motor has a unique footprint on vibration, heat, operational range, expand/contract margins, etc.  So there are a number of factors that will help you decide what is best, I would contact the company if possible (probably Chinese and ZFG, but always an early consideration if possible).


What counts towards the official weight of a drone?

Unmanned aircraft weighing less than 55 pounds and more than 0.55 pounds (250 grams) on takeoff, including everything that is on board or otherwise attached to the aircraft and operated outdoors in the national airspace system must register.

Federal Drone Registration: FAQ (Updated February 02, 2020)


Are there any alternatives to BetaFlight?

Among open-source options, Emuflight, INav & Ardupilot (different focus) Butterflight (mostly defunct), Raceflight (mostly defunct).  There is still Cleanflight and Baseflight, which are the shared lineage for Betaflight, Emu, Butr, and others.
Of these, EMU has an EKF (Enhanced Kalman Filter) variant of filtering, and also in older releases captures a lot of the 'goodness' of BF 3.5.7 filtering with a few PID adjustments that some pilots prefer for feel - and is now where Project Mockingbird does custom whoop tune development.

FlightOne, especially FalcoX adds a lot of ease of use for OSD configuration, and on most setups the stock tune out of the box or selected from a short list of defaults will fly really well.  Require FL1 FC or FL1 license on specific F411 targets (at the moment) - FL1 makes great ESCs if you can get ahold of them.

KISS is also a closed source, with mostly one source of ongoing development, but provides impressively good flight performance as well.  Needs KISS FC to work, I do recommend other ESCs.

For closed source options, the biggest downside is cost and need to re-learn some of the approaches if you're familiar with Betaflight already.

The biggest feature-set of Betaflight lies in the RPM Filtering capability (which in my experience can make cheap builds outperform most people's expensive stuff), although for many users without a background that makes the tuning process very accessible, the 3.5.7 builds of BetaFlight will fly better in stock or near-stock tunes.


These four wires are most likely a USB connection, red and black are power, and the other two are USB data. That means you can't use it neither with today's analog gear, like the VTX that you linked to (as it requires an oldschool analog TV signal), nor any of the new and shiny digital FPV systems, like the DJI system (as they use their own proprietary protocols).

However, you can use it for flying if you really want or need it — you just need a computer on your drone that can process the video feed, such as a Raspberry Pi, and some kind of digital wireless connection, such as WiFi or even 4G to stream that feed to your phone or computer on the ground. That's how a lot of commercial drones work, and you can build something similar yourself.

Whether you really need or want it is another question. Using such a setup for the "sporty" kind of FPV flying, i.e. racing or freestyle, is probably a bad idea, as the latency will be very high, on the order of hundreds of milliseconds if not more. Such systems are more suited for long range and autonomous craft, which generally fly themselves (and sometimes even do some onboard analysis of the video feed), or just as a not necessarily practical but fun-to-make project to prove to yourself how cool of a hacker you are (or as part of such a project, like a facial-recognition patrol drone or whatever).


Do aerodynamics scale for model aircraft?

Aircraft do not scale linearly, for a number of reasons.

Most aerodynamic equations include a speed squared term. If you halve the speed, you only get a quarter of the lift and drag, so models tend to be lighter and less powerful than you'd expect.

If you make a plane with half the wing span, it only has a quarter of the wing area. So now a half size model flying at half speed only has one sixteenth of the lift.

Reynolds number - air flowing over a surface will start with laminar flow but turn turbulent after a while, depending on the speed (and density and viscosity but we can ignore those). A smaller, slower model will see more laminar flow than the full size aircraft. In practice, this is a relatively small effect compared to the previous factors but it's an important consideration for high performance models.

Example
A full size Spitfire had a wing span of 36 ft 10 in (11.23 m), a weight of 5-6000lbs (2-3,000kg) and between 1000hp and 2000hp depending on the variant, with a maximum speed of around 400mph.
If you scaled it linearly, a 1/10th scale model would have a 1.1m span, weigh 200kg, have 100hp and fly at 40mph!
If (as Phil suggests) you scale weight and power by 1000 (10 cubed) it would weigh 5-6lbs and have 1-2hp, which is close but still heavier and more powerful than real models:
A 72" model (1/6th scale) might weigh 15lbs and have 2.5-3KW (4hp) and fly at 100mph.
A 44" model (1/10th scale) might weigh 2-3lb and have 2-300W (1/2hp) and fly at 60mph.
A 36" model (1/12th scale) might weigh about 1lb and have 100W (1/7th hp) and fly at 50mph


Does the Ryze Tello have an adjustable camera angle?

The Ryze Tello has a fixed-angle camera and is not adjustable (without modification). There are, however, a few ways to get pictures at different angles. The first way is to accelerate to get a downward camera angle or to go backwards to get an upward angle with the camera. This method is great if you don't want to modify anything. If you want a permanent adjustment to the camera angle and you don't mind modifying the Tello, you can follow the instructions here to modify the camera position.


Can a 45C 11.1V 4500mah battery damage a 30A esc if a motor draws less amps?

A larger battery can supply more current, but will only supply as much as the motor and ESC pull from it. Once the battery is large enough to provide the power the motor needs, making it larger won't increase the power - much like increasing the size the fuel tank in a car won't change the engine power.
If your motors will only draw 11amps, you don't really need 30amp ESCs. 20amp ESCs would be fine, unless there's a chance that you might want to switch to 4s (14.8v) batteries and push the motors harder in the future.

will the battery power motors, raspberry pi and Arduino all together?

I think the pi and Arduino require 5v, and they would probably like to be isolated from the electrical noise of the motors and ESCs. The usual solution for this is a PDB (Power Distribution Board) with a 5v output. This is a simple circuit board that splits the battery power to multiple solder pads to simplify connecting up the motors. They often also contain filtered 5v and 12v outputs. They're only about $5 so most people use them.
The alternative is to use fixed-wing ESCs, which usually have a 5v supply built in. This is called a BEC (Battery Eliminator Circuit, from the days when people carried a separate 5v radio battery) and supplies 5v on the red wire of the receiver plug. I wouldn't recommend this though, as multicopter ESCs will generally have a faster and smoother response, and support a range of high-speed protocols such as DSHOT. Also it's generally considered a bad idea to connect more than one 5v supply, so you'd be paying for and carrying 2 unused BECs.
I should add that a 4500mAh battery is a lot for three 2204 motors to lift. Half that size would be considered a large battery. I would suggest starting with batteries that are half or even a third of that size. That way you can have a light machine for testing and then parallel two (or three) batteries together to see how it flies with the extra weight. Two smaller batteries will probably cost about the same as one larger battery.


What impact does a shroud have on the torque produced by the prop(s) of a multirotor that incorporates a properly designed shouded propeller?

Ducts and shrouds reduce tip losses; the reason we don't see them more often is that a well designed propeller should not have large tip losses.

Tip losses are greatest when there's a large pressure differential between the upper and lower side of the blade. In other words, the blade is highly loaded.

This question explains why large, lightly loaded blades are more efficient than smaller, faster spinning blades:

Why are larger propellers generally more efficient than smaller ones?

Notice that none of the answers mention tip losses - they're not the reason the small, fast prop is inefficient, they're just an extra problem. Fixing that extra problem with a shroud still leave you with fundamentally inefficient system.

  assuming that the shroud can be manufactured with a mass that is less that the incremental improvement in thrust,

For a well designed prop, a shroud adds more weight that it saves - and it's usually better to use that weight to increase the diameter of the prop instead of adding a shroud.

  significant benefit of having some built-in protection from horizontal impact with obstacles.

To work well, a shroud needs to have very little clearance from the blades. That means that if you hit anything, the shroud will hit the blades. You might cause less damage to the thing you hit, but you're still going to crash.


Holy Stone HS110D Drone PCB Wiring - What does the circled wire in the picture do?

That is almost certainly the antenna and one end is supposed to be free. This is increasingly likely as the square black chip next to it (labelled PAN159CY) is the wireless transciever (radio) - and I can't see another antenna candidate or connector.


Which settings in Betaflight allows me to do flips and rolls?

It’s called Airmode.

It keeps the motors in a constant idle state, even at zero throttle. This gives you pitch, roll and yaw authority even when you are upside down.

You can enable it in the configuration tab in Betaflight.

You will notice that your props may try to spool up even when your quad is just sitting armed on the floor, but this is called I term ramp-up and is completely normal.

I tend to keep Airmode on all the time so have it set up in the configuration tab, but you can also map it to a switch in the modes tab so you only have it on when you want it.


Is there a way to estimate battery discharge duration from mAh and C-rating?

The capacity and C-rating don't tell you how long the battery will last, just how quickly you could discharge it if you really wanted to do it as fast as possible.
It's perhaps analogous to the top speed of a car - just because your car can reach 120mph doesn't mean that a journey of 120miles will take an hour. Most of the time you'll never reach that top speed.
A high C rating is like having a really big fuel hose on a car. You might need it on a dragster but it won't make an ordinary car any faster, or use fuel any faster.


multi-protocol transmitters -- difference between stm32 4-in-1 and stm32 c2500 only ? [tx16s Vs. tx12]

1> The "4 in 1 chip" does handle more protocols than the CC2500. It has the CC2500, nRF24L01, A7105 and CYR all integrated in one package and allows you to bind with and use Flysky, Spektrum, HUBSAN, DSM2, DEVO, J6PRO, ESKY, HISKY, KN, V2X2 and ASSAN receivers which you won't be able to do with just the CC2500.
2> Spektrum is basically a brand of high end radios. (At least they are high end for me)


What type of connector is used on the DJI manifold 2?

Based on some searches, I think this connector is purposely undocumented as DJI states clearly on sites that sell the official cable:

Note: Only use a DJI UART cable such as this one to connect your own device to the Matrice 100's UART port, as other cables can cause the flight controller or your device to be damaged from excess voltage.

In any case, I think I've figured it out. Using this image:

And the dimensions from the manual:

I was able to establish that the connector pitch is probably 1.25mm. I use a lot of JST brand connectors in my work, and I think the GH series are a spot-on match:

Based on this, the connectors would be part numbers:

4-pin header (device socket): SM04B-GHS-TB
4-pin housing (cable end): GHR-04V-S
10-pin header (device socket): SM10B-GHS-TB
10-pin housing (cable end): GHR-10V-S
Contacts (crimp pins): SSHL-002T-P0.2

This is only a best-guess and I must reiterate once again that DJI does not appear to support nor endorse third-party cables, let alone DIY solutions.


What is the power draw of an average 5 inch FPV-drone?

No, those figures are likely quite correct. High peak current draw is one of the reasons why drone battery leads are 12 AWG. Although, the MQTB motor test you're looking at is a fairly synthetic test that isn't 100% representative of what you'd see on an actual multirotor drone.
Here are some reasons why:

The astronomical current figures at roughly >2/3 throttle aren't that common to see in a given flight. High throttle punches are usually a small percentage of total flight time.
When the current draw gets too high, the internal resistance of the battery causes voltage sag that tempers/lowers power output. You can think of this as a kind of negative feedback loop.
The peak figures he cites are almost certainly highly transient affairs, like for example when the motor initially starts from a stagnant state and requires a bunch of juice for a fraction of a second. I haven't tested this myself, but I'm fairly confident that peak current draw over a more reasonably long period of time would be a lot lower.

That said, peak current draw during a flight can still be quite high. Once well-calibrated, the current sensor on many flight controllers can paint a rather flattering picture of total current draw, sometimes showing hovering in the ~20-50A range and peaks in the ~100-150A range during punch-outs for reasonably powerful setups.


DJI Mavic Mini 2 can't record 2.7k 60fps after update

After a bit of research it seems like the problem is likely to be that you don't have the latest version of the DJI fly app. As per this forum thread you will need version 1.2.4 or later to see the option for 2.7k 60fps.
On android you will need to download the latest APK, while on iOS it can be downloaded from the App Store. Links to both version are availible on the dji website.


What is the typical glide ratio of Kline-Fogleman airfoils?

This paper (AE-74-1054-1) from the University of Tennessee has a graph on page 15 showing the best they managed was 1:17 with the step on top and 1:8 with the step on the bottom but these were at a very specific angle of attack, and about half that was more typical.

  The wind tunnel results show that for this new airfoil the lift/drag ratio is lower  than for the flat plate, and the pressure data show that the airfoil derives its lift the same way as the inclined flat plate. This airfoil offers no discernable advantages over the conventional airfoil.

Anecdotally, that matches my experience with KFm gliders. They work about as well as a flat plate. Ok for a light-wieght park-flier with loads of thrust but no good for a glider.


Can I use two 2s lipo batteries to power my FPV DJI goggles?

Connecting the batteries in series or parallel brings two new issues with it that you need to consider before you start using them and they are both related to batteries that are not on the same charge level.

If you put two batteries of a different charge in parallel, the battery with the highest charge will charge the other one and very high currents can flow which can damage your batteries.
If you put two batteries of a different charge in series they will both discharge at the same rate during use and the battery with the lowest charge will discharge a lot deeper potentially causing a lot of damage.

So if you want to do this always take this into consideration:
Parallel connection: Make sure the batteries are at the same charge level.
Series connection: Make sure the batteries are at the same charge level AND of the same capacity.
So if you want the higher capacity and don't necessary need the higher voltage (in this case you don't): parallel connection is better and safer than in series.


What mission planner should I be using for INav?

This is the official recommended mission planner for INav on the Google Play Store:

https://play.google.com/store/apps/details?id=com.eziosoft.ezgui.inav&hl=en

From the official GitHub repo:

  Mission Planner for INAV can be downloaded from the Google Play Store. There is a free version which limits number of waypoints to 5 and (very reasonably priced) paid-for version with additional functionality. The application is not open source. For questions and help the RCG "Mission Planner for INAV" thread can be used: RC Groups support forum.

There used to be an app for iOS as well, but this is no longer available on the app store.

Another app for Android is Droid Planner 2:

  Droid Planner 2 can also be downloaded from the GitHub. It is free and released under GNU Public License v3.

  Droid Planner only supports iNav's one-way MAVLink protocol. The following telemetry data is displayed:

  Vehicle position on map, active flight mode, heading, altitude, speed.
  A broken connection recovers once restored after any amount of time. The flight track remains on screen even when data link is broken -> lost model recovery. Log files can be opened in PC software Mission Planner.

More options can be found in the GitHub repo.


TL;DR:
You probably need to get a new battery pack. Your current one is likely at the end of its life.

My guess is that your battery is just old and has reached the end of its useful lifetime. The fully-charged capacity, and by extension flight time, of all batteries decreases over time when charged and discharged repeatedly. This process is sped along by pulling large amounts of current from the battery like what is sometimes necessary for multirotor flight.
Your description of the slow charge rate after the battery reaches ~12V points to the battery being degraded. At 12V, a 3S battery pack is sitting at ~4.0V/cell. This means that the charger has likely entered the balance charging phase where it tries to equalize the voltage of all 3 cells in the pack and slowly bring them up to 4.2V/cell.
Well-worn batteries will take longer in the balance charging phase because the internal resistances of each cell in the pack (which generally increases during the battery's lifetime at different rates for each cell in the pack) are different, resulting in more energy and time that must be used to fully balance the pack.
Tips for Increasing the Lifetime of a LiPo Battery
Although you mentioned that you're not really on a tight budget, you're probably interested in how you can make your next battery packs last longer. Here are some of the more common things that cause wear:

Sustained and peak current draw:
It's generally an excellent idea to avoid drawing excessive amounts of current from a battery. Full throttle punches are fun and sound amazing, but do contribute to accelerating the degradation of the battery, so maybe don't do them as often. :)

Charging beyond 4.2V/cell:
LiPo batteries shouldn't be charged beyond their full capacity of 4.20V/cell. Aside from increasing the risk of spontaneous combustion, the increased cell voltage puts extra strain on the internals of the battery, which accelerates battery degradation. It's true that some specific variants of LiPo have higher full-capacity voltages, like LiHv with ~4.25V/cell, but the vast majority of LiPo cells shouldn't be charged beyond 4.20V/cell.

Not storing packs at ~1/2 capacity (~3.7-2.8V/cell) for extended periods of time:
Much like how over-charging LiPo batteries beyond their full charge level increases the internal strain on the pack, the same is true to a lesser extent for lower cell voltages over a long period of time. It's generally accepted that storing LiPo batteries at ~1/2 capacity, which translates to ~3.7-3.8V/cell, is safe and is a happy medium for minimizing battery degradation over a long period. If you plan on not flying your batteries for more than a few days, it's a good idea to run what's often called a "storage charge program" on your balance charger to charge/discharge your battery to ~3.7-3.8V/cell. Then recharge your batteries back to full as soon as is practical before flying.

Draining a pack too far:
For normal use, permanent damage is caused to the battery if any of the cells drop below 3.0V/cell. Although as I mentioned previously, multirotors demand a lot from our batteries, so it's important that we try to keep the average cell voltage above ~3.4-3.6V during flight. This is because LiPo battery cell voltages drop off quite quickly below ~3.6V/cell, and so it's important to keep some margin to prevent any of the cell voltages from momentarily dipping below 3.0V before we can get the chance to land safely.


Seeking a cheap, programmable drone, for use in the UK

The sort of sensors and equipment you are talking about would fit on a 250mm frame (measured diagonally, between motor centres), which is the 'standard' outdoor FPV size that takes 5" props. Below this size, things don't really get any cheaper but it gets a lot harder to squeeze everything into the frame.
If you need more space, the F450 'Flame wheel' frame is pretty standard for development drones. It's heavy and fragile compared to an 250 frame though, and everything is more expensive, so I'd stay small if you can.
'Programmable' is difficult to define as almost no one writes their own code, yet the dividing line between 'toy' and 'hobby' grade is when the flight controller has a USB port and can be flashed with (usually) open-source firmware such as BetaFlight, and the PIDs and flight modes configured.
Most flight controllers only have a couple of inputs, one of which is used for the RC receiver. The other is for people who want to add a GPS (and some flight controllers have built in barometer and compass). In theory then, you could do everything with a standard flight controller, if you can work out how to modify BetaFlight, and you're careful to leave it enough CPU time to fly the drone.
Alternately you could handle navigation on the ESP32 and just send the flight controller commands as if it was the RC receiver.
The next step up are the flight controllers designed for autonomous flight, such as the Pixhawk. They have a lot more inputs and software for setting up waypoint missions. They are more expensive though.
You probably don't need the button cell, most people use a PDB (power distribution board) to split the battery wires to the motors, and they usually also contain a 12v and 5v regulator, and often the flight controller also provides 3.3v
Finally, you're unlikely to stay under 250g so you need to register as a drone operator with the CAA. It's only £9 and you can take the multi-choice test as many times as necessary. The rules are pretty simple (don't fly near things you wouldn't want to hit!) but you should read them to make sure you don't accidently break the law. You should also consider buying some insurance, it's pretty cheap from FPVUK or the BMFA.


What are the advantages and disadvantages of a "plus" and "cross" quadcopter configuration? Is one preferred over another?

According to this paper:

pitch and roll control authority is up to about 30% greater for the
cross-conﬁguration since all four (as opposed to only two) rotors are
used.

However this is a bit academic as you often combine pitch and roll in normal flying, or fly at an angle, so it only matters if you want to do the fastest possible flips or rolls.
For most FPV people, the main advantage is that the camera can look out between the front two rotors. A lot of the time you're flying at an angle anyway (because you're looking where you're going while cornering), so the theoretical differences are minor.


How can I become a contributor developer for the Betaflight project?

Normally the best way is to start by forking the betaflight or configurator repository and work on some issues in their respective GitHub issue trackers and submit pull requests. Given that you are an Android developer, I assume you are familiar with UI development so I'm sure they would appreciate some feedback on the configurator UX.


I'm looking for photo sets from drone photography to convert to 3D models. Where can I find them?

Search youtube for videos with the keywords FPV and ORBIT.  This is the motion that will give you 360 degree frames of an object.  Also search for Point of Interest mode for DJI drones.  Then you can use a youtube downloader service to download the video and then extract the frames you are interested in. Here is an example of a dji poi video https://youtu.be/hlfOVBAmc4o?t=48
If you put -tutorial in your search terms it will weed out all the tutorial videos that you don't want
https://www.youtube.com/results?search_query=dji+point+of+interest+examples+-tutorial


Even if the antennae didn't feel loose, it probably was. After removing and screwing it back in I haven't had this problem again.


You would have to be careful so as not to lose some functionality like current sensing or affect the CG of your quadcopter.

One way I could see this working is leaving the 4-in-1 ESC on the stack in your quadcopter frame and only wiring up one ESC, with the signal wire from the standalone ESC wired to the pad on the 4-in-1 ESC where the dead channel's signal comes in from the flight controller.

In order to not affect the center of gravity of your quadcopter and destabilize it, you would likely need to mount this extra ESC in the middle of your frame. Normally with either a 4-in-1 or standalone set of ESCs, the symmetry the ESC components on the frame balances out and you wouldn't have to worry about messing with the CG too much.

You would also have to make sure that the 4-in-1 and standalone ESC operate on the same signaling protocol (Oneshot, Multishot, DShot, etc.), because Betaflight (et. al) expect all ESCs to be capable of the same signaling method.


Under normal conditions operating on the arms of a multirotor, ESC temperature should be well-controlled by the airflow passing over them from the propeller exhaust. It isn't a good sign that your ESCs are reaching dramatically higher temperatures like 70° C, given that you should be expecting to see ~30-40° C on a regular basis. One of three things is likely to be happening here:

The ESCs are undersized for the motors/propellers you have them driving
The ESCs are broken and showing signs of wear
The design of the ESCs is bad
Your PID/filter tuning settings are bad

Without knowing the exact details of your multirotor setup, it could be the first case due to the ease of accidentally picking too small an ESC to pair with power-thirsty motors and propellers. If this is true, then you should replace the ESCs with properly sized units, because there is a possibility that the overworked ESCs could catch fire or spontaneously die on you. (not to mention the fact that the efficency of the ESC will be decreased at higher temperatures)

However, the likely causes change because you're running a smaller multirotor, as @QuadMcFly said in his comment below: (emphasis mine)

  I would add for a 5" inch drone it is extremely unlikely that the issue is related to the ESC. I have run even tiny 20A ESCs on some ridiculously overpowered setups and they come down cool when tuned properly. In the vast majority of cases, hot ESCs come from bad tuning or bad hardware, either in the FC or the ESC. The first place I would look is at flight logs of the gyro and motor output data. That will tell you right away if it's a tuning issue.

Mitigation of ESC Related Thermal Issues

The heat you see generated by the ESCs is likely related to the power losses in the MOSFETs (metal–oxide–semiconductor field-effect transistors) used by most all ESCs to switch the power and off between the three phases of the BLDC motor. There are calculable power losses that occur while the MOSFET is conducting electricity and when it switches state between on and off, both of which contribute to heat generation.

The most direct solution to the issue is to add a decently-sized heatsink to the top surface of the MOSFET transistors and capacitors on the ESCs. You'd likely have to remove the plastic insulation for the best heat transfer between the MOSFET casings and heatsink.

Something like this, although with the insulation removed:

(cit.)

When you re-affix the ESC to the frame arm, it would be best if you also put down some kind of electrical insulation like electrical tape on the surface of the arm between it and the ESC so that there isn't a risk of the frame shorting out an ESC component.


I've had the same thing happen when using these same motors and props (and others). The propeller seems to get stuck at a point where it is not flush with the motor. From my experience, the solution is as simple as tightening it further. I use a tool like this that allows me to apply more force than a typical prop-tightening tool.
It should look like this when fastened correctly, with the prop sitting flush with both the motor bell and the nut:


At what point does drone insurance become worth it?

This is a personal approach, so like all financial advice it's only worth what you paid for it.
If you have worked with the insurance industry in the past, you will likely have learned that good insurance companies pay out almost (if not more) money than they take in. This is because what the insurer is doing is taking the premiums and lending them out, much like a bank. So when you spend \$100/yr across 10 years, and you get back a $1000 payout in the tenth year, the entire time the insurance company has been earning interest on the money you gave them. This 5-10% overhead is quite profitable when you have tens and hundreds of billions of dollars stockpiled.
The upshot is that insurance won't save you any money in the long-term, it only spreads the financial pressure of a decision across time. I only consider insurance worthwhile when the cost of throwing my hands up and walking away is unacceptably high. Put another way, I only get insurance when either required by contract or law (e.g. renters insurance or auto insurance), or when I cannot afford the consequences of not having the thing which is insured.

Worthwhile: I have a pet, and I don't want to have a financial consideration enter into whether life-saving surgery is an option.
Not worthwhile: I have a television which I don't really watch and wouldn't miss it if it weren't there anymore.

You'll have to decide for yourself the impact of losing your drone. If it's just a toy and you don't think you'll fly it again if you lose it, then maybe it's not worth the cost of insurance. If it's your livelihood and you can't afford to buy another one then insurance is a guarantee against catastrophic business failure.
Or if the amount of damage you could do with your drone could cause you to be personally liable for a ruinous amount, then you might want to protect yourself with insurance.
P.S. Drop a comment when you decide, I'd love to know how your reasoning went.


Is it possible to control an RC plane using reaction wheels?

Yes, it would!

A YouTuber, Thomas Stanton, has actually built a helicopter controlled with reaction wheels in this video!

However, for a fixed wing aircraft, you would also have to consider the location of the reaction wheels relative to the centre of mass, and you would most likely want to house the reaction wheels internally to avoid adding extra drag.

You would also have to have a reaction wheel for each axis.

However, this should be relatively simple, so the answer to your question is yes, it is possible.

In answer to your other question of practicality, as far as I am aware there have not been enough studies in the RC sector to say for certain, but my personal opinion is no, it would not be practical due to the added mass and the problem of weight distribution, plus the limited space inside an RC aircraft and the advantageous torque traditional control surfaces give.


Keep quadcopter steady when taking a video

I'm going to assume that you don't need your videos to be stabilised realtime. In this case you need image stabilisation software.

I'm not an expert in this field particularly, so I'll copy the most relevant answers on this reddit thread.

  If it's a smoothing stabilisation like this, you can use VirtualDub
  and Deshaker (Tutorial), Ffmpeg (Tutorial) which are all free or you
  can use After Effects - Tutorial. There's also a Stabilization bot
  called stabbot which will do it for free. Check out r/stabbot. It uses
  Ffmpeg/vid.stab.


Is there a method to wire a brushless motor to an ESC so it spins in the required direction first time?

Without disassembling the motor, and figuring out how the ESC is designed, there is no other way then simply testing it.
From my experience, the motors spinning clockwise has usually been the ones that can be wired without crossing any wires and without having to reverse the direction. But that could just be a coincidence, so you should definitely not trust that it will always be like that, even if that might be the most common.
The simplest and fastest way to get the motors to spin the right way is usually to just solder all the wires so that it looks the best, and then reverse the direction in for example BLHeliSuite as needed.


How do ducted fans impact drone performance?

There are a number of performance differences between open-blade quadcopters and ducted fan quadcopters and they may depend on the shape of the ducts.

In theory, the most notable difference in performance is the increase in either hovering efficiency or increase in thrust. When a drone has the ideal shape of ducted fan, it could decrease battery consumption by up to 60% or almost double the thrust. However, many drones that use ducted fans don't have a duct with optimal shape or have heavy ducts and thus won't get as significant advantages.

An optimal duct for hovering would look something like this:

Also, for larger quadcopters, creating a ducted fan with tight tolerances would probably add a lot of weight which would diminish the usefulness of the duct and if a duct is severely limiting the prop size, it can limit the quadcopter's efficiency because larger props tend to be more efficient.

Another impact on performance that can be caused from using ducts, (especially for high performance or fast quadcopters), is ducts can cause the drone to react strangely when performing aerobatic maneuvers because the ducts can catch the wind and push the quadcopter off course. The ducts can also add more drag when the quadcopter moves fast. An open-blade quadcopter is generally smoother when performing fast movements or maneuvers.

In the end, If a drone has ducts with tight tolerances that don't add too much weight and has the "optimal" duct shape, it will probably be more efficient and get better flight time than a similar open-blade quadcopter but it may be less smooth and efficient when performing fast aerobatic maneuvers.

Source of information about efficiency: The ideal shape for a multirotor ducted-fan


What obstacles are there in making ultra long range model aircraft?

So many different issues, I study a lot of the FAA regulations closely but have contemplated this same question a few times.

The power and weight of gear is heavy, so to make it all the way around is not going to happen without a lot of hacking. (solar proof of concept or similar notwithstanding)
Efficiency of solar (power and weight) is a problem. The square feet of solar needed is quite large and disproportionate to how we conceptualize "RC planes".  These vehicles are really flying solar arrays.
Data, control, tracking, and safety assurance is much harder half way around the globe, even with satellite assistance
Satellite data and connectivity hardware is still big leaving this type of task to the larger military units.

From a regulatory perspective

FAA would rather cut their hand off than issue BVLOS waiver
Patchwork problems internationally exist not only for aviation, but also frequency use . EU radio freqs are very different.  When traveling to fly internationally it takes a while to make sure I am legal or have legal gear.
Clearing airspace for these attempts is a serious undertaking of budget and politics.

Weather is a huge concern for small craft.  Variations in the strata of the atmosphere are no joke. Be it thin air at altitude, winds, cold/hot temperature, all of this is hard for a small craft to manage.

"At heights of 7 to 16 kilometres, the air in jet streams moves at constant speeds of over 25 metres per second or 90 kilometres an hour." source

The US Military is one of a few orgs with advanced programs for this type of use.  The U.S. military UAS groups has Tier 2+ with a '3,000-nautical-mile (6,000 km) radius'. These devices range from sub-55 pound to 2+ ton vehicles.
For what it is worth, I know some people with ties to a successful cross Atlantic RC plane flight.  The stories they tell underscore how hard this task is to pull off.
In the end our number one obstacle for extended UAV flight of this type (and in general) is the same problem we have for phones and laptops.  Our current battery tech is 20-30 years behind the technology it powers.  The physics community engaged with this part of the equation frequently try to move the ball forward but the person to make a lighter and better battery will win the internet.  Other fuel types are out there too, for example Hydrogen where the MFD3000 by Bobby Watts is available with a hydrogen fuel cell providing some interesting performance and represents good progress.
I think anyone with some expertise in mechanical, electrical, aviation, fuel and power train, etc could stop by and add a few more reasons why this is generally out of reach right now.


What is a good 3'' BNF that can be used indoors and outdoors?

Depending on your budget I would look at these two options:
Diatone Taycan: this is a cinewhoop style quad that is more than powerful enough to fly outside as well. It will not be as nimble as your typical freestyle quad but if that is not what you are looking for you can't go wrong with this one.
HGLRC Sector 132: This option will be cheaper as the Taycan but seems to offer good value for the money. By default there are prop guards that allow you to fly indoors (not ducts) and if you remove them, you can start freestyling outdoors.
Just based on the type of build, the Taycan should be more sturdy and last longer than the Sector 132. That being said, this is a hobby where you can destroy any type of quad if you start pushing your limits and try out more challenging moves.
Regardless of your choice, it is a good idea to implement throttle scaling when you fly indoors. This can be done on your remote and tied to a switch.


Why is the noise in my accelerometer more pronounced than the noise in my gyro?

A large change in acceleration doesn't necessarily mean a big change in velocity. Since the chance in velocity is the integral of the acceleration, the velocity will barely change if the acceleration spikes quickly for example.
Something that I don't quite understand is why the two graphs doesn't match. By definition, the acceleration should be negative when the velocity decreases, and positive when it increases. It doesn't look like that is the case here, even if the zero value on the y-axis is wrong. So it is possible that One of the sensors gives you the wrong readout.


Is owning, but not flying, a drone in Germany illegal?

According to the rules you linked to, the only thing you are required to do is stick a label on your drone with your contact details, so stick a label on!
All the remaining rules relate to when, where and how you fly, and what certificates you might need to do so. Since you won't be flying, none of these apply.
Nowhere is there any prohibition on simply owning a drone.
However, since this is your new hobby I find it unlikely that you won't want to fly in your home country. You'd need a 'certificate of knowledge' which doesn't seem to be too hard to get, and permission if your drone is over 5kg. Why not just get what you need and fly in Germany?
Note that flying drones in other countries may have rules that are much more stringent than Germany. Don't expect to just rock up on some foreign beach and fly.


What is a good alternative to carbon fibre for a quadcopter frame?

There's no denying the fact that when it comes to materials that are both strong and light, carbon fiber is king. If you're looking for performance, there's no way around it. If performance is not the most important thing in the world, however... Well, there are several options.

Before I start listing these, I must point out that all frames break. Some break more easily than others, but you can't have an unbreakable frame. The question is how you repair it when it breaks, and the answer depends on the material.

Plastic frames are actually a very popular in micro-sized quadcopters that only weigh tens of grams. These frames usually feature a non-flat shape, prop guards and other features built into them. A subset of those, 3D printed frames, are also quite widely used in DIY designs of larger sizes, and, as everything 3D printed, are a good way of prototyping your design before moving on to other materials. In some cases, where performance is not a concern (for example, if your main focus is having a platform for testing algorithms for which flight time or range is not important at the time, or just want to play around with a design) you may even never move on from a 3D printed frame. While these tend to be heavy and/or brittle compared to other materials, there are some exceptionally light designs, and depending on the intended applications, durability might not be a concern. Besides, if you do break one of these, you can always just print a new one.
Wood and metal/fiberglass is a staple of "oldschool", large (8"+) DIY drones from the KK2/Naze era. As the drone gets larger, heavier frames tend to impede it less, and any crash could easily break an arm just due to the sheer weight of the drone, no matter what the arm is made of. So this kind of design focuses on repairability instead: one can easily make tens of wooden arms in their workshop for very cheap and swap them when they break. Potentially, one could try to make a smaller wooden frame as well, although at this point lightness becomes important, so I'd advise to stick to lighter woods if you do, maybe even balsa.

Full-metal frames are not unheard of as well; Aluminium and titanium are widely-accepted building materials in aviation and actually tend to have better performance than wood. You'll have to use tubes/other non-flat shapes, though.
Even though you've ruled out carbon fiber, you may be referring specifically to CNC machining of frames out of flat sheets. Premade carbon fiber elements such as strips and tubing are readily available for order online and are easy to work with using basic tools. A lot of larger drones today are made using the same basic technologies as the wood/metal drones of old, but with arms out of carbon fiber tubes instead of wooden planks. If you're building a large drone, this is a very feasible yet high-performance option.

Finally, Pure fiberglass composite. Glass fiber composites are significantly less stiff than their carbon fiber counterparts, and heavier as well, but there are a few niche uses even in performance designs.

The first of them is very large drones, which tend not to care as much about neither weight nor aerobatic performance. Here, glass fiber can shine if you've got the facilities to manufacture it. That said, if you're building a very large drone, the frame is likely to be a very small fraction of its cost, so you might just as well order a custom carbon design (or use premade carbon parts like the previous option).
The more interesting option, however, is a PCB frame, in which the frame is
not just a structural component, but also carries all the electronics
that the drone needs to fly. Some fantastically lightweight designs
have been made using this technique: even though the glass fiber
itself is heavier than carbon, the design makes up for it by ditching
all the other PCB and wiring weight that would otherwise be carried
on top of the frame in a traditional design. While not optimal for sporty flying, this is the perfect option for a long-range/long
flight time autonomous design... if you can lay out the PCB, as all
of the components (flight controller, ESCs, vTX, etc) have to be
integrated into it.

Some examples for inspiration:

Plastic frames

Brushed micro quadcopter (tiny whoop) with molded plastic frame:

2" brushless copter with molded frame:

Brushless quadcopters with 3D printed frame:

Foldable brushed 3D printed frame:

Wood/metal homemade frames:

Typical fully wooden H frames of the KK2/Naze era:

Classic X frame with metal/plastic center plate and wooden arms:

Very simple wooden X frame:

Small drone with laser cut wooden frame:

Frame with metal arms:

Full-metal frame:

H frame out of carbon tubing with 3D printed brackets holding it together:

Glass fiber frames

Molded fiberglass tricopter:

PCB frames:


How do I attach a propeller to this motor?

You use one of the provided rubber rings to attach the propeller to the motor. The 2 protruding screws with their heads are the anchor points for the rubber before it is wrapped over the prop to the other screw.
The propeller motor connection is not rigid but has a little give and comes loose on impact, hopefully saving the propeller and the motor that way.


Where to find model aircraft with moving parts?

I don't have any models from this site, but it appears from their product descriptions that they have what you're looking for on themotorpool.net. They have a plethora of models of all nationalities, classes, and time periods.

For example, their Bf 110E-2 model is described as having "spinning propellers"
and the "ability to display the model with landing gear in either extended or retracted mode".

From what I can tell, almost all of their models have movable parts of some sort.


Is it feasible to use an RC car transmitter/receiver to control an RC plane?

Maybe!
Let's take a look at what parts from the RC car we might be able to reuse for an RC plane. You haven't shared any specific details about the RC car you're looking to salvage parts from or the RC plane you're trying to build, so I'm not able to make a specific analysis.
RC Transmitter and Receiver
One major concern is how many analog channels the RC car control system has. analog channels allow you to vary the control signal through its range of motion as opposed to having the channel just be on or off. AFAIK most RC cars only have two channels (throttle and steering) and you'll need 4-6 channels to control an RC plane (ailerons, elevator, rudder, throttle, any aux channels you may need). You probably won't be able to reuse the RC TX/RX if it doesn't have >=4 analog channels.
The RC transmitter might also be less than ideal for RC aircraft control. If you have one of the pistol-grip type devices, like this one from Radiolink, you might find that operating the plane becomes cumbersome even if it has enough analog channels.

Most RC aircraft controllers are shaped quite differently for ergonomic reasons, like this one:

ESC
The major factor to consider here is the current rating of the RC car ESC. Depending on the size plane you plan to pair it with, you may need an ESC capable of supplying >20-30 amps of current to the propeller's driving motor.
Assuming that the current rating for the ESC is adequate, the chances are good you'll be able to reuse the RC car ESC.
BLDC Motor
This is something you didn't ask about in your question, but I thought it would be good to mention anyway. It's possible you may be able to reuse the drive motor to power the propeller, although it's difficult to give a more specific answer without knowing more about the RC car and airplane.
As @JacobB notes:

Most RC cars use comparatively low voltage batteries (2-3s) and have a spur and pinion gear to create the necessary torque.
And because many RC cars use brushed motors (and the ones that are brushless have high Kv, like 4000 or 5000), having the motor attached directly to a propeller would likely result in either a severe lack of power and/or extremely inefficient performance.

Servos
Servos are rated for how much deflection they can provide (e.g. 45°, 90°, 180°, 360°, continuous/infinite rotation) and how much force/torque they can deliver. Again depending on the size of the aircraft you want to control, the servos you find in your RC car may be too small to control the aerodynamic surfaces of your plane.
The aerodynamic surfaces on a plane can experience decently strong forces in-flight, so it's important to make sure your servos are appropriately sized and can maintain control.