Can I configure Betaflight in the field without using a laptop?

Yes, you actually can! There are a couple of options for the Betaflight firmware which are mostly plug and play.

OpenTX Lua Scripts

Radios running OpenTX can use scripts written in Lua to control these parameters with the radio's scroll wheel and buttons. Please see @PaulKendall's answer for an explanation of how this method works.

Betaflight OSD

If you have an FPV setup on your quad, you can make use of the Betaflight OSD to tweak the vast majority of settings you would need access to during a flight session, like the PID values and rate profiles you mentioned. The OSD makes use of the sticks on your transmitter to navigate the menus, which appear as an overlay on top of the camera feed from the quad.

As described in this article by Oscar Liang, the stick combination mid throttle, yaw left, pitch forward, roll centered is used to enter the menu, which should look something like this:

From here, pitch up/down is used to navigate up and down in the current menu, and roll right to select an option or enter a sub-menu. (items with arrows on the right side are sub-menus) The roll axis is used to change a value up/down.

I know this sounds confusing, but it makes a lot more sense once you try it out and gain experience working with it. Remember to select the SAVE REBOOT after making a change and before going back to flying, or your change likely won't take effect!

Speedy Bee Smartphone App

If you want an experience that is closest to using the desktop Betaflight configurator or don't have an FPV setup on your quad to use, the Speedy Bee smartphone app for iOS and Android may be a good option. You can connect to the quad either using Bluetooth (you must have a Bluetooth module already installed on your quad), or over USB (USB OTG mode is only supported on Android).

Because this app is developed and maintained by a third-party and not the Betaflight project, some new settings aren't likely to be accessible with this app, but it should work for most common settings. I suggest watching Joshua Bardwell's video about this app for more details.


What is the background of remote controller modules?

There are multiple purposes for the pins on the back. One of them is to connect an external transmitter module like crossfire, which gives you a longer range.

Another use is to flash a receiver with another firmware by connecting it with a servo cable (on FrSky transmitters at least) to the bottom three pins.

There are transmitters from several different manufacturers, and I think some of them are compatible with each other, but you shouldn't trust that all of them are.


How does frame stiffness affect a quadcopter?

Background

Stiffness, oscillation, and vibration damping are separate concepts but all somewhat relevant to the question. Each affects flight dynamics, and each is (somewhat) a function of material choice.

For example, titanium is very stiff, and it rings like a bell. CF is very stiff, but it rings like mud. Aluminum is in between.

Stiffness

Stiffness is a good thing when you don't want your structure to be compliant[*]. Stiffness means that the frame isn't deflecting, which is important for the presumed laws of motion. Put another way, a quadcopter's autopilot assumes that all the motors are pointed in a fixed direction[**]. If those directions change, i.e. because of flexing, then the most fundamental assumptions of its flight dynamics are false. It's no surprise flight flight performance degrades!

Of course, any structure will bend/deflect/twist under load, no matter how stiff. What's important is whether it does any of these sufficiently enough to compromise your flight goal. If you're trying to do telescopic videography then a little bit of error is immediately visible in the results. If you're learning to fly, then so long as the autopilot keeps things sunny-side up you're fine.

Oscillation

Oscillation, such as control-surface flutter, occur when the  entire structure starts deforming like a spring. This can dramatically build up and quickly rip the airframe to shreds. (It really is that fast, that violent, and that scary.)

If a quad's arms are insufficiently damped, oscillations can occur. Most frequently, it will occur as a twist about the arm axis, but I've also seen it happen in the flapping direction. Because of differences in dynamics between a multirotor and a fixed-wing, it's unlikely that your quad will suffer inflight failure but it's also very hard to keep control, both for you and the autopilot.

The closer you get to instability, the more energy you have to expend to keep things stable. In the case of a multi, at the very least this translates into decreased flight time.

The more powerful your input (e.g. a powerful quadcopter), the easier it is to drive a system past a critical damping threshold. The Tacoma Narrows Bridge was famous for this when the wind  was blowing and the bridge was slightly wet. More recently, these out of control harmonic oscillations have famously been seen on the Millennium Bridge in London (surprisingly due to human footfall) and the Humen Perl River Bridge in China (heavy winds).

A full discussion of how to control oscillation is outside the scope of this answer. However, it is relevant to note that some materials are springier than others, and these materials are the ones more likely to have pathological  oscillation.

Vibration

Vibration is a form of oscillation, but I'm breaking it out so I can specifically address how it affects sensor data. Vibration is an issue when it causes signal integrity issues with the IMU (i.e. accelerometer + gyrometer)[***]. This can confuse the autopilot, leading it to make incorrect estimations of the aircraft's roll-pitch-yaw, and as a result respond inappropriately. This can and does cause crashes.

Again, a full discussion of how to control vibration is outside the scope of this answer. And once again, the springier the material is, the more it is likely to have pathological vibration.

Composites vs. metals

Composites (e.g. carbon fiber, fiberglass, kevlar, ...) have what are called anisotropic properties. This means that they don't have the same properties in all directions. Consider a square sheet of aluminum. No matter which axis you pull it on, it has the same strength, stiffness, heat transfer, etc... This is isotropic.

Now consider the same sheet made out of unidirectional carbon fiber. Along the carbon fiber axis it's incredibly strong. But turned 90 degrees, where the only thing holding it together is the glue, you'll find that it has no strength whatsoever.

You would be correct in assuming that this difference in properties extends to oscillation and vibration. The upshot is that composites can be tricky to chose because two seemingly similar structures (e.g. a tube) can have very different properties.

[*] Able to move and bend, like the Goshawk.

[**] See math in https://drones.stackexchange.com/a/323/46.

[***] For example, aliasing and clipping.


Can you charge 1s and 2s batteries at the same time with the tinyhawk 2 included charger?

According to this source, yes you can charge both 1s and 2s batteries at the same time. This is because, like you mentioned, there are separate charge controllers. You can also choose whether you want them to charge to 4.2v or 4.35v per cell.


Connect mavlink (on RPi) with QGroundControl

After referring to lot of documents and mavlink message format, I was finally able to send heartbeat to the QGroundControl from the sample code without Ardupilot/PX4. I modified the code
mavlink_msg_heartbeat_pack(1, 1, &msg, MAV_TYPE_HELICOPTER, MAV_AUTOPILOT_GENERIC, 0, 0, 0);

and was able to receive the data at QGC end. It seems like default system id and component id values was key to communicate with the QGC. Now, since the first step is accomplished would look in transmitting other various parameters to QGC from mavlink RPi.
Thanks


How to get a licence to fly a drone in the city of Prague for a week?

(Caveat - I don't understand Czech either!)
There does indeed appear to be a restriction on flying around Prague - this is mentioned in English on the Czech CAA website.
There also appears to be a country wide airspace restriction/notice relating to Unmanned Aircraft - see this PDF Document for a 'working translation' in English. There are details on flying within a number of airspace types, but it looks like Section I.2(e) is relevant for the Prague restriction:

Prohibited  areas  (LKP),  restricted  areas  (LKR),  dangerous
areas  (LKD),  and temporary  segregated  areas  (TSA)  and  temporary
reserved  areas  (TRA)  activated  by another user
The  UA  operation
shall  not  be  conducted  in  LKP,  LKR,  LKD,  and  in  other  TSA
and  TRA activated  by  another  user,  or  in  restricted  areas
published  in  AIP  SUP  or  NOTAM,  except when it is permitted by an
appropriate operational authorization issued by the CAA.
The  UA operation  UA  in  the  LKR9  restricted  area  may  be  conducted
without  the  CAA approval, provided that conditions of operation in
CTR, in a congested area, or the conditions of other local areas are
observed. The UA operation in LKR9 restricted area is also possible
based on the operational authorization issued by the CAA.

Based on this (while accepting that the translation might not be finalised/accurate), you should just need an Operational Authorisation from the Czech CAA.
I believe this page contains documentation relating to Operational Authorisations - but is all in Czech!
It looks likely you will need some understanding of the language to be sure you are compliant. Your best chance for official guidance on this would be to contact one of the people listed at the bottom of the page from my first link; but again there is no guarantee you have a common language.


what HD cameras should be used and for what purpose?

I don't know about all the cameras, but here is my opinion.

If you want the best video quality you can get today and not be too heavy for a 5" FPV drone, then you want the GoPro Hero 8.  That will obviously change quickly if something better comes along.  The Hero 8 also has great stability with Hypersmooth or you can opt to use ReelSteady Go with it.  Downsides are the size, weight, form factor, and cost.

The benefit of the GoPro Hero 7 is that it is slightly lighter than the 8, has a removable lens, accepts ND filters better, and Hypersmooth is still quite good.  However, it does not work as well with ReelSteady Go.

The GoPro Session 5 was the go-to camera for FPV drones for years and lots of people still use it because of its size, weight, and form factor.  However, GoPro does not make them anymore so they are not as easy to get.

If you want stabilized HD footage for a drone that can't carry a GoPro, then consider using the Insta360 Go.  They are tiny, lightweight, and can thus be used on very small drones.

If you are not looking for the best quality, and the cost is more important, then consider some of the cheaper options like the Caddx camera.

I'd say the best way to choose a camera all depends on what your needs are, what you want out of it etc.  If you just want decent videos for Instagram, you will have a different answer than someone shooting cinematic footage for commercial use.

And one final bit, be sure to get some sort of protection plan on the camera because you will most likely kill it at some point.  If you can buy the camera from Best Buy, they offer a great Geek Squad protection plan.  Another option for the GoPro cameras is the GoPro plan.


Why does the video cut out on HX-100 drone?

This sounds like a loose video wire from your camera to your flight controller.  Since you still see OSD, but a gray screen, it indicates that the connection from the flight controller to the VTX is good (that's why you are seeing OSD), but the gray indicates that something is wrong on the camera end.

A warm camera is a little alarming.  A warm VTX is normal.

I would check all of your connections to your camera.  If they all seem fine, then the problem could be the camera (the warm camera seems culprit to me).


I fixed it by re-uploading the font to the flight controller.

Some people report they need to power the quad with a LiPo for this to work. I did not need to.

Go to Betaflight's "OSD" tab
Click "Font Manager"
Click "Upload Font"

I'm now back with a normal crosshair.

As @3k pointed out,

  The reason for this is that the character set has changed between versions so it has to be updated on the copter.


What sort of fuel do model jet plane engines take?

I think the only real answer is... it depends on the model.  Obviously producers can make whatever they like, though I've never actually heard of a model plane which runs on actual jet fuel (probably because it's far more difficult to get your hands on jet fuel than on the fuel you can get at any gas station).

In fact, some models even come with an electric option (Electric Duct Fan).


What factors affect the noise level of propellers?

Since only you can be the judge of what you feel is too loud, and noise level from a drone is the result of many parts working together, this is a tough question to answer and I'm not sure you will succeed in finding "the" prop to suit your needs.  That said, if reduced noise is what you are looking for, and we remove the other parts of the drone from the equation, I'd suggest stiffer props from the manufacturer of your choice.  After flying for a while, if you feel that the noise level is getting worse, change props (all of them).

In my experience, as long as the props are balanced (most manufacturers have gotten better and producing consistently balanced props), on the stiffer side, clean, and free of any kinks or abrasions, they will be as quiet as they can be.  This is why drones sound quieter when they are first flown compared to after a few landings when they get grass and other debris sticking to them, along with abrasions and kinks on them - making them less efficient and more noisy as a result.

I use DALPROP T5045C and HQ R38 props.  Both are on the stiffer side and both are quieter than the thinner, more flexible props I had used a few years ago.

I hope this helps.  Good luck!


Determining the mAh of 4S battery for a 3 inch drone

As @JacobB suggested in his comment, first check all the parts of your aircraft that connect directly to the battery can accept 3S or 4S voltages (usually the flight-controller and esc.) This may immediately constrain your choice.
First you need to calculate the maximum weight your drone can lift; you can work backwards from this question:

Get the thrust per motor for your drone
Multiply this by the number of motors
Divide by two
Weigh the drone without the battery and subtract from number above

This will give you a target maximum weight for your battery. You can then choose the highest capacity battery that falls in this range.
More expensive batteries will give slightly higher mAh ratings at slightly lower weights - to work out what is good cost/flight time balance in your situation, you can estimate the flight time for a given mAh rating with the following formula:
$$Endurance = \frac{ 0.8 * Ah }{ AverageAmpsDraw } * 60 $$
[source]
Note - if your battery is labelled in mAh, divide by 1000 for Ah


Autonomous operation isn't a technical problem.  For example, DJI's drones are capable of flying themselves back to the launch point if the signal from the remote control fails.
The problem is legality.
Most drones are required to be flown within the operator's line of sight.  The reason for this isn't because the operator has to be actively controlling the drone -- a fully-autonomous drone is perfectly legal -- but because the operator is responsible for collision avoidance.
If you've got an operator watching things, ready to take over if a problem arises, a self-piloting drone is perfectly legal.  If, on the other hand, you want something that's completely unattended, you'll need to work with your local regulatory agency to get permission, and that's going to be a difficult thing to do.


Drone Controller, PID Is Enough, or ODE Modeling Required?

You can certainly use a PID based design and tune it from there. This design is simple enough that if you just have XYZ PID controllers and proper output mixing it should work just fine.
I'd recommend doing something similar to his design where the motors are far from the center of mass which will increase the torque generated by tilting the motors.
Good luck!


I don't think that you can edit the throttle limit directly in the goggles, but you can change rateprofile in the betaflight OSD.

So what you can do is to set up a few different (up to six) rateprofiles in betaflight, and then change between those in the OSD. It won't be as granular as changing the limit directly, but it should be enough if you just want to change between a few different values.

To make it even faster to switch between the different limits, you can even assign a switch to do it.

Here is a guide on how it is done.


TL;DR:
Probably, but you'd likely be spending a lot of time locating a programming interface on the flight controller board and custom-compiling or custom-writing firmware to work on the special board configuration.
Some more expensive toy-grade drones may have accompanying smartphone apps that allow you to change some flight parameters, but very few of those exist for a variety of reasons.
In general, I wouldn't recommend trying to modify the firmware of cheap toy-grade drones...  It's almost certainly a better use of your time and money to begin tinkering with DIY drone parts that are meant to be tweaked.

In the case of the OP's example toy drone, the Blade Inductrix FPV RTF, there is some limited good news. From looking at this image of the FC board, it can be discerned that the FC uses an off-the-shelf Atmel processor of some kind:

I've tried rotating and zooming the image to find what exact model the chip is, but the image is too coarse to read the finer print. The fact that it uses an Atmel processor means that you could probably use a UART/ISP programmer device to upload new code once you locate and solder thin wires to the appropriate legs of the LQFP package. However, you'd likely run into other challenges when trying to modify or change the existing firmware:

Attempting to de-compile the firmware on the FC to change PID constants or alike would be exceedingly difficult
The Atmel chip is likely 8-bit, which would require adaptation of open-source FC firmware like Betaflight to work (Betaflight is designed to work with STM32/ARM 32-bit processors)
The Atmel chip is almost certainly running at a slower clock speed and has less IPC capability than the chips used for DIY FPV drones and would require firmware with severely slimmed-down feature sets
The pinout of the FC board is entirely unknown; you'd have to manually trace out the circuit to find which pins on the processor lead to what parts of the drone, or locate a schematic
Critical peripherals like the IMU (accelerometers + gyroscopes), ESCs/H-bridges, RC RX, and FPV TX are likely non-standard components communicating with custom proprietary (or at least unknown) protocols that would require reverse-engineering
etc.


I'm going to give you the answer first:
Yes, it is worth calculating the approximate range if you're building a (very) long-range aircraft. However, theoretical calculations will usually be such a crude approximation of reality that you can't rely on them, and the actual maximum range will depend on a myriad of factors besides what you can consider in the approximation.

If you're flying short-range, near obstacles (or inside of them!), doing acrobatic maneuvers, etc, the local conditions, attitude of your drone, its proximity to things, etc at each particular moment will affect the performance of your video link orders of magnitude more than any theoretical range. Things such as where your antennae are pointed and how directional they are, what kind of noise reduction your receiver has, and where you are standing suddenly become extremely important and intimately connected to the configuration of the building or other environment that you're flying. Sometimes, just moving your receiver several centimeters to the side could make a huge difference in the clarity of the received signal, or lowering your transmitter's power can make the link perform better due to less multipath interference.

So what people usually do is they have some known values of range and general performance for certain combinations of gear, and they consider how much a certain change to that gear will affect this performance. As in, swapping a 3dB antenna for an 11dB option might give you an 8dB stronger signal, which corresponds to 4dB (~2.5x) more range in open air versus the alternative under the same conditions; or it might give you the ability to punch through that wall (while your less-directional omni antenna covers the rest of your surroundings). Maybe upping the VTX power twofold could give you sqrt(2)=1.41x more range even through that wall. Or it might give you more interference instead. So after making that hypothesis, you implement the change, go and test it and see if it really works this way; It might or it might not!

Okay, now comes the nerdy part.

I don't know the exact formula you're speaking about, but I'm going to reason my way through this just for kicks.

So, our goal is to have a signal at the receiver that's higher than its sensitivity (which represents the weakest signal that the receiver can detect and demodulate). Our signal path is as follows:

Original signal -> transmitter's amplifier gain -> (feedline loss) -> TX antenna gain -> path loss from travel between antennas -> RX antenna gain -> (preamp?) -> receiver.

(scroll it).

The signal is multiplied at every step except where it is reduced while traveling through the air, so we need the product of all gains multiplied by the loss factor to be higher than the sensitivity.

So our max-range equation is:

AmpGain*Gain1*Gain2*PathLoss = RxSens.

In the simplest, ideal case, where we assume that there's nothing around except the two antennas (free space), the losses from transmission are inverse-square:
FSPL = wavelength/(4Pi * distance²), from which we get:

AllGains*wavelength/(4Pi * Distance²) = RxSens;
4Pi * Distance² = AllGains*wavelength/RxSens;
MaxDistance = sqrt(AllGains*wavelength/(4Pi*RxSens)).

And here comes the caveat.

We don't live in an ideal world.

We're not transmitting in free space. There might be buildings or trees or rain or anything else in the way, too. Worse, it doesn't even have to block your line of sight, as objects near the line will cause diffraction of your signal which can interfere with your reception if they are within the fresnel zone. The ground itself will often be the biggest source of such interference at larger distances, sometimes reducing the propagation factor to an effective inverse-fourth-power law.

Also, consider that the max-gain vectors of your antennas are not always pointed at each other; while you can use an antenna-pointing device on the ground, your aircraft will pitch and roll, reducing the transmitting antenna's gain in your direction.

The earth's atmosphere can affect your signal significantly as well, and it will do so differently each day, both in terms of how much and in terms of how.

For all of those factors, while you can predict roughly how they are going to affect your range, there's a lot of things about them in each particular location and at each particular time that you simply don't know. And since signal-modifying factors stack by multiplying, the discrepancy can get pretty big pretty fast.

Your receiver isn't ideal, either. It will probably start to corrupt the signal in various ways as you approach its sensitivity level. You're unlikely to ever see this, however, because long before it reaches that critical stat of your receiver, your precious signal will drown in...

Noise.

Noise is the worst enemy of anyone working with high-gain systems. It's the third law of thermodynamics personified. Thousands of manmade and natural radiation sources around the planet bombard everything around with random radio waves. Even if you exclude those, outer space is actually quite noisy as well. Your receiver itself generates noise internally; in fact, for most gear including your receiver sensitivity itself is defined by that internal noise level. Still, external noise tends to be orders of magnitude more powerful, and whatever the source, you don't want any of it.

The total strength of noise as received by your gear is called the noise floor, and that floor is lava. No matter how sensitive the receiver, the moment your signal level dips below the noise floor, it's gone: still out there, but you can't tell it apart from the noise to decode it properly anymore.

Depending on the nature of your signal, it can even become totally unintelligible before it reaches the noise floor, at several dB above it. Usually, the higher a signal's bandwidth (which is closely linked to data transmission rate), the sooner it's overcome by noise. Some advanced digital transmission modes can extract a message from well under the noise floor, but in order to do that they transmit several bytes per minute.

Video is a notoriously high-bandwidth type of data, both in the internet and in radio. Analog video especially so, as it has zero compression. So it gets disrupted by noise rather easily. On the other hand, zero compression implies lots of redundancy in the data, and redundancy allows for error correction. Do you know what's one of the best signal processing and error correction machines in the world? The human brain! Even when a video signal is almost entirely eaten up by static, we can still see shapes in it. So for video, the intelligibility threshold seems to be just at the noise floor mark, give or take a couple dB.

You can fight noise in various ways. Usually, if most of it is coming from certain sources on the ground, just using a directional antenna helps by attenuating signals from unwanted directions. Sometimes, if the noise is just the regular background entropy kind, it doesn't. A good frequency selectivity of your receiver (i.e. its ability to ignore signals that aren´t the target frequency) also helps tremendously.

An interesting way of fighting noise is signal-stacking, as done by receivers like RapidFire or True-D X: Instead of simply switching to the antenna with the highest signal, they use signals from both and merge them into one. Since the useful signal is the same and noise is random, the signal adds up while the noise mostly cancels out. Astrophotographers use the same technique to get stunning clear pictures of the sky, except that they have to stack hundreds of extremely noisy photos. The digital communication modes I mentioned earlier also work this way by sending multiple copies of the same transmission.

Video is still a high-bandwidth signal, however, and you've only got two copies of it to stack, so the gains from the likes of Rapidfire are far from dramatic, but it'll win you a couple of additional dB over the noise floor. Beware, though: such receivers depend on a special sync signal in the video to do the adding correctly, and when the noise eats that... You'll probably get a blank screen.
That's a common caveat with any technology that does the error correction for you, by the way, including digital video: it gives a cleaner and more pleasant signal while it does, but will probably give up before you do. So this kind of error correction might not be the best for very long-range flights.

Final word

To appreciate the sheer expanse of outcomes that you might possibly encounter once you start pushing the range envelope, it's probably best to look at ham radio operators.
On one end, they will casually use 50-watt transmitters to just talk locally within a range of no more than several tens of kilometers, and sometimes use kilowatts to punch through unfavorable conditions. On the other end, they can literally talk to the other side of the earth on fractions of a watt and bounce signals off of the Moon. They've been doing all of that crazy stuff for decades and had invented things that were thought impossible.

And the reason for all of those achievements is lots and lots of experimentation. Without testing what works and what doesn't, a theory is useless. So, if you decide to jump down this rabbit hole, start to think like a ham. Make that estimate to the best of your ability, assemble what you think should work best, and then go out and fly it. See how it works in reality, and how it differs from your expectations. Make hypotheses. Go out on different days and in different places to see how the conditions change your outcome. Then change something in your system and see what happens. Write it down. Make your own theory.

And, most important of all,

Have fun!


How common is it for drone operators to actually follow all the regulations?

As with any new technology, at first things are quite chaotic. Take my country for example, a few years ago, the regulating agency of flight introduced a scheduled drone flying program. This meant you had to make an appointment in advance with 60 days (!!!) and specify where and for how long you were flying. Also it was necessary to have your vehicle registered which at the time was a terrible process. Of course, people would still fly their vehicles and eventually this implementation has been removed and now people can fly freely.
Latest generation drones have a no-fly zone in their memory, so whenever you try to go in those areas they won't go. Flying over buildings in cities is quite challenging due to various factors I won't go into. Flying over forests while inside forests is also challenging. If you build a custom drone using a different transmission channel you can theoretically fly over building, forests and so on without any technical issues.
These rules are made for the future. As the technology will become more available for the general public, there needs to be an controlled environment so that anyone can fly their vehicle safely. Take aerial transportation for example: they have very specific routes and timeslots when each plane can fly. They most likely said the same thing as you about planes back in the 40's or 50's.
Also certain bans are against people that cannot breathe and think at the same time and would cause serious damage to their surroundings. This why I support the idea of pilot testing and registration. Pilots and decision-making people are expected to know what they are doing.
People in my country still fly these vehicles without any permits or registrations, but also the governing body doesn't care and the city police is clueless about required paperwork when flying a UAV.
Yes...people in the city I live in fly drones in the city...and post it on our favorite social platform...and I know some of them, and no, they don't have registration or permits...they also don't care...


In drone racing with multirotors, how does one navigate the course?

I've gone out and asked an actual racer about this one.

There are usually some visual cues. The course is laid out in cones, the flags are asymmetrical so you know on which side you should pass, the gates you must fly twice through usually have something (bright fabric, a flag, ...) next to them on the side that you fly around before going back into the gate, etc. and the cones themselves are not just a flat string but are laid out in certain patterns; for example, when the sequence of cones terminates in an arrow this usually indicates you should go upwards (or higher, i.e. in the case of two gates stacked on top of each other), while a perpendicular line marks the entry point of the cone string.

Sometimes the track elements are colored differently (and asymmetrically) to indicate the direction. Some elements just have canonical uses based on their shape: you should pass through gates, fly besides flags, dive the dive gates... The Jacob's ladder specifically is almost always passed top to bottom in sequence (while whether you should go right or left varies track to track).

That said, there is no universally accepted, set-in-stone standard for these cues. Which ones are used depends a lot on the materials available to the race organizers, how much time they have to set up the track, the track's layout, etc. For instance, flags can either be fabric on a springy pole (in which case you fly on the convex side), or they can be inflatable and curved to hang over the ground (in which case you fly under the overhang on the concave side).

Most drone racing leagues have their own standard of how different track elements and maneuvers should be marked on the racetrack and try to stick to it. Even then some smaller local or regional events of the league may be limited by the organizers not having all the required materials. Also, one cannot mark everything on the more complicated, technical tracks, or mark it in an unambiguous manner. Sometimes some parts of the track are formed by the terrain itself, i.e. an abandoned building or natural rock formation. In that case there's no standard and the track designers must get creative to even make it clear where the gates are!

So visual cues aren't everything. A lot of races rely on communicating to pilots how the track should be flown before the race. Sometimes it's a verbal briefing, sometimes a printed schematic. Sometimes the track is published well in advance so pilots could train on their home field or in a simulator. There could even be a video walkthrough or a DVR of a practice run. Additionally, many leagues actually perform a physical walk through of the track on or before race day (much like Formula 1 car racing), so that the marshals and pilots can discuss challenges, specific items and hazards, etc.

In the end, for serious racing, how the track itself is marked does not matter. During the actual race the speed at which everything is happening does not leave you any time to think or figure out what's coming next by parsing the track. The racer just knows in advance which gate or maneuver comes after the one he's flying through, and anticipates it, most likely along with the one after that as well. Knowing the track is a very important factor in winning a race, as in order to have the fastest (which usually means shortest) trajectory, the pilot must start the necessary maneuvers for passing the next gate while he has not yet passed the previous one.

Besides training beforehand and learning the track from the given materials, the racers are always allowed to fly some laps around the actual final track before the race in order to memorize its layout. This is critical as the actual racetrack will always slightly differ from whatever training track they might have constructed for themselves. During these training laps they also figure out the best way to tackle the elements of that particular track in the speediest way possible, and thus develop an optimal trajectory, or racing line, as it's usually called.

So, in the end, the on-track markings are there just to aid in the initial few laps as the track is being learned by the pilot, and after that, memory takes over.


Technically, you don't need a rangefinder or a barometer for a normal drone but if you want to make a drone with altitude hold, I would recommend a barometer over a rangefinder because a rangefinder will not work very high up or when you are flying over certain surfaces like water. A rangefinder would only be good if you stay close to the ground and don't fly over reflective surfaces.


There are two parts to mapping with a Mini 2.

Autopilot App - No autopilot apps will work with the Mini 2 until DJI releases the SDK. Expect Dronelink to provide support for the Mini 2 quickly after DJI introduces the SDK.
You'll need a photogrammetry processing software.

Here's two posts I've written examining what maps and accuracy you can expect with the original Mini.  The mini 2 should be similar.

Can I Map and Survey with the Mavic Mini
How Accurate is Mapping with the DJI Mavic Mini

Full Disclosure I'm the founder of Civil Tracker.


Why do brushless motors have 3 wires compared to 2 wires on a brushed motor?

A standard brushed motor has a positive, negative and a commutator that switches the polarity of the rotor.

Conversely, a brushless motor has a multiple of three sets of coils. These coils are energized in a particular order to create a series of electromagnetic forces that ‘pull’ the bell (which contains permanent magnets) around.

As there are three sets of coils, there are three wires - one for each set of coils so they can be individually energized.

The ESC is what controls the sequence in which the coils are energized.


What is circular polarization in antennae?

This is going to be a long answer but bear with me.
First, I must discuss the phenomenon of polarization itself. As you probably know, radio waves are a subtype of electromagnetic waves, which in turn are nothing more than alternating electrical and magnetic fields in space. The laws of this universe dictate that a changing electric field creates a (perpendicular) magnetic field, and a changing magnetic field will create an (also perpendicular) electric field. This allows the two fields to feed on each other.
Now, the most common picture of an EM wave that you have probably seen is something like this:

This is a linearly polarized wave.
Here, the electric field is red and the magnetic field is blue. Each of those always oscillates in a particular direction (e.g. the electric field vertically and the magnetic horizontally). This kind of wave is typically generated with straight antennae, in which electrons move back and forth along the antenna and thus generate the linearly-oscillating electric field. The wave on the picture is vertically polarized (The polarization direction is the electric field's orientation) and was probably generated by a vertically oriented antenna.
While an antenna only generates waves of its corresponding polarization, it can receive differently polarized waves, as long as the electrons inside the antenna can move in the same direction that the electric field of the wave is oscillating.
For example, an antenna oriented at 45° relative to the incoming wave's polarization, will still receive the signal, as it is still oscillating somewhat in the same direction, just (1.41 times) weaker: the electrons in the antenna can still be moved up and down by the wave, as long as they're also moving side to side so that they could stay within the antenna. In fact, the only situation in which such an antenna will reject (not receive) a signal entirely is if the antenna is perpendicular to the polarization direction: electrons can't move vertically in a horizontal antenna at all. In all the other antenna positions will have varying strengths of reception (or gains) depending on how much the directions differ (in mathematical terms, it's proportional to the inner product of the two directions).
The other main type of polarization is circular polarization, in which the electric field does not just oscillate in one direction, but instead rotates around an axis (along which the wave travels), with the magnetic field following, forming a spiral as it propagates through space. A circularly-polarized signal can be thought of as a sum of two perpendicular linearly-polarized ones, one lagging a bit behind the other:

(note that the red and blue colors do not mean electric and magnetic fields here, this is the same field viewed from two directions.)
(for completeness, there is also elliptical polarization, which can be thought of as a sum of a circular and linear signal, or a sum of two linear signals that are not equally strong).
Any rotation must have a direction (or handedness), clockwise or counterclockwise, relative to the direction in which the wave is traveling. That is what distinguishes Right-hand circularly polarized (RHCP) waves from Left-hand circularly polarized (LHCP) ones. There are antennae specifically designed to emit and receive circularly-polarized waves, and they, too, must have one of the two possible directions. An RHCP antenna emits RHCP waves, and the same is true for LHCP.
Reception is more complicated. A linear antenna would receive circularly-polarized signals no problem, in any orientation (except if pointed directly at the signal's source), since, as we discussed, exactly half of that signal's power is linearly polarized in the antenna's direction (and the other half perpendicularly, so that part is rejected, hence the gain is 1/2 or lower). The opposite is also true: a circularly-polarized antenna of either handedness will happily receive (though at no more than half gain) any linearly polarized signal (which is half of a circularly polarized one).
The one kind of signal a circularly polarized antenna will reject is a signal of the opposite handedness. an RHCP antenna will refuse to receive an LHCP signal, and vice-versa. And that is the main reason why these antennae are used in FPV.
The bane of video signals in industrial and other environments rich in RF-reflecting surfaces is multipath propagation, or multipathing, as it's more commonly known. This is a situation in which the transmitted signal, besides a direct line of sight path, gets received via reflections from nearby surfaces. Since the reflected signal takes a longer path, it gets to your goggles a bit later than the direct signal. This delay is enough for the very fast scan rate of analog video to go forward several pixels, so overlaying this delayed signal (of which there are usually several) on top of the original one produces severe distortions in the image.
The good thing for us about reflected signals, however, is that a circularly polarized wave reverses its handedness whenever it bounces off of something, so a reflected RHCP wave becomes LHCP and vice versa (as the direction of travel changes, while the direction of rotation stays the same), which means that the antenna will reject all reflections (it will not reject double reflections, which will have the original polarization, but those are weaker than single ones).
Another bonus is that it allows sharing bandwidth better if pilots on close channels fly opposite-handed signals, as possible interference from the neighboring band will be rejected by one's receiving antenna.
As far as the difference between RHCP vs LHCP goes, as you can see, neither is better than the other physically, they are two sides of the same coin.
However, in actual practice there are differences if you don't fly alone. Historically, RHCP has been several times more common than LHCP, and that means two things:

With an RHCP antenna, you can spectate almost anyone, and they can spectate you, while with an LHCP antenna it's far less likely both ways (although one can spectate both kinds of circular polarization with a linear antenna and face the multipathing).
It's easier to get interference from others (and cause interference to them) if you're flying the same polarization, so RHCP is going to be generally noisier and more interference-prone for everyone around.

If you're not racing, the second point probably doesn't matter, just move a couple of channels away. If you are, you should probably have both RHCP and LHCP antennae with you and be ready to switch as directed by the organizers.
Still, even if you're not a racer, it can be worth having a couple of LHCP antennae in your flight bag for when a lot of people are in the air; you'll get a much cleaner signal that way (and they will, too)!
Some receiving antennae, such as the Triple-feed patch, can be either RHCP or LHCP depending on where you attach the feedline. Those can be worth having if you can't decide or switch polarizations often, or just to reduce the number of antennae to own.


How to focus a newly-replaced lens on an FPV camera?

I like to set the drone in a window or take it outside.  Then adjust the focus while looking through the goggles until I get it where it looks good to me.

You may also wish to print out a focus test pattern or follow this detailed tutorial on focusing an FPV camera.


What would a basic fixed-wing electronic setup look like?

The main things you need are a motor, battery, electronic speed controller (ESC) and servos.
The ESC sits between the battery and motor, and it also has a thinner cable that goes to the receiver. The wires from ESCs are often labelled, but if not, there will be two big wires on one end that are the input from the battery (red for positive, black for negative), and three big wires on the other end that supply the motor. The motor wires can be any colour and it doesn't really matter how you connect them. If the motor runs backwards, switch any two motor wires. The cable to the receiver will be three wires joined together in a ribbon, similar to the cable on servos (and with the same plug)
The ESC also provides 5v to power the receiver and servos. All the positive and negative pins in the receiver are connected. The part of the ESC that does this is known as a BEC or 'battery eliminator circuit' because 'back in the day' you needed a separate 5v battery for the radio.
Servo cables have three wires, positive, negative and signal. Positive is in the middle so that plugging it in backwards doesn't cause short circuit. Servo plugs are pretty standard, although some have a little tab on one side to prevent them being reversed - but this requires that the receiver has a slot for the tab (and many don't, and the tabs get cut off).
Servo plugs are small and need a special crimp tool to make, so they are nearly always done for you - and they all use the same pins, so different brands are compatible.
You might also want plugs to connect the motor and ESC, either because the design routes the motor wires through the firewall, or so you can move components between models. 3.5mm bullet connectors are pretty standard here, with the male plugs on the motor (so you can safely power up an ESC without a motor).
If you shop carefully, you can probably find an ESC and motor with all the relevant plugs already attached but many only have the servo plug, either to reduce cost or to provide you with more choice. If it comes without plugs, or you need to change a plug, you will need to solder.
The Tiny Trainer is just about able to use the little red JST battery plugs that use the same pins as servo plugs, but it's right at the limit and old plugs tend to overheat, melt and short circuit. XT30 plugs would be a safer choice. XT60 plugs are much bigger than you need but a common standard for chargers and larger models. There are lots of other types of plugs these are the most common now.
Beware that the connectors such as the motor plugs or XT60 battery plugs require a larger soldering iron than the 20w irons that are usually sold for hobby electronics. Also, if you're soldering, you'll need some heat shrink tube to cover the joint. 5mm diameter fits XT60s and 3.5mm motor plugs. Remember to put the tube on the wire before you solder the joint, and slide it away from the heat or it'll shrink before you are ready.
If you are lucky enough to have a local hobby shop, they are usually very helpful in finding parts that will work together. Take a list of the recommended gear and they'll find equivalents, and may even do the soldering for a small fee.
When you're in the hobby shop, buy some spare props, some piano wire for the pushrods, control horns and swing keepers to make the attaching the push rods easier, and some Velcro strap to hold the battery in place. And a battery charger (with power supply). And some screws to hold the motor to the firewall.


Can boosting the Caddx vista to 1200mw cause damage to the unit?

As far as I can tell, after doing some digging, There are a few things to consider when boosting a Caddx Vista to 1200mw. First, the Caddx Vista tends to get hotter that the DJI air unit, so heat is one thing to consider. If the drone is flying, there should be enough airflow to dissipate the heat, so 1200mw shouldn't cause any overheating issues. Just make sure the drone isn't sitting somewhere with the Vista on a high-power setting.

Another thing that seems to happen when using a higher power setting (whether that be 1200mw or just 1000mw) is that the bit rate drops. Several people claim that when flying with 1000/1200mw, the bit rate drops and causes lower image quality.

See here starting with post #84 on page 6 and then the beginning of page 7.


Is it possible to get a FAA Part 107 drone license if one has had a pilot license revoked or suspended?

I can't find anything that says losing your general aviation license would prohibit you from being unable to obtain a drone pilot's license. In fact, the two seem separate in every regard but one: The Integrated Airman Certification and Rating Application (IACRA) . Per the Part 107 page you need to make an IACRA application. That application will ask if you have ever had an IACRA revoked

  Have you ever had an Airman Certificate Revoked?

It's unclear what answering Yes will do (they want you to explain why it was revoked), but it doesn't seem to be an automatic disqualifier.


How to determine the lowest safe ESC current rating for a drone build?

I think most people just copy other builds, and step up a size if they burn out any ESCs.

If you want to be more scientific you could test one motor with a watt-meter to measure the current draw. Throttle up slowly so you can watch the current build, to protect the ESC you're using for the test. Also check the motor temperature after 30 seconds or so.

This will give you the maximum current that each ESC needs to handle.

There are lots of different designs of watt-meters available for hobby use. They typically measure up to 100amps so should all be adequate for this test, and cost less than a set of ESCs.

A typical multi-meter will only measure up to 10amps and would not be adequate. Clamp meters typically only measure AC current, and we need to measure the DC current from the battery to the ESC.

Fix the motor to something solid, like a plank of wood that is clamped to a bench. Be careful that the wires and other loose material will not be sucked into the prop, and don't stand in-line with the prop, just in case breaks and throws the blades out at high speed. Even a couple of hundred watts is quite intimidating in a bench test.

Alternately, you could search the forums to see if anyone else has tested a similar motor with a similar voltage battery and prop.


It should be possible so long as the aircraft has enough spare thrust and lift to counteract the drag and weight. Many model aircraft have good power-to-weight compared to their full size equivalent.

For the banner attachment, you need a secure point to attach it to the aircraft, well away from any propellers or control surfaces. Usually this is on the bottom rear of the fuselage.
It is a good idea to include a 'weak link', which will break if the banner snags; this leaves the banner behind, but saves the aircraft being pulled from the sky.
You will also need to weight the banner to hang in the correct orientation - making the vertical part of the banner support bottom heavy should suffice.

You can fly banners from both fixed-wing or rotary wing aircraft, in slightly different ways. Fixed wing banner extends behing the aircraft, whereas on a helicopter or multirotor the banner hangs below.

Image source


My DIY Project - My motors and voltage regulator for water pump does not work at the same time

If you are using the same battery for everything, the drone rotors might be drawing too many amps causing your pumps to not spin. Try using an external battery for it, or get a bigger battery for the whole drone. And if this doesn't help, please explain your setup a bit better.


The night eagle (and other 'night' FPV cameras) use visible light, but can work with very low light levels. Moonlight looks like daylight and starlight is flyable. There are plenty of videos that demonstrate what you would see.
Even many ordinary FPV cameras will often work better than normal vision. It's quite common to finish an evening flight and be surprised at how dark it is.
As far as I know, the Night Eagle is still the best but new cameras are released every few months so it's worth checking the latest reviews.
'Night' cameras might pick up some of the nearest IR frequencies, but they're not real IR cameras. You won't see any contrast between hot/cold objects. Real FLIR cameras tend to be expensive, and the more affordable ones are low-resolution and designed for smart phones rather than FPV video transmitters.
I don't think that putting a light on a drone is going to help much until you reach car headlamp brightness, and even then you might find it's too narrow.
I found this commercial FLIR drone but at under 9 frames per second it's not really FPV, and it still looks expensive.


How can I safely parallel charge my LiPo batteries?

Only parallel-charge batteries with the same number of cells.

Safe parallel charging requires that the voltages on individual cells be close to each other. A difference of 0.1V per cell is considered to be the maximum safe difference (I found it online, and have used ever since, although I have no scientific proof of that).

One way to compare the packs before parallel-charging is to compare their total voltages and make sure that the maximum difference between any pair of packs does not exceed 0.1V times the number of cells. However, this may be unsafe due to the fact that some packs may have more unbalanced cells than others, so it is better to compare each pair of cells that will be charged in parallel.

With many packs of uneven charge, it may be possible to divide them into groups whose cell voltages are close enough to be safe to parallel-charge and then charge each group separately.

A parallel-charging board with enough protection is recommended, with fuses limiting the total current of each pack being the minimum. Some boards also have fuses or current indicators for each cell of each pack for more safety.

Choose an appropriate charge current, which equals the sum of charge currents of all parallel-charged packs. Charging at 1C times the number of packs seems to be reasonable and safe if there are no large differences in capacities and cell degradation between packs.

Regular safety rules of charging single LiPo packs apply, for example never leaving the batteries unattended while charging, keeping away from flammable materials, etc.


What is the best drone for beginner fliers?

There's no single 'best' starter drone, as new ones are released every few months. However there are lots of good options under \$30, and some as cheap as $15.
What you're looking for is something small and light, so you can fly it indoors without breaking things. It should have rings around the props to protect them and anything it hits.
Syma, Eachine and Hubsan are popular brands but there are others.
Do get more than one battery. They typically only last about 5 minutes and take an hour to charge. A charger that can charge more than one battery at a time is a bonus too. That said, 10-20 minutes is enough for most people - at least after the first time.
'Headless mode' is quite a useful feature for beginners, it allows you to ignore yaw. Forwards is always 'away from you', whatever direction the drone is facing. It's not essential though, and can be confusing or lead to bad habits.
'Altitude hold' should be useful but I've found these drones always need an experienced pilot to trim them first. It's good for an experienced pilot to teaching kids under 10, but not for teaching yourself.
Many cheap quads have tiny 'jelly bean' transmitters, which can be difficult for those with big hands. If you think this might be a problem, you can spend a bit more for a proper sized controller. There are various hacks for extending the sticks too.
Don't buy one with a simple recording camera. Until you pay enough for one that can transmit the signal to you in real time, the footage will be rubbish because you can't point the camera and frame the shot. The first time you do it, it's cool to see a blurry, poorly coloured picture from the air but the novelty quickly wears off. View your first drone as a trainer and indoor toy that will take the knocks of learning while you save up for something better.
Don't bother buying loads of spares, these little things don't break very often, and it's as likely to be a motor burning out as a broken prop, and a pack of spare motors can cost more than the drone.
That said, they are very prone to picking up stray hairs and winding them around the props and motors. Try not to spend all your time bumping around on the floor, and check for hairs after each crash.
When you're learning, try to pop up into a hover a foot or so off the ground and concentrate on holding the right height. When it has drifted half-way to an obstruction, cut the power and land. Don't try to fight it because you'll just crash harder. Put it back in the middle of the room, review what you should have done and try again. When you can control the height, try correcting the drift but still land and review when it gets half-way to an obstruction. If you struggle, find a larger space to practice for a while.


Does the LaForge V3 Fat Shark Receiver receive audio from VTX?

The V4 has audio, but I can't find any information about whether or not the V3 has it. I did however see that a manual mentions a VTX audio option in the menu. But since it doesn't say which version, the easiest thing to do would be to see if you can find that option and make sure to turn it on.


Huh. I never thought about this before, but charging identical LiPo batteries in series with one another seems logical enough. In fact, here's an article which discusses how to do it, which I'll summarize here.

Prerequisite requirements:

The charger must support the series battery (e.g. for 2x 4s batteries, the charger must work with at least 8s)
You must create a balance lead adapter (and separate main discharge lead adapter) that combines the leads from the batteries into one. The highest voltage cell on one battery will have the same balance connector pin as the ground wire in the next battery's balance lead. (graphic below shows this in more detail)
All batteries MUST be the same capacity, but not necessarily the same cell count. (e.g. not 1s 1000 mAh and 1s 1500 mAh, but 1s 1000 mAh and 4s 1000 mAh is fine)
All batteries MUST have per-cell voltages that are close to one another. A difference of 0.05 V is usually okay.

Advantages

Time savings from charging all at once and hooking up batteries fewer times
Can allow for faster batch charging than parallel charging if the charger's power limit is less of a bottleneck than the current limit
Unlike parallel charging, each individual cell is monitored and balanced separately by the charger

Disadvantages/Risks

If something were to go wrong during the charge cycle it would effect multiple packs. (e.g. an internal cell short in one pack could light all packs on fire)
All batteries MUST be properly connected to both the main discharge lead adapter and balance connector adapter, (and connected in the right order) or funky stuff like short circuits and fires could happen!
Can be a slower batch charging method than parallel charging if the charger's power limit is more of a bottleneck than the current limit


Are there any extremely small brushless drone motors?

Theoretically, I don’t see why we couldn’t make current motors even smaller than they are today - we already have brushless motors with stators less than 10mm across (for example these 0802 motors), and we could change the format by using inrunner motors instead. However there are certain factors that we need to consider.

The first of these is that we can’t just have the motors - we also need the ESC boards. This is either done with a 4-in-1 ESC or 4 individual ESCs. This adds complexity, weight, size and heat buildup.

We must also consider whether or not these negatives outweigh the positives. This is up to personal opinion, and in some instances it may well be worth it, however with miniaturisation comes increases manufacturing complexity with smaller tolerances.

These are all important factors to consider: is it possible and is it worth it.

I believe it is possible, however finding enough demand may be a limiting factor.


Is it better to have a bottom heavy quadcopter or a top heavy quadcopter when it comes to a quadcopter stability?

It depends on what kind of stability you're after. Nicolas Petit wrote a paper on this several years ago, the graphics below are taken from Pages 4 and 5.

The summary is that a higher CG is better for x-y position stability, a lower CG is better for speed stability.

Background

Before we begin, let's assume we're only talking about a drone flying purely forward and so all dynamics are related to pitch.[*]

Second, let's define the CG, center of gravity, relative to the aerodynamic center (similarly to in https://aviation.stackexchange.com/questions/19388/what-is-the-difference-between-centre-of-pressure-aerodynamic-centre-and-neutra). The aerodynamic center is the idealized point where all the drag forces are acting around it evenly. In other words, it can be thought of as the spot where half the drag rotates the quadcopter into a pitch up orientation, and half rotates it into a pitch down orientation.

Here's the relevant image. It shows forward flight, and that's the same as position-holding flight with a backwards wind.[**] The red arrow is for when the CG is above the aerodynamic center, and the blue arrow is for when it's below the aerodynamic center.

Top-heavy CG: increases position stability, decreases velocity stability

When the CG is above the the aerodynamic center, the drone tends to tilt forward when there's a gust of wind. This ensures positional stability, because as the drone is pushed backwards it naturally leans forward which reduces the speed at which it accelerates backwards.

However, since the CG is above the aerodynamic center, when flying forward the drone wants to trip over its AC and pitch even more forward. This means that forward velocity is somewhat underdamped, results in a less positive-- or even negative-- feedback cycle.

Bottom-heavy CG: decreases position stability, increases velocity stability

In this case, the CG is below the aerodynamic center. When there's a sudden gust which destabilizes the drone, the drone tilts away from its holding position, which combined with the gust's force, accentuates the positional error.

But on the other when flying steadily forward the drone tends to want to pitch back toward the vertical, instead of tripping over its own feet. This means that forward velocity is well damped without a positive feedback cycle.

[*] Roll functions exactly the same way, but it's distracting to write "pitch/roll" each time we're talking about a rotation.
[**] This is a basic tenet of aerodynamics and fluid flow, which is that it's indistinguishable if the object is moving through the fluid or the fluid is moving around the object.


Why are quadcopters inherently unstable?

A stable system is one where there's a force that pushes the system back towards it's original state when the system is moved from its balance point.

For example, when the driver gets into a car, there's more weight on one side of the car than the other but the car doesn't flip over. The springs on that side of the car compress and push harder on the wheels. The tyres push harder on the ground and the ground pushes back. The extra force from the ground exactly balances the extra weight and (after possibly bouncing for a moment) the car stops moving.

In an unstable system, say a broom balanced vertically, once it moves from the perfect balance point gravity will pull it further away and it accelerates.

For a quadcopter, there is no force that returns it to its original location, holding it horizontal, or even preventing a gradual roll.

Even if the thrust from each motor was exactly the same, and the weight was perfectly balanced, it's very unlikely to be perfectly stationary. If there's even the slight amount of pitch or roll, the angle from horizontal will gradually increase. As the angle increases, the sideways thrust increases and the drone accelerates.

So the roll rate is constant, the angle and acceleration increase linearly, the horizontal speed increases faster as the acceleration builds, and the distance from the starting position increases even faster as the speed builds. To the pilot, it looks like the drone is accelerating away from its original position much like the broom falling over.

To a systems engineer, the rotation of the drone is neutrally stable - there are no forces increasing or decreasing the rotation rate. However the speed and position are unstable - once it starts moving, it accelerates.

I should mention that there are second-order (i.e. relatively minor) forces too. If you had a large fin on top of the drone, aerodynamic drag on the fin would return the drone to upright as the speed builds and limit the maximum speed - in practice this isn't useful as it doesn't return the drone to its original position.


How to drive a servo for a tricopter with a flight controller?

This has likely been answered here: https://drones.stackexchange.com/a/230/50, but to paraphrase:

A standard flight controller can easily be set up to control a servo, as per your edit.

You first need to plug in your flight controller (with props off, if you’re using one with motors already attached) and enable ’servo tilt’ in the configuration tab, then go into the CLI.

You then need to type ‘resource’ (assuming you are on the latest version of Betaflight.)

As you want a tricopter and your flight controller is likely for a quadcopter at least, you can reassign a motor output (or any other PDB with a built-in timed) to become a servo output. If you don’t know how to re-map resources in Betaflight, the answer I have linked gives an excellent rundown.

You should then see a servo tab appear in Betaflight, and from there you should be able to configure everything you need.


Can I replace the switches on my taranis qx7s with switches from a X9D?

Technically speaking, the electrical functionality of all normally-open SPDT (single pole double throw) toggle switches are exactly the same; it's just a matter of the physical dimensions of the part. The important dimensions for matching RC TXs with toggle switches are the diameter and pitch of the threads cut into the cylindrical section, but they're unlikely to differ between radios.
 (cit.)
The kinds of SPDT toggle switches used in RC transmitters and like are so common and standardized that it's unlikely any manufacturer would go out and make their own instead of sourcing standard ones.

In regards to possible differences between FrSky Taranis QX7S and Taranis X9D switches, this RCGroups thread seems to indicate that they are interchangeable.
I don't have either product, so I can't speak from experience, but the primary plausible difference seems to be between an M6-0.75mm and an 1/4-40 (inch-TPI) thread on the switch. The replacement switches I've purchased for my own transmitters usually come with matching nuts, so I don't think the possible difference thread pitch is really a concern. M6 and 1/4 in diameters are pretty close in size, so I don't think that the diameter difference would pose much of an issue when sticking the new switch into the clearance hole in the transmitter housing.


How do thrust requirements change as a multirotor leaves ground effect?

To fly above ground effect, you need thrust equal to the weight of the aircraft (or slightly more, as you want to climb).

Ground effect doesn't hold aircraft down, it just makes it easier to fly when they are close to the ground. The exact amount varies quite widely depending on exactly how close the wing/rotor is to the ground. For a plane or helicopter it may only be 10% extra lift, but a vehicle designed to maximise ground effect (like a hovercraft) can fly with vastly less power than would be required to fly without ground effect.

Ground effect is particularly noticeable if you hover a model helicopter inverted over a smooth surface. As it descends towards the floor, it looks as if it lands in a big soft cushion - decelerating and bobbing around at about half to a third of rotor diameter. You can hover without the constant adjustments in collective pitch that are normally required, and need to drastically reduce thrust to go lower. When you're half an inch or less off the floor, it's probably only using 10-20% of the pitch required for normal hovering, and there's a self-levelling effect so it's much easier than it looks!
For a typical FPV quadcopter with 5" props on top of the motors, you almost can't feel ground effect if you put the battery under the frame. With the battery on top you can feel a noticeable cushion and you can find a throttle setting that holds the quad off the ground but won't climb out of ground effect. I'd estimate that it's flying at about 80-90% of normal power.

While the power may be reduced, it can be harder to hold a multi-rotor or helicopter in place when in ground effect. The bubble of higher pressure tries to escape out from underneath the aircraft, pushing it in the opposite direction.

A general rule of thumb is that the reduction in power due to ground effect starts to be noticeable when the altitude is less than around half or a third of the wing span or rotor diameter. The 'air bubble' effect on position holding while hovering is noticeable one or two diameters above the ground. Both tail off gradually so there's no hard limit.

See also:
* What is ground effect?
* How high does the ground effect last?
* Does ground effect scale 1:1 with wingspan?
and lots more, over at the Aviation stack-exchange.

Also this excellent academic paper:
Characterization of the Aerodynamic Ground Effect and Its Influence in Multirotor Control


How do I test if my DX5e antenna is still good?

That's probably OK, but you should do a range test.
All transmitters have a mode that transmits a much lower power than normal, and if that is sufficient to control the model at 30 paces, full power will be enough to fly further than you can see.
From page 9 of the DX5e manual:

Face the model with the transmitter in your normal flying position. Pull and hold the trainer switch while toggling the HI/LO Rate Switch four times. The LEDs will flash and the alarm will sound indicating the system is in range check mode. note: The trainer switch must be held the entire time during the range check process; releasing the trainer switch will exit the range check mode.

You should  have    total   control of  the model   with    the trainer switch  pulled  at  30  paces   (90 feet/28 meters).

Now an Inductrix probably won't have the range of a normal receiver so don't worry if it's a little short, but you should range test every receiver you buy every time you install it in a different model. Ideally from a range of angles to make sure the antenna can't be hidden behind a large metal or carbon-fibre part.
Before doing a range test, it's important to either remove the props or ensure that the model will shut down if it loses signal, to prevent it flying away.


Crossfire/Unify - Change Settings in TX and in OSD

I believe the answer is no, you cannot use both. When you set the vtx up for lua in the radio, you change the communication protocol from SA to CRSF. You also remove the SA wire from the FC and move it to the Rx channel 1. So having just done this twice, it seems you get one or the other.


The big problem with learning to fly is that some of characteristics you need for a good trainer are the opposite of other requirements.

Large models are less affected by wind, because their flying speed is proportionally higher relative to the wind speed. However small models are a lot more robust in a crash. and cheaper, so less discouraging when you crash.

Highly stable models return the model to straight and level flight but neutrally stable models (like a flying wing) are less affected by gusts.

Traditionally this leave you with the option of a small, stable model and waiting patiently for calm weather, or a faster, less stable model and an instructor (or lots of crashes and repairs!)
These days it's possible to use electronic stabilisation to tame a neutrally stable aircraft, and to reduce the effect of gusts on a small model (although it won't stop it disappearing down wind!)
IME it takes an hour or two (on a simulator, or accumulated flight time) before a beginner can keep a model under control and steer the right way when it's flying towards them. Most people can only concentrate for 5-10 minutes at a time, so that's 10-20 flights (on a reasonably stable trainer, suitable weather and an instructor avoiding crashes). It then takes few months to a year before progressing on to an aileron model that isn't stable in roll - maybe 10 hours of flight time, say 100 flights of 5-10 minutes each.
Obviously if you don't have an instructor and are just doing short glides, those first couple of hours will translate to a lot of flights. If you're starting with an aileron plane (like a flying wing) you'll need to reach that 10 hour mark before you're really flying.
Older people take longer to learn, and missing a few weeks due to bad weather or other commitments puts them back a few steps. It's common for people to struggle through weeks of poor conditions then suddenly 'get it' when we have a nice calm day because they're not fighting the gusts.
I've taught a couple of people who practiced extensively on a simulator and flew a trainer competently on their first flight and an aerobatic model 5 minutes later.


Why is 5.8ghz used for FPV and 2.4ghz use for transmitters?

As is discussed in this Oscar Liang article, FPV hardware exists for all different kinds of frequency bands, stretching from 900 MHz to 5.8 GHz, with 5.8 GHz being the most popular today. People in our hobby have tried nearly everything under the sun. :)

Probably the most well-known tradeoff in RF is that between frequency and penetrating power. In general, higher frequency transmissions will not be able to travel as far and penetrate as many obstacles as lower frequencies.

This would make it sound like lower frequencies would always be better, but this is not the case. Especially in the case of FPV video, available bandwidth and interference issues become more of a concern than pure range.

With increased frequency comes higher bandwidth and reduced latency, which is to say that the 5.8 GHz band accessible to us can support more data throughput at a lower latency than the 2.4 GHz band. This is critical because video transmission requires several orders of magnitude more data to be sent than RC controls and telemetry do because it sends entire pictures instead of a few numbers per transmission.

The Troubles With 2.4 GHz FPV Radios

The need for more bandwidth in the 5.8 GHz range is further compounded by the fact that our analog FPV systems usually aren't frequency-hopping. In the old days of the RC hobby when airplanes were the only option, most RC transmitter/receiver systems had a fixed operating frequency. I have a couple of these transmitters, which actually have a slot where you need to insert a special crystal oscillator to select which frequency the transmitter uses!

This became a problem once people started coming together to fly and the RC hobby became more widespread. All of a sudden, people would begin turning on their transmitters at the flying field and their transmitter would talk over other people's transmitters using the same frequency, blasting them out of the sky and interrupting the controls to their plane. (does this sound familiar? XD) This issue was worsened by the fact that the 2.4 GHz band we are allowed to use doesn't allow for very many non-overlapping channels and because the 2.4 GHz band is used by many other different kinds of devices and appliances which can also step on RC transmitters' signals.

The solution to this problem, which has been implemented in the vast majority of all 2.4 GHz RC systems, is to implement frequency-hopping. This feature allows the RC system to dynamically switch between frequencies on the fly as needed, without any input required from the pilot.

These same issues caused by transmitters stomping on each others' signals are also a massive issue in the 5.8 GHz band, but to a far lesser extent due to the vast array of different non-overlapping channels available for use.

 (cit.)


How does a propeller with more blades create more "grip"

Aside from fancy material science which can result in propellers that interact with air more cleanly and is highly proprietary, the primary way a propeller could gain more "grip" on the air is by being more aggressively pitched or having more blades.

In a propeller spec which can be displayed either as:

[diameter in tenths of an inch][pitch in tenths of an inch], e.g. 5042
[diameter in inches]x[pitch in inches]x[number of blades per prop], e.g. 5x4.2x3

the pitch of the propeller theoretically represents how far forward the propeller would move over the course of one rotation if it were curring through a solid material. More aggressively (higher) pitched propellers can be said to "grip" the air more than less aggressive pitches because they will go further per rotation.

The same is true for high blade count propellers, which will also go further per rotation due to the fact that there's more surface area for air to exert a reaction force against.

But I think that the idea that propellers need to trade increased pitch and increased blade count for rotation speed is technically wrong. Just like how saying that higher kV motors require lower-voltage batteries isn't technically correct but in practice holds true, I believe the same holds true for propellers.

It is true that aggressively pitched and higher blade count propellers require more powerful motors to drive because they do more work per rotation. This is the reason why in practice these propellers are usually paired with lower-kV motors which spin slower per volt because they require less rotational velocity to do the same amount of work.

It should be noted that some (well-designed) propellers mitigate the increased work required to spin propellers with more than three blades by decreasing the chord length of each blade, thus decreasing the area of the blade and the work required to rotate it. This is where reading the spec sheet of the propellers you want to use and taking a careful look at the aerofoil geometry is key. :)

 (cit.)

However, nothing would stop you from pairing a high-voltage battery with a high-kV motor and an aggressively pitched propeller with many blades. The setup would, of course, consume ungodly amounts of power and require some pretty special materials to ensure the propeller doesn't shatter from the intense forces it would feel at those high speeds, but it could technically be done and there isn't anything wrong with doing so.


How much does mounting an antenna near a carbon fiber frame degrade signal reception?

This is a huge question, which is why a complete answer is not really possible without specifics on your drone frame, antennas, antenna placement etc.

Long story short - Carbon Fibre is pretty much opaque to 2.4GHz radio waves. It's not interference, as such, it just blocks it, which is why guidance is always given to ensure your antenna placement is such that the frame is not between them and you in normal flight.

For a camera platform drone, like an Inspire, you can be confident of the orientation of the drone so the antennas will always have line of sight to you, but for a racing drone you need to make decisions based on "least obscured" placement, as well as robustness, which is why they are typically in the plane of the frame (but not attached along frame members) or well above the frame.

Pic from getfpv.com showing antenna in the plane of the frame:

And a pic of one of mine with a mushroom antenna raised above the frame:


Is Psychiatric History an Obstacle to Obtaining a Part 107 Certificate?

There are no specific medical restrictions placed on drone pilots in the US, however the pilot should not have reason to believe their state of health (physical and/or mental) will affect the flight:

§ 107.17 Medical condition.
No person may manipulate the flight controls of a small unmanned
aircraft system or act as a remote pilot in command, visual observer,
or direct participant in the operation of the small unmanned aircraft
if he or she knows or has reason to know that he or she has a physical
or mental condition that would interfere with the safe operation of
the small unmanned aircraft system.

Source: eCFR 14.I.F.107.B.17


Difference between analog and digital FPV systems?

tl; dr:

Analog is:

Cheaper;
Lower resolution;
Has a noisier, more staticky picture that breaks up gradually when the signal gets weaker. Analog image breakup, while unpleasant to the eye, is generally easier to see through.
Has a constant low latency;
Is an entirely open standard, with anyone free to produce compatible equipment

Digital is:

Currently rather expensive,
Has (significantly) higher resolution,
Has a way cleaner image, usually without a speck of static right up to the very limit of its range, but when it starts breaking up, it does so suddenly and in weird ways. The broken image is usually harder to see through than an analog signal. With DJI, for instance, the picture will suddenly get very blocky and low-res when it gets out of range.
Also, digital systems can have inconsistent latency that changes in flight (DJI in particular is known for this), although this mostly concerns the highest-performance pilots, such as racers.
The only digital system currently in widespread use is the DJI system, which is a proprietary standard with only two manufacturers authorized to make the gear.

Analog and digital systems have about the same usable range per unit of VTX power.

To compare how they look, it's better to just see the videos:

DJI HD system range & breakup vs analog
ByteFrost image & breakup

How they work and the reasons for noise/breakup related differences:

Anything that is encoded in a radio signal at a point in time can be represented as a level, which can be higher or lower; Thus the "primary" information encoded in the signal can be represented as a graph of this level over time:

In an analog system that level can take any value, within some range, which we'll think of as 0 to 100% for simplicity(*). It can be 20%, 70% or 48.573498%, theoretically to any degree of accuracy. The drawback to that is that any noise that's floating around in the radio spectrum will add to whatever signal you're receiving, so in practice, you'll get something like 40% (true signal) + 5% (noise) = 45% (received signal), and since the original signal could be anything, the receiver has no way of knowing which part of what it got is noise.

The image that your goggles receive is made up of pixels, and the color that each of them has is encoded as percentages of red, green and blue: zero to 100%. An analog TV system encodes this picture by "scanning" over all of the pixels in sequence, line by line, and setting the signal's level equal to each pixel's brightness in turn. For example, this oscilloscope picture represents the TV signal for the "colored bars" picture:

The ladder you see represents how the brightness level of the pixels changes from left to right in each line of the image.
There are also other signals overlaid on top of it, these correspond to the lines closer to the bottom of the image, which have other sequences of colors in them (the oscilloscope displays all the lines together on one screen).

The consequence of all that is that whenever there's another signal on your frequency around, it gets added to your VTX's signal and through that directly to your picture. Since usually that other signal is noise(**), your picture is overlaid with noise.
When your signal is strong, the noise is a very small percentage of it and is usually unnoticeable by the eye. As the useful signal gets weaker, though, the noise (which stays the same) will get stronger compared to it, so the percentage of noise in your picture increases. At some point, it will become so strong that you couldn't see any useful signal in the picture anymore.

In a digital system, however, only two levels are available: 0 and 1, represented by 0% or 100%(***). The image is encoded into a sequence of bits using a special algorithm, called a codec, and decoded from that sequence on the other end, almost exactly like online video. Besides simply encoding the video feed, one of the most important functions of modern codecs is to compress it, so that it can be communicated in a significantly smaller amount of bits, which in turn means that higher-resolution video can be transmitted with the same bitrate (number of bits sent per second).

Since radio signals are by their nature analog, on the receiving end noise will still be added to the signal, so if a 0 was transmitted and 10% of noise is added by the environment, the received result is 10% — but since the receiver knows that only either 0 or 100% could have been transmitted, it picks the closest possible level, in this case, 0.

This means that when you transmit digital data, noise is just ignored until a certain point: Even when noise levels are as high as half of the analog signal (which creates a picture that's rather hard to parse), with digital you receive exactly the same information as you would if there were no noise, with the resulting video perfectly clear. However, when the relative noise level does pass the critical threshold, ones will start suddenly (and randomly) becoming zeroes and vice-versa. What happens to the video after that depends on the particular digital system and how the video is encoded in it.

With the DJI system, the whole screen suddenly becomes low-res and blocky, occasionally skipping frames, and the latency increases. The cumulative effect of this can range from "just an unpleasant-looking image" to "totally unflyable".

With Fatshark's Bytefrost system (currently in development/beta testing), random bits of the image will flash with colored blocks, while the rest remains high-resolution. The manufacturer touts this "analog-like" style of image breakup as a feature of the system.

In both cases, digital breakup, when it does happen, is arguably harder to see through than analog breakup, because our brain still has better error correction software than seen in these systems. This is also due to the fact that digital images, while crystal clear up to a point, start breaking up sharply after that point and quickly get to "very hard to parse" levels, while an analog picture is already quite static-ridden by then and just continues to gradually get worse.

That said, digital systems can employ error-correcting codes and other tricks to be able to recover from a certain percentage of errors, at the expense of reduced bitrates, yielding a lower-quality image which can be decoded from a longer distance.  you'll typically see them start to gradually reduce image quality before breaking down completely, giving you some warning signs that you are going out of range. There is also usually a link health indicator somewhere in the corner that is similar to the RSSI reading for RC control systems which you could look at to see if you're going out of range. You probably aren't going to be looking at it all the time, though, and a low signal indication can be missed, while it's hard to miss an image full of static and getting worse.

Footnotes:

(*): The analog signal can technically be any real number, and can sometimes go beyond its designated range, i.e. in our example be less than 0 or bigger than 100%. How the system handles the signal going out of its designated boundaries is the system's business.
(**): the other common kind of interfering signal is your own signal that's reflected off of something. In which case you will get two of the same picture overlaid on top of each other, but one of them which is slightly late and is shifted to the right because of that. This is called multipath interference, or multipathing, and is a major source of image degradation with analog FPV systems. Cutting down on multipath signals is also the primary reason why circular-polarized antennae are used in FPV..
(***): Actually, a digital system can have more than two values, and there are different ways of representing them as analog levels; the main distinguishing factor is that there's a fixed (integer) number of them. For example, for a simple digital system with 4 levels, they would be 0, 33%, 66%, and 100%, with corresponding ranges of -∞ to 16%, 16-50%, 50-83%, and 83%-∞.


Determing the frame's hole mount length, for the right mount size motors

This blog post by Oscar Liang has a table identifying the type of motor that is used for each frame on 4S (other voltages would only affect the KV). Here is a screenshot of it:

And yes, 22XX means any motor with a stator diameter of 22mm and with any height, as it is the stator diameter that will determine the mounting pattern.


How to Simultaneously View and Record GoPro Content While Flying

I don't have any experience with that API, but I read through the documentation so I will try to give the best answer I can based on this and the comments below the original question.
I would recommend using an either an Arduino or Raspberry PI pico. If you do that you can use the arduino library based on the goprowifihack API (Arduino libraries works for the Pi pico).
The only downside I see to this is that it does not support live streaming, so you would need a separate solution for that. The best solution here would probably be to mount an FPV camera in the same direction as the GoPro and then use a regular video transmitter. You could also use the HDMI output of the GoPro I think, but I don't know of a way to transmit that video without adding considerable weight.
To control the Arduino (or any other microcontroller of your choosing), the same rx reciever that you already have can be used. I don't want to comment on exactly how to do this as I don't feel qualified, but there are libraries for both SBUS (You can split the cable to go to both your flight controller and the Arduino, you just have to know which channels should do what) and PPM for example.
If you then for example set up your transmitter so that a switch corresponds to channel 8 and then program the Arduino to activate video recording, the switch on the transmitter will work as a start button for video. You could also set it up so that video is on when the switch is in one position, and that it turns off in the other direction. Repeat this for all the functions you need and it will be as close to a DJI drone as possible. As a bonus it will be a lot more flexible than a DJI drone.
This shouldn't add too much weight if you use a lightweight microcontroller (just remember that you need WiFi as well, either built in or a separate board).


The problem you have found is that you are trying to calculate the efficiency and lift coefficient of your fan, which isn't possible. There's no simple calculation to convert power to thrust because it will depend on the angle of attack of the blades to the airstream.
The mass flow rate is calculated on the velocity of air through the fan, not the motor speed.
The 'Free Stream' velocity is the air speed of the aircraft, or zero for a static test rig. You could measure the exhaust velocity with a pitot tube - a simple length of clear tube with some water held in a U-bend would work to compare static and dynamic pressure. However if you're measuring things, you might as well measure the thrust directly.
The alternative is to look up the thrust figures for commercially available EDFs, and scale them to allow for any difference in power (I'd guess that a 3D printed fan can't run as fast as a commercial unit)
Having said that, it must be possible to take the normal lift equation (1/2 * lift coefficient * air density * area * air speed squared) and treat the fan blades as wings and integrate for the change of speed with radius. I guess the problem is that you could estimate the maximum lift-coefficient to 1 but it could be 30% higher or lower, and you probably want a more accurate answer.
If you look on-line, you'll find a number of power-to-thrust calculators for RC propeller aircraft, but they all rely on measured values for specific propellers. I've not seen one that includes values for EDFs, presumably because EDF thrust is highly dependant on inlet and exhaust shape.


Cannot configure ESC: BLHeli-Configuator connects but with blank screen

Try using BLHeliSuite32 instead, since it is a BLHeli_32. I don't think BLHeli configurator works with 32 bit ESC:s, correct me if I'm wrong.

If that does not work, try having Betaflight configurator open WITHOUT connecting. This has helped me sometimes.

If it still does not work, you might need to connect to the ESC without using passthrough (if possible) and try to reflash it with a newer BLHeli_32 version.


First radio transmitter. Taranis X-Lite, X9 Lite or Jumper T8GS?

At this point I would recommend the radiomaster TX16S. It goes for about the same price as the other radios you list but has everything you need to get you going for a very long time.
FrSky (Taranis) is doing everything it can these days to lock you into their ecosystem and you need to start hacking once you are further in the hobby and want to switch over to crossfire for example.
With the TX16S you can use almost all receivers that are on the market.
Whatever radio you pick eventually, just make sure it runs openTX. Almost all instruction videos you will find online are based on openTX.


When looking for camera drones, there are several things to look out for:

GPS is a big one. It allows the quad to hold its position so you can line up the shot perfectly. It also allows auto-return functions and flight tracking.

A 3 axis gimbal is also important. You can get away with 2 axis, but three is much smoother. This allows you to angle the camera properly and holds it steady so you can get the perfect shot. It is also very useful for filming as it removes the actions of pitch and roll, and reduces yaw.

Large prop are helpful as a larger wheelbase generally means more stability.

Not that it needs saying, but brushless motors are a must.

And FPV feed to a phone or other monitor is essential to line up your shots, and also relays useful information about the state of the aircraft.

For filming, you’ll want a neutral density or ND filter in bright conditions to make the footage smoother and add motion blur (not essential but can make footage look better).

Finally, you’d obviously want excellent battery life (at least 25 minutes) to give you time to set up and get the shot you want.


What are controllers of Parrot AR drone 2?

After reading relevant materials, i think I have find the answers for this question.
Q1:Since AR drone2 is a proprietary system,  this company didn't publish any documentation about the internal structure.
Q2:According to my understanding and guess, the controllers will use the estimated states of AR drone2 and the estimated states may from EKF(I am not 100% sure)
Q3:Since AR drone2 is a proprietary system, it seems there is no way you can control the motors directly by using your own controllers. But there is an alternative way:
using Simulink and Optitrack to control AR Drone
using Simulink and Vicon to control AR Drone


Try lowering your $D$ coefficients by $10$ in each direction. If that doesn’t help, slowly try increasing your $I$ coefficients.
[Make sure you first implement (and TEST!) a kill switch any time you are modifying PID terms (particularly $I$ terms) as you can create an unrecoverable flyaway situation!]


Is it possible to enable pit mode without using the button with SmartAudio?

I believe you can do it w/scripting on FR Sky and other OpenTX system.  They are a good bunch of people: https://www.open-tx.org/


Drone in Italy as a tourist - Where/what to do?

So far as I can tell (as of 30th September 2020) Italian drone regulations are available first-hand here - but only in Italian; as I am not an Italian speaker you will need to locate a reputable translator (I have browsed briefly with Google Translate, but due to the risks of mistranslation will not copy it here.)
The rules across Europe have largely been harmonised under EASA, so you are likely to find them to be quite similar to your home (German) rules - and so unfortunately flying close to a busy tourist location is likely to be difficult for a recreational flyer.


How to know if a place is a no-drone zone in the UK?

There are two considerations on whether you can fly somewhere - airspace restrictions and land restrictions.

Details on the airspace can be acquired from a number of common sources - the CAA has a page with lots of in-depth information here1, but there are a lot of third-party websites and applications which present information in a more accessible format (for example, I personally use Drone Safety Map and the NATS Drone Assist app.) Remember to check for NOTAMS, although many websites and apps provide this functionality too.

Land restrictions are not so neatly centralised, so you will need to manually investigate whether your local authority or playing field owners have any restrictions for flying drones from their land. As drone use has increased in recent years, larger numbers of public spaces are making this information available on websites or signs but in many cases, you will need to find out who is the owner and contact them directly for permission.

[1] Wayback machine link.