Crossfire, Unify VTX settings are being overwritten I have a crossfire nano hooked up properly to a unify pro32 HV.  Everything works as expected and perfectly except when I use the Crossfire Lite OSD to change the VTX settings.

When I change the VTX settings, such as channel, I can see the signal switching in the goggles (goes from perfect image to static) and I can even change the channel in the goggles to the new channel being blasted from the VTX and I can see the video.  But about 5-10 seconds later, the VTX settings automatically change back to the initial settings.  In the example above, the channel reverts back to the initial channel and I can change my goggles back to that channel to see it again.

I've tried hooking up to the TBS agent to switch the channels and that works as well, but again, it reverts back in 5-10 seconds.

Here's a video to show what's going on: https://photos.app.goo.gl/p6Dt7A11EWjBS3Vz9

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


AliExpress drone vs DJI Pro 2 I can buy a drone in US or Canada like the DJI Pro 2 for over 1000$. But when I search professional drones on Ali they advertise many that share many features such as 4K video, dual camera, 20minute flying time. The one main thing I don’t see on many drones on Ali is the long range ability which DJI has. Is that really what I’m paying for with DJI, the range? It would be great if someone here has bought a drone from Ali and can lend some advice. And if you know of any decent Ali brands of drones please recommend.

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


Isn’t flying FPV drones illegal accoding to FAA rules? I understand there is First Person View (FPV) drone flying which involves putting on a headset and controlling the drone from the live camera feed,

The FAA Rules For Unmanned Aircraft Part 107 state:

  Visual line-of-sight (VLOS) only; the unmanned aircraft must remain within VLOS of the remote pilot in command and the
  person manipulating the flight controls of the small UAS.

If that is true, then surely FPV flying is illegal as you don’t maintain a visual line of sight with the aircraft?

FPV is not actually illegal because that document also states:

  Part 107 does not apply to model aircraft that satisfy all of
  the criteria specified in section 336 of Public Law 112-95.

and the criteria specified in section 336 of Public Law 112-95 is:

  (1) the aircraft is flown strictly for hobby or recreational use;

  (2) the aircraft is operated in accordance with a community-based set
  of safety guidelines and within the programming of a nationwide
  community-based organization;

  (3) the aircraft is limited to not more than 55 pounds unless
  otherwise certified through a design, construction, inspection, flight
  test, and operational safety program administered by a community-based
  organization;

  (4) the aircraft is operated in a manner that does not interfere with
  and gives way to any manned aircraft; and

  (5) when flown within 5 miles of an airport, the operator of the
  aircraft provides the airport operator and the airport air traffic
  control tower (when an air traffic facility is located at the airport)
  with prior notice of the operation (model aircraft operators flying
  from a permanent location within 5 miles of an airport should
  establish a mutually-agreed upon operating procedure with the airport
  operator and the airport air traffic control tower (when an air
  traffic facility is located at the airport)).

This is essentially saying that if it is a recreational drone, follows community guidelines, is not more than 55lbs, doesn't interfere with manned aircraft, and notice is given to airports before flying within 5 miles of them, that FPV is perfectly Legal


How to straighten a motor shaft? I have a drone with a motor shaft that got slightly bent. I can't buy a replacement bell because they only sell the motor as a whole. Is there any way to bend the motor shaft back into position?

This will be very hard to do.

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


DJI Mini 2 Mapping Capabilities I planned to buy my first drone to ease my work. Recently, I found the DJI Mini 2, which I can afford to buy. I've already done some research about drone mapping. Based on my findings on the internet, I could use this technique since it does not require me to install some APK on an android phone. The link is below:
DJI mini mapping tutorial
So, I want to use this technique for my first drone. My question is, can I do this with DJI Mini 2?
Thanks

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 LiPo batteries get puffy? I have used many liPo batteries with my drones and other RCs over the years and have had a few of them puff up. What are the main reasons that LiPo batteries puff up? And are there ways to avoid LiPo batteries getting like this?

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

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

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

More in-depth information here.


Can "toy" drone FC firmware be tweaked? Can the flight controller of cheap "toy" grade quads, such as the Blade Inductrix, be programmed at all? I'd assume that even if it's possible, it wouldn't be practical. I don't have any experience with FC programming (yet).

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.


ESC power up beeps sound twice: du-du-du...du-du-du beep-beep Just recently, my power-up beep sounds have changed. The normal sequence is just du-du-du beep-beep for ESC power, then throttle signal, right?
Now I get du-du-du, pause, du-du-du (again) beep-beep.
What could this be indicative of? This is an NBD BeeBrain BL AIO FC/ESC running BF4.1 and JazzMaverick 16.77 48k.
This changed without any change in config or setup.
I might have crashed it a bit harder than usual?

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

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

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

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

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

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


Which battery do I need for my drone? I am totally new to the drone space and I recently acquired a hand-me-down drone without a battery.
It has a XT60 male connector type with 4 motors (DL2205 KV=2300).

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

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


What is the established drone racing etiquette? As I understand it, at drone races there are lots of rules to follow pre and post flight, an obvious one that jumps to mind is to shout ‘plugging in’ before plugging in.

What other rules and best practices are there to make sure races operate smoothly?

I understand that different clubs may have slightly different rules, but as long as there is a consensus on the general kind of rules there are, I’d appreciate it. Thanks!

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

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

And some more rules we incorporate:

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


"Rogallo wing" balsa rubber-band model plan - do you have one? Several decades ago - 4 or 5 at least - I saw a plan in a model aircraft magazine for a very simple rubber-band driven plane that used a piece of a dry cleaner bag (thin plastic wrap style) as a Rogallo wing.  Wingspan was maybe 2 feet.  Wing spars were simple 3/16" (approx balsa sticks), the body was simple balsa too.  Flew great - I built several back then!

Now, I'd like to experiment with that again and would really like to have some tips - the original plans would be great if someone has a stash or online scan of it in whichever model aircraft magazine it was ... but any information on model-scale Rogallo wings would be useful to get me started.

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


Is voltage sag safe if the voltage isn't too low? I know that a liPo cell should never be below 3.0 volts, but can voltage sag during high throttle maneuvers damage the battery excessively even if the voltage it not below that level?

And if that is the case; should you try to limit how much the voltage sags by getting a battery with a higher C-rating, or will the difference be negligible if you go from, for example, 75 C to 100 C on a 1500 mAh battery.

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

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


Why do brushless motors have 3 wires compared to 2 wires on a brushed motor? I'm familiar with regular DC motors, which is what brushed motors appear to be. They have fixed stator magnets, rotor coils connected to the shaft and two brushes that power the coils at the right time via a commutator. By varying the amount of power via the two leads connected to the brushes I can vary the speed the motor spins at (more or less).

Yet brushless motors typically have 3 input wires. I can see an arrangement of coils and magnets still, but why does it need 3 wires and a special ESC to drive it?

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 sort of license would I need to fly a small drone (e.g. DJI Spark) in the Israeli Golan? I live in the Israeli Golan. There are a bunch of military bases around, and regulations about drones - since, naturally, you don't want to get your drone shot down or cause a national security incident.

I'm wondering about a small drone - specifically, the DJI Spark drone. It's 143×143×55mm in size, and weighs 300g.

What sort of license would I need to fly this drone in the Israeli Golan?

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

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

  Do not:

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

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


How common is it for drone operators to actually follow all the regulations? If you look at the list of rules for drone operators, it seems like the hobby is getting harder and harder to enjoy. Registration requirements, ID broadcasting, bans on flying over buildings of any kind, bans on flying over national forests/parks, professional licensing requirements, etc. But in practice, how common is it for people to actually follow all these rules for casual drone operations? Are there any surveys available that measure the compliance rate?

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...


What does it mean for SBUS to be an inverted protocol? I've heard talk about how F4-based flight controllers can't natively support the SBUS receiver protocol without special circuitry. What does this mean and what hardware is needed to invert an inverted protocol like SBUS?

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

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

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


Kk2.1.6 falling in mid air and losing control I am recently flying my DIY quadcopter built with lightweight aluminum bars and a kk2.1.6.

It was a very good experience until it suddenly started losing control and it fell midair. It tried to gain control as just before hitting the ground and it rose up again for a brief moment before falling again. I also heard vibrations from loose nuts. What might be the reason for this odd behavior?

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

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


Why do brushless drone motors tend to be more expensive than brushed? The most common motors used for modern drones are brushless outrunner motors and I have heard that they are simpler to manufacture and are less susceptible to issues when being mass-produced than brushed motors because the only parts of the brushless outrunner that rub together are in the bearings (they don't require a brush to be at just the right spot or pressure or anything). If this is the case, why do brushless motors tend to be more expensive than brushed motors?

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

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


How can I know the laws of a specific country regarding drones? Reworded from: https://travel.stackexchange.com/q/112261/

If travelling to a few countries, I was wondering if there is one source for all the drone rules/laws of the countries I am travelling to instead of doing a specific search for each country.

Does such a source exist?

There are a large number of websites which collate drone rules in different countries; to list a few:

Dronegenuity
UAV Coach
Drone Rush
UAV Systems International

However, due to the rapidly evolving nature of drone rules these could quickly become out of date - in fact, at time of writing, some of the sites already contain old (now incorrect) information. The safest option is to check the website of the country's aviation regulator before you travel.


Putting decorations onto a drone? I want to jazz my quadcopter up a bit, maybe put some stickers or little sequins.

Will these affect the flying capability of the drone, even though they aren’t that heavy?
Will this affect balance?

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

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

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


What is circular polarization in antennae? I've seen references to left-hand (LHCP) and right-hand circular polarization (RHCP) with respect to antennae, especially in reference to mushroom antennae. What's the difference between the two polarizations and what does it mean for an antenna to be circularly polarized?

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.


Do drones with inflight API access for control inputs exist? I am a software engineer brand new to drones.

I want a drone where I have full access to all of the flight controls programmatically. For example, I can run a python script and once executed, it sends a command to the drone and it moves up or something like that.

I also want to be able to do this outside and not require something like a wifi signal.

I think the cheap 100 dollar DJI Ryze Tello sounds good, but I can't tell from the demos how much I actually have access to the drone programmatically or if DJI is a good option to start with. Any suggestions would be great.

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

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

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

Serial interface

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

MSP

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

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

Ground control software

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

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

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


How does having bent antenna impact reception? I have heard that when an antenna is bent, it can reduce the performance of the antenna and thus limit the range of a drone. This is mentioned in this video at 5:29. I am wondering why a bend in an antenna would cause this loss of RF signal? And how significantly would having slightly bent antennas on an XM+ receiver actually impact the range of a drone?

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

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

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


How can I take aerial photography without leaving my house? I have found a place I can fly my drone, and want to take some photos of it. At the moment it's ill-advised to leave one's home unless it's necessary, and aerial photography of abandoned places is not necessary, so I want to try to do this by releasing piloting my drone from the doorstep.

Assume that this place is within battery and radio distance of my house, and can be reached by a public footpath that joins a road that reaches my house. How can I get the drone there and back with minimal inconvenience (and risk) to myself and others, and under what conditions?

(Creative solutions are encouraged; e.g., some safe way to get a drone to count as a vehicle suitable for road travel.)

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

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

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

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

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

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

(Emphesis mine)

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


What sort of fuel do model jet plane engines take? I've seen several gas powered model jet airplanes at air shows. Do these smaller engines take the same type of fuel as their full-size counterparts, or do they just use 2-cycle gas like their propeller brethren?

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 aviation regulations govern the use of drones in Ireland? I am thinking about starting to fly drones in Ireland.  What are the rules that are going to apply, and where can I find the latest information about the laws that govern the hobby in Ireland.

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

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

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

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

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

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


What is Commutation Enhancement in BLHeli_M and what should i set it to? In the BLHeli_M version of the BLHeli Configurator, theres an Option for Commutation Enhancement. By default its set to Async-PWM but it can be set from 3%-CE to 100%-CE. What does this Parameter do and what should i set it to on a Mobula 6?

The BLHeli_M Commutation Enhancement feature is intended to increase motor power response times by "overdriving" (my words, may not be the most appropriate analogy) the coils to charge/discharge them more quickly.

Source: (rgroups.om)

Commutation is a job to charge two of three coils, A B and C, sequentially.
Enhancement is an attempt to make the charge time shorter.
Sync-PWM try to apply power at the very right moment without delay.
Enhancement try to make the charge speed faster.

Source: (rcgroups.com)

Commutation Enhancement(CE)
Use more power when charging the coils to speed up settling time of motor current. RPM will be higher with the same throttle.
Many request for normal RPM after disarm. So CE will be off right after startup. Then is on after a certain dshot transistion detected. After that, CE will be off after arming(dshot=0).

As for advice on whether or not to enable it on your Mobula 6, I'm not quite sure. From reading the development descriptions written by JazzMaverick (the primary maintainer of BLHeli_M), it appears that the feature was built to "accelerate" motors/ESCs in the 5" class, although I do see a couple mentions of applications for whoop-sized drones in the RCGroups thread.
I'd try leaving the defaults as-is and then coming back to investigate your options for changing the commutation enhancement setting later on if you feel like the quad's motor performance is lacking. Granted, I've never used the feature before, so take my recommendation here with a grain of salt. :)


Li-ion battery will not charge on ISDT Q8 Smart Charger The Li-ion Pack has 4 18650 lithium-ion batteries with a soldered Balancer and XT60.
On my ISDT charger, the read out voltage for each battery cell is:

3.17V
3.18V
3.29V
2.93V

Whenever I try to charge the Li-ion Pack, I get:

"This operation is not supported"

My Question:
How can I bypass this using my ISDT charger?
Auline 14.8V 4S 18650 2600mAh
ISDT Q8

Note: This answer does not consider whether this is a 'good idea', as there can be safety concerns with over-discharged lithium batteries (e.g. see this question.)
I aim here to address the 'how' of the question, but you must decide if you consider the risks acceptable (for this battery, or others in the future) and follow all the usual charging safety measures.
It looks like the charger in question supports unbalanced charging, so you may be able to set the battery to charge in unbalanced mode which should ignore the per-cell voltage. In this case, I would set the charge current as low as possible and charge somewhere very fireproof - in case the low cell voltage has caused damage - and plan to stop the charge once the faulty cell has exceeded about 3.1V. You should then be able to try balance charging again.
However, the charger may check the sum of the individual cell voltages for the unbalanced charging mode and could therefore still decide that the total voltage is too low.


Is there an open source video processing/sensor based follow me technology? I'm constructing a multirotor drone and want to introduce follow me feature, are there any open source solutions I can use for that purpose?
Please note that I want the tracking algorithm to be autonomous based on some video processing and sensors.

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

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

MAVLink's Follow Me plugin

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


DJI FPV Air Unit mistakenly declares "slow card" Getting a slow sd card in my DJI goggles screen indicating the message for the air unit.

Using a 64gb SanDisk Extreme Pro Card.

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


What happens when a multirotor runs out of battery mid-air? If I am flying a multirotor drone and it runs out of battery mid-air, what will happen to it? Will it just fall down from the sky or will it give me a low-battery warning?

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

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

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

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


Report a lost drone in the UK? I recently lost my drone in the UK, it is quite big and sure someone must have picked it up.

Is there any way to try and recover it, an online lost drone database? What steps can be taken in the future so if my drone is lost it can be returned to me?

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

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

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

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

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

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

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


How do laterally folding propellers affect multirotor flight? There are a number of multirotors on the market which have propellers which fold at the hub, primarily to make storage and transport easier. Centrifugal force keeps the blade extended when the motor is spinning.

However, there is still a flexible coupling between the blade and the hub; what effect does this have on the aircraft during flight?

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

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

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


How do I change GPS elements on the Betaflight OSD for my Acrobee Lite? Right now I'm on Betaflight trying to change the OSD for my Acrobee Lite and it's being really weird, I can't seem to get more than three little thingies to be shown at once. Is there some sort of secret code I need or something? This is really confusing. I'm trying to have the altitude, artificial horizon, battery usage, compass bar, crosshair, GPS speed, home direction, home distance, RSSI value, and warnings shown.
I got everything laid out on Betaflight, but when I clicked save and then checked it on my drone it wasn't right. Currently, I have the horizon, crosshair, compass bar, and altitude showing but I don't know how to get the rest on. Sometimes when I change a box something will appear and another disappears.
How do I change my OSD?

EDIT: apparently some of the elements were just getting cut off the edge of the screen so I moved everything to the center and a few more appeared but I still cant get home distance/direction and speed.
EDIT2: I have the following elements on the OSD; compass bar, battery usage, crosshair, altitude, rssi value, and I think warnings. I want to add the gps speed, home direction and distance but am unsure how. Every element I have enabled right now has boxes 2 and 3 checked, but I can't get the gps speed and home direction/distance to show up. What do I need to change?
EDIT3: acro bee lite apparently does not have gps so problem solved i guess.

Turns out the AcroBee Lite does not have gps functions so adding gps functions to the osd is impossible.


Optical sensors vs Ultrasonic sensors on a drone Are there any performance differences between optical sensors and ultrasonic sensors? How do I make the right choice for my drone? I will be using them in a object detection system for collision prevention.

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

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

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

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


FrSky receiver snap-on connection problem The snap connector thingy (picture included) on the xm+ receiver (although it is the same with nearly every other receiver) I have stupidly disfigured, as well as the antenna end. Is it possible to fix this or solder the wires on directly? Do you have to get a new receiver because of some tiny connector which has been slightly (ok, fine, quite a lot) bent?

P.S.: Yes, I know this was very stupid and I will not make this mistake again!

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

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

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

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

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


Is it possible to enable pit mode without using the button with SmartAudio? I know that pit mode can't be enabled from Betaflight after power up with SmartAudio, but I think it is a bit annoying that you have to hold the button on the VTX during power up if you want to enable pit mode.
I am fine with pit mode being on every time I plug in the battery, since it can be disabled with a switch. My question is therefore if it is possible to have the VTX, specifically the TBS Unify pro v3, turn on with pit mode already enabled. I had the race variant before, and that one could do it. I just haven't found a way to do it on the v3.

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/


Should Crossfire transmitter antenna orientation match receiver antenna orientation? I've always heard that you should orient the antenna on the Crossfire transmitter vertically, with the ends pointing up and down. I've also heard that you should do the same with the receiver antenna (immortal T), but on many builds that is difficult or less durable, so I mount the antenna horizontally on the drone (under an arm, for example).

For the best signal when flying a drone that has a horizontally mounted Crossfire antenna, should I also rotate the transmitter antenna horizontal to match, or is it better to keep it vertical regardless?

It would be great to understand the logic either way as well. Thanks!

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

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

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

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

[1] The main exceptions probably being stunt aircraft


What makes mushroom antennae so good at receiving and transmitting omnidirectionally? I've seen many references saying that mushroom antennae (like the one shown below) are excellent at receiving and transmitting omnidirectionally. I have them on my drones' video transmitters and one paired with a patch antenna on my FPV goggles.

What makes them so good at this?

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


What are the usual engineering considerations for a UAV current sensor? Current sensors are a popular feature in both multicopter and fixed wing flight control stacks.

Most of them are analog, i.e. convert the current into a voltage using a small-value shunt resistor (and possibly a voltage amplifier). That voltage is then read by the flight controller via an ADC pin. I've been trying to make my own, and in such cases it is useful to know the best practices in the industry, however my Google-Fu was not strong enough to find those, and reverse-engineering is a rather tedious proposition, since I only have an ESC-integrated one.

The main questions I couldn't find answers to are:

Is the shunt resistor usually mounted high-side (i.e. at the positive battery terminal), or low-side (at the negative terminal), and why? Does it matter?
What class of amplifier, in terms of precision, is used for the sensed voltage?
What range of output voltages does the FC expect?
Is there any smoothing (lowpass filtering) applied to the sensor's output to reduce aliasing from possible high frequency current ripple?
If there is any difference between standard sensors used in betaflight FC stacks vs., e.g. APM or PX4 flight controllers, what are they?

If someone could provide a circuit schematic of a "typical" current sensor, it would be very much appreciated as well.

Clarification on the purpose of this question:

I'm asking because, while I've got my own ideas about each choice, I have no way of verifying whether I've considered everything in the decisions and the relative importance of all the factors involved (without building one first and seeing how things play out). Thus, I'd like to learn from field-tested commercial designs and see how they solved all these questions.

i.e. my thoughts on where to mount the shunt: Low side is simpler, but if done this way, ripple currents from ESCs may induce voltage ripple in the ground, which is worse than voltage ripple at VBAT, since everything in the drone is connected to it directly as a common terminal, while VBAT typically goes through a regulator before hitting sensitive elements. On the other hand, I may be overthinking the significance of this aspect and it might be fine either way; the best way to learn that would be to see how others do it; if they only mount the resistor high-side (or only do it low-side), then it must be significant, if they don't care I shouldn't care either.

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

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

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

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

What range of output voltages does the FC expect?

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

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

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


How to control a gimbal remotely? Is there a simple way to remotely control the panning of a gimbal (preferably with a separate stick/joystick, controlled by another person)? I'm planning to do a budget cinematography/utility camera drone built out of the ZD550 frame and I don't really need more than a GoPro. I'd probably get a smaller gimbal like this. It'd also be nice if I could adjust the amount of "inertia" the panning has, sensitivity, etc.
According to the description of this gimbal controller board, "Each axis could connect analog joystick." Not sure how I would make that remotely controllable though. Something out of Raspberry Pi? Separate tx/rx? Just throwing out ideas.

The easiest way would be to use the sliders or the rotary knobs on a reasonably advanced transmitter - although it might not feel natural and the pilot would be busy flying.
Alternately you could plug another transmitter in via the buddy box port, and map two channels from the sticks on the 'student' transmitter to a couple of spare channels on the 'master'.
Transmitters running OpenTx have quite powerful buddy box settings, you can choose to only take a couple of channels from the student, and to send the signals from channel 1 on the student to channel 7 on the master - neither of which are possible on low-end transmitters. They can also accept quite a wide range of student transmitters, so some mixing of brands is possible.
It's also possible to build a head-tracker, that uses accelerometers to detect the motion of your head, and outputs a signal that you can send to the buddy box port.
As you mentioned, a second transmitter and receiver could be used to send signals directly to the drone, without going through the buddy box port. This would allow you to use two low-end transmitters. The only disadvantage I can see is the slight extra weight of a second receiver.


Visual references when driving a Quad Copter through gates I am a complete beginner, starting to learn to fly. So far I managed to control the altitude and do some basic turns, I can also do figure eights if the flags are well far from each other.
I realised that a trick to go around flags and objects is to use them as a visual reference, what I mean with this is to have them at sight at every moment. This trick for me works with flags, trees and objects that are well apart from each other. But when I try going through gates I struggle a bit, I can go through gates that don't have sharp turns or where the gates are  well far away from each other.
What kind of visual reference do more experience pilots use when racing? Where do they look at to when turning and going through gates? I feel I am looking at the wrong places and that's why I keep crashing.

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


Does active braking help protect propellers from damage? As far as I know with active braking option enabled in BLHeli32 the speed controller will counteract the free spinning of the rotor according to throttle value hence making it slow down quicker when needed. Could this help protect propellers upon crash?

No matter how quickly you slow down the propellers after an impact with the ground, much of the damage actually comes from the impact directly into the ground and the force from the mass of the quad, so the speed of the props shouldn’t make too much of a difference.

If you mean the slowing down the props after it’s on the ground would save them, it may prevent further damage to the props, however the current draw if the props get caught in something could easily fry an ESC so it is likely more economical to sacrifice the props.

I generally go by the rule that if I crash, disarm as fast as possible. This stops me from burning ESCs as I’m usually disarmed before I hit the ground, though this comes with the negative that i can’t recover after a crash because I’ve disarmed.


First radio transmitter. Taranis X-Lite, X9 Lite or Jumper T8GS? I'm looking for a nice budget radio transmitter to start in the hooby and stay with it for some good time. I'm found these three good options but I'm tottaly newbie so I don't know the pros and cons of each one
I would like some radio that could make at lease abou 1-2 Km with no mods by default. And the most versatile about receivers.
I read the T8GS supports almost all protocols and it leads at this specification, but whay about the other ones?
I intend to some FPV

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.


Differences between fiberglass-reinforced plastic and polycarbonate propeller materials? What are the differences between drone props that are built from fiberglass-reinforced plastics and props made from polycarbonate?

The main differences are:

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


Is it feasible to use an RC car transmitter/receiver to control an RC plane? I am a beginner to the hobby and I've been constructing chuck gliders out of foam board and card for a few years. I'd like to start making models that I can control, but I don't have much of a budget. I do, however, have a hobby-grade RC car with a receiver/esc/servo combo and matching transmitter. I am planning on removing this unit and using it to control the one channel (pitch) of a foam board glider. Is this a feasible idea? What are some major problems I might come across?

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

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

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

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

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


Wiring a 4-in-1 ESC to an Arduino Uno I have four brushless motors that I want to control using an Arduino Nano. I was looking at getting a 4-in-1 ESC controller similar to this one on amazon.
At this point in my project, I'm more interested in toying around with components and understanding how to control them using an Arduino rather than having a fully functional drone. I was wondering if anyone had a good resource on how to communicate with and control ESCs using an Arduino. Is this something that can easily be done?

It seems pretty straightforward how to solder the actual brushless motors to the ESC, however I was mainly wondering how this 8-wire connector can be used to control them from an Arduino. If I directly connect the M1/M2/M3/M4 wires to pins on my Arduino, are there libraries which can be used to control the motors?

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


Are there any extremely small brushless drone motors? I have seen many tiny brushed drones, about an inch from motor to motor, and was wondering whether it would be possible to create a brushless drone that small. Are there any brushless motors available that are extremely small, ones that could fit on the size of drone below? And is there anything else to consider if making a drone so small?

(I realize that you could put oversized motors on a small drone but I am asking about motors that would be proportionally normal)

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.


Flight controller or ESCs on Eachine Viswhoop not booting properly After last flight my Eachine Viswhoop stopped booting properly and arming (link to gfycat with sound). When connecting battery to drone I can hear FC beeping but don't get second set of "beeps" from ESCs. Also red led on flight controller starts blinking. First thing I checked configuration in Betaflight and noticed that tab "Modes" was empty and didn't have ARM or beeper configured. Thankfully I had my settings backed up so I was able to restore them. But after connecting to battery once again I still get the same result as earlier. BLHeli Configurator also sees all 4 ESCs but I cannot spin them. So, what can be problem here?

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

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


Why is the noise in my accelerometer more pronounced than the noise in my gyro? I have a small quadcopter equipped with the MPU6050, which has a built-in accelerometer and gyroscope. It is able to sample the acceleration at 1 kHz and the angular velocities at 8 kHz. However, I only sample them at 200 Hz. I placed my drone indoors before a large fan to see how it responds, and the typical readings look like this:

These are raw signals. They have only been converted to the proper units. Why is the angular velocity so much smoother than the acceleration? The only guess I have is that the MPU6050 is averaging the measurements each time I sample: so for acceleration it can average 5 measurements (1000/200), while for the angular velocity it can average 40 measurements (8000/200).

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


Getting Started with Aerial Photography There are a large number of camera drones on the market covering a vast range of cost, from hundreds to thousands of £/€/$/etc.

What are the things to look out for in a photography drone for someone looking to try amateur drone photography?

Please focus on the drone specific elements, such as camera gimbals, stabilisation, etc.
Discussion on cameras is welcome and relevant, but when going into detail consider that this is a drones question rather than Photography.

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.


Does motor KV requirement scale linearly with voltage? There have been myriad questions regarding motor KV, such as this one. Many of these come close to answering my question, however none have given me a definite answer.

If I am picking motors for a build, will the motor KV required scale linearly with voltage?

For example, if I know that for a 3” build on 3S (11.1V) I will need 3000KV motors, to find the required KV for similar motors to spin the same propeller on 2S, would I multiply the KV by 3/2, giving me a KV if 4500? If so, does this hold true for larger motors as well?

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


How to clean dirt out of my miniquad motors? I accidentally crashed my 250 class quadcopter into the dirt today and got dirt all over it, including into the motors. I managed to clean the dirt off of the frame and electronics, but I have no idea how to go about dealing with the dirt inside the motors.

Do I have to take apart my motors in order to clean them? If so, how could I do that and what tools/cleaning supplies should I have to do this well.

There are several steps to cleaning the motor:
Clean the chassis and speed control
An air compressor can be used to blow off the dust and dirt that has accumulated on the chassis and speed control. It’s best to work with a clean car and workspace to prevent crud from getting into the motor. If necessary, use an old toothbrush to remove stubborn dirt deposits.
Inspect the wiring

Check the speed control for frayed wires and damaged insulation; these can lead to radio interference and, worse, short-circuiting, which can burn up the motor and speed control. Repair damaged wires with heat-shrink tubing or liquid electrical tape.

Remove the motor

You do not need to unsolder the motor’s power leads or unplug the sensor harness, but doing so will give you more freedom to work. After removing the motor, wipe it with a rag to remove dust and dirt.

Disassemble the motor
Take the motor apart (this is quite easy to do) then you can wipe the individual parts of the motor.
Clean the bearings

Remove the rear bearing from the motor for cleaning. The front bearing on Novak brushless motors is glued to the front endbell and cannot be removed; leave the front bearing in place, and give it a good dousing with motor spray.

(Steps adapted from this link)
How to take the motor apart: (added per OP's request)

IMPORTANT: Before disassembling the motor, if your item carries a warranty, opening and attempting your own repair, will void whatever warranty you have.

First, get to know the parts of your brushless motor:

(Image Source)
The parts needed/recommended for disassembly:

(Image Source)
Step 1:

The first step involves in removing the circlip (E-ring), followed by the collar ring from the rear shaft using the pliers. Be cautious when removing the circlip because it might spring out. If you do not have any spares, do not lose it or bend it out of shape.

(Image Source)
Step 2:

The second step is to pull and remove the rotor (the bell) from the stator (the body).

(Image Source)
Step 3:

Notice that there are two bearings attached to the rear end and front end of the stator. The next step is to remove the bearings from the stator. Sometimes, the bearings are easy to remove, and some have a tight fit. Remove one of the bearings using barbeque skewer or any wooden stick to plug out the bearing from the stator. Usually, the front end bearing is easier to remove.

(Image Source)

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Step 4:

After you have managed to remove one of the bearings, use a pin punch tool to push the other bearing out from the stator.

(Image Source)
Step 5:

Place the stator on the plastic wire spool / solder bobbin and carefully tap the pin punch tool using the hammer until the bearing drops out from the stator.

(Image Source)

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Step 6:

The next procedure is to remove the shaft from the rotor (the bell). Use the allen key to remove the set screw on the rotor.

(Image Source)

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Step 7:

The final step is to push the shaft out from the rotor using the similar pin punch tool technique implemented previously to remove the bearings.

(Image Source)

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Motor disassembly steps adapted from this article.


Is flying a drone in Puerto Rico the same as flying one in the continental USA? Puerto Rico, a US territory in the Caribbean, is under the jurisdiction of the USA but is exempt from some of its laws - for example, it is considered within the USA under immigration law (e.g. the requirements to enter are the same as the mainland USA, flights between Puerto Rico and the mainland are considered domestic, etc.) but is considered overseas for Federal tax purposes (residents are exempt from many Federal taxes).

Are drones in Puerto Rico regulated under the same FAA rules as the rest of the USA, or are there different rules to follow (e.g. different licenses, fewer or additional prohibitions, etc.) that would not be typical of a US State?

Yes.

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

Puerto Rico is considered as within the United States.


Full throttle oscillations When I give full throttle with a full 4s battery I get vibrations.

After a while of flying the vibrations disappear.

Everything is fixed (no loose parts)
Filters are on 1x multipler.
RPM filters are enabled.

2207 2750kv iFlight Xing motors
TBS Source One frame
JohnyFPV Props

This is not a new build. I am flying this quad since 1/2 a year now. RPM filters were on 2x originally.

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

tpa will lower pid-values at higher throttle.


Is it possible to transmit a high-resolution image over an analogue signal? As digital video signals become more popular, there is a growing question; Why can't we just create a higher resolution analogue system? Are there actual limitations of the technology that prevent it?
So far, I have thought of a few potential reasons why analogue transmissions remain low-resolution:

Existing equipment (people already have goggles/cameras with set resolutions)
Not wanting to give up bandwidth (racers in particular need to have enough bandwidth for multiple pilots)
Lack of interest (people might just not care enough to change analogue)
Cost (depending on what would have to happen to create a high-resolution analogue system, it may end up being even more expensive than digital)
Not possible (it might just be beyond the capabilities of current technology)

Several of these reasons, however, would also prevent people from changing to a high-resolution digital system. They would need new (expensive) equipment for digital, and most digital systems have less available channels because they use more bandwidth.
Would it be possible, though, to create a new analogue system that transmits a high-resolution image? Even if it took so much bandwidth that only two people could fly simultaneously and it required new cameras/goggles/VTXs, it could still be worth it.
And if it was possible, would it actually be worth it, or might it be better to go digital?

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


4 in 1 ESC and FC won't turn on when connected together After crashing my drone recently, my ESC and flight controller won't turn on (the power led lights up on the FC, but nothing else works either it doesn't anymore, or I was wrong before) any more when they are connected together. When I disconnect the ribbon cable between the FC and the ESC, the ESC turns on as it should (Beeps when the battery is connected), and the FC turns on when I connect it to USB, and it seems to function correctly.

I have measured the connections going between the two, and all of them appears to be okay, except that 5 V from the ESC drops to about 0.5 V when the flight controller is connected. If I connect both the ribbon cable and a USB cable, the FC turns on and the video feed goes from showing black (no OSD) to show the actual video. The ESC still won't turn on though.

I noticed that the capacitor had come loose from one terminal, but it seems unlikely that that caused these problems. Unless it caused voltage spikes that damaged something?

My FC is a HobbyWing FC F4 G2 and the ESC is a HobbyWing XRotor Micro 60A 4in1.

Any help is appreciated, I can't even figure out which board causes the problems.

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