Can I run two motors off of one ESC? Normally, a dual-motor plane has two ESCs but could I connect two brushless motors to one ESC? What would happen to the torque and speed of each of the two motors connected to the ESC?

Technically, you could connect 2 motors if they're exactly the same and it will probably work. However, there is a good possibility of various weird things happening due to this.
The problem with connecting more than one motor to a single ESC stems from how these motors are driven. An electric motor works by using electromagnetic coils to pull permanent magnets and create a force on the rotor. However if you just supply current to a coil and leave it as is, at first a closeby magnet will get attracted to the coil, rotating the motor bell, but then it will just sit there in the nearest position after oscillating back and forth a bit like a pendulum.
To create a continuous rotation, you need to stop the current as the magnet passes by and then reverse it to now repel the magnet, adding more rotational momentum to the system. In a brushed motor this switching is done mechanically using contacts (brushes) inside of the motor. A brushless motor, on the other hand, does not have this kind of mechanical switching mechanism inside, so the switching must be done externally, and this is the brushless ESC's job.
To determine when to switch the currents around, the ESC must detect when the magnets are passing the coils. To do that it constantly measures the voltages on its three terminals (as a magnet passes by an electromagnetic coil, it creates a voltage in it), and uses that information to switch the currents in the motor in perfect sync with the motor's rotation, speeding up the current pulses as the motor speeds up and slowing them down as it slows down. Real-world ESCs and motors are not perfect, though. sometimes the ESC is incorrect in its assumptions about what the motor is doing right now or how it's going to react, and sends pulses that don't correspond to the motor's actual position. This is called desynchronization, or, colloquially, desync, and results in the motor suddenly losing almost all of its power for a short amount of time (usually less than a second) while the ESC tries to determine the correct state of the motor and reapply power in sync with its rotation. With quadcopters this condition is most common when the motors are spinning at very low RPM, loaded by a reverse airflow (e.g. falling out of the sky bottom-down) and need to suddenly speed up, all of which mess with the ESC's expectations, and can lead to a violent uncontrolled spin.
Now, if you connect two motors, the ESC will still think it's one motor. If these motors are sufficiently different, the most likely thing is that one (or both) of them won't work. Different motors will both create voltages of different magnitudes when rotated, and react with different speed to currents flowing through their coils, so one of the motors will invariably get far ahead of the other in response to a current pulse, making synchronization of the two near-impossible, and the ESC very confused. Depending on the ESC and motor combination, it can either give up entirely, or try to drive this weird "motor" anyway, likely leading to one of the motors spinning at reduced efficiency and the other just jerking.
If the motors are identical, though, e.g. of the same brand and model, and with identical propellers, they are likely to want to rotate at about the same speed, in which case they will produce more or less the same induced voltages in their wires and react the same to what's supplied by the ESC. So this setup will probably work, at least on the ground. Even then, nothing is perfectly matched. One motor will likely be slightly ahead of the other, or otherwise slightly off, which is probably not enough for triggering a desync, but means a more confused ESC and less efficiency overall, as one of the motors (or both) is driven suboptimally.
Further, when you mount the motors on an actual airplane, conditions may be even more unequal. Suppose that one of these "identical" propellers is chipped, or the plane is flying in a crosswind, or even that it's just in a banked turn, so one wing moves through the air slightly faster than the other. Then the load on the motors can differ considerably even though they are the same. Because of that differing load, one of them might get just enough ahead of the other to confuse the ESC into either giving up entirely, or making erratic assumptions about this weird combined "motor", leading to wrong signals being sent, which might, for example, slow down one motor and speed up the other, possibly ending in one of them desyncing and stopping entirely. This, in turn, can potentially lead to very unpredictable behavior of the model in flight.
So, to conclude this long-winded explanation, the answer to your question is:
Don't. Just don't.


DJI Mavic Mini 2 can't record 2.7k 60fps after update As announced, DJI Mavic Mini 2 should support 2.7k 60fps.
I have the latest update of 01.02.0300 but still do not see the option.
Does anybody have the same problem ?
Thank you!

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


What is the background of remote controller modules? I've seen that in many remote controllers you can plug in an external module under it.

Are there other purposes for these modules than just being transceivers?
Are the module bays offered by multiple brands, or is it just a single brand thing?
Are there different styles of modules that look similar but don't work interchangeably?

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

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

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


XM+ bound to FRSKY x9 lite, but commands not showing up in betaflight I'm very new to making drones, so sorry in advance for lack of proper terminology or if this is a dumb question. I've bound my xm+ to my transmitter (solid green light showing on receiver), but for some reason my inputs aren't showing up in betaflight. I have the correct serialRX activated on the correct UART (and have also tried both) and my receiver mode and serial receiver provider are set correctly (to serial based receiver and SBUS, respectively). This problem arose after I accidently connected my receiver to my flight controller backwards, and have already tried replacing the receiver, as I thought I might have fried my XM+, but that didn't solve the issue. Any help is much appreciated!!

Since you connected power to the SBUS pad it is possible that the inverter that sits between the SBUS pad and the UART pin on the microcontroller is fried.
To get around this, I would suggest connecting SBUS to another RX pad. However, since SBUS is an inverted protocol, you will first need to get the uninverted signal somehow. There are two ways to do this with the XM+ (other SBUS receivers have similar methods). The first option is to solder the wire that goes to the RX pad to the leg circled in red in the image below.

source
The other option is to build your own inverter using a transistor and two resistors.

source
The first option is probably the easiest if you feel confident enough to solder to that small pad, but it does come with an increased risk of damaging the receiver.


Spray painting a drone, what to look out for? I don't really like the colours on my drone and found a few bright spray paint colours around my house. I would like to spray paint my drone, but I obviously do not want to damage any parts of my drone (motors, electrical components etc.)

Is it safe to do this?
What should I look out for when painting? Any specific areas of the drone I should avoid touching?
How can I get the paint to not wash off it the drone crashes etc.? Anything I can apply to it?

If you have the technical know how, I would suggest you remove the shell from the drone and spray paint that separately to avoid contamination any of the electronics or mechanical components.

If you aren’t able to do that, then apply masking tape and/or plastic or any other protective layer over any exposed electronics, cameras, ports, motors etc. Basically anything functional. Also make sure not to point the propellers as this could lead to them being unbalanced.

Before you paint the drones, you may want to scuff the surface with some sand paper. This will provide better adhesion for the paint - if you’ve ever tried writing in marker on a smooth surface, you know how difficult it is to stick.

After you have applied your paint, you can add a clear coat and a fixative to protect the paint.

Here is a video showing how to spray paint a drone: https://youtu.be/5aSsd3tCJ-c, notice how the person completely disassembles the drone before applying the paint.

You may also want to consider a vinyl wrap of hydro dipping as decent alternatives to spray paint.

Here is another example of how to spray paint a more premium drone: https://youtu.be/mP4oyjipwhQ


How do you figure out which motors to use for your UAV? There are a ton of electric motors on the market. Is there a mathematical formula or some other means of figuring out which size of motor you will need for your airplane, helicopter, or drone?

N.B.: This applies particularly to drones, and to a lesser extent to helicopters.  Airplanes are a totally different field and would require a separate answer.
Obviously the more power per weight... the better.  But basically you want to be able to get enough energy out of the motor to be able to hover at around half throttle.  There's a good tutorial here which explains the full process, but basically you'll have to do the following:
1. Make an estimation of the total weight
Be sure to include the weight of your motors in this calculation.
2. Multiply that weight by 2
This compensates for the fact that the drone has not only to hover but also to move upwards through the air.  If you build your drone and find it's too sluggish for what you want, simply increase this factor.
3. Find an engine that will generate 1/n thrust
...where n is the number of engines that you will have on the drone.  So for instance, if you calculated that the weight of your drone was going to be 800g, you'll want each motor to have roughly 400g of thrust.
For most propellor motors designed for flight, you'll be able to find what is called a "thrust table" which will list propellor size (be sure your body is big enough to leave space for the propellors to spin independently!), the amount of current it burns, thrust (usually measured in grams), etc.
Do the comparison, and you're set!


How do I attach a propeller to this motor? As many of you already know, I am getting into RC planes. I went to my hobby shop today and bought a motor and some props, but I can't figure out how to attach a propeller to the motor. Unlike what I've seen on YouTube videos, there is no shaft on the front of the motor. The motor is a Dualsky 2304c 1850kv brushless motor.
Apologies for bad pictures, I only have my laptop with me.
The front of the motor:

The back of the motor:

The motor accessories:

The propeller and accessories:

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


Difference between T700 carbon frames and 3k carbon frames? I have heard that iflight's Green Hornet uses T700 carbon instead of 3k carbon. What difference does the new T700 carbon make?

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

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


How to build a buddy box cable for FlySky FS-i6X RC transmitters? I have a couple of FlySky FS-i6X RC transmitters that I'd like to use to teach my brother how to fly. I've seen other people use "buddy box" cables that connect two transmitters and allow one person to temporarily give the other control of the aircraft while in flight.

The back of the transmitter has a port that looks like an old S-video receptacle. Can I use that to make a buddy box cable?

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

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

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

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

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

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

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

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

Now, go and fly!


Budget drone capable of lifting 0.5 - 1 kg and flying ~2km radius I'm looking for some budget multirotor drone (up to $300) which can lift 500g-1kg of weight and has long enough range to fly a ~2 km radius, it should also have a small camera just for control. Is there a ready to fly model which I can buy, or can I somehow easily construct a drone with the following parameters?

The flight you describe is 100% illegal in the US requiring 107/commercial license + BVLOS waiver.  BVLOS waivers are very rare, 61 issued searching the DB for "107.31" here: https://www.faa.gov/uas/commercial_operators/part_107_waivers/waivers_issued/.  The rules are similar in most countries.
Your budget is off by a zero as well.  Depending on your duty needs, I think minimum \$1k for the core + $ for peripherals.
A Phantom could probably limp that payload - but you need to dig into the powertrain math:

What ratio of thrust do you want? Are you carrying a load form point A to B or will you require some performance?
Weight of unit AUW desired? Payload and performance are correlated to thrust and other performance features.
Flight time desired?

Camera for control at 2km requires high power VTX (FAA HAM license required). A 1 watt VTX in an urban environment will struggle to get you signal without a lot of extra gear (signal tracking from a ground station). Also, 1w is the legal max for a HAM / FCC licensed person, else you are 25mw (0.025 watt) limited and can't mod anything legally.  To get more watts you need to dip into FCC Part 15 regulations, not cheap.
I see you are in Armenia - I don't know the local there laws but my international flight experience informs me that most of the world is either unregulated (3rd world) or as strict or more strict than the US.


Estimate position of a balloon attached to a quadcopter I'm trying to measure/estimate the position of a balloon, for that I need the two angles shown in the picture below. enter image description here

The idea is to place a helium filled balloon on a quadcopter to provide lift. As a first step, I imagined a fixed platform with the balloon attached. Next, I would like to estimate the value of the angles α and φ, since the length of the strap is fixed, getting the position of the globe is simply trigonometry.
The problem is that I am not sure how to measure such angles, what would be better and what type of sensor to use. I was thinking of some kind of optical sensor, but I'm not sure.
Can anyone suggest me some ideas?

The simple answer would be a potentiometer, as used for position feedback in a servo. You apply a voltage to the outer two contacts and measure the voltage at the moving, middle connection.
The next step would be a rotary encoder. These should give you less friction and more accuracy, if you need it.
A camera would require significant processing power, but might be viable on a larger drone. Measuring α would be tricky with a camera on the drone, but you could calculate it from the distance from the centre and the length of the tether.


Will LiPo batteries deteriorate outside with the temperature changes due to day-night cycle? I am new to the hobby and I'm concerned about LiPo batteries. I live in an apartment (this is Europe, blocks of apartments are the norm in my country) so I can not put the batteries in a dedicated room or garage. Finding a spot in the house where I can put the LiPos 1 meter away from any flammable material seems impossible.

I do have, however, a convenient small balcony in which I could put a box with the batteries for safe(st) storage, something like a bat-safe, ammo or metal tool box, which have kind of proven they can withstand a LiPo fire; in a small plastic closet to protect them from rain insulating the metal box from the plastic closet with some drywall and maybe some sand.

The balcony does not receive direct sunlight, but temperatures vary from ~ 15C/59F (min) ~ 35C/95F (max) in summer and ~0C/32F (min) ~ 15C/59F in winter. Temperatures could very exceptionally reach a maximum of 45°C a couple of days at peak hours. From what I have read batteries can bare these ranges of temperatures without too much of a problem but, Will they suffer from the everyday variation of temperatures and humidity and will this increase the risk of them exploding?

The idea here is to maximize safety, I do not really care that much if their performance deteriorates or if I have to renew them more often, as long as this does not come with a higher risk.

Note: The batteries I have ordered are two AHTECH Infinity 4S 14.8V 1300mAh 85C Graphene.

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

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

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

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


What should I look for when buying FPV goggles? What features should I look for when I’m buying a new set of FPV goggles?

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


Drone in Italy as a tourist - Where/what to do? I am a German citizen and have a hobby drone dji mavic 2 pro. I would like to know how/where I have to do all the paperwork so that I can as a tourist fly my toy over Pisa tower and make a short video and take some pictures....
For doing this in Germany, the process is pretty clear. But what about Italy?
I want to avoid things like this here:
 French tourist could face €113,000 fine...

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


What does KV mean on motors? I've seen ratings on brushless DC motors like this one for something called KV. What does that rating mean?

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

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

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

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

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


How high can the Mavic Mini fly? I was wondering how high the DJI Mavic Mini could fly before the air became too thin. I know that there are restrictions in the app (and probably in the drone/controller itself) but if you were able to override those things, how high would it go (not including battery life)? Thanks in advance!

The maximum altitude is 3000m (about 9800 ft.)
(Note that this is relative to mean sea level, not the ground.)
Sources:

DJI Mavic Mini "Specs" page (under "Max Takeoff Altitude")
DJI Mavic Mini "FAQ" page (under "Propulsion" > "What is Mavic Mini's maximum service ceiling?")


How to visually inspect the magnitude of PID gains? The defaults in Betaflight generally work very well, but I own and fly very different quads ranging from 2" to 6". It may easily be that there is room for little adjustments I can make with the tuning sliders in the Configurator. How can I use graphs and plots generated from blackbox data to determine whether the PID gains on a certain quad are excessive or too low or about okay? How do these scenarios manifest visually in Blackbox Viewer or other visualization tools?

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

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

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


What is a turtle mode? Could you explain what is a turtle mode?

I'm wondering what the use of it is? Does it work on all quadcopters?

Turtle Mode, Flip Over After Crash in Betaflight, is a mode that allows your drone to flip over if it is upside down by reversing the prop direction and spinning up very rapidly in order to lift a side of the drone so it flips to the upright position.

This is a manual process - the drone doesn't do this automatically when it flips over (you need to enable turtle mode after the crash, manipulate the sticks to flip the drone over, and disable turtle mode in order to resume flight).  It's beneficial because if your drone goes down 100' away, for example, you can put your drone in turtle mode and flip it back to the normal position, and resume flying.

There are some cases where turtle mode won't work, like when you land upside down in tall grass, the grass could bind the motors/props, preventing turtle mode from working effectively.


How useful is knowing the torque generated by a motor prop combination? I'm designing a series of small drones for use in a swarm configuration in combination with terrestrial / marine / submarine robots.  As part of the process, I've designed a test rig to measure the thrust performance of various motor / propeller combinations.  The general layout of the rig is shown in the Sketchup diagram below:

 (click to enlarge)

The motor mount is attached to a plate, which is bolted to a MGN12C linear bearing block mounted on a 100mm rail. (If anyone is interested, the details of the MG series linear bearings can be downloaded here).  This particular bearing was just one I happened to have available.

At the other end of the plate, a load cell protrudes through a slot, and this is used to measure the thrust force generated by the motor / propeller combination.  The mount is designed to allow the load cell to be easily changed (I have quite a few 1kg, 3kg, 5kg, and 10kg load cells).

The data, together with temperature and vibration measurements from an accelerometer, will be gathered using an Arduino (or possibly a Raspberry Pi if I decide to integrate the rig with other test gear).

The length of the plate means there is ample room to mount the ESC between the motor mount and the load cell.

It occurs to me that it would be fairly straightforward to modify the rig to allow the twisting force, or torque, generated by the motor / propeller combination to be measured.  Probably the simplest way with the parts I have available now would be a pair of load cells fitted between a modified motor mount and the plate attached to the linear bearing as shown here:

 (Click to enlarge)

Minimal modification to the electronics would be required to add this to the design, and I have more than enough load cells available to add this to the rig.

We had to re-design arms in a previous model when we found the forces generated by the motors caused them to deform while testing the unit on a static rig, which is why this modification appeals to me.  However, I wonder whether anyone else has any experience of using this kind of data in drone design.

So, how useful is it to know the torque generated by a motor prop combination when designing multi-rotor drones?

Not very useful.

If you're designing at the absolute limit of what a frame can handle, such that it's as light as possible, then maybe you'll want to know the maximum torque, but you should be designing for orders of magnitude more strength than that (It's not hard, small BLDC motors aren't very torquey).

There are control algorithms that take into account frame torque and how the forces are applied. I forget off-hand, but it was a matrix-solving control algorithm, as opposed to PID. Unless you're writing your own flight control software, that's not an issue.

When changing motors, it may give minor insight into tuning the yaw axis (these motors have more torque, so less P-Gain may be needed), but yaw is done by applying torque to a propeller, so again unless the motor can barely spin the propeller (this should not be the case in a design), then this isn't a concern.

Where you may see an advantage is when computing efficiency - ie how much torque do I have to apply to propeller x to get y grams of thrust? However, this is generally done by using current figures since those are more relevant to power system design.

Another possibility is how much braking/accelerating torque a given ESC can provide - high performance machines needs rapid changes in motor RPM, so a given ESC may be better (due to FET latency, resistance) at control algorithms than others. This would be interesting data to see, but I don't think any ESC is necessarily better than another, since braking is just "FETS on" and accelerating is just pushing the control algorithm to its max. Noisiness is much more important than acceleration torque in practice.


Video being indexed on SD card, but no duration time and no actual video recorded I am using an HS-Series Holy Stone drone. I was able to get one video recording, but every subsequent video recording indexes the recording, but no duration time, and no actual video in it even when it says it has 23 Mb of video.
I am using VLC to view it since my MacBook Pro does not support .avi files.
What is going on?
Here is the VLC UI:

Notice how the first recording shows 00:59 seconds, but each subsequent recording I tried, shows no Duration, yet at least one of them has 25MB...of what I have no idea.

Given that the drone camera is a cheap model from China, I think it's entirely possible that the camera modules is broken and could be outputting incorrectly terminated .avi files.
In earlier comments, you explain that no files are created when a known-good SD card is used to record video with the drone. My prior experience with camera modules for cheap Chinese-made RTF (ready-to-fly) drones has shown that there can be serious quality issues with the recorders. I used to have a Syma X5 model which I upgraded with the OEM camera module. The whole camera solution was shoddily made and implemented.
Based on the observations with the files created on the first SD card, one of which is unplayable but >0 bytes in size and the others which are entirely empty, this seems likely. Zero-size or otherwise corrupted video files are a common symptom of an incorrectly-terminated video stream. The .avi file format supports many kinds of video streams (wiki), but the drone camera most likely uses some kind of MPEG codec with interframe compression, which is more susceptible to corruption or incorrect stream termination than a full-frame uncompressed stream.


Why does the video cut out on HX-100 drone? I have a BetaFPV HX-100 drone which I recently got. When I tried to fly it FPV the video feed would cut out and leave a grey screen. The OSD still showed up in the goggles so I'm not sure if there is a problem with the camera or video transmitter.

The VTX was hot to the touch and the camera was warm. It was not the output power setting of the VTX because it cuts out even at a close distance to my goggles. I would appreciate any ideas on how to fix this issue.

This happens when flying normally or even hovering and the grey screen is a solid grey, not static. I have not had any big crashes with it.

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

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

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


30 year old water-slide decals. Are they still usable? Not strictly a drone or model aircraft question, but I have seen multiple questions here about painting models, and this is also about the aesthetics.
When clearing my deceased cousins house, I found several plane and car models he was in the process of building 30 years ago, but he never finished them.
In 1992 he got in a motorcycle accident, which shattered his right hand leaving him with severely restricted movement of that hand. He never got back into building models, because of the loss of dexterity.
As a tribute to him I would like to finish those models.
What has me worried are the water-slide decals.
There are a bunch of them to be placed over painted areas (Humbrol enamel paint).
They have been stored at room temperature in a closed (dark) cupboard all this time.
I have used some 5 year old decals in the past without any issues, but these are 30 years old.
Would these still be usable after 30 years or is it better not to try to put them on at all?

It's probably safe to assume that they will no longer work. This source suggests that water-slide decals have shelf life of about 3 years. "After a few years the glue holding the decal onto the paper breaks down, fusing decal and paper together."
Because there is some variation between different decals from different manufacturers, they may last slightly shorter or longer than that in storage (like 3-5 years), but 30 years is probably too long. I think it's worth trying out one of the smaller decals just to be sure of whether it will work or not.


What is antenna gain? As I understand it, generally, a higher gain antenna will result in a longer range and better reception for FPV flying. What is antenna gain? What makes a low-gain patch antenna different that a high-gain patch antenna?

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

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

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

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

1dB corresponds to about 1.25:1

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

7dB ~= 5:1

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

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

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

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

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


Difference between analog and digital servos? I've noticed that there is often a price gap between analog and digital servos. What is the difference between an analog and digital servo and why is a digital servo better?

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

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


How long can a LiPo pack be kept fully discharged before storage charging? There is a reason LiPo packs are recommended to be stored at a storage charge. It is commonly known that storing a battery fully charged reduces its capacity and performance. What about storing batteries discharged after flights for extended periods of time?

Has any research been done on this topic?

Suppose I flew today and the next time I'll be flying in 7 days.
Which is better:

leave the packs discharged, and charge after 7 days,
storage-charge immediately, fully charge after 7 days,
fully charge immediately, leave charged for 7 days?

What about 3 days or 2 weeks instead?

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


How does frame stiffness affect a quadcopter? When choosing a frame for a drone, something I hear mentioned occasionally is how flexible the carbon fiber is. It seems that the general opinion is: more flex in a frame is worse. How would having a frame with flex in it be bad for a high-powered quadcopter? Would it impact the efficiency and performance of the quadcopter?

Background

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

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

Stiffness

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

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

Oscillation

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

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

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

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

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

Vibration

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

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

Composites vs. metals

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

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

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

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

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

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


How to reset connection between drone and RC for MAVIC Air 2 My new dji Mavic Air 2 arrived, but it was not connected to the remote control as it is supposed to be connected upon arrival. I then tried connecting it manually following the on-screen instructions of the DJI Fly App, but the Mavic Air 2 doesn't go into the pairing mode when I press the on/off button for 4 seconds or more.

The connection between the App and RC works. The drone is turned on, it does its self-test and briefly beeps. The 4 LEDs are constantly on. Holding down the power button makes it flash slowly, but there is no beep and the LEDs will stop flashing when I release the button.

Is there any other way to reset / pair the drone to the remote control?

According to page 36 of the DJI Mavic Air 2 manual the aircraft should be linked to the remote controller on arrival.

If not, you should follow these steps:

  Power on the remote controller and the aircraft.
  Launch DJI Fly.
  In camera view, tap (three dots)  and select Control and Connect to Aircraft.
  Press and hold the power button of the aircraft for more than four seconds. The aircraft beeps once indicating it is ready to link. The aircraft beeps twice indicating linking is successful. The battery level LEDs of the remote controller will glow solid.

I think the issue was you didn't try to link it through the DJI Fly app.


Taranis X9D Plus 2019: Switches SE,SF,SG,SH, and SI are not working/responding on Betaflight So switches SA,SB,SC, and SD work on Betaflight. Aux channels 1 through 4.
But switches SE,SF,SG,SH, and SI are not working/responding on Betaflight. Aux channel 5 through 9, I believe
I am using TBS Crossfire lite, with crossfire nano receiver.
On my Taranis I have already setup Mixers to activate all switches and nobs.
When I first binded to my reciever I did not set up the rear switches(SE,SF,SG,SH, and SI), but I already  setup 3 of the front switches(SA,SB, and SC) before binding to my model.
Things I have tried:

Connected Taranis via usb mini, opened Devices and Printers on windows and ran through the wizard to re-setup my Taranis
Reconfigured my Input and  Mixer page on my defined model. Simply defined each switch in order in the Inputs page, and then on Mixer page defined the channels
Rebinded my crossfire transmitter receiver by holding down the button and plugging in my battery to the quad. Releasing the button on the receiver and pressed and held the button till it binded again and  updated again. This time my model was setup but same results
On Taranis, long pressed MENU and went to page HARDWARE: ran calibration and also Debuged switches. Everything seems normal

Considerations:

I have the latest Betaflight-configurator and I am using the latest Beta-flight firmware on my quad

Flight Controller: STABLE PRO F7, Model:STABLE PRO F7 35A

Transmitter Receiver: TBS crossfire nano

Everything else works as expected

This is occurring with two of my first-build quads

Followed this reference to solder and config the transmitter receiver: https://geprc.com/download/en/GEPRC-STABLE-PRO-F7-DUAL-MANUAL-V1_0.pdf

Taranis is in 1-16 channel mode. Also I am not missing switches, all are activated, I checked in the HARDWARE's page

Taranis Firmware: opentx-x9d+2019, version: 2.3.6

How my Mixer is setup:
CH1 Rud
CH2 Ele
CH3 Ail
CH4 Thr
CH5 SA
CH6 SB
CH7 SC
CH8 SD
CH9 SE
CH10 SF
CH11 SG
CH12 SH
CH13 SI
CH14 S1
CH15 S2
CH16 LS
CH17 LR

Any suggestions would be much appreciated.

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


What is the proper method of tuning PID filters for recent BetaFlight (4.x) on micro-sUAS WITHOUT Blackbox? I've included my specific platform below, but this question should be applicable to other platforms just as well.

Board:
Matek F411

Platform:
BetaFPV Beta65 Pro 1S (65mm m2m)

I know there are serial logging devices specifically to provide BlackBox for boards without onboard flash, but my concern is that given the very narrow power envelope of these micros that any data collected with an external logging solution would be scaled improperly once the micro is flown without the logger onboard.

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

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


What does the full scale range in a IMU MPU9250 datasheet signify? I have MPU9250 IMU, which I am working on to get orientations from the sensor output data. While exploring the MPU9250 (also other sensor such as BNO055) datasheet I came across the full scale range for accel & gyro given as ±2 ±4 ±8 ±16 (g) and ±250 ±500 ±1000 ±2000 (deg/s). I did not understand what it signifies, but seems important.

I tried searching online related to full scale range but did not get explanation regarding it. I request if someone could help me in understanding by providing good explanation or a source which I can go through, I'd really appreciate that.

Also which one would be better to use?

I have asked the same question on Stack Overflow. I really need help on this.

Thanks.

TL;DR:
The configurable scale ranges for the accelerometer, gyroscope, and magnetometer on the MPU-9250 IMU affect the maximum resolution and range of the reported values.
As you increase the full-scale range setting, the maximum value/rate that can be reported by the IMU before the IMU is saturated increases, but the resolution of the value/rate decreases.

Section 3.1 of the datasheet for the Invensense MPU-9250 has a table that states the "Gyroscope ADC Word Length", which represents how large the values reported over the I2C or SPI interfaces are. In this case, the gyro word length is 16 bits. A 16 bit two's complement binary number (explained wonderfully in this video by Ben Eater) can range from -32768 to +32767.

Take the example of the ±500 °/s full-scale range. The unit resolution of the two's complement value reported by the gyro (aka Least Significant Bit/LSB resolution) in the ±500 °/s full-scale range is: $\frac{32767}{500} = 65.534$ LSB/(°/s) (degrees per second per LSB). This is not coincidentally labeled in the graph as the "Sensitivity Scale Factor" for the FS_SEL=1 condition.


Good beginner 6-ch radio with configurable controls under $80 Probably a bit early to be asking this question since I actually don't have a plane that flies yet, but this way I can keep an eye out for sales.
I'm looking for a (minimum) 6-ch radio with controls setting under $80 for a beginner. Should also come with a receiver as well. More channels is fine and even preferred but it's not something I want to break the bank on. If I can save a set of controls for a certain aircraft and just move the receiver between planes that's a plus to. Pretty simple, thanks for your input.

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

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

FS-i6X

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

FS-iA6B

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

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


Can I use the trainer port on a DJI Phantom 2 controller for a buddy box connection? I am trying to teach my wife how to fly drones, and as I have an old DJI Phantom 2 with two controllers, I wanted to buddy box her on it.

I know there is a trainer port on both transmitters, but are they the same function as on Spektrum or other transmitters? And, if so, which switch changes it between master and slave?

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

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

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


Reasonably priced FPV Drone for beginner I'm looking for the best, reasonably priced FPV racing drone to start with. I would use a local shop but sadly there are none in a good distance from where I live. I have looked around quite a bit and it seems there is a wide variety of price and quality.
What would be good options to start with as I think it's really interesting but don't want to fully commit to it by spending a huge stack of money on the best stuff and then figuring out it wasn't quite what I expected? I've had some fly time with an FPV drone so no need to tell me to try one out before buying.

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


How to connect ESC to serial? I am looking for a way to access to configuration of Blheli-S firmware running on a Diatone Mamba F30Mini:

It seems possible using :

blheli-configurator
BLHeliSuite

There is explanation on different site about FC Passthrough. This seems to use connection to a Flight Controller.
As I am planning to use an Arduino nano to connect to the ESC, I am wondering how to make the connection.
Is it possible to connect more or less to connect the ESC to a PC ? or connecting through the arduino  ?

Thanks to informations given by @ifconfig, using BLHeliSuite connecting :

GND to Arduino GND
ESC1 to Arduino D11

It allow to upload to Arduino the "Arduino Bootloader"

Then selecting "ATMEL BLHeli Bootloader (USB/Com)", and go to "ESC Setup" tab

Finally quite straight-forward, with a tool that may be more user-friendly.


Beginner looking to try a DIY plane, any suggestions on a kit? Title pretty much sums it up, but I would like to try to make my own RC plane. I'm looking for a kit or bundle I can get that would basically include all the supplies to make a plane, so I guess that would be a controller, receiver, a motor or two, some servos, and a battery(I think?). Built in FPV camera is a plus but not necessary. Price is where I'm mainly concerned, and unless the circuitry is pretty indestructible I only really want to pay twenty dollars for it. If I can crash it without too much damage then I can do up to 50 dollars, but preferably you would have some experience with the kit so I don't have just amazon reviews to go off of. I can solder any connections that are needed if the kit isn't preassembled.
If you need any more details, just ask and I'll add it.
I am in the US, state of Tennessee.

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


How to fix OSD displaying weird symbols I upgraded from betaflight 3.X to betaflight 4.1.X and now my OSD displays weird symbols.

It seems to have replaced the crosshair. (The 3 L shapes in the center)

Expected result:

How can I fix it?

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

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

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

I'm now back with a normal crosshair.

As @3k pointed out,

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


How to Simultaneously View and Record GoPro Content While Flying I want to be able to remotely take pictures and start and stop video using a GoPro Hero 9 Black while it's attached to a quadcopter. The pictures will need to be saved to the GoPro's internal memory card.
I'll also need to be able to simultaneously view a live video feed from the GoPro so that I'm able to properly align the camera for taking pictures.
I have never used a GoPro before but I've read that it has a wireless live video stream feature built in where you can see a live video feed from the camera on your phone using the GoPro app. If this is true does this work well while the drone is moving around. The drone will be used for surveying land. It will not be used for FPV racing if that has any relevance.
Ideally I would like to be able to connect a Nvidia Jetson Nano or alternatively a Raspberry Pi to the GoPro and use something like OpenCV in combination with the "unofficial" GoPro API project on GitHub to automatically take the pictures and while I view a live video feed from the drone. That's the main goal but if this isn't possible then can I do it manually using the GoPro app on my phone? Any guidance or suggestions would be greatly appreciated. Thank you.
UPDATE
Perhaps this will help clarify my objective. I'm basically wanting to make a DJI style controller with the phone screen that let's you see where you're going and to have a feature that lets you take pictures manually or autonomously using AI/OpenCV.

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


What do the numbers mean on a brushless DC motor? Brushless DC (BLDC) motors can have a range of numbers on them:

A four-digit number (e.g. 2212)
A Kv number (already explained on this site)
A number with N and P (e.g 12N14P)

What do they mean?

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

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

A Kv number (already explained on this site)

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

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

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


What is the power draw of an average 5 inch FPV-drone? I'm doing a project in school where I'm comparing different batteries for use with an FPV drone. To do that I need to know a how many watts the battery needs to be able to provide.
I found this motor test, but if I understand it correctly, the motor draws about 30 - 40 A on average depending on the propeller, and 150 - 190 A maximum. If this test is with only one motor (I assume it is, correct me if I'm wrong) that seems like a lot, considering that my drones draw about that much in total (4 motors + VTX, camera etc.) during flight. The voltage is about the same as my drones (4S), so that would result in a much higher power draw (About 9600 W vs. 2400 W).
Could someone help me figure out what is correct? Considering a typical 1550 4S battery can deliver about 200 A burst, it seems like I'm wrong about something.

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

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

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


Drones for spelunking? I do a lot of spelunking and I would like to know if anyone has some recommendation as to what type of drone would be the most useful in cave exploration with light and camera possibilities in total darkness.

I am only interested in drone usage where the cave chambers are quite spacious and not for usage in narrow passageways.

If it were me, there are two different kinds of drones I would want. A tiny whoop and a cinewhoop.

Tinywhoop is a loose definition of a very small drone that's about 3 inches wide with guards/ducts around its props and weighing anywhere from 20-50g. They are very durable and fly in places nothing else could. You can assemble them yourself (very easy) or buy one pre-assembled for around $100. You can add a small, powerful LED that allows some pretty fun exploration in dark and very tiny spaces without the risk of loosing a larger, more expensive piece of equipment. Example video of someone flying in their house with the lights out

A cinewhoop is a larger drone, more like 6-7 inches wide and weighing 200-400g, also with ducts. Typically it's called a cinewhoop because it's about the smallest thing you would want to fly that can comfortably carry a GoPro. You could equip this with a much more powerful light and have a real blast flying around. Like tavis mentioned the DJI FPV system would be an excellent setup for caves as it has great video quality and is very resistant to signal multipathing which is a very common affliction of analog video systems like what you would install on a tiny whoop. Multipathing is likely to be a very real issue inside a cave. If you could live with the analog break up though there are few 'starlight' type analog cameras that can operate in very dark environments. However, I don't think that would be nearly as enjoyable or as useful as a 1000+ lumen flashlight strapped to a DJI equipped cinewhoop. That would be awesome. Video here of DRL pilot NURK doing a better job of explaining what a cinewhoop is.

I would say your typical DJI consumer drone would not be well suited to a dark cave. Without GPS they would rely on downward-firing cameras for position hold which doesn't work in the dark. You could possibly rig up light to assist the cameras but I would guess that it still wouldn't work very well.


How hot is too hot for a VTX? I have heard that when a VTX gets too hot, it can cause irreversible damage to the VTX. I have sometimes noticed that the VTX on my 5-inch drone gets very hot to the touch. How hot is too hot for a VTX and what are some signs that the VTX on my drone might be getting too hot?

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

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

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

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

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

Possible solutions

As Oscar Liang points out, you could try:

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

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


What is the difference between FPV or LOS, angle or acro drones? What is the difference between FPV or LOS, angle or acro drones? How are they different?

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

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

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

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

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

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

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

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


What is a safe amount of time a LiPo pack can be kept fully charged before it is discharged to storage capacity? Is there any research that shows the actual decrease in capacity/performance/longevity of high discharge current LiPo batteries, like those used in quadcopters, depending on the total time they spent being fully charged? Maybe there are some chemical processes that start to occur after some time of being charged which should be avoided and the time until they start being harmful can be measured or calculated...

What rule of thumb (that can be scientifically backed, or would at least be an educated guess) can we assume to keep the batteries in good shape without being too extreme?

You should store the battery fully charged as little as possible; a few days is probably fine, but when it gets closer to a week or more you might start to see it's health degrading. That said, there is no hard limit where it will stop working, so my advice is to charge it when you are going to use it, and then charge / discharge it to 3.8 V / cell when you are done.

  A battery sitting at room temperature fully charged will lose about 20% of its capacity in a year. A battery sitting at 60 degrees Celsius will lose 40% of its capacity in just 3 months.

https://brushlesswhoop.com/storing-fully-charged-lipo-batteries/


What types of cameras don't have wide angle view? Are those a good fit for a racing drone? I am looking to make an fpv racing drone but I want to find a small, lightweight camera without the wide angle lens. Should I use this camera as my main fpv camera or mount a wide angle camera for my racing drone?
I am thinking of just mounting one camera (the one without a wide angle lens equipped), but if a wide angle lens is better than I will mount both cameras. One for recording faraway footage (essentially use it as a hobby camera when not using it for racing), and the other one as my primary camera.
so, should I aim of using the wide angle lens or a normal angle lens camera? Also: what's the name of cameras without a wide angle lens?

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


How do I calculate thrust for a quadrocopter? I am designing a quadrocopter style aircraft that uses a 20 inch propeller 1.625 inches in width and has a pitch of 8 inches. How do I calculate static lift / thrust and the dynamic thrust / lift forces when it would be moving in a direction (not diagonal in an upward or downward).

Thank you.

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

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

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