Why did a quadcopter in vertical descent appear to lose thrust and descend rapidly? Towards the end of a flight with a quadcopter, I started a vertical descent to come in and land. There was no wind on that particular day so I just came straight down.

Shortly after starting to descend, the quadcopter started falling faster. I increased the throttle to compensate, but it only seemed to fall faster.

After a rather unceremonious landing, I checked the quad over and found it still had power enough to take off and hover, so I don't think it outright lost power. It came down level with the motors going.

What would have caused the rapid loss of vertical thrust?

It sounds like you've encountered what's called "vortex ring state", when a rotorcraft descends into its own wake -- essentially, a self-created downdraft.

There are two ways to escape from such a situation:

Power out of it.  Climbing out of the craft's wake vortex requires about twice as much power as hovering, so while most full-sized helicopters can't do this, most quadcopters and other small-scale craft can.
Move forwards (or backward, or sideways) out of the craft's wake.  This is the standard procedure for helicopters.

You can also avoid it entirely by not descending too fast, or by not descending straight down.


Do aerodynamics scale for model aircraft? If I wanted to make a scale model plane that looked like a real aircraft, could I just scale down all of the measurements of the actual aircraft to make a model aircraft with similar flight characteristics? If not, what is the reason?

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

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

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

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

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


How to PID tune a 10" drone? I have a 10" inch drone that I have trouble tuning. On the default Betaflight PIDs it feels really unstable, and I have tried to raise P, I and D both together and one by one without noticing much difference. Some things I almost doubled even. Since Betaflight is tuned for 5" inch quads (I think); are there more parameters than those three that I should change when tuning a larger quad? Or should I just keep increasing everything?

I should probably also add that I run Betaflight 4.1 with bidirectional dshot and RPM filtering.

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

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

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

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

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

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

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


What counts towards the official weight of a drone? In the United States, a drone that weighs 55 pounds or more needs to be registered using a different process than lighter drones.  What counts towards that weight -- is it dry weight, fueled weight, maximum takeoff weight, or something else entirely?

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

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


What props for a 2207 2750KV motor? I have a 4s Nazgul5 drone. It is a 5'' drone and uses 2207 2750kv brushless motors. What is the most aggressive prop I can use on the drone without causing it to overheat or cause the battery to sag?

This table provides guidance to propellor size:

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

(table source)

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


How to know if a place is a no-drone zone in the UK? I live in the UK and often hear that a lot of places are "no-drone zones" meaning flying a drone there could get you a penalty.

Is there any database for these zones as often signage isn't clear?

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

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

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

[1] Wayback machine link.


Barometer vs Rangefinder On a small 6-7inch drone do I need a barometer, rangefinder or both? Is there a standard practice or does it just depend on use case?

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


How can I become a contributor developer for the Betaflight project? I am a mobile developer for the Android platform and recently found I highly growing passion for self-built drones.

I have also noticed the community around this hobby is very collaborative and it would be extremely helpful to know a viable way of becoming a contributor to the Betaflight project to start doing my part for the community.

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


What does the TVL rating mean for FPV cameras? I see listings for analog FPV cameras like this one which have ratings in units of TVL. (e.g. 1000 TVL or 1200 TVL) What is TVL?

TVL (TeleVision Lines) is a measurement of the "resolution" of an analog camera. Digital video systems use pixel resolution to specify how many pixels wide and tall the image is (e.g. 1080p = 1920 pixels wide by 1080 pixels tall), but analog video systems don't technically have pixels so there has to be some other way to represent their resolution capabilities.

 (cit.)

The horizontal resolution power of analog video (TVL) measures how many distinct vertical bars can be depicted by the device (either a camera or display screen) across a horizontal length equal to the height of the picture. The example diagram above shows analog video with 6 TVL.

Cameras with higher TVL ratings will be able to capture smaller details than other cameras, although this is also dependent on the capabilities of the other equipment in between the camera and your FPV goggles or screen.


Quadcopter return to home function not working I have a small S49 quadcopter. Whenever I am flying it and initiate Return to Home on the controller, it always returns to the wrong location compared to where it launched from. Sometimes maybe 5/10 metres away. How can this be fixed?

Having Googled the S49 drone, it appears that your drone uses ‘headless rth’.

This means that instead of using GPS, the drone goes into ‘headless mode’ then pitches backwards towards you.

Unfortunately, due to the nature of how this works and the inaccuracies of the accelerometer, this process is rarely accurate.

I don’t know of any way to fix this as it is a hardware limitation as far as I know, so I suggest you think of it more as a way to move the drone roughly towards yourself to recover it if you lose orientation, rather than a way to have the drone return and land accurately.


EDF Simplified Thrust Calculation? So I am making my own EDFs that are 3D printed. The inside volume of the tube is 412,132 cubic millimeters I believe: Inner diameter is 80mm and length was 82mm
The hub diameter is: 40mm
Calculated FSA: 69.28
I’m using a standard 9v dc motor.
The calculated Kv for this motor is approximately 2,222
The calculation I found was:
Thrust = Mass flow rate * (Exhaust Velocity - Free Stream Velocity)
I went to calculate for mass flow rate as a start but that requires velocity and I can’t find a resource that tells me how to calculate for velocity without asking for mass flow rate which requires Velocity to calculate for ♂ .
Is that the velocity that the motor/fan spins at or is it the velocity that the air moves through the fan, or is it something else?
I know the density of the fluid is a part of that Velocity calculation and the Mass Flow Rate calculation. Given the fluid is just air I found that it’s 12.01 Pascals at sea level. I’m about 157ft above sea level.
I will then need to find Free Stream and Exhaust Velocity and I am not sure how to do that!
Can you give me an equation that gives me Thrust that uses any combination of only: motor RPM, motor Kv, Tube Volume, FSA, Wattage Motor Uses, number of rotors, Diameter of interior EDF tube, Diameter of Hub, Voltage, Air density, length of EDF.
I have the numbers for all these and that should be enough, that’s a lot of details.
If not can you explain what Velocity it’s talking about and also what Exhaust and free stream velocities are?
THANKS!

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


Would I be able to program a smartphone app to control a smartphone-controlled drone? I want to buy my first drone, I have no prior experience. I'm interesting in getting a cheap drone with a camera that can be controlled from a smartphone app. Something like the Tello Ryze would be at the higher end of my price range. I have experience programming (including smartphone apps). I noticed the Tello Ryze specifically has an SDK. Would it be possible to use this SDK from an Android app I wrote? And is it usually possible to program drones that are controllable from a smartphone yourself (I am finding it difficult to Google good information about this)?

To clarify, what I would like to access in my program:

Video stream from the drone to my smartphone app
Send commands from my smartphone app to the drone to control it

This certainly seems doable. The SDK documentation is not very clear but basically it's a simple string command over UDP. So it looks like the drone creates a WiFi access point which you connect to with your phone and you have a dedicated IP address/port number to send commands to the drone.

There is a simple python application linked in their SDK document that shows the basics. And there is a github repository that also has a python program that shows how to get the video feed from the drone over the command API described in the SDK.

Given the SDK and the example projects in the github repository I think it would be a feasible project by someone with some Android development skills.


How to program drones that have old flight controllers? I have a few components of drones like some old FCs and old ESCs and I don't want them to go to waste. Is there any way to configure a drone that I built with those parts, as I don't think the FCs are capable of running BetaFlight. (are there any really simplistic, light things that the FC can run instead of BetaFlight?)

You can always run an older version of betaflight. Also, if you have older F3 controllers you may also like to like to look at https://github.com/spatzengr/betaflight/releases which are maintained by UAV Tech.

I have used these on some older F3 boards and it breathes new life into these old boards.


What can I do with a LiPo whose balance lead has been destroyed? I'm starting a bit of a collection of batteries which have been ejected during a crash and ended up with a destroyed balance lead. Is there anything other than disposing of them that I can do, assuming the cells are still in good condition?

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

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

 (cit.)

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

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

Repair instructions:

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

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

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


What are the advantages and disadvantages of a "plus" and "cross" quadcopter configuration? Is one preferred over another? I'm not really an expert when it comes to quadcopters, I heard about the "plus" and "cross" configurations and that the "cross" configuration provides more stability.
I would appreciate any sort of help on the advantages and disadvantages of those configurations and the physics/maths behind why one is better than the other.

According to this paper:

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

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


Serial Communication Not working in APM 2.8 I have being trying to establish Serial Communication connection between APM 2.8 and Raspberry Pi Board. But the connection is not being recognized in the Raspberry Terminal Console.

Configuration Setup:

Model: Quad-copter

Type: X

Flight Controller: APM 2.8 FC

Companion Computer: Raspberry Pi 3 Model B+

I have being digging through web archives and found only MAVProxy Connection with Pixhawk & Raspi. Any help, advice, or guidance would be helpful.

Current Refereed:Here

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

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

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

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

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

enable_uart=1

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

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


How does a propeller with more blades create more "grip" I have heard that the more blades a prop has, the more "grip" it has in the air at the expense of speed.

How do more blades create more grip but lose top speed?

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

In a propeller spec which can be displayed either as:

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

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

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

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

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

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

 (cit.)

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


Can I salvage a 4in1 ESC? I have a 4in1 Brushless ESC that only one ESC burned out on, but the rest should still be usable. Is there a way to salvage it and use a single ESC to replace the burned one or something? If so, how would I go about doing that?

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

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

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

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


Could a drone collect a river sample? I know very little about drones, but I'm trying to learn if collecting river samples is something that could be done autonomously using commercial drones?

Is it possible to give the drone some coordinates of the river and have it go and collect without further input?
Is it even legal to do such a thing without someone controlling?
If it's not possible to do autonomously, how much input would be needed from a controller?

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


Why don't multiprotocol transmitters work with Radiolink receivers? I have a Jumper T16 and am wondering why it or any other multiprotocol transmitters don't work with Radiolink receivers. Is there a reason for this?

The Jumper T16 multiprotocol module doesn't support Radiolink receivers, as you know. This is because the module doesn't have any hardware to recreate the Radiolink protocol.

As can be seen on the product listing for the T16 Pro here,

  RF Chip Data

  Cypress Semiconductor CYRF6936: DSM/DSMX, Walkera Devo

  Texas Instruments CC2500: FrSky, Futaba SFHSS

  Amiccom A7105: FlySky, FlySky AFHDS2A, Hubsan

  Nordic Semiconductor NRF24L01: HiSky, Syma, ASSAN and most other Chinese models

The reason for this is likely either that:

The necessary hardware to create the Radiolink protocol either isn't available to third parties or isn't cost-effective to license
Radiolink receivers aren't popular enough to warrant the R&D required to reverse-engineer the protocol or include in a radio


Methodology and sources for obtaining the correct replacement batteries for low-end RC aircraft According to Mark's answer to this Drones and Model Aircraft SE question, I likely need to buy quite a few replacement LiPo batteries.

Finding large LiPo batteries for expensive aircraft is usually rather simple.  There are scores of websites that sell them, as well as many brick-and-mortar RC/hobby stores.  Standards exist to help guide one's purchases.

But for the small "non-replaceable" batteries that are included in inexpensive RC aircraft, what are some good sources?  By "non-replaceable", I mean that the batteries are not designed to be replaced, but, of course, can be replaced by someone experienced with electronics and aware of the required safety protocols.

What methodology is appropriate for getting the right battery for each aircraft?

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


At what per-cell voltage should I land? I built 5" quad and I'm using 4S 1300mAh LiPo batteries. My question is: at what voltage should I land? Let assume that I'm trying to land when my battery is as low as it should go without causing damage to the pack.

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

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

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

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


What is the difference between a patch and helical antenna? I have seen both patch and helical antenna when shopping for video receiver antennae. All I know is that they are both directional antennae. Do these types of antennae have any inherent performance differences? (I´m talking specifically about 6-turn helical antennae compared to patch antennae)

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

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


What is the analog video input pinout on the DJI FPV goggle? I want to build an adapter cable for DJI FPV goggles analog video input. What is the correct connector type and the pinout for the analog signals?

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

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

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


What is the point of a dual-gyro setup for a FC? When looking for a flight controller for a drone build, I have seen some FCs with dual-gyros. What is the purpose of having two gyros on a FC? (Are there any advantages or disadvantages)

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

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

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


What happens if i select the wrong number of motor poles in BetaFlight? what kind of effect has on my gear and flights to have set a different number of motor poles that I actually have installed. for example, my motors have 14 poles but I set 12 on BetaFlight or vice-versa.

This is a setting that is needed for RPM sensing and RPM filtering. An ESC can not sense the RPM of your motors directly, but can sense how often the magnets in the motor bell pass by the coils in its stator (this is called eRPM), and will report that number to the flight controller if you've got bidirectional DSHOT enabled. If your motor bell has x magnets (or poles) in it, then they will pass a coil x times per full revolution of the bell. Thus,
    eRPM = x * RPM.

In other words, to calculate the RPM of a motor, the flight controller will read the eRPM value reported by the corresponding ESC, and divide it by the number of poles. This means that if you set the wrong number of poles, your flight controller will read the wrong RPM from the motors, and the RPM filter will not work properly.

If you don't use RPM filtering, you probably should not care. If you do, it won't work as intended (and may cause your copter to fly worse) unless you put in the correct number.


Can motors be "too big" for any reason? I'm wondering, what would make a certain motor "too powerful/large" for a build? Could software/firmware of FC/ESCs be a problem? The only thing I could come up with is strength of the frame and weight limitations. Just looking for anything a beginner might miss.

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


What is the smallest drone ever made? There are nanocars and nanomachines in general.

I would be very impressed if a nano drone or nano-UAV existed, however, I would not be surprised if there has been something built on the microscopic scale.

What is the world record for the smallest drone ever made?

Previous answers have covered commercial ones. I believe the smallest self-powered drone is the piccolissimo. It is 2.5g and passively stable.

Here is the Wikipedia article on it:
https://en.m.wikipedia.org/wiki/Piccolissimo


How do you build and use a smoke stopper? What is the right way to build and use a "smoke stopper" device to prevent a short or other improper wiring from damaging your electronics?

Is it different depending on what voltage you are using?

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

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

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

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


Do propellers layered on top of each other have twice the thrust of one propeller? If you had two of the same propellers layered on top of each other going at the same RPM, with some space in between, would the thrust be double the amount of a single propeller at the same RPM?

If not, what factors affect this?

The propellers are pushing down on the ground by the way.

At least at the small/model scale, the answer is no. There is significant efficiency loss due to the stacked propellers not receiving the same clean air as they would if they were horizontally separated.

This is demonstrated experimentally in a Youtube video by rctestflight which compares the efficiency/thrust production of three different propeller configurations sharing the same propeller type and motor type:

Coaxial — What you're talking about where two props are stacked vertically so that they share the same rotational axis.
Overlapping — Where two props' discs overlap each other by roughly half a prop diameter
Normal — Where two props are positioned side-by-side without any overlap

To quote Daniel from rctestflight in his video:

  Any prop overlap is significant in efficiency loss. Traditional side-by-side
  motors are 25% more efficient than coaxial and 8.4% more efficient
  than overlapping props.

  To me, losing 25% efficiency is not worth the small form-factor and increased ease of transportability that a coaxial system produces.


Why do most drones have upward facing motors? Most freestyle quadcopters I have seen are specifically designed to fly with the motors facing upward and have the arms of the drone underneath the propellers like this:

As you can see in the picture ^ , the arms of the drone are fairly wide (as is common in lots of quadcopters of this type) and it seems to me that the arms would block a portion of the air coming from the props. I imagine that having the arms like this would reduce efficiency or top speed and maybe cause some amount of prop wash.

There are significantly less quadcopters which have the motors mounted under the arms like this:

(and almost all of them are home-made projects like this one which modified an existing frame to work upside down)

What I am wondering is, why are so many manufacturers and hobbyists making so many drones with upward-facing motors as it seems that downward-facing motors would be better in many ways? What are the advantages and disadvantages of upward-facing motors vs downward-facing motors?

One practical advantage is increased ground clearance, reducing the likelihood of a prop strike on landing or take-off.

Aerodynamically, a pusher1 design can be less efficient because the propeller is spinning through the wake of the fuselage - this results in 'lumpy' air, causing vibrations. I don't have exact figures, but I've heard anecdotally that this can be a 10 to 15% drop for fixed-wing designs and the exact fuselage configuration will make a difference!

1: Pushers are often defined as '[motor/engine] shaft under compression'; which seems to apply to the 'upside down' motor configuration.


DJI FPV Goggles Max Power option not available. Location: USA I have a set of DJI FPV goggles but cannot change the Max Power option. It shows on the menu under the device section but I cannot select it.

I am in the USA
updated to latest firmware: V01.00.0500
2S 3000mAh battery
.txt file on sd card narco_pwr.txt with pwr_2 attribute in txt file
I have not powered or setup my Caddx Vista.

Max Power is  set to 25mW and I cannot change it.

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

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

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


What are the specs of the DJI Mavic 2 Pro motors? Does anyone know the specifications of the motors on the DJI Mavic 2 Pro? I'm asking about things like stator size, KV, etc.

The most information I could find and definitely link back to the Mavic 2 is the following from the EU CE testing report:

1.5.1   TABLE: List of critical components
╔═════════════╦═══════════════════╦═══════════════╦═══════════════════╗
║ Object/part ║ Manufacturer/     ║ Type/model    ║ Technical data    ║
║ No.         ║ trademark         ║               ║                   ║
╠═════════════╬═══════════════════╬═══════════════╬═══════════════════╣
~             ~                   ~               ~                   ~
╠═════════════╬═══════════════════╬═══════════════╬═══════════════════╣
║ Motor of    ║ DZ DJI Technology ║ WM240-2009    ║ 14,8 V, 0,5 A     ║
║ airscrew    ║ Co Ltd            ║               ║ max. 1040 rpm/V   ║
╚═════════════╩═══════════════════╩═══════════════╩═══════════════════╝


Eliminate effect of Vibrations on IMU I am testing pitch orientation correction of Quadcopter and ended up in a issue.
I have a test setup with one degree of freedom to test orientation correction. I have fixed the drone to the setup and basically trying to test pitch correction by feeding a desired pitch angle and current pitch angle from IMU. The AHRS code uses Mahony filter to fuse the sensor data from Accelerometer, Gyroscope and Magnetometer. When I start my testing and spin the motors, can see the current pitch angle varying (or Oscillating). When I test without spinning the motors, the pitch angle is much precise. I observed that when the motors spin, a lot of vibration is generated and i doubt this is affecting the sensor to result in inaccurate output.
My question is

How can the vibration effect be nullified on the IMU sensor? By using some hardware component?
If it can be handled using Software, please let me know.

Thanks

You need to be specific about what IMU hardware you are using, like a general purpose IC like MPU6050 (gyro + accelerometer), or flight controllers like  Pixhawk, pixRacer, etc ..
In Order for you to achieve a fine tuned, linear input-output response, you need to take care of few things firsts and accept some constraints :

you cannot fully get rid of vibrations, but hopefully you can reduce it to an extent by using  anti-vibration absorber like this

You cannot achieve a linear relationship between input and output signals. This is mainly due to limitations of The flight hardware you are using
In the end, the goal is get proper output signals rather than reduce vibrations. this can be achieved  by

1)Research more about your IMU hardware, like manufacturers Datasheet, or asking with the community

The reason why you are getting a change in pitch angle is due to its internal offset of various parameters associated with its IC manufacturing.

Coming to UAVs, you need to implement your own PID algorithms by using digital filters like Kalman Filter, or you can choose Open Source flight firmwares for your IMU hardware.

After implementing and testing your filter, you need to fine tune it by hooking the output to a DSO and modifying the filter coefficients to suit your needs. A lot of trial and error goes into finding the tuning parameters like Kp, Ki, and Kd.


Drone with SDR for HF antenna field strength measurements, I need a Raspberry Pi based drone kit that I can just buy I have very little time until I get to the USA to buy the material to put together a drone which I want to use to measure antenna gain patterns for HF radio. Example, a 20m band half-wavelength dipole antenna is 10 m wide and should be mounted 10 m above ground, probably it will have to be less. Additional parasitic elements of about 10 m wide spans are added around the main driven element to achieve directional gain.
The issue is measuring this gain for which one needs to get two wave lengths away and up all around the antenna assembly, which is easily 100 m all around. And for 40m, 80m, 160m band antennas it would scale accordingly.
The measurement device we would need to lift up is an SDR (software defined radio) device, which can be as small as a USB dongle, along with some small receiving antenna. The drone would fly around the transmitting antenna and would measure the antenna's gain pattern on a set of chosen frequencies and transmit power while we would be intermittently transmitting our call sign in Morse code like a beacon. Given the fact that it's an experimental antenna assembly, the drone should re-take the same flight path several times as we adjust the setup.
Since an SDR dongle needs to be plugged into a USB port of a computer running some Linux, the Raspberry PI comes to mind. And to avoid having to carry that as extra payload, I want to have a drone which has a PI as its main control and communication module.
I have seen several PI drone projects, dating as far back as 2016, but even after all that time I have not seen an affordable kit. It should be around $200, as even in 2016 a ZDnet article talked about a DIY PI drone in that budget. Since I have very little time to get the parts together while on a short visit to the USA before returning to South America where everything is twice as expensive, I would like to find something that I can just buy. Do you know of a kit or a state of the art shopping list that is based on a generic drone kit?
[Hey Stack-Exchange police: this is not just a shopping question, it is a state of the art question.]
What is the state of the art in Raspberry Pi drones now? Much has happened since 2016.
How can I tell from looking at drone kits on Amazon for example, how much payload they are able to carry? This is not reported in the specs that I can see.
I would like to avoid the RC gear and just use WiFi to control the drone and receive the telemetry data.

You might check out this video on using a Pixhawk and a rPi with Python.  The video just walks you through the serial connection and some Python code for basic control.  The Pixhawk is doing all the real time control (Linux isn't good for that part) and the rPi is talking to it over a serial port.
You might also look at the MicroHawk reference in:  Do drones with inflight API access for control inputs exist? (although the link seems to be broken at the moment).


Quadcopter rotates too slow When I flew my drone yesterday, all the manoeuvres I did felt really slow. I then went into the OSD and changed the rates a bit. The drone then acted as it should, but when I then changed back it still felt fine.

When reviewing the blackbox logs afterwards I noticed that the gyro only registered around 340 deg/s rotation at full stick deflection (See the last part of the bad blackbox log)

After going into the OSD to change what I said above, the rotation speed was 900 deg/s at full stick deflection, as it should be with the rates I have.

To clarify a bit: It felt like I was using rates that were much lower than they actually were. The problem disappeared when I went in to the OSD menu and out again, but the next day I had the same problem again. It is not a major issue as it is easily fixable, but I would be glad if I could get some help to figure out what the problem could be.

The flight controller uses betaflight 4.1.

Here is a link to the blackbox logs.

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


How to restore LiPo-powered aircraft that have not been flown for years? I have a bunch of relatively inexpensive battery-powered radio-controlled aircraft that I haven't flown in over a year.  I haven't flown some of them in over 3 years.

The aircraft are airplanes, helicopters, and drones.

I believe they are all powered by LiPo batteries that are designed to be recharged, not replaced.

From what I've learned about LiPo batteries, they tend to go bad if not used for a long duration.

What general steps do I take to resurrect all these aircraft so I can fly them again?

You may need new batteries.

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

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

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


F450 frame all legs are not stable I have one of the so called popular F450 frames. Not the original dji one, but chinese part. Now, I have assembled motors and ESCs. The assembling process was harder than it should be, probably because of not being original product. Anyway, I achieved to match holes with each other and installed the top plate. However, there is a problem. To cross legs(front left and rear right) cannot touch to the ground at the same time, i.e, one of the legs stay in the above the ground, approximately 2 cm. Is this something I should worry about? Can a neat flight be achieved with this problem existing?

Do not worry, I also have an F450 frame(not original) and one or 2 of the legs stick up slightly. I have had a perfect flying experience even if I was using an Arduino as a Flight Controller. Your FC(Flight Controller) should smoothe it out. But, if it sticks up more than like 5cm, then you might have a slight concern.


Estimate safe ESC curerent for 4108 600kv motors? Is there a way to safely guess what current ESC's will need to be with this motor (or any motor in general)? The motors are rated 23A in the table, but that's only with a 4s; I might use a 6s in the future. If I were to use, say, a 30A ESC, should it be enough to not burn out mid-flight or something tragic? Would I get some kind of fair warning (obvious smoke, buzzer, etc.) before my investment falls out of the sky? Thanks!
Edit - I'll be using 1555 props as recommended, and the ZD550 frame with a small gopro gimbal

Myriad factors contribute to the current draw of a drone motor including supply voltage, motor kV, the propeller geometry (i.e. diameter, number of blades, pitch), ambient atmospheric conditions, etc.
One can make educated guesses and estimations of the current a motor will draw under known conditions, but the best and most accurate/reliable method is to test the desired setup and measure the current draw under the expected operating conditions. If you can, you should try this by assembling the motor and propeller on a thrust stand and powering the motor while measuring the current draw with an ammeter.

From the Banggood listing linked to in the OP:

The motor of interest here is the 600 kV variant, which appears to be rated for up to 6S voltage, but power draw is only displayed for 4S voltages. If we take the rough assumption that the motor's current draw will increase linearly with the supply voltage (also assuming all other conditions are unchanged), then we can estimate the current draw for the listed propellers running on a 6S batery:
$$\begin{align}
\text{New Current Draw} &= \frac{\text{New Voltage}}{\text{Old Voltage}} \times \text{Old Current Draw}
\\ &= \frac{6s}{4s} \times \text{Old Current Draw}
\\ &= 1.5 \times \text{Old Current Draw}
\end{align}
$$

APC1238 (12" prop, 3.8" pitch): 25.8 A
APC1447 (14" prop, 4.7" pitch): 33.75 A
1555CF (15" prop, 5.5" pitch): 34.5 A

NOTE: These are incredibly rough estimates. I highly encourage experimentally determining the true current draw of your intended setup.
NOTE: These calculations will be irrelevant if you're not using the same propellers as the ones cited in the experimental performance data.
As @Kralc mentions in his answer, you should add a roughly 20% safety margin on top of the expected current draw when spec'ing out an ESC to account for any unforeseen situations.


Tuning with notchy motors on Betaflight I'm having trouble tuning a custom 3-inch build on Betaflight.  I can't get general vibrations to go away.  I'm using RC in Power 1506 4100KV motors on 4S.  These are very notchy motors.  I've been told notchy motors are difficult to tune.  Are there any pointers out there for such a build with notchy motors?

I've tried several versions of Betaflight 3.5.7 through 4.1.  I've turned down the filtering and the motors stay cool.  I've tweaked the PIDs a lot and it hasn't helped much.  I've even changed to a completely different Frame to get rid of any possible resonance.  I've tried soft mounting the FC, and hard mount.  TIA

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

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

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


Seeking a cheap, programmable drone, for use in the UK This meta question states that recommendation questions are on-topic. However, as I am new here, I don't want to commit a fox's foot (faux pas ;-), so please vote to close, rather than downvote, if I have posted wrongly (or, just ask me to delete the question). Thanks.
As soon as I get a single answer here, I will delete as duplicate my as yet unanswered Hardware Recommendations question Cheap programmable drone, for use in the UK, which reads:

I am looking for a cheap, programmable drone, for use in UK, to use in
a Minimal Viable Product for a system that will later use more highly
spec’ed drones.
• Needs to accept a programmed flight plan (*)
• Needs to carry an
ESP32 (approx. 10g) and a coin cell battery (2.9g), possibly an
altimeter of GPS sensor (I would guess 5g, max 10g)
• Long flight time is not important, but I would like 5, preferably 10 minutes
• Outdoor use, not indoor, although I can live with being restricted
to calm days
(*) I am not sure if UK law requires a human drone operator, rather
than a programmed flight plan. If that’s the law, I will comply, but I
have a strong preference for programming a series of waypoints.

[Update] do I need to be licensed to fly a drone in the UK?

I hope that this question is specific enough to be acceptable. Basically, I am looking for a quick & dirty demo (don't want to spend £ hundreds, in case no one bites at the idea), to demonstrate how a drone with an ESP32, or similar, can be automated to fly over sensors and collect data via Bluetooth Low Energy (BLE).

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


Connect mavlink (on RPi) with QGroundControl I am trying to connect Mavlink (installed on RPi) with QGroundControl via 3DR Radio Telemetry as to transmit telemetry data from RPi without dependency of Ardupilot/Px4 flightcontrollers. I want to achieve transmitting data such as GPS or Postion using the message format of mavlink without dependency of ardupilot/PX4.
So far, I have taken the example code from mavlink git and have built the code as per instructions. I start the QGroundControl and was able receive the System ID and component ID from QGC. I modified the code to transmit a heartbeat from RPI-mavlink to QGroundControl.
mavlink_msg_heartbeat_pack(system_id, 200, &msg, MAV_TYPE_HELICOPTER, MAV_AUTOPILOT_GENERIC, MAV_MODE_GUIDED_ARMED, 0, MAV_STATE_ACTIVE);

int len_hb = write_message(msg);

After building and executing this code, I was expecting the QGroundControl to connect/show received message, but nothing was available at the QGroundControl.
I need to understand if what I am doing is the right way to do or if there is predefined method to establish such kind of communication. Kindly, help me out with documentations (if any) available to accomplish message transmission without ardupilot/Px4.  If someone could guide me on this issue, it would be really helpful.
P.S: I have posted the same Question on StackOverflow
Thanks

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

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


Do low temperatures damage LiPo batteries? I'm wondering if low temperatures affect LiPo batteries in any way? Can colder temperatures damage the cells? Can I store my batteries in bat safe box outside, in cold/hot temperatures?

Low temperatures do affect LiPo batteries in a number of ways, first and foremost being the increase in internal resistance as the battery gets colder. This is why LiPos need to be warmed up before flying in cold weather, as otherwise they will be able to sustain a much lower discharge current than usual and will not give their full rated capacity:

In flight the current flowing through the pack itself will hopefully generate enough heat internally to keep it warm, though for longer-flying platforms with low current draw (i.e. long-range fixed wing) an onboard heater or thermal insulation may be required to keep the cells warm.

As for damage, it does not occur in storage or while discharging even when a LiPo battery is brought below freezing; the rated temperature range for discharging is -20° to +60°C, and the electrolyte itself does not freeze until around -40°.

In fact, storing lithium batteries at 0°C prolongs their life compared to when you store them at higher temperatures. Some owners of Li-Ion batteries even claim to "revive" dead cells by placing them in a freezer for a week or so.

The one thing you should be really careful about when storing batteries at low temperatures is the possibility of moisture condensation on the battery's terminals when you bring the cold battery into a warm room afterwards or simply if the weather changes. This can cause a short circuit, which, while unlikely to damage the battery (as the water will likely evaporate from the current before anything truly bad happens), it is still unsafe and will corrode the battery's contacts through electrolysis.

So if you decide to store your batteries in the cold, and especially outside, put them in a sealed bag together with a baggy of desiccant (e.g. silica gel) so that the air around the battery is always as dry as possible, and when bringing them inside, do not remove them from the bag until they've warmed up to the ambient temperature.

However, if you try charging a lithium battery at a below-zero temperature, the battery will be rapidly and permanently damaged via the same lithium plating process that damages cells whose voltage is brought too low. The rate of damage is proportional to the charging current (i.e. more current damages your batteries faster), and the extent of it is such that manufacturers state that the battery is completely dead and should be disposed of if it's charged below zero even once.

The allowable charging temperature for lithium batteries is 0-45°C, but I'd recommend sticking to the middle of the range and only charging at room temperature.

Please note that the exact numbers (i.e. rated temperatures) are for Li-Ion cells, but they should be close for the whole lithium rechargeable family.


Recommendations for a BLDC motor and battery for a 28x20 propeller? I'm looking for a motor and battery set up that can last for over an hour and spin the propeller at 6000+ rpm. The propeller is a 28x20 single blade being connected to a second blade for a total of 56 inches. The weight of the whole propeller is 0.56lbs.

The first issue with your question is that you can't spin a 56" prop at 6000RPMs without it tearing itself apart. The tip speed of a 56" prop at 6000 RPMs is 1000mph, well over mach speeds. Practically speaking a 56" prop will probably be good up to about 3500 RPMs safely, with an absolute maximum of about 3700 RPMs (0.80 mach).

56x20 is massive, so you're going to need a significant motor. Based on the specs and what you're looking to accomplish it could vary some, but this motor is probably very close to what you're looking for: https://www.getfpv.com/lumenier-lu15-ii-80kv-professional-motor.html  The 40" props they've tested are only hitting about 3500 RPMs, and barely within the motor's wattage limits, so you will probably not quite be able to hit the maximum RPMs for the prop, but should be workable.


Does the performance of a brushless motor decrease over time? The brushes wear out in brushed motors and their performance decreases until they stop working, but brushless motors have fewer parts in contact and thus don't wear out nearly as fast.

Does the performance of a brushless motor decrease over time or does its performance remain practically the same throughout its life?

Like most things, it depends on how you treat it.

As far as I know, you should never need to worry about a brushless motor wearing out, especially for our purposes where we will likely demolish them before they have a chance to wear out.

You do have to watch out for the bearings and magnets, however.

With the bearings, like any mechanical component, they will eventually wear down, however this will likely not be for a long time. It is still a consideration, though.

You also have to watch out for the magnets.

Under normal circumstances, the magnets are fine to last a long time, however excessive heat can eventually demagnetise them, leading to reduced performance and eventually the motor not producing enough power for us to use.

If anyone has any other ideas or wants to correct me, please do!


What is DSHOT in ESCs? I've heard that modern quadcopters use a DSHOT protocol. Could you describe what it is?

What's its purpose?

DShot is a protocol that flight controllers use for communicating with ESCs. It stands for 'Digital Shot'. It is a new digital communication protocol developed by Flyduino in collaboration with Betaflight as an alternative to Oneshot and Multishot.

Oneshot and Multishot (and standard PWM) all use analogue signals. The length of an electrical pulse determines the value being sent from the flight controller to the ESC. This can lead to problems with accuracy if there are slight differences between the speeds of the clock signals in the ESC and the flight controller (which is why ESC calibration is required with analogue protocols). Also, these signals have a fixed length range of 1 to 2 milliseconds, which means they can't be sent more often than once every 4 ms or so, or 250 times a second.

Digital signals can also be more resilient in the presence of electrical noise.

The number associated with the protocol indicates how much data is sent per second.  There are several speed options available:

DShot150 – 150,000 bits/Sec
DShot300 – 300,000 bits/Sec
DShot600 – 600,000 bits/Sec
DShot1200 – 1,200,000 bits/Sec


What technology offers the longest range for RC controls? After watching a video demonstrating a jet-powered RC aircraft, I started to wonder about the maximum range available with modern RC equipment.

What is the maximum range currently available, and which technology does it implement?

On which frequencies does it operate?

If you have any data regarding the control-response lag (delay), please include it too.

I'm more interested in controller-based distances, and not FPV.

If you are not interested in hard, real-time control and can accept some autonomy in the drone, then your best choice is satellite communication. The iridium based modules are lightweight, (45 grams), and are easy to integrate (they have USB and Serial connections). One example is RockBlock. You pay per message, and one message (50 bytes) is just about enough to send coordinates of the next way-point and to get status information from the drone.

EDIT:
In response to the comment about LEO: if your drone is about to go further than low-earth orbit, take a look at this S-Band transceiver. Perfect choice at 194 grams. However, you will need to build your own tracking station, but then even the sky is no longer the limit.


Can I replace the switches on my taranis qx7s with switches from a X9D? One of the switches on my taranis qx7s recently broke, and when looking at replacement switches I found two alternatives; X9D and X10/X12. The X9D switches are much cheaper than the other, so my question is therefore: are the X9D and qx7s switches the same?

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

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


How can I learn to fly RC planes using a simulator? I intend to learn how to fly RC fixed-wing planes (I have previously worked on semi-autonomous aircraft, but had never learned how to fly RC planes). However, I currently live in a small place where there are no suitable places for flying planes (due to population density, lack of facilities, and laws). There are no "empty places" in the entire country. For now, I will be satisfied with flying in a simulator, leaving real-life practice for the future. I don't mind buying the RC remote control. What kind of simulator should I get? Is there any criteria I should use to find a suitable simulator for fixed-wing RC aircraft?

I've enjoyed the Great Planes RealFlight simulator for a number of years. It has lasted well and I like the included transmitter controller. It is not free nor cheap, but it will save you a lot more in RC models that you can bring home from the flying field ready to fly again (i.e. it will save you many crashes).


Why do multirotors usually have four propellors? I've observed that hover-capable drones almost always have four propellers. There are helicopters of course, which have two, but in general, the vast majority of UAVs seem to go with four propellers.

It seems to me intuitively that this must be less efficient: More points of friction, more wiring, more weight, etc...

Why is it that we see an overwhelming predominance of four propeller drones rather than, for instance, three propellers?  Is it just because it's easier to control or gives a smoother flight?  Or is there some physical reason?

With aircraft, there are 6 degrees of freedom (DoF) we want to control (roll-pitch-yaw, and x-y-z), but for hovering vehicles (i.e. rotorcraft) we can get away with controlling two degrees of freedom (x-y) by combining the 4 other degrees (roll-pitch-yaw + z).

There are many ways of controlling those four DoF, typically:

helicopters, which use variable pitch blades and some combinations of main and tail rotors, to result in a combination which results in control over roll, pitch, yaw, and thrust
tricopters, quadcopters, hexacopters, octocopters, etc..., all of which use their blades to control the roll, pitch, yaw, and thrust.

Somewhere it is inescapable that for stabilized flight on all four DoF you must have the ability to control each DoF. Mathematically, the quadcopter provides the lowest number of simple actuators that accomplish the job. Each actuator can work by simply speeding up or slowing down, whereas helicopters and tricopters require some other kind of actuation, usually a complex one. It turns out from a pragmatic viewpoint speeding up and slowing down motors is much more robust than changing angles of spinning blades.

Quadcopter

Formally, it looks like this:

So quadcopters reign over helicopters and tripcoters because they're simple. Each axis is controlled by a certain combination of motor speeds and all axes are independent, i.e. when you increase thrust you don't have to also worry about changing the pitch.

So why not hexas, octos, etc...?

Hexacopters

Hexas can lift more, but provide no additional safety margin because there is no way to stabilize all four axes when an arbitrary motor goes out (there are certain configurations which can continue to fly if one motor out of a set of four fails, but cannot fly if either of the other two fail).

Hexas also are less efficient, since they use smaller propellers. (Without diving into blade theory, and simplifying somewhat, the most efficient blade is a single blade which is infinitely large and moving infinitely slowly.)

Hexas also have 6 motors for four degrees of freedom, so when they're all flat you get what is called an "overconstrained" system. You can actually angle them to interesting effect, e.g. CyPhy Work's LVL 1 drone.

Octocopters

Octos theoretically can continue to fly if any arbitrary motor fails, but in reality, you'll likely encounter blade stalling effects or other pathologies that will not allow the octo to sustain flight. This happens because the typical octo is loaded past the point where 4 motors can carry it safely. However, certain high-value vehicles, such as those carrying expensive cameras, are octos because it's cheaper to over-spec the octo's batteries and motors than it is to replace a $50k camera.

Octos are also less efficient, for the same reason as hexas.


Is owning, but not flying, a drone in Germany illegal? I live in Germany but I travel a lot around the world; my new hobby is aerial photography, for this I am considering buying a drone.

Since Germany has some rules about drones, can I have a drone in Germany without a "mandatory label marking," "without insurance," etc?
I mean, the rules are clear to me IF AND ONLY IF I am flying the drone in Germany, but what if I use it only when on vacation in other countries? Am I breaking any rules?

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


Motor size and Kv advice for ZD550? I'm building my first drone out of the ZD550 frame. Most of the extra weight would be from a small gimbal like this (180g) and a gopro hero 4 (83g). What kv/size of motor would be ideal for this config, plus some extra? Some speed would also be nice :D I'm hearing around 600-900 kv, but I have no idea if that range is too vague. Also, shouldn't I be more concerned about torque? I think that's estimated through physical motor size.
Thanks!

There are a lot variables when it comes to building your own drone, so there will be a variety of motor sizes and Kvs that will work.
I think the recommended motor size for the ZD550 is 3508 (like this motor) but that doesn't mean you can't use something different, like a 2814 motor, because a 2814 is fairly similar in stator volume to a 3508. This answer should help you figure out what motors you want. Torque is mostly related to motor size, so you shouldn't have to worry about not having enough torque with either a 3508 or 2814 motor. And these motors should easily be able to carry some extra weight with no problem.
As for Kv, the prop size greatly affects the recommended Kv. Here's a good explanation of Kv and how to choose the correct Kv. The ZD550 frame can fit 10''-15'' propellers, so I would recommend lower Kv (~600) for 15'' and higher Kv (~850) for 10''. Also, Kv will greatly depend on what battery you intend to use. Generally a higher voltage battery = lower Kv motor, and vice versa.


Determining the mAh of 4S battery for a 3 inch drone I have a 3" drone frame, flight controller that can take 3S or 4S LiPo, and 1507 4200KV motors. How do I find the right capacity of battery to purchase? Should I purchase 4S 850mAh or 4S 1500mAh? And why?

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

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

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


How to avoid a drone going out of range? I have a simple drone (less than 250 grams), I want to fly it as high as possible to take photos with it, how can I make sure it doesn’t go out of range considering it has no altitude indicator? What would happen if it does go out of range, would it just fly off?

The first stage is to make sure you read your local laws about altitude and VLOS restrictions.

Once you know what you can legally do, the first stage is to set up a failsafe. Do this by setting failsafe conditions in your radio and in whatever firmware your quad uses. Choose what you want the quad to do during a failsafe, whether it should drop out of the sky or slowly lower.

You can also install GPS and set the failsafe as a return to home, and depending on your firmware there are different ways of doing this.

Another way to make sure you stay in the range is to set up RSSI or LQ depending on your system. This tells you how strong the signal is so you can monitor it.

You can also get an RF meter to make sure there is no interference where you intend to fly.

Finally, you can use a specialty long-range system like TBS Crossfire or FrSKY R9, as long as it is legal in your region.


What happened to DSHOT1200 support in Betaflight? I just went to update my flight controller from 3.5.3 to 4.1.0, and I noticed that some time in the past two years the option to enable DSHOT1200 was removed from Betaflight.

Does anyone know why this happened? Was it something to do with the release of RPM filtering?

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

  DShot1200 is officially removed from Betaflight 4.1:

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

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


How to focus a newly-replaced lens on an FPV camera? I cracked the lens on my FPV camera after a metal pipe jumped in front of my quad...Fortunately, I had another lens from another FPV camera sitting around.  But I'm not sure how far away I should be trying to focus this lens.  Are there any best practices or rules of thumb to follow?

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

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


Betaflight Camera Angle What does the camera angle setting in betaflight effect?  Does it affect roles in acro?  Does it affect angle mode?

I feel like it's just a setting that does not affect my life.  Am I wrong?

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

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

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


Can I re-flash the Hubsan fpv1 controller with built-in fpv screen? the only radio transmitters, also called remote controllers, that embed a fpv screen, for seeing what the drone sees, near the joysticks, instead of having to get obstructed vision by goggles,
that I found, are :

https://www.youtube.com/watch?v=df0Cvv3T69Y called fpv1 (i'm not interested into h7000 as it runs android and requires wifi rather than RC)
https://www.youtube.com/watch?v=XBbptOuXXdA called fpv2

However they only seem to be able to bind Hubsan drones...

Would it exist a way to flash them in order to make them run openTX firmware, or anything that could make them become universal controllers?

Alternatively, what parts from Hubsan should I buy if I want to put in my own custom drone build and be compatible with the DIY ecosystem ?

Can I build from scratch a controller with arduinos so that I could use openTX, embed an fpv screen, while still getting decent range (>5km) ?

I'll start by warning that you may have misconceptions that are leading you in the wrong direction. The reason that FPV screens built into transmitters are not more popular is that screens aren't bright enough to compete with sunlight. The primary function of goggles is to cut out bright reflections, so you can see the picture.
Also, transmitters are comfortable when held down at your waist, which would make the picture on these transmitters too small. You'd need something the size of a laptop or tablet computer screen to work well at arms length. The picture from an FPV camera is pretty low quality by modern TV standards, and needs all the help it can get.
The transmitters you link can't be flashed to run OpenTx. They just weren't designed to be modified like that. They may not even have enough flash memory or processing power, as OpenTx has vastly more features. Similarly Hubsan doesn't sell a stand-alone receiver for their transmitters.
You probably could buy an OpenTx radio and transplant the electronics into one of these cases, but I think you'd have to find something clever to make their TV format screens display information that was designed for a dot-matrix screen. You could look at the boards used for OSD (On Screen Display) but I imagine it would get very technical.
I think your best bet would be one of the many FPV monitors, mounted to the top of a normal transmitter - just be sure to stand in a shady spot.


Why is 900 MHz better for long range than 2.4 GHz? There are various transmitter modules like the FrSky R9M system which operate at around 900MHz rather than the more commonly seen 2.4GHz. Why is 900MHz better for long range drones, and why is 2.4GHz so common if 900MHz can provide better range?

The strength of an electromagnetic signal will attenuate (decrease in strength) over long distances between the receiver and the transmitter due to the inverse square law, but this phenomenon applies equally to all wavelengths/frequencies.

The main difference in transmission distance capability between long-range RC systems like TBS Crossfire and FrSky R9M (which use the 900 MHz ISM band) and run-of-the-mill RX systems that use the 2.4 GHz band is due to the diffraction (and reflection) of the signal by obstructions in and around the line of sight between the transmitter and receiver. This process is explained well in this Electronics.SE answer. Essentially, lower frequency (longer wavelength) signals are less susceptible to signal strength losses by reflection and diffraction because they require larger objects to affect the signal in the same way.

As the Electronics.SE answer points out, this becomes easier to conceptualize if you look at two different frequencies like 1 MHz (wavelength of 300 m) and 100 MHz (wavelength of only 3 m). If there's, for example, a house or other large object in the vicinity of the path between the transmitter and receiver, the 1 MHz signal with a wavelength of 300 m will find it a lot easier to avoid being disturbed and reflected than the 100 MHz signal with a wavelength of 3 m for the simple reason that it is just so much larger in comparison to the house.

At extremely long ranges (like over the horizon), other effects also come into play like the ability of lower frequency signals to more easily reflect off the curved surface of the earth and the ionosphere, but these kinds of transmission distances aren't an issue for RC aviation for more practical reasons. :)

2.4 GHz RC solutions are far more ubiquitous because the antennae required to transmit and receive signals at a frequency of 2.4 GHz are far smaller than those required for 900 MHz. This is due to the change in wavelength between the two frequencies and is evident in the size difference between 2.4 GHz receiver antennae and those for 900 MHz:

TBS Crossfire Immortal-T antenna: (The active section runs horizontally in the picture)

Futaba 2.4 GHz receiver and antenna: (The active section is silver/white in the picture)


Storing drones during the winter Up here in Canada, it can get kinda cold during the winter. And just because of where I tend to use my drone, it can be easier for me to store it in an outdoor (dry, but unheated) shelter than to haul it back to the house all the time.

Is it okay for drones (Eachine E520S) to stay outside at like -30°C, or should I continually bring it inside?  My guess is that it's the batteries that will suffer the most, so should I just bring them in? (lithium-ion batteries)

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

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

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

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


My ESC in my stack just randomly exploded, any ideas why? I was casually flying my drone and my ESC board in my stack let out the magic smoke, and then fell from the sky randomly. I hadn't changed any tuning settings, and the stack was only ~3 weeks old. I am using a Mamba F405 MK2 stack, and the Flashhobby Samguk WU 2206 2700Kv motors. As far as i am aware, the Flight controller portion of the stack still works, but the ESC board is dead. The tune was essentially stock, except for modified serial port configuration.
What could have caused it to explode, and what should I do?

From your description I assume that the ESC in the stack blew up, not necessary the FC. This makes most sense as that is the part where the biggest currents are flowing.
Check if the electrolytic capacitor on the battery connection of the ESC still is properly connected as that absorbs the voltage spikes your motors generate during flying. If that capacitor is loose or located too far away, the voltage spikes can destroy the FETs on your ESC and you will burn up the ESC and the motor connected to that ESC.


How to decide drone motors? I'm planning to design a quadcopter with a payload capacity of 5 kg. I would need the thrust power from the motors to cater for at-least 7 kg with extra part weights.
I would like to know,

How do I decide the motor power? (I can do only one-off investment for that with my budget and no chance for trial and error.)
How do I decide the blade sizes? (for this I can spend little extra trying a few different blades.)
Deciding the battery. But when I know the motors, I guess I could do the maths to decide the battery, but any comments on that are most welcome.

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

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

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


Is there an advantage to having more LiPo cells in a low-power drone? A major reason that high performance racing or freestyle drones use LiPo batteries with high cell count (6s rather that 3s or 4s) is so that the voltage won't sag as much. This allows high performance drones to operate longer when pushing the limits of their power. Is there, however, any advantage to using a high cell count LiPo in a drone that doesn't pull enough power to cause any significant battery sag?

The advantage of running at higher voltage is that you can use thinner wires and lower current (i.e. cheaper) ESCs, as power lost to resistance is proportional to current (squared) and not voltage.

Another reason can be to reach higher prop RPM when you can't find motors with sufficiently high kV, or merely want to continue using the motors you already have.

Much of the discussion about increasing cell count is confused because people increase the total battery size, for example, going from 4s 1500mAh to 6s 1500. With 50% more battery it's not surprising that both flight time and the ability to provide bursts of power without voltage sag have increased. Similarly pushing more power through the same motor is not a free upgrade. Either it's using up a safety margin or the motors were larger and heavier than necessary in the original design.


How to measure the RPM of a drone motor using Betaflight? I am interested in finding out the RPM of the motors on my drone with certain amounts of load and without load. Is there some way to measure the RPM of a drone motor using BetaFlight? If not, is there another way?

Not sure if it matters, but I am using the SucceX-E 45A 2-6S BLHeli_S Dshot600 4-in-1 ESC and some Xing-E 2207 2750kv motors.

Bi-directional DShot can help you here. After updating the BLHeli firmware on your BLHeli_32 ESCs to the latest version (support was added only recently) and the firmware on your flight controller to Betaflight >4.0 (PR on Github), you can go into the Betaflight Configurator app and enable Bi-directional DShot under the Configuration tab.

It must be said that the easy BLHeli ESC firmware upgrade is only available for BLHeli_32 ESCs. There are ways to get support for bi-directional DShot on BLHeli_S ESCs, but they're more complicated. Instructions are linked to on the Betaflight Wiki page.

Motors tab in Betaflight Configurator

The ESC-reported motor RPM values will then be displayed under the R label next to each motor in the Motors tab.

OSD Widget

There is also an OSD widget for viewing motor RPMs, called OSD_ESC_RPM in the code, which can be enabled under the OSD tab.

BlackBox

As @BenWilson points out in his comment on this answer, you can also set the BlackBox debug mode to DSHOT_RPM_TELEMETRY and get motor RPM values recorded in your BlackBox log The implementation is described on the Betaflight Wiki here.