Why was 32kHz gyro sampling removed from Betaflight?

In order to access the 32kHz gyro sampling mode, the gyroscopes in question (e.g. ICM 20689) would be switched to an experimental mode where there is higher gyro data jitter, and that sampling at the lower 8kHz frequency made use of lower overall latency lowpass filtering built into the gyro.
Often to get usable gyro data from the 32kHz sampling rate, more filtering was required, and the overall phase delay response meant that the FC would not be able to start compensating for a change in craft orientation any faster.

The other part is that in terms of pure filtering performance, the net gain achievable assuming identical gyro signal-to-noise ratio performance is on the order of tens of microseconds.  The bigger potential improvement would be to use the higher sampling rate to enable enhanced/fast Kalman filtering where the PID calculations can take into account more data, however, the input noise in that experimental 32kHz mode means the net improvement is minor for the computational cost involved, and that better performance is achievable at the lower sampling rate the IMU units available right now (that meet the cost and form factor requirements to install onto cheap FPV UAS systems)

Practically, lower overall phase delay response with RPM notch filtering and moving lowpass filters well beyond the prop wash and flight input frequency space, which results in much better overall performance, and often reduced motor heat in flight across a wide range of flight envelopes because less amplification of gyro noise is occurring, particularly through the D term of a PID controller which amplifies higher frequency oscillations.


Why is it bad to power up a drone without an antenna on the VTX?

Short answer: Yes, this is very much true. The VTX sends out radio-frequency (RF) energy through the antenna connector and pigtail, and if nothing (or the wrong thing) is connected at the other end, all the energy gets reflected back and could damage your VTX (or just heat it up).

Long answer:

At high frequencies a thing called "impedance matching" becomes very important.

Basically, when a radio wave travels through a wire, the wire presents a certain impedance (a more "generalized" form of resistance) to the signal. Whenever the travelling wave encounters a change in this impedance as it goes through the cable, connectors and other hardware, i.e. it was 50 ohms before a certain connector and becomes 75 ohms after, part of the wave gets reflected back at that point, and the bigger the mismatch, the bigger the percentage that gets reflected.

Any properly designed antenna will have the same impedance (in the frequency range that it was designed for, i.e. 5.8 GHz for a video antenna) as the coaxial cable before it, and dissipate the received radio-frequency energy by radiating it out in the form of radio waves, thus nothing gets reflected towards the VTX. However, If there is nothing at the end of your pigtail, the mismatch is as big as it can possibly be, since nothing connected means infinite resistance. Thus all of the power that the VTX emits gets reflected back towards it and has nowhere to go other than to heat it up. If the VTX is poorly designed, the reflected power could even burn some of the components in it!

Obviously, this gets more important with the more powerful VTXes. If you've got a 25 mW or even a 200 mW VTX, it's probably not going to damage itself if left without an antenna for a while (although you better not risk it anyway). An 800+ mW VTX could do much more damage; in the best case it will just get very very hot.

If you really need to run without an antenna for some reason (which is rare, but possible if you're testing something that needs the VTX to be powered up, but don't want to actually transmit anything), there is an option to terminate the transmission line with a "dummy load" (which is just an powerful resistor that's impedance-matched to the coax and dissipates all the RF power that it gets) instead of an antenna. Note that shorting the connector is also going to reflect all the power back to the VTX; zero resistance is no better than infinite resistance.

Also, beware of antennae that are designed for other frequency ranges. i.e. a 2.4 GHz antenna from your router or radio control might look like it's compatible and will probably even have the same (SMA or RP-SMA) connector, so it'll fit mechanically just fine, but at 5.8 GHz it will have a drastically different impedance and thus transmit only a small portion of the power it receives, with the rest being – you guessed it! – reflected back and heating up your VTX.


How do I set up LUA scripts on my OpenTX radio?

Oscar Liang has a really good page for this specific use case https://oscarliang.com/vtx-control/

And Joshua Bardwell has some really good videos that you can follow along with
https://www.youtube.com/watch?v=zFb5IJpM1XU


Drone tipping over when attempting to fly (possible accelerometer problem?)

Some simple things to look for before we go deeper:

Did you check that all four props are mounted correctly? (CW on top left + bottom right, CCW on top right + bottom left)
With your props off(!), your drone connected to the Betaflight Configurator, and your control inputs neutral, are all three control axes all centered (at $1500$) in the Receiver pane?


Is superglue (cyanoacrylate) a viable means to repair quadcopter frames?

It depends on the risk you are willing to take, but for such a small drone being flown indoors I'd give it a go1 and accept it may crash - after all, it has already done so once so it can't be made worse! The repair will, however, be a weak point in future crashes.
How well it flies will depend on how accurately you reposition the part, but you're likely to get 'close enough' (again, probably 'worth a try'.) I would also, once the glue has fully set, try to hold the drone only by the broken bit and give it a shake - if it can hold the whole weight, it should be OK when the weight is divided between the propellers.
For larger drones you should replace the part to manufacturer specs or get a professional to take a look.
1 Subject to the normal indoor flying considerations - avoid pets, other people and anything expensive or valuable, etc.


Flysky FS-ia10b output more than 2000 microsecond pulse

I don't think that will be possible. The protocol for analogue radio was a 20ms packet, consisting of up to 9 pulses and a gap. So there were 10 things in a 20ms packet. The receiver would split each packet into 2ms blocks and send each block to a servo. Any pulse longer than 2ms would overlap the pulse for the next channel.
Obviously with digital radio, it's entirely possible to exceed this limit but I think it's unlikely that there will be an easy, built-in way to do it. Once you've set the end points as high as they'll go, I think that's all you can do.
You could use an Arduino to scale up the pulses you're getting from the receiver to the size you need for the servo. An Arduino Nano is a similar size and price to a servo, and easily able to handle that sort of task.
However I think you'd be better to look at why your servo needs such a long pulse. Presumably it worked OK before you modified it, so it should be able to work from a standard pulse. It's possible that you've damaged it during the modification, or that the resistance of your voltage divider is too different to the original potentiometer, or that you've connected it incorrectly.


Why does motor not want to go above 2/3 thrust?

It was the battery.
Bought a new battery, charged it, all 3 lights turned green, and motor/esc has no trouble going to full thrust. Lots of power now, so that should really help the plane fly.
Problem solved!
And on the bright side, I get to tear apart the broken 3s battery and see how to fix it or turn it into a 2s. I'll probably put a question on electrical engineering SE about that later.


How do I set up IRC Tramp for my HGLRC Forward VTX?

Since you confirmed that the protocol+vtx table is correct I'd focus on the physical connection. One explaination would be that the signal is disturbed and the vtx has a hard time decoding it.

Try to disconnect all other components and focus just on the fc+vtx. Try to isolate the SMA connector of the vtx from any carbon parts/metal parts of the frame. Another option is to twist the cables going from the fc to the vtx. You could also add capacitors to the power supply.

If nothing works I'd try a different vtx and check if the problems carry over the new vtx. Adding a phono of your wiring could also help to identify the issue.


What is the correct process for transferring to a new transmitter?

Transferring models shouldn't be a problem since both radios run OpenTX.

However,  you cannot transfer bindings unless you are also moving an external module to the new radio.  For example, if you move a crossfire module from the QX7 to the Radiomaster, all your models are still bound at the module level, no re-binding required.  But if you are going to be using the internal module of the new radio then re-binding is required.


Crossfire signal strength - comparing Immortal "L" vs "T"

The short answer is, yes. According to testing done here there is an observed 3db loss when folding the bottom half of a dipole which would be the same effect as cutting your output power in half.

See this graph from the link above. At the bottom in red is the bend of the antenna and the blue line is the gain.

The 'T' antenna is called a dipole and it's a very common antenna design. Changing the shape of the antenna will almost always change the antenna's gain and radiation pattern. Unless you have tested very carefully or are following a known and tested antenna design, chances are you aren't improving its performance. This article describes what one person went through to test folding a dipole by 90 degrees. Other things to keep in mind, the L shape allows your antenna to be polarized in two directions, while it might have worse reception under certain circumstances it may be better in others. This could be especially useful on a quad in situations where the orientation of the antenna would not be straight up and down.


How do I change GPS elements on the Betaflight OSD for my Acrobee Lite?

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


On a multirotor, how effective is braiding of power wires from the ESC to the BLDC Motor(s) in reducing or eliminate emf?

The braiding of motor wires should only be done for looks.  It looks cool and that's about it.  It may actually cause problems with smaller motors.  I once had braided motor wires on a brushless whoop size quad and the braiding of the wires caused the motors to behave erratically at start up.  Sometimes they would start spinning.  After removing the braids they started up perfectly every time.


Video being indexed on SD card, but no duration time and no actual video recorded

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


How do I re-charge the batteries of my S49 drone faster when outside?

I couldn't find manual or just the specifications for this exact drone, but from what you wrote and what I see on the images it uses 1-Cell batteries charged with some USB adapter.
You didn't specify which USB power-bank you have or which power adapter you use to charge from the mains.
I assume it's some-kind of phone charger. These days phone chargers typically outputs current of 1.0-2.0A. So my guess is that your power-bank isn't capable of outputting these kinds of current.
Try looking for a power-bank with QuickCharge (QC). They are capable of giving out currents up to 4.5A


Recommend safe distance from other drone flyers

In this situation, I would approach the other flyer(s) to say hello and discuss this with them to prevent misunderstandings or differences of opinion - for example, if the other flyer is new they might want much larger separation than you think is enough, whereas if you are both experienced you might be happy flying in close quarters.

Also, it might make life easier if you stand close to each other; I've done this before (albeit with people I knew) and it is useful to regularly share information like "I'm at 200 ft by the tree on the left", "Thanks, I'm at 50 ft moving across to the right" - and of course warnings like "Unleashed dog approaching!" and "Helicopter!"

If you are looking for official guidance, you could check CAP 393 Article 95:

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

I am not aware of any specific rule for separation in relation to other drones, unless they were to be considered a vehicle for rule 95(2)(c) in the quote but elsewhere in CAP393 they use "flying machine" to refer to (any) aircraft.

However, as always, there is the 'catch-all' in Articles 94 and 241 should something go wrong:

  94.—(1) A person must not cause or permit any article or animal (whether or not attached to a parachute) to be dropped from a  small unmanned aircraft so as to endanger persons or property.
  (2) The  remote pilot of  a  small  unmanned aircraft  may  only  fly  the  aircraft  if  reasonably satisfied that the flight can safely be made.
  (3) The  remote  pilot of  a  small  unmanned aircraft  must  maintain  direct,  unaided visual contact with the aircraft sufficient to monitor its flight path in relation to other aircraft, persons, vehicles, vessels and structures for the purpose of avoiding collisions.

and

  A person must not recklessly or negligently cause or permit an aircraft to endanger any person or property.


How can I build a drone similar to a DJI Mavic 2?

Everything you requested is possible in a home-built unit (EXCEPT: object sensing and tracking.)

I'd recommend the painless360 youtube channel.  Lee has detailed build videos on just about every kind of RC model that flies as well as tutorials about radio gear, GPS, ESC, batteries, camera gear, etc.  They are excellent and you can follow along with the builds.  It makes building your first model a great chance of success.  His channel is amazing especially if you are new.  All the lingo, terminology, etc are explained there in some way in literally hundreds of videos.  This may sound like a shameless plug for his youtube channel but I've literally spent hundreds of hours watching his stuff since 2015 and learned so much in the process.  If you are new to the hobby, this is a must.

Painless 360 YouTube Channel

As for the sensing and tracking features, it would be very hard or impossible to build one that would do all those things effectively for less trouble and money the aforementioned Mavic series could.  Things like object sensing and tracking are still very proprietary software options and you're not going to make this yourself currently.  As stated in the previous answer: Just buy it from DJI if you really need those features.  Otherwise, build your own.  It's a blast.


Is it possible to use the head tracking in a FPV goggle with a computer?

I have an off the shelf PPM to USB adapter. It isn't this one specifically, but works on the same principle:
RC USB-Interface III

You can either wire it directly to an R/C Receiver or to a buddy box barrel connector.
My Fatshark googles have a buddy box output for the head tracking output, which is what I'd typically run into my R/C transmitter. But it should work equally well directly into this.
The difficult part would be remapping the channels. If I remember correctly the buddy box connection gets roll, pitch, and yaw from channels 5, 6, and 7. And the USB device remaps the inputs to appear as a joystick controller with multiple axis and buttons/sliders.


For custom multirotors, what is the most useful method of collecting flight data?

If you're using a flight controller running either the Cleanflight or Betaflight firmware, you can make use of the builtin "Blackbox" logging features. These firmwares rely on the flight controller to have a MicroSD card slot which you can populate or an onboard flash chip to save the flight data to.

In order to configure the flight controller to use Blackbox logging, you'll need to go into the Betaflight/Cleanflight Configurator app and follow the steps of this tutorial.

After you fly around and get back to your computer, you'll want to download the logs and view them. If you recorded to a flash chip on the flight controller, you'll need to use the Betaflight/Cleanflight Configurator app to pull them off, but you can also just pull the SD card out of the quad and put it into an SD card reader if you used the SD card slot.

Then, you can view and process the logs using the Betaflight/Cleanflight Blackbox Viewer app. Joshua Bardwell on Youtube has an excellent video playlist discussing how to analyze them, as well as Mark Spatz from UAV Tech. The UAV Tech channel is an excellent resource for this kind of technical analysis.


What are the exact specifications of a RushFPV SMA connector?

The Rush cable is slightly non-standard as it lacks the threads on the holes that the TBS connectors have. It works fine though. The SMA is a standard SMA thread, so the normal hole size for SMA connectors works well. In my designs I usually use a 6.5mm hole for SMA, though some of that depends on the tolerances you're dealing with on your machining. The hole spacing for the small mounting holes is 11.5 mm from center to center. I usually use a 1.5mm hole for the bolt for the Rush SMA mounts, as you can thread an M2 bolt right into the carbon without needing an additional nut, but if you're wanting to use a nut, you'll want to increase that to a 2mm hole.


How to keep my prop from flying off my motor?

It looks like you are missing a piece of the prop adapter.
There should be three pieces, the nut, the central shaft with a thread for the nut on one end and a taper on the other, and a collar that matches the taper. You're missing the collar.
The nut forces the collar onto the taper, which squeezes the slots together, gripping the shaft. They're normally very secure but won't work without all three parts.
Here's a picture (I get an error when I try to embed it) or you could just look for pictures of 'collet prop adapter'.
https://shop.pichler.de/Collet-Cone-Prop-Adapter-60mm


30 year old water-slide decals. Are they still usable?

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


What is the meaning of 'control of a building' in the UK Air Navigation Order on Unmanned Aircraft?

I found what looks to be a reasonable answer in CAP 722 - the UK's drone rules guidance document.
In short, it appears a vessel, vehicle or structure can be under your control if you have the permission of the captain/driver/owner, and any people are under your control. For people to be under your control, they must be willing and able to follow guidance and safety instructions of the pilot.
The long version (correct April 2020):

Article 95(2)(c) makes reference vessels, vehicles and structures being under the control of a remote pilot or operator.  A vessel or vehicle could be said to be ‘under the control’ of a person if:

That vehicle or vessel is present for the purpose of participating in the flight operation; and
The operator of the vehicle or vessel (for example the driver of a car or captain of a ship) and any passengers are under the control of the remote pilot and can reasonably be expected to follow directions and safety precautions to avoid unplanned interactions with the small unmanned aircraft; and
The owner or other person with an interest in the vessel or vehicle (such as a lessee) has granted permission for a UAS to operate within 50 metres of that vehicle or vessel.

A structure could be said to be ‘under the control’ of a person if:

The owner or other person with an interest in the structure (such as a lessee) has granted permission for a UAS to operate within 50 metres of that structure; and
Any occupants of the structure are under the control of the remote pilot.

For completeness, as the document referes to control of people:

Persons under the control of the SUA operator or remote pilot can generally be defined as:

Persons solely present for the purpose of participating in the flight operation.
Persons under the control of the event or site manager who can reasonably be expected to follow directions and safety precautions to avoid unplanned interactions with the small unmanned aircraft. Such persons could include building-site or other industrial workers, film and TV production staff and any other pre-briefed, nominated individuals with an essential task to perform in relation to the event.

Spectators or other persons gathered for sports or other mass public events that have not been specifically established for the purpose of the flying operation are not regarded as being ‘under the control’ of the of the SUA operator or remote pilot.
In principle, persons under the control of the SUA operator or remote pilot at a mass public event must be able to:

elect to participate or not to participate with the small unmanned aircraft flight operations;
understand the risk posed to them inherent in the small unmanned aircraft flight operations;
have reasonable safeguards instituted for them by the site manager and SUA operator during the period of any flight operations;
not have restrictions placed on their engagement with the purpose of the event or activity for which they are present if they do not elect to participate with the small unmanned aircraft operation.

Note: As an example, it is not sufficient for persons at a public event to have been informed of the operations of the small unmanned aircraft via such means as public address systems, website publishing, e-mail, text and electronic or other means of ticketing, etc. without being also able to satisfy the points above. Permissions have, however, occasionally been granted for small unmanned aircraft flights at public events where these involved a segregated take-off site within the main event, with the aircraft operating only vertically within strict lateral limits that keep it directly overhead the take-off site. Such flights may also be limited by a height restriction and the tolerance of the aircraft to wind effects and battery endurance.


This depends - do you want to fly FPV or LOS, angle or acro?

For the LOS angle, I absolutely love the Hubsan X4 H107C. It’s relatively cheap, flies smoothly and is really robust. There is also a version without a camera called the H107L.

If you want to fly FPV acro (or angle), the Tinyhawk, Mobula 6 and Meteor 65 (though you may need a throttle cap) are all said to be very good.


Is it a good practice to use a throttle limit of 90% for racing builds?

The reason why this is recommended for racers is likely due to the fact that during these high-throttle punches, the voltage of the battery sags from the load and so more current is pulled from the battery to increase the motor RPM.

Power is equal to VI when V is voltage and I is current, which can be rewritten (by substitution of Ohm's law V = IR in for V) as I^2 * R. This means that if more current is drawn to compensate for a voltage sag, the power consumed by the motors increases by a greater amount because the resistance of the circuit is constant. (for all intents and purposes) For example, in the hypothetical scenareo that twice the current is drawn, four times the power would be consumed. This results in quicker battery drain.

The 10% figure that you've heard isn't a hard line, but rather just a suggestion based on experience with miniquad hardware and its capabilities. This recommendation isn't just specific to racing; all quads would benefit from the efficency (energy extraction from the battery) boost from rarely using the top XX% of the throttle range. However, this is often more important in racing scenareos where it is often necessary to make the most of the energy in the battery pack you're using because you've selected the smallest and lightest one possible and need as much energy as you can get to complete the course in record time.


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


Does the Meteor65 Acro support Spektrum RSSI?

Most serial receivers support RSSI through a feature called RSSI channel.

This means, unlike in receivers with a dedicated RSSI output, you can map one of the channels to carry the RSSI signal.

For example, on a FrSKY receiver, the telemetry channel is channel 16, or aux 12.

So you would go into the receiver tab in Betaflight and set RSSI channel to aux 12, and then place the RSSI icon in the OSD.

It is likely a similar process for Spektrum, so I would recommend consulting your transmitter’s manual.


What technology offers the longest range for RC controls?

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.


When flying FPV with a 5.8ghz video transmitter, is an Amateur Radio License (Ham license) required?

No, you do not require an Amateur licence to use this equipment.

Most RC equipment that operates at 5.8GHz (or 2.4GHz) makes use of what is known as an ISM band, a series of frequencies made available for anyone to use (historically for Industrial, Scientific and Medical purposes) subject to certain restrictions - such as transmitter power.

These restrictions have been set so that everyone gets fair use (rather than becoming an RF shouting match) and to minimise the harm caused if misused.

So long as the equipment complies with your local regulations for ISM equipment you are able to use it. The difficulty can be verifying this compliance without test equipment; reputable products will usually be correctly tested and constructed, whereas some inexpensive/unbranded clones might have cut corners to save money which means they are no longer legal. It's generally safe to assume it complies if it is supposed to, but different countries may be more strict on enforcing this.

However, having an Amateur licence may allow you to do more. If your jurisdiction allows using Ham gear on drones then - so long as you comply with all requirements (usually things like regular ID, no secret encryption and not commercial use) - you would be free to use the power limits and frequencies granted in your licence.


How do you figure out which motors to use for your UAV?

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


You mentioned that you tried SBUS, but that receiver only supports PPM and IBUS (not SBUS).  Try setting your flight controller to use IBUS.  As for the RX getting hot - I'm not sure about that.  The photo on banggood shows that it takes VCC, but be careful.  The description details indicate that it only accepts 3.3 - 5 volts for voltage.  If your using anything more than a 1S Lipo, you will certainly fry the RX unless you have it soldered to either a 3.3V or 5V pad.

Here is the protocol and voltage information from Banggood.

Signal output: IBUS, PPM

Power input: 3.3-5V


What registration is required for 250g to 2kg drones in Australia?

According to this page you don't need to register your drone "you’re only flying for sport or recreation, including model aircraft flown at CASA-approved model airfields". Since you're only intending to fly your drone in recreational purposes it seems like you don't need to register your drone even if it weighs over 250 grams.
I am no legal expert though, and if I were you I would contact CASA and ask for clarification if you are want to be absolutely certain about how you should do.


There is a software called UgCS which I believe should fit your requirements. It allows you to create waypoints with precise location and altitude. There are options like terrain following which allows the UAV to maintain a relatively constant altitude above ground level, a Facade scan tool which allows you to plan a flight path along a vertical plane at a certain distance from a wall, or there is the option to easily create custom waypoints with specific altitudes as shown in this flight path:


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


What affects the total flight time for a drone?

I'd watch out for several factors:
1. Weight
Drop any additional weight that you don't absolutely need for functioning.  This may mean downsizing to smaller motors, which obviously means that you have to balance weight vs. power; you may be able to get your flight time much longer but at the expense of speed.
2. Battery
Obviously the more juice you got, the longer you can go for.  Again, though it's going to be a balance between the weight of your battery... go for lighter technologies like LiPo.
Also, make sure you've got new batteries on your drone; as your batteries age, they'll lose juice and capacity.
3. Motors
If you really want to increase the flight time... go for lighter less powerful motors.  Of course, you won't be able to go as far, so that may not be a tradeoff you want to make.
4. Weather
Make sure you're flying your drone in dry weather... Condensation or water droplets will both add resistance to any moving parts and weight.  Also, when the weather is well below freezing you'll notice that your batteries will start performing less well, as well as having increased friction.
5. Maintenance
Keep all moving parts well greased!  As friction increases, the amount of battery power you use will increase in consequence.
Also, make sure all contacts remain solid.  Though it's not a big issue with newer drones, rust in the contacts / loose contacts will eat power.


Can I re-flash the Hubsan fpv1 controller with built-in fpv screen?

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.


Where can I find a computational fluid dynamics model of a full quadcopter or multirotor?

You can make your own using free software.

Let’s take Fusion 360 as an example, in its simulation suite it has a Computational Fluid Dynamics study option.

If you’re willing to design an accurate-ish model of the aircraft you want, or pay someone to design one for you, you can run the CFD simulation and get the results for yourself.

This is obviously CPU intensive so there is an option to run it in the cloud on Autodesk’s servers.


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

GND to Arduino GND
ESC1 to Arduino D11

It allow to upload to Arduino the "Arduino Bootloader"

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

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


How does one wire an OSD-enabled external DVR into Fat Shark goggles?

I have an external fpv monitor with this DVR and if I watch the DVR live feed a delay in the video image is noticable. If the DVR does lock up while flying you are in big trouble. So I'd strongly advise against the "fly via DVR output" solution.

But there is another solution: You can connect the DVR output to the usual AV input of the goggles (next to the power plug). On the bottom side of the googles there is a switch to turn off the receiver module. When the internal module is deactivated the goggles show the video from the AV plug.

This picture shows the pinout of the AV cable (comes with the googles). I don't know if the DVR can be connected to the AV plug permanently or if this would cause interference if the module is active.


Restore over-discharged LiPo batteries ~2.0V/cell

Firstly, I'll add the usual statement that mistreated LiPos can be dangerous and you are almost always better getting new ones.

In some cases, a 'smart' battery charger will refuse to charge cells which have dropped between a certain (manufacturer-determined) voltage. In this case, you can connect the battery to a current limited voltage source and charge the batteries manually with a very low trickle current - 0.1 to 0.2C is more than enough. It will take a while, but speed is not the aim here. You might be able to do this with the charger, by telling it the battery is a different type, or by using a separate supply. In either case, you must ensure the max voltage is within the normal range for the LiPo (4.2V/cell.)

You should only attempt recovery of a suspect battery in a safe, supervised environment - charging is the most likely time for such a battery to fail, and particularly so if it has been over-discharged.


How to prevent a 5V supply from sagging to 4.6V when the FPV camera is on?

I've pulled a few specs together from the parts you linked to:

Flight controller:

Input: 2S to 6S LiPo (7V to 20V)
Output: 5V 3A

VTX:

Input: 7V to 24V
Output: 5V 0.3A

Camera:

Input: 5V 0.65A  --OR-- 12V 0.27A

You don't mention what size battery you are using, but I will assume it is in the FCs range of 2S to 6S.

The camera needs too much current (0.65A) to get it's 5V from the VTX (max 0.3A)
The VTX should be connected to the battery
The FC should be connected to the battery
The camera should be connected to the FC 5V supply

This wiring should work, if you have no other large 5V loads connected to the FC.

    ╔══════════════════╗  ╭───────╮
    ║ LiPo Battery     ╟──┤ 7-20V ├────┬──────────┐
    ╚══════════════════╝  ╰───────╯╔═══╧═══╗  ╔═══╧═══╗  ╭────╮
                                   ║  VTX  ║  ║  FC   ╟──┤ 5V ├────┐
                                   ╚═══════╝  ╚═══════╝  ╰────╯╔═══╧═══╗
                                                               ║  Cam  ║
                                                               ╚═══════╝

Alternatively you will need a separate DC-DC converter or regulator to supply the 5V. A capacitor is great for smoothing out surges in demand, but won't hold up the 5V rail if the load is constantly too large.


How do I safely dispose of my LiPo batteries?

First is to discharge the battery to zero volts which can be done by using a LiPo charger or a light bulb. It can then be thrown into a recycle bin, but check with your municipality before doing so as some will not take them. There may be a local or regional centre where they can be dropped off.

It is not advised to use salt water as it takes a long time and its corrosive nature to metals can lead to incomplete discharge.


What is a safe amount of time a LiPo pack can be kept fully charged before it is discharged to storage capacity?

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

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

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


Empty Cells on LiPo - Balance Charger or Battery Problem?

It sounds like the balance lead for the first cell isn't connected somewhere. The wire may have come off inside the charger, or it could be the balance plug, the wire, or an external parallel board (if you're using one).

I'd open up the charger and test the voltage at the back of the balance plug, and see if you can spot any broken solder joints.


How to clean dirt out of my miniquad motors?

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

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

Remove the motor

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

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

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

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

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

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

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

(Image Source)
Step 1:

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

(Image Source)
Step 2:

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

(Image Source)
Step 3:

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

(Image Source)

(Image Source)
Step 4:

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

(Image Source)
Step 5:

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

(Image Source)

(Image Source)
Step 6:

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

(Image Source)

(Image Source)
Step 7:

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

(Image Source)

(Image Source)
Motor disassembly steps adapted from this article.


How to pick parts for a quadcopter flying on Mars?

TL;DR:

You wouldn't want to be responsible for the design of such a project. :) Between the environmental constraints (atmospheric pressure, temperature, etc.) and physical requirements (extreme radiation tolerance, etc.), this would be an incredibly difficult thing to make.

Believe it or not, there is actual flight hardware being prepared to do something just like this on Mars in the near future as a part of the NASA Mars 2020/Perseverance mission. The Mars Helicopter Ingenuity is a fully-autonomous helicopter that is planned to be used for aerial reconnaissance and surveying of the Perseverance rover's path. It's a truly insane piece of engineering:

Some fun facts about the craft: (sourced from the Wikipedia articles linked to above and the YouTube video linked to below)

1.8 kg total mass, including 273 g of batteries
Set of two counterrotating 1.20 m diameter propellers

Maximums at ~2400 RPM and rotor tip speeds < 0.7 mach

Max power consumption of 350 W
Each full flight is expected to be < 90 sec, at a max altitude of 5 m above the martian surface
Total battery capacity between 35-40 Wh
Uses a blanket of CO2 gas around the avionics and batteries as an insulation layer

Operating conditions: (again sourced from the Wikipedia articles linked to above and the YouTube video linked to below)

Ambient atmospheric pressure ~0.01 atmospheres (one-hundredth of the atmospheric pressure at sea level on Earth)
Gravitational force 38% as strong as on Earth
Weak solar energy, resulting in the charging process requiring an entire Mars day
Night-time temperatures at about -80 to -100 °C, resulting in ~ 2/3 of the battery's energy stores being used to keep the electronics and actuators from freezing through
Dramatically higher ambient radiation levels due to Mars' lack of a magnetic field and ionosphere to deflect the solar wind and electromagnetic emissions

The YouTube channel Veritasium has an excellent video discussing this project and the development of the helicopter design with the CalTech/JPL engineers who built the thing, which I think is well worth watching.


I found some information here which may be helpful to read through. They mentioned the issue of video lag and seemed to find a few potential solutions which were: to use a wifi extender, use a different app (mentioned in the link) to connect to the drone, or adjust settings of your phone like screen resolution and processor power if possible.
As I don't own this drone, I'm not sure that these solutions will fix the issue. But if the video lag persists, just know that you really can't expect very low-latency video from a drone this price.


Why do a quadcopter's opposite rotors spin in the same direction?

This design is to prevent unwanted yaw when moving, or vice-versa.

Consider the following two actions:

When a quadcopter yaws, it does so by creating a speed difference between the opposite-rotating rotors; creates a torque that rotates the quad.
When a quadcopter moves left/right, it does so by slowing all the motors on one side of the frame and/or speeding those on the other, imparting a slight roll - the thrust is now slightly sideways, so it moves. (This is the same for front/back.)

From this, you can infer that if the same-direction rotors were on the same side as each other, when creating a speed difference to yaw the aircraft you would also impart a roll. Similarly, when trying to roll the aircraft would yaw.

The image below shows visually how an opposite-corner rotation quadcopter moves:


Why does moving the centre of gravity forward increase stability of a plane?

TLDR;
Aft CG means stalls are unrecoverable!
Why is CG relevant?
The plane's elevator is the control surface for pitch angle. The elevator is its own proper wing, and as such only has a certain amount of force it can develop before it stalls. Once it stalls, then pitch control is lost. Not a happy day.
The force that the elevator must develop is the lever arm required to balance the torque stemming from the distance between the airframes center of gravity (CG) and the wing's aerodynamic center.

Think about it as a see-saw with an elephant sitting close to the center, being balanced by a mouse 100x farther out.
The upshot is that if the CG is aft of the aerodynamic center, then the plane's nose naturally wants to rotate up, and the elevator has to push the tail up to counterbalance.
And if the CG is fore of the aerodynamic center, then the plane's nose naturally wants to rotate down, and the elevator has to push the tail down to counterbalance.
Aft CG
Aft CG is a huge no-no. It all comes down to stall recovery. If the CG is behind the aerodynamic center, then if the plane stalls the nose will simply not come down. The upshot is that you can never regain airspeed and you will enter into what's called a deep stall or leaf stall.
One common reason for cargo aircraft crashes is improperly secured loads which shift backwards immediately after rotation on takeoff. This moves the CG aft, and the plane can no longer keep its nose down and very quickly stalls unrecoverably. There is a famous video a decade years back of a US military cargo jet which had its cargo load shift back on takeoff while departing a Middle East airbase. All aboard died, and I won't post the link out of sensitivity.
Excessive forward CG
If it's too far forward then your elevator will have to produce a ton of downward force to keep the nose up. This is draggy and inefficient. But it's also (within the limits of reason) survivable.
So where to put the CG?
As far back as you safely can! Competition glider pilots move the CG to the very limits of safety because they want to stop the elevator from producing downward thrust which costs precious meters of altitude.
Jetliners want an aft CG because it saves fuel. But don't panic, they have strict limits on how far back they can move it. Because in equal parts no pilot wants to die and no amount of saved jet fuel ever covers the cost of a crashed plane.


2D/3D/fpv video glasses to use with external camera

Ordinary FPV cameras output an analogue PAL or NTSC TV signal, which you should be able to plug straight into a TV or goggles that are intended for watching TV.
Some FPV goggles only have a receiver and no input socket, but the good ones generally allow you to change the receiver, and thus have a socket for the AV signal. (Also some of the very cheap ones are just a box for a screen from an automotive reversing camera system, and expect you to provide your own receiver)
So, skipping the radio link should be easy enough, which is good because you do get some radio 'noise', especially if you're indoors where the signal can reflect from metal objects, or where there's interference from WiFi and other electronics.
Good quality FPV cameras are generally optimised for a wide dynamic range (so you can still see details on the ground when the sun is in the picture) and low light (it's common to finish an evening flight and be surprised by how dark it really is) and low latency (for racing) but the resolution is usually fairly limited because the radio bandwidth is limited.
It gets harder if you want better resolution. The only real option from the FPV world is the DJI digital FPV system, but it's quite expensive and I don't think you can use it without the radio, which is integrated into the goggles. The goggles may have an HDMI socket, but the camera doesn't.
Then again, there must be HDMI cameras outside of the FPV world. A quick google shows lots of people using DSLRs, and lots of action-cams (like GoPros) have HDMI ports - although I'm not sure if they support real-time video, or just replay. Also, any form of digital processing introduces some lag, and it doesn't take much to make hand-eye coordination difficult.


How does the center of gravity affect a quadcopter?

Altering the location of the CG (center of gravity) also changes the CM(center of mass) because the force of gravity acts roughly on the CM. Normally, the motor arms are equally sized so that all motors have equal control authority and all contribute equally to providing thrust while in level flight when the CM is close to the center of the multirotor. Another way to say this is that multirotor frames are constructed so that the CG, CM, and CL (center of lift/thrust) are all as close as possible to one another.

However, this is no longer the case because now each of the motors now has a slightly different lever arm distance to the CM of the multirotor. Consequently, not all of the motors have the same control authority and some of the motors will have to remain at higher/lower throttle than others while in level flight.
This will manifest itself in diminished flight performance. If all motors are running at the same throttle, the multirotor will naturally begin rotating with the longer lever arm(s) going upward and the shorter one(s) going down. If the distance between the CG/CM and CL isn't too great so as to not make the multirotor immediately flip over after raising the throttle, the I term in the PID controller of your flight controller should be able to account for the discrepancy after takeoff. The ability to pitch/roll in different directions will be different due to the asymmetric thrust distribution around the CG/CM.
Even so, it isn't ever recommended to fly with a significant distance between the CG/CM and CL. All (half-decent) frames are designed to minimize the risk that component placement will move the three far from one another, but this is an important consideration for scratch-built frames.
In the example from the OP, a case where the CG/CM was moved from the center to a location right behind the front two motors, the effects would be quite severe indeed. It's difficult for me to come up with a practical setup to create such a situation, but this is a good example of a worst-case situation, with the CG/CM to CL distance being close to 2/3 the arm length. (depending on the angle between the arms on the frame)


Accessing the video feed of a cheap FPV drone

I am a little familiar with this - the task is hard and the suggestion to talk to the hacking side is very wise, to wit - here is a primer from my messing around. (also a software developer w/over 20 years experience).
To find out the IP, that is very easy - look to your router and check on leases or devices.  Unless you have static IP assignments or similar, the IP will change overtime but generally stay the same on a calm network.
For capturing the stream - Wireshark is in your future (https://www.wireshark.org/) and you will need to know the protocol being used and  various transport mechanisms then reconstruct that data for your end use.  It is safe to assume that the guys in Shenzhen took the fast, easy, hacky, free/open source route.  Chips in used will also provide clues.
I have worked a bit in wifi security hiring people to corrupt wifi data for tech presentations - your task is not one I would ever take on (out of my wheelhouse).  Still I have tried this exact hack a few times and have been defeated universally.
I got around this by using a USB HDMI capture box and porting in the analog signal.  $200 part and 1h set up and I can stream from goggle to internet. The capture box is something I am glad I have and the 10days of work I skipped hacking signals is not forgotten.


How do you use the PID controller to compensate for a higher weight?

I'm afraid there is no simple answer to this question. Adding weight to the quad will change the way it responds so in theory you would need to redo the tune from scratch.

That being said, there are a few assumptions you can make.
Since the weight goes up, the inertia goes up as well. So it will respond slower to stick movements and will have a bigger tendency to overshoot. To compensate for the slower reaction you can increase the P-gain and/or feedforward. To fight the overshoot (and the effect of the bigger P-gain) you will need to increase the D-gain as well.
I assume I-gain will not be impacted a lot.


Why do LiHV batteries degrade faster than an equivalent LiPo?

LiHV and LiPo batteries have different battery chemistry. If you were to charge a normal LiPo up to 4.35v per cell it would likely explode or at least puff up. The different battery chemistry in LiHV batteries allows the batteries to have a higher capacity to weight ratio. (For example, a 1500mah LiPo would be noticeably heavier than a 1500mah LiHV) A LiHV battery can hold more power and performance in a battery that weighs less.

This ability to pack more power into the battery does come at a cost though as it does reduce the longevity of the battery significantly. Any good quality standard Lipo should hold up to 300+ charge cycles (If taken care of properly) whereas a LiHV battery may begin to swell up at only 30-40 cycles.

LiHV batteries are very useful when it comes to serious racing though. The high performance and increased power per weight ratio give racers an advantage when racing, they will just need to replace batteries a lot more often.

It just depends on whether you want a little bit longer flight at the expense of needing to replace your batteries a lot more often.

You can read more about it here:
https://oscarliang.com/lihv-lipo-drone-battery-hvli/


Do flexible propellers affect multirotor efficiency?

It's really a challenging question to answer because flexible and stiff propellers both have their advantages and disadvantages in terms of efficiency.

Flexible propellers are typically lighter than stiff propellers, so they spin-up more efficiently than stiff propellers.  On the other hand, when a propeller is spinning at higher speed, a flexible propeller will have a tendency to flex upward and flatten out vs a stiff propeller that is designed to resist flexing (stiff propellers will still flex, but not as much as a flexible propeller).  With a flexible propeller, this could result in a scenario where you have more throttle to give, but the drone won't go any faster.  To some degree, you might also notice that the drone is less controllable - might not take a corner as well as a stiff propeller would.

If you have a lower powered motor, a flexible (lighter) propeller might be your best option because you may never reach the point where efficiency is an issue.  However, if you have powerful motors, and your desire is for speed, then a stiff propeller will probably be more efficient at higher speeds than a flexible one.


How do I tell when to replace the motors on my tinywhoop?

As someone who loves collecting data for analysis, I do track all of my flights.  I have found that, under normal conditions, brushed motors should last for at least a few hundred flights.  You can get higher if you don't fly hard or get things caught in the props or motor shaft that will make the motor heat up more than normal.

How to determine whether a brushed motor needs to be replaced is a tough question and it really depends on your willingness to fly with a drone that isn't operating perfectly.  With mine, the telltale indicator is that I notice less power than usual and yaw is off a bit (twitchy or lethargic).  This makes sense since all motors perform to the level of the least capable motor.  If you experience less performance, first, ensure that it's not just the battery.  This is easy to do - just use a newer battery, or one that you know is still providing good power.  The next thing to do, if the battery checks out, is to replace the motors.  I recommend replacing all of the motors at once for brushed motors since they are all getting some form of wear while in use.  Replacing them all ensures that you are starting with peak performance.


How to measure the RPM of a drone motor using Betaflight?

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.


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.


What is a filtered power pad on flight controller?

Without knowing more specifics about the board(s) you're asking about, it's more than likely that the filtered power pads on your flight controllers are just the normal output from the 5v voltage regulator on the board which has a capacitor between the voltage regulator output and ground.

Because the 5v regulator's output is shared by all kinds of devices connected to the board, all of which consume different amounts of power and at different times, it's entirely likely that the actual voltage on the regulator's output will have some "ripple" where the voltage oscillates around 5v.

 (cit.)

Some components on the drone which deal with analog signals, like the camera and VTX, may introduce visual artifacts into the image or intermittently drop out if they are fed power with too much ripple. The aim of the filtered voltage output is to smooth out the ripple so the voltage oscillations are far less pronounced. In the above graph, the solid black line is the filtered output.


Drone with SDR for HF antenna field strength measurements, I need a Raspberry Pi based drone kit that I can just buy

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).


How to determine the minimum thrust to weight ratio for a fixed-wing aircraft?

I'm afraid it's not that simple. As you mention, planes can fly with a fairly low thrust to weight ratio. Obviously gliders can fly with no power at all, so there's no critical lower-limit.
The other issue is that pitch-speed is important - the prop needs to be producing thrust when the plane is moving, not stationary. Measuring static thrust only tells you half the story.
Instead of thrust, it's better to look at power. Assuming that your motor and prop are reasonably efficient (and most are) measure the power going into the motor with a watt meter (or run it for a minute and see how much energy your charger puts back into the battery). Then weigh the complete model, including the battery.

50 w/lb is about the minimum for gentle cruising and slow climbs.
100 W/lb is 'sporty', say for a warbird. You don't have to worry about conserving energy and can easily climb out of trouble. It might not have unlimited vertical climb though.
200 W/lb is unlimited vertical climb, hovering and climbing vertically out of the hover.

For ducted fans, you should double these numbers. i.e. at 100 w/lb you'll need to be careful to conserve energy.
However foam board planes aren't super-efficient or super-fast, so you can usually do a simple thrust test to check it'll be OK. Dangle it by a wing tip and gradually apply power.

If the model has enough power to support its own weight, it's probably around 100 w/lb
If it can only reach about 45 degrees, its about 50 w/lb and will fly if you're careful.

About pitch speed
If a prop has 6" of pitch and is turning at 1000rpm, under no load it would advance 6000 inches per minute. Or to put it another way, if the model reaches that speed, it won't be producing any power!

'Slow' props have a pitch of about half their diameter (e.g. 8x4")
'Sport' props are 2/3 to 3/4 of the diameter (e.g. 8x6")
'Speed' props are 'square' with the pitch equal to the diameter (8x8)

If you find your plane accelerates well from the launch but has a low top speed, you might need a higher pitch prop.
If it doesn't have enough thrust to take off, go to a larger diameter.
If you change the prop, you should measure the current drawn to check you're not exceeding the limit of the ESC. Also check that the motor and battery don't overheat.


What Requirements Do I Need to Comply with in Order to Fly a Custom Drone Commercially in Texas

In general Drone is considered as UAS (Unmanned Aircraft System)
The FAA has basic information on their website https://www.faa.gov/uas/
In order to fly the drone, the drone need to be registered it cost US $ 5 for 3 years.
and for personnel flying the drone need to have adequate knowledge on regulations, procedures including map and charts, notam, etc if you intent to operate commercially then Part 107 of US FAA regulations applies.
For carrying goods especially cargo,  as far as airmen certification personnel detail is required, although picture is not displayed in the license however for security reason personal details including address is required.  see the license information here  and official FAA page for license registration, the FAA is cover for all United states, including Texas.
I hope this help


How to identify and address multirotor motor overheating

Assuming that you've spec'ed a good propeller-motor match and the heat isn't generated by a tiny motor being forced to drive a giant propeller, the heat generation is likely due to a physical issue with the drone, mistaken filter settings, or a tight PID tune.

First of all, it's a good idea to determine whether your motors are actually hot enough to matter. A good rule of thumb I've heard commonly thrown around is that if your motors are SO hot to the touch that you want to take your fingers off after a few seconds, then that's too hot. If you feel like you could keep them there forever, then it's probably not something that will ever be a problem.

Physical Causes

It's possible that you have a mechanical connection issue somewhere on your drone which is causing problems, like if one of the motor fastener screws is protruding into the motor casing and making contact with the windings like in this video from Joshua Bardwell which also goes through troubleshooting steps and how to rectify the issue.

Other physical causes can include locations on the drone where parts or wires (like motor phase wires) that are touching conductive things they shouldn't be. Giving your drone a look over to make sure any of these aren't the problem is worth a try.

Also, make sure that the hot motor is still balanced. If the motor has been damaged, that is almost certainly the cause of the elevated temperature of the motor.

Filtering Issues

If you ever went into the configurator for your flight controller firmware (e.g. Betaflight, Cleanflight, Raceflight, iNav, etc.) and disabled or restricted the default gyroscope filtering settings, it's possible that these are the culprits. Insufficient filtering on the noisy gyroscope signals can result in hot motors because the flight controller tries to respond to the noise it sees in the gyroscope data and not just the pilot's inputs. The inconsistent nature of the gyro noise means that motor commands would be constantly changing up and down, which can be a cause of hot motors.

If you ever tweaked the filtering settings to try and improve their latency, it's possible that any crashes you've since experienced have beat up the drone sufficiently that there is now extra noise that wasn't there before when you adjusted the filters, causing hot motors. In that case, I'd recommend raising the filter sensitivity. The Betaflight Configurator for >4.0 versions of Betaflight makes this super easy with a slider that you could push towards increased filtering.

PID Tuning Factors

As noted in this (admittedly old, for Betaflight <2.x) Betaflight Tuning Guide, motors will start getting hot if your D-gains are too high. I would recommend checking all of the previous situations before pursuing this case because issues with the PID gains are often more subtle and difficult to track down, but a solution here would be to slowly back off on the D-gains in your PID controller until the motors cool down sufficiently.

The D term in the PID controller tries to look ahead into the future and consider if the controller is trying to reach the pilot's input commands quickly enough, attempting to slow the reaction time or speed it up as necessary to minimize overshoot. This system can be completely screwed up if excess noise enters, so checking the filtering settings to make sure nothing is wrong there first is a good idea.


How does a quadcopter flip without falling very much?

The trick is speed. The reason why multirotors don't lose (very much) altitude while performing quick mid-air aerobatics is because of the speed of the move and the skillful management of throttle by the pilot.

You're correct in presuming that a multirotor performs a flip by changing the speed of the motors on each side of the craft. Freestyle pilots usually make use of near-total stick deflection and extreme rate profiles targeting ~600-1200 deg/s of angular velocity. The fact that the flight controller speeds up motors on the side which needs to rotate "up" and slowing down those on the side which needs to rotate "down" ensures that the move occurs as fast as possible because all four motors contribute to the torque which rotates the multirotor.

The other factor which helps the multirotor not descend too much is the skillful management of the throttle during the move. By lowering the throttle to near idle while the craft is inverted, the pilot minimizes the amount of thrust the craft generates which acts to pull it down towards the ground. As I discuss in my answer to "What's the purpose of airmode and idle up?", the idle up and airmode features (of the RC transmitter and flight controller respectively) help ensure that the multirotor is in control during these times of low-throttle.

Because of these two factors, the duration in which gravity has the power to suck the craft towards the ground is often too little to matter, and the multirotor flips in-place.

Of course, there's nothing preventing you as the pilot from slowing down the flip and performing an "O"-shaped loop instead of a flip in-place. This can be done by easing off the roll/pitch axis and turning down rate profiles.


LiPo 4S battery's output voltage higher than 14.8v

As others pointed out, 16.8 volt is what that battery should be reading (unless there is a specific chemical composition that makes that different).
Discharging a battery to use with your application is an anti-pattern, please avoid.  Put a voltage regulator in there with the correct amp rating (draw) and you will be all set.
Pololu has some 1-2a 12v step-downs that cost $3-10. You can also go adjustable, here is a 9-30v 1 amp step up/down.
I know a few good ones on Amazon. Bottom line, this is a common part, easy to use, and I have had high reliability for nearly identical situations.


I can't bind my S-FHSS Corona SBUS receiver with a jumper t16 transmitter. Are they compatible?

There could be many reasons why it won't bind.  Unfortunately, there doesn't seem to be a clear reason.

Did you follow the binding instructions in the Corona R6SF manual?

http://img.banggood.com/file/products/20160907060650R8SF%20R6SF%20and%20R4SF%20Instruction%20manual%20V1.2.docx

You might also try to upgrade the Jumper T16 internal multi-protocol module firmware.  Here is a link to a video by Joshua Bardwell that outlines the process.

https://www.youtube.com/watch?v=qT_wmbxTBp8


Are these methods of Drone Localization Techniques for a dark environment plausible?

With sufficiently accurate and stable IMUs (think ADIS 164xx) on both vehicle and drone you should be able to calculate the relative offset between the two. In any way this will have to be a custom solution.
What GPS denied environment are you referring to?


What is the purpose of adjusting the thrust angle for model aircraft?

Typically the motor will point down and to the right. There can be various reasons for this.

I've included a plans excerpt from https://aerofred.com/details.php?image_id=91062

Downward angle

Sometimes it's to align the motor with the true flight direction, because the wings need some AOA (Angle Of AttacK) greater than 0. In the excerpt above, you can see that there's a 3 degree downward tilt. If you look at the original plan, the wings are mounted flat to the fuselage. A 3 degree AOA is pretty normal for cruise flight, so this motor is mounted so that it's pointed in the direction of movement.

If it were not pointing downward, then when you changed thrust it would have a tendency to momentarily sink/climb.

Another advantage of pointing down is that it increases pitch down tendency (or decreases pitch up tendency) when power is applied. Pitching down is safer since it drives you away from stall, instead of toward it. This is especially valuable in the case of positive pitch stability craft where there is a natural tendency to pitch up as airspeed increases.

You'll find that full-scale aircraft have the motor aligned with the fuselage, but their wings mounted at a 2-3 degree incidence angle so that the fuselage is level while the plane is flying forward. It turns out that the squishy humans seated inside prefer not to be seated on a slope.

Pointing to the right

In some models, the motor can be turned to the right so that its thrust counteracts the p-factor, which tends to turn the plane to the left. This makes the plane easier to handle when under high power at slower speeds when the rudder is less effective.


I have used the FS-i6, it was my first radio. The throttle is generally on the left, and it is generally mode 2. In your case, it doesn't appear to be the case. So it might be useful to check in the transmitter settings if you have channel assigned correctly.
You can always see the signals sent to the FC in the Mission planner Setup tap under RC calibration. Ensure throttle signal moves as you desire.
From what I can see, there is probably some issue with the RC link and the signal being sent to the Pixhawk.


Your question asks about the XT30 connector. I found this diagram which lists the specs of the XT30 connector:

However, this source suggests that the Matrice 600 pro has an XT60, not an XT30. It wouldn't make a difference in power though, as you are correct that the Matrice 600 can only supply 10 Amps and 18 Volts for a total of 180 Watts. This means that the only difference between using an XT30 and an XT60 would be the size of the connector. You can tell which connector it uses by looking at the side of the connector, in small letters it will say either XT30 or XT60.
The specs of the XT60 are as follows:

If your total power draw will be 7A at 18V, it should work fine. Just make sure that the total wattage you are trying to pull is under 180W.


Does active braking help protect propellers from damage?

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

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

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


What is Commutation Enhancement in BLHeli_M and what should i set it to?

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

Source: (rgroups.om)

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

Source: (rcgroups.com)

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

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


In what direction should I orient the dipole antenna on my 2.4 GHz RC transmitter?

The antenna should be aligned to match the orientation of the antenna on your aircraft; generally this will be vertical, but for more acrobatic aircraft a compromise of 45° might be appropriate for greater reliability but lower performance. For more detail see this question.

Some manufacturers of off-the-shelf equipment will recommend an orientation in their user manuals - in this case, follow the instructions they give.


Affordable ways to crimp your own connectors

I crimp my JST connectors using needle-nose pliers. It’s probably not be best way to do it, but it’s cheap and works well for me.

Place the wire with the stripped end in the metal connector piece.

Squeeze perpendicular to the tabs on the connector with the pliers.

Now squeeze parallel to the original direction of the tabs.

Finally, squeeze perpendicularly to the original position again.

Give the wire a tug to make sure it is held securely.

Like I say, probably not the best way and I can’t vouch for how reliable it is, but it hasn’t let me down yet.


The difference between outrunner and inrunner BLDC (brushless DC) motors is that an outrunner's rotor is outside the stator while an inrunner's rotor is inside the stator.

 (cit.)

In general, outrunners have more torque and rotate slower than inrunners, which usually have less torque and rotate significantly quicker. The two different designs are possible because it is relatively easy to conceptualize how to flip the order of the stator and rotor within the motor due to the fact that no mechanical commutation is necessary, unlike in a brushed motor.

Almost all brushed DC motors would be classified as inrunners and very few brushed outrunner designs exist, as you note. The Wikipedia page for Brushed DC motors makes no reference to an "outrunner" brushed motor and almost no search results appear for them.

I'm not a designer of electric motors, but I have a theory for why this might be the case: the usefulness of brushed motors is mostly because they are inrunners.

 (cit.)  (cit.)

The above images show "inrunner" brushed motors and the commutation mechanisms they require. If you were to make them into outrunners, the only difference they would have with brushless outrunners would be the swapped location of the coils/magnets. This setup would also increase the size of the commutation mechanism and forcing the brushes to be crammed inside the commutation ring, making it more complicated and susceptible to breaking.


How can I run 2207 2450Kv motors on 4S with 7” props?

7" props have a disk area that is almost twice as large as 5" props, so the motors will be heavily overloaded. (7 squared is 49, 5 squared is 25, the other terms in the equation for the area of a circle cancel out)

Assuming you use 5x4.5 3-blade props on a 5" quad, you could reduce the load 33% by switching to 2-blade props. You can reduce the load further with a lower pitch prop. 7x3 is about as low as you can find, and will give you another 33% reduction, so now the motors are only running at 133%.

I think that's worth a try, but if the motors get hot you may need to step down to 6" props. 6x4 props may work better than 7x3, giving you more speed.

Reducing throttle to 70% will reduce the maximum voltage, similar to switching to 3s batteries. However torque and heating are related to current. Reducing voltage will limit the maximum power and thus the maximum current, but won't make any difference at part throttle.


EU Citizens: range settings of DJI Digital FPV system (FCC vs CE)?

The difference is due to regulation - in the EU, the permitted radio power output is lower than that allowed under the FCC regulations, which results in reduced range.
Without a licence from your countries telecoms regulator, it would be illegal to operate the FCC equipment in Europe.
It is likely to be a software setting, but there might be hardware differences if the costs make business sense (possibly cheaper hardware Vs. different production process.)


How to determine the correct specification of servo for a control surface

Most forces in aerodynamics are proportional to area, speed squared, air density and a 'coefficient' that describes how efficient your object is. There's also usually a 1/2 for obscure theoretical reasons.

If you work in metric, the density of air at sea level is basically 1, which takes that out of the equation.

The maximum lift coefficient of an average wing is roughly 1. The drag coefficient of a non-aerodynamic object is also (very) roughly 1 (see link for examples), so for a first approximation, we'll use 1.

Now you've got to guess the maximum speed of your model, maybe by multiplying the motor Kv, battery voltage and prop pitch.

That should give you the maximum force on the control surface. We've no idea how it's distributed but we'll assume it's in the middle. Multiply the force by half the chord to get the torque.

Then allow for differences in the length of control horns. If they're the same, the servo needs the calculated torque. If the servo horn is half as long as the control surface horn, the servo will only need half the torque.

Finally, double or triple that value to give yourself a safety factor.

Alternately, just copy a similar model.

Then when you test fly, don't try any high-speed dives until after you'd tried a level full speed pass. If the controls feel sluggish at speed, you need larger servos.


Does the FS-R6B receiver work with the JJPRO EM-16 transmitter?

There are various digital protocols, and many receivers are able to handle more than one protocol (and so can a few transmitters).
It looks like that transmitter uses "AFHDS 2A", so you need a receiver that can handle that.
That receiver (FS-R6B) says "AFHDS" on the label, and there's a big complicated chart (the 5th image) showing which receivers handle AFHDS 2A, but I can't see the FS-R6B on the chart so it's not much help. The binding instructions imply that it might do both.


How to tune a Deadcat Quadcopter with 10 inch props and asymmetrical mass distribution along the roll axis with a low power to weight ratio?

The discovery frame uses the old DJI plastic arms and the arms are not very stiff. So you get a lot of flex in the arms. This leads to two major problems: vibrations and the quad has a relative long delay until force of one motor really starts to rotate the quad. This problem gets amplified if you are using out of balance motors or very felxible propellers. This would explain the unstable flight you are describing.
I'm afraid that this is a "hardware problem" and not a "software problem" that you can fix with some PID settings. Modern flight controller firmware is built with certain assumptions about the used frames/escs/motors. A 10 inch prop on a 810KV motor with 3S and a plastic frame isn't a target platform for betaflight or any other race/freestyle oriented FC firmware.
What are your options? Make the frame as rigid as possible and try to dampen the FC so you get rid of the vibrations (e.g. put double sided tape under it). I don't know what FC you are using but try to get some blackbox logs to see the frequency range of the vibrations and try to add more filtering to clean up the signal for the FC. After that you can start to tune the PIDs and maybe get a flyable quad. You can tune the quad without any flips and rolls because with a low power/weight ratio the quad will probably crash in the process. So just tune the quad for regular forward flight.


Can a lipo battery be charged with more than 0.5 amps?

You definitely can charge your 1205 mAh 6S LiPo at a current higher than 0.5A. For most of the LiPos we use on drones, the recommended charge rate is 1C, which for your battery, would be about 1.2Amps. (A 1C charge rate is 1 Amp for every 1000mAh)
A 1C charge rate is a good middle ground, but you can use a lower charge rate (~0.5C) for safer but slower charging, or a higher charge rate (~2C) for faster charging at the expense of the battery's longevity (and more risk of fire).
And as @ifconfig mentioned, this similar question should also help answer your question.


Is there a way to override the distance limit on a DJI Mavic Mini without a mobile phone app?

Currently no, barring exploiting vulnerabilities on the platform (which DJI are pretty quick at fixing) there is no way around the distance limit. The wide field really won't be your issue here. At 60m you can't see it safely line of sight.

Why not get a cheap phone just for this purpose? Doesn't need to be fully featured, or indeed that current - just check compatibility with DJI first.


Does FPV transmitter antenna need to match the goggle receiver antenna?

No, specific brand and specs don't matter for interoperability (aside from making sure that both antennae are built to transmit/receive the same frequency). Specific specifications about an antenna can help you figure out which will work best for your use case, like the antenna's gain, but mixing brands and types is fine.

However, there are certain mixes of antenna polarizations that will work better than others. You can learn more about this in Can circularly and linearly polarized antennae be used together for FPV? and about what antenna polarization is in What is circular polarization in antennae?.


Any vehicle yaws (i.e. turns) by having a net torque applied. What's interesting about a quadcopter is not just how it yaws, but how it yaws and doesn't roll, pitch, or climb at the same time.
To understand how this works, we need to briefly look at the math. We'll use a plus configuration, but really any mutirotor configuration works.

The thing to keep in mind is that thrusts and torques are related to propeller speeds. If you speed a propeller up, it's intuitively obvious that it will create more thrust. And likewise, if you spin it faster, you need more torque. So changing motor speeds changes the net forces and torques on the airframe.
(Pedantically, it goes with the square of speed. So if you double the speed you quadruple the thrust and torque. But that's not important to this analysis.)
Here's the high-level driving equation. If you've ever messed around with mixers, you'll notice that the 4x4 matrix in the middle looks really familiar:

What this does is it maps rotor speeds (squared) to torques about the roll, pitch, and yaw axes, as well as the net vertical thrust.
For hover, let's assume all motors are spinning at the same speed, W. So W = w1 = w2 = w3 = w4
Yaw
What happens if we speed up the first and third and slow down the second and fourth by the same (squared) amount dW?
torque_x = 0*(W^2 + dW) + 1*(W^2 - dW) + 0*(W^2 + dW) - 1*(W^2 + dW) = 0
torque_y = 1*(W^2 + dW) + 0*(W^2 - dW) - 1*(W^2 + dW) + 0*(W^2 + dW) = 0
torque_z = 1*(W^2 + dW) - 1*(W^2 - dW) + 1*(W^2 + dW) - 1*(W^2 + dW) = 4*dW
F_z      = 1*(W^2 + dW) + 1*(W^2 - dW) + 1*(W^2 + dW) + 1*(W^2 + dW) = 4*W^2

So the net force doesn't change (all the dW cancel out), and neither do the net rolls and pitches, but voila we have 4*dW worth of torque!

For completeness, here's what happens when you want to change the other axes as well.
Pitch
Let's change the front and back motors by the same (squared) speed, but we'll leave the other two motors alone:
torque_x = 0*(W^2 + 0) + 1*(W^2 - dW) + 0*(W^2 + 0) - 1*(W^2 + dW) = 2*dW
torque_y = 1*(W^2 + 0) + 0*(W^2 - dW) - 1*(W^2 + 0) + 0*(W^2 + dW) = 0
torque_z = 1*(W^2 + 0) - 1*(W^2 - dW) + 1*(W^2 + 0) - 1*(W^2 + dW) = 0
F_z      = 1*(W^2 + 0) + 1*(W^2 - dW) + 1*(W^2 + 0) + 1*(W^2 + dW) = 4*W^2

Notice that, again, z-thrust stays constant, but this time only a pitching torque appears.
Roll
Let's change the left and right motors by the same (squared) speed, but we'll leave the other two motors alone:
torque_x = 0*(W^2 + dW) + 1*(W^2 + 0) + 0*(W^2 + dW) - 1*(W^2 + 0) = 0
torque_y = 1*(W^2 + dW) + 0*(W^2 + 0) - 1*(W^2 + dW) + 0*(W^2 + 0) = 2*dW
torque_z = 1*(W^2 + dW) - 1*(W^2 + 0) + 1*(W^2 + dW) - 1*(W^2 + 0) = 0
F_z      = 1*(W^2 + dW) + 1*(W^2 + 0) + 1*(W^2 + dW) + 1*(W^2 + 0) = 4*W^2

As always again, z-thrust stays constant, but this time only a rolling torque appears.
Thrust
Finally, what happens if we speed up all four motors by the same (squared) speed?
torque_x = 0*(W^2 + dW) + 1*(W^2 + dW) + 0*(W^2 + dW) - 1*(W^2 + dW) = 0
torque_y = 1*(W^2 + dW) + 0*(W^2 + dW) - 1*(W^2 + dW) + 0*(W^2 + dW) = 0
torque_z = 1*(W^2 + dW) - 1*(W^2 + dW) + 1*(W^2 + dW) - 1*(W^2 + dW) = 0
F_z      = 1*(W^2 + dW) + 1*(W^2 - dW) + 1*(W^2 + dW) + 1*(W^2 + dW) = 4*W^2 + 4*dW

So only in this case do we see an increase in vertical thrust (by 4*dW). Notice how the net torques about each axis cancel out.


How long does a LiPo battery last and how do I know if it needs replacing?

How long is a LiPo battery usually supposed to last?

Most LiPo batteries are not rated to last longer than 300 charge cycles. That is the battery fully draining and charging again. However, measuring a LiPo battery’s lifespan may not be practical, as batteries go through varying depths of discharge when they are used.

Many manufacturers have stated that their LiPo batteries will last 2 or 3 years. This is a somewhat realistic approximation for a scenario where a battery is regularly used and charged around 2 or 3 times a week.

  How will one know if it needs to be replaced?

After a LiPo battery goes past its lifespan, it can't hold as much charge anymore, so look for signs that it doesn't last as long as it used to on 'full charge'.


What's the difference between the Spektrum DX6 and DX6e?

I found this comparison chart in an rcgroups thread. The DX6e and DX6 (G3 in this case) are very similar, but the biggest differences that I could find is that the DX6e doesn't have voice alerts and that it doesn't support wired trainer, which will make it harder to use with for example a simulator on a PC. The DX6e also supports mode 5, where the throttle centers with a spring.
I see no compelling reason to upgrade from one to the other. If you want to use it with a simulator (which you probably want, at least as a beginner) it is cheaper to just buy a wireless adapter to be able to connect the transmitter to the PC via the wireless trainer link.


How do aerodynamics for small model aircraft differ from that of larger aircraft?

The largest difference is that in Reynolds number, which is the same issue that wind tunnel models have. The Reynolds number is linear in length, which for the wing is the chord length. So for example, a quarter-scale model will have a Reynolds number equal to one-forth that of the full-scale aircraft (in the same atmospheric conditions). The reason this matters is that airflow tends to transition from laminar to turbulent at a specific Reynolds number, so if one uses the same airfoil on the model as in the full-scale aircraft, then usually the performance will be different near stall (usually much worse). That is why reduced-scale models usually change to an airfoil that is designed for lower Reynolds numbers.
BTW, Intro to Aerodynamics by Anderson is excellent. That was my first textbook when I started aero engineering over 30 years ago.


Do propellers at different angles work the same? And do intersecting flows of air work the same?

The amount of force generated remains the same, however it is applied in a different direction; you therefore get an effective reduction in force for a given axis. To use the two extremes:

If the airflow is straight down, all of the force is used for lift
If the airflow is completely horizontal, none of the force is used for lift

Aligning the motor at angles between these splits the force between horizontal and vertical. The 'best' angle for your design of course depends on the type of aircraft; for a rotary wing aircraft you need to generate a lot of lift from the rotors but for a fixed wing you want thrust, while the aerofoils create the lift.

In your image, the airflow would remain the same but the two motors would be pulling against each other - this decreases efficiency, however the advantage can be improved manoeuvrability in flight as rather than just relying on gravity to roll and pitch the aircraft you can use the angled thrust to 'push' the aircraft around.


Li-ion battery will not charge on ISDT Q8 Smart Charger

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


What is the performance difference between a brushless motor with a wider vs taller stator?

The simple answer is "more width equals more torque".

More torque let's you drive a higher pitch or larger diameter prop effectively. Given the same size prop, it will drive it with more effectiveness. Added torque allows the prop to spin up or slow down faster, which ultimate results in a better flying quad because it can more quickly and accurately respond to your stick inputs.

In the miniquad community, this is the reason a lot of freestyle pilots trend toward wider motors on a 5" prop compared to racers. It provides a higher level of smoothness because of how it reacts.

Another effect of more width is more efficiency especially with big crafts that are targeting flight time. You'll see lots of the big (15" or larger) propeller quadcopters with very wide motors. For instance a 5010 stator size. High performance isn't usually the goal here so a wider stator allows them to drive the big prop without lots of excess weight and power that they don't need.

Oscar Liang also has a good concise explanation of different motor stator sizes here

This is an endurance quad I built years ago with the 5010 motors I mentioned. Just to give you an idea of the scale and the uses of a very wide motor.


The first step I take to help protect my gear from shorting out from water is to apply silicone conformal coating to all the electronics I can.  I cover the ESC, flight controller, vtx, and any other exposed electronics.  I find that this also provides some protection from random solder drips or conductive grains of sand.

However you have to be very careful because conformal coating will block electrical signals that you want to work, like your USB port.  Also don't coat the barometer if you FC has one.

What other tips do people have for protecting their gear from bad weather?