Building, flying and crashing a large QuadPlane

Not all of the adventures that CanberraUAV have with experimental aircraft go as well as we might hope. This is the story of our recent build of a large QuadPlane and how the flight ended in a crash.

As part of our efforts in developing aircraft for the Outback Challenge 2016 competition CanberraUAV has been building both large helicopters and large QuadPlanes. The competition calls for a fast, long range VTOL aircraft, and those two airframe types are the obvious contenders.

This particular QuadPlane was the first that we've built that is of the size and endurance that we thought it would easily handle the OBC mission. We based it on the aircraft we used to win the OBC'2014 competition, a 2.7m wingspan VQ Porter with a 35cc DLE35 petrol engine. This is the type of aircraft you commonly see at RC flying clubs for people who want to fly larger scale civilian aircraft. It flies well and the fuselage and is easy to work on with plenty of room for additional payloads.

VQ Porter QuadPlane Build

The base airframe has a typical takeoff weight of a bit over 7kg. In the configuration we used in the 2014 competition it weighed over 11kg as we had a lot of extra equipment onboard like long range radios, the bottle drop system and onboard computers, plus lots of fuel. When rebuilt as a QuadPlane it weighed around 15kg, which is really stretching the base airframe to close to its limits.

To convert the porter to a QuadPlane we started by glueing 300x100 1mm thick carbon fibre sheets to the under-surface of the wings, and added 800x20x20 square section carbon fibre tubes as motor arms. This basic design was inspired by what Sander did for his QuadRanger build.

in the above image you can see the CF sheet and the CF tubes being glued to the wing. We used silicon sealant between the sheet and the wing, and epoxy for gluing the two 800mm tubes together and attaching them to the wing. This worked really well and we will be using it again.

For the batteries of the quad part of the plane we initially thought we'd put them in the fuselage as that is the easiest way to do the build, but after some further thought we ended up putting them out on the wings:

They are held on using velcro and cup-hooks epoxied to the CF sheet and spars, with rubber bands for securing them. That works really well and we will also be using it again.

The reason for the change to wing mounted batteries is twofold. The first is concerns of induction on the long wires needed in such a big plane leading to the ESCs being damaged (see for example http://www.rcgroups.com/forums/showthread.php?t=952523&highlight=engin+wire). The second is that we think the weight being out on the wings will reduce the stress on the wing root when doing turns in fixed wing mode.

We used 4S 5Ah 65C batteries in a 2S 2P arrangement, giving us 10Ah of 8S in total to the ESCs. We didn't cross-couple the batteries between left and right side, although we may do so in future builds.

For quad motors we used NTM Prop Drive 50-60 motors at 380kV. That is overkill really, but we wanted this plane to stay steady while hovering 25 knot winds, and for that you need a pretty high power to weight ratio to overcome the wind on the big wings. It certainly worked, flying this 15kg QuadPlane did not feel cumbersome at all. The plane responded very quickly to the sticks despite its size.

We wired it with 10AWG wire, which helped keep the voltage drop down, and tried to keep the battery cables short. Soldering lots of 10AWG connectors is a pain, but worth it. We mostly used 4mm bullets, with some HXT-4mm for the battery connectors. The Y connections needed to split the 8S across two ESCs was done with direct spliced solder connections.

For the ESCs we used RotorStar 120A HV. It seemed a good choice as it had plenty of headroom over the expected 30A hover current per motor, and 75A full throttle current. This ESC was our only major regret in the build, for reasons which will become clear later.

For props we used APC 18x5.5 propellers, largely because they are quite cheap and are big enough to be efficient, while not being too unwieldy in the build.

For fixed wing flight we didn't change anything over the setup we used for OBC'2014, apart from losing the ability to use the flaps due to the position of the quad arms. A VTOL aircraft doesn't really need flaps though, so it was no big loss. We expected the 35cc petrol engine would pull the plane along fine with our usual 20x10 prop.

We did reduce the maximum bank angle allowed in fixed wing flight, down from 55 degrees to 45 degrees. The aim was to keep the wing loading in reasonable limits in turns given we were pushing the airframe well beyond the normal flying weight. This worked out really well, with no signs of stress during fixed wing flight.

Test flights

The first test flight went fine. It was just a short hover test, with a nervous pilot (me!) at the sticks. I hadn't flown such a big quadcopter before (it is 15kg takeoff weight) and I muffed up the landing when I realised I should try and land off the runway to keep out of the way of other aircraft using the strip. I tried to reposition a few feet while landing and it landed heavier than it should have. No damage to anything except my pride.

The logs showed it was flying perfectly. Our initial guess of 0.5 for roll and pitch gains worked great, with the desired and achieved attitude matching far better than I ever expected to see in an aircraft of this type. The feel on the sticks was great too - it really responded well. That is what comes from having 10kW of power in an aircraft.

The build was meant to have a sustained hover time of around 4 minutes (using ecalc), and the battery we actually used for the flight showed we were doing a fair bit better than we predicted. A QuadPlane doesn't need much hover time. For a one hour mission for OBC'2016 we reckon we need less than 2 minutes of VTOL flight, so 4 minutes is lots of safety margin.

Unfortunately the second test flight didn't go so well. It started off perfectly, with a great vertical takeoff, and a perfect transition to forward flight as the petrol engine engaged.

The plane was then flown for a bit in FBWA mode, and it responded beautifully. After that we switched to full auto and it flew the mission without any problems. It did run the throttle on the petrol engine at almost full throttle the entire time, as we were aiming for 28m/s and it was struggling a bit with the drag of the quad motors and the extra weight, but the tuning was great and we were already celebrating as we started the landing run.

The transition back to hover mode also went really well, with none of the issues we thought we might have had. Then during the descent for landing the rear left motor stopped, and we once again proved that a quadcopter doesn't fly well on 3 motors.

Unfortunately there wasn't time to switch back to fixed wing flight and the plane came down hard nose first. Rather a sad moment for the CanberraUAV team as this was the aircraft that had won the OBC for us in 2014. It was hard to see it in so many pieces.

We looked at the logs to try to see what had happened and Peter immediately noticed the tell tale sign of motor failure (one PWM channel going to maximum and staying there). We then looked carefully at the motors and ESCs, and after initially suspecting a cabling issue we found the cause was a burnt out ESC:

The middle FET is dead and shows burn marks. Tests later showed the FETs on either side in the same row were also dead. This was a surprise to us as the ESC was so over spec for our setup. We did discover one possible contributing cause:

that red quality control sticker is placed over the FET on the other side of the board from the dead one, and the design of the ESC is such that the heat from the dead FET has to travel via that covered FET to the heatsink. The sticker was between the FET and the heatsink, preventing heat from getting out.

All we can say for certain is the ESC failed though, so of course we started to think about motor redundancy. We're building two more large QuadPlanes now, one of them based on an OctaQuad design, in an X8 configuration with the same base airframe (a spare VQ Porter 2.7m that we had built for OBC'2014). The ArduPilot QuadPlane code already supports octa configs (along with hexa and several others). For this build we're using T-Motor MT3520-11 400kV motors, and will probably use t-motor ESCs. We will also still use the 18x5.5 props, just more of them!

Strangely enough, the better power to weight ratio of the t-motor motors means the new octa X8 build will be a bit lighter than the quad build. We're hoping it will come in at around 13.7kg, which will help reduce the load on the forward motor for fixed wing flight.

Many thanks to everyone involved in building this plane, and especially to Grant Morphett for all his building work and Jack Pittar for lots of good advice.

Building and flying a large QuadPlane has been a lot of fun, and we've learnt a lot. I hope to do a blog post of a more successful flight of our next QuadPlane creation soon!

Thank's for the explanation John A.B

For John D..and anyone else who might be interested…

To do a full analysis "black box" data would be helpful (aka logs) but in the meantime lets run through a scenario of possible causes just for the fun of it. Ideally a side-by- side comparison between logs and video would be helpful to see exactly what transpired when.

BTW sorry for the long post - lots of copy pasting! This is only intended as a mind exercise, not as evidence to incriminate the culprits responsible for this incident. :-|

RC Air Crash Investigation Quadplane Porter ...

Chapter 1: Dynamics of a Quadplane

There a various factors that make a quadplane different in both construction and in operation from either aircraft and mulitcopter designs. By amalgamating the various components of each, the effects either components have on each other are largely unknown and require further study. I see this whole development process is a "learning exercise" to understand exactly how they actually interact, and how they do not. Here a list of factors described so far, by others and myself:

Quad motor autorotation without ESC brake - Please see John ABs post above. At first glance this might seem trivial, however in this particular case the Porter's forward motor is in fact a 35cc gasser with significant power. The other parameter of importance is that the flight was done, effectively, at maximum forward throttle and set at 28m/s (100kmh). The prop wash also likely to be higher still. The difference to a X8 multicopter motor layout is threefold: 1) the energy output of the gasser is significantly more 2) the angle of attack is different 3) the volume of air moving across the airframe is greater in comparison. It's unlikely any X8 motor would produce the same level of energy, and as John AB said due to the inefficiency of the symmetrical props and motors. In this case it's conceivable that the speed and the duration of the flight caused the quad props to auto rotate at enough speed that it damaged the ESC, at least partially. I think the extra load finished it off.
Quad pitch/roll angle induced negative lift from wing surfaces - when the quad tilts to move laterally the wings are pitched down in the direction of travel which produces a downforce the quad motors in turn need to overcome with more thrust. The more angle of attack, or lateral speed commanded, the more downforce produced by the wings. In this case both the wings and the tail load the rear motors the most, and it is they that are producing the most lift. This is also due to the tail having a longer lever but smaller surface than the main wing, and the rear motor essentially becomes the fulcrum of both. Solution: only allow a quadplane to fly laterally at low pitch/roll angles and use forward motor/prop for most lateral movements, in the direction the wings produce lift, not downforce. Planes don't fly backwards well, and this might of well contributed to the ESC failure because of the extra load through reversing.
Another thing that is noticeable from looking at the video again (and again!) is that whilst the quadplane is reversing and the left rear motor fails, the whole aircraft banks left due to the loss of lift. On a non-winged quad the loss of a motor would have likely resulted in the quad pitching in the direction of the lost motor. In the quadplane it resulted in pitching predominately to the left ie between both left side motors. This would indicate that the rear left motor was creating most of the lift whilst moving backwards, which in turn could have been the cause of it's premature failure. This is likely due to the negative lift from the wing/tail being born by the left rear motor, which the left front motor only had to start supporting after the rear left motor failed and the aircraft became level (likely due to the pilot responding to the motor failure). Without current sensing or log it's a bit hard to tell exactly. It would be interesting to see what the angle was when it was traveling backwards as all of attitude states are hard to tell from the perspective of the video itself, along with the PWM signals to the ESC starting just before that time.

Chapter 2: ESC Design and Propulsion (Copied text in italic)

If the FETs used are as Darius indicated then these comments copied from the spec sheet should hint at how thermal management should be done:

a) In using surface mount devices such as the TO-263 package, the environment in which it is applied will have a significant influence on the part’s current and maximum power dissipation ratings. Precise determination of PDM is complex and influenced by many factors:

1. Mounting pad area onto which the device is attached and whether there is copper on one side or both sides of the board.

2. The number of copper layers and the thickness of the board.

3. The use of external heat sinks.

4. The use of thermal vias.

5. Air flow and board orientation.

6. For non steady state applications, the pulse width, the duty cycle and the transient thermal response of the part, the board and the environment they are in. Fairchild provides thermal information to assist the designer’s preliminary application evaluation. Figure 21 defines the RθJA for the device as a function of the top copper (component side) area. This is for a horizontally positioned FR-4 board with 1oz copper after 1000 seconds of steady state power with no air flow. This graph provides the necessary information for calculation of the steady state junction temperature or power dissipation. Pulse applications can be evaluated using the Fairchild device Spice thermal model or manually utilizing the normalized maximum transient thermal impedance curve.

Based on the above description the primary heat path for this surface mount package would be through the base (component side) which means the PCB, not the top side of the TO case. Thermal conductivity of copper is also 385W/m k which I'd assume is higher than the casing itself (I'm still looking for exact specs on this). In saying that though it is noteworthy that exactly the middle FETs have blown, indicating they couldn't cool as well as the exterior ones, either through the PCB or heatsink, and because they are surrounded by other heat sources (FETs). Thermal conductivity of paper sticker is only 0.05 ...so not good if it's relying on case for heat transfer, which it shouldn't be doing as a surface mount designed device though. Let alone a wrapped one. From the extra ESC photos there seems to be breaks in the case heatsink over the affected FET, and even some sort of foil cover over the metal itself, and eve more bizarre, the case side heatsink has yet another PCB on top. All this can't be helpful. As Paul pointed out as well, this is a bad thermal design and execution overall despite them relying heavily on multiple FETs to reduce heat generation.
Theoretically the design of the ESC is such that they parallel multiple FETS as the heat dissipation of each FET is reduced by the square of the number of parts. So if one FET dissipates 10W, then two would dissipate 5W total or 2.5W each. With an ESC for aircraft the idea is to reduce weight as much as possible, so having heavy heatsinks is worse than adding extra electronic components. This typically also allows for better peak performance too before it runs into thermal issues.
FETs are typically voltage sensitive - autorotation voltage spikes are likely, especially if the motors are low kV, which essentially means they are also good low RPM generators depending on the props. A higher kV would be lees likely to achieve the same high voltages.

So in conclusion although it is possible that the FET(s) on the ESC was faulty since manufacturing, I think its likely that the autorotation spikes, the extra load from the wings when reversing, the poor cooling design of the ESC and even possibly the sticker all contributed to the premature failure and crash.

Regards

You need to be a member of diydrones to add comments!

Join diydrones

Comments

Developer

Andrew Tridgell February 28, 2016 at 8:35pm

sure, its the same order as an X quad. ch5 is right front. ch6 is rear-left. ch7 is front-left, ch8 is rear-right

so before the failure there was more power being applied on the left side than the right side.
JB February 28, 2016 at 7:03pm

No probs Tridge!

Thx for the log files, I'll see if I can dig out some more "evidence"! ;-)

I'm a little surprised the motor managed to remain stationary and not autorotate.

From the PWM graph above it is evident the the left rear was working much harder that the rest from the outset. I'm assuming from the graph Ch5 or Ch7 is right rear? If so then both these being higher leads to the conclusion they were in fact influenced by the reversing maneuver and negative lift from the wings or that the left rear motor might of been defect in some way. The left rear was running at some 75% throttle during the reversing. Seeing that the PWM signal is not linear to ESC current load, rather more indicative of demanded rpm, I'd say that the ESC was operating at fairly high currents during the reversing.

The other comparison is of course Ch8 which in the end, and during the reverse, was less than half of Ch5.

Before I go any further can you please indicate which channels were which motors? Thx
Developer

Andrew Tridgell February 28, 2016 at 3:44pm

@JB and others,

I should also say that from zooming into the raw video it is clear that the quad propellers were not turning at all in fixed wing flight, even when flying at full speed. The NTM 50-60 motors have a very strong magnet "click" per pole as you turn them, and the video clearly shows that this was sufficient to prevent the motors from turning even though we had ESC braking off.

Cheers, Tridge
Developer

Andrew Tridgell February 28, 2016 at 3:39pm

@JB, wow, thank you for thinking about this so carefully!

I have put the log file here:

http://uav.tridgell.net/PorterQuad/crash-2016-02-21.bin

Here are some graphs of the PWM values. First the takeoff in QHOVER:

now the PWM values during the landing transition (also QHOVER), descent and crash:

the point where the rear left motor (RC6) fails is fairly evident.

http://uav.tridgell.net/PorterQuad/crash-2016-02-21.bin
Justin L February 28, 2016 at 3:35pm

Tridge, thanks for all your efforts. There are many of us that are following your progress. Post like this definitely help with not only what to do but what not to do. Im currently building a mugin 2.6 quadplane. The below video is on my test quadplane on an old aero. Everything so far has been working as expected.

YouTube
Warren Eather February 28, 2016 at 3:05am

Thanks for the report Tridge,

The CUAV's VQ Porter will go down in history as one of the great UAV S&R pioneer aircraft. I was privileged to see her fly in Kingaroy in 2014.

Are you planning to post the flight logs you speak about in your report?
JB February 27, 2016 at 9:51am

Thx for the math to support this Jure.

Accordingly I'd expect a measely ESC Pwm of around 60% to already get it in the critical 48A range....I wonder what it actually was.
Jure M February 27, 2016 at 9:34am

So 8S battery gives you ~30V, @30A this is 900W on each ESC/motor. If we assume 95% efficiency for motor and ESC, you get 45W on ESC and ~45W on motor. If each FET dissipates equal amount of heat, we get 2,5W on each FET. With the setup that this ESC has, most of the heat is transferred over the case, which is by far the worst way you can use to cool the device. But let's be generous and say that each FET has ~1 in2 copper area on the PCB for heat dissipation. According to datasheet, this gives us 43°C/W thermal resistance from junction to ambient, and with 2,5W on each FET, this gets us to 107,5 °C + whatever the ambient temperature is on junction. This should be OK.

If we go in the other direction, with 175°C max junction temperature, we have max ~4W on each FET, max. 72W of heat loss on the ESC, with 95% efficiency this is 1440W or 48A on each ESC. If you go over 48A, you are in trouble and that little label will get you there sooner. Not to mention that heatshrink wrap. And this is if you are flying in freezing 0°C weather.

Basically the numbers for this ESC look nice, but when you calculate a bit, they are pure BS.

So in conclusion I think the world needs an ESC that has a little more "brain" than taking PWM input and (hopefully) converting it to motor rotation, one that would be able to send you feedback on the overall health of the ESC and motor, that would report ESC temperature, that would report motor phase currents and battery current and that would be able to tell you that it has a problem and you better land soon or (at the very least) to turn off the motors and prevent the crashing quad from becoming out of control lawnmower of death.
JB February 27, 2016 at 7:47am
Thank's for the explanation John A.B

--

For John D..and anyone else who might be interested…

To do a full analysis "black box" data would be helpful (aka logs) but in the meantime lets run through a scenario of possible causes just for the fun of it. Ideally a side-by- side comparison between logs and video would be helpful to see exactly what transpired when.

BTW sorry for the long post - lots of copy pasting! This is only intended as a mind exercise, not as evidence to incriminate the culprits responsible for this incident. :-|

RC Air Crash Investigation Quadplane Porter ...

Chapter 1: Dynamics of a Quadplane

There a various factors that make a quadplane different in both construction and in operation from either aircraft and mulitcopter designs. By amalgamating the various components of each, the effects either components have on each other are largely unknown and require further study. I see this whole development process is a "learning exercise" to understand exactly how they actually interact, and how they do not. Here a list of factors described so far, by others and myself:
- Quad motor autorotation without ESC brake - Please see John ABs post above. At first glance this might seem trivial, however in this particular case the Porter's forward motor is in fact a 35cc gasser with significant power. The other parameter of importance is that the flight was done, effectively, at maximum forward throttle and set at 28m/s (100kmh). The prop wash also likely to be higher still. The difference to a X8 multicopter motor layout is threefold: 1) the energy output of the gasser is significantly more 2) the angle of attack is different 3) the volume of air moving across the airframe is greater in comparison. It's unlikely any X8 motor would produce the same level of energy, and as John AB said due to the inefficiency of the symmetrical props and motors. In this case it's conceivable that the speed and the duration of the flight caused the quad props to auto rotate at enough speed that it damaged the ESC, at least partially. I think the extra load finished it off.
- Quad pitch/roll angle induced negative lift from wing surfaces - when the quad tilts to move laterally the wings are pitched down in the direction of travel which produces a downforce the quad motors in turn need to overcome with more thrust. The more angle of attack, or lateral speed commanded, the more downforce produced by the wings. In this case both the wings and the tail load the rear motors the most, and it is they that are producing the most lift. This is also due to the tail having a longer lever but smaller surface than the main wing, and the rear motor essentially becomes the fulcrum of both. Solution: only allow a quadplane to fly laterally at low pitch/roll angles and use forward motor/prop for most lateral movements, in the direction the wings produce lift, not downforce. Planes don't fly backwards well, and this might of well contributed to the ESC failure because of the extra load through reversing.
- Another thing that is noticeable from looking at the video again (and again!) is that whilst the quadplane is reversing and the left rear motor fails, the whole aircraft banks left due to the loss of lift. On a non-winged quad the loss of a motor would have likely resulted in the quad pitching in the direction of the lost motor. In the quadplane it resulted in pitching predominately to the left ie between both left side motors. This would indicate that the rear left motor was creating most of the lift whilst moving backwards, which in turn could have been the cause of it's premature failure. This is likely due to the negative lift from the wing/tail being born by the left rear motor, which the left front motor only had to start supporting after the rear left motor failed and the aircraft became level (likely due to the pilot responding to the motor failure). Without current sensing or log it's a bit hard to tell exactly. It would be interesting to see what the angle was when it was traveling backwards as all of attitude states are hard to tell from the perspective of the video itself, along with the PWM signals to the ESC starting just before that time.
Chapter 2: ESC Design and Propulsion (Copied text in italic)

If the FETs used are as Darius indicated then these comments copied from the spec sheet should hint at how thermal management should be done:

a) In using surface mount devices such as the TO-263 package, the environment in which it is applied will have a significant influence on the part’s current and maximum power dissipation ratings. Precise determination of PDM is complex and influenced by many factors:

1. Mounting pad area onto which the device is attached and whether there is copper on one side or both sides of the board.

2. The number of copper layers and the thickness of the board.

3. The use of external heat sinks.

4. The use of thermal vias.

5. Air flow and board orientation.

6. For non steady state applications, the pulse width, the duty cycle and the transient thermal response of the part, the board and the environment they are in. Fairchild provides thermal information to assist the designer’s preliminary application evaluation. Figure 21 defines the RθJA for the device as a function of the top copper (component side) area. This is for a horizontally positioned FR-4 board with 1oz copper after 1000 seconds of steady state power with no air flow. This graph provides the necessary information for calculation of the steady state junction temperature or power dissipation. Pulse applications can be evaluated using the Fairchild device Spice thermal model or manually utilizing the normalized maximum transient thermal impedance curve.
- Based on the above description the primary heat path for this surface mount package would be through the base (component side) which means the PCB, not the top side of the TO case. Thermal conductivity of copper is also 385W/m k which I'd assume is higher than the casing itself (I'm still looking for exact specs on this). In saying that though it is noteworthy that exactly the middle FETs have blown, indicating they couldn't cool as well as the exterior ones, either through the PCB or heatsink, and because they are surrounded by other heat sources (FETs). Thermal conductivity of paper sticker is only 0.05 ...so not good if it's relying on case for heat transfer, which it shouldn't be doing as a surface mount designed device though. Let alone a wrapped one. From the extra ESC photos there seems to be breaks in the case heatsink over the affected FET, and even some sort of foil cover over the metal itself, and eve more bizarre, the case side heatsink has yet another PCB on top. All this can't be helpful. As Paul pointed out as well, this is a bad thermal design and execution overall despite them relying heavily on multiple FETs to reduce heat generation.
- Theoretically the design of the ESC is such that they parallel multiple FETS as the heat dissipation of each FET is reduced by the square of the number of parts. So if one FET dissipates 10W, then two would dissipate 5W total or 2.5W each. With an ESC for aircraft the idea is to reduce weight as much as possible, so having heavy heatsinks is worse than adding extra electronic components. This typically also allows for better peak performance too before it runs into thermal issues.
- FETs are typically voltage sensitive - autorotation voltage spikes are likely, especially if the motors are low kV, which essentially means they are also good low RPM generators depending on the props. A higher kV would be lees likely to achieve the same high voltages.
So in conclusion although it is possible that the FET(s) on the ESC was faulty since manufacturing, I think its likely that the autorotation spikes, the extra load from the wings when reversing, the poor cooling design of the ESC and even possibly the sticker all contributed to the premature failure and crash.

Regards
Developer

John Arne Birkeland February 27, 2016 at 3:23am

@John D.

It's not so much about thrust, but motor timing. Basically brushless motors suck at low rpm, high torque applications (especially the RC kind that does not have hall position sensors). Going from still standing to spinning is a hard task for a brushless motor, even with just a little bit of opposition. A propeller spinning the wrong way, makes the task even harder, and large propellers/low kv motors impacts the problem.

As for the 'after the plane was already hovering' reasoning, I see two cases where this still applies.

1. Restarting counter rotating propellers puts a huge strain on the ESC (RC controllers are not current limited and will happily self destruct if told to do so). So even if the motor was able to re-start and hover, it may have been the contributing factor that caused the FET failure.

2. A ESC is kind dumb. Once it is up and running and the propeller is sort of rotating the right way, it will just continue even if the motor timing is wrong. Resulting in limited power, and eventual failure. A common problem for people putting to large propellers on high kv motors, and/or misconfiguring the ESC timing settings.

But I don't think 2. happened in this case. Large motors with timing missteps, make a very distinct loud 'screming' sound, that I think we would have been able to hear in the video.

And I've actually never heard about the "auto-rotating bottom prop" myth before. But it's one of those instantly debunk ones. For that to happen the bottom prop would have to auto rotate and 'steal' enough energy to prevent the top from generating enough lift. And simply put, propellers aren't even close to that kind of efficiency.

of 9

This reply was deleted.