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!
Comments
@ Tridge Thanks for the feedback.
It would seem that it was a simple matter of the ESC failing thermally. But if there was little wind then the effect of reversing might of been more pronounced than usual(with feathering and wind the quadplane would have simply drifted back with the wind). Maybe a video/log in-sync analysis might shed more light on if at all the wings increased the quad load in reverse flight? I'm still a bit concerned about flying the quadplane around in any direction and prefer to only move laterally in the direction of the nose using the forward motor (so yaw to heading and then forward to WP). Do you have any concerns regarding the quad pitch angle inducing negative lift on the wings when not flying forward?
Out of interest I was also wondering if you are using a ESC brake or similar on the quad props in forward flight or how you are managing the lift created by the autorotation of the quad props? Stopping the props should increase available forward thrust ie reduce drag. Did the Porter climb much at full forwards thrust? Also was the forward gasser motor running the whole time including through the reverse hover? Is so could the Porter electric start the gasser yet to comply with OBC regs?
I note that attaching the quad arms with silicone sealer also likely resulted in you being able to recover the wings mostly intact as the the arms detached on impact. Nice! I always do that with my battery mounts...allowing the things detach on impact typically means you get to fly another day without major repairs, especially on a electric vehicle.
Going forwards do you think there's a possibility to recover from a quad motor failure, if the aircraft has enough altitude, using the plane wings? When the motor fails at height it seems that the remaining quad motors do retard the decent in the beginning but it gains momentum as it falls leaving no altitude left to recover. Had the quad motors all disengaged, upon one failing, maybe there would have been enough altitude and forward velocity had the quad motors not interfered, to recover from the "quad induced stall" using the plane? This might be helpful as a skill to learn for other quadplane users. With new flight characteristics also comes new flight control methods I suppose.
Another alternative to a X8 multicopter, if the quad motors are powerful enough, might be to use reversing ESCs for the quad motors for reverse thrust. Upon one motor/ESC failing the opposite motor could maintain level attitude by switching between reverse/forwards whilst the remaining two quad motors go full thrust to slow decent. It might yaw but it's likely to impact the ground much slower resulting in less damage overall. That way there isn't an increase in components etc, only a reversing ESC and some extra code is required to handle the failure more readily. In fact taking this a step further in conjunction with the above, if at enough altitude the remaining quad motors could force the plane to pitch down faster on motor failure, then even back up again once winged flight speed is achieved allowing the aircraft to land in plane mode. Hybrid quadplanes seem to have a few more options!
Just an idea.
Regards JB
@Paul and Gary, thanks for your comments. I agree the design isn't great, and in fact we emailed hobbyking in the hope they may improve things for other users.
@Michael, while this aircraft is big by diydrones standards, it isn't actually the scariest aircraft flying out of my local flying field. That would be the larger jet turbines or the big helicopters we fly. Our precautions including having fencing for pilots, large gaps to the areas spectators are in, a remote ignition cut for the petrol engine, proper insurance coverage (thanks to MAAA), heavy model inspections (again, MAAA) and the pilots do in fact have RPAS qualifications (although this flight was under the part G hobby rules). We are in the process of getting an OC for CanberraUAV, although we expect most of our flying to be under part G anyway.
There are still risks, and we've had some close calls over the years that we've learnt from.
@Mark, just to be clear, by X8 in this post I mean an 8 motor octaquad in X8 motor layout, not the X8 foam flying wing. We also have a couple of those, but we haven't tried to convert them for VTOL.
A pity about the crash. It didn't seem to hit so hard, but the damage was major.
A few thoughts about ESCs.
The sticker placement is pretty silly on the part of the manufacturer, but not the only problem. The failed FET is in the centre of all the other heat generating FETs and heat-sinking is minimal to none. Definitely not a 120A ESC unless exposed to a gale force air-flow.
The heat generated (energy dissipated) by the FETs has to go somewhere and the little alloy plates can only absorb so much without getting very hot. Heat transfer to the plates is also poor. The alloy plates cannot dissipate much heat through the heat-shrink covering. As supplied, this design is never going to be reliable as current increases unless receiving a lot of forced cooling air.
I have modified smaller ESC heat-sinking in the past to improve reliability with minimal air flow. It's not difficult but is time consuming.
Think of a 20 watt power resistor with a heat-shrink covering at full dissipation; you are going to have a melt-down. Screw it to a big metal heat-sink with airflow and it will operate at full power for years. Just don't use those finned alloy resistors without a heat-sink. I remember the one that exploded in the workshop with a heck of a bang 25+ years ago.
Hi Tridge, very sorry to see outcome, especially when likely caused by presence of a QC sticker which was almost certainly not present when they tested and spec'd their ESC.
I know your operation was well below maximum allowed, so it is likely safe to assume this problem is linked to the sticker, the presence of that sticker would form an effective thermal gap between the Darlington and the Heat sink cutting heat dissipation to a very small fraction and heat build up could be very fast and very quickly destructive even at loads way below maximum.
I think the liability for this falls squarely with the ESC manufacturers a a really stupid error on their part.
And I believe it should be pointed out to them that all the ESCs they have been selling with this feature are at incredible risk of failure and that it is entirely their fault.
Clearly none of the ESCs can be relied on without removing that stupid sticker and regreasing the heat sink with thermal grease.
The weak link in the chain is always the problem and with a plane, that link is usually fatal.
The government and the military generally get around this by rigorously retesting everything and not relying on supplied specs.
Unfortunately for the rest of us who live in the real world this is often not an option.
Seems like you had a great plane, really sorry to see it downed by such an unnecessary event.
Best regards,
Gary
Hi Tridge - sorry to hear about the crash, but looks like you've got to the bottom of it, and will have a better solution going forward. As always it's better to discover these issues during testing rather than in the competition!
I was wondering if you could explain a bit about the safety precautions you're taking - as operating an experimental 13kg aircraft starts to get a bit more serious than the small foam aircraft. For example when we were testing with a similar sized J3 Cub during my Sydney Uni days we had barricades and fire extinguishers at the ready, and conducted the testing in a remote farm.
Even though we are flying recreationally, <150kg, and thus not required to obtain an Operators Certificate - I feel like it is worth taking a proactive approach to the safety and potentially engaging with CASA on this type of testing.
Team Thunder were using X8's. https://www.youtube.com/channel/UCnqKB6Liq7O1ioZvtHhE77Q. We performed PC based simulations on various configurations and found the wing to be optimal. We also coined one air-frame the "rocket sled" due to its stubby wing design. The challenge calls for something that is reasonably fast enough to complete the course. Weight is the key especially if not using gas for forward flight :)
What a pitty that silly esc, Good luck next time. Thank's for shared so detailed description :)
Bummer about that ESC, hard to believe it's not a QC or just bad model/manufacturer issue, especially if you were only at around 30A. You way want to take a look at the Hobbywing Platinum ESCs, known to be very reliable.100s of hours on those at mostly 2/3rd their ratings, never an issue, and I think many others have had similar experience. They are also fairly rated, with their nominal ratings well under the peak they can sustain for a few seconds.
btw, when I said "2S 2P" batteries, I mean we had a total of four 5Ah 4S batteries. Each pair of 4S batteries was in series, to create a 8S battery. Then we had one of these pairs per wing. So the ESCs were getting 8S voltage. Each motor had 2.5Ah to use.
I'm clarifying this as I noticed someone quoting this blog post and thinking we were on 2S.
@David, we don't know for sure if the sticker was a factor or not. We're sure it didn't help, but we may have just had bad luck that the particular FET failed.
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