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.

IMG_20160212_143527.jpg?width=600in 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:

IMG_20160218_200021.jpg?width=600They 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 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.

IMG_20160221_113920.jpg?width=600Unfortunately 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:

IMG_20160221_131749.jpg?width=600The 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:

IMG_20160221_131435.jpg?width=600that 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!

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  • Developer

    @JB, unfortunately we only had current monitoring on one motor (front-right). That is due to separation of batteries between left and right wing, and limits of the current sensor. So all we can do is look at current on front-right and extrapolate based on PWM value. That extrapolation indicates the current to left-rear was quite normal before the failure, but there is plenty of room for error in that of course.

    The wind was quite low, and we don't think contributed significantly, but there is room for error in that too. We think the current to the failed ESC was around 30A, and the ESC was rated to 120A. A bit of wind shouldn't have been an issue.

    @turdsurfer, the batteries hold on nicely with a single failure of the clips (we checked that), and actually hold without the clips at all due to velcro, although with less certainty. Regarding X8 vs Quad, we're building two more aircraft, one a quad and one a X8. Jack is building one based on a valient model and going for the "test the ESCs and motors lots on ground" approach. Grant and I are doing the X8, going for redundancy.

    @Hughes, the problem with smaller props is the efficiency is lower, so you need more battery, which adds weight. Thanks for the glue suggestion!

  • I agree with hugues,

    For large props the bottom line is to use large amp esc prevents things like this happen.

    I have a similar design like yours I use 2212 motor running 11 inch prop.                                                              The 2212 motor

     are designed to drive 8 or 9 inch props 20a esc is enough. But working with large blades like 11 inch, you needs to increase the amps of the esc to 40a otherwise they will be over heat and shut off.

    Try the hobbywing 120a hv they are reliable

  • Developer

    @Darius, there are more photos of the ESC showing heatsink etc here:

    @Max, burn in to lower the chance of infant mortality may well be a good idea, but it isn't easy to run them under load for a significant period of time. Just the time to charge the batteries would be significant! Each 3 mins of flight takes 1 hour of charging

    Index of /PorterQuad
  • Moderator

    Its making omelettes and eggs, this is way ahead of some military work. Well done Tridge and team.

  • Maybe it's just me but is the sticker really the cause of the problem?  I had a bad Castle ESC right out of the box once.  Obviously its a ESC failure but I don't know if the sticker caused it.  Great work Tridge!


    David R. Boulanger

  • MR60

    Great post, thx for sharing.

    Also from an armchair perspective, some points come immediately to mind:

    -18" props on 380KV motors is really not an ideal setup for resisting such as winds as seen in your video. You'd be better off with smaller props and higher RPM motors

    -In quadcopter mode, when landing the huge surfaces of the plane in the wind are clearly not managed by the four motors and props (you see it moving all around)=>the ESCs were propbably stressed too much

    -I would not use the glue you used for gluing the quad frame as you might get some surprises after a few flights with fatigue problems. I would advise 3M 2216 rather, which is a bit expensive but best epoxy resisting fatigue (used in real planes).

    Impatient to see your future X8 build !

  • Oh dear ... :)

  • sorry about your plane and thanks for all your work wouldn't  it lighten the load on the quad if the plane was in heading hold stab set into the wind again thanks ; )

  • It looks so good to see you flying Tridge ! Thanks for sharing here.

  • Ouch. That brings back memories of the balsa RC days.
    Anyway, that's a hard lesson to learn to test, inspect, and build things as if your life would depend on it.

    E.g.: stress test ESCs and motors on the ground first way beyond the expected stress they'll receive in flight etc.

    Look at the way the batteries are hanging on the wing: If one of those plastic clips break or the elastic breaks, then the batteries gone, followed by the plane. Rubbery velcro battery ties with anti-slip padding on the wing is much safer.

    I wouldn't opt for an X8 setup though, because of the added drag and points of failure, even though there's some redundancy. A thoroughly tried and stress tested X4 setup is imho the best tradeoff.

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