After the unfortunate crash of our last QuadPlane build with a failed ESC CanberraUAV wanted to build a new version with more redundancy in the VTOL part of the build. The result is the above OctaQuadPlane which we successfully flew for the first time yesterday.
When we were first designing a large QuadPlane we did consider an octa design, but rejected it due to what we thought would be a high degree of complexity and unnecessary weight. Needing wiring for both 8 vertical lift motors plus 4 controls for fixed wing flight, and extra controls for ignition cut and auxiliary functions like remote engine start, choke and payload control we worked out we'd need 15 PWM outputs. The Pixhawk only has 14 outputs.
After the failed ESC on the previous plane lost us the aircraft we looked again at an octa design and found that it would not only be possible, but could potentially be simpler for wiring than our last aircraft and be lighter as well, while having more lift.
To start with we looked for motors with a better power to weight ratio than the NTM Prop Drive 50-60 motors we used on the last build. We found them in the t-motor 3520-11 400kV motors. These very well regarded motors have a considerably better power to weight ratio, and back-to-back mounting of them was extremely simple and light with the above clamping arrangement.
To manage the complexity of the wiring we added support in ArduPilot for high speed SBUS output, and used one of these SBUS to PWM adapters embedded in each wing:
http://www.hobbyking.com/hobbyking/store/__24482__SBD4_4_Channel_S_BUS_Decoder.html
that allowed us to have a single wire from a Y-lead on the SBUS output port of the Pixhawk going to each wing. We modified the BRD_SBUS_OUT parameter in ArduPilot to allow setting of the SBUS frame rate, with up to 300Hz SBUS output. For a large QuadPlane 300Hz is plenty for multi-rotor control.
The arm mounting system we used was the same as for the previous plane, with two 20x20x800 CF square section tubes per wing, mounted on a 300x100x1 CF flat plate. The flat plate is glued to the wing with silicon sealant, and the CF tubes are glued to that plate with epoxy.
For ESCs we used the HobbyWing 40A, which has a burst rating of 60A. That is well above our expected hover current of 15A per motor. Combined with the redundancy of the OctaQuad and the better reputation of HobbyWing ESCs we were confident we wouldn't have a repeat of our previous crash.
For batteries we switched to 6S, using one 5Ah battery per wing. That gives us over 4 minutes of hover flying time while keeping the weight well below our last build (thanks largely to the lighter motors and ESCs).
For this initial test flight we had the fuel tank mounted horizontally unlike the previous vertical arrangement. This was OK as we only filled it a small amount for these test flights. We will be converting to a vertical arrangement again for future fights to reduce the impact of fuel slosh causing CoG oscillations. We may also fill it with fuel anti-slosh foam as we have done on some other aircraft (particularly the helicopters).
Overall the build came out about 1.5kg lighter than the previous build, coming in at 12.5kg dry weight. With a full load of fuel we'd expect to be about 13.5kg.
We did three test flights yesterday. The first was just a quick hover test to confirm everything was working as expected. After that we did the first transition test under manual control, which worked very nicely.
The copter part of the tuning could definitely do with some work, but we thought it was stable enough to do a full auto mission.
It was a short mission, with just two full circuits before landing, but it nicely demonstrated autonomous VTOL takeoff, transition to fixed wing flight, transition back to hover and auto landing. The landing came in within a meter of the desired landing point.
We had set the distance between the transition point and landing point a bit short, and we hadn't included a mission item for the plane to slow down before it transitioned which led to a more abrupt transition than is really good for the airframe. Going from 100km/hr to zero over a distance of 77 meters really puts a lot of stress on the wings. We'll fix that for future missions. It is nice to know it can handle it though.
The transition to hover also caused it to climb a fair bit which meant it spent more time in VTOL landing than we would have liked, chewing through the battery. That was caused by it having to pitch up to slow down enough to achieve the stopping point in the mission which caused it to climb as it still had a lot of lift from the wings, combined with a high angle of attack. It didn't help that we had a slow slew rate on the petrol motor, reducing the motor from full throttle to zero over a 3 second period. We chose a slow slew rate to reduce the chance of the engine cutting due to fast throttle changes. We can fix that with a bit of engine tuning and a longer transition distance - probably 130 meters would work better for this aircraft.
The climb on transition also took the aircraft into the sun from the pilots point of view, which isn't ideal but did result in a quite picturesque video of the plane silhouetted against the sun.
The full flight log is available here if anyone wants to see it. The new features (new SBUS output support and OctaQuad support for quadplanes) will be in the 3.5.1 plane release.
We're not done yet with QuadPlanes. Jack is building another one based on the Valiant, which you can see here next to the Porter:
Combined with the GX9 tradheli build that Greg has put together:
we are all set for lots of VTOL fun!
Many thanks to everyone who helped with the build and as always to CMAC for providing a great flying field!
Comments
another factor to some angle of the motors is to achieve hover with the wing at a slight positive angle. that way, as was mentioned before,the rotors are relatively neutral to the airflow in forward flight, the lift motors will also aid speed during transition, and when hovering at zero airspeed, the wing can help lift a little, instead of creating downforce. It also helps if you have to move forward in forward flight. in either of the last two modes, the efficiency of hover will be increased slightly by the wing, as wind speed increases, helping offset the inherent loss of efficiency in windy or gusty conditions. well, that's my thinking anyway. I'm experimenting with a easy glider in the x-4 configuration with a single electric motor up front, and the guts from a phantom 1 and phantom 3. i did not make any changes to the flight controller, and just stuck it it. i use two control systems, and two pilots. did the build in about 3 days. just to see how hard it would be. turns out VTOL is pretty easy, at least in it's simple form. plane flew very well.
https://www.youtube.com/watch?v=HYe160rU31I
I appreciate seeing your build, and an autonomous flight. any more info about settings, or setup would be appreciated much, as i have a similar size design on the drawing board. thanks for sharing!
Hi Tridge,
I tried SBUS using Futaba R7008SB.
In Arducopter heli 3.3.3 stable I had to set BRD_SBUS_OUT = 1 and BRD_SAFETYENABLE = 0 to get signals on the SBUS out of the Pixhawk.
In Arduplane 3.5.3 stable I had to use BRD_SBUS_OUT = 4, using other values (1,2,3,5,6,7) I didn't get any signal.
Results were not promising, don't think it will work without channel remapping.
Could you please be so kind and provide the heli firmware you use for download ?
Thank You
Hello! What was your experience with QLoiter, did you have to change any parameters to make it work?
What parameters did you change? My plane is starting to oscillate when I enter loiter mode and I'm sure that GPS is set correctly.
Thank you ang good job! :)
I tend to like the Hybrid Electric approach. These guys have a very good example as do several others. Not winged tiltrotor yet, and not particularly high translational speed, but the writing is on the wall; https://www.youtube.com/watch?v=TUYzANXd4Ys
@Jaime and @Tridge
Agree with both.
If comparing full electric quadplane and full electric tilt-rotor: They are both more or less the same in terms of performance. One is more similar to multicopters the other is more similar to fix-wing. It depends on how you fly it.
While matching props-motors for tilting-propellers is difficult, it is possible: We did it and it works. The only thing is you don't have any of the fly modes optimized, while in quadplanes you do but has to add the extra weight.
Petrol engine: It is the best solution if your PL is not heavy. They have a lower PL/MTOW ratio than fully electric. Again, it depends on application. If you just want a camera, go for @Tridge quadplane. If you want to carry a heavy load, for for fully electric.
We are developing the software for tilrotors (3 propellers): I think we could help on the firmware development @Tridge Send me a PM if you think we could help
Regards
@Tridge, very interesting points regarding quad planes and tiltrotor
@Jaime, I do plan on adding tiltrotor support, and I'd particularly like to support the FireFly6. Team Tiltrotor has done some great work on that, but it isn't yet in master, and getting it in the quadplane code would be a very nice option.
As to the general question of whether tiltrotors are a good idea I think it depends on the scale of the aircraft.
The biggest problem I see with tiltrotors is that the prop and motor that is ideal for vertical flight is rarely the ideal prop and motor for forward flight. A typical RC scale tiltrotor won't have variable pitch props, and will typically use brushless electric motors. This is because variable pitch prop systems for small scale RC aircraft add quite a lot of complexity and cost.
On motor type, I think that if you are really after long range then you need to go to a petrol forward engine. Making a pure petrol tiltrotor is tricky at small scales (not impossible perhaps, but certainly not easy).
In our OctaQuad plane the petrol forward motor can give us over an hour of flight at high speed (58 knots). The vertical lift motors last 3 minutes. The batteries for that vertical lift weigh quite a bit more than the petrol for the forward motor.
So I think non-tiltrotor is actually the best option for really long range. Tiltrotors are still interesting aircraft however, and if you want to go purely electric they are a reasonable option if you don't want to use a helicopter. So we will add support, but I can't promise a date for it.
Cheers, Tridge
This is great news Tridge. I'm really grateful that you're sharing your progression. It did bring some ideas to mind about VTOL planes.
I've seen in other Flight Controllers that they're adopting a tilt rotor setup, is that planned for Arduplane? Is my assumption that it would be more efficient than the quad plane due to weight savings correct? What would be the pros and cons of a setup like that?
I've been reading the low-down of the QuadPlane code and I must say it is impressive. Thanks to the developers for doing such a great job! Now to design an airframe to put it in :)
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