The MicroWing UAV is an epic and ongoing project to discover if it is possible to build a small, fully autonomous wing using a KFm wing section and foam board building techniques.
This article is the first in a series to be released over the next few weeks detailing the design and build of this truly awesome little UAV. In this article we’ll look at the reasons for building such an airframe and the design considerations that went into it.
Introduction
The MicroWing UAV came as a result of two other successful projects that I had completed – the Versa Drone and the MicroWing.
With the Versa Drone I had tried to design and build a fully autonomous flying wing under the hypothesis that it was possible to build a truly “disposable drone” both quicker and cheaper than those teams that had claimed to do similar using 3D printed designs and composite materials.
The MicroWing was my own little KFm4 flying wing, built using a single sheet of foam board and created purely for fun flying.
Both projects were a huge success and have been built and flown by other pilots and UAV operators for both fun and commercial use.
With the MicroWing UAV I had intended to use the KFm4 MicroWing as a starting point to recreate the Versa Drone in a smaller platform. I was aware that UAVs of this size already existed in airframes such as the Sensefly eBee, the Trimble Gatewing and the Lehmann Aviation LA300 – but with a price tag on these models of anywhere between $2,000 and $40,000 I was pretty sure I could bring it home in a budget that would make such an airframe more affordable to everyone.
Gear Selection
Gear selection for the MicroWing UAV was going to be critical if I was going to fit it all in. The answer came in the form of the HobbyKing HKPilot Mini. This miniaturised APM 2.5.2 is a full-featured autopilot, running the same Ardupilot as its big brother. Combined with a smaller than normal power module and uncased GPS unit both the weight and size could be minimised. I would lesson the burden still further by using an uncased 6ch Orange Rx and also remove the casing from the telemetry module. In fact, the only thing that was going to increase in size from the original MicroWing was the flight pack and the motor, which would see the original Turnigy Park 300 replaced by a slightly larger Turnigy 2205/34 1500kv motor.
Designing the MicroWing UAV
When I first sat down to design the MicroWing UAV there were a couple of things that I was sure of:
- It needed to be around the same size as the original MicroWing, with a span of around 850mm.
- It would need to carry a full autopilot, telemetry, GPS, and camera as well as the normal rx, servos and flight battery.
What all this meant was that for the same size as the original, the MicroWing UAV was going to be both heavier and need more useable internal space. On top of this it would still need to have a docile performance and good slow speed handling.
The easiest way to increase internal space was to add a blunt-nose center section. I settled for adding around 80mm to the center as this seemed about right and would easily accommodate the width of a 1000mah LiPo. This however only took care of horizontal space and I soon discovered that the KFm4 wing section with only 3 layers of foam simply wasn’t deep enough to contain all the gear.
I took another look at the KFm family of airfoils and my eye was drawn to the KFm7. This section uses a double step on the top of the wing and a single step on the bottom. It claims to be ideal for flying wings and solved a few of my problems both increasing the thickness of the wing and also creating a semi-symetrical section that should improve slow speed handling and stability.
Even with the inclusion of the center section I retained the 850mm wingspan. I did increase the chord of the wing and pull everything around so the leading edge and trailing edge ran parallel.
The result was a wing that, to be honest, looked like a winner before it was even committed to foam…
Next time…
With the plans drawn and printed we will what happened when the design was transferred to the trusty foam board. What should have been a simple build soon became complicated as I had to deal with gear placement and new gluing methods to save weight…
I like your thinking. So true that all those expensive autopilot wings are about the 800-900mm wingspan.
I have a Teksumo running the HKPilot Mini and it flies really well. I was one of the first people to test that board, and there were a few dodgy ones sold at the time!
So, I run an FrSky Rx on CPPM. You can run CPPM into the HKPilot by jumping the 2-3 inputs and input into pin 1. OR – and this is what I now fly with jumpers soldered on the bottom of the board as per DIY Drones instructions. Why? Because the CPPM goes straight onto the main processor and does not pass through the PMM chip and code.
What nice about CPPM is you can place the Rx on the wing tip and run 3 wires back to the FC.
The FrSky D4R-II is perfect for the job if you want to reduce space even more. I’m happy with the FrSky D8R-XP, but have used the 4 on CPPM and its just as good and smaller (a bit cheaper too)!
The other thing I found to reduce wire lengths was to solder the Power Module direct to the ESC. The truth is the wire gauges that come on the PM are on the heavy side!
I like your choice of airfoil – be interesting how it goes with the extra centre. Of course the COG will move forward a bit. I would like to try with that foam too. Do you like it better than foam board?
One other thing. The HKMini may not allow you to run both telemetry and an osd. Mine refuses to. But maybe they fixed the power issue.
I recommend metal gear servos – the APM keeps them busy on the auto modes.
Looking forward to following you progress, between home life that is…
Rob
Hi Rob,
I’m certainly interested in the CPPM idea. At the moment I’m running a normal FRSky Rx and it was a bit of a pain routing the wires through the wing so CPPM would be good. It sounds like the inputs work exactly the same as CPPM input on the full size APM so I might give it a go.
I don’t want to give too much away before I write the following articles but it does fly very well and that is actually normal foam board!
I running this as a full UAV, no FPV camera down-link at all so I haven’t really thought about OSD anyway but I know there was always a problem with the full size APM and running both (although I believe the new HKPilot 2.7 can do it).
Cheers,
Mike
Hi Mike
Once you go to CPPM you won’t go back! At the most you only need 4 wires, the 4th being RSSI.
I forgot to mention that I don’t use an external Mag – so no cutting the trace wire and ending up cutting the SDA trace at the same time! But the truth is I have no issues with mag on board, and the FC is just under the PM and near the ESC!
Mine tracks like its on rails. Doing a grid is like watching the thing run along the waypoint tracks with no variation. I run the cheaper GPS with no problems – just give it time to warm up after GPS lock.
The OSD issue was not a problem on any of the full sized boards even before HK’s 2.7. Their version incorporated fixes to overcome the 3.3 regulator issue, which was not an issue if you did not power the board from your PC USB and remove or plug in items while connected. (still rather have the 2.7 fix though!).
Anyway, if you are running UAV then you will have little wind resistance and the thing should fly for ages on the 1000 mA pack! Like the eBee!
But it is good to have Telemetry link for more control, and tuning. I now use 915 Mhz and like it.
TEC tuning is the one to get right, and it matters most on waypoint turns. A friend in NZ uses the airpeed sensor, and from what I can see it really makes a big difference. Personally I think the TEC algorithm needs tuning!
To a certain extent APM has reached its limits and 32bit is the way to go – if you can find one that does not take up the backyard! I’ve been hoping the mini VRBrain would do the job, but not satisfied yet.
I would be interested to see where you place the FC. I’ve seen a few wings with it off centre to allow for a still camera in the centre (like the eBee).
I really like the wing shape you have chosen. And I am convinced you are right on track with the wingspan. If eBee can do it so can you. The key is bury you gear in the wing and keep it smooth.
Oh, one other thing I have noticed. The servo linkages on mine need to be lower geared. That is I believe its better to have more movement on your servos – so link to the inner holes on the servo arm, and longer horn on the control surface. Of course the gain has to be raised. But I believe it will give a better response and less over shoot and hunting. So fast metal gear servos!
Well I’ve waffled enough
Thanks for sharing – I look at your site a few times a week.
Rob
Can you produce short videos showing the construction of these foam model aircraft?
I’ll do my best. Usually I put them together so quickly I even forget to take photos!