The MicroWing UAV was 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

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.

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:

1. It needed to be around the same size as the original MicroWing, with a span of around 850mm.
2. 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…

The Build…

Working with foam board is always enjoyable. Building a KFm wing in foam board even more so.

Cutting the small channels for wires and equipment bays was a little time consuming but I still had all the parts cut within a couple of hours work. Sadly the MicroWing UAV was no longer a 1 sheet airframe but I only needed a little more than 1 sheet and it still worked out at around $12 of material – even using expensive Australian foam board.

Finishing Off

Not only does the MicroWing UAV have to perform, it also has to look the part.

Foam board is not known for its weatherproofing abilities so I gave the entire model a spray coat of black enamel paint and then a final layer of clear polyurethane. This finish was easy to apply and seems to have added an extra dimension of strength to the skin of the wing with very little extra weight gained.

I wrapped the leading edge in cloth tape and also covered the open GPS bay. The red stripe not only looks good, it also aids orientation on an otherwise “stealth” airframe!

With everything hooked up and including flight battery, but not camera, the final weight was 530g. Although heavier than the original MicroWing (350g), this is still considerably lighter than other commercial platforms of the same size so I was hopeful of a good performance in the air.

On to the flying then!

As is usual with the weather around here the conditions were less than perfect when I headed off to the LMMAC field for test flights. It was grey and blustery with the threat of rain in the air but I simply couldn’t wait any longer.

First flights were conducted under full manual control and the result was something of an anticlimax – the MicroWing UAV flew perfectly!

The KFm7 section performs very well in the air generating more lift and with better slow flight handling characteristics than the symmetrical sections.

With the 1000mah 3S packs I was getting around 8 minutes of flight time. That is a little less than I had hoped for but it is a very small pack and a larger motor than the original MicroWing.

As a final test for the day I decided to make some flights in Autotune on the APM to try to get those PIDs locked in. That’s when things went wrong…

Using Autotune

Autotune in Ardupilot places the aircraft into FBWA (Fly-By-Wire). You have to push the sticks all the way over and hold for two seconds for the APM to “learn” the extremes of movement and then tune itself accordingly. This just wasn’t happening with the MicroWing UAV. No matter how much I threw the sticks around the airframe barely responded.

The problem, as it turns out, is the OrangeRx software. When you switch on one of the wing mixes in the menu it severely restricts the servo movement to account for the mixing. This means that, as far as the APM is concerned, the PWMs never reach the required extremes to register a response.

Time for some modifications

Back in the workshop I had some decisions to make. I really wanted to use the OrangeRx, just to prove it could be done but things just weren’t going to progress quickly enough. I decided to remove the OrangeRx unit and replace with a Taranis receiver. Using the Taranis would give me much better control over the settings and flight modes anyway.

…and back to the field!

So, the Taranis gave me the required PWM outputs to make the Autotune process work. Things went pretty well and I soon had the MicroWing UAV flying around in Stabilize and FBWA without a problem.

A quick test of the Return to Launch function showed us that the navigation was working fine so it was time to plug in some waypoints and see how it flew under full autonomous control…

…not so good!

Basically, whilst it was tracking to waypoints okay, I kept losing altitude particularly after the turn. I had put this down to a speed issue but a similar problem recently with a multirotor has highlighted the fact that I probably had absolute altitude checked in mission planner and this can cause just such a problem.

At this point the project took a bit of a backseat as I got frustrated with trying to get the damn thing to work. A few months passed before I finally dusted off the old wing and decided to give it another go...

It works!

Yep, if that video proves nothing more then it at least proves it flies autonomously. It turns out that the problem I had originally was to do with the altitude setting in Mission Planner. At the time it was too easy to set “absolute” altitude and this is what was causing the wing to drop out of the sky as soon as it reached its first waypoint and reset the target altitude. In more recent releases of MP however this has been addressed and you are hit with a big warning popup if you try to send a flightplan to the flight controller with absolute altitude checked.

The flight performance though was still a little marginal and flying as I did in high winds resulted in a very bumpy ride and a necessity for nearly 100% throttle for most of the flight. The one thing that does buoy my enthusiasm though is the fact that the launch and landing, despite the conditions, were exceedingly easy and smooth.

Is there a future for the MicroWing UAV?

In its current format I’m not so sure. The power is marginal to say the least and I just feel that if I actually tried to mount a payload on this wing it would simply be too heavy to perform with any success. Obviously the 1000mah 3S pack just isn’t cutting the mustard with regards to power but a larger pack would just add to the weight problem.

There are also a few concerns about the KFM wing section that have been highlighted in this and other wings:

1. They just don’t glide. Well, they do but with a pretty steep glidepath. This doesn’t help with the efficiency if you can’t just coast a bit when you want to.
2. They are very sensitive in pitch. It is probably due to the mainly flat nature of the section but small elevator inputs can result in large changes in pitch even if you have the expo dialed way up. My guess is that the wing starts to pitch and then “catches” the wind on the flat surface which causes the bigger deflection.

Are there alternatives?

I wanted a small FPV wing anyway and my birthday was looming so I asked the family for a TBS Caipirinha. This fully molded EPP wing is pretty much identical in size to the MicroWing UAV but the setup and power system couldn’t be more different.

The biggest surprise is that they both weigh the same. The Caipirinha however is fitting full FPV gear, a GoPro and a 3300mah 2S pack into the same space so the wing itself is obviously lighter.

The Caipirinha uses a high KV motor (I’m using a 2206/2150kv mutirotor motor) and 8 inch prop. This is giving anything up to 25-30 minutes of flight from a single pack.

It also flies well. It still gets knocked around in the wind but it has a huge speed range (15 – 80kph roughly) and handles well.

The problem is that this project wasn’t to see what could be done with $100 of foam that someone else had made. It was to create something that was essentially a “disposable drone”.

Conclusion

So, is there a reason to pursue this further? Obviously a “proper” wing section molded in foam is the way to go but is it worth the effort?

I mean, when compared to the $40,000 you’d spend on a Trimble Gatewing, is $100 for a Caipirinha that could be modified to serve the purpose such a bad deal? Of course, then there’s also plenty of cheap EPP wings on the HobbyKing site that could be sourced if only for the wing panels.

I don’t know. The idea of a truly disposable foam board “drone” appeals to me but sometimes I wonder if the cheapest alternative commands the same respect and serious consideration that the overpriced systems do purely because of the price-tag.

Anyway, if you’d like to give it a go I’ve decided not to pursue this as a commercial venture so I’m making the drawings free to anyone who wants to take a look:

MicroWing-RPAS – drawings pack (zip)

Inside the zip file is two PDF’s and a Sketchup file. These aren’t plans as such (as there is no writing or dimensions), but the PDF’s are to scale and you should get all the info you need about dimensions from the Sketchup file. All you really need to know is that it is made entirely from 5mm foam board with a bit of 3mm “Corflute” plastic top and bottom.

PS - the CG is 135mm from the leading edge of the blunt nose section.

If you've made it this far then thanks for reading the whole thing! If you are still craving more silliness you can always check out www.red20rc.com.au and see what I spend all my free time and my kids inheritance on...