After finally getting a chance to do some more test flying (and filming) of the foam board UAV, I sat down and thought about the difficulties in successful small UAV design – and why the foam board UAV may NOT be the answer…

Let’s start by watching the foam board UAV flight video:

My apologies for the quality of some of the footage – the GoPro was filming at 720/60, which isn’t the best for distance work. You can understand though why I have been so far reluctant to put my APM controller into this model!

So is full-size UAV design the best pattern for a small UAV?

There is actually no real reason why a full-size UAV design would NOT work on a model. Outside of the world of UAVs and Drones, there are thousands of scale flying models out there that fly perfectly well every weekend. So why did my Foam Board UAV design give me so many problems?

I think the first and most important reason is that the Foam Board UAV is NOT a scale model of any full-size drone. Yes, it took inspiration from the “Predator” and “Reaper” drones but at no stage did I look at a 3-view (like the one above) and take measurements that were then translated to the model design. Let’s take a look then at some of the problems I faced and see if we can learn anything from the real thing…

1. Difficulties in hand-launching

A pusher motor is always going to give you problems when you hand-launch a model. Of course the full-size has long, retractable undercarriage. If I had fitted undercarriage to the model then I wouldn’t have experienced a problem.

2. Vertical take-off performance

Again, not really a full-size comparison issue. The model was badly setup initially and very overpowered. Both the Predator and the Reaper are pusher-prop driven aircraft although neither have more power than they need. UAVs of this kind are built for economy and endurance, not 3D aerobatic performance!

3. Difficult to make smooth, coordinated turns

This is possibly my first obvious design flaw. in the 3-view above you can see that the wing is mounted at around 50% of the fuselage length. This allows for plenty of room at the front of the aircraft to store heavy imaging equipment away from the engine systems. This reduces the possibility of interference but also make it easier to balance the airframe.

Compare this then to my UAV design and you can see that I have taken it to the extreme. The wing sits at around 66-70% of the fuselage length, leaving little or no distance between wing and tail surfaces. The outcome of this is that the tail moment-arm is very small and the yaw effect of the stabilisers is minimal. Moving the wing forward, even by a few inches, would greatly improve the turn performance.

4. Sudden unexpected maneuvers and wing flex

These two are linked and don’t really reflect full-size design. The truth of the matter is that foam bends. As soon as airspeed increases and the nose pitches down a long flexible wing will bend down at the tips. On a design such as this with very long and tapered wings this anhedral flex is pronounced and the effect on flight performance profound. The solution here is to simply increase the strength of the wing and/or reduce the wingspan.

5. Poor glide performance

Even with the large equipment “bubble” in the nose, the Predator is still a very sleek aircraft with a low drag coefficient. The Foam Board UAV however was designed with simplicity in mind and those curved cheeks hide a large flat plate that was used to mount the camera. Put a large flat plate on the front of any small model aircraft and you are going to increase drag dramatically – and as we all know, drag is the arch enemy of glide performance!

So what IS the best small UAV design?

There is still a lot of merit in the Foam Board UAV design but I keep asking myself if it will ever truly deliver the results I had looked for in the beginning. When searching for a successful small UAV design (fixed wing) there are a few key points that it simply must address – in my humble opinion:

1. Size – Obviously larger models tend to perform better than small ones but we always need to consider how we are going to transport the model. The wing on my Foam Board UAV design was nearly 2 meters and in one piece, which was not easy to move!
2. Power - Whilst we don’t want amp sucking 3D power-to-weight ratios, we do want enough power to get up there quickly and out of trouble if required. Flight times are limited by battery capacity and spending the first 2 minutes of any flight climbing to altitude is going to badly effect mission success.
3. Launching – Small UAVs rarely perform missions within the small radius of the local airfield (or even some short grass for that matter). Taking off and landing on spindly undercarriage is simply not practical so a small UAV design MUST be able to be hand-launched with a near 100% success rate.
4. Performance – Every control input effects performance and reduces flight efficiency (just ask the DLG guys). A small UAV ideally needs the minimum number of servos, driving the minimum number of control surfaces over the smallest distance. All this shouldn’t effect flight performance though and the UAV needs to be stable (especially if you are filming or mapping), have a gentle stall and a reasonable glide. What it doesn’t need is the ability to snap roll, flat spin, hover or fly sustained inverted!
5. Space – UAVs need to carry a lot more gear than your average RC model so extended internal space for efficient layout of electronics is essential.
6. Durability – Operating in often harsh conditions means a small UAV needs to be able to take the odd knock without falling to pieces.

What’s out there already?

Do a quick image search on Google for “Small UAV” or “Model UAV” and see what you get. A few designs are immediately obvious…

The most obvious thing we see is that all small UAVs have:

1. NO undercarriage
2. Pusher motor configuration

A lot of the time UAV and FPV go hand-in-hand and it is the necessity of a clear forward field of view that has driven the pusher revolution.

Of the more traditional designs present we are seeing a lot of “pod and boom” UAV design. This can either be a single boom with a high wing (as in the original Skywalker), or a twin boom (as in the Aersonde). My only concern here is durability. Unless you are going to invest in expensive build techniques and materials I worry that a flex free, durable boom would be difficult in a small UAV.

Where these designs really wing though is the ability to break them down for transport. Removable two-piece wings are a great advantage in small UAV design. It is even better if the often flimsy tail feathers can be broken down for storage as well.

So now we turn to the wings. I have to admit that the more I think about it, the more I like flying wings for UAV design.

…and here’s why:

• Wings can provide a large surface area within a relatively small wingspan.
• When setup correctly they are smooth and efficient aircraft with good glide performance.
• They are relatively easy to launch once trimmed and can be solo launched as well.
• Some wings have a lot of internal space in their design, making it easy to space out electronics. This is espescially true of designs such as the Skywalker X8 or “snub nose” wings like the Zephyr II.
• Wings are inherently strong being one piece formats with no thin or flimsy booms.
• With only two moving surfaces and two servos, wings can be economical on control inputs and power drain.
• They look cool!

Conclusion

For now I have decided not to pursue the foam board UAV design any further, although the plans remain available for those that want to have a play. I will be focusing my attentions on pushing the envelope of what is possible with flying wings – particularly those that can be scratch built from cheap materials.

I hope you have found this rather long article interesting and maybe even thought provoking. I look forward to hearing your views in the comments!

You can learn more about the UAVs I design and build plus lots of other great content at www.red20rc.com.au