I’ve become more and more fascinated recently by the function and purpose of UAV’s and Drones. Not so much the military versions, but the smaller civilian units capable of aerial photography, surveillance, support etc.
My interest was really inspired when I started watching videos from, and reading about Conservation Drones (watch THIS YouTube video for a good overview of what they do). They are taking very affordable airframes and electronics and turning them into high-functioning surveillance platforms, operating in some of the most remote parts of the world to support conservation efforts. When I then also read about the Drone User Group Networks “Drone Social Innovation Award” my brain went into overdrive.
Could I use the same Flite Test techniques I’ve been using to turn out foam board parkflyers to create a stable, efficient foam board UAV?
Searching for inspiration…
I knew from the start that this was going to be a plane rather than a multirotor. I had been watching a number of documentaries on military drones to see what the really high-end stuff looked like and the simple design concept of the Reaper and Predator drones really caught my eye. These were basically just glider wings, set back on the fuselage to counter the weight of the camera equipment and sensors in the nose. The anhedral tail surfaces on the Predator are intended to assist in the prevention of prop-strike during take-off and landing – plus they look cool.
I had just finished building a Flite Test Simple Soarer, so I knew a larger span folded wing was possible. This was enough inspiration for me and I needed and excuse to try out Sketchup for something…
Using Sketchup to visualise the design
I’d played around with Sketchup a long time ago when it was still Google Sketchup but never really got that into it. I couldn’t get my head around using polygons to build up complex shapes. Since then the software has evolved and, along with some excellent training videos, it really is quite user friendly.
I had already sketched out a design on paper so I knew the rough dimensions. A single evening later and I had a 3D model in the computer – this was looking good!
Once I was happy with the 3D model I then went back and redrew the components in 2D ready for printing. The Flattery plugin for Sketchup allows you to export 2D drawings to a .svg format. With the aid of Adobe Illustrator I easily converted the whole lot to pdf for printing.
In fact, if you are really clever you can design in 3D then use Flattery to “unwrap” your model into a 2D shape for printing.
Wing Design Failure
In an effort to keep things as simple as possible you’ll notice the wing on the 3D visualisation is a simple arched design, similar to that on the FT Old Speedster and Old Fogey. I thought I would try this out as a very high-left undercambered wing might be just the thing for this airframe. Unfortunately over 72 inches the strength of the arch is simply overcome by the weight of the material. Although it did indeed generate a massive amount of lift, as soon as I slung a fuselage underneath it would have folded.
For this reason the final design used the same folded wing as the Simple Soarer and other FT designs. I also included the open undercambered section on the final 10 inches of each wing to provide extra lift and reduce tip-stall tendencies that might be inherent in a tapered wing.
Putting it all together
As always with these designs, once the plan is printed and cutting starts it all happens very quickly. I got the second wing cut and glued over the New Year. Full length flaperons are included with a single servo built into each wing to drive them. As with other wings like this I built the two halves separately and brought them together at the last minute, the foam providing incredible strength a rigidity even at the join.
The fuselage came together very quickly with a minimal part count. The whole thing is built up of “A” and “B” style folds with a few formers towards the rear to aid rigidity where the tail and motor will mount.
The nose section is a second piece that is a push fit over the main fuselage tube. This double thickness adds strength to the nose area to protect the expensive camera and flight control equipment that will be house there.
The tail again was very simple with the fin tabbing into the fuselage top surface and the horizontal stabilisers sitting against shaped formers in the fuselage.
A prototype is born…
In just a few hours I had gone from a Sketchup model to a full size prototype, ready for the electronics.
Some of the specs are:
- Wingspan: 72 inches (1828mm)
- Wing chord at root: 8 inches (203mm)
- Wing chord at tip: 4 inches (101mm)
- Fuselage Length: 30 inches (762mm) – without motor or camera
- Bare airframe weight: 15.8 ounces (450g)
Overall, I am very happy with the way this has transferred from paper to prototype. At 450 grams for the airframe I am not worried at all about the weight. Even from “heavy” Australian foam board this is still a light model.
Next job is to fit the electronics. This is having a Futaba Radio for control, HKMegaPilot with GPS for flight control. I will still use the GoPro initially with FatShark or Immersion RC for the FPV setup; although I might add a second camera with headtracking later on when the GoPro is going to be used for mapping etc. Power will come from a single Park 480 motor in the tail.