Upgraded FT Spitfire: More durable and lightweight

My upgraded Spitfire, with blue core fuselage, larger control surfaces, a carbon spar wing, top of wing mounted servos, and an epoxy resin impregnated foamboard wing for improved strength and waterproofing.

The purpose of this article is to introduce new build techniques and design changes that I made to the FT Spitfire to improve its durability, performance and flexibility.

My first FT Spitfire was the first Flite Test Plane I made. Naturally I spent a lot of time building and painting it, like any proud beginner. I was devastated when on its maiden flight it was caught by a gust of wind and crashed on take-off. The area around the wingbox crumpled, and whilst it did fly again, I was frustrated by how foamboard crumples easily and loses its rigidity with age. 

My first spitfire lost its strength and rigidity in the fuselage after a couple of bad landings, due to the foamboard buckling and tearing at areas of stress concentration.

I build several other planes and quadcopters before coming back to the FT Spitfire design. I knew the design had potential, but felt it had a few flaws;

• The 'wingbox' (the area where the wings met the fuselage) was too weak, and I've heard plenty of stories of people who have had their wings tear through their fuselage on a hard landing. 
• The power pod greatly increased how long it took to get up and flying, swap out batteries etc. Also the barbeque skewers holding the pod in had a tendency to widen their holes in the foamboard and tear out. The power pod reduces the available useable space too.
• The length of the nose required a light build and a heavy lipo (2200mah) to get it balanced. Using heavier foamboard or painting the plane worsened this problem.
• I had a servo break on me after a landing because of their exposed location under the wing. 

Despite these issues, I felt the wing design was exceptional - strong, light and with amazing stall characteristics. And as a Brit, I had to have a spitfire!

For my second spitfire, I used two sheets of Dow company blue core foam for the fuselage, extending the length of the nose and ditching the power pod. I cut the foam and hollowed out the fuselage using a hot wire cutter I made from a poundland hacksaw and a brushed ESC.

Basic fuselage cut out using sketchy-looking Poundland hacksaw hot wire cutter.

I cut the tail out of only one sheet so that I would be able to split the plane in half for easier transport/post-crash reconstruction. I then built the wing as in FT's design, but I decided against a one piece wing. I added a section of carbon spar ahead of the foamboard spar to increase strength and stiffness.

Hollowed out fuselage to save weight and move CG further forward. Nose was left solid for better impact stress distribution.

I then glued the foamboard wings into the blue core fuselage using epoxy resin, and glued the tail pieces to the one of the pieces. The two fuselage halves are held together by bbq skewers, but they are not load bearing and haven't damaged the blue core foam thus far. I installed the servos vertically, so that they stick out of the top of the wings. This protects them from damage. 

Wing sections were joined to the fuselage by epoxy resin. The plane can be separated into two halves down the centre line of the fuselage. Note how the servos in the wing are installed vertically, with the control horns sticking out the top of the wing to protect them.

I cut an electronics bay on the upper surface of the nose, with plenty of room so that electronics can be moved around and the plane can be reconfigured. I used velcro and rubber bands to secure any electronics or cargo in place. If this was deemed unsightly/draggy, it would be easy to add a foamboard hatch to the bay.

The setup worked well with;

Turnigy NTM 28-26 1350kv motor, spinning a 9x5 prop
Zippy 1800 3S 20C
Turnigy plush 40A ESC (would've worked just as well with 30A)

I then decided I wanted more speed, so upgraded the battery to a 4 cell and downsized the prop to an 8x4. It seems much faster, but I haven't been able to test it at full pelt as the prop I'm using flutters at high speeds!

I've also mounted my Mobius action camera on the top of the cockpit before - see the video.

To make the plane more agile, I increased the size of the tail control surfaces. You can see the difference most clearly by looking at the top two photos in this article. I know the elevator doesn't look as realistic any more, but it enables you to take advantage of that excellent wing, pulling really tight turns without stalling! It also flies inverted quite happily :D 

The elevator was enlarged to increase turn rate, and the push rods were stiffened by feeding them through guide holes in the fuselage. This prevented them buckling.

The final major change I made was to waterproof the foamboard. I've done this in the past using tape or PVA glue, but for this plane I did something that is lighter and makes the plane resistant to scratches and dings. I used a squeegee to impregnate the foamboard with a very thin layer of epoxy resin (from Poundland :D). I found that I only needed a very small amount to cover all the foamboard, and it is now very waterproof! Water beads up and runs off, and the solution is as light as tape.

Wing impregnated with epoxy resin. This was done after the wing was assembled, using a squeegee to create a very thin layer of epoxy that could soak into the foamboard's paper. It gives the foamboard a nice shine :)

The consequence of all these changes is a plane that is lighter, more fun to fly, much more resilient to bumpy landings, easier to reconfigure (e.g for FPV), and is waterproof. Blue Core foam is an excellent material to build planes out of, and whilst the finish on mine is a bit rough and ready, with more time and effort you could carve as intricate design as you wish. 

I hope this proves useful to you, as a proof of concept for using blue core, foamboard and epoxy resin together to build better planes! As this is my first article, let me know if you have any questions or comments and I'll do my best to answer them :)