Strengthening leading edges and undercarriages

Many of the planes on Flite Test use wings with undercambered ends to stop tip stalling, and they work beautifully. The first one I saw and built was, I think, David's Spitfire. But although the wings work as intended, the undercamber brings a problem - a weak spot where the folded leading edge becomes a raw bit of foamboard. Everyone of my planes seems very quickly to acquire either a tear, or at best a bend at the end of each wing. So I inevitably end up splinting the ends with lolly sticks or bits of carbon fibre and lashings of hot glue.

There's a second problem - the folded edge is always a few millimetres back from the raw edge, leaving a step in the leading edge of the wing.

So I came up with a way of adapting all the FT (and others' plans) to solve these problems. It involves creating the leading edge around a carbon rod that goes all the way to the end of the wing, giving a uniform, rounded leading edge that also provides significant resistance to tip crumpling.

The photos are of a FT Mustang I've just built, which also incorporates a design for undercarriage that is very robust - much stronger than the paint stick and glue method of the original. It uses 3d printed parts and I've included links to the .STL files as well as putting them on Thingiverse.

Of course, as we'll be altering the size of the wings slightly, this is only really useful for the scratchbuilders. Kits from the store don't really lend themselves to the adaptation.

We need to create about a quarter of an inch (6mm) of extra room at the folded leading edge, so you need to extend the chord of the underside of the wing by this amount and then add a 50% scorecut a quarter of an inch behind the leading edge scorecut.

So the underside of the wing is the same size as it was, but there's an extra slot to dig the foam out of, just like you would for an A or B fold. The 90 degree chamfer on the leading edge now becomes two 45 degree chamfers, one either side of the slot.

When you're adding the quarter of an inch to the underside of the wing, remember to move any other cuts down by the same amount (eg spar slots, undercarriage marks, servo horn slots etc).

Then, using the hot tip of your glue gun, run it along the foam on the raw leading edge at the tip of the wing to produce a semi-circular groove.

Lay a piece of 3mm carbon rod in the slot along the length of the leading edge, all the way to the end so it can tuck into the groove. If you're miserly, like me and don't want to use a whole carbon rod, use a skewer for the inboard half and carbon for the outboard, Use hot glue to stick the sticks in place.

Then, when you get to the part of the build where you fold the wing over to glue the top to the bottom, the rod becomes wrapped in the paper at the fold and pokes out all the way to the end of the wing. Cover the exposed rod with a paper address label and it can be invisibly decorated.

Another thing to consider is that when you're sliding the wing into the slot in the fuselage, you might need to make the slot a little longer to accommodate the extra thickness at the leading edge.

The undercarriage mod uses two parts for each wing, There's a large flat section that fits inside the wing where the paint stick would go, and another part that goes on the outside of the wing that accepts the wire undercarriage.
First, 3d print two sets of plates. Then, stick the larger plate inside the wing with the chamfer facing upwards and towards the leading edge (so as not to get in the way when you fold the top over). This means there are three holes at the back and two at the front. Using a barbecue skewer, poke through all the five holes so you can see them on the underneath of the wing. Make sure the plate is well stuck down, Slide it around a bit while the glue is still hot to spread the glue around. You don't want this bit to fall off inside the wing once the wing is glued together,

You'll probably need to freehand cut a relief in the underside of the spar to fit over the plate.

Once the wing is complete and inserted into the fuselage, you can screw on the outside, smaller plate, using the holes you poked through with the skewer to line it up perfectly.

Use four cheese head screws to screw the plates together, making a foamboard sandwich. Remember to drill out the holes in both plates so the screws hold tight, but don't crack the plastic. Make sure you use screws of the correct length, so they don't emerge from the top of the wing.

Drill out the fifth, back middle hole to the exact diameter of your wire undercarriage, Bend the wire to the shape shown, making sure the tail end of the wire isn't so long that it pokes through the top of the wing, but is long enough to go well into the plate inside . Then add the two final cheese head screws along the edge of the slot so that they clamp the wire into the slot. There are no corresponding holes on the inside plate, so using the holes in the smaller plate as a guide, drill through the inside plate to ensure you get a really strong fix for the wire-clamping screws.

Because the inside plate is much bigger than the outside one, it spreads the impact of landing over a larger area of foam without creating a shear line. I've successfully flown a Spitfire using this design for five months now without any problem - a record for me. And there have been a few hard landings in that time. If the wire does bend, it's so well clamped, you can just bend it back.

In case you're interested, I've also created an STL file for the spinner. There are clockwise and counter-clockwise versions and I've posted the link to those on Thingiverse too. The design is an adaptation of a brilliant spinner design done by Aero330.

UCplates.stl

SpinnerCCW.stl
SpinnerCW.stl

Undercarriage plates: http://www.thingiverse.com/thing:2166026
Spinner: http://www.thingiverse.com/thing:2166061