Slots for Wing Spars:

1. Scribe 2 lines 1/16” apart along each spar position on the wings lower surface.
2. With a piece of tape as a depth gauge on a modelling knife, depth set for the specific spar, make a cut along one line perpendicular to the wing surface.
3. Make a 2nd cut on the other line 1/16” away at an angle to cut out a wedge of foam. Remove the wedge with the point of the blade.
4. Use a 1/16” dia. grinding spindle in a Dremel to turn the triangular slot into a rectangular slot. It helps to place or tape a straight edge along the slot line for both the knife and Dremel cuts.

Wing Spar Design:

I considered the following:

a) The Kevlar skin provides significant structural strength and durability so the need for a wing spar at all is somewhat debatable. However I hate to build a model and later find it is structurally weak, so decided to include a minimal spar(s).
b) The foam alone seems stiff enough for flight loads even without the Kevlar, so a spar will primarily benefit crash resistance. If a tow hook is added, I would not be confident of sustaining launch loads without a spar.
c) I'll use balsa/CF/epoxy composite spars, and as the CF tensile strength is significantly higher than the balsa, the balsa can be minimal.
d) A flat spar fitted tightly in the foam is well supported from deflection, buckling and twisting in the fore/aft direction, so it doesn't matter too much if it is weak in that direction before insertion in the foam. Stiffness against bending in the roll direction is most important.
e) To be stiffness/weight efficient I'd like to have a larger vertical spar dimension at wing root than tip. But it is difficult to cut a varying depth groove in the foam, so decided to have a constant depth spar.
f) Foam removed to accommodate the spars needs to be minimal.
g) Because this is a swept wing, a spar parallel to a % of chord line will have a kink at the root, which will be weak at the root where bending moment is highest. So I decided to have a spar at 33% of chord along each wing, and 2 short length spars perpendicular to the chord line at the wing root, spanning across the wing joint. These short spars are effectively also wing joiners. One of the short spars is at the CG so loads via a tow hook can be transferred directly to that spar.

I eventually decided on spars that are a sandwich of CF tow in-between 2 strips of 1/32” hard balsa glued with epoxy. So only 1/16” wide slots are needed for each spar. The spar depth is dictated by the minimum wing section along each spar line. This is 10 mm for the long spars and 15 mm and 20 mm for the short spars spanning the wing root line. The spars are fitted flush to the wing lower surface as that is where the maximum tensile bending stress is (when not flying inverted).

Wing LE

Elevon hinges:

The elevon hinges will be the Kevlar lower skin. However Kevlar skin at the hinges can de-laminate over time with flexing of the hinge. This is a pain to repair so I'm adding some ripstop nylon secondary hinges. These secondary hinges are trapped between a strip of light 1/32" balsa and both the TE and LE of the wing and elevon respectively. So if the Kevlar de-laminates, the elevon will not sag below the wing and alter the wing lower surface geometry.

Wing Leading Edge:

The LE is reinforced with 2 strips of hard 1/16” balsa. The foam wing as cut is slightly flat at the LE, so I cut off 1/16”, glued on the 1/16” strips and planed/sanded a LE.

Early build pics cont.

Early build pics