[sfwa]

Airframe assembly

Our airframe is the Skywalker X8: a flying wing design with approx 2m wingspan, 800g frame only, ∼5kg maximum take-off weight, and EPO foam construction with carbonfibre rods to stiffen wings and plywood wing and motor mounts.

We picked the X8 for its relatively large payload capacity, expected ability to cope with high winds (ideally 50km/h for the OBC), and its low price; we plan to destroy a few airframes during development.

The flying wing approach is also nice from the perspective of our intended control approach (NMPC), since the computational load varies with the number of control outputs. In the case of the X8, we only have to worry about the the two elevons and the throttle.

Most people seem to be using 11–14″ props and 4S ∼800KV motors. Several have reported flight times well over an hour when using multiple 10Ah batteries, so endurance should not be a problem assuming we can get the right combination of prop and motor.

While planning the build, our objective was to make as few modifications to the stock airframe as possible, since we’re treating them as disposable. Obviously there’s no point getting a cheap airframe if each one requires hours of modifications to be useable.

We stuck everything together with foam-safe CA for foam/foam joins, and two-part epoxy for the foam-plywood and foam-carbonfibre joins. There seems to be some debate online over whether it’s best to use foam-safe or non-foam-safe CA (and then whether or not kicker should be used), but nothing major has broken yet so our approach seems good enough.

Servos were mounted at the top of the wings (cutouts are at the bottom, but that seems like a very bad idea). Slits were cut in the top surface and control horns attached; the servos were glued in from the bottom and then the gaps were covered with fibreglass tape. The servos used were Hitec 225MGs, which are small, sufficiently powerful, and very cheap (just as well since they're stuck in the wings now).

Photo showing servo placement on underside of wing, covered in fibreglass tape Photo showing control rod placement and slit on top of wing

All seams (there were a few big ones, QC doesn’t appear to be especially good) were covered with fibreglass tape. After wings are attached prior to flight (wire through the holes in the plywood mounts), we also cover the wing seam with fibreglass tape, and add a couple of lateral strips across the top and bottom of the fuselage reaching the wings—during some of the early flights the wings started to detach.

Fibreglass tape hinges seem good for elevon attachment; initially we used 3M Blenderm tape but that stretches too much in flight.

Lessons learned from test flights:

Photo showing spinner, motor and stock motor mount

Lessons remaining to be learnt:

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