The high strength to weight and stiffness of carbon-fiber-reinforced tubes have made them the obvious choice for spars in small UAVs. They are cheap, and available in a wide variety of diameters, thicknesses, and ply orientations. Perhaps the only issue we find when using such tubes for spars is that they are not simple to drill or machine since the material tends to splinter rather than cut cleanly - therefore we try and avoid making holes in them or doing anything other than cutting them to the required length. Instead, we typically use selective laser-sintered (SLS) nylon clamps and bushes to join them to the rest of the aircraft - since nylon is very much less stiff than typical carbon-reinforced plastic, this provides suitable cushioning at any joints and reduces stress concentrations in the spars. The loads that can be carried by such spars when correctly engineered into designs is prodigious - Figure 3.5 shows the main spar and wing assembly of the SPOTTER UAV under sandbag static loading - a single 35 mm diameter spar is here carrying a total mass of nearly 100 kg on each wing, equivalent to a 7 g gust loading. Ultimately, the foam parts of the wing failed before the carbon spar in this test.

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