Projects – Detailed design

Strut design

Struts are the braces that hold the wing in place and carry virtually all loads on the wing. Negative lift on the wing trying to buckle the struts is usually the limiting load case, driving a cross-section that’s optimized for it’s buckling resistance.

Original strut concept. Heavy, complex and draggy.

While high-strength tubes with an airfoil section are available on the market, they are expensive, relatively heavy and still require lots of bits and pieces to be mounted to the fuselage and wing.

Instead, using an airfoil-cross-section with an extra trailing part for the glue joint, we made the entire strut in a single part, with the end points directly glued in when the airfoil section was glued shut. Finishing was a mere matter of trimming back the trailing edge and the weight savings were substantial (>50%).

New concept for a wing strut; one-step fabrication.

BRS parachute system attachment

Parachute rescue systems for the entire airframe are an option that gained popularity in recent years. For certain classes of aircraft they are mandatory.

The airframe originally had a Kevlar strap wrapped around the central bulkhead to transfer the parachute loads (up to 10 kN, 2200 lbsf) to the rest of the airframe. Testing showed that the strap loads resulted in the skin being pulled away from the bulkhead, resulting in buckling and ultimately failure of the skin.

Originally the parachute strap entered from the lower left and wrapped all the way around the central Kevlar bulkhead (yellow).
BRS parachute attachment load test. Buckling of the front half of pylon skin is obvious.

 

 

Total failure of the pylon due to the parachute strap pulling the central bulkhead from the skin.

We sought an alternative approach where the separate strap was replaced by extra strands of carbon and Kevlar integrated in the fuselage skin. Our big concern was transferring the loads to a shackle such that the parachute could be attached. The only conceivable way to do this was bolting it to a metal transfer-piece. With bolt holes being a traditional weak point of composites, we optimized the laminate at the end of the fiber strands for a bolted connection and tested it to destruction.

Testing of the bolted integrated BRS parachute strap. Failure didn’t occur until 18 kN (4000 lbsf)

The design proved to be satisfactory, yielding a reduction in parts count, cost and build time.