Eurocopter's Tiger helicopter incorporates an engine cowling stiffened with hat stringers made with ROHACELL XT polymethacrylimide (PMI) foam from Degussa Performance Plastics (Darmstadt, Germany). Previously, the approximately 1.5m/4.8-ft long and 0.8m/2.6-ft wide cowling was a solid laminate made with bismaleimide (BMI) prepregs that could withstand the high engine temperatures (160°C/320°F). The stringers allow a lighter cowling with a thinner BMI skin.
The 80 mm/3.2-inch wide and 1.5m/4.8-ft long stringers are CNC machined and thermoformed to shape by Eurocopter. Layup is done on a male tool and the entire cowling including the stringers is co-cured. The XT (extended temperature) grade PMI foam is the highest performing foam offered, with a high enough heat distortion temperature to withstand the BMI's autoclave cure at 190°C/374°F and 90 psi and 230°C/446°F post-cure. The PMI foam offers excellent compression strength (1.7 to 3.6 MPa/246 to 522 psi), creep resistance and buckling resistance as well as light weight. The cowling now weighs significantly less, saving overall aircraft weight, and manufacturing costs have decreased as well.ROHACELL structural foam has also been developed for resin infusion processes. Watch for the RI family of PMI foams, including RIMA (resin infusion manufacturing aid) and RIST (resin infusion structural sandwich core). Both are low-density, low creep foams designed for use as a mandrel or as a stay-in-place core for complex parts made in a closed mold, with zero resin uptake.