Applications and Mechanics of Pressure Adaptive Honeycomb

One of the most compliant structures in aerospace applications that does not suffer from certification constraints is plain honeycomb. It is widely used in primary and secondary structure of FAR 23/25 certified aircraft. In this research, the compliant nature of this material is being exploited by inserting pouches in each of the honeycomb cells. Pressurizing these pouches results in a stiffening of the overall structure. By having an external (spring) force act on the honeycomb structure, this variable stiffness results in an overall deformation of the honeycomb. Strains in excess of 50% can be achieved through this mechanism without encountering the material (yield) limits. It can be shown that based on the maximum pressure that can be extracted from the High-Pressure Compressor in a typical jet engine, the energy density of pressure adaptive honeycomb is on the par with that of shape memory alloy, while exhibiting strains that are an order of magnitude larger at a transfer efficiency that is close to 1. The paper discusses the mechanics of pressure adaptive honeycomb and describes a simple reduced order model that can be used to simplify the geometric model in a finite element environment. The theory that underpins this reduced order model is shown to correlate well to experimental tests. In addition, a proof-of-concept application is presented where pressure-adaptive honeycomb is integrated over the aft 35% of a wing section. It is demonstrated that camber variations in excess of 5% can be generated by a pressure differential of 40kPa. Results of subsequent wind tunnel test show an increase in lift coefficient of 0.3 at a wind speed of 45kts across an angle of attack ranging from −6° to +20°.