The fixed- or rotary-wings of aerospace vehicles comprise of airfoil sections with carefully designed profiles that are optimum for a certain design flight condition, or represent a compromise with acceptable performance over some range of conditions. The ability of an airfoil section to morph or change its profile, as required, can provide future aerospace vehicles with tremendously improved efficiency and versatility. Morphing airfoil sections would allow a single aircraft to be viable for very low-speed loiter/surveillance and high-speed combat missions; or allow a helicopter in high-speed cruise to have a high figure of merit even though the outboard regions of the blades encounter compressibility effects on the advancing side and are close to stall on the retreating side, in every rotor revolution. However, airfoil sections are conventionally designed to be rigid, and must remain so under aerodynamic pressure distributions, even while morphing under actuation loads. This represents a considerable challenge.
In the present project, an innovative smart-material based actuation scheme is proposed in conjunction with special skins that are ultra-flexible in extension yet have significant flexural rigidity. The actuation scheme comprises of an array of linkages or "vertebrae" (with both active and passive members) connected to the upper and lower skins of the airfoil section. In each linkage, extension of the upper active members produces stretching of the upper skin, while contraction of the lower active members produces shrinking of the lower skin, and together they produce a rotation of the passive member. Progressively larger rotations of the successive linkages in the array result in a camber of the airfoil section. Leading-edge droop of the airfoil section is produced in a similar manner. Skins that are highly flexible in extension and compression and can accommodate large elastic strains will be required to achieve significant profile changes. Yet it is required that the skins be flexurally stiff so sections between supports do not deform under aerodynamic surface pressures. Such skins are to be designed using Shape Memory Alloys or Flexible Matrix Composites, in conjunction with innovative geometric configuration.
A finite element analysis of the airfoil section with spar, flexible skin and linkages (comprising of active and passive members), will be developed as part of this project. Using design optimization, the geometry of the linkages and the properties of the skin that produce both target as well as maximum profile changes will be determined. Design constraints will include actuator saturation limits, maximum strains on the skin, and rigidity under aerodynamic loading. Both Shape Memory Alloy actuators (for maximum actuation strain capabilities) as well as piezoelectric actuators (for high-frequency capabilities required for helicopters) will be examined. For high-frequency applications, amplification of actuation inputs due to structural resonance will be considered.
