Bioinspired UAV Wing

Abstract

The operational range of UAVs is typically related to aircraft size; smaller low-cost UAVs cannot be used for certain missions that requi re extended range. A concept for extending the range of small UAVs is to fold their large, fragile wings and transport them to their operational region. Local range extension is achieved through high lift to drag ratios, hence large span. Disadvantages of pre-existing des igns include: non-compact wing storage, lack of wing stiffness, low reliability and long de ployment time. We improve upon the design of deployable wings by using a method inspired by moth wing expansion during eclosion. We rapidly rigidize a soft fabric wing shortly after d eployment through a combination of two separate self-contained chemical reactions. The pri mary reaction is generation of UV radiation by combustion of metallic magnesium with a solid oxidizer. The radiation activates UV hardening of an acrylic adhesive in a UV-transmissive quartz-fiber wing structure. The resulting wing structure has mechanical properties similar to a non-deployable wing but deploys rapidly and reliably from a very compact package. We have successfully demonstrated a tubular composite structure that can be inflated and cured to full strength in less than 20 seconds in an inert atmosphere without external energy sources. I. Introduction eployable wings have been devised using various design concepts over a period of many years. The most notable technologies have been mechanically hinged wings, pressurized inflatable fabric wings and post rigidized inflatable wings. Mechanical hinging is t he simplest and most common method for folding a traditional aircraft wing. This design has the advantage of sim plicity and ease of adaptation to thin chord wings. However each mechanical hinge can only reduce the wingspan by a maximum of 50%, therefore each additional reduction in stowed length doubles the number of hinged joints. This exponential increase in mass causes structural deficiencies and leads to reliability problems. On the opposite end of the spectrum are inflatable wings. They are composed of flexible fabric materia l that is fabricated into a segmented compartmentalized struc ture and is pneumatically inflated to extend to its full size, supported entirely by internal pressure. Since infl atable wings are made of fabric they are capable of high length and volume reduction ratios. Their low mass allows them to be deployed in seconds or less. Like the hinged wing, inflatable wings are restowable and redeployable. While inflatable wings have very high deployment reliability, continuous positive pressure is required to maintain structural integrity of the wing. This res ults in a vulnerability to loss of pressure from le aks or punctures. Another major drawback to the inflatable wing is on set of buckling at a lower bending moment compared to rigid wings. The two fundamental disadvantages of positively inflated wing structures are stiffness (i.e. re sistance to buckling) and vulnerability to pressure loss. Both can be improved upon by rigidizing the flexible win g fabric shortly after inflation. By encapsulating the fabri c fibers in an adhesive matrix and curing the adhes ive after the wings have been deployed, the assembly becomes a structural composite. The main disadvantage to this design is the long cu ring time of the adhesive. The hardening of an adhe sive is time intensive, and typically requires evaporation or a catalytic reaction. UV cured adhesives can har den orders of magnitude more rapidly, but require exposure to lar ge quantities of UV energy. Successful experiments have been conducted using sunlight as the UV source [1]. Howe ver since sunlight’s UV power is limited the cure t ime during these experiments was approximately 7-10 minutes. Dependence on sunlight also has a considerable disad vantage in that curing is impossible during nighttime or cloud y conditions. Onboard UV generation from battery powered UV