A tip gap control system for a ducted aircraft includes a flight control computer including a blade length control module configured to generate a blade tip actuator command and a proprotor system in data communication with the flight control computer. The proprotor system includes a duct and proprotor blades surrounded by the duct. Each of the proprotor blades includes an active blade tip movable into various positions including a retracted position and an extended position. The tip gap control system also includes one or more actuators coupled to the active blade tips. The one or more actuators move the active blade tips between the various positions based on the blade tip actuator command, thereby controlling a tip gap between the proprotor blades and the duct.

A variable pitch propeller is designed to adjust the pitch of the propeller blade during flight to maximize the propeller efficiency. The propeller blade may comprise airfoil cross-sections. Each cross-section may be composed of different materials at the leading edge and trailing edge. In various embodiments, these materials are selected and oriented to achieve the necessary elastic moduli of the leading and trailing edge for the airfoil cross-section. During liftoff, the airfoil at the blade tip possesses a high blade pitch (e.g. 20 degrees), thereby increasing the generated lift on the propeller blades. During flight or hover conditions when maximal lift is no longer required, the trailing edge of the airfoil displaces upward and reduces the blade pitch to minimize the drag forces on the blade tip.

A variable pitch propeller is designed to adjust the pitch of the propeller blade during flight to maximize the propeller efficiency. The propeller blade may comprise airfoil cross-sections. Each cross-section may be composed of different materials at the leading edge and trailing edge. In various embodiments, these materials are selected and oriented to achieve the necessary elastic moduli of the leading and trailing edge for the airfoil cross-section. During liftoff, the airfoil at the blade tip possesses a high blade pitch (e.g. 20 degrees), thereby increasing the generated lift on the propeller blades. During flight or hover conditions when maximal lift is no longer required, the trailing edge of the airfoil displaces upward and reduces the blade pitch to minimize the drag forces on the blade tip.

A propeller system combines innovative strategies to create a new methodology to reduce propeller or rotor noise. The propeller is specifically aimed for ultra-quiet electrically powered aircraft for use in high proximity aviation, but its low-noise advantages will extend to other purposes. The propeller blade includes geometries, along with size and operational limitations that minimize rotational and vortex noise, vibration and span-wise air flow on the blade. To further reduce noise, the propeller provides greater relative thrust on the inboard portions of the blade than do conventional propellers and provides less than conventional relative thrust including negative thrust at the outermost portions of the blade. The propeller blade includes stepped changes in local blade stiffness at calculated intervals that can reduce resonant blade vibrations and their resultant noise. This ultra-quiet propeller design can also be used for quieting hovercraft, drones, surveillance aircraft, indoor fans, wind tunnels and other applications.

A propeller system combines innovative strategies to create a new methodology to reduce propeller or rotor noise. The propeller is specifically aimed for ultra-quiet electrically powered aircraft for use in high proximity aviation, but its low-noise advantages will extend to other purposes. The propeller blade includes geometries, along with size and operational limitations that minimize rotational and vortex noise, vibration and span-wise air flow on the blade. To further reduce noise, the propeller provides greater relative thrust on the inboard portions of the blade than do conventional propellers and provides less than conventional relative thrust including negative thrust at the outermost portions of the blade. The propeller blade includes stepped changes in local blade stiffness at calculated intervals that can reduce resonant blade vibrations and their resultant noise. This ultra-quiet propeller design can also be used for quieting hovercraft, drones, surveillance aircraft, indoor fans, wind tunnels and other applications.

A method for separating a first mechanical part from a second mechanical part is described, wherein the second mechanical part is bonded to the first mechanical part by an adhesive film along a connecting area, the first mechanical part having a first specific thermal conductivity and the second mechanical part having a second thermal conductivity that is higher than the first thermal conductivity. The method includes at least one cooling step during which the second mechanical part is cooled to a negative temperature and at least one stressing step during which the second mechanical part is subjected to mechanical stress in order to cause the adhesive film to break.

A method for separating a first mechanical part from a second mechanical part is described, wherein the second mechanical part is bonded to the first mechanical part by an adhesive film along a connecting area, the first mechanical part having a first specific thermal conductivity and the second mechanical part having a second thermal conductivity that is higher than the first thermal conductivity. The method includes at least one cooling step during which the second mechanical part is cooled to a negative temperature and at least one stressing step during which the second mechanical part is subjected to mechanical stress in order to cause the adhesive film to break.

A propeller having a plurality of blades extending radially outward from a hub, the blades forming a loop. Each loop having an intake portion, an exhaust portion and a tip portion extending radially outward from the hub and a gap between the intake root and the exhaust root. The tip portion of each of the blades is 30%-75% of the blade, the tip portion beginning at a first deviation from zero of the roll value and extending to 90 degrees, wherein roll value is zero in a plane parallel to the hub axis, and wherein the blades have a vertical angle between −45 degrees and 45 degrees throughout.

A propeller having a plurality of blades extending radially outward from a hub, the blades forming a loop. Each loop having an intake portion, an exhaust portion and a tip portion extending radially outward from the hub and a gap between the intake root and the exhaust root. The tip portion of each of the blades is 30%-75% of the blade, the tip portion beginning at a first deviation from zero of the roll value and extending to 90 degrees, wherein roll value is zero in a plane parallel to the hub axis, and wherein the blades have a vertical angle between −45 degrees and 45 degrees throughout.

A three-dimensional auxetic structure, comprising a plurality of adjoining hollow cells, each hollow cell having cell walls and a transversal cross section of the plurality hollow cells following a two-dimensional auxetic pattern, each cell wall comprising folding lines parallel to a plane containing the auxetic pattern such that peaks and valleys are defined in the cell walls and the cell walls being foldable along the folding lines.