A method of generating a pressure wave proximate an airflow surface and altering airflow to promote a localized lowering of skin friction over the airflow surface is described herein. A series of pressure waves may be configured to create a virtual riblet to control turbulent vortices in a boundary layer adjacent to the airflow surface creating a virtual riblet. The pressure waves may be configured to prevent disruption of the flow of air relative to at least one of a step or a gap associated with the airflow surface. The pressure wave generating system may be comprised of at least one of a thermoacoustic material, a piezoelectric material and a semiconductor material, and a microelectric circuit.

Aerial vehicles may include propulsion units having motors with drive shafts that may be aligned at a variety of orientations, propellers with variable pitch blades, and common operators for aligning the drive shafts at one or more orientations and for varying the pitch angles of the blades. The common operators may include plate elements to which a propeller hub is rotatably joined, and which may be supported by one or more linear actuators that may extend or retract to vary both the orientations of the drive shafts and the pitch angles of the blades. Operating the motors and propellers at varying speeds, gimbal angles or pitch angles enables the motors to generate forces in any number of directions and at any magnitudes. Attributes of the propulsion units may be selected in order to shape or control the noise generated thereby.

The rotary airfoil 100 defines a cross section and a span, wherein the cross section is a function of the point along the span (e.g., spanwise point) and defines an upper surface and a lower surface at each spanwise point. The rotary airfoil 100 also defines, at a cross section, a lift coefficient (C.sub.L) that is a function of the angle of attack at which the airfoil is rotated through the air. The system can optionally include: a rotor hub to mount the rotary airfoil, a tilt mechanism to pivot the rotary airfoil between a forward configuration and a hover configuration, and a pitching mechanism to change the angle of attack of the rotary airfoil 100.

A method and to a system of reducing the noise generated in-flight by a vortex wake caused by each first blade of a main rotor of a hybrid helicopter. The hybrid helicopter includes a main rotor, at least two wings and at least one propeller. The method enables a stabilized flight phase on the level or with a non-zero aerodynamic slope to be implemented by determining a first value of the pitch of the second blades of each propeller and an angle of incidence of the main rotor as function of the flight conditions, then by applying the first pitch value to each propeller and by applying the angle of incidence to the main rotor so as to direct the vortex wake to limit the noisy interactions between the vortex wake and the other first blades and/or the second blades.

An unmanned aerial vehicle (UAV) includes an audio source collecting microphone detecting a target audio signal, background noise-producing components producing background noise different from the target audio signal, background microphones collecting the background noise, and noise emitters each configured to reduce the background noise by emitting an audio signal having a reverse phase of the collected background noise collected by the corresponding background microphone. The background microphones further collect reduced background noise. The UAV further includes a processor receiving signals which comprise the target audio signal and the reduced background noise, and generating a processed signal based on the collected reduced background noise to reduce interference by the background noise to the target audio signal.

Disclosed is a system, method, and aircraft for controlling acoustic radiation from an aircraft comprising a plurality of rotor systems and a noise controller configured to regulate acoustic radiation from the plurality of rotor systems. The noise controller can be configured to regulate a commanded flight setting from the flight control system and to output a regulated flight setting to the plurality of rotor systems. Based on the regulated flight setting, the plurality of rotor systems are configured to generate, individually and in aggregate, acoustic radiation having a target acoustic behavior. In certain aspects, the noise controller can change the directionality of acoustic radiation from the plurality of rotor systems, or otherwise tune the acoustic radiation to reduce detectability or annoyance.

Disclosed is an aerial vehicle having a reduced noise signature. The aerial vehicle may be a vertical take-off and landing (VTOL) aerial vehicle. The aerial vehicle comprises an airframe and a plurality of rotors operatively coupled with one or more motors. The plurality of rotors may comprise a first, second, third, and fourth rotor. Each of the first, second, third, and fourth rotors may be arranged in a single plane and oriented to direct thrust downward relative to the airframe. In certain aspects, at least two of the plurality of rotors employ a different geometry to generate a targeted noise signature.

Rotor noise cancellation through the use of mechanical means for a personal aerial drone vehicle. Active noise cancellation is achieved by creating an antiphase amplitude wave by modulation of the propeller blades, by utilizing embedded magnets through an electromagnetic coil encircling the propeller blades. A noise level sensor signals the rotor control system to adjust the frequency of the electromagnetic field surrounding the rotor and control the speed of the rotor. An additional method comprises of incorporating a phase lock loop within the control system configured to determine the frequencies corresponding to the rotors and generate corrective audio signals to achieve active noise cancellation.

A system for reducing rotor acoustics of an aircraft. The system includes a structural feature of an aircraft and a propulsor configured for fixed-wing flight mounted on the structural feature. The system further includes a plurality of rotors mounted on the structural feature. The plurality of rotors are configured to include a first rotor, a first motor mechanically coupled to the first rotor, a second rotor, and a second motor mechanically coupled to the second rotor. The system further includes an aircraft controller communicating with the first motor, second motor, and the propulsor. The system includes an alignment module configured to place the second rotor in alignment with the first rotor. The system further includes a rotational control module configured to initiate rotation of the plurality of rotors. Initiating rotation of the plurality of rotors further includes initiating rotation of the first rotor and the second rotor.

Systems and methods control sounds produced by a remote-controlled vehicle during operation of the remote-controlled vehicle are described herein. One or more components of the remote-controlled vehicle may be controlled and/or manipulated so that the remote-controlled vehicle produces sounds during operation that match a predetermined set of sounds. The control and/or manipulation of the one or more components of the remote-controlled vehicle may be effectuated without alternating a path of the remote-controlled vehicle.