Fixed-wing aircraft, such as airplanes, are capable of flight using wings that generate lift responsive to the forward airspeed of the aircraft, which is generated by thrust from one or more jet engines or propellers.  The wings generally have an airfoil cross section that deflects air downward as the aircraft moves forward, generating the lift force to support the aircraft in flight.  Fixed-wing aircraft, however, typically require a runway that is hundreds or thousands of feet long for takeoff and landing. 
Unlike fixed-wing aircraft, vertical takeoff and landing (VTOL) aircraft do not require runways.  Instead, VTOL aircraft are capable of taking off, hovering and landing vertically.  One example of a VTOL aircraft is a helicopter which is a rotorcraft having one or more rotors that provide lift and thrust to the aircraft.  The rotors not only enable hovering and vertical takeoff and landing, but also enable forward, backward and lateral flight.  These attributes make helicopters highly versatile for use in congested, isolated or remote areas.  Helicopters, however, typically lack the forward airspeed of fixed-wing aircraft due to the phenomena of retreating blade stall and advancing blade compression. 
Tiltrotor aircraft attempt to overcome this drawback by including a set of proprotors that can change their plane of rotation based on the operation being performed.  Tiltrotor aircraft generate lift and propulsion using proprotors that are typically coupled to nacelles mounted near the ends of a fixed wing.  The nacelles rotate relative to the fixed wing such that the proprotors have a generally horizontal plane of rotation for vertical takeoff, hovering and landing and a generally vertical plane of rotation while cruising in forward flight, wherein the fixed wing provides lift and the proprotors provide forward thrust.  In this manner, tiltrotor aircraft combine the vertical lift capability of a helicopter with the speed and range of fixed-wing aircraft.  Tiltrotor aircraft, however, utilize openly exposed proprotors, which may present a number of drawbacks.  For example, openly exposed proprotors can lead to blade tip thrust losses during flight, thrust vectoring capabilities are limited and use of pressure differentials to augment thrust is limited.