Rotorcraft including auxiliary propulsor positioned to ingest boundary layer flow

1. A rotorcraft, comprising:

a fuselage having a longitudinal axis, the fuselage including drag generating portions, a main rotor assembly having an upper rotor, a lower rotor and a pylon arranged between the upper and lower rotor, the pylon defining a pylon axis that is perpendicular to the longitudinal axis, and an auxiliary propulsor having an expected propulsion efficiency at least a portion of the auxiliary propulsor extending above a midpoint of the pylon axis;

the auxiliary propulsor being disposed to ingest boundary layer flows at least a portion of which flows along the pylon axis and in wake regions associated with the drag generating portions establishing an increase in the expected propulsion efficiency of the rotorcraft by canceling a pressure drag increase associated with the auxiliary propulsor, wherein the drag generating portions include a main rotor pylon supporting the main rotor assembly, and aerodynamic features disposed on sides of the fuselage on the main rotor pylon, the drag generating features creating axisymmetric and near axisymmetric boundary layer flows and wake regions during forward flight of the rotorcraft.

2. The rotorcraft according to claim 1 , wherein the main rotor assembly comprises coaxial, counter-rotating main rotors.

3. The rotorcraft according to claim 1 , wherein the auxiliary propulsor is disposed aft of a rotor disk defined by the main rotor assembly.

4. The rotorcraft according to claim 1 , wherein the auxiliary propulsor is disposed to intersect with a rotor of the main rotor assembly and respective rotations of the auxiliary propulsor and the rotor are timed to avoid mutual collisions.

5. The rotorcraft according to claim 1 , further comprising:

additional drag generating portions and additional auxiliary propulsory having respective expected propulsion efficiencies disposed on the fuselage;

the additional auxiliary propulsors being disposed in additional wake regions associated with the additional drag generating portions and being provided with corresponding increases in the respective expected propulsion efficiencies thereof.

6. The rotorcraft according to claim 5 , wherein the additional drag generating portions comprise at least one or more of landing gear elements, skids and removable payloads.

7. The rotorcraft according to claim 5 , wherein the additional auxiliary propulsors comprise ducted rotors.

8. A method of designing a rotorcraft, comprising:

assembling a fuselage having a longitudinal axis including drag generating portions, a main rotor assembly having an upper rotor, a lower rotor and a pylon arranged between the upper and lower rotor, the pylon defining a pylon axis that is perpendicular to the longitudinal axis, and an auxiliary propulsor, wherein the drag generating portions include a main rotor pylon supporting the main rotor assembly, and aerodynamic features disposed on sides of the fuselage on the main rotor pylon, the drag generating portions creating axisymmetric and near axisymmetric boundary layer flows and wake regions during forward flight of the rotorcraft;

identifying the axisymmetric and near axisymmetric boundary layer flows and wake regions associated with the drag generating portions; and

positioning the auxiliary propulsor such that at least a portion of the auxiliary propulsor extends above a midpoint of the pylon axis so as to ingest boundary layer flows in the wake regions to provide an increase in an expected propulsion efficiency by canceling a pressure drag increase associated with the auxiliary propulsor of the auxiliary propulsor.

9. The method according to claim 8 , wherein the positioning comprises positioning the auxiliary propulsor aft of a rotor disk defined by the main rotor assembly.

10. The method according to claim 8 , wherein the positioning comprises:

positioning the auxiliary propulsor to intersect with a rotor of the main rotor assembly; and

timing respective rotations of the auxiliary propulsor and the rotor to avoid mutual collisions.

11. The method according to claim 8 , wherein the positioning comprises:

identifying that an initial design position of the auxiliary propulsor will lead to an expected increase in an efficiency benefit cancelling pressure drag increase; and

re-positioning the auxiliary propulsor increasingly aft of the initial design position until the expected increase in the efficiency benefit cancelling the pressure drag increase is reduced.

12. The method according to claim 8 , further comprising:

assembling additional drag generating portions and additional auxiliary propulsors onto the fuselage;

identifying additional wake regions associated with the additional drag generating portions; and

positioning the additional auxiliary propulsors in the additional wake regions to provide an increase in respective expected propulsion efficiencies of the additional auxiliary propulsors.