Method and system for improving transition lift-fan performance

1. A method of operating a propulsion system in a vertical take-off and landing (VTOL) aircraft, the VTOL aircraft having a forward thrust propulsion system and a plurality of ducted lift fans, the method comprising:

controlling the forward thrust propulsion system to generate a forward thrust during a transition from vertical flight to forward flight;

selectively allocating, during the transition from vertical flight to forward flight, each of the plurality of ducted lift fans into a first operation group configured to operate at substantially full throttle, a second operation group configured to operate at intermediate throttle, or a third operation group configured to operate at substantially zero throttle,

wherein:

each of the plurality of ducted lift fans is configured to be controlled independently, and

the first operation group, the second operation group, and the third operation group are configured to generate, in aggregate, a first desired thrust as a function of momentum drag; and

selectively reallocating, during the transition from vertical flight to forward flight, one or more of the plurality of ducted lift fans from one of the first operation group, the second operation group, or the third operation group to a different one of the first operation group, the second operation group, or the third operation group to generate a second desired thrust as a function of momentum drag.

2. The method of claim 1 , wherein to generate the first desired thrust, the method further comprises:

operating each ducted lift fan of the first operation group at full throttle;

operating each ducted lift fan of the second operation group at intermediate throttle, wherein the intermediate throttle is configured to balance thrust during the transition from vertical flight to forward flight; and

shutting off each ducted lift fan of the third operation group such that each ducted lift fans of the third operation group operate substantially zero throttle.

3. The method of claim 1 , further comprising controlling the forward thrust propulsion system to generate a forward thrust during forward flight.

4. The method of claim 3 , further comprising allocating each of the plurality of ducted lift fans to the third operation group during forward flight.

5. The method of claim 1 , wherein the second operation group is configured to operate at intermediate throttle by:

varying a rotation per minute (RPM) of one or more of the ducted lift fans of the second operation group;

varying a pitch of one or more of the ducted lift fans of the second operation group; or

a combination thereof.

6. The method of claim 1 , wherein the VTOL aircraft has a disc loading value of about 50 pounds per square foot or less.

7. The method of claim 1 , further comprising covering all of the ducted lift fans of the third operation group, wherein the covering is configured to decrease aerodynamic drag on the VTOL aircraft.

8. The method of claim 1 , further comprising the step of directing a flow of fan exhaust aft of the VTOL aircraft.

9. The method of claim 8 , wherein one or more ducted lift fans of the plurality of ducted lift fans include an adjustable vane, and wherein the step of directing airflow of the fan exhaust aft of the VTOL aircraft comprises adjusting one or more of the adjustable vanes of the one or more ducted lift fans.

10. The method of claim 1 , wherein the plurality of ducted lift fans comprises six or more ducted lift fans.

11. The method of claim 1 , wherein the VTOL aircraft is configured to use a hybrid of turbine thrusting and electric thrusting.

12. The method of claim 11 , wherein the forward thrust propulsion system is configured to employ the turbine thrusting and the plurality of ducted lift fans are configured to employ the electric thrusting.

13. The method of claim 1 , wherein the selectively reallocating, during the transition from vertical flight to forward flight, one or more of the plurality of ducted lift fans from one of the first operation group, the second operation group, or the third operation group to a different one of the first operation group, the second operation group, or the third operation group to generate a second desired thrust comprises:

throttling at least one ducted lift fan of the first operation group from substantially full throttle to substantially zero throttle to reallocate the at least one ducted lift fan of the first operation group to the third operation group; and

maintaining the ducted lift fans of the third operation group at substantially zero throttle.

14. A system for operating a vertical take-off and landing (VTOL) aircraft comprising:

a forward propulsion system;

a plurality of ducted lift fans; and

a controller, wherein the controller is configured to:

control the forward thrust propulsion system to generate a forward thrust during a transition from vertical flight to forward flight;

selectively allocate, during the transition from vertical flight to forward flight, each of the plurality of ducted lift fans into a first operation group configured to operate at substantially full throttle, a second operation group configured to operate at intermediate throttle, or a third operation group configured to operate at substantially zero throttle,

wherein:

each of the plurality of ducted lift fans is configured to be controlled independently, and

the first operation group, the second operation group, and the third operation group are configured to generate, in aggregate, a first desired thrust;

selectively reallocate, during the transition from vertical flight to forward flight, one or more of the plurality of ducted lift fans from one of the first operation group, the second operation group, or the third operation group to a different one of the first operation group, the second operation group, or the third operation group to generate a second desired thrust as a function of momentum drag;

control the forward thrust propulsion system to generate a forward thrust during forward flight; and

allocate each of the plurality of ducted lift fans to the third operation group during forward flight.

15. The system of claim 14 , wherein to generate the first desired thrust, the controller is configured to:

operate each ducted lift fan of the first operation group at full throttle;

operate each ducted lift fan of the second operation group at intermediate throttle, wherein the intermediate throttle is configured to balance thrust during the transition from vertical flight to forward flight; and

shut off each ducted lift fan of the third operation group such that each ducted lift fans of the third operation group operate substantially zero throttle.

16. The system of claim 14 , wherein the second operation group is configured to operate at intermediate throttle by:

varying a rotation per minute (RPM) of one or more of the ducted lift fans of the second operation group;

varying a pitch of one or more of the ducted lift fans of the second operation group; or

a combination thereof.

17. The system of claim 14 , wherein the VTOL aircraft has a disc loading value of about 50 pounds per square foot or less.

18. The system of claim 14 , wherein all of the ducted lift fans of the third operation group are covered to decrease aerodynamic drag on the VTOL aircraft.

19. The system of claim 14 , wherein one of more ducted lift fans of the plurality of ducted lift fans comprise one or more adjustable vanes, and wherein the controller is further configured to adjust one or more of the adjustable vanes to direct a flow of fan exhaust aft of the VTOL aircraft.

20. The system of claim 14 , wherein the VTOL aircraft is configured to use a hybrid of turbine thrusting and electric thrusting.