Distributed propulsion system and method of control

1. A method of controlling power distribution in an electrical aircraft propulsion system, the electrical aircraft propulsion system including: (i) a plurality of electrical propulsion units, each of the plurality of electrical propulsion units having a plurality of rotatable blades on a rotor, each blade of the plurality of blades having an adjustable pitch; (ii) a pitch adjusting mechanism for each of the plurality of electrical propulsion units, each pitch adjusting mechanism being configured to adjust the pitch of the plurality of blades of each of the plurality of propulsion units simultaneously so as to avoid a non-symmetrical load profile on a hub; (iii) at least one electrical machine electrically connected to the plurality of electrical propulsion units so as to provide electrical power when in use; and (iv) a control system, the method comprising steps of:

determining a required propulsion;

determining whether the plurality of propulsion units are collectively delivering the required propulsion; and

simultaneously adjusting a pitch angle of the plurality of blades of at least one of the plurality of propulsion units so as to increase or decrease the propulsion provided by that propulsion unit.

2. The method of controlling power distribution in the electrical aircraft propulsive system as claimed in claim 1 , wherein the step of determining whether the plurality of propulsion units is delivering the required propulsion includes, for each propulsion unit:

monitoring a rotational state and pitch angle of the rotatable blades; and

determining whether the rotational state is within a predetermined range.

3. The method of controlling power distribution in the electrical aircraft propulsive system as claimed in claim 2 , wherein the monitoring the rotational state includes:

monitoring an angular position of the blades; and

determining either or both of a rotational speed and angular acceleration of the rotatable blades.

4. The method of controlling power distribution in the electrical aircraft propulsive system as claimed in claim 1 , further comprising the step of:

determining whether parasitic oscillations exist in the rotation of the blades in one or more of the plurality of propulsion units, and adjusting the pitch of the blades to damp the oscillations.

5. The method of controlling power distribution in the electrical aircraft propulsive system as claimed in claim 4 , wherein the determining whether parasitic oscillations exist includes a step of monitoring an acceleration and deceleration of the rotor.

6. The method of controlling power distribution in the electrical aircraft propulsive system as claimed in claim 4 , wherein only a portion of the propulsion units in the plurality of propulsion units is used to act as dampers.

7. The method of controlling power distribution in the electrical aircraft propulsive system as claimed in claim 1 , further comprising adjusting the pitch of the blades in one or more propulsion units of the plurality of propulsion units to generate a reverse thrust in order to generate electrical energy for an electrical network.

8. An electrical propulsion system for an aircraft, comprising:

a plurality of electrical propulsion units each having a plurality of rotatable blades, each blade of the plurality of blades having an adjustable pitch;

a pitch adjustment mechanism for each of the plurality of electrical propulsion units, each pitch adjusting mechanism being configured to adjust the pitch of the plurality of blades of each of the plurality or propulsion units simultaneously so as to avoid a non-symmetrical load profile on a hub;

at least one electrical machine electrically connected to the plurality of propulsion units so as to provide electrical power in use; and

a control system configured to; (i) determine a required propulsion and propulsion being delivered by the propulsion system, and (ii) simultaneously adjust the pitch of the plurality of blades of at least one propulsion unit of the plurality of propulsion units so as to increase or decrease the propulsion provided by that propulsion unit.

9. The electrical propulsion system for the aircraft as claimed in claim 8 , wherein the control system is configured to: (i) monitor a rotational state and pitch angle of the rotatable blades, and (ii) determine whether the rotational state is within a predetermined range.

10. The electrical propulsion system for the aircraft as claimed in claim 9 , wherein the monitoring of the rotational state includes: (i) monitoring an angular position of the blades, and (ii) determining either or both of a rotational speed and angular acceleration of the rotatable blades.

11. The electrical propulsion system for the aircraft as claimed in claim 8 , wherein the control system is further configured to: (i) determine whether parasitic oscillations exist in the rotation of the blades in one or more of the plurality of propulsion units, and (ii)issue a control signal to the pitch adjustment mechanism of the one or more of the plurality of propusion units to adjust the pitch of the blades so as to damp the oscillations.

12. The electrical propulsion system for the aircraft as claimed in claim 8 , wherein the control system includes: (i) a central control system configured to control a distribution of power between the plurality of propulsion units, and (ii) a secondary control unit at each of the plurality of propulsion units, the secondary control unit configured to determine whether the propulsion units are operating within predetermined rated limits.