Main rotor rotational speed control for rotorcraft

1. An aircraft comprising:

an airframe having an extending tail;

a counter rotating, coaxial main rotor assembly disposed at the airframe including an upper rotor assembly and a lower rotor assembly;

a translational thrust system positioned at the extending tail and providing translational thrust to the airframe; and

a flight control system including an engine controller configured to control a main rotor rotational speed of the upper and the lower rotor assemblies of the main rotor assembly as a function of airspeed of the aircraft to maintain rotor blade tip speed of the upper and lower rotor assemblies below a threshold rotor tip speed.

2. The aircraft of claim 1 , wherein the flight control system includes a flight control computer configured to:

obtain a threshold main rotor rotational speed; and

maintain the main rotor rotational speed at or below the threshold main rotor speed.

3. The aircraft of claim 2 , wherein the threshold main rotor rotational speed is a function of airspeed of the aircraft and advancing rotor blade tip Mach number.

4. The aircraft of claim 3 , wherein a maximum advancing rotor blade tip Mach number is 0.9 Mach.

5. The aircraft of claim 2 , wherein the flight control computer further utilizes one or more of outside air temperature, wind speed and/or direction to determine the threshold main rotor rotational speed.

6. The aircraft of claim 1 , wherein the flight control computer is configured to set the main rotor assembly to a slowed main rotor rotational speed when the airspeed exceeds a selected airspeed, the slowed main rotor rotational speed lower than a baseline main rotor rotational speed at an airspeed less than or equal to the selected airspeed.

7. The aircraft of claim 6 , wherein the selected airspeed is about 200 Knots.

8. A method of operating an aircraft having a dual coaxial main rotor assembly and a translation thrust system, the method comprising:

calculating a threshold main rotor rotational speed of the dual coaxial main rotor assembly at a flight control computer of a flight control system of the aircraft;

calculating, via the flight control computer, an actual main rotor rotational speed according to an environment of the aircraft; and

maintaining, using an engine controller of the flight control system, the actual main rotor rotational speed to remain at or below the threshold main rotor speed according to an airspeed of the aircraft.

9. The method of claim 8 , wherein the threshold main rotor rotational speed is a function of one or more of airspeed of the aircraft and advancing rotor blade tip Mach number.

10. The method of claim 9 , wherein a maximum advancing rotor blade tip Mach number is 0.9 Mach.

11. The method of claim 8 , further comprising utilizing one or more of outside air temperature, wind speed and/or direction to determine the threshold main rotor rotational speed.

12. The method of claim 8 , further comprising setting the main rotor assembly to a slowed main rotor rotational speed when the airspeed exceeds a selected airspeed, the slowed main rotor rotational speed lower than a baseline main rotor rotational speed at an airspeed less than or equal to the selected airspeed.

13. The method of claim 12 , wherein the selected airspeed is about 200 Knots.